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  • Instrumentation and Control Lab

    Position control

    Ex 6: Study of a Position Control System

    6.1 Objective:

    To study the O/P response characteristics of a position control servo system

    6.2 Apparatus:

    (i) Power supply (PS150E) (ii) Servo amplifier (SA150D) (iii) DC motor (DCM150F) (iv) Reduction gear tacho unit (GT150X), (v) Op Amp unit (OA150A) (vi) Input potentiometer (IP 150H) (vii) Output potentiometer (OP 150H) (viii) Connecting wires.

    6.3 Procedure and Experimental Results:

    (i) Connect the +15 and -15 volt with input potentiometer. Take the angular position (from 00 to 1800 both clockwise and anti clockwise direction) vs output voltage readings. Plot the data in a graph paper and calculate the slope.

    (ii) Do the step (i) for output potentiometer.

    (iii) Now connect the input potentiometer and o/p potentiometer with the Op Amp unit as shown in Fig 1. Put both the potentiometer at zero angle position and observe the O/P. If the O/P voltage is not zero adjust the zero setting knob to bring the output voltage at zero. Vary the input potentiometer position in step keeping the output potentiometer fixed at zero degree. Plot error angle vs output voltage keeping Op Amp gain at unity.

    (iv) Now connect the position control circuit as shown in Fig 2. Draw the block diagram of the circuit in control system point of view. Check your circuit with instructor. Set the Op Amp in unity gain position (position 1). If necessary adjust the zero setting knob.

    (v) Suddenly change the input potentiometer angle approx to 200, 400 etc. upto 1400 in both the directions. Observe the steady state O/P potentiometer readings. Also draw the transient response available through the optical encoder and computer interface for at least two step inputs. From the plot calculate the rise time, settling time, peak overshoot, delay-time, damping ratio, conditional frequency and steady state error.

    (vi) Now vary the input gain as 0.1, 0.2, …, 0.9 and repeat the step (v).

    (vii) Put the amplifier gain control knob in position 2. Repeat the step (v). Is there any difference observe in transient response and steady state response?

    (viii) Now apply the eddy current break in three different positions. Repeat the step (v). ). Is there any difference observe in transient response and steady state response?

    (ix) Calculate the transfer function of the overall system. Observe theoretical step response of the transfer function and compare with that of obtained through experiment.

    Note: Please collect the circuit diagram from the instructor and before switching on the circuit verify it from the instructor.

    6.4 Few Questions for Viva Voce :

    (i) Can you say few applications where servo position control is being used?
    (ii) From the system transfer function, calculate the gain for which the system becomes unstable?
    (iii) How the eddy current damping works?
    (iv) Can you represent DC motor using state variables?

    6.5 Reference:

    (i) Katsuhiko Ogata, “Modern Control Engineering”, Prentice-hall Of India Pvt Ltd, 4th edition, 2007.
    (ii) Feedback manual, “Modular Servo System MS 150”.

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