Servo Motor Speed Discrepancy Issue with PCI-7340 in Velocity Mode

[similars2006]

Hi all,

I’m currently using a PCI-7340 card to control four servo motors and have been troubleshooting an issue for some time. When I use the “1D Interactive” function in NI-MAX to configure one of the motors in velocity mode, the motor speed from the encoder information consistently reaches only about 70% of the target value I set. For example, setting the target speed to 200 rpm results in a steady measured speed of around 140 rpm, and setting 400 rpm gives about 280 rpm. Because of this discrepancy, the following error accumulates quickly and eventually exceeds the threshold, causing the test to stop.

I’ve tried adjusting the “counts per revolution” setting under Encoder Setup, but it doesn’t seem to make any difference—the position still indicates 4000 counts per revolution even when I manually rotate the motor after changing the setting.

Any suggestions or insights would be greatly appreciated!

Thank you in advance.

Best,
Yin

[RIObotics]

Hi Yin,

It looks like you're making progress, good to hear!  For your problem, I have some suggestions:

1. You can do a quick sanity check for the "encoder counts per rev" setting.  While servo is off, use your finger to turn the motor exactly 1 revolution.  Is the position count = 4000, as expected?  Or is it something else?  The actual count should be consistent with the “counts per revolution” setting under Encoder Setup.  If this is not consistent, then change your setting to match the actual count per rev.

2.  Assuming that the above setting is correct, have you observed the encoder speed at lower velocities?  You are testing at 200rpm, which is quite high.  Have you tried to observe at lower speeds, such as 10rpm or 50rpm?  Does it also show the same 70% issue for low velocities?  Some motors may actually have a limit for max rotating speed, so you can double check the specifications of the motor.  

3. If all else fails, maybe you can rewire the motor to another axis on the PCI-7340 and try again.  I doubt the PCI-7340 is broken, but you never know...

BR

John

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