Digital Drivers and Stall Detection

Hi Guys,
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I have a little issue with my X (long) axis stalling occasionally (I ran a 2.5 hour job yesterday with no problems, but the subsequent 10 minute pocketing routine produced a nasty stall) and I think I have traced this to a slightly banana shaped ballscrew, possibly made worse by my misting solution messing with the lubrication of the machine generally.
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Anyway I've been looking at some of the new digital drivers out there, which claim to have stall detection (AM882 etc.), but what does this actually mean in practise? I'd like to think the driver (in concert with the BoB and MACH3?) can automatically compensate for stalls/missed steps, but I suspect its a case of the machine stopping, whilst the mechanical fault is cleared and then being able to run from that point, without a new and unintended offset being introduced halfway through a job.
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Anyone with practical experience of their behaviour would be much appreciated, as I may get the drivers anyway, just for peace of mind and use my 752's on something else (I have plans for a larger 2nd machine for wood and also have been giving my brand new SC4 lathe the evil eye, since I found out its main leadscrew is Oldham coupled at one end and pillow blocked at the other :-) ).
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I also see Leadshine now list the AM882 as discontinued, so will have to scout out what the alternative is for a machine running its motors at ~68V.
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Cheers
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Chris
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PS. Merry Christmas in case I don't post again before the season gets into full swing.

Chris,
I tried a stall with my AMM882 and motor just lying on the bench, when it stalled the red light blinked and the motor stopped, I had to cycle the power to that particular driver to reset the fault.
Because I don't have my machine completed yet I can't tell you what the consequences would be in a working situation but you would think the other stepper motors would keep working.
The AM882 has a relay output that operates under fault conditions, in my design I have these connected to stop the whole machine including the spindle when one or more AM882's fault.

Read this link re. AM882 discontinued
http://www.mycncuk.com/forums/genera...ite-am882.html

Merry Christmas to you too

My Leadshine drives are AM882H The difference I see is that they accept an AC input as well as DC up to 100V it does not matter which way round the input cables are connected. ..Clive

If the stepper motor stalls the driver no longer knows the position on the rotor, since there is no feedback. The only thing the driver can therefore do if it detects this, is output a fault signal which you would generally use to stop the rest of the machine. To recover the position of the stalled motor, the corresponding axis would need to be homed.

The way to 'automatically compensate' is to have an encoder on the motor, which gives feedback of the rotor position - that's a servo motor.

You should first try and fix the mechanical cause of the stalling and consider stall detection as a last resort, since if the motors are tuned properly and the mechanical side is properly maintained the motors are only likely to stall if the machine crashes. You could check your motor tuning with the following spreadsheet, but unless you've changed it recently, the bent ballscrew is more likely to be the cause:

http://www.mycncuk.com/forums/gcode/...or-tuning.html

Thanks guys, as ever.

That makes sense then - stall detection = a way of reducing scrap metal, whereas servo motors = actual error correction

I will definitely be replacing the slightly bent ballscrew - it was the first one I purchased and I probably bent it due to uninformed rushing when I originally put the machine together. Chalked up as experience and the old one may be able to be used in a shortened format on a new z axis or on the lathe if I convert that (I guess I can machine it or have it machined), so not a complete loss.

Cheers


Chris

Jons correct with everything he says but I'll add to it.!!. . Mach Can't use Encoder feed back and correct for any error even with Servos. Also often Servos without Control participation don't correct for error they just have Encoder following Error value set in the drive software which they then give an output Fault signal if Encoder shows is outside this value. This is pretty much what the Am882 does.! Which by the way are great drives and worth paying the extra for even if your not using Stall detect.

If you want drive/Motors that actually correct for any error then look at the Leadshine Hybrid Servo system. These correct any position error internally to the drive so the Control is not involved. Nice bit of kit just not cheap but a lot less than Servos. (Some where on the Forum I posted a Link to a Video I made just do a search.)

Thanks Dean,

The hybrid drives maybe something to look at in the future then - a straight ballscrew first though.

BTW - will have a video up of the machine cutting shortly - I'll post in my build log for a bit of festive humour :-)

Cheers


Chris