NI pwm output servo control problems

[Dagonza]

Hi all,
I am trying to control some RC servo motors using a data acquisition card and simulink (real time windows target). I have noted some very strange things happening however.

I do have some electronics experience but mostly deal with mech stuff. I am hoping someone here can help me out.
 
I am trying to control 2 RC servo motors using my NI DAQ (PCI-6025E) and simulink. To do this i have both motors powered from a labratory power supply plugged in to the power socket in the wall. I then use the DAQ card to send PWM signals (through simulinks "frequency output" block) to the each servo to control its position.

I have had success controlling one of the motors but for some reason when i connect the signal cable up for the second motor either one or both of the motors (not always consistent) begin to oscillate quite fast. sometimes this will last for a second or two, and other times it will go on for 30 seconds.

Could this have something to do with the grounding? Do i need to ground the DAQ to the power supply? if so what pin to i ground?

Cheers

[Kyle_A.]

Hello,

How are you connecting your motors to the DAQ card?  Is the PWM being made with an analog output?  Can you describe with more detail what is happening when the motors oscillate?

If you measure the voltage level of your line, do you see any residual voltage or voltage jump?  I don't believe hooking up grounds will be the best solution since you can cause a ground loop between your PC's power supply reference and your RC motor's power reference. 

Does it make a difference if you connect the motors to your device when they are powered off first?  Do the motors move once powered on?  Sometimes, motors move to position themselves if they are made to do this, and after a few seconds they become initialized and work for the duration.  Can you still use the motors after they stop oscillating?