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Author Topic: How to handle servo faults  (Read 5747 times)

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Offline simpson36

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Re: How to handle servo faults
« Reply #10 on: September 25, 2013, 05:56:00 AM »
There is a lot to consider when setting up an E-stop scheme. I would advise that before copying an existing scheme, make sure that whatever hardware you are using has the same features and behavior as the setup you are copying or you may get a very different result.

A well designed E-stop setup will stop the motors as fast as possible, but also retain the machine coordinates, otherwise every E-stop is going to require rehoming the machine.

I make a 4th axis product and often put customer's motors (and drives) on them, so I have exposure to a lot of different stuff and have read a lot of manuals and had conversations with manuf reps and engineers to the point where it all starts to blend together so I try not to quote specifics on a particular brand. That being said, IF my memory serves, the Allan Bradley drives that Hood uses will retain their location and continue to track the encoders while disabled. My Mitsubishi drives and I think also Yaskawa and some others do not.

So, to use Mistubishi as the example, when the drive receives ENABLE, it considers whatever the actual position is  . .  to be the current commanded position. Depending on the conditions present when the drive was disabled, the difference between where the drive IS and where it should be will vary from a lot to a tiny bit, but the error will be there.

Before I became aware of this phenomena, I was using disable/enable to avoid faulting when the 4th axis spindle was locked while cutting a gear or pulley tooth. The error was tiny per tooth, but after 360 degrees and 72 teeth, the error has acculated to a few degrees. Not even visible to the naked eye sometimes, but enough to ruin a gear if more than one pass is required to cut the teeth.

This took a long time, a lot of ruined parts and a lot of phone time with Mitsubishi engineers to finally figure out what was hapenning, so I just pass it along FWIW.  I use ENABLE/DISABLE all the time on the spindle and the 4th axis (when it is in LATHE mode), but there can be pitfalls to using ENABLE/DISABLE on an axis drive, depending on how the feature is implemented on your hardware.

Speaking of FWIW, for my E-stop setup, I have the error lines from each drive 'daisy chained' thru a bank of opto isolators  and then a single wire from the end of the chain to the Kflop E-stop input. This arrangment takes care of the voltage difference, the isolation, and multiplexing the inputs in one step in one device. If any drive faults, it breaks the chain and the Kflop gets the E-stop (think christmas tree lights). It is set up active OFF so a broken wire or power problem will also cause the E-stop.

Also consider recovery from the E-stop. Make sure that unexpected instant restarts are supressed (spindle primarily). Another difference between drives comes into play here again. My Mistubishi axis drives require a reset after a fault whereas my Copley Controls spindle drive need only a disable/enable cycle and they are ready to rock again.

Offline mc

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Re: How to handle servo faults
« Reply #11 on: September 25, 2013, 02:38:03 PM »
I'm going to be using the analogue inputs on the drive via a Kanalog, so all the position control is going to be handled by the KFlop. Having a scan of the manual, the drive simply passes through the encoder signals from the motor, and provided the drive logic power remains, the encoder signals should remain.

I think I'm going to opt to keeping the e-stop circuit seperate.
Each drive fault output will control a seperate DPDT relay.
So if I connect all the stop signals through one of the NO in each relay to a 24V source, then should a drive fault then the stop signal gets triggered on all drives, and then use the other contacts to control a notifier to the KFlop to let it know a drive has faulted. I could probably just use a SPST relay, as the drive outputs are open collector so can be linked direct to the Kanalog optos without interfering with each other.
By cutting the stop signal, the drives require re-enabling to exit the stop, so there's no need to kill the enables as well, so I can programme a delay into the KFlop to kill the enable, even if the KFlop doesn't respond for whatever reason the drives will stay stopped.
Then I can connect the Enable and Reset direct from the Kanalog to the drives, and the Ready and Faults direct to the Kanalog.

That way I only need 2 to 4 inputs (depending on if I join the ready/faults on each drive or keep them seperate), and 2 outputs (enable is shared, and issuing a fault reset just clears any faults so no need to trigger drives seperately).

The only issue outstanding, is how to tie in the VFD, however I'll probably just link another relay into the VFD direction signals.