I'm closing a current PI loop with a DSP and need a test method to determine the loop bandwidth. This will be the current loop in a motor driver that is sampled at 100uS so according to nyquest the max frequency is 5Khz and a realistic cuttoff should be 1-2KHz.
With a digital control process how do I measure the system response? Sine sweep the input and look at the output for DB cross? Is this done open loop and calculate the closed loop based on the open loop and PID gains? Step response?
What I have so far is that a step input (0 to max) makes the output rise (10% to 90%) in 1.5mS 15 samples (~650 Hz). Open loop plant gain (Vref/Iout) is about 10. When I close the PI loop around this I can get the ramp to lower values to rise much faster (the point of the PI control correct) how do i know that my loop bandwidth is below 1/4 to 1/5 the nyquist frequency?