analog feedback oscillation

[DanBl]

I am using a PCI-7354 controller and have the encoder mode working now and doing exactly what I need. I am attempting to develop the analog feedback mode but have run into a problem. The controller is connected to a UMI-7774 interface box and currently I have the input to ADC1 shorted so it should be 0 all the time. In looking at the position counts with NI-MAX, they are swinging around by as much as 60 counts. It seems that no matter what I try, the position won't calm down and if I enable the motor it buzzes like crazy and keeps trying to get in position.

I'm not sure what could be causing this. If I switch back to encoder feedback, the position sits rock still and doesn't move at all. As soon as I change to analog feedback, the position starts jumping around. If I set the PID to 0, the motor won't react to the oscillation but the position is still quite jittery so it's not a tuning problem. Any ideas what I'm missing here?

[Poolean]

1. What voltage range are you using?

The resolution varies with the range you have selected. And, the largest resolution value is 305 uV/LSB. this would mean a difference of 18 mV in your analog signal could cause the 60 count swing. The input ranges and their corresponding ranges are on page 5-12 of the spec sheet.

2. What is your analog signal being generated by?

3. Did you observe the signal?

It'd be good to check the integrity of the signal. If it is moving up and down by about 18 mV while it's supposed to be at rest that would explain the behavior you're seeing.

Mike