PLC unintentionally triggering E-stop circuit

Hi, this is a problem that has been dogging me for months and I thought I'd ask here.

I have a Direct Logic 05 PLC that I am using to interface a high-speed spindle with a Haas CNC machine. The CNC machine has a relay output that it uses to call peripheral devices, and it watches for a feedback signal on another pin. The PLC watches the spindle motor drive for errors and sends the feedback signal if there are none. If there is an error, the PLC opens a NC relay which is wired into the CNC's E-stop circuit.

We had been having weird errors where the CNC would stop running in the middle of a job without going into E-stop, so we removed the NC relay and replaced it with a jumper. So far the mystery error has not come back (one month).

Do I need some kind of transient protection on the NC relay output? I am fairly certain that the PLC was not sending the E-stop command because when I force an error on the drive the CNC machine goes into E-stop, not the mystery state.

Change your PLC logic to make the relay energize when in a no alarm or normal running condition, then use a N.O. contact on the now energized relay to prove all ok from PLC side. That is what is called fail safe mode. N.O. circuit contacts are normally less likely to open from dirty contacts or vibration also.

opaji said:

We had been having weird errors where the CNC would stop running in the middle of a job without going into E-stop, so we removed the NC relay and replaced it with a jumper. So far the mystery error has not come back (one month).

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This kind of statement scares me.

I can't help but cringe when I hear about a jumper going in where something "safety circuit related" is supposed to be.

Does the CNC have a purpose-built E-Stop circuit, with features like external device monitoring and short circuit detection ?

I recently worked on a system that had a momentary voltage loss on one input channel of a Safety Relay. The Input was only low for about 100 microseconds, but that was enough to trip the Safety Relay.

This led the Safety Relay to go into a "lockout mode", where it could only be reset by cycling power, or by making both Safety Inputs go false for more than one second.

This is a different mode from the usual "E-Stop", where a set of contacts would open and stay open, because somebody had pushed an E-Stop pushbutton or broken a light curtain beam.

I agree that using a general-purpose PLC to perform a monitoring function that is part of the Emergency Stop feature of a machine might be inadequate for some levels of protection, but I don't know your machine and I'm not going to weigh in on that.

dploof23 said:

This kind of statement scares me.

I can't help but cringe when I hear about a jumper going in where something "safety circuit related" is supposed to be.

Click to expand...

I wired into the auxiliary e-stop input on the CNC's control board, it's shipped with a jumper in place for just this purpose. I should have said I reverted it back to factory standard configuration.

Ken:
Yes, the CNC has a purpose built e-stop circuit which when opened will stop the machine.

These ice cube relay contacts are available with different types of contacts
- different material - usually gold for DC
- there is also low current - Bifurcated - for communications
I am thinking that as you are using the CNC wiring it is both low current and might be 5 Vdc
The standard relay contacts can be sensitive to a dusty environment
They are designed to self clean - but - as suggested - normally closed means it never even gets a regular movement.
As you are not getting a fault indication I suspect it is dust between the contacts

dploof23 said:

This kind of statement scares me.

I can't help but cringe when I hear about a jumper going in where something "safety circuit related" is supposed to be.

Click to expand...

Sounds like this is more to protect the machine than people though.

most CNC equipment has an Emergency Stop system.
AND
associated shutdown system to protect the machine itself
I believe you are 100% correct

Try this, maybe...

If the error the PLC is monitoring is a quick transient that "disappears" and thus isn't a critical "game over" condition (I think you said the CNC machine is found stopped in the middle of an operation without being in an E-Stopped condition) why don't you try putting a delay on the output coming on to your relay. It might only require a fraction of a second or so to "filter" out the false ghost shutdown errors.

Okay thanks for the suggestions guys, I'm going to try reversing the PLC logic so the coil of the relay is energized when there are no alarms. I am also looking through the CNC manual to see if I can have the PLC send a feed-hold command instead of e-stop.