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Table of Contents

Actuators:

Introduction

I – Electrical Actuators

The advantages of the electric systems The disadvantages of the electric actuators Classification of the actuators

1-Linear Actuator

Terms used in linear actuators

2 – Motors

A.C. motors D.C. motors Stepper motors Detailed analysis of D.C. motors Starting Large D.C. motors The effect of inertia and inductance

3 - Solenoid

Definition of Solenoid . Technical considerations . Types of solenoids . Example on Application of Solenoids .

II – Hydraulic Actuators

Components of Hydraulic system . Types of valves used . Types of cylinders used . Advantages & Disadvantages of Hydraulic actuators .

III – Pneumatic Actuators

Some basic characteristics of pneumatic systems . Some symbols for pneumatic systems .

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Advantages and Disadvantages of pneumatic actuators .

Sensors:

Introduction

I – Proximity Sensors

Definition of Proximity Sensor . Detection Mechanism . Working range . Classification . Features of Proximity Sensor .

II - Strain Gauge

Introduction . Definition of Strain .

Strain Gauge Measurement .

Calibration .

III -Thermocouple

Introduction . Definition of thermocouple . Thermoelectric characteristics . Thermocouple Calibration Procedures . Applications . Appendix 1 .

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Actuators

Introduction

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Actuators are a type of tool which is used to put something into automatic action. It is used on a wide variety of sources, from humans putting something into action to computers starting up a program. Whichever type of actuator you are in need of, there are several different tools that can help you achieve putting something into motion.

There are several reasons why actuators may be used in an office work area. Most of these will be used as the thing that positions or moves a certain object. For example, many will be used to move or position valve doors in certain systems. They may also be used to maneuver certain mechanical devices that are used in a work area. By using actuators, it is easier for one to work in their area, as well as easier to maneuver something around in the certain area. The main way in which actuators are divided for best use is through their shape and style. Through the several different types of styles available, one is able to determine which type of actuator will be best for their task . In this Report we will focus on hydraulic , pneumatic &electric actuators.

Definition of actuator:

An actuator is the device that brings about the mechanical movements required for any physical process in the factory. Internally, actuators can be broken down into two separate modules: the signal amplifier and the transducer. The amplifier converts the (low power) control signal into a high power signal that is fed into the transducer; the transducer converts the energy of the amplified control signal into work; this process usually involves converting from one form of energy into another, e.g. electrical motors convert electrical energy into kinetic energy.

Types of Actuators

Here we will focus only on :

Electric Actuators

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Hydraulic Actuators

Pneumatic Actuators

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I – Electric Actuators

Electrically actuated systems are very widely used in control systems because they are easy to interface with the control systems which are also electric and because electricity is easily available unlike fluid power which require pumps and compressors .

Electric actuators mount on valves which, in response to a signal, automatically move to a desired position using an outside power source. Single-phase or three-phase AC or DC motors drive a combination of gears to generate the desired torque level.

The advantages of the electric systems are :

Electricity is easily routed to the actuators ; cables are simpler than pipe work .

Electricity is easily controlled by electronic units .Electricity faults are often easier to diagnose .

The disadvantages of the electric actuators are :

Electrical equipment is more of a fire hazard than other systems unless made intrinsically safe in which case it becomes expensive .

Electric actuators have a poor torque – speed characteristics at low speed .

Electric actuators are all basically rotary motion and complicated mechanisms are needed to convert rotation into other forms of motion .

The power to weight ratio is inferior to hydraulic motors.

Classification of the actuators according to :

Movement Multi-turn actuators Part-turn actuators

Linear actuators

Design Motor (1) Limit and torque sensors (2)

Gearing (3) 6

Valve attachment (4)

Manual operation (5)

Actuator controls (6)

Electrical connection (7)

Field bus connection (8)

Functions Automatic switching off in the end positions Safety functions

Process control functions

Diagnosis

Duty types Open-close duty Positioning duty

Modulating duty

Service conditions Enclosure protection Ambient temperatures

Explosion protection

We will focus only on the types given below :

Linear actuators Motors Solenoid

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Linear Actuators

Electric linear actuators provide linear motion via a motor-driven ball screw, lead screw, or acme screw assembly. The load is attached to the end of the screw and is unsupported. Acme screws are lead screws with matching threads on both the screw and nut. Ball screws are lead screw and ball nut combinations that enable the balls in the nut to circulate when the actuator is in motion. Electric linear actuators with belt drives, geared drives, and direct drives are also available. Belt drives connect the motor to the actuator with a belt. Geared drives connect the motor to the actuator with a set of gears. With direct drives, the motor is connected directly to the electric linear actuator. In terms of performance, important specifications include stroke, rated force or load, system backlash, and rated speed. Stroke is the

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maximum distance that the shaft travels from a fully extended position to a fully retracted position. Backlash is the position error due to directional change. The maximum rated speed is typically a low or no load amount. 

Electric linear actuators vary in terms of motor type, power, and features. DC brush motors feature built-in commutation so that as the motor rotates, mechanical brushes automatically actuate coils on the rotor. By contrast, brushless DC motors use an external power drive that allows commutation of the coils on the stator. DC servomotors have an output shaft that is positioned when a coded signal is sent to the motor. Electric linear actuators also use a variety of AC motors. Common types include single, multiphase, universal, induction, gear, and synchronous motors. AC servomotors are permanent magnet synchronous motors that have low torque-to-inertia ratios for high acceleration ratings. AC stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. Motor voltage and continuous power are important performance specifications for both AC and DC motors. Motor features include motor encoder feedback, linear position feedback, position switches, and integral brakes.  

There are several mounting options for electric linear actuators. Some cylinders are equipped with a clevis or eye attachment that connects to the extended end of the piston. Others are equipped with a mounting flange or bracket, a floating mount bracket, tapped holes, or threaded holes. Foot brackets are flanges that rest underneath the cylinder. Lugs are short blocks with holes that attach to the side of the cylinder and allow mounting to another surface. Cylinders equipped with trunnion mounts feature specially designed mounting blocks that are located at the cylinder cap or head. Face mount, nose mount, and rear mount electric linear actuators are also available.  

Electric linear actuators provide many optional features. Some devices provide adjustable stroke, holding brakes, or shock absorbers. Other devices include double-ended rods, a multi-position endplate, an integrated overload slip clutch or torque limiter, and a protective boot. Integral sensors monitor position and proximity. Integral flow control incorporates a valve that limits the amount of air of fluid that enters the cylinder. Magnetic switches indicate the thruster’s position. Thermal overload protection trips a switch when a preset temperature is exceeded. Bumpers or cushions soften the impact at the ends of a stroke. Intrinsically safe electric linear actuators can be used in hazardous environments such as chemical processing facilities. Water

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resistant devices are sealed to prevent corrosion. Typically, body materials consist of aluminum, steel, plastic, or stainless steel.There are several things to consider when choosing a linear actuator. These factors include the speed, stroke length and load rating of the actuator. Also evaluate the duty cycle accuracy and programmability of the actuator. Decide what the desired lifetime of the end product of the linear actuator system will be. Are there particular safety mechanisms required, environmental concerns to be addressed or space issues to deal with? If the system is not battery-run, the size and kind of motor (AC, DC or special) are important considerations. Compare the different available motors, which include stepper, brushed DC or brushless servomotors. Design system flexibility is determined by the anticipated specification revisions.

Terms used in Linear Actuators :

Accuracy The difference from the precise value of the intended velocity or position. ACME Screw A threaded screw utilizing sliding friction surfaces between the nut and the screw. It is self-locking and is about 30-40% efficient. Back Drive Torque produced by the applied load on a drive resulting in the reversal of rotation of the nut. Backlash The space between the interactive elements in a drive train or leadscrew assembly that creates a mechanical “deadband” when shifting directions. Ball Bearing Screw A screw that operates on ball bearings. Ball bearing screws have a low starting torque, are approximately 90% efficient and can be back driven. Bi-directional Repeatability The divergence in the ending position attained by moving away and then returning to a regular point from both plus and minus directions. The error or non-repeatability factor is determined from the sum of the hysteresis, the backlash and one unit of the system resolution. Cantilevered Load Loads or forces that are not symmetrically placed on the center of the positioner table.

Compression Load

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A load that leads toward compressing the positioner. Continuous Motor Torque The torque created by the motor at rated constant current. Cycle A complete positioner extension and retraction returned to the beginning point. Duty Cycle The amount of time a positioner can run and how much time it needs to cool. It is on time to cooling time, meaning a duty cycle of 25% is a cycle in which a positioner operates continually for ten seconds must rest for thirty seconds.

Dynamic Load Rating A design constant used in calculating the estimated travel life of the roller screw; the dynamic men load is the load at which the device will perform one million revolutions. Efficiency The ratio of input power to output power. Error The difference between the actual and the intended condition. Error typically refers to the position but could refer to velocity. Extension Rate The speed at which the positioner extends or retracts. Extension rate differs with the load on DC positioners but differs very little on AC positioners or step-motor positioners. Force Rating The linear force created by the actuator at constant motor torque. Hardwired Signals Electrical signals traveling between two control devices that are connected with dedicated conductors. Holding Brake A brake that works against back driving to hold the positioner in place under compression loads or tension. Hysteresis The opposing force accumulated in an elastic material or mechanism after the outside forces acting on it have been changed (e.g. the mechanical wind-up in the lead-screw assembly). Jog Moving or positioning a load in incremental steps. Lead The distance the leadscrew nut travels for every rotation of the leadscrew. Limit Switch

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A switch that limits the travel or motion in a specific direction. Linear Movement Movement in a straight line. Linear Position Accuracy The error between the intended shift and real position attained by a linear positioning component or stage system. The linear accuracy of components and stage systems, which includes motor accuracy, leadscrew accuracy, stage accuracy (pitch and yaw) and thermal expansion, varies with complexity and number of components.

Linear Rate Rate of movement of a linear component. Load The amount of force axially put on the positioner. Max Velocity The linear velocity the actuator will attain at a given motor rpm. Maximum Static Load The mechanical load limit of the actuator if recirculated oil or other cooling method is used to allow higher than rated torque from the motor.

Microstepping The technique of electronically subdividing every complete step of a stepping motor. Multiplex System A system that utilizes two lead-screws in order to actuate several three-piece pump modules, the combination of which drives the pistons in a linear motion to create displacement. Each system uses a pneumatic rotary actuator to drive its main function. Optical Encoder A linear or rotary element that has alternating opaque and clear spaces. Detectors calculate the light and dark changes, and the position is determined by counting the amount of changes. Resolution The lowest exact positioning movement attainable from a system. Stroke Length The complete movement of the positioning table from complete retraction to full extension.   Thrust The complete force necessary to move a load, taking into account friction, acceleration and gravity.   Unidirectional Repeatability The capability of a system to return to an intended position, nearing that position from a plus and minus direction.  

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Motors

There are three types of motors used in control applications:

• A.C. motors.• D.C. motors.• Stepper motors.

1. A.C. motors

A.C. motors are mainly used for producing large power outputs at a fixed speed. Typically these are 1420 or 2900 rev/min. Such motors are controlled by switching them on and off.

Increasingly, speed control is being used with A.C. motors on applications such as pumps where it is found to be more economical to control the flow rate by changing speed rather than by opening or closing a pipe line valve. Speed control is achieved electronically by varying the frequency or by chopping the power supply.

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These motors are usually geared down in order to produce a greater torque and increase the control range. They may also have the rotation converted into linear movement by a lead screw mechanism.

Lead screws are used to convert rotation into linear motion as shown. Rotation screws the carriage back and forth along the lead screw.

2. D.C. motors

Direct current motors are more widely used in control applications and they are usually referred to as SERVO MOTORS. These are covered in detail later in the tutorial. The development of more powerful magnets is improving the power to weight ratio but they are still not as good as hydraulic motors in this respect. Servo motors usually have a transducer connected to them in order to measure the speed or angle of rotation. The diagram shows a typical arrangement

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3. Stepper motors

Basically a stepper motor rotates a precise angle according to the number of pulses of electricity sent to it. Because there is confidence that the shaft rotates to the position requested, no transducer is needed to measure and check the position and so they are common on open loop systems.

There are 3 types of stepper motor in common use and these are:

1. The PERMANENT MAGNET TYPE.2. The VARIABLE RELUCTANCE TYPE.3. The HYBRID TYPE.

3.1 THE PERMANENT MAGNET TYPE.

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The rotor is a permanent magnet with a North and South poles as shown. Two pairs of poles are placed on the stator and energized to produce a pattern of N - S - N - S (starting at the top). The rotor will take up a position in between the poles due to equal and opposite torques being exerted on it.

If the polarity of both pairs of poles are reversed the pattern will change to S - N - S - N and the rotor will flip 45o to a new position of balance. In order to obtain more steps, more pairs of poles are used but there are only two windings. Reversing the polarity of both windings moves the rotor on one step. Stepping is produced by simply reversing the polarity.

The rotor is held in position even when the poles are not energized. In order to obtain many steps, the poles are often stacked one behind the other and not in a single ring. The number of steps may also be increased by using a gear box on the output shaft.

3.2. VARIABLE RELUCTANCE TYPE

The rotor is constructed of soft iron with a number of teeth which are unequal in number to the number of poles on the stator. The stator has multiple poles which are energized by several separate phases. The diagram shows a

system with three phases.

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When a current is applied to the stator windings, the rotor aligns itself in the position of least magnetic reluctance. This position depends upon the number of phases energized.The rotor retains very little magnetism so there is no holding torque when the current is removed. The number of steps is given by N = SR/(S-R) where S is the number of stator slots and R the number of rotor slots.

3.3 HYBRID MOTORS

Hybrid motors are a combination of the last two types. Each pole is divided into slots as shown. The rotor has two sets of slots, one behind the other with one set offset to the other by 1/2 slot pitch. The rotor is magnetized longitudinally. This produces a high resolution.

In general all stepper motors are controlled electronically.

4. DETAILED ANALYSIS OF D.C. MOTORS

The theory of electrical machines is based on two basic discoveries called the motor principle and the generator principle. In any given machine, the two go together.

4.1 THE MOTOR PRINCIPLE

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When a conductor is placed in a magnetic field at a right angle to it and current flows in the conductor, a force is exerted on the conductor. The force F (Newton) is related to the flux density B (Tesla), the current I (Amps) and the length l (metres) by the formula: F = B ℓ I

The flux density is the flux per unit area so B = φ/A

, where; φ is the flux in Webers and A is the cross sectional area of the flux path in m2.This force acting at a radius produces the torque T to rotate the motor. The current in the conductor is Ia.

For a given motor the area, lengths and radius are constant so the equation reduces to

T = k1 φ Ia ................ (1)

4.2 THE GENERATOR PRINCIPLE

When a conductor moves at velocity v m/s through a flux of density B Tesla, an e.m.f is generated in the conductor such that E = B ℓ v. This e.m.f opposes the flow of the applied current so a forward voltage is required to overcome it. This effect is produced in the conductors of motors as well as generators since the motor has moving conductors passing through a flux.The conductor is part of a coil rotating at speed N rev/s and v = 2πNR

For a given motor in which the area and length may be considered constant, the equation becomes

E = K2φ N ......................... (2)

4.3 GENERAL PRINCIPLES OF MOTORS

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A basic motor is constructed of a rotor carrying the conductors in the form of a loop. The loop is placed in a flux as shown. The flux bends around the two sides and produces a torque. The loop would flip to a

vertical position and no torque would be produced. In order to make the loop rotate continuously, the current must be reversed by use of a commutator. In reality, the conductor is made of many loops and the current is switched to ensure that it flows in the loop normal to the flux.

All DC motors are based on these principles. The flux may be produced by permanent magnetic poles or by separate coils called the FIELD WINDINGS. The rotor in the diagram would be called the ARMATURE. It is possible to have brushless motors and for the stator to be the armature. A typical design is the use of two pairs of poles and many loops on the rotor which are energized through the commutator.

Equation 2 may be deduced by equating mechanical and electric power. The mechanical power produced is P = ωT = 2πNTWhere ω is the speed in radians/s and N is the speed in rev/s. Note that ω = 2πN

Ea is the armature e.m.f. The electric power converted into mechanical power is P = Ea Ia

Equating mechanical and electric power we have 2πNT = IaEa Ea = 2πNT/Ia ........ (2a)Or T = IaEa /2πN ........ (2b)

Since T = k1φIa we may substitute into (2a) Ea = k1φ2πN = k2φN ...... (2)

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Due to losses in the armature windings, the terminal voltage Va is given by

Ea = Va – IaRa ................... (3)Substitute for Ea N = (Va - IaRa)/2πk1φ .......................... (4)

4.4 CONSTANT ARMATURE CURRENT or FIELD CONTROL

If Ia is kept constant it follows that N = constant/φ = constant/If

Since T = k1 φ Ia and for constant Ia it follows that T = constant x φHence N=constant/T

It follows that the torque and speed may be controlled by varying the field current. This has an advantage that a relatively large power may be controlled by a small field current and the power amplifier needed in the control circuit is relatively small. The diagram shows the relationship between torque and speed for constant field current.

4.5 CONSTANT FIELD CURRENT OR ARMATURE CONTROL

In this case the field current is maintained constant or a permanent magnet is used to produce constant flux.

Since Ea = 2πNT/Ia (equation 2a) and T ∝ φ Ia then Ea ∝ Nφ

If φ is constant then Ea = 2πN K ......................... (5) and N = Ea/2πk

Substituting for N in equation (2a) gives T=kIa hence T = ktIa

Or Ia = T/kt ..................................... (6)20

Equating (3) and (5) we have Ea = Va - IaRa = 2πNK = Ke NSubstitute (6) for Ia and

Va - TRa /kt = N Ke T = ( Va - Nke )kt/Ra T = C1Va - C2N ........................... (7) Or Va = C3T + C4N ........................... (8)

These are the equations commonly used to explain the steady state characteristic of a DC motor with armature control.

The diagrams above show equation 8 plotted for constant T and equation 7 for constant Va.

4.6 MIXED CONTROL

So far you have studied motors with separate field and armature windings and looked at the characteristics of these motors.We will now study the characteristics of various motor configurations of the type mainly used on large D.C. Machines. In the previous work it was shown that

V = Ea + Ia Ra E = k1φ2πN = kNφ T = K1 φ Ia

Ea is the back e.m.f on the armature, φ is the flux per pole, T is the torque and N the speed (rev/s).

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The Mechanical Power output of the motor is less than the Electrical Power at the terminals because of losses. The efficiency of the motor is defined as

η = Mechanical Power/Electrical Power

Mechanical Power = 2πNT and Electrical Power is terminal Volts x Amperes.

4.7 SERIES MOTOR

In this case, the field winding is in series with the armature. The same current flows through the armature and the field winding.

Equating mechanical and electrical powers we have P = 2πNT = Ea IaRearranging we have T = EaIa /2πN

If the electric power is constant, EaIa are constant so T = Constant /N

In this case for constant electrical power the relationship between torque and speed is inversely proportional.

The torque - speed characteristic shows that at low torque (no load conditions) the motor is liable to over speed and become damaged.

At low speed there is a high torque (starting torque) which is ideal for servo applications

4.8 SHUNT MOTOR

In this case the field winding is connected across the supply. Since the field current is constant, the characteristics are those of an armature controlled motor covered earlier.

Ea = V - IaRa

T = k1φ Ia so Ia = T/φk1

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Ea = V - TRa/φk1 = k Nφ but φ is constant so everything is a constant except T and N so for constant electrical power this reduces to T = C1 - C2 N

This shows that at zero speed the starting torque is C1 and as speed increases, the torque drops off. The ideal Torque - Speed characteristic is as shown. In reality the line is curved down due to other effects not considered.

4.9 COMPOUND MOTOR

The compound motor is a cross between the other two with both a parallel and series field winding. For constant electric power, the Torque - speed characteristic is between that of the other two.

4.10 D.C. SERVO MOTORS MANUFACTURERS APPROACH

Smaller servo motors are used for robotic applications, that is, the control of position and speed of a shaft. They may use field control or

armature control or both.

FIELD CONTROL

The armature current Ia is maintained constant and the field current If is supplied through a power amplifier and controls the torque. The torque is unaffected by the speed. The relationship between torque and current is

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T = k If

ARMATURE CONTROL

This is quite common since smaller servo motors use permanent magnets. With the development of more powerful permanent magnets, DC servo motors are improving their power to weight ratio but are still not as good as hydraulic motors in this respect.

Manufacturers of such motors present the steady state characteristics based on equations 7 and 8. However some practical aspects must be brought in. First, there is a permanent loss of torque due to friction and

a current is needed to overcome friction torque before any useful torque is produced. This is expressed as Tf in catalogues. There is also a loss of torque due to damping which is directly proportional to the speed of the motor. This torque is Td and is found by

Td = kd N where N is in 1000 rev/min.

The other important constants quoted for such motors are the Torque constant kt and the e.m.f. constant ke. Torque is normally quoted in N cm which is not a recommended SI unit and the shaft speeds are quoted in 1000 rev/min.

The current required to operate such a motor is given by the equation

I = (TL + Tf + Td)/kt where TL is the load torque.

The useful torque from the motor is TL = ktI - Tf - Td

The voltage required at the terminals is V = (N ke/1000) + (IaRa)

DATA TABLES FOR MOTORS

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5. STARTING LARGE D.C. MOTORS

Consider a basic D.C Motor. The terminal voltage is V. The back e.m.f on the armature is E. The armature resistance is Ra. The field resistance is Rf.The flux per pole is Φ. N is the motor speed.

From earlier work it was shown that V = E + Ia Ra E = K1 NΦ T = K2 Φ If

When the motor is started, the speed is zero so there is no back e.m.f. It follows that V = Ia Ra

The starting current without protection would be V/Ra and this would be very large. In addition to this, there will be a load with inertia connected to the motor and a large current is needed to provide the torque.

In order to limit the current, it is normal to insert a variable resistance in series with the armature which is gradually reduced as the motor speeds up and then latched in place in the zero resistance position. In the event of an interruption to the power, the starting resistance is

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unlatched and springs back to the starting position. We should consider how the starting resistance is used with different motor field configurations.

The three types or configurations are shown below.

SERIES MOTOR

In the case of a series wound motor, the starting resistance is placed in series as shown.

SHUNT MOTOR

In the case of a shunt wound motor, the starting resistance is placed as shown so that the field is initially connected to the supply and the armature is in series with it. As the motor speeds up the field resistance is gradually increased and the resistance in series with the armature is reduced.

COMPOUND MOTOR

Compound motor has a starting resistance as shown.

6. THE EFFECT OF INERTIA AND INDUCTANCE

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Most of the work covered so far has been concerned with the steady state analysis of motors. When sudden changes are made to the speed or torque, it takes time for the system to respond because of the time dependant effects such as inertia, damping and inductance. A motor may have damping torque which is directly proportional to speed. This was given previously as:

Td = Kd x N/1000 where N is rev/min and T is in Ncm.

If we use S.I. units the damping torque would be given by:

Td = Kd ω where ω is rad/s and T is Nm

ω is the rate of change of angle per second and may be expressed in calculus form as dθ/dt. Hence: Td = Kd dθ/dt.

If the motor is accelerated, torque is needed to overcome the inertia Ti and this is directly proportional to the angular acceleration. Ti = Iα

α is in rad/s2 and I is the moment of inertia in kg m2. (Many manufacturers use the symbol J for moment of inertia)The acceleration is the second derivative of angle with respect to time so we may write

Ti = I d2θ/dt2

When a motor is producing acceleration the total torque acting on it is

T = TL + Tf + Kd dθ/dt + I d2θ/dt2

This results in a mechanical time constant τm which governs how quickly a motor will accelerate. This is defined by manufacturers as the time taken to accelerate the motor up to 63% of the required speed with no load on it. This will be analysed and defined in detail in another tutorial. Now consider that the terminal voltage of a servo motor was defined earlier as V = Ea + IaRaIf the current is changing, the inductance of the armature winding also produces a voltage (back emf) given by L di/dt where L is the inductance in Henries and i the transient current.The total terminal voltage is then V = L di/dt + Ea + IaRaThis gives rise to an electrical time constant defined as τe = L/Ra

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This will be explained in another tutorial. If the motor uses field control, the electrical time constant is based on the field winding inductance and resistance.

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Solenoids

Definition of Solenoid

A solenoid is an electromechanical device that converts electrical energy into linear or rotary mechanical motion.

All solenoids include a coil for conducting current and generating a magnetic field, an iron or steel shell or case to complete the magnetic circuit, and a plunger or armature for translating motion. Solenoids can be actuated by either direct current (DC) or rectified alternating current (AC).

Solenoids are built with conductive paths that transmit maximum magnetic flux density with minimum electrical energy input. The mechanical action performed by the solenoid depends on the design of the plunger in a linear solenoid or the armature in a rotary solenoid. Linear solenoid plungers are either spring-loaded or use external methods to restrain axial movement caused by the magnetic flux when the coil is energized and restore it to its initial position when the current is switched off.

Cutaway drawing Figure 1-50 illustrates how pull-in and push-out actions are performed by a linear solenoid. When the coil is energized , the plunger pulls in against the spring, and this motion can be translated into either a “pull-in” or a “push-out” response. All solenoids are basically pull-in-type actuators, but the location of the plunger extension with respect to the coil and spring determines its function. For example, the plunger extension on the left end (end A) provides “push-out” motion against the load, while a plunger extension on the right end terminated by a clevis (end B) provides “pull-in” motion. Commercial solenoids

perform only one of these functions. Figure 1-51 is a cross-sectional view of a typical pull-in commercial linear solenoid.

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Rotary solenoids operate on the same principle as linear solenoids except that the axial movement of the armature is converted into rotary movement by various mechanical devices. One of these is the use of internal lands or ball bearings and slots or races that convert a pull-in stroke to rotary or twisting motion.

Motion control and process automation systems use many different kinds of solenoids to provide motions ranging from simply turning an event on or off to the performance of extremely complex sequencing. When there are requirements for linear or rotary motion, solenoids should be considered because of their relatively small size and low cost when compared with alternatives such as motors or actuators. Solenoids are easy to install and use, and they are both versatile and reliable.

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Solenoids are the most common actuator components. The basic principle of operation is there is a moving ferrous core (a piston) that will move inside wire coil as shown in Figure 5.1. Normally the piston is held outside the coil by a spring. When a voltage is applied to the coil and current flows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of the coil. The piston can be used to supply a linear force. Well known applications of these include pneumatic values and car door openers.

Technical Considerations

Important factors to consider when selecting solenoids are their rated torque/force, duty cycles, estimated working lives, performance curves, ambient temperature range, and temperature rise. The solenoid must have a magnetic return path capable of transmitting the maximum amount of magnetic flux density with minimum energy input. Magnetic flux lines are transmitted to the plunger or armature through the bobbin and air gap back through the iron or steel shell. A ferrous metal path is more efficient than air, but the air gap is needed to permit plunger or armature movement. The force or torque of a solenoid is

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inversely proportional to the square of the distance between pole faces. By optimizing the ferrous path area, the shape of the plunger or armature, and the magnetic circuit material, the output torque/force can be increased . The torque/force characteristic is an important solenoid specification. In most applications the force can be a minimum at the start of the plunger or armature stroke but must increase at a rapid rate to reach the maximum value before the plunger or armature reaches the backstop. The magnetizing force of the solenoid is proportional to the number of copper wire turns in its coil, the magnitude of the current, and the presence of the magnetic circuit. The pull force required by the load must not be greater than the force developed by the solenoid during any portion of its required stroke, or the plunger or armature will not pull in completely. As a result, the load will not be moved the required distance. Heat buildup in a solenoid is a function of power and the length of time the power is applied. The permissible temperature rise limits themagnitude of the input power. If constant voltage is applied, heat buildup can degrade the efficiency of the coil by effectively reducing its number of ampere turns. This, in turn, reduces flux density and torque/force output. If the temperature of the coil is permitted to rise above the temperature rating of its insulation, performance will suffer and the solenoid could fail prematurely. Ambient temperature in excess of the specified limits will limit the and conduction. Heat can be dissipated by cooling the solenoid with forced air from a fan or blower, mounting the solenoid on a heat sink, or circulating a liquid coolant through a heat sink. Alternatively, a larger solenoid than the one actually needed could be used .The heating of the solenoid is affected by the duty cycle, which is specified from 10 to 100%, and is directly proportional to solenoid on time. The highest starting and ending torque are obtained with the lowest duty cycle and on time. Duty cycle is defined as the ratio of on time to the sum of on time and off time. For example, if a solenoid is energized for 30 s and then turned off for 90 s, its duty cycle is 30⁄120 = 1⁄4, or 25%.

The amount of work performed by a solenoid is directly related to its size. A large solenoid can develop more force at a given stroke than a small one with the same coil current because it has more turns of wire in its coil.

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Types of solenoids:

Open-Frame Solenoids

Open-frame solenoids are the simplest and least expensive models. They have open steel frames, exposed coils, and movable plungers centered in their coils. Their simple design permits them to be made inexpensively in high-volume production runs so that they can be sold at low cost. The two forms of open-frame solenoid are the C-frame solenoid and the box frame solenoid. They are usually specified for applications where very long life and precise positioning are not critical requirements.

C-Frame Solenoids

C-frame solenoids are low-cost commercial solenoids intended for light duty applications. The frames are typically laminated steel formed in the shape of the letter C to complete the magnetic circuit through the core, but they leave the coil windings without a complete protective cover. The plungers are typically made as laminated steel bars. However, the coils are usually potted to resist airborne and liquid contaminants. These solenoids can be found in appliances, printers, coin dispensers, security door locks, cameras, and vending machines. They can be powered with either AC or DC current. Nevertheless, C-frame solenoids can have operational lives of millions of cycles, and some standard catalog models are capable of strokes up to 0.5 in. (13 mm).

Box-Frame Solenoids

Box-frame solenoids have steel frames that enclose their coils on two sides, improving their mechanical strength. The coils are wound on phenolic bobbins, and the plungers are typically made from solid bar stock .The frames of some box-type solenoids are made from stacks of thin insulated sheets of steel to control eddy currents as well as keep stray circulating currents confined in solenoids powered by AC. Box-frame solenoids are specified for higher-end applications such as tape decks, industrial controls, tape recorders, and business machines because they offer mechanical and electrical performance that is superior to those of C frame solenoids. Standard catalog commercial

33

box-frame solenoids can be powered by AC or DC current, and can have strokes that exceed 0.5 in. (13 mm).

Tubular Solenoids

The coils of tubular solenoids have coils that are completely enclosed in cylindrical metal cases that provide improved magnetic circuit return and better protection against accidental damage or liquid spillage. These DC solenoids offer the highest volumetric efficiency of any commercial solenoids, and they are specified for industrial and military/aerospace equipment where the space permitted for their installation is restricted. These solenoids are specified for printers, computer disk-and tape drives, and military weapons systems; both

pull-in and push-out styles are available. Some commercial tubular linear solenoids in this class have strokes up to 1.5 in. (38 mm), and some can provide 30 lbf (14 kgf) from a unit less than 2.25 in (57 mm) long. Linear solenoids find applications in vending machines, photocopy machines, door locks, pumps, coin-changing mechanisms, and film processors.

Rotary Solenoids

Rotary solenoid operation is based on the same electromagnetic principles as linear solenoids except that their input electrical energy is converted to rotary or twisting rather than linear motion. Rotary actuators should be considered if controlled speed is a requirement in a rotary stroke application. One style of rotary solenoid is shown in the exploded view Figure 1-52. It includes an armature-plate assembly that rotates when it is pulled into the housing by magnetic flux from the coil. Axial stroke is the linear distance that the armature travels to the center of the coil as the solenoid is energized. The three ball bearings travel to the lower ends of the races in which they are positioned. The operation of this rotary solenoid is shown in Figure 1-53. The rotary solenoid armature is supported by three ball bearings that travel around and down the three inclined ball races. The de-energized state is shown in (a). When power is applied, a linear electromagnetic force pulls in the armature and twists the armature plate, as shown in (b).

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Rotation continues until the balls have traveled to the deep ends of the races, completing the conversion of linear to rotary motion.

This type of rotary solenoid has a steel case that surrounds and protects the coil, and the coil is wound so that the maximum amount of copper wire is located in the allowed space. The steel housing provides the high permeability path and low residual flux needed for the efficient conversion of electrical energy to mechanical motion. Rotary solenoids can provide well over 100 lb-in. (115 kgf-cm) of torque from a unit less than 2.25 in. (57 mm) long.

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Rotary solenoids are found in counters, circuit breakers, electronic component pick-and-place machines, ATM machines, machine tools, ticket-dispensing machines , and photocopiers.

Example on Application of Solenoids :

The flow of fluids and air can be controlled with solenoid controlled valves. An example of a solenoid controlled valve is shown below . The solenoid is mounted on the side. When actuated it will drive the central spool left. The top of the valve body has two ports that will be connected to a device such as a hydraulic cylinder. The bottom of the valve body has a single pressure line in the center with two exhausts to the side. In the top drawing the power flows in through the center to the right hand cylinder port. The left hand cylinder port is allowed to exit through an exhaust port. In the bottom drawing the solenoid is in a new position and the pressure is now applied to the left hand port on the top, and the right hand port can exhaust. The symbols to the left of the figure show the schematic equivalent of the actual valve positions. Valves are also available that allow the valves to be blocked when unused.

II – Hydraulic Actuators

Hydraulic systems are used in applications requiring a large amount of force and slow speeds. When used for continuous actuation they are mainly used with position feedback .They also suffer from maintenance problems (e.g. leakage of the hydraulic fluid, dirt/contamination of fluid.) Hydraulic actuators may be linear, or rotary.

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An example system is shown below. The controller examines the position of the hydraulic system, and drivers a servo valve. This controls the flow of fluid to the actuator. The remainder of the provides the hydraulic power to drive the system.

Hydraulic systems normally contain the following components :

1. Hydraulic Fluid .2. An Oil Reservoir .3. A Pump to Move Oil, and Apply Pressure .4. Pressure Lines .5. Control Valves - to regulate fluid flow .6. Piston and Cylinder - to actuate external mechanisms .

The hydraulic fluid is often a noncorrosive oil chosen so that it lubricates the components.

Types of control valves used :

In the standard terminology, the ’n-way’ designates the number of connections for inlets and outlets. In some cases there are redundant ports for exhausts. The normally open/closed designation indicates the valve condition when power is off. All of the valves listed are two position valve, but three position valves are also available.

2-way normally closed - these have one inlet, and one outlet. When unenergized, the valve is closed. When energized, the valve will open, allowing flow. These are used to permit flows.

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2-way normally open - these have one inlet, and one outlet. When unenergized, the valve is open, allowing flow. When energized, the valve will close. These are used to stop flows. When system power is off, flow will be allowed.3-way normally closed - these have inlet, outlet, and exhaust ports. When unenergized , the outlet port is connected to the exhaust port. When energized, the inlet is connected to the outlet port. These are used for single acting cylinders.

3-way normally open - these have inlet, outlet and exhaust ports. When unenergized , the inlet is connected to the outlet. Energizing the valve connects the outlet to the exhaust. These are used for single acting cylinders .

3-way universal - these have three ports. One of the ports acts as an inlet or outlet, and is connected to one of the other two, when energized/unenergized. These can be used to divert flows, or select alternating sources .

4-way - These valves have four ports, two inlets and two outlets. Energizing the valve causes connection between the inlets and outlets to be reversed. These are used for double acting cylinders.

Some of the ISO symbols for valves are shown below . When using the symbols in drawings the connections are shown for the unenergized state. The arrows show the flow paths in different positions. The small triangles indicate an exhaust port.

ISO Valve Symbols

When selecting valves there are a number of details that should be considered, as listed below :

pipe size - inlets and outlets are typically threaded to accept NPT (national pipe thread).

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flow rate - the maximum flow rate is often provided to hydraulic valves.

operating pressure - a maximum operating pressure will be indicated. Some valves will also require a minimum pressure to operate.

electrical - the solenoid coil will have a fixed supply voltage (AC or DC) and current.

response time - this is the time for the valve to fully open/close. Typical times for valves range from 5ms to 150ms.

enclosure - the housing for the valve will be rated as :

type 1 or 2 - for indoor use, requires protection against splashes . type 3 - for outdoor use, will resists some dirt and weathering . type 3R or 3S or 4 - water and dirt tight . type 4X - water and dirt tight, corrosion resistant .

Types of cylinders used :

A cylinder uses pressurized fluid or air to create a linear force/motion as shown below. In the figure a fluid is pumped into one side of the cylinder under pressure, causing that side of the cylinder to expand, and advancing the piston. The fluid on the other side of the piston must be allowed to escape freely - if the incompressible fluid was trapped the cylinder could not advance. The force the cylinder can exert is proportional to the cross sectional area of the cylinder.

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A Cross Section of a Hydraulic Cylinder

Single acting cylinders apply force when extending and typically use a spring to retract the cylinder. Double acting cylinders apply force in both direction.

Schematic Symbols for Cylinders

Advantages and Disadvantages of Hydraulic Actuators :

Hydraulic actuators have brute strength, essentially no compressibility and excellent power-to-weight ratio. However, they tend to leak, have lower reliability, are higher maintenance, expensive and loud, use flammable fluids and generate heat.

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III – Pneumatic Actuators

Pneumatic systems are very common, and have much in common with hydraulic systems with a few key differences. The reservoir is eliminated as there is no need to collect and store the air between uses in the system. Also because air is a gas it is compressible and regulators are not needed to recirculate flow. But, the compressibility also means that the systems are not as stiff or strong. Pneumatic systems respond very quickly, and are commonly used for low force applications in many locations on the factory floor.

Some basic characteristics of pneumatic systems :

- stroke from a few millimeters to meters in length (longer strokes have more springiness .- the actuators will give a bit - they are springy .- pressures are typically up to 85psi above normal atmosphere .- the weight of cylinders can be quite low .- additional equipment is required for a pressurized air supply- linear and rotatory actuators are available.- dampers can be used to cushion impact at ends of cylinder travel.

When designing pneumatic systems care must be taken to verify the operating location. In particular the elevation above sea level will result in a dramatically different air pressure.

Some symbols for pneumatic systems :

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42

The flow control valve is used to restrict the flow, typically to slow motions.

The shuttle valve allows flow in one direction, but blocks it in the other.

The receiver tank allows pressurized air to be accumulated. The dryer and filter help remove dust and moisture from the air,

prolonging the life of the valves and cylinders.

Advantages and Disadvantages of pneumatic actuators :

pneumatic actuators are inexpensive, have rapid response and are simple and easy to control, they are also loud and their position is difficult to control.

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Sensors

44

Introduction

Sensors convert physical phenomena to measurable signals , typically voltages or currents .

The basic physical phenomena measured with sensors include :

Angular or linear position .

Acceleration .

Tempreature.

Pressure or flow rates .

Light intensity .

Sound .

Most of these sensors based on electrical properties of materials and devices . As a result signals often require signal conditioners . these are amplifiers that boost currents and voltages to larger values .

Types of Sensors :

In this report we will focus only on the types mentioned below :

Proximity Sensors

Strain Gauge

Thermocouple

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Proximity Sensor

Definition of Proximity Sensor

It is a device to detect object with no contact, making use of changes in a magnetic/ electric field.

Detection Mechanism

A proximity sensor create a net of electro/magnetic field and detects an object which enters the field , just as a spider form its web and catches its prey. The net is created by the magnetic lines originated from the oscillation circuit. When a metallic object comes into the field, the magnetic lines get disordered, which is transmitted to the oscillating circuit .The oscillating circuit will detect the object approaching and output the decision.

Working range

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Classification

• Detection mode and principles

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Shape

48

Features of Proximity Sensor

1. Stable operation, unsusceptible to water, oil, dust, light, etc..

Be able to use for machine tools splashed with cutting oil or food processing machine washed with water (magnetic type).

2. Resistant to vibration and shock

Anti-vibration/shock since the whole circuit can be coated with resin.

3. Able to detect without any contact

Detection distance is bout 0-30mm. No damage on an object.

4. Higher speed/performance compared with limit switch

Long life and quick response.

5. Magnetic type is for metal detection, capacitance is for everything except fluid

Liquid in a paper cup can be also detectable.

6. Susceptible to magnet effect

High possibility of malfunction in an area where large amount of electric current flows such as welding or electro magnetism.

Major Characteristics

(1) Effect according to materials of object

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(2) Size of object and detection distance (Model E2E-XIR5E1)

(3) Thickness of object and detection distance (Model E2E-X10E1)

Surface effect

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Electric current flows equally in a conductor when frequency is low (DC), but flows densely on the surface and sparsely inside with high frequency. The higher the magnetic transparent ratio is, the higher this tendency is.

Whirling electric current in the object

- Aluminum

Whirling current flows deep inside of an object due to low transparent ratio. Small anti-magnetic bundle occurs from the surface and inside as well.

Therefore a proximity sensor can only detect an object within a short distance.

- Steel

Whirling current flows densely on the very surface due to high transparent ratio. Therefore, large anti-magnetic bundle the occurs on surface. As a result a proximity from the detect distance.

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STRAIN GAUGE

INTRODUCTION

The strain gauge has been in use for many years and is the fundamental sensing element for many types of sensors, including pressure sensors, load cells, torque sensors, position sensors, etc.The majority of strain gauges are foil types, available in a wide choiceof shapes and sizes to suit a variety of applications. They consist of apattern of resistive foil which is mounted on a backing material. Theyoperate on the principle that as the foil is subjected to stress, theresistance of the foil changes in a defined way.

Strain gauges are sensing devices used in a variety of physical test and measurement applications. They change resistance at their output terminals when stretched or compressed. Because of this characteristic, the gauges are typically bonded to the surface of a solid material and measure its minute dimensional changes when put in compression or tension. Strain gauges and strain gauge principles are often used in devices for measuring acceleration, pressure, tension, and force. Strain is a dimensionless unit,defined as a change in length per unit length. Strain gauges have a characteristic gauge factor, defined as the fractional change in resistance divided by the strain.Common gauge resistance values typically range from 120 to 350(, but some devices are as low as 30O or as high as 3 k).

Definition Of Strain

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1 below.

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Figure 1. Definition of Strain

Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of

measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10-6.When a bar is strained with a uniaxial force, as in Figure 1, a phenomenon known as Poisson Strain causes the girth of the bar, D, to contract in the transverse, or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's Ratio. The Poisson's Ratio n of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force) to the strain in the axial direction (parallel to the force), or n = eT/e. Poisson's Ratio for steel, for example, ranges from 0.25 to 0.3.

The Strain Gauge

While there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gauge is the bonded metallic strain gauge.

The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure 2). The cross sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. Strain gauges are available commercially with nominal resistance values from 30 to 3000 Ω, with 120, 350, and 1000 Ω being the most common values.

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Figure 2. Bonded Metallic Strain Gauge

It is very important that the strain gauge be properly mounted onto the test specimen so that the strain is accurately transferred from the test specimen, through the adhesive and strain gauge backing, to the foil itself.A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain):

The Gauge Factor for metallic strain gauges is typically around 2.

Strain Gauge Measurement

In practice, the strain measurements rarely involve quantities larger than a few millistrain(e x 10-3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. For example, suppose a test specimen undergoes a strain of 500 me. A strain gauge with a gauge factor of 2 will exhibit a change in electrical resistance of only 2 (500 x 10-6) = 0.1%. For a 120 W gauge, this is a change of only 0.12 W.

To measure such small changes in resistance, strain gauges are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge, illustrated below, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.

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Figure 3. Wheatstone Bridge

The output voltage of the bridge, VO, will be equal to:

From this equation, it is apparent that when R1/R2 = R4/R3, the voltage output VO will be zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero output voltage.

Therefore, if we replace R4 in Figure 3 with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain gauge is designated as RG, then the strain-induced change in resistance, DR, can be expressed as DR = RG·GF·e. Assuming that R1 = R2 and R3 = RG, the bridge equation above can be rewritten to express VO/VEX as a function of strain (see Figure 4). Note the presence of the 1/(1+GF·e/2) term that indicates the nonlinearity of the quarter-bridge output with respect to strain.

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Figure 4. Quarter-Bridge Circuit

Ideally, we would like the resistance of the strain gauge to change only in response to applied strain. However, strain gauge material, as well as the specimen material to which the gauge is applied, will also respond to changes in temperature. Strain gauge manufacturers attempt to minimize sensitivity to temperature by processing the gauge material to compensate for the thermal expansion of the specimen material for which the gauge is intended. While compensated gauges reduce the thermal sensitivity, they do not totally remove it.

By using two strain gauges in the bridge, the effect of temperature can be further minimized. For example, Figure 5 illustrates a strain gauge configuration where one gauge is active (RG + DR), and a second gauge is placed transverse to the applied strain. Therefore, the strain has little effect on the second gauge, called the dummy gauge. However, any changes in temperature will affect both gauges in the same way. Because the temperature changes are identical in the two gauges, the ratio of their resistance does not change, the voltage VO does not change, and the effects of the temperature change are minimized.

Figure 5. Use of Dummy Gauge to Eliminate Temperature Effects

The sensitivity of the bridge to strain can be doubled by making both gauges active in a half-bridge configuration. For example, Figure 6 illustrates a bending beam application with one bridge mounted in tension (RG + DR) and the other mounted in compression (RG - DR). This half-bridge configuration, whose circuit diagram is also illustrated in

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Figure 6, yields an output voltage that is linear and approximately doubles the output of the quarter-bridge circuit.

Figure 6. Half-Bridge Circuit

Finally, you can further increase the sensitivity of the circuit by making all four of the arms of the bridge active strain gauges in a full-bridge configuration. The full-bridge circuit is shown in Figure 7.

Figure 7. Full-Bridge Circuit

The equations given here for the Wheatstone bridge circuits assume an initially balanced bridge that generates zero output when no strain is applied. In practice however, resistance tolerances and strain induced by gauge application will generate some initial offset voltage. This initial offset voltage is typically handled in two ways. First, you can use a special offset-nulling, or balancing, circuit to adjust the resistance in the bridge to rebalance the bridge to zero output. Alternatively, you can measure the initial unstrained output of the circuit and compensate in software.

The equations given above for quarter, half, and full-bridge strain gauge configurations assume that the lead wire resistance is negligible. While ignoring the lead resistances may be beneficial to understanding the basics of strain gauge measurements, doing so in practice can be a

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major source of error. For example, consider the 2-wire connection of a strain gauge shown in Figure 8a. Suppose each lead wire connected to the strain gauge is 15 m long with lead resistance RL equal to 1 W. Therefore, the lead resistance adds 2 W of resistance to that arm of the bridge. Besides adding an offset error, the lead resistance also desensitizes the output of the bridge.

You can compensate for this error by measuring the lead resistance RL and accounting for it in the strain calculations. However, a more difficult problem arises from changes in the lead resistance due to temperature fluctuations. Given typical temperature coefficients for copper wire, a slight change in temperature can generate a measurement error of several me.

Using a 3-wire connection can eliminate the effects of variable lead wire resistance because the lead resistances affect adjacent legs of the bridge. As seen in Figure 8b, changes in lead wire resistance, R2, do not change the ratio of the bridge legs R3 and RG. Therefore, any changes in resistance due to temperature cancel each other.

Figure 8. 2-Wire and 3-Wire Connections of Quarter-Bridge Circuit

Heating

Resistive heating in strain gauges also should be considered because the gauges respond to temperature as well as stress. In most standard circuits, the heat that each gauge dissipates is less than 100 mW, so it's not usually a problem. This is especiallytrue when the strain gauge is bonded to a material that conducts heat

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quickly, such as metal. However, because most wood, plastic, or glass materials do not conduct heat away as rapidly, use the lowest excitation voltage possible without introducing noise problems. Also, heat can become a problem when the strain gauges are uncommonly small, or numerous gauges occupy a limited space.

Calibration

The signal-conditioning module also typically provides a shunt calibration feature. See Figure 7. It lets users switch their own shunt resistors into either one of the two lower legs of the bridge under software control. For example, a shunt resistor can be calculated to simulate a full load. Applying a shunt resistor is a convenient way to simulate an unbalance without having to apply a physical load. For any balanced bridge, a specific resistor can be connected in parallel with one of the four bridge elements to obtain a predictable unbalance and output voltage.

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Thermocouple

INTRODUCTION

Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. The basic theory of thermocouples dates back to 1821 when T.J. Seebeck discovered that a current is induced into a closed circuit of two dissimilar metals by heating one of the two junctions. And, as long as the temperature differences exists between the two junctions, current will continue flowing through the circuit.

While the theory is nearly 150 years old, incorrect application of thermocouples still affects today's sophisticated industrial processes. In any temperature control system, the heart of that system is the temperature sensing device -- in this case, thermocouple. Without proper application or understanding of basic thermocouple circuits, even the most complicated system cannot function.

In his discovery, Seebeck also concluded that any two metals can be used. However, the magnitude and direction of the generated current are functions of the magnitude of the temperature difference between the junctions and the thermal properties of the metals used in the circuit. Therefore, not every combination of metals is acceptable for thermocouple usage.

Definition Of Thermocouple

Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient.

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THERMOELECTRIC CHARACTERISTICS

A thermocouple should have thermoelectric characteristics such that the electromotive force (emf) produced per degree of temperature change is sufficient to be detected by standard measuring instruments. The device must also be capable of withstanding temperature extremes for prolonged periods, rapid temperature changes, and corrosive atmospheres while exhibiting reproducibility and a high degree of accuracy.

Thermocouple types can be identified based on wire insulation color.

Type

Temperature range

°c (continuo

us)

Temperature range °c (short

term)

Tolerance class 1

(°c)

Tolerance class 2

(°c)

IEC Colour code

BS Colour code

ANSI Colou

r code

K 0 to +1100-180 to +1300

-40 to +375 ± 1.5 °c, 375 to 1000 ± 0.004*[t]°c

-40 to +333 ± 2.5 °c, 333 to 1200 ± 0.0075*[t]°c

J 0 to +700-180 to +800

-40 to +375 ± 1.5 °c, 375 to 750 ± 0.004*[t]°c

-40 to +333 ± 2.5 °c, 333 to 750 ± 0.0075*[t]°c

N 0 to +1100 -270 to +1300

-40 to +375 ± 1.5 °c, 375 to

-40 to +333 ± 2.5°c, 333 to

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1000 ± 0.004*[t]°c

1200 ± 0.0075*[t]°c

R 0 to +1600-50 to +1700

0 to +1100 ± 1.0°c, 1100 to 1600 ± (1+0.003 (t-1100))*[t]°c

0 to +600 ± 1.5 °c, 600 to 1600 ± 0.0025*[t]°c

Not defined.

S 0 to 1600-50 to +1750

0 to +1100 ± 1.0 °c, 1100 to 1600 ± (1+0.003(t-1100))*[t]°c

0 to +600 ± 1.5°c, 600 to 1600 ± 0.0025*[t]°c

Not defined.

B+200 to +1700

0 to +1820Not Available

600 to 1700 ± 0.0025*[t]°c

No standard use copper wire

No standard use copper wire

Not defined.

T-185 to +300

-250 to +400

-40 to +125 ± 0.5°c, 125 to 350 ± 0.004*[t]°c

-40 to +133 ± 1.0°c,133 to 350 ± 0.0075*[t]°c

E 0 to +800 -40 to +900

-40 to + 375 ± 1.5°c, 375 to 800 ± 0.004*[t]°c

-40 to +333 ± 2.5°c, 333 to 900 ± 0.0075*[t]°c

For further information on thermocouple types & other colour codes see Appendix 1.

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LOCATION

Proper location of the thermocouple is probably the most important factor in obtaining accurate temperature control. Thermocouples should be in a position to have a definite temperature relationship to the heat source and workload. A good 'rule of thumb' in locating thermocouples is to place them between the workload and heat source. The thermocouple should be located 1/3 the distance from the heat source and 2/3 the distance to the workload.

If a thermocouple is located too close to the heaters, a long warm-up time will result. The thermocouple will sense the heat before it reaches the workload, and this means rapid on/off action of the controller. In effect, the controller is controlling the heater and not the workload. In rare cases, voltage will be induced into the thermocouple circuit at high temperatures when located too near the heaters.

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When a thermocouple is located too close to the workload, there is a substantial delay in sensing the proper control point and the result is overshooting the temperature. In most cases, it is better to be too close to the heaters than the workload as once a temperature point is passed, it becomes difficult to cool the workload unless a forced cooling system is used. Two thermocouples connected in parallel could be used, one located near the heaters and the other near the workload. Both will balance these two factors and provide closer control.

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Another consideration in location is when locating a thermocouple in a thermocouple well. If it is not bottomed correctly, located at the bottom of the well, the thermocouple will be reading the air temperature around it and not the temperature of the workload.

COMPENSATION

The compensation method used by all millivoltmeter manufacturers is to attach a bi-metallic spiral to the top hairspring of the coil suspension system. This spiral is selected according to the range of the instrument and will deflect the indicating pointer correspondingly with changes in ambient temperature. Once ambient is set mechanically, using a zero adjust screw, it is not necessary to change the setting during the

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operation of the instrument. In solid state instruments, the compensation is achieved electronically by placing a temperature sensor, such as a thermistor or RTD, at the cold junction to monitor its temperature. The signal from this sensor is used to compensate for variations in cold junction temperature.

The automatic compensation for ambient temperature is sufficient in most industrial applications. However, in laboratory experiments or critical control situations, when maximum accuracy is desired, one of two cold junction compensation methods are used. One method is to place the cold junction in an agitated ice bath, shown below. The instrument will then be set at 32 deg F (0 deg C), which is the temperature of the ice bath.

In the other method, the cold junction is situated in a precisely controlled temperature above ambient, as shown below. In this case, ambient compensation is not necessary. The mechanical zero adjustment is set at the cold junction temperature being maintained. The normal temperature being maintained is 150 to 200 degrees F at the cold junction.

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In normal applications, if the cold junction is located too close to the heat source, conduction and radiation heating will cause inaccurate readings. Errors will also occur when using copper wire or the wrong thermocouple lead wire. When copper wire is used, the cold junction in effect remains at the thermocouple connector block instead of the instrument. This will cause the instrument to read low in most cases unless the cold junction and instrument are known to have the same ambient temperature.

There is one application where cold junction compensation is not a factor. When two thermocouples are connected in series opposing, as shown below, a millivoltage is produced which is the difference in millivolts between the temperature at both thermocouples. As the difference in degrees between the two thermocouples is being measured, cold junction compensation is not necessary.

Each millivolt measuring instrument is calibrated for both the type of thermocouple being used and the length and gauge of the lead wire. The thermocouple lead wire is in effect in series with the thermocouple wire and the meter movement. Using wrong thermocouple lead wire can be avoided by simply following the color-coding used by all manufacturers (Table, below). A solid state controller can be used with up to 100 ohms of external resistance without having to be recalibrated.

Calibration symbols and color codes for thermocouple and extension wire

TypeISA

SymbolPositive

(+)Polarity-

Color CodeConductor

Negative (-)

Overall

Thermocou J -- Iron + White Constantan - Re Brown

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ple (Magnetic) d

Extension JX -- Iron + White Constantan - Red

Black

Thermocouple

T -- Copper

+ Blue Constantan - Red

Brown

ExtensionTX --

Copper+ Blue Constantan -

Red

Blue

Thermocouple

E -- Chromel

+ Tan Constantan - Red

Brown

ExtensionEX --

Chromel+ Tan Constantan -

Red

Brown

Thermocouple

K -- chromel

+ YellowAlumel

(Magnetic)-

Red

Yellow

ExtensionKK --

Chromel+ Yellow Alumel -

Red

Brown

Thermocouple

S -- PT 10% RH

+ -- Platinum - -- --

Thermocouple

R -- PT 13% RH

+ -- Platinum - -- --

ExtensionSX --

Copper+ Black Alloy 11 -

Red

Green

When a millivoltmeter is calibrated, a series resistance (commonly called a calibrating spool) is used between the moving element coil of the instrument and the thermocouple tip. The resistance of the wire must be determined and used in the calibration of the instrument. If the resistance of the thermocouple wire and extension wire is higher than the instrument is calibrated for, the temperature readings will be low and if the resistance is lower, the temperature readings will be high.

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Where the thermocouple and extension wire are a significant portion of the circuit, then we must also consider the resistance change of the thermocouple wire at elevated temperatures. It may be necessary to calibrate instruments at the operating temperature. As an example: 5 feet of .020 dia. platinum vs. platinum 10% Rhodium thermocouple wire, would have a resistance of 2.3 ohms. At 2500 deg F, the resistance would be 2.3 x 3.5 ohms, or 8.5 ohms. A millivoltmeter with a sensitivity of 10 ohms per volt would have an error of approximately 4% at 2500 deg F. The effects of temperature on the thermocouple and thermocouple extension wire are shown in Table below.

To illustrate the effects of incorrect lead length calibration on the millivoltmeters, we have charted the errors that can result for various ranges and thermocouples by deviating in resistance from the calibrated lead length. Table below is based upon 10 ohms per millivolt sensitivity instrument. Instruments with less sensitivity would show greater errors. A meter with a 5 ohm per millivolt sensitivity would have errors twice as great.

Thermocouple resistance change with temperature

Multiplying factor for various temps; both wires same gauge

200 deg F

400 deg F

800 deg F

1600 deg F

2500 deg F

Iron-Constantan 1.02 1.05 1.11 1.22 ----

Chromel Alumel 1.05 1.14 1.30 1.62 2.01

Chromel-Constantan

1.13 1.33 1.7 2.5 ----

Plat. 10% RH - Platinum

1.13 1.34 1.83 2.67 3.50

Plat. 13% RH - Platinum

1.13 1.33 1.80 2.60 3.40

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Deviation in Ohms from calibrated lead length

1 -- 0-2000 deg F C/A (4.02% at 20 Ohms)

2 -- 0-1200 deg F I/C, 0-1600 deg F C/A (4.97% at 20 Ohms)

3 -- 0-800 deg F I/C, 0-600 deg F C/A (7.65% at 20 Ohms)

4 -- 0-500 deg F CU/C, 0-2200 deg F PLT/PLT + 13% RH

0-2400 deg F PLT/PLT + 10% RH (14.32% at 20 Ohms)

5 -- 0-300 deg F I/C, o-350 deg F CU/C (22.15% at 20 Ohms)

THERMOCOUPLE CONNECTION

There are two common errors in connection thermocouple circuits. One is to connect the extension lead wire completely reversed. In this case, you would receive a low reading because the reversal causes the emf generated at the connection of the thermocouple and extension lead wire to be subtracted from the emf generated by the thermocouple. A more obvious error is to completely reverse the thermocouple. The instrument in this case will read downscale with an increase in temperature.

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Some control instruments feature 'thermocouple break protection' which means that in the event of an open or broken thermocouple, a small voltage is applied to the instrument which will cause it to read full scale and turn off the external circuit. Thus, in the event the thermocouple breaks because of a mechanical shock or vibration or if it is over-exposed to extremely high temperature and deterioration sets in, an unattended process will not overheat because of the loss of control.

Another consideration in the thermocouple use is that the leads wires should never run in the same conduit with electrical lines. This may induce currents in the thermocouple wire, resulting in instrument errors and poor control. However, if this cannot be avoided, or if the induced currents are being picked up at the thermocouple itself, then one side of the thermocouple lead wire should be grounded through a 1.0 microfared paper capacitor at one of the thermocouple terminals in the instrument. In emergencies, a direct ground will sometimes work as well.

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Occasionally, because of atmospheric conditions, corroding may occur on connections which cause a loss of the millivolt signal. Or, a poor connection between the lead wire and thermocouple could cause loss of signal.

The gauge size of the wire used in thermocouples is again dependent upon the application. Usually, when longer life is required, for the higher temperature ranges, the larger size wires are chosen. When sensitivity is the prime concern, the smaller sizes should be used.

THERMOCOUPLE CALIBRATION PROCEDURE

The following information is intended to give the reader a review, in some detail, of the equipment requirements and proper techniques needed to accurately calibrate thermocouples and thermocouple materials.

Branom Instrument calibrates thermocouple and thermocouple wire in accordance with one of the following American Society for Testing and Material (ASTM) Standards: E207-88, standard method of Thermal EMF Test of single thermoelement materials by comparison with a secondary standard of similar EMF temperature properties. E220-86, standard method for calibration of thermocouples by comparison techniques.

In general these standards describe the type of temperature source, measuring equipment, standards, and procedures needed to accurately

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perform a calibration. Each of these elements will now be looked at more closely.

CONTROLLED TEMPERATURE SOURCE

The temperature source used in the process of calibrating should as a minimum be stable enough to provide a constant temperature (approx.+/-.2 degrees F) for a short length of time (approx.20 mm.) at any temperature at which the temperature bath or other source is to be used. The temperature source should have a zone of uniform temperature into which the thermocouple measuring junction may be inserted. The length of the temperature source must be adequate to permit a depth of immersion sufficient to assure that the measuring junction temperature is not affected by a temperature gradient along the thermocouple wires.

CONTROLLED TEMPERATURE SOURCES

Fixed Point: When highly accurate measurements must be made, fixed point cells are used. A fixed point cell consists of a metal sample inside a graphite crucible with a graphite thermometer well submerged into the metal sample. When the metal sample is heated to the freezing point, it will produce a very stable and constant temperature. In order to better understand the operation of fixed point cells, the following definitions are useful.

Fixed Point: A reproducible temperature of equilibrium between different phases of a material.

Freezing Point: The fixed point between the solid and liquid phases of a material.

REFERENCE JUNCTIONS

A thermocouple's output is based on the difference in temperature between the measuring junction (hot junction) and the reference junction (cold junction). See Figure A.

REFERENCE JUNCTION TEMPERATURE

A controlled temperature must be provided in which the reference junction is maintained at a constant chosen temperature. The reference junction temperature should be controlled to a better accuracy than that expected from the thermocouple calibration. The most commonly

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used reference temperature is 32 degrees F., but other temperatures may be used if desired.

ICE BATH

One of the most common reference junctions is the ice bath. The ice bath is made up of a mixture of melting shaved ice and water. The ice bath is a convenient and inexpensive way to achieve an ice point, it can be reproduced with ease and with exceptional accuracy. Junctions formed between the thermocouple materials and instrument leads can be simply immersed into the slush mixture, or alternatively glass "U" tubes containing a quantity of mercury approximately 3/4" to 1" depth can be placed into the slush mixture. Quick electrical connection can then be made between thermocouple and instrument leads through the mercury. (Figure B).

Note: An improperly used ice bath can result in serious errors. The largest error which is likely to occur arises due to melting of the ice at the bottom of the bath until the reference junctions are below the ice level and surrounded by water alone. This water may be as much as 7 degrees F above the ice point.

AUTOMATIC ICE POINT

The automatic ice point is an electrical refrigerated device in which an equilibrium between ice and water is constantly maintained. The change of volume of water in freezing is used to control heat transfer. Some commercially available devices provide wells into which the user may insert reference junctions formed from his own calibrated wire. Others are provided with many reference junction pairs brought out to terminals which the user may connect into his system.

ELECTRONIC COMPENSATION

This method employs a compensation circuit containing a source of current and a combination of fixed resistors and a temperature sensitive resistor (TSR). This device can be designed to produce similar EMF to that of the thermocouple being calibrated. The Electronic Compensator will make EMF compensations to the thermocouple circuit based in the difference in EMF from 32 to ambient temperature.

MEASURING INSTRUMENTS

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The choice of a specific instrument to use for measuring the thermocouple output will depend on the accuracy required of the calibration being performed. In general, an instrument such as the Fluke 702 calibrator or Altek 422 is sufficient for most thermocouple calibrations.

REFERENCE THERMOMETERS

The reference thermometer to be used for the comparison calibration of a thermocouple will depend upon the temperature range covered, the accuracy desired, the capabilities, or the preference of the calibration laboratory. The following are different examples of reference thermometers.

PLATINUM RESISTANCE THERMOMETERS

A standard platinum resistance thermometer (SPRT) is the most accurate standard available, however, it is the most expensive standard, and other standards are acceptable alternatives depending upon the temperature range covered, the accuracy desired, the capabilities, or the preference of the calibration laboratory. The following are different examples of reference thermometers.

LIQUID-IN-GLASS THERMOMETERS

Liquid-in-glass thermometers are available to cover the range from -300 to 950 degrees Fahrenheit. with an accuracy of from .1 to 3 Fahrenheit depending on the type of thermometer and the width of the range covered. They are relatively inexpensive but they are fragile, and if the highest degree of accuracy of which they are capable is to be achieved, an individual thermometer must cover a very narrow temperature range so that the graduation intervals can be as large as possible. A further disadvantage of the liquid-in-glass thermometer is that because of their fine graduations reading errors are a distinct possibility. Taylor Instruments offers Superior Grade Certified Secondary Reference Thermometers individually or in matched Celsius or Fahrenheit sets, which Branom stocks.

TEST ASSEMBLY PLACEMENT IN THE FURNACE

Depth of immersion is the most important consideration if accurate calibration results are to be obtained. The depth of immersion must be

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sufficient to eliminate the effects of heat transfer away from the junction. It is impossible to establish a minimum depth of immersion that would be useable under all circumstances since heat transfer characteristics are dependent on the mass of material being put into the temperature source.

WIRING CONNECTION FROM TEST ASSEMBLY TO READOUT INSTRUMENT

The actual wiring necessary to connect the test assembly, reference junction and readout instrument will depend on the quantity of thermoelements in the test assembly, the type of reference junction used and whether or not a switching device is used, but the basic requirements are the same. Thermocouple extension wire is used to connect the thermoelements to the reference junction. Copper wires are used between the reference junction and readout instrument.

THERMOCOUPLE WIRE, WIRING PROCEDURE

Ideally, the samples of the thermocouple material to be calibrated and the standard thermocouple element should be cut long enough so that they reach directly from the temperature source to the reference junction without the need for extension wires. If this is not possible extension wires may be used, but they must be securely connected directly to the test assembly conductors. If extension wires must be used, remove any oxide layer that may be on the surface of the test assembly conductors and attach an extension wire of the same material to each conductor by laying the extension wire alongside the conductors and joining them securely by means of an alligator clip.

THERMOCOUPLE CALIBRATION WIRING PROCEDURE

When calibrating thermocouples, it is faster and more convenient to use a thermocouple switching box. The extension wires from the thermocouples are placed into one side of the reference junction. Multiple pairs of copper leadwire will exit the reference junction and will be connected to the switch box. One pair of copper leadwires will run from the readout instrument to the thermocouple switch box.

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For a more in-depth look at thermocouples and thermocouple calibration the reader is encouraged to read ASTM STP 470, manual on the use of thermocouple in temperature measurement.

JUNCTION LOCATION

One of the primary advantages of calibrating thermocouple materials against a base-metal standard of similar EMF output is that the sample(s) to be calibrated are welded to the base-metal standard forming a common junction thus achieving good isothermal conditions between the test thermoelement and the standard. Furthermore, because the test thermoelement and the standard produce nominally the same EMF vs. platinum (pt-67) the EMF output changes little over a fairly broad temperature range, thereby reducing the need for precise temperature source control. See Figure C.

MEASUREMENT

Set your controlled temperature source to the specified temperature and allow it to adequately stabilize. Immerse the test assembly into the test temperature medium and provide sufficient time for the test assembly to stabilize. Once the test assembly is stable the EMF generated between the test specimen and the reference standard can be recorded. Avoid soaking the test assembly at temperature for a prolonged period of time, as it can cause permanent changes to occur in the thermoelements.

Once the reading is taken, raise the test temperature to the next higher temperature, first removing the test assembly from the temperature source, or advance the test assembly to the next temperature source. Allow the temperature source and the test assembly to stabilize as before, and take a second set of readings at the new temperature.

In all cases take the reading in sequence from the lowest to the highest temperature. A base metal reference standard shall be used for one series of temperature changes only.

ASTM E 220 THERMOCOUPLE CALIBRATION

The Test thermocouple junction should be located so that it is in intimate contact with the junction of the standard. Without making a

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radiograph of the thermocouple it is impossible to know exactly where the junction is located. A few generalizations can be made which enables junctions to be located quite closely. First, the cap weld on a metal sheathed thermocouple is normally about as thick as one-half the sheathed diameter. Second, a "U" junction is normally about one-half the sheathed diameter. Using these generalizations, a thermocouple .125" diameter, will have a grounded junction approximately .063" below the tip of the cap. The thermocouple standard should be tied to the thermocouple (s) with a fine gauge wire. The junction of the standard should be bent so that it is in contact or at least very close to the point where it has been calculated that the junction is located. See Figure D.

PLATINUM VERSUS PLATINUM RHODIUM THERMOCOUPLE

Platinum vs. platinum 10% Rhodium standard thermocouples (ANSI Type S) are exceptionally accurate and stable devices. NIST offers calibration uncertainty of.9 F. from 0 to 1112 F, and 1.26 F. from 1112 to 2012 F. When used as a working standard, understandably due to the high cost of these materials, they cannot be discarded after each use. Consequently, care must be taken to avoid contamination, work hardening and other sources of de-calibration.

BASE METAL THERMOCOUPLE STANDARDS

An alternate approach to calibrating thermocouples and thermocouple materials, and one which gives a high degree of accuracy, is calibrating with a secondary standard that has similar FMF properties to those of the test element. That is, calibrating a KP element against a KP standard.

All thermocouple materials in the USA are referenced to a pure platinum element (PT-67) which is retained at the National Institute of Standards and Technology. It is important that regardless of the type of standard used that traceability to this NIST standard be accomplished.

CALIBRATION PROCEDURE

The instruments mentioned previously as standards, fixed point cells, platinum resistance thermometers and liquid-in-glass thermometers

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can all be used to accurately calibrate thermocouples if the proper calibration procedures are followed. As previously mentioned, the two most common procedures are ASTM E207 & P220. We will now examine both more closely.

ASTM E207 WIRE TO WIRE CALIBRATION

In order to achieve the maximum amount of accuracy when using base-metal standards, it is desirable whenever possible to make wire to wire readings, that is, reading the EMF developed between the thermocouple material to be calibrated and the base-metal standard of similar material.

Thermocouple material should be free of contaminants, insulated wire should be stripped of insulation. Insulation should be removed carefully in order to avoid cold working by nicking or stretching. Any one of these conditions could cause erroneous calibration results.

FLUIDIZED BEDS

The fluidized bed is a unique method of providing closely controlled temperatures. The bath consists of a very fine mesh aluminum oxide, a heated chamber into which the medium is placed, and a means for slowly agitating the bath by introducing a flow of air. By careful control of heat input and air flow, temperatures of the bath can be controlled within close limits thereby producing isothermal conditions between the calibration standard and the test setup. Fluidized beds are useful for calibrating over the temperature range from ambient to 1600 degrees F.

STIRRED LIQUID BATHS

Stirred liquid baths operate on the same principal as fluidized beds and are an excellent means of establishing closely controlled temperatures. Although stirred liquid baths using molten salts or liquid tin are available with a temperature range as high as 932 degrees F., the most common application is in the range of ambient to 500 degrees F. utilizing silicone oil as the bath material.

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TUBE-TYPE HEATING ELEMENT FURNACE

For temperatures above 500 degrees F. an electrically heated tube furnace is recommended. Tube furnaces operate in the range of 500 to 3100 Degrees. F.

Figure A:

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Figure B:

Figure C:

Figure D:

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Applications

Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and RTDs are more suitable.

Steel Industry

Type B, S, R and K thermocouples are used extensively in the steel and iron industry to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, Type S thermocouples are regularly used in the electric arc furnace process to accurately measure the steel's temperature before tapping. The cooling curve of a

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small steel sample can be analyzed and used to estimate the carbon content of molten steel.

Heating appliance safety

Many gas-fed heating appliances like ovens and water heaters make use of a pilot light to ignite the main gas burner as required. If the pilot light becomes extinguished for any reason, there is the potential for un-combusted gas to be released into the surrounding area,

thereby creating both risk of fire and a health hazard. To prevent such a danger, some appliances use a thermocouple as a fail-safe control to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame. The resultant voltage, typically around 20 mV, operates the gas supply valve responsible for feeding the pilot. So long as the pilot flame remains lit, the thermocouple remains hot and holds the pilot gas valve open. If the pilot light goes out, the temperature will fall along with a corresponding drop in voltage across the thermocouple leads, removing power from the valve. The valve closes, shutting off the gas and halting this unsafe condition.

Many systems (Millivolt control systems) extend this concept to the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters.

A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

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Out of concern for wasted energy, many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. This eliminates the need for a standing pilot flame but loses the benefit of any operation without a continuous source of electricity.

Thermopile radiation sensors

Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile sensors. For example, laser power meters are based on such sensors.

Radioisotope thermoelectric generators (RTGs)

Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.

Appendix 1

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Thermocouple Types

The selection of the optimum thermocouple type (metals used in their construction) is based on application temperature, atmosphere, required length of service, accuracy and cost. When a replacement thermocouple is required, it is of the utmost importance that the type of thermocouple type used in the replacement matches that of the measuring instrument. Different thermocouple types have very different voltage output curves. It is also required that thermocouple or thermocouple extension wire, of the proper type, be used all the way from the sensing element to the measuring element. Large errors can develop if this practice is not followed.

The thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

K

Type K (chromel–alumel) is the most commonly used general purpose thermocouple. It is inexpensive and, owing to its popularity, available in a wide variety of probes. They are available in the −200 °C to +1200 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals, nickel, is magnetic. The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354°C. Sensitivity is approximately 41 µV/°C.

E

Type E (chromel–constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.

J

Type J (iron–constantan) is less popular than type K due to its limited range (−40 to +750 °C). The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration. The magnetic properties also prevent use in some applications. Type J thermocouples have a sensitivity of about 52 µV/°C.

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N

Type N (nicrosil–nisil) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/°C at 900°C, slightly lower than a Type K. Designed to be an improved type K, it is becoming more popular.

B, R, and S

Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity, approximately 10 µV/°C, than other types. The high cost of these thermocouple types makes them unsuitable for general use. Generally, type B, R, and S thermocouples are used only for high temperature measurements.

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

Type S thermocouples use a platinum–rhodium alloy containing 10% rhodium for one conductor and pure platinum for the other conductor. Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

T

Type T (copper–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. As both conductors are non-magnetic, type T thermocouples are a popular choice for applications such as electrical generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of about 43 µV/°C.

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C

Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0& °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 500°F.

M

Type M thermocouples use a nickel alloy for each wire. The positive wire contains 18% molybdenum while the negative wire contains 0.8% cobalt [1] . These thermocouples are used in the vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. Though it is a less common type of thermocouple, look-up tables to correlate temperature to EMF (milli-volt output) are available.

Thermocouple Color Codes

Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown.

United States ASTM:

British BS1843: 1952:

British BS4937: Part 30: 1993:

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French NFE:

German DIN:

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