By: Engr. Irfan Ahmed HalepotoAssistant Professor

LECTURE#06POSITION SENSORS

(DISPLACEMENT) TRANSDUCERS

AUTOMATION & ROBOTICS

POSITION (DISPLACEMENT) SENSORS• A position sensor is any device that permits position

measurement. • It can either be an absolute position sensor or a relative one

(displacement sensor). – Gear Mechanism

• Position sensors can be either linear , rotary, or angular . – Revolution per minutes.

linear position sensors Rotary Sensors

Position Sensor Types• Linear variable differential transformer (LVDT)• Encoders • Potentiometer• Capacitive transducer• Eddy-current sensor• Hall effect sensor• Proximity sensor (optical)• Inductive Non-Contact Position Sensors• Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

• LVDT is a non contact transducer based on Electromechanical device that measures Displacement.

• LVDT are responsible for electrical output, proportional to displacement of a movable core.– Movements are usually measured in range of ±12 inches

(Rotary , Linear) – Some LVDT’s have capabilities to measure up to ±20

inches (Range: 0.01-20 inches).• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer• LVDT transducer comprises a coil

former on to which three coils are wounded. • One primary and two secondary

coils.• Primary transformer coils wound on

non-magnetic cylindrical-coil forms.• Two secondary transformer coils

are wound on top of the primary.• LVDT consists of a ferromagnetic

core which moves inside the coil form.– Transfer of current b/w primary

and secondaries of the LVDT displacement transducer is controlled by the position of a magnetic core called as armature (core).

LVDT Internal Parts

Signal conditioning circuitry

Primary coil

Secondary coil

Secondary coil

Bore shaft

Ferrite core Epoxy encapsulation

Stainless steel end caps

High density glass filled coil forms

Magnetic shielding

LVDT Transducer Construction• Primary coil typically is excited at 5-

8V AC b/w 60 Hz and 20 kHz,

– causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.

• Two secondary windings are located on either side of the primary winding

• The A.C in the primary winding creates an axial (Lines) magnetic flux field that is concentrated in the core.

• This flux is coupled to the secondary windings through the core.

• Core causes the magnetic field generated by the primary to be coupled to the secondaries.

LVDT Operation• When the core is centered b/w the

two secondary windings the voltage induced in each is identical.– Voltage Va induced in secondary

a and voltage Vb induced in secondary b will be in phases (both voltages cancel), output (VaVb) will be zero.

• Core position where the output voltage is zero is referred to as the null position

• As core move to the left, first secondary is more strongly coupled to the primary than the second secondary.– Voltage of the first secondary

causes an output voltage which is in phase with the primary voltage.

LVDT Operation …………. • Likewise, when core moves to the

right, second secondary is more strongly coupled to the primary than the first secondary.– Voltage of the second secondary

causes an output voltage to be out-of-phase with the primary voltage.

• LVDT signal conditioners generate a sine wave for the primary and synchronously demodulate the secondary output signal, so that the DC voltage that results is proportional to core displacement.

• Sign of the DC voltage indicates whether the displacement is to the left or right.

• In one direction, output signal will be in phase with the excitation and 180° out of phase in the other direction

LVDT Transducer-Operation

LVDT Applications • Automation Machinery• Civil/Structural Engineering• Power Generation• Manufacturing• Metal Stamping/Forming• Pulp (soft tissue) and Paper• Industrial Valves• R & D and Tests• Automotive Racing

LVDT Application Circuits • Signal conditioning associated with differential transformers

includes filtering and amplification

Amplification

Filtration

Why use a LVDT?• Friction – Free Operation

– No mechanical contact between core and coil (usually)• Infinite Mechanical Life

• Infinite Resolution– Electromagnetic coupling

• Limited only by electrical noise• Low risk of damage

– Most LVDT’s have open bore holes• Null Point Repeatability

– Zero displacement can be measured• Single Axis Sensitivity

– Effects of other axes are not felt on the axis of interest• Environmentally Robust

– Stable/Strong sensors – good for structural engineering tests!

ENCODERS • An encoder is a device, circuit, transducer, software program,

algorithm that converts information from one format or code to another, for the purposes of standardization, speed, space accommodation, security etc.

• In Automation Industry, Encoders are sensors that generate digital signals in response to movement. – Rotary (Shaft) encoders: respond to rotation, – Linear encoders: respond to motion in a line.

• Rotary encoder (shaft encoder) is an electro-mechanical device that converts the angular position or motion of a shaft to an analog or digital code.

• Linear encoder is a rotary device that outputs digital pulses in response to incremental angular motion. – When used in conjunction with mechanical conversion

devices, such as rack-and-pinions, measuring wheels, or spindles, shaft encoders can also be used to measure linear movement, speed, and position.

Encoder Applications• Encoders have many uses in positioning applications.

– For example, a rotary encoder attached to a DC motor can be used to keep track of the number of revolutions the motor has rotated from its initial position.

• One of the simplest applications of rotary encoders is the mechanical computer mouse. – A mechanical mouse has 2 rotary encoders: One for X

position and one for Y position.– As the mouse moves, each encoder outputs square wave

pulses. – The number of pulses indicate how far the mouse has

moved in X or Y direction.• Encoders are also used in Computer Numerical Control

( CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology • Encoders can use either optical or magnetic sensing technology.

• Optical sensing: provides high resolutions, high operating speeds, reliability, long life operation in most industrial environments.

– Typical incremental scale periods vary from hundreds down to a few micrometres.

– Light sources used include infrared LEDs, visible LEDs, miniature light-bulbs and laser diodes.

• Magnetic sensing: often used in such rugged applications as steel and paper mills, provides good resolution, high operating speeds, and maximum resistance to dust, moisture, and thermal and mechanical shock.

– Magnetic linear encoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils, Hall Effect or magnetoresistive readheads.

Optical Encoders• Optical encoders use a glass disk with a pattern of lines

deposited on it.

– a metal or plastic disk with slots (in a rotary encoder), or a glass or metal strip (in a linear encoder).

• Light from an LED shines through the disk or strip onto one or more photo detectors, which produce the encoder’s output.

• An incremental encoder has one or more of these tracks, while an absolute encoder has as many tracks as it has output bits.

Optical Encoders• Optical encoder's disc is made of

glass or plastic with transparent and obscure areas.

• A light source and photo detector array reads the optical pattern that results from the disc's position at any one time.

• This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft.

• The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm).

Magnetic Encoders• Magnetic sensing technology is very resistant to dust,

grease, moisture, and other contaminants common in industrial environments, and to shock and vibration.

Magnetic Encoders Types • There are several types of magnetic sensors. • Variable reluctance sensors: detect changes in the magnetic field

caused by the presence or movement of a ferromagnetic object.– Variable-reluctance rotary sensor (magnetic pickup) consists of a

coil wound around a permanent magnet, generates a voltage pulse when a gear tooth moves past it.

• Another type of sensor uses a permanent magnet and a magneto resistive device to produce a change in either voltage or electrical resistance in the presence of ferromagnetic material, which can be in the form of a gear tooth (in a rotary encoder) or a metal band with slots (in a linear encoder). – This type of sensor will work down to zero speed, and is available

in both rotary and linear forms. • Another type of magnetic sensor uses a magneto resistive device to

detect the presence or absence of magnetized “stripes,” either on the rim of a drum or on a nonmagnetic strip.

Encoders Classification • Encoders are available with a choice of outputs.

– Incremental (relative-linear) & Absolute (Rotary) • Incremental encoders: generate a series of pulses as they

move. – These pulses can be used to measure speed, or be fed

to a counter to keep track of position. – Output of incremental encoders provides information

about the motion of the shaft which is further processed elsewhere into information such as speed, distance, RPM and position.

• Absolute encoders: generate multi-bit digital words that indicate actual position directly.– Output of absolute encoders indicates the current

position of the shaft, making them angle transducers.

Rotary Encoder….inside• Typical rotary encoder looks like a

potentiometer, it has infinite rotation.• You can turn it in either direction

without hitting an end point. • Inside, the encoder, there is a slotted

disc and 2 optical interrupters.• Two opto interrupters are mounted on

the slotted disk. • As the slotted disk turns, the light beam

between the LED and the phototransistor of the opto interrupters are connected and disconnected.

• This results in a pulse outputs from each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders

• Absolute encoder uses multiple groups of segments that form concentric circles on the encoder wheel.

• The concentric circles start in the middle of the encoder wheel and as the rings go out toward the outside of the ring they each have double the number of segments than the previous inner ring.

• The first ring, which is the innermost ring, has one transparent and one opaque segment.

• The second ring out from the middle has two transparent and two opaque segments, and the third ring has four of each segment.

• If the encoder has 10 rings, its outermost ring will have 512 segments, and if it has 16 rings it will have 32,767 segments.

Absolute Encoders• Since each ring of the absolute encoder has double the

number of segments of the prior ring, the values form numbers for a binary counting system.

• Since the absolute encoder produces only one distinct number or bit pattern for each position within its range.– it knows where it is at every point between the two ends

of its travel, – and it does not need to be homed to the machine each

time its power is turned off and on (like in Incremental Encoder).

Rotary binary encoding-Example• An example of a binary code, in an extremely simplified encoder with

only three contacts, is shown below.• In general, where there are n contacts, the number of distinct positions

of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions

Sector Contact 1 Contact 2 Contact 3 Angle

1 off off off 0° to 45°

2 off off ON 45° to 90°

3 off ON off 90° to 135°

4 off ON ON 135° to 180°

5 ON off off 180° to 225°

6 ON off ON 225° to 270°

7 ON ON off 270° to 315°

8 ON ON ON 315° to 360

Rotary encoder for angle-measuring devices marked in 3-bit binary.

Rotary binary encoding……issue• In Rotary binary encoding, contacts produce a standard binary count as

the disc rotates. • However, this has the drawback that if the disc stops between two

adjacent sectors, or the contacts are not perfectly aligned, so switches can have a different moment, thus it become impossible to determine the angle of the shaft. – If contact 1 switches first, followed by contact 3 and then contact 2,

for example, the sequence of codes will be :– off-on-on (starting position) on-on-on (first, contact 1 switches on)

on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2 switches off)

• This will produce sectors sequence as 4, 8, 7 and then 5 from table• In addition contracts can have different sequence and momentum, this

behavior can be undesirable and could cause the system to fail. – i.e.: if the encoder were used in a robot arm, controller would think

that the arm was in the wrong position, and try to correct the error by turning it through 180°, perhaps causing damage to the arm.

Linear Encoder• A linear encoder is a sensor that is based on pair of

different scales to encodes position.

• The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position motion controller.

• Motion can be determined by change in position over time.

• Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current.

• Linear encoders are used in metrology instruments, motion systems and high precision machining tools ranging from digital calipers to coordinate measuring machines.

INCREMENTAL ENCODERS• Incremental encoders are used to track motion and can be

used to determine position and velocity. • Incremental encoders provide a specific number of equally

spaced pulses per revolution (PPR) or per inch or millimeter of linear motion.

• This can be either linear or rotary motion. – A single channel output is used for applications where

sensing the direction of movement is not important. – Where direction sensing is required, quadrature output is

used, with two channels 90 electrical degrees out of phase; circuitry determines direction of movement based on the phase relationship between them.

• With Incremental Encoders as direction can be determined, very accurate measurements can be made.

Incremental Encoder Classification • They can be either mechanical or optical.

– Mechanical type requires debouncing and is typically used as digital potentiometers on equipment including consumer devices.

• Most modern home and car stereos use mechanical rotary encoders for volume control.

• Mechanical type Incremental encoders are limited up to 10,000 counts per revolution.– Optical type is used when higher RPMs are encountered

or a higher degree of precision is required.

Incremental rotary encoder---output• An incremental encoder’s output

indicates motion. • Incremental rotary encoder

provides cyclical outputs, when the encoder is rotated.

• They employ two outputs called A & B, known as quadrature outputs, as they are 90 degrees out of phase.

• These signals are decoded to produce a count-up pulse or count-down pulse.

• Since two sets of pulses are out of phase from each other, it is possible to determine which direction the shaft is rotating by the amount of phase shift b/w the first set and second set of pulses.

Incremental encoder---output• Incremental encoder’s output

indicates motion.

• To determine position, its pulses must be accumulated by a counter.

• Counting is subject to loss during a power interruption by electrical transients.

• When starting up, the equipment must be driven to a home position to initialize the position counters.

– Some incremental encoders also produce another signal known as the “Z channel, this signal, produced once per revolution of a shaft encoder.

•

Incremental Encoders…..issue• One of the major drawbacks of the incremental encoder is that

the number of pulses that are counted are stored in a buffer or external counter.

• If power loss occurs, the count will be lost. • This means that if a machine with an encoder has its electricity

turned off each night or for maintenance, the encoder will not know its exact position when power is restored.

• The encoder must use a home-detection switch to indicate the correct machine position.

• The incremental encoder uses a homing routine that forces the motor to move until a home limit switch is activated.

• When the home limit switch is activated, the buffer or counter is zeroed and the system knows where it is relative to fixed positional points.

• The absolute encoder is designed to correct this problem.• Absolute encoder is designed in such a way that the machine will

always know its location.

Incremental v/s absolute encoder …constructionTraditional absolute encoders• Traditional absolute encoders have multiple code rings with various

binary weightings which provide a data word representing the absolute position of the encoder within one revolution.

• This type of encoder is often referred to as a parallel absolute encoder.• The distinguishing feature of the absolute encoder is that it reports the

absolute position of the encoder to the electronics immediately upon power-up with no need for indexing.

Traditional incremental encoders• A traditional incremental encoder works differently by providing an A

and a B pulse output that provide no usable count information in their own right.

• Rather, the counting is done in the external electronics. • The point where the counting begins depends on the counter in the

external electronics and not on the position of the encoder. • To provide useful position information, the encoder position must be

referenced to the device to which it is attached, generally using an index pulse.

• The distinguishing feature of the incremental encoder is that it reports an incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation• The difference between incremental and absolute encoders is

analogous to the difference between a stop watch and a clock.

• A stop watch measures the incremental time that elapses between its start and stop, much as an incremental encoder will provide a known number of pulses relative to an amount of movement.

• If you knew the actual time when you started the watch, you can tell what time it is later by adding the elapsed time value from the stop watch.

• For position control, adding incremental pulses to a known starting position will measure the current position.

• When an absolute encoder is used, the actual position will constantly be transmitted, just as a clock will tell you the current time

Encoders Applications• Encoders can be used in a wide variety of applications. They act as

feedback transducers for motor-speed control, as sensors for measuring, cutting and positioning, and as input for speed and rate controls. Some examples are listed below

• Door control devices • Assembly machines• Robotics • Labeling machines• Lens grinding machines • x/y indication• Plotters • Analysis devices• Testing machines • Drilling machines• Ultrasonic welding • Mixing machines• Converting machinery • Medical equipment