Positive Feedback

In this section, you will put the system in positive feedback and examine the resulting instability.

Turn off power.
Switch the leads from the tach between GND and the controller.
Set the function generator to a DC voltage of zero.
Turn back on the power. Be careful not to touch the motor.
The motor should be still at this point. If not, turn off power, wait until the motor stops, and turn it on again.
Set the oscilloscope to trigger on source 2 in single trigger mode. Set the trigger level to about 1V.
Twist the motor shaft slightly. The motor should spin out of control.
Turn off the power.
You should be able to clearly see the response on the oscilloscope. If not, adjust the vertical position and voltage scales of source 2 and try again.
When you can clearly see the response, print the screen.

Frequency Response Plots

Now, you will generate a set of Bode plots for both open and closed loop using the signal analyzer. Sample plots appear below this section.

Turn off power supply.
Reverse the tach leads so the system will again be stable.
While leaving everything connected, move the T connector from the function generator output to the signal analyzer source.
Move the cables from inputs 1 and 2 on the oscilloscope to inputs 1 and 2 on the signal analyzer.
Turn off the oscilloscope and turn on the signal analyzer.
Press Inst Mode button and select Swept Sine.
Press Freq button and set the start and stop frequencies to 1Hz and 100Hz.
Select Resolution Setup and set Auto Res. on and Min Res to 101 pts/sweep.
Press Source button and set level to 4Vpk.
Press Scale button and set Autoscale on.
Turn on power supply and hit Start button.
While the motor is spinning, press Disp. Format button and select Bode Diagram.
This is the closed-loop frequency response. Print it out.
Put the system in open loop by moving the wire connecting the op-amp to the amplifier from the op-amp output to the row with the function generator output (this bypasses the op-amp circuit entirely).
Press Source button and set level to 200mVpk. (The open-loop has a natural amplification of 20 over the closed-loop.)
Press Start, and while the motor is spinning, press Disp. Format button and select Bode Diagram.
This is the open-loop frequency response. Print it out.
Click to Enlarge Click to Enlarge
Closed-Loop Frequency Response Open-Loop Frequency Response

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