Closed Loop Response

  You will go through this section once for each of the three gains, changing the potentiometer in between.

  1. Step Response
    Check your connections. It is particularly important that the power be applied to the op-amp in the right direction, and that none of the pins on the chip are shorted to each other. Be careful that none of the resistor or capacitor leads touch each other.
    Set the function generator to a square input of amplitude 1V, offset 1V, and frequency 1Hz. This will be a square wave alternating between 1V and 3V.
    Set the trigger to source 1 at a level of 2V with upward slope.
    Set both voltage scales on the oscilloscope to 1V/div.
    Set the time scale to 50ms/div, and the time delay to 200ms.
    Adjust the vertical position of traces 1 and 2 so that the origin of each is at the origin of the screen.
    Set trace 2 to invert mode. This will get rid of the negative sign when viewing the signal.
    Turn on the power. If the voltage levels on the power supply do not go up to 20V, then turn it off immediately since something is most likely shorted. If this is the case check your connections.
    If the motor spins uncontrollably, you are in positive feedback. If this is the case, interchange the two leads from the tachometer.
    Measure the error by using the math function on the oscilloscope. To do this, press the +/- button, turn the math function on, select menu, and choose the ``-'' function. Set units/div=1V and offset=3V using the knob normally used for cursors.
    Use the cursors to measure the following:
    Steady State Error
    Peak Time, Tp.
    98% Settling Time, Ts.
    % Overshoot, %OV.
    Print this screen.
    Click to Enlarge Click to Enlarge
    Closed-Loop Step Response - Low Gain Closed-Loop Step Response - High Gain

  2. Disturbance Rejection
    Apply a disturbance to the system by lightly holding the motor shaft, allowing it to spin between your fingers. Do not hold it so hard that the power supply voltage drops significantly.
    View the oscilloscope. Note (qualitatively) any changes in Ts, steady-state error, %OV, and Tp.
    Print this screen.
  3. Ramp Response
    Hit the sawtooth button on the function generator (this is the one with vertical lines, as opposed to the triangular wave button).
    Set the scope to trigger on a downward slope.
    Use the cursors to measure the steady-state error to a ramp.
    Print this screen.
    Click to Enlarge
    Closed-Loop Ramp Response

  4. Change Gain
    Detach the potentiometer from the circuit (remember where it went).
    Measure it's resistance and set it to the next of the values calculated earlier.
    Replace the potentiometer in the circuit.
    Repeat section 3.4 for the new gain (there will be three gains in total).
Ajay Juneja
Tue Apr 20 13:30:59 EST 1999



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