Noise problem causes Stepper motors to creep

[Capnden]

I have run into a tricky problem. I have had a homemade CNC mill, made from a drill press, for several years. I use a PC with TurboCNC software driving a much modified Chinese Stepper controller with the Toshiba 6560 driver chip.
I recently modified the Mill to use a commercial column and spindle head. I retained the AC drill motor but got rid of the belt and pulleys by using a direct gear drive. To control the speed I built a variable frequency drive based on a paper on the microchip website that synthesizes two sine waves 90 degrees apart using PWM at 12kHz.
My problem is that when the motor is running it is generating noise that causes the Y and Z steppers to creep. The co-ordinates in the software are not changing so the software isn't causing the creep. If I disconnect the LPT cable and run the motor I do not get the creep so I don't think it is coming from the Chinese controller. I suspect that I am either getting induced noise in the LPT cable although I believe it is shielded, or I am getting noise on the AC line that is getting back into the computer. I looked for noise on the AC line with a scope but didn't see anything obvious. I tried moving the cable away from the motor and control without any noticeable difference. If I replace VF control with the original start capacitor there is no problem so the noise is definitely coming from the VF controller. Does anyone have an idea how to fix this or isolate what is causing it?

Similar Threads:

• Newbie- Stepper motors problem
• Need Help!- small tuning problem with stepper motors
• Newbie- Stepper motor creep
• Should my motors make noise?

[H500]

Try adding a 1n capacitor between the step pin and ground, at the controller end.

[Capnden]

Thanks for your response. After I posted here I found a site that discussed noise issues with VF controls. Apparently they are well known for interfering with CNC controls. They advised shielding the motor wires. I ran the motor cable through grounded flex conduit and my problem appears to have been resolved. I have a line filter that I got out of an old printer that I plan on putting on the AC line to the VF control. If the problem comes back I will take a look at putting a cap on the step line but hopefully I have it fixed. Thanks for your help.