Basics of Servo system and simulation

8/13/2019 Basics of Servo system and simulation

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By- Gaurav Khanna13/PIT/082Neel kamal Singh13/PIT/085Rohit Kapoor13/PIT/090

M.Tech ( WCN)


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What is an automatic control system ?

A system in which we control the state ofa process.

We provide the reference to the systemwhich is the required state of the process.

At the output the transducers are used to

generate similar signals to represent theactual values at the output of a process.

Compare the actual values at the outputto the reference or required values andgenerate an error signal.

This error signal is fed to the actuator

and gearing arrangement to make theactual output closer to the referenceinput.

This process ends if the error signal iszero or if the output becomes exactlyequal to the reference input.


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What is an analog closed loop system?

InputTransducer

ComparatorAmplifier Geared

Motor

Outputtransducer

Voltage signals

Error signal


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What is a Digital closed loop system?

InputTransducer

DigitalController

GearedMotor

DigitalOutput

transducer

Digital signals

Computer


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What is a Servo motor?

A servo motor is an electromechanical device in which anelectrical input determines the position of the armature ofthe motor.The position of armature is determined by the duty cycle

of a periodic rectangular pulse train.


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What is a servo motor?

A Servo is a small device that has an output shaft. This shaftcan be positioned to specific angular positions by sendingthe servo coded signal. As long as the coded signal exists onthe input line, the servo will maintain the angular position

of the shaft. As the coded signal changes, the angularposition of the shaft changes

Futaba S 148 Servo
http://www.seattlerobotics.org/guide/images/servo1.jpg


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What is a servo motor?The servo motors are small, have built in control circuitry, and are extremely

powerful for their size.A standard servo such as the Futaba S-148 has 42 oz/inches of torque, whichis pretty strong for its size.It also draws power proportional to the mechanical load. Thus a lightlyloaded servo does not consume much power.

The maincomponents of aservo motor are:1. Electric Motor

2. position feedbackpotentiometer

3. Reduction gear4. Actuator arm

5. Control CircuitryThree sets of wires for input, Control Signal,

Power (+5 V) and Ground.
http://www.seattlerobotics.org/guide/images/servo3c.jpghttp://www.seattlerobotics.org/guide/images/servo3c.jpghttp://www.seattlerobotics.org/guide/images/servo3c.jpg


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How does Servo Motor Work?

Potentiometer is connected to the output shaft of the motor through gears.As the motor rotates the resistance of the pot changes, so that the circuitcan measure exactly in what direction motor shaft is pointing.This pot allows the control circuitry to monitor the current angle of theservo motor. If the shaft is at the correct angle, then the motor shuts off. If

the circuit finds that the angle is not correct, it will turn the motor in thecorrect direction until the angle is correct.The output shaft of the servo is capable of travelling somewhere around 180degrees.A normal servo is used to control an angular motion between 0 and 180

degrees. A normal servo is mechanically not capable of turning any fartherdue to a mechanical stop built on to the main output gear.The amount of power applied to the motor is proportional to the distance itneeds to travel. So, if the shaft needs to turn a large distance, the motor willrun at full speed. If it needs to turn only a small amount, the motor will runat a slower speed. This is called proportional control.


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How does a servo motor work?

The control wire is used tocommunicate the angle. The angle isdetermined by the duration of a pulsethat is applied to the control wire. Thisis called Pulse Coded Modulation.

The servo expects to see a pulse every20 milliseconds (.02 seconds).The length of the pulse willdetermine how far the motor turns.A 1.5 millisecond pulse, for example,will make the motor turn to the 90

degree position (often called theneutral position). If the pulse is shorterthan 1.5 ms, then the motor will turnthe shaft to closer to 0 degress. If thepulse is longer than 1.5ms, the shaftturns closer to 180 degress.


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When these servos are commanded to move they will move to the position andhold that position. If an external force pushes against the servo while the servo isholding a position, the servo will resist from moving out of that position.The maximum amount of force the servo can exert is the torque rating of theservo. Servos will not hold their position forever though; the position pulse must be

repeated to instruct the servo to stay in position.

When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to aposition and holds its output shaft some number of degrees counterclockwisefrom the neutral point towards zero. When the pulse is wider than 1.5 ms theopposite occurs towards 180 degrees.


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Control Circuitry in servo motor

The control circuitry contains a chip M51660L , whichcompares the error in positioning the motor.The chip contains a timer that produces pulse signals fromthe potentiometer.These signals are similar to the ones you supply.These two pulse signals (the ones you are sending and theones generated by the potentiometer) are fed into a pulse

width comparator.

This comparator produces the signals indicating whichdirection the motor should turn in.


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Control Circuitry in servo motor


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Types of servo motors

AC Servo Motors based on Induction MotorDesigns:

AC servos can handle higher current surges and tend tobe used in industrial machinery.

DC Servo Motors based on DC Motor DesignsDC servos are not designed for high current surges.Generally speaking, DC motors are less expensive thantheir AC counterparts.

AC Brushless Servo Motors based on SynchronousMotor Designs.


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What is servo mechanism?

A servomechanism, or servo is an automatic device thatuses error-sensing negative feedback to correct theperformance of a mechanism.The term correctly applies only to systems where thefeedback or error-correction signals help control mechanicalposition or other parameters.A servomechanism may or may not use a servomotor.For example, a household furnace controlled by a thermostatis a servomechanism, yet there is no motor being controlled

directly by the servomechanism.A common type of servo provides position and velocitycontrol.Servos use electric motor as the primary means of creating

mechanical force. Other types of servos use hydraulics,pneumatics or magnetic principles.


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Uses of servo mechanism

Servomechanisms were first used in military, fire controland marine navigation equipment. Also used in automatic machine tools, satellite trackingantennas, remote control air planes, automatic navigation

systems in boats and planes, anti air craft gun controlsystems.Radio controlled models,Auto focus cameras to accurately position the lens.Modern hard disk drives use magnetic servo systems withsub micrometer positioning accuracy,Width and thickness control equipment in industry.Opening and closing valves in furnaces.


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Summary of servo mechanism

Servos operate on the principle of negative feedback wherethe control input is compared to the actual position of themechanical system as measured by some sort of transducer

at the output. Any difference between the actual andwanted values (an "error signal") is amplified and used to

drive the system in the direction necessary to reduce oreliminate the error.


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What is the difference between steppermotor and servo motor control?

A stepper motor has winding insuch a way that the rotation has acertain number of discrete "steps".These steps are where the magnetic

fields cause the motor to want tosettle in one of these positions.The number of steps per revolutionis rather high, around two hundred orso, and varies by model andmanufacturer.Often operated in open loop. Whichcan lead to errors if the motor isunable to comply with the controllercommandCost is lesser

Servo motor is much smoother inmotion as compared to steppermotor.

operated in closed loop set ups, soprovide better control and accuracy.Cost is high


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What is the difference between steppermotor and servo motor control?

Produce more noise andvibrations than servos.Not suitable for high speedused for small torques andspeeds.Slow linear responseSimpler in design.Not suitable for systems with

varying loads.low powers upto 1 hp and speedupto 2000 rpm.

Faster linear responses.Higher torque, low armatureinertia.marginal coolingsmooth and precise controluse mainly for transientresponse improvement.Reduction in steady state

errors.Reduction in sensitivity.Complex in design due tofeedbackhigh torque in comparison to

its size


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Types of Feedback Devices

1. Analog Tachometers2. Digital Tachometers (Optical Encoders)

3. Resolver

Absolute Encoder Incremental Encoder


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Types of feedback Devices

Analog Tachometer:These are miniature motors, wire gauge is thin so that less current carryingcapability is there.Reverse of motor operation.The faster shaft is turned larger is the magnitude of the voltage.Polarity of the voltage is indicates the direction of rotation.They provide directional and rotational information.If tachometer voltage gradient = 2.5 V /1000 rpm, motor shaft rotates at 3600rpm then voltage shown on the tachometer is = 3. 6 krpm x 2.5 V / Krpm = 9VVoltage gradient is sensitivity of tachometer.


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Types of feedback Devices

Digital Tachometer:Usually called optical encoder is also a mechanical to electrical conversiondeviceThe encoders shaft is rotated and an output signal results which isproportional to the distance or angle the shaft is rotated through.

The output signal may be sinusoidal or square wave providing an absoluteposition.Two types 1) absolute encoder

2) Incremental encoder


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Types of feedbacks

Absolute encoder:This type provides a specific address for each shaft position throughout 360degrees..It might have a contact (brush) or non contact type of sensing position.The contact scheme incorporates a brush assembly to make direct electricalcontact with the electrically conducive path of the coded disc to read information.The non contact scheme uses photoelectric detection.The number of tracks on the disc can be increased to desired accuracy.The disc has a built in memory system, so the disc does not return to home orstart position upon re energizing power.


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Types of feedbacks

Absolute encoder:


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Types of feedbacks

Incremental encoder:The Incremental encoder provides either pulses or sinusoidal output signal as itis rotated through 360 degrees. The distance data is obtained by counting thepulses. The disk is manufactured with opaque lines. A light source passes the beamthrough the transparent segments onto a photosensor which outputs a square

pulse train.


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Types of feedbacks

Incremental encoder: The number of opaque lines on the disc measure the positional accuracy of theencoder. Either one or two channel outputs can be provided. The two channel versionprovides a signal relationship to obtain motion direction.A zero index pulse can be provided to determine the home position.


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Types of feedbacks

Resolver:

it is a rotating transformer.simplest resolver contains a single winding on rotor and two windings onstator located 90 degrees apart.

Any reference signal applied to rotor winding causes voltage on thestator winding to change through transformer action, the change in thestator voltage is proportional to the angle gone through by rotor.One electrical cycle of output voltage is equal to each 360 degrees ofmechanical rotation. This is fed to controller and inside it a R to D (

resolver to digital) converter analyzes the signal and produces an outputrepresenting angle of rotor movement and also the speed.


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Types of feedbacks

Resolver:Resolver with three stator windings 120 degrees apart is called as synchro.


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Types of actuators

Air Motors : use compressed air to create motion, Pressureand f low determine the speed and torque.Hydraulic Motors: use pressurize oil to move piston.Stepper Motors: Electromechanical device to convert one

digital pulse into a specific rotational movement ordisplacement.AC Induction Motors: widely used for constant speedrequirements. Servo motors: A motor with a feedback device, controls

speed and position accuracy.


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Simulation of servo system Servo system is used to control load.

Main application of control system is to vary andmaintain the speed of motor.

The servo system must provide

a) Required torque

b) Required speed

c) Required accuracy


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System modelling is used to understand the working of

control system. Consider a direct current position control servo system.

System is made up of an electric motor with an outputshaft.

Let the shaft internal load is W, electric circuit inputvoltage Vwhich is transformed by the motor in torque .

Friction exist between bearing of the motor and load.

an equation can be written as.

2

2+

=

Where, ais constant representing friction.

of servo output shaft.


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The objective is to control the shaft position . It can be achieved by simulation run for different

set of values of gain and time constant.

Gain K and time are related to voltage by

differential eqn.

2

2+

=KV

Different values of gains and time constants are

evaluated by simulation run by using differentcontrol system design tools


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Different values of gains and time constants areevaluated by simulation run by using differentcontrol system design tools.

Rearranging the system transfer model gives the

state space form, for position and velocity of servosystem output shaft.

Software contains all the required mathematicalformulas used to determine the inertia of the load

and critical parameter used in selecting a servosystem.


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Uses of Servo controllers

Use in manufacturing company as production linesneed to be controlled.

Useful in transportation system because of the electric

motors. In communication network using hard disk drives.

In focusing and tracking of the laser beam of the CD.


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SIMULATORS ON SERVO SYSTEM

CE 110


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Self-contained and compact bench-mounting unit

Ideal for classroom demonstrations and student project work

Shows problems of speed and position control of a servomotor

under different loads Mimics industrial, transport and aeronautical problems - with

realistic results

All inputs and outputs buffered for connection toTecQuipmentsoptional controllers or other suitable controllers

Front panel includes a mimic diagram of the process so thatstudents can clearly see what they are controlling

Shows basic control of speed, with advanced studies of non-linear effects of hysteresis, deadzone and saturation


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Essential Base Unit Controller (CE120) A controller with analogue and

digital controls and instruments or

Digital Interface (CE122) An interface which connectsbetween most products in the Control Engineering range

and a suitable computer (not included) or

Other suitable controller with 10 V inputs and outputs

Both the CE120 and the CE122 include TecQuipmentsCE2000 Control Software (see separate datasheet) with

editable, pre-made control experiments for use with theCE110.


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The CE110 Servo Trainer shows d.c. servo position andspeed control systems using typical industrialtechniques.

It has a d.c. servomotor, a d.c. generator and aflywheel mounted on a common shaft.

Analogue 0 to 10 V control signals vary the

servomotor shaft speed in either direction. An opticalsensor measures the speed and shows it on a panel-mounted digital meter.


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Operating Conditions

Operating environment: Laboratory environment

Storage temperature range: 25C to +55C (packed) Operating temperature range: +5C to +40C

Operating relative humidity range: 80% attemperatures < 31C decreasing linearly to 50% at 40C


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CE 120 CONTROLLER


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Digital display of voltage measured (+/- 15 V) orfrequency of function generator. Function generatorgives square, sinusoidal or sawtooth signals at variable

frequency. Signals can be offset and amplitudeadjusted.

Frequency range 0.01 to 100 Hz

Display and function generator:


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FUNCTIONS

Temperature (thermal) control Level control

Engine speed control

Servo control


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OTHER SIMULATORS Thermal Process Control (CE103)

Coupled Tanks (CE105 and CE105MV)

Ball and Beam (CE106) Engine Speed Control (CE107)

Coupled Drives (CE108)

Ball and Hoop (CE109)

Servo Trainer (CE110)


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Basics of Servo system and simulation

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