robot actuators and feedback components(main7)

Unit-VII

Robot Actuators & Feed back components

IntroductionPosition & Velocity sensors are used in robotics as feed back

devices, while actuators & power transmission devices are used to accomplish the control actions indicated by the controller.

Actuators are the muscles of robots. If you imagine that the links and the joints are the skeleton of the robot, the actuators act as muscles, which moves or rotate the links to change the configuration of robots. The actuators must have enough power to accelerate and decelerate the links and to carry the loads, yet be light, economical, accurate, responsive, reliable and easy to maintain.

Position sensors provide the necessary means for determining whether the joints have moved to correct linear or rotational locations in order to achieve the required position & orientation.

The speed with which the manipulator is moved is another performance feature which must be regulated. Robots utilize a feedback system to ensure proper speed control.

It is important that a sophisticated control system has to be developed to fine tune the dynamic performance of manipulator during acceleration & deceleration as it moves between the points in work space.

Position Sensors PotentiometersPotentiometers are analog devices whose output

voltage is proportional to the position of wiper.Potentiometers offer a low cost method of contact

displacement measurement. Depending upon their design, they may be used to

measure either rotary or linear motion. In either case, a movable slide or wiper is in contact

with a resistive material or wire winding. The slide is attached to the target object in motion.

A DC or an AC voltage is applied to the resistive material.

When the slide moves relative to the material, the output voltage varies linearly with the total resistance included within the span of the slide.

An advantage of potentiometers is that they can be used in applications with a large travel requirement.

It is possible to use pots to provide a limited amount of feedback control in robots where high proportional resolution and accuracy are not required.

Benefits:Relatively Inexpensive

Can be used for Distance and Direction

Low software overhead – Not dependent on interrupts

Can Sense Speed and Distance

Drawbacks:Sample Rate issues can limit max useable Speed

Small “Dead” zone, though usually not a big problem

Position Sensors ResolversA resolver is a type of rotary electrical transformer

used for measuring degrees of rotation. It is an analog device whose output is proportional to the angle of rotating element with respect to fixed element.

The primary winding of the transformer, located on rotor shaft, is excited by a sinusoidal electric current, which by electromagnetic induction induces current to flow through the secondary windings located on the stator.

The two two-phase windings, fixed at right (90°) angles to each other on the stator, produce a sine and cosine feedback current by the same induction process.

The relative magnitudes of the two-phase voltages are measured and used to determine the angle of the rotor relative to the stator.

Since a resolver is a rotary transformer we must require an AC signal for excitation. If Dc signal is used there will be no output signal.

Position Sensors EncodersEncoders are sensors which converts linear or angular

displacement in to digital code or pulse signals.Encoders are mainly classified as Linear encoder and rotary

encoder.They are also classified as Absolute encoder and incremental

encoders.Rotary encoders are used to measure the angular position

and direction of a motor or mechanical drive shaft. Linear encoders measure linear position and direction. They

are often used in linear stages or in linear motors.Absolute encoders provide actual position relative to a fixed

reference position.Incremental encoders sense the position from previous

position. A robot utilizing an incremental encoder must execute a calibration sequence before position information obtained.

In a rotary encoder, a glass or metal disk is attached to a motor or mechanical drive shaft. The disk has a pattern of opaque and transparent sectors known as a code track.

A light source is placed on one side of the disk and a photo detector is placed on the other side.

As the disk rotates with the motor shaft, the code track interrupts the light emitted onto the photo detector, generating a digital signal output.

The number of opaque/transparent sector pairs, also known as line pairs, on the code track corresponds to the number of cycles the encoder will output per revolution. The number of cycles per revolution (CPR) defines the base resolution of the encoder.

An absolute encoder consists of the disk which has multiple concentric code tracks and a separate photo detector is used with each code track. The number of code tracks is equivalent to the binary resolution of the encoder., as shown in fig 12.4.

An 8-bit absolute encoder has eight code tracks. The 8-bit output is read to form an 8-bitword indicating absolute position.

While absolute encoders are available in a wide variety of resolutions, 8-, 10-, and 12-bit binary are the most common.

Due to their complexity, absolute encoders are typically more expensive than incremental encoders. Absolute encoders may output position in either parallel or serial format.

An incremental rotary encoder, also known as a quadrature encoder or a relative rotary encoder, has two outputs called quadrature outputs.

Incremental encoders are used to track motion and can be used to determine position and velocity. This can be either linear or rotary motion. Because the direction can be determined, very accurate measurements can be made.

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

The two output wave forms are 90 degrees out of phase, which is all that the quadrature term means. These signals are decoded to produce a count up pulse or a count down pulse and tables are used to decode the direction.

For example if the last value was 00 and the current value is 01, the device has moved one half step in the clockwise direction.

Gray coding for clockwise rotation

Phase

A B

1 0 0

2 0 1

3 1 1

4 1 0

Gray coding for ccw rotation

Phase

A B

1 1 0

2 1 1

3 0 1

4 0 0

Benefits:Can be used to sense Speed, Distance, and Direction.

Drawbacks:Can be expensive

Can be cheaply built but at the expense of time!

Uses more I/O Lines

Use of Interrupts can be problematic

Mechanical Noise Issues

Velocity Sensors Tachometers

A tachometer (also called a revolution-counter, rev-counter, or RPM gauge) is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common.

Tachometers can be divided in to1.DC (Digital)tachometer2.AC (analog) tachometerIn robotics mostly DC tachometer is used.

vo

Commutator Speed

RotatingCoil

PermanentMagnet

SNRotatingCoil

2r

h

DC Tachometer (Angular Velocity)

• A tachometer is essentially DC generator providing an output voltage proportional to the angular velocity of the armature

• The rotor is directly connected to the rotating object.

• The output signal that is induced at the rotating coil is picked up using a commutator device (consists of low resistance carbon brushes)

• Commutator is stationary but makes contact with the split slip rings

+

-

Outputvo

AC Carrier Source

vref

Secondary Stator

PrimaryStator

~

Permanent-Magnet

Rotor

Permanent Magnet AC Tachometer• When the rotor is stationary or moving in a quasi-static manner the

output voltage will be constant

• As the rotor moves, an additional voltage, proportional to the speed of the rotor will be induced

• The output is an amplitude modulated signal proportional to the rotor speed and demodulation is necessary

• Direction is obtained from the phase angle

• Due to commutator in DC tachometers a slight ripples will appear in output voltage which can not be filtered out, this can overcome by using AC tachometer

• For low frequency applications (~5Hz), supply with 60Hz is adequate

• Sensitivity is in the range 50 – 100mV/rad/s

Benefits:Can be implemented with limited resources - sensors included in last year’s kit

Can Sense Speed and Distance

Drawbacks:Cannot sense Direction

Use of Interrupts can be problematic.

Mechanical Noise can bea problem

Types of Actuators1. Electrical actuators:Electric actuators are

simply electro-mechanical devices which allow movement through the use of an electrically controlled systems of gears. Electric motors

DC servomotors AC motors Stepper motors

Solenoids2. Hydraulic actuators

Hydraulic actuators allow a robot to move by the use of fluids moving under pressure through a series of valves by the use of pumps. The hydraulic fluids normally consist of oils which are reasonably non-compressible.

3. Pneumatic Actuators Pneumatic actuators use compressed gas to

force the movement of pistons through the use of pumps and valves and so allow movement of the robotic part.

Hydraulic and Pneumatic actuators are classified as

Linear Actuators (Cylinders)Rotary Actuators (Motors)

Issues in choosing actuatorsLoad (e.g. torque to overcome own inertia)Speed (fast enough but not too fast)Accuracy (will it move to where you want?)Resolution (can you specify exactly where?)Repeatability (will it do this every time?)Reliability (mean time between failures)Power consumption (how to feed it)Energy supply & its weight

Types of Hydraulic actuators: Cylinders & Motors

Cylinder types:

Single acting: work can be done only in one direction

Piston

Double acting piston:

Piston rod on both sides

Plunger

Work is done in both directions

Telescopic

Telescopic

Fast moving

Tandem

Fast moving

Hydraulic cylinders

Cylinder types: plunger

smaller force, better efficiency, axial support is necessary but cylinder does not have to be so good quality

Without inner buffer With inner buffer (pilot piston)

Hydraulic cylinders

Cylinder types: piston, single acting, pressing-cylinder

Spring

inside outside

Hydraulic cylinders

Cylinder types: piston, single acting, puller

Spring

inside outside

Hydraulic cylinders

Cylinder types: piston, double acting

Velocity is different

Hydraulic cylinders

Cylinder types: piston, double actingPiston rod on both side

Velocity is the same in both directions

Hydraulic cylinders

Cylinder types: piston, double actingPiston rod on both side

Velocity is different in the two directions depending on cross section ratio of the rods

Hydraulic cylinders

Cylinder types: double acting piston, tandem

Hydraulic cylinders

Cylinder types: single acting, fast moving

Hydraulic cylinders

Cylinder types: double acting, fast moving

Hydraulic cylinders

Cylinder types: telescopic, single acting

Large working paths or limited space

Hydraulic cylinders

Cylinder types: telescopic, double acting

Large working paths or limited space

Hydraulic MotorsA Hydraulic motor is a mechanical

actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation).

Several types of hydraulic motors:Gear MotorsVane MotorsGerotor MotorsRadial Piston Motors

Gear MotorA gear motor consists of two gears, the

driven gear (attached to the output shaft by way of a key, etc) and the idler gear.

High pressure oil is ported into one side of the gears, where it flows around the periphery of the gears, between the gear tips and the wall housings in which it resides, to the outlet port.

The gears then mesh, not allowing the oil from the outlet side to flow back to the inlet side.

Vane MotorA vane motor consists of a housing with an

eccentric bore, in which runs a rotor with vanes in it that slide in and out. The force differential created by the unbalanced force of the pressurized fluid on the vanes causes the rotor to spin in one direction.

Most vane motors have spring clips behind the vanes to keep them in contact with the cam ring.

Gerotor motorUses an internal gear that rides inside a gearThe rotor has 1 tooth less than the external gearThe rotor is offset to the center of the motor and

travels in a orbital fashion.Because of the way they are designed, they are

a low speed-high torque motorMany gerotor motors use rollers on the lobes of the external gear to lower the friction caused by the rotor lobes hitting the external gear lobesClearances in gerotor motors are between .000” to .001”

Radial Piston MotorNewest type of motorUses pistons which are placed around a

crankshaftProduce higher torque than gerotor motorsRequire special valves on the shaft

In-line piston motorThe most efficient type of high speed motorCan be fixed displacement or variable

displacementRequires a case drain for internal leakageTight tolerances require high filtration

Pneumatic ActuatorsUse Pressurized Air to achieve motionAdd great deal of power and speed to any

actuation system.Variety of Actuation mechanisms available

CylindersMotors

Why go the Pneumatics’ Way?Weight

Cylinders much lighter than motorsSimple

Much easier to mount than motorsMuch simpler and more durable than rack

and pinion for linear motionFast on/off type tasks Big forces with elasticityNo leak problemsNo burnout

Cylinders can be stalled indefinitely without damage

What can deter you on your way?All the components are quite expensiveA properly designed system is more

complex than an equivalent electromechanical system (electric motors, power screws, linear actuators, etc.). 

All these components take up quite a bit of valuable space within a robot.

No weight advantage if only one cylinder used (still need compressor, reservoir, pressure sensors, regulator)

Electric MotorsElectric motors are the most common

source of torque for mobility and/or manipulation in robotics

The physical principle of all electric motors is that when an electric current is passed through a conductor (usually a coil of wire) placed within a magnetic field, a force is exerted on the wire causing it to move

DC Motors: DC motors are very common in industry and have been

used for a long time. In DC motors, the stator is a set of fixed permanent magnets, creating a fixed magnetic field, while the rotor carries a current. Through brushes and commutators, the direction of current is changed continuously, causing the rotor to rotate continuously.

AC Motors: Electric AC motors are similar DC motors except that the

rotor is permanent magnet, the stator houses the windings, and all commutators and brushes are eliminated.

Servo Motors:

A Servomotor is a DC,AC, brushless, or even stepper motor with feedback that can be controlled to move at a desired speed (and consequently, torque), for a desired angle of rotation. To do this, a feedback device sends signals to the controller circuit of the servomotor reporting its angular position and velocity. If as a result of higher loads, the velocity is larger than desired set value, the current is increased until the speed is equal to the desired value. If the speed signal shows that the velocity is larger than the desired, the current is reduced accordingly. If position feedback is used as well, the position signal is used to shut off the motor as the rotor approaches the desired angular position.

Components of a DC Electric Motor

The principle components of an electric motor are:North and south magnetic poles to provide

a strong magnetic field. Being made of bulky ferrous material they traditionally form the outer casing of the motor and collectively form the stator

An armature, which is a cylindrical ferrous core rotating within the stator and carries a large number of windings made from one or more conductors

Components Of An Electric Motor (cont…)

A commutator, which rotates with the armature and consists of copper contacts attached to the end of the windings

Brushes in fixed positions and in contact with the rotating commutator contacts. They carry direct current to the coils, resulting in the required motion

Components Of An Electric Motor (cont…)

(Rotating) Commutato

r

Stator

Brushes

Armature

How Do Electric Motors Work?The classic DC motor has a rotating armature in

the form of an electromagnetA rotary switch called a commutator reverses the

direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor

As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet.

During that instant of switching polarity, inertia keeps the motor going in the proper direction

How Do Electric Motors Work? (cont…)

A simple DC electric motor: when the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation

How Do Electric Motors Work? (cont…)

The armature continues to rotate

How Do Electric Motors Work? (cont…)

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic

field. The process then repeats.

Electric MotorsElectric motors usually have a small rating,

ranging up to a few horsepowerThey are used in small appliances, battery

operated vehicles, for medical purposes and in other medical equipment like x-ray machines

Electric motors are also used in toys, and in automobiles as auxiliary motors for the purposes of seat adjustment, power windows, sunroof, mirror adjustment, blower motors, engine cooling fans and the like

Stepper MotorsWhen incremental rotary motion is required in

a robot, it is possible to use stepper motorsA stepper motor possesses the ability to move

a specified number of revolutions or fraction of a revolution in order to achieve a fixed and consistent angular movement

This is achieved by increasing the numbers of poles on both rotor and stator

Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor)

Stepper MotorsThis figure illustrates the design

of a stepper motor, arranged with four magnetic poles arranged around a central rotor

Note that the teeth on the rotor have a slightly tighter spacing to those on the stator, this ensures that the two sets of teeth are close to each other but not quite aligned throughout

Stepper Motors (cont…)Movement is achieved when

power is applied for short periods to successive magnets

Where pairs of teeth are least offset, the electromagnetic pulse causes alignment and a small rotation is achieved, typically 1-2o

How Does A Stepper Motor Work?

The top electromagnet (1) is charged, attracting the topmost four teeth of a sprocket.

How Does A Stepper Motor Work? (cont…)

The top electromagnet (1) is turned off, and the right electromagnet (2) is charged, pulling the nearest four teeth to the right. This results in a

rotation of 3.6°

How Does A Stepper Motor Work? (cont…)

The bottom electromagnet (3) is charged; another 3.6° rotation occurs.

How Does A Stepper Motor Work? (cont…)

The left electromagnet (4) is enabled, rotating again by 3.6°. When the top electromagnet (1) is again charged, the

teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to

make a full rotation.

Stepper MotorStepper motors have several advantages:

Their control is directly compatible with digital technology

They can be operated open loop by counting steps, with an accuracy of 1 step.

They can be used as holding devices, since they exhibit a high holding torque when the rotor is stationary

Electric Motors: MountingWhen used with rotary joint systems, motors can

produce torque by being mounted directly on the joints or by pulling on cables

The cables can be thought of as tendons that connect the actuator (muscle) to the link being moved

Since cables can apply force only when pulled, it is necessary to use a pair of cables to obtain bidirectional motion around a joint, this implies mechanical complexity

Mounting motors directly on joints allows for bidirectional rotation, but such mounting may increase the physical size and weight of the joint, and this may be undesirable in some applications

Electric Motors: Linear MovementThe fact that electric motors produce

rotational motion raises an issue with regard to their use in robots

For linear translation it is necessary to translate rotational to linear motionFor example, prismatic joints require linear

translation rather than rotation from the motorLeadscrews, belt-and-pulley systems, rack-

and-pinion systems, or gears and chains are typically used to transform rotational to translational motion

Comparison of Actuating Systems:

Hydraulic Electric Pneumatic

+ Good for large robots and heavy payload

+ Good for all size of Robots

+ Many components are usually off-the-shelf

+Highest Power/Weight Ratio

+Better control, good for high precision robots

+Reliable components.

+Stiff system, High accuracy, better response

+Higher Compliance that Hydraulics

+No leaks or sparks

+No reduction gear needed

+Reduction gears used reduce inertia on the motor

+Inexpensive and simple

+Can work in wide range of speeds without difficulty

+does not leak, good for clean room

+Low pressure compared to hydraulics

+Can be left in position without any damage

+Reliable, low maintenance

+ Good for on-off applications and for pick and place

Comparison of Actuating Systems: (contd.)

Hydraulic Electric Pneumatic

- May leak. Not fit for clean room application

+Can be spark-free. Good for explosive environment.

+Complaint systems.

-Requires pump, reservoir, motor, hoses etc.

-Low stiffness -Noisy systems.

-Can be expensive and noisy, requires maintenance.

-Needs reduction gears, increased backlash, cost, weight, etc.

- Require air pressure, filter, etc.

-Viscosity of oil changes with temperature

-Motor needs braking device when not powered. Otherwise, the arm will fail.

-Difficult to control their linear position

-Very susceptible to dirt and other foreign material in oil

- -Deform under load constantly

-Low compliance - -Very low stiffness. Inaccurate response.

-High torque, High pressure, large inertia on the actuator.

- -Lowest power to weight ratio

Types of Stepper Motors There are three main types of stepper motors:

Variable Reluctance stepper motor

Permanent Magnet stepper motor

Hybrid Synchronous stepper motor

Smriti Chopra

This type of motor consists of a soft iron multi-toothed

rotor and a wound stator.

When the stator windings are energizedwith DC Current, the poles become magnetized.

Rotation occurs when the rotor teethare attracted to the energized statorpoles.

Variable Reluctance motor

Smriti Chopra

Permanent Magnet motorThe rotor no longer has teeth as withthe VR motor.

Instead the rotor ismagnetized with alternating northand south poles situated in a straightline parallel to the rotor shaft.

These magnetized rotor poles provide an increased

magnetic flux intensity and because of this

the PM motor exhibits improved torque characteristics

when compared with the VR type.

Smriti Chopra

Hybrid Synchronous motorThe rotor is multi-toothed like the VR motor andcontains an axially magnetized concentricmagnet around its shaft.

The teeth on the rotor provide an evenbetter path which helps guide themagnetic flux to preferred locations inthe air gap.

Smriti Chopra

Applications

Stepper motors can be a good choice whenever controlled movement is required.

They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism.

These include printers plotters medical equipment fax machines automotive and scientific equipment etc.

Smriti Chopra

Electric Motors

Electric motors are everywhere!

In your house, almost every mechanical movement that you see around you is caused by an AC or DC electric motor.

AC Motors Two main types of AC motor, Synchronous

and Induction. Synchronous motors supply power to both

the rotor and the stator, where induction motors only supply power to the stator coils, and rely on induction to generate torque.

72Sean DeHart

73

AC Induction Motors (3 Phase) Use poly-phase (usually 3) AC current to create a

rotating magnetic field on the stator This induces a magnetic field on the rotor, which tries to

follow stator - slipping required to produce torque Workhorses of the industry - high powered applications

Sean DeHart

AC induction MotorsInduction motors only supply current to the

stator, and rely on a second induced current in the rotor coils.

This requires a relative speed between the rotating magnetic field and the rotor. If the rotor somehow matches or exceeds the magnetic field speed, there is condition called slip.

Slip is required to produce torque, if there is no slip, there is no difference between the induced pole and the powered pole, and therefore no torque on the shaft.

74Sean DeHart

Synchronous AC Motors Current is applied to both the Rotor and the

Stator. This allows for precise control (stepper

motors), but requires mechanical brushes or slip rings to supply DC current to the rotor.

There is no slip since the rotor does not rely on induction to produce torque.

75Sean DeHart

Direct Current Motors

overall plan of a simple 2-pole DC electric motor

A simple motor has 6 parts, as shown in the diagram

Direct Current MotorsDirect Current Motors

Direct Current Motors

Brushed DC Motor

The brushed DC motor is one of the earliest electric motor designs

Easy to understand design

Easy to control speed

DC STEPPER MOTORS

Stepping motors are electric motors without commutators

Commutation is handled externally by the motor controller

Controller charges opposite coils attracting the center rotor magnets

DC STEPPER MOTORS

DC STEPPER MOTORS

Voltage Rating provides desired torque

Resistance-per-winding determinesthe current draw of the motorMaximum operating speed

Degrees per StepSets the number of degrees the shaft

will rotate for each full step