View Full Version : Preempting RF noise

11-12-2003, 11:21 PM
I am designing my cnc retrofit layout, and am planning to build a metal cabinet to hold a computer, relay rack, a 24v isolated power supply, 3 PWM servo amps (AMC brand), and a Galil interconnect module. The servo amps are standalone units, with a metal housing which "roughly" closes the unit in, but not really;)

My question is about the potential for RF noise from the servo amps perhaps being picked up somewhere amongst all these electronic chips. Should I try to mount these amps and the isolated power supply in their own compartment (all metal)? How well fitted does a box have to be to prevent RF noise from leaking out? Obviously, I am going to have to have holes for cables in and out of this area.

Thanks for your suggestions and experience :)

The computer and the servo amps will be sharing the same 110v AC line. The computer will be running from a battery backup unit (not a true power conditioner), whereas the amps will not. But, they will share the same input power.


11-13-2003, 10:36 AM
Dear Murray,
No need to worry about the RF noise from the Servo-Amps. You can mount them in same cabinet.
The care you need to take are as follows:

All the Cables carrying precision Signals like, Encoder or Scale feedback, Axes Command lines etc. must be Shielded cables and of suitable gauge size (as per Drive and CNC manufacturer's recommenations).

Use Spike Suppressors across relay coils and across the power lines of coolant, fan motor etc.

If you are using low DC control voltage (24V or 12V etc.) use a SMPS power supply from market.


11-14-2003, 07:19 PM
Hey Murry, Darek again.

The rule for shielded cables in this application is ground one end only to avoid ground loops. It is best to ground the end at the controller since
it is usually closer to the main ground. Hook these connections to the chassis instead of the board ground. Board grounds usually have bypass capacitors to Chassis ground.

The wires you will want to watch out for are the motor power leads, since they are chopped DC. Keep them away from the shielded cables. I know this makes it a task since the motor power leads and encoder cable have to basically follow each other all the way to the motor. Just take care to separate them, do not place them in the same conduit together. Use two instead, this also is common practice.

I like the AMC products and price.

Hope this helps.

11-14-2003, 09:15 PM
Thanks guys, I knew about the motor power/encoder cable precaution, but was not sure about how much noise the amps were broadcasting.