Modular Servo system MS150

ASSIGNMENT 7

Object Use of the tachogenerator in a speed control system.

Equipment required Ouantity

1

Apparatus

Operational Unit 150A > Attenuator Unit 1508 Pre-Amp Unit 150C Servo Amplifier 1500 Power Supply ,l 50E Motor Unit 150F

Voltmeter (30-0

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PRELIMINARY PROCEOURE

In Practica/ 7. 1:

1. Attach the Operational Unit 150A, Atten-uator Unit 1508, Servo Amplifier 1500, Power Supply 150E and Motor Unit to the Baseplate by means of the magnetic fixings.

2. Connect the plug of the Servo Amplifier to the Power Supply. '

3. Connect 'the plug of the Motor Unit to the Servo Amplifier.

4. Connect the Power Supply to the mains supply line. DO NOT switch on yet.

DISCUSSION ANO EXPERIMENTAL PROCEDURE Assignments 5 and 6 showed how simple open-loop position control systems could be con-structed using the kit. In this assignment, we are going to deal in the same way with speed control.

Referring back to Assignment 3 on motor characteristics, you will see that you drew a curve of the signa! inputs into the Servo Amplifier against speed. This means that with-out any load you can set the motor to run at a specified speed by determining the value of the input signa!.

07. 1 Can you say what sort of speed control system this was in Assignment 3?

07.2 Looking now at the torque/speed graphs in Assignment 3, say what will happen if a load is placed on the motor shaft and is then varied.

With an open-loop system the results showed that there can be reasonable speed control when operating without or with a fixed load but the system would be very unsuitable where the

lm~d was varying. In this assignment we are going to show the improvement that can result from closing the loop and using feedback. That is, the actual speed wlll : be compared with the required speed. This produces an error signa! to actuate the Servo Amplifier output so that the motor maintains a more constant speed .

As a first experiment we will simply feed back a signa! proportional to the speed , using the Tachogenerator. We then compare it with a reference signa! of opposite polarity, so that the sum will produce an input signa! into the

Modular Servo system MS150

Servo Amplifier of the required value. As comparator, we will use an Operotional Amplif-1er. Set upas in fig 7 .1 and switch on power.

Practica! 7.1 Simple closed loop speed control system

Connect the Servo Amplifier far armature con-trol. The field coil driving transistors on the Servo Amplifier require positive inputs, so that a negative reference plus a positive tacho voltage when summed and inverted in the Operational Amplifier will produce a positive output, the error voltage V e.

01.3 Explain why the Tacho voltage has to be less than the Reference voltage.

Connect the potentiometer slider to an input of the Operational Amplifier. Befare connecting the Tacho Generator to the input of the Oper?-tional Amplifier, turn up the slider on the potenometer so that the motor revolves and on your voltmeter determine which is the Tacho's positive output. The correct side is then connected to the Operational Amplifier input and the other side to COM. Reset the reference voltage to zero and then gradually increase the reference voltage so that you can take readings over the motor speed range of up to 5000rev/min for the Reference, Tacho and error voltages.

Record as in fig 7. 2 usi ng the tacho cal ibration of Assignment 3 to calculate the speed. El. 1 Plot the Error voltage against Speed, far the data_ recorded in fig 7.2. Compare the Error/Speed resu/ts with those you obtained in Assignment 3 Motor characteristics, for match.

07.4 What would happen f you reversed the Tacho-generator connections to th8 0ptJrational Amplifier and what sort of feedback would you cal/ it? To find o~t the effect of load on speed we can use the magnetic brake as a load. The change in speed for a change in load will give us the regulaton. Disconnect Motor Unit plug and place one of the hubs in the Load Unit l50L onto the motor shaft to within 1/8" of the bearing and lock

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Feedback lnstruments L td

1~'01'1 1 1 1 '

11 1 ' 1 1 3 w-

Fig 7.1

reference tachogenerator error speed voltat)8 . voltage voltage rev/min

Fig 7.2 onto it the spacer using the spanner provided. Make sure the shaft is inside the split end of the hub before tightening. Screw the other hub through the brake disc and into the other end of the spacer. Refix Motor Unit and reconnect plug in Servo Amplifier. The second part of the assignment is concerned to show how an increase in the for:ward path gain wiH cause a given fall in speed to cause a larger increase in the value of the error V 0 , so that for any change of load the speed drop or 'droop' will decrease with increasing gain as in fig 7.3.

52

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SEUO AMPllflU UH SA 1SDI

COH N810t - 1ACMG iWll HllSIF

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Far a gain control we can use the circuit of fig 7.4, which was shown in Assignment 2 to have a

1 gain of--.

a

So set upas in fig 7.5. (see overleaf)

In Practica/ 7.2: Apparatus as in Practica! 1.

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brake torque 1 o

Fig 1.4

Practica! 7.2 To show effect of gain on speed change for load change.

On the Magnetic Brake swing the magnets clear. Move the feedback selector switch on the Operatonal Amplfier to the externa! feedback position. lnitially set the gain to unity, that is to positon 1 O and adjust the reference volts ti 11 the motor runs at 1 OOOrev/min. Then take read:ngs of the Reference voltage, Servo 1 nput error voltage Ve and the Tacho voltage, using the voltmeter, over the range of brake positions O 10 and then tabulate as in fg 7.6. Be careful that you do not exceed the 2A limiting current. Repeat the readings for a gain of 10, that is set the gain potentiometer to position 1. Re-adjust the reference potentiometer to give a no-load motor speed of 5000rev/min. Repeat the experment for a maximum speed of 500rev/min.

El.2 Plot your results in the form of graphs of Error voltage against Brake Setting and Speed against Brake Settng far gain values of 1 and 10.

Fig 1.3

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Feedback lnstruments Ltd

ATIEIUIOR

COM MDIU-UCHO UllT MIHIF

Fig 7.5

brake reference error position volts volts

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Fig 7.6

AC1

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SElfO AM l fl[I um SA 1101

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tachogenerator speed volts rev/min

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brake reference position gain volts

Fig 7.7

In the last part of the assignment the kit will be assembled to form a simple reversible speed control system. From your readings you will have seen that a high gain decreases the mnimum reference signa! needed for the motor to respond so on this third experimen~ we will use high gain.

llHMUATOR UMll lU1508

PRESH o

rtEAMP UfllT PA 150 e

tOH MOTOR - llCMO UMIT Hl150F

Modular Servo system MS150

The inputs into the Servo Amplifier will drive the motor in opposite directions but both inputs require positive voltages. As the output of the Operational Amplifier varies from positive to negative it is necessary to use the Pre Amplifier that is so designed that a negative input give~ a positive voltage on one output and a negative input gives a positive voltage on the other output, see Assignment 6. The gain is about 25, as you will have found.

In Practica! 7.3:

Apparatus to be as in Practica! 1 but replace the Operational Unit with the Pre-Amp Unit 150C. Set up as in fig 7.8 adjusting the reference to zero output befare coupling to the Pre-Amplifier.

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Feedback lnstruments Ltd

Practica! 7 .3 Demonstration of a simple rever-sible speed control system

Set the si ider on the reference potentiometer to position 5.

With no load on the motor, now find that you can invert the sign of the reference signal so that you can reverse the direction of mtor rota~ion, by slowly' turning the reference sllder to e1ther side of the centre position. Record the reference voltage that just causes the motor to rotate, as in fig 7.9.

Set the speed of rotation in one direction to 1000rev/min and then take readings over the brake positions 0-10, as in fig 7 .1 O. Far measuring the error voltage place the voltmeter across both the Pre-Amplifier outputs.

Then reverse direction and repeat the readings.

PRACTICAL CONSIDERATIONS & APPL ICA TIONS So important has the tachogenerator been o sidered in speed control, that it has very of been made an integral part of the motor.

Examples of speed control can be seen in ev' branch of industry and transport. They h; become particularly important in continU< processes such as in the control of sheet-mE thickness in hot rolling mills, in generators a most industrial motors. In guidance systems, automatic pilots, lifts overhead hoists both reverse speed and positio1 control may be used.

minimum signal needed far motor response

forward reverse

Fig 7.9

forward reverse brake tacho- reference error speed tacho- reference error speed position generator volts volts rev/min generator volts volts rev/mi

volts volts

Fig 7.10

56 '

Feedback lnstruments ltd

Assignment 7

Answers to Ouestions etc

07.1 The speed control in Assignment 3 was an openlpop system

07.2 A variable load upan the output shaft of aff open-loop system will cause large speed fluctuations

07 .3 1 f the tachogenerator voltage was greater than the reference there would be no drive to the motor. Then the motor would decelerate until the tacho voltage was less than the reference.

07.4 In this case (reversa! of tacho connec-tions) the motor would continue to increase in speed. This is positive feed-back.

07 .5 1 f you continue to i ncrease the motor load at sorne point the motor drive amplifier will limit and the speed will then rapidly decrease. This indicates that automatic controls must be operated within specified limits.

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