Calibration and Control of Servo Trainer

8/19/2019 Calibration and Control of Servo Trainer

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American University of Sharjah

Department of Electrical Engineering

Department of Mechanical Engineering

ELE 353LMCE 415L

Calibration and Control of Servo Trainer

Objectives

To calibrate the circuits of the Servo Trainer Apparatus, namely the input actuator (the motor

circuit) and also the output sensors (the speed and angular position sensors).

To learn how to control the servo trainer using P and PI methods by selecting appropriate gain

factors.

Introduction

The CE110 Servo Trainer shown in Figure 1 relates specifically to velocity control and angular

position control problems as they would typically occur in industry. It may also, however, be

used as a practical introduction to the design, operation and application of control systems in general.

Figure 1: CE 110 Servo Trainer system.

The CE110 Servo Trainer comprises a motor driven rotating shaft upon which is mounted, (from left to

right):

1. An inertial load flywheel

2. A tachometer to measure the shaft speed

3. A generator that provides an electrically variable load upon the motor.

4. An electrically driven motor that provides the motive power which rotates the shaft.

5. An electrically operated clutch to enable the motor driven shaft to be connected to a secondary

shaft called here the position output shaft, which connects to:-I. A 30:1 ratio reduction gearbox.


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II. An output shaft position sensor and calibrated visual indicator.

III. Adjacent to the visual indicator of output shaft position is a manually operated position

dial which can be used for setting desired (set-point) angular positions.

The CE110 includes power amplifiers for the drive motor and load generator and power supplies/signal

conditioning circuits for the associated speed and velocity sensors.

The motor speed is determined by the voltage applied to the drive amplifier input socket on the front

panel. Likewise, the generator load is determined by the external load input. Both inputs are arranged

to operate in the range 10V (0 to 10V in the case of the generator).

The shaft velocity sensor and the output shaft position sensor are sealed to give outputs calibrated

in the range 10V. A door at the rear of the left hand side allows access to change the size of the

inertial load by adding or removing the inertia discs supplied. For safety, a micro-switch mounted in

the door disables the drive amplifier when the access door is open or not fully latched.

In addition to the main rotating components, a further facility for investigating servomechanism control

is provided in the form of a set of typical servo-system non-linear elements. These are situated at

the top of the unit and, as shown in Figure 2, from left to right comprise: -1. An anti-dead-zone block, to eliminate any dead-zone deliberately introduced or inherent in the

CE110 motor.

2. A dead-zone block, to introduce additional dead-zone so it may be simulated and studied.

3. A saturation block, to allow servo-drive amplifier saturation to be simulated and studied.

4. A hysteresis block, to allow gearbox and servo-drive train backlash to be simulated and studied.

Control Principles

Consider a simple system where a motor is used to rotate a load, via a rigid shaft, at a constant speed,

as shown in Figure 3.

Figure 2: Simple Motor & Load system.

The load will conventionally consist of two elements,

1. A flywheel or inertial load, which will assist in removing rapid fluctuations in shaft speed.

2. An electrical generator from which electrical power is removed by a load.

Under equilibrium conditions with a constant shaft speed, we have:


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When this condition is achieved the system is said to be in equilibrium since the shaft speed will be

maintained for as long as both the motor input energy and the generator and frictional losses remain

unchanged. If the motor input and/or the load were to be changed, whether deliberately or otherwise,

the shaft speed would self-adjust to achieve a new equilibrium. That is, the speed would increase if the

input power exceeded the losses or reduce in speed if the losses exceeded the input power.

Open Loop Control and Operator Dependency

When operated in this way the system is an example of an open-loop control system, because no

information concerning shaft speed is fed back to the motor drive circuit to compensate for changes in

shaft speed.

The same configuration exists in many industrial applications or as part of a much larger and

sophisticated plant. As such the load and losses may be varied by external effects and considerations

which are not directly controlled by the motor/load arrangement. In such a system an operator may be

tasked to observe any changes in the shaft speed and make manual adjustments to the motor drive

when the shaft speed is changed. In this example the operator provides:

I. The measurement of speed by observing the actual speed against a calibrated scale.

II. The computation of what remedial action is required by using their knowledge to increase or

decrease the motor input a certain amount.

III. The manual effort to accomplish the load adjustment, required to achieve the desired changes

in the system performance, or by adjusting the supply to the motor.

Again, reliance is made on the operators experience and concentration to achieve the necessary

adjustment with minimum delay and disturbance to the system.

This manual action will be time consuming and expensive, since an operator is required whenever the

system is operating. Throughout a plant, even of small size, many such operators would be required

giving rise to poor efficiency and high running costs. This may cause the process to be an uneconomic

proposition, if it can be made to work at all!

There are additional practical considerations associated with this type of manual control of a system in

that an operator cannot maintain concentration for long periods of time and also that they may not be

able to respond quickly enough to maintain the required system parameters.

Closed Loop Control

Figure 4 shows a typical arrangement for a closed-loop control system that includes a feedback loop.The tachometer gives feedback about the current speed of the motor shaft, electronic circuits would

then generate an Error Signal which is equal to the difference between the Measured Signal and the

Reference Signal.

The Reference Signal is chosen to achieve the shaft speed required. It is also termed the Set Point (or

Set Speed in the case of a servo speed control system).

The Error Signal is then used, with suitable power amplification, to drive the motor and so automatically

adjust the actual performance of the system. The use of a signal measured at the output of a system to

control the input condition is termed Feedback.

In this way the information contained in the electrical signal concerning the shaft speed, whether it be

constant or varying, is used to control the motor input to maintain the speed as constant as possible


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under varying load conditions. This is then termed a Closed-Loop Control System because the output

state is used to control the input condition.

Figure 3: Feedback Closed Loop Control.

Proportional plus Integral “PI” Controller

In order to maintain a non-zero input to the motor drive, there must always be a non-zero error signal

at the input to the proportional amplifier. Hence, on its own Proportional Control cannot maintain the

shaft speed at the desired level with zero error, other than by manual adjustment of the Reference.

Moreover, proportional gain alone would not be able to compensate fully for any changes made to the

operating conditions.

Operating with zero Error may, however, be achieved by using a controller which is capable of

Proportional and Integral Control “PI”. Figure 5 shows a typical schematic diagram of a PI Controller.

Figure 4: PI controller

Achieving Equilibrium with Zero Steady State ErrorThe Proportional Amplifier on its own will leave an Error at the instance of the change in speed.

However, with the Integrator output signal increasing, ramping upwards in response to this error, the

supply to the drive motor and the motor torque will correspondingly increase. The shaft speed will rise

until the Set speed is achieved and the Error is zero. At this condition the motor and loads are equal and

the system is in equilibrium.

This new operating condition will be maintained until another disturbance causes the speed to change

once again, whether upwards or downwards, and the controller automatically adjusts its output to

compensate. In practice the PI Controller constantly monitors the system performance and makes the

necessary adjustments to keep it within specified operating limits.


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Figure 5: Overall Response of the PI Controller to a step change in Set Speed.

Effect of Increasing Integral Gain on PI Response

The amount of Integral Action will affect the response capability of the system to compensate for a

change. Figure 7 shows the typical response of a system with constant Proportional and varying levels ofIntegral Action.

Figure 6: Typical system response with constant proportional

but varying integral gains.In general,

(a) Any increase in the amount of integral action would cause the system to accelerate more quickly

in the direction required to reduce the Error and have a tendency to increase instability.

(b) Decreasing the integral action would cause the system to respond more slowly to disturbances

and so take longer to achieve equilibrium.

Proportional + Integral + Derivative “PID” Controller

Where fast response is required with minimum overshoot a Three-Term Controller is used. This consists

of the previous PI Controller with a Differential Amplifier included to give a PID (or Three-Term)

Controller.

The performance of a Differential Amplifier is that the output is the differential of the input. Figure 8

shows the characteristic of a Differentiator supplied with a square wave input.


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Figure 7: Differentiator supplied with Square wave Input

Each time the input level is reversed the output responds by generating a large peak which then decays

to zero until the next change occurs. In a practical Differentiator the maximum peak value would be

achieved at the power supply rail voltage levels to the Differentiator itself.In a PID Controller the polarity of the output would be configured to actually oppose any change and

thereby dampen the response of the system. The gain of the Differentiator would control the amount of

damping provided, both in amplitude and duration.

Differentiator Improves System Transient Response

The damping required for the situation described in Figure 10 could also, therefore, be achieved by

including a Differentiator in the control loop to suppress the high acceleration caused by the Integrator

without affecting it's ability to remove the Error. It is the balance between the Integral and Differential

Action, which now controls the overall system response to a step change in Set Level.

The speed and manner with which a system can overcome disturbances is termed the Transient

Response. By careful selection of the parameters of the proportional, integral and differential amplifiers

it is possible to produce a system Transient Response to suit the specific application.

Equipment

CE110 : Servo Trainer Apparatus

CE122 : I/O Digital Interface (Serially connected to a PC)

CE2000L : Digital Controller (Lite Software) Installed on a PC

Procedure

PART A: Motor Calibration Characteristics

1. Initial CE110 settings:

Clutch disengaged (i.e. position shaft not connected).

Rear access panel firmly closed.

Smallest inertial load installed (1 disc installed).

2. Make the following connections between CE122 and CE108 while all equipment remain off:

Table 1

CE122 CE110 CE122 CE110

A/D Channel 1

Output from

Tachometer ω D/A Channel 1

±10V input to Drive

Motor A/D Channel 2 Output Shaft D/A Channel 2 Not connected


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PositionIndicator θ

A/D Channel 3 ±10V Reference SetPotentiometer

GND GND

3. On the desktop of your PC, start CE2000 Lite

4. Open CE110 file as saved in the home directory of this software.

5. Go to Options>Circuit options>General and tick on “allow editing”

6. Make the necessary connection as in the figure below:


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7. After completing the necessary connections, run your circuit.

8. Slowly increase the fine potentiometer voltage until the motor just starts to turn. This is the size of

the positive dead-zone for the motor drive amplifier; enter it into the first row of the Table 2

provided.Increase the potentiometer to 1V; record the corresponding motor speed from the speed display on

the CE110 front panel.

9. Increase the coarse potentiometer voltage in 1V steps to 10V and record the corresponding speed in

Table 2.

Table 2

Motor Drive Voltages (Positive) Motor Speed (rpm)

Dead zone=---- (volt to barely start rotation)

1

2

3

4

5

6

7

8

9

10

10. Repeat the above procedure with the clutch engaged, and complete Table 3. Avoid running the

Servo Trainer at high speed for prolonged periods with the clutch engaged, as this may cause

excessive wear of the gearbox.

Table 3



1

2

3

4

5

6

7

8

9

10

PART B: Speed Sensor Settings

11. CE110 settings:

Clutch disengaged.


Smallest inertial load installed.


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12. Readjust the previous connection as in the figure below.

13. Set the target potentiometer to the speed sensor output (initially 1V) that you require then adjusts

the coarse and fine potentiometers until the error bar graph is at a minimum. Enter the

corresponding speed reading in Table 4. Repeat the process in steps of 1V for positive speed sensor

readings.

Table 4

Motor Speed Positive (rpm) Speed Sensor Output (V)

1

2

3

4

5

6

7

89

PART C: Angular Position Transducer Calibration

14. CE110 settings:

Clutch engaged.


Smallest inertial load installed.

15. Connect the circuit as shown in the figure below

16. Open and close the switches connected to the 'fast' and 'slow' potentiometers to turn the output

shaft to the specified angles and enter the corresponding position sensor output in Table 5.

Table 5

Indicated Angle (º) Position Sensor Output (V)

-150


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-120

-90

-60

-30

0

30

60

90

120

150

PART D: Effect of Integral Action on Steady State Errors

Note: you have to capture the system responses whenever needed by using the chart

recorder and to save them to a word document file in your folder.

17. CE110 settings:

Clutch disengaged.


Largest inertial load installed.

18. To record the graph presses the “record” button.

19. To save the captured graph, open a word document file and copy the figure there, to copy the figure

press the “camera” button on the top toolbar then paste the figure in the document.

20. Make sure to clear the record memory by going to Options>Circuit options>Recording and tick on

“clear all series” .

21. Make the necessary connections as in figure below

22. Slowly increase the potentiometer output voltage to 4V, and observe the steady state error. (for

KP=1 this should be approximately 2V). Observe the error signal as integral action takes effect, as

follows: - with Ki =0.1, press the integrator reset button and switch the integrator into the

controller. (Note: it is most important to press the reset button each time an integrator is switched

into a circuit. Failure to do so can cause unpredictable results). Observe how the speed slowly

increases and the error signal slowly decreases to zero as the integrator output increases so as to

cancel the error. Switch the integrator out of the circuit.


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23. Repeat the above procedure for Ki =0.5, 1, 2, 4, 6, and 10. Note that as Ki is increased the error is

reduced to zero more rapidly until a point is reached when the error overshoots zero, and oscillates

before settling to zero. The oscillations became more pronounced as the Ki is increased.

24. Save all graphs to your word document file.

PART E: Selection of Integral and Proportional Controller Gains

25. CE110 settings:

Clutch disengaged.


Largest inertial load installed.

26. Readjust the previous connection as in figure below

27. Double click the function generator and make the following settings

28. The square wave generator signal provides a series of step changes in the reference signal, which

can be used to investigate the step response of the servo-speed control system. With KI=3,

investigate the effect of proportional gain upon the control system step response by recording and

printing the response for values of Kp =1, 0.1, and 0.01. Comment on the shape of the results in

terms of speed of response and amount of overshoot.

29. Investigate the effect of integral gain upon the control system step response by setting Kp =1, and

plotting the step response for values of Ki =0.5, 1, 5, and 10. Comment on the shape of the

resulting step responses in terms of speed of response and amount of overshoot.


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30. Save all graphs to your word document file.

Lab Report

The report should include the following information:

All results and graphs.

Analysis to sensor calibration graphs in terms of sensitivity, linearity, resolution, etc.

Analysis of transient response for P and PI controllers.

Discuss why the motor drive characteristic differs with the clutch engaged and disengaged.

Discuss the effect of integral gain on steady state error.

Discuss also the effect of changing proportional and integral gains on overall system response.

Your conclusions and observations.


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DATA SHEET

Note: Make sure your instructor signs your data sheet and later enclose it with

your laboratory report. (Reports with no attached data sheet will not beaccepted).

Table 6



1

2

3

4

5

6

7

8

9

10

Table 7



1

2

3

4

5

6

7

8

9

10

Table 8

Motor Speed Positive (rpm) Speed Sensor Output (V)

1

2

3

4

5

6

7

8

9


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Table 9

Indicated Angle (º) Position Sensor Output (V)

-150

-120

-90

-60

-30

0

30

60

90

120

150

Group Members:

1-

2-

3-

Signature:Instructor’s

Documents

Calibration and Control of Servo Trainer