We recently had some problems with noise making is way into the CW and CCW stepper motor controller signal and thus making the motor rotate when it should have been stationary . We discovered various noise sources including 24V clutches and an impoperly grounded shield; however,we were unable to remove all of the noise. The motor would turn each time when the machine got to a particular step; therefore, we assumed it was another motor/supply generating the noise. When I placed a homemade Spectrum Analyzer (limited to 50kHz)on the CW and CCW, no noise was seen (but could be seen on an oscilloscope). I suspected that the noise was probably beyond this range and suggested tying in some capacitors from the CW and CCW to ground to allow the high frequency content to drain off. My suggestion was not easily accepted but when we tried it, it worked. The controller has an input resistance of 230 ohms and I selected a 0.1uF capacitor to allow the content to roll off above 20kHz. Although this is working and this is the third week it has been working, other people are starting to doubt this technique. I need feedback on this and any short comings I should expect. Thanks for your time.
A parallel capacitor to shunt HF noise is a time-honored technique that can often work perfectly adequately.
You say that the controller input impedance is 230 ohms. Is that the input to the controller built-in to the stepper motor itself? The actual amount of attenuation of the noise will also be dependent upon the source impedance for the control signal.
Your spectrum analyzer results indicate that the noise is over 50kHz or so, and you say that you could see this noise on a CRO. Did you manage to expand the timebase on the 'scope sufficiently to get an approximate frequency reading from that?
#128259 - 01/05/0308:33 AMRe: Capacitor to drain HF noise
As Paul suggests, try to roll off this noise with bypass caps.
In addition, I could suggest a filtered supply (filtered to the Hz of noise + harmonics), drain caps at the load ends - drain to shields, create an Isolated grounding setup, and finally an Isolated power system.
Be sure all the shields and other grounding items are star connected at one point ONLY!!! Otherwise you will never get rid of the loop currents, and end up adding more noise to your system than removing.
Drive everything to Earth Ground at the same single star point.
If you plan to include AC Line filters, or other Line "PI" filters, connect the stuff driven to ground via an Isolated Grounding Conductor, which also terminates to the star point.
Looks like you need to place Low Pass filters at each crucial point, plus drain each circuit's conductor to ground at each crucial point, via a ceramic cap connected to the shield. One per conductor is needed.
Let us know the progress!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!