RC Servo strange problems

[mechag]

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.

Here are some strange things i have encountered and i am not sure why they happen or if i have fixed them correctly.

Servo grounding
I am powering the RC servo's through a labratory power supply connected to the mains power socket. The PS has - GND and + outputs. If i simply connect the servo + and - to the PS + and - then the regardless of the PWM I send the servo it moves to its most extreme position (and draws lots of current as it tries to keep moving). When i connect the - and GND terminals on the PS and then connect the servo i can control it normally and without any issues. Why is this happening? and is my "fix" ok?

Servo Control
Now comes something strange with simulink and the DAQ. I am using some frequency output pins on our DAQ unit (from a computer) and it is fine with one motor. But when i connect the signal cable for the second servo motor up it starts to oscilate about its position quite fast (despite being given a constant signal). I have hooked up a CRO to the signal and observed it before and after connecting the second motors signal cable and the signal seems to be the same. It seems like it is some sort of grounding issues with the DAQ. Does anyone have any suggestions? this one has me totally stumped.

Cheers

 

[OperaHouse]

Usually people have the opposite problem, connecting the ground causes it to go crazy due to a ground loop problem.

If I had to take a guess, the first problem sounds like the controller input is not grounded and grounding the servo power provides the ground for the input.