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PRELIMINARY PROCEDURE

/ n Practica/ 6. 1:

1. Attach the Attenuator Unit 150A, Pr~-Amp Unit 150C and Power Supply 150E to the Base­plate by means of the magnetic fixings.

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

DISCUSSION ANO EXPERlMENTAL PROCEDURE

In the previous assignment we found that our input signal could vary from positive to negative but that we could not use its polarity as a means of determining the direction of motor rotation; the magnitude of the signa! controlled the speed of rotation. On examining the field windings we found that one transistor would energise one winding for drive in one direction and the other would cause reverse rotation. Now the Pre­Amplifier is able to provide this type of control because if there is a positive voltage on either of its inputs, then one of its outputs becomes positive; whilst if one of its inputs becomes negative, then the other output becomes positive.

Befare we use the Pre-Amplifier to control the motor so that it can rotate in both forward and reverse directions, we will carry out a practica! to find its characteristics.

Set up as in fig 6.1 and switch on power.

Fig 6.1

Practica! 6.1

Modular Servo system MS150

To find characteristics of the Pre-Ampl ifier

On Potentiometer 1, with the voltmeter set the output between the slider and COM to +1 V.

To be able to obtain accurate readings we need a fi r ·~ control and so we are going to use Poten­tiometer 2 to give us fractions of the one volt that we have selected.

Connect Terminal 6 of Potentiometer 2 to the slider of Potentiometer 1. This means that the positions 1 to 1 O gives us input values in tenths of a volt.

We are now in a position to take output values of the Pre-Amplifier for different values of inputs to see how the amp lifier behaves.

For each slider position on Potentiometer 2 take readings with the voltrneter of each of the Pre­Amplifier outputs and COM as in fig 6.2. After you have done this repeat for the different inputs with the voltmeter across both outputs.

Now connect Terminal 3 of Potentiometer 1 to the -15V supply and repeat your readings.

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o+1SV

oCOM

0 - 15V

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Feedback lnstruments Ltd

1 input pre-amplifier output ~ignal

. volts Vo (3) Vo (L.) Vo (L.-3)

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1

Fig 6.2

E6. 1 Plot graphs of the input volts against the output volts as in fig 6.3 and fig 6.4. The curves will depend on how you have corinected the voltmeter across the amplifier outputs.

The ratio of the output voltage V 0 to the input voltage V¡ gives the gain K.

E6.2 Using the straight part of the curves find the gain of the Pre-Amplifiér.

06. 1 State why you should measure the gain on the straight part ofthe curve.

06.2 Explain the reasons for the non-linear portions of the curves.

06.3 State the range of signals that you think the input should be kept to.

06.4 What input value will give a ni/ voltage across the outputs?

1 f the answer to this is not zero it is because there is a zero offset in PA 150C. This can be eliminated using the set zero control.

We are now in a position to use the outputs of the Pre-Amplifier to control the motor rotation , and provide- to it an error signa! from the Operational Amplifier~ This can be done by combining the circuits of Practical 1 of Assign­ment 5 with that of this assignment.

Set upas in fig 6.5 using the circuits of Practicals t aod 2.

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15

pre-amplifier output V0 (3) pre-amplifrer output V0 (4)

Fig6.3

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Modular Servo system MS150

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voltage across pre -~lifier outputs V0 (4-3)

Fig 6.4

In Practica/ 6.2:

1. Retain the units of item 1 in the first practica! and also attach the Operational Unit 150A, Motor Unit 150F and 1 nput and Output Potentiometers 150H and 150K to the Baseplate by means of the magnetic fixings.

2. Connect the Servo Amplifier connector plug into the Power Supply.

3. Connect the Motor Unit connector plug into the Servo Amplifier.

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Practica! 6.2 To demonstrate a simple motor driven closed-loop position con­trol system

This time we shall utilise the error signal output V 0 of the Operational Amplifier ·to drive the output potentiometer via the Pre-Amp and motor.

The potentiometer on the Attenuator can now be used as a gain control and should initially be set to zero (i .e sl ider to common) befare switching on the power. The slider should be connected to the input of the Pre-Amplifier. With the gain (Attenuator) set to zero adjust the PrecAmpl ifier zero so that the motor does not rotate.

Now set the input potentiometer 150W to sorne arbitrary angle and i ncrease the Attenuator setting. The output shaft should rotate to an angle -nearly equal t o that of the 1 nput shaft.

1 f the output cursor stops befare arriving at the set posit ion, one is faced with the fact that the system is tolerant to an error and the motor will not respond till the error exceeds a certain value. In the next assignment we shall study th is effect, which is known as 'deadband'. For the purpose of this practica!, increase the gain so that this tolerance is overcome and you get the correct alignment.

E6.3 Note the different r1>.<:.11/t.<:. nht::iínPrf in

Feedback lnstruments ltd

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output cursor position 1n degrees

required actual misalignme

Fig 6.6

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PRACTICAL CONSIDERATIONS & APPLICATIONS

1 n a position control system it is of great importance that there should be a smooth stop at any required position. 1 f there is an overshoot then there must be a reverse movement to co"mpensate. 1 f movement is rapid where the system gain is high, it becomes very difficult to get smooth, accurate arrest. 1 ndeed, the correct­ing movement might also produce an overshoot that will again need corr_ection and in an extreme case a system could be moving to and fro across a required position in a state of instability.

Future assignments will deal with the problems of high gain causing fast movement resulting in overshoot and even instability and how they can be dealt with _

So far we have dealt with motors in the unloaded state. The Speed/Torque characteristics of a motor may not produce a su itable response under load. To overcome this problem, gearboxes are very often used. However, they then introduce the further problem of free play in the gears or backlash.

Modular Servo system MS150

Feedback lnstruments Ltd

Assignment 6

Answers to Ouestions etc.

06.1 The gain must be measured on the straight pai1 of the curve because this is the part that will be used in practice.

06.2 The non-linear parts of the curve are due to the pre-amplifier circuit limiting at voltages nearing the ±15V supply line.

06.3 The input must be kept within the range ±0.3V.

06.4 The input will give zero output unless, as discussed in the text, the amplifier has not been zeroed correctly.

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