sdcexp-1(position and speed control)
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NGEE ANN POLYTECHNIC
Diploma in Aerospace Technology
System Dynamics & Control
EXPERIMENT (SDCEXP-1): POSITION & SPEED CONTROL OF DC MOTOR
SYSTEMS
OBJECTIVE:To enable the student to understand the following concepts:
(a) Open loop and closed-loop control of position and speed
(b) Velocity feedback in position control
(c) Effects of increasing system gain
(d) Effects of load changes on a speed control system
EQUIPMENT: Servo Demonstrator SD 155
MS-I 50 Feedback Control System
Hand-held tachometer (optional)
220 KOhm resistor block
INTRODUCTION:
In Position Control, the objective is to maintain (control) the physical position (linear or angular)of the mechanism. For example, an electric motor could be used to change the position of a
rudder of an airplane, to operate a lift, to alter the launch angle of a missile or to position a
robotic arm. In Speed Control systems, the controlled variable is speed (again, either linear or
angular). They are used in continuous processes such as sheet material rewinder systems,
synchronisation of shaft rotation in printing and hot rolling mills, and in applications such asmissile guidance systems, automatic piloting systems, lifts and overhead hoist control.
Control can be achieved either in open or closed loop. In an Open Loop control system, no
mechanism for implementing feedback exists. Hence if the actual position deviates from thedesired position, no automatic corrective action will take place. In a Closed Loop control system,
the system is error actuated. This means that the systems actuator output depends on the
difference between the actual controlled value (feedback) to the desired value (setpoint). Withclosed loop control, the system can automatically compensate for changes in the load. At higher
loads, more energy will be supplied to maintain the controlled variable at the setpoint.
Due to system inertia, a position control system with only position feedback, may exhibit an
undesirable oscillatory output. This is because the output position tends to overshoot its desiredposition. Oscillations can be dampened out if the system is able to reverse its torque before the
position error reaches zero. This can be achieved by adding to the position error signal another
signal, which would counter-act the error signal when required, but diminish when the error hasbeen reduced to zero. The velocity signal is normally used as the counteracting signal. Velocity
feedback, when applied to a closed-loop position control system will have a dampening effect on
oscillations in the position variable.
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PART 1- POSITION CONTROL PROCEDURE / OBSERVATION:
PROCEDURE/OBSERVATION :
A - OPEN LOOP POSITION CONTROL
1. Connect the wires from the servo panel to the motor (paper roller) according to theirrespective color codes.
2. Disconnect the yellow lead (Y) from the position feedback potentiometer.
3. Plug in Re, 10 K only. Do not plug in Rvel.
4. Lift up the pen-holder, switch on the power supply to the motor and the servo panel. Do notactivate the paper roller.
5. Try to adjust the INPUT potentiometer until the OUTPUT (motor shaft/aileron position ) is in
the middle position. Is this possible ? Why?
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B - CLOSED LOOP POSITION CONTROL
1. Without switching off the power supply, connect the Yellow lead (Y) from the Position
Feedback potentiometer.
[Notice that the motor shaft/aileron is now oscillating. Continuous oscillations can wear
out the feedback potentiometer. Hence, do not leave the system running for more than 10
seconds continuously! Switch off the power after each step is completed]
2. Set the input potentiometer to position 0. Vary the INPUT signal (desired value) by changing
the position potentiometer (from 0 to 4 and 0 to -4). Notice that the motor shaft/aileron
follows the changes in the input this time.
3. Lower the pen-holder and switch on the paper roller. Apply a STEP INPUT to the system by
quickly turning the potentiometer from 0 to 4. Plot the response curve on the paper roll.
4. Switch off the power supply, stop the paper roller and lift up the pen-holder.
5. Tear out the response curve, stick the response curve at the space provided.
Please cut and paste the response curve here;
C - CLOSED LOOP POSITION CONTROL WITH VELOCITY FEEDBACK
In this part of the experiment, anadditional feedbacksignal calledVelocity Feedbackis added.It has the effect of increasing the damping in the system.
1. Plug in 5.6 K resistor as the Rvel to provide for velocity feedback.
2. Turn the INPUT potentiometer to 0 position and switch on the power supply.
3. Lower the pen-holder and switch on the paper roller. Input a STEP INPUT to the system by
quickly turning the INPUT potentiometer to 4 position .Plot the response curve on the
paper roll. Stop the paper roller and label the resistor values on the response recorded.Please cut and paste the response curve here;
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4. Increase the velocity feedback by changing the value of the Rvel resistor to l KOhms. Repeatstep (3) of this section. Note the change in the speed of response. Is the speed of response
faster or slower?
Please paste the response curve here;
5. Decrease the velocity feedback by changing the value of the Rvel resistor to lOO KOhms andrepeat step (3). Note the change in speed of response as compared to the plots taken in the
previous two steps. Is the response faster or slower?
Please paste the response curve here;
D - CLOSED LOOP POSITION CONTROL WITH INCREASED GAIN
In this experiment set-up, the error signal is amplified before reaching the motor. If the gain of
the amplifier is high, the system will become more sensitive, resulting in a faster system
response.
1. Plug in 5.6K resistor into the Rvel (for velocity feedback).
2. Set the INPUT potentiometer to 0 position.
3. Increase thegainby changing the value of theRe resistor to 1K (Re= Gain). Switch on the
paper roller. Give the system a step change at the input andtake a plot of the system response .Switch off the paper roller and label the resistor values on the plot taken.
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4. Decrease the gain by changing the value of the Re resistor to lOOK andtake another plotof a step response. Note the difference in speed of response between the two gain settings. Is
this response faster or slower than that obtained in the previous step (3)?
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Please cut and paste the response curve here;
5. Switch off all the power supply to the Servo Demonstrator and disconnect all the resistors.
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PART 2 - SPEED CONTROL
A - OPEN LOOP SPEED CONTROL
In this section, we shall study the effect of load changes on a speed control system operated
under open-loop control.
1. Set up the Speed Control apparatus for the MS-ISO system as shown in figure 2.
2. Disconnect the tacho feedback signal from the Op Amp Unit and swing the magnetic brakesclear from the brake disc. Ensure that the feedback selector is turned to External Feedback.
3. Set the Input Attenuator to 0. Turn on the power supply and adjust the Attenuator until themotor speed is about 1000 rpm. Use the hand-held tachometer to register the speed. [ not
allow the motor current to exceed 2 Amps/
4. Stop the motor by turning off the power supply. Swing in the magnetic brakes until the edge of
the magnets coincides at the 4th mark.
5. Restart the motor and record again the motor speed in Table 1.
Initial Speed (rpm)
( o oa
Final Speed (rpm)
( t oa
% Speed Reduction
Open Loop
Closed Loop
B - CLOSED-LOOP SPEED CONTROL
The procedure carried out in the previous section is repeated with the system under closed- loop
control.
1. Connect the tachogenerator feedback signal back to the Op Amp Unit and swing the magneticbrakes clear from the brake disc.
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2. Repeat the steps (3) - (5) in the previous section A , recording your results again in table l.
DISCUSSION:
Part 1 - Position Control
1. Attach all the plots taken from the Servo Demonstrator into your report. Ensure each plot is
neatly labeled with their respective resistor values (Reand Rvel). With reference to these plots,answer the following questions:
a. Why is the output of the Closed-loop position control system (without velocity feedback) so
oscillatory?
b. Comment on the improvement in position control after velocity feedback is added into thecontrol system.
c. How does the gain setting affect the speed of response? What would you expect if the gain is
set too high?
d. From all the plots taken, which combination of Re and Rvel would you consider to give the
best output for position control? Explain your answer.
Part 2 - Speed Control
Using the results recorded in table 1, comment on the effect of load change under the open andclosed-loop control. Suggest one method to further reduce the percentage drop in speed under
closed-loop control.
CONCLUSION
Briefly conclude on the findings of this experiment.
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