Electronic – Ground induced noise between two connected boards

interferencepower

I have a CNC system that consists of a CNC controller connected to a PC via USB. The controller is in turn connected to a stepper motor driver board. My problem is that some time the turning off/on of the stepper motor drive power supply induced larges spikes in the power distribution of the CNC controller, some times to the extent of resetting the system.

When the drives are switched on you can even see the 20khz PWM frequency of the drivers switching on/off on the CNC controllers power distribution as thin voltage spikes. If the drivers board is disconnected from the controller board there is no interference.

I am assuming that since both boards are connected with a common ground that this is the main cause of the induced spikes. Also the CNC to driver interface is via digital isolators, so in fact the CNC board and driver boards are electrically isolated.

I would like to know what I could do to minimize this problem. Any suggestions are greatly appreciated.

The controller uses the USB power supply and is isolated from the drivers. But only when the drivers are connected to the controller do I see noise being injected into the controles (usb powered) power supply.

A diagram to illustrate the architecture:

Best Answer

My first knee jerk reaction is that the ground connection between the controller and the motor doesn't belong there. You have a isolator block, which makes sense, but then you defeat the isolation by tying the grounds together.

What is the nature of the STEP and DIR lines to the motor controller, and what exaclty is the isolator block? Most likely the isolators are opto-couplers. In their simplest form, these have floating NPN transistor collector and emitter outputs. If the motor driver has passively pulled up inputs, then tie the isolator output transistors between the motor driver ground and these lines. There are other ways they could be hooked up too, but it makes sense to get into that after you explain the electrical interface to the motor driver and what exactly these isolators are.

In no case should there be a connection accross the isolation.