Laser – Close Loop Stepper Alarm Interlock

Previously I had upgraded my OMTech 3555 130W CO2 laser from open loop to closed loop stepper drive on the X axis. See:

The closed loop stepper driver has an alarm output. This is used to signal the controller that the stepper driver has encountered an error that it cannot correct. The particular driver I am using will halt on an alarm condition. This is different from the case of an open loop stepper losing steps and continuing merrily on its way (in the wrong location).

I was having some intermittent alarm issues with the stepper. The problem is that without the interlock, the X axis stops, the laser keeps firing and the Y axis keeps moving, basically destroying the part being engraved. I needed to get to the bottom of the alarm condition, but with it being highly intermittent (once or twice a day), I also needed to keep using the laser. So this drove me to do a bit more research and found a Youtube video on how to do this. It seemed like a reasonable approach, so I took the idea and applied it to my machine.

Interlock process

The Ruida 6442 controller used in my laser does not have a separate stepper driver alarm input. So the idea is to add onto an existing alarm / interlock input. For this the door interlock signal is used. It will now trigger an error if either the door is opened or the stepper driver alarms. To do this, one more 24 relay is added. The relay is driven by the alarm output signal. One set of normally closed contacts is connected in series with the door interlock signal. The schematic addition is shown below.

CN1 and CN2 are connectors on the Ruida controller. Remove the existing wire on CN2-3 (was black in my case). Splice it to the wire from pin 9 of the relay (solder and heat shrink). Pin 1 of the relay is now connected to CN2-3. This completes placing the relay in series with the door protect interlock switch.

Interlock connections on the Ruida controller

CN1-6 (+24v) connects to pin 14 of the relay (coil +)

Stepper driver ALM+ connects to pin 13 of the relay (coil-)

Stepper driver ALM- connects to CN1-1 (0v)

X alarm added to the other relay additions

That is all it takes.

When the laser powers on the relay will flash for a moment (as does the red alarm light on the driver). That is normal. Open and close the lid and the lights should appear as normal. Run a small test job to verify that everything is working normally.

The Alarm is a bit hard to trigger on demand. There is too much torque to do it by hand. You have to carefully block the travel or fool the laser into driving past the home position. Turn on the laser and hit escape as it is homing. This aborts the homing. You can now drive the X axis to the left into the stop. It will alarm and you WON’T hear that awful grinding noise of the motor skipping steps. Now if you try to run a job it will immediately beep and halt on the Ruida controller. To reset the alarm, the laser must be power cycled.


Youtube video by Jim Alexander: Incorporation of the Alarm Function for Hybrid Stepper Driver


24v relay:

Relay DIN mounting rail:

Wire ferrules and crimper:

20 gauge wire; Red: , Black, White, Yellow

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