Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: ethos on March 11, 2010, 04:36:39 AM
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I am assembling a home-brew Mill using NEMA 34 steppers and have hit an interesting issue whilst trying to calibrate my X and Y axes.
I ran a simple test using a pen in the chuck to test the accuracy of rapid and interpolated moves using incremental G0 X10 and G1 X10 F30 steps respectively.
The result was rather surprising, as rapid moves appear to be accurate, but slow moves GAIN steps!
Pen Trace 1<http://www.kill-9.co.uk/tdc/frankenmill/ruler.jpg>
I ran another series of tests, changing one parameter at a time to see what influenced the problem.
Pen Trace 2<http://www.kill-9.co.uk/tdc/frankenmill/ruler2.jpg>
The test run parameters are as follows:
vel = max velocity under motor tuning for X & Y axes
accn = acceleration under motor tuning for X & Y axes
current = max stepper driver current
clock = Mach3 kernel speed
1. G1 F300 vel=2500mm/min accn=1000mm/s/s current=3.8A clock=60KHz
2. G1 F150 vel=2500mm/min accn=1000mm/s/s current=3.8A clock=60KHz
3. G1 F75 vel=2500mm/min accn=1000mm/s/s current=3.8A clock=60KHz
4. G1 F75 vel=2500mm/min accn=250mm/s/s current=3.8A clock=60KHz
5. G1 F75 vel=2500mm/min accn=50mm/s/s current=3.8A clock=60KHz
6. G1 F75 vel=2500mm/min accn=50mm/s/s current=4.58A clock=60KHz
7. G1 F75 vel=500mm/min accn=50mm/s/s current=4.58A clock=60KHz
8. G1 F75 vel=500mm/min accn=50mm/s/s current=4.58A clock=25KHz
9. G1 F75 vel=500mm/min accn=50mm/s/s current=3.8A clock=75KHz
10. As above, but loosened off the X axis gib strip
11. G1 F1000 vel=2500mm/min accn=50mm/s/s current=3.8A clock=75KHz
12. G1 F1000 vel=2500mm/min accn=1000mm/s/s current=3.8A clock=75KHz
So, it would seem that the table is losing calibration slightly at 300mm/min feed rate and is really bad by 75mm/min.
Lowering the acceleration marginally improves the error.
Increasing the driver current makes things somewhat worse. This is a pair of allegedly 9A drivers with 350NM steppers rated at 4.5A. The steppers barely get warm when run at 4.58A, but after five minutes at this current, one of the drivers went into thermal shutdown so I dropped back to 3.8A.
Lowering the kernel speed from 60kHz to 25kHz made the problem much worse. Pushing it up to 75kHz made little difference to 60kHz.
Loosening the gib strip on the X axis made things significantly worse.
Ramping the feed rate up to 1000mm/min brought the output back to 10mm steps again, but the reduced acceleration rounds the corners.
Running at 1000mm/min with an acceleration of 1000mm/s/s gives an output that is accurate to within the error bounds of the felt tip pen.
So, running fast makes it more accurate. Increasing the power or reducing the load makes it less accurate.
The steppers must be moving more than one microstep per step input at lower speeds, but I can't see how this can happen unless the inertia of the load was stronger than the holding torque of the stepper. If this were the case, I'd expect it to get worse with speed, not better.
Has anyone seen this before?
Thanks,
Terry
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Terry,
How to check and also setup your mill accurately is described in Using Mach3 Mill manual.
Once you have the motor tuning done and have confirmed movements, then you can try ploting out some
programs using you pen holder. Don't use it for calibrating an axis. There is a calibration feature in the Mill program.
RICH
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Can you attach your xml please.
Hood
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RICH, you misunderstand me. I have followed the setup procedure in the manual, however the results seemed inconsistent so I was trying to validate the calibration process by tracing known distances on paper to track down where the error was creeping in.
I have attached a copy of my XML. I shall be interested to see if this is a config problem or a hardware one.
Thanks all,
Terry
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First glimpse of the xml looks fine.
Are you in the UK?
Hood
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Yes, South Wales.
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Could send you a loan of a G201 to try out if you wished?
Hood
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Thanks. I might take you up on that if I can't diagnose the problem with things I have to hand here. I will try putting a scope on the pulse train to see if there is obvious noise or jitter and I will try measuring the current through the coils to see if I am getting what I expect to see. I guess I ought to try a different PC to eliminate the parallel ports etc. The other thing that might be worth trying is disabling the microstepping to see if it is a problem in that area.
Terry
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Sort of sounds like the microstepping is not working properly, you wil maybe pick that up with the scope.
Hood
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Ok, here are some scope traces...
http://redpool1.trance.co.uk/tdc/frankenmill/trace1.html
The Blue trace is the X axis step output from the BOB.
The Tan trace is a current clamp placed around one of the phase wires from the driver to the stepper.
The first four traces are 4 runs of the code G1 X0.5 F2 (2mm/minute) with the clamp meter on each of the stepper wires in turn.
Listening to this very slow traverse, you hear a steady "clonk-clonk-clonk" until the point shown on the B+ trace where there is a steep asymmetric drop in current. At this point, you hear a sputter of steps, then back to the slow steady pace.
Can anyone tell what the problem is from the shape of the traces?
Terry
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Link should read http://www.kill-9.co.uk/tdc/frankenmill/trace1.html
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Cant seem to open the view of the trace, can you attach it to your post?
Hood
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Traces attached...
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And traces for rapid movement...
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Closeup of artefacts in slow speed trace...
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Have you run the driver test to see what your pulse is like?
Hood
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Yes. It's a fast machine that runs with a stable clock up to 65kHz. There may appear to be some pulses missing in these traces, but this is because the width of the pulses is close to the sampling rate at this timebase, so the scope is not catching all of them. Sampling at higher rates shows a certain amount of jitter between pulses, but an average pulse rate that matches the desired feed rate.
What concerns me is the asymmetric profile of one channel of the stepper drive. I was expecting to see two phase shifted sinusoids, but the B channel appears to collapse each time it crosses the peak and at this point you can hear the stepper jump several steps. I am guessing that the motor lets go as the current falls sharply and gets pulled towards the next whole step.
Not sure if this might be due to a wiring issue in the stepper, or a driver fault?
I have checked my wiring against the manuals for the steppers and the drivers and all appears to be correct, but I have found at least one error in the driver docs so anything could be happening.
Terry
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If its the 9 amp drives from Arc Euro then I am guessing the error you found is the CW/CCW and Ste/Dir dip switch, it was the wrong way round in the docs on the drive I got. Then again they could have been changed as I got mine not long after they were out.
Dont really know enough about steppers to say but I would think what you expect to see would be correct but where the problem lies I dont know :( Hopefully someone with more experience of steppers can help. If not the loan of the Gecko to test is still open.
Hood
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Listening to this very slow traverse, you hear a steady "clonk-clonk-clonk" until the point shown on the B+ trace where there is a steep asymmetric drop in current. At this point, you hear a sputter of steps, then back to the slow steady pace.
Hi I think I have a similar issue but I'm not sure wether my motor or my ballscrew the one that is making that tap tap tap noise after 20+ hours running.
do you have a recorded sound of that clonk clonk clonk?
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In this particular instance, the feedrate was set to 2mm/minute, so each 'clonk' is the sound of a single microstep. In my case, the problem is the fact that I get a nice regular pace of steps, then at a particular angle the motor spins forward several microsteps in quick succession.
Terry
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Was there any resolution to this?
Cheers Brian
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tap tap tap noise after 20+ hours running
- what's the temperture like after 20 hours of running since steppers can start to act funny when they get hot
feedrate was set to 2mm/minute
- thats around .001" / sec or one "clonk" / pulse per second
- the slowest feedrate i ever tried running my steppers with a G201 was 1 "tik" / sec........
I was just fooling around to test a peck drilling cycle, but they just tiked like a clock and left them run like that for a half hour time.
Sounded just like a clock and in perfect harmony with the stop watch. ( good grief what i do when i get bored ???)
Never experienced what you experienced so don't know what to say. Maybe take Hood up on his offer to try a different drive.
Sound is a good indicator and you know what's right when you hear it.
Sorry don't know what to tell ya......
RICH
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I'm far....far from being an expert, but I had a similar problem with my homebrewed machine. I'll save you the long story of all I went through. I was using only four of the six wire Nema 34 steppers, so I rewired them and voila, the problem vanished. I had originally wired them for torque rather than speed. I switched two wires and that solved my problem. As I said in the first sentence I'm no expert, not even close to intermediate, but I hope I can help somehow, 'cause this forum has been exceptionally generous to me. If you already tried this pardon me.
Richard B.