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Author Topic: I seem to be gaining steps...  (Read 8208 times)

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Offline ethos

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I seem to be gaining steps...
« on: March 11, 2010, 04:36:39 AM »
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

Offline RICH

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Re: I seem to be gaining steps...
« Reply #1 on: March 11, 2010, 07:55:18 AM »
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

Offline Hood

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Re: I seem to be gaining steps...
« Reply #2 on: March 11, 2010, 08:10:47 AM »
Can you attach your xml please.
Hood

Offline ethos

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Re: I seem to be gaining steps...
« Reply #3 on: March 11, 2010, 08:59:03 AM »
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

Offline Hood

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Re: I seem to be gaining steps...
« Reply #4 on: March 11, 2010, 10:14:39 AM »
First glimpse of the xml looks fine.
Are you in the UK?

Hood

Offline ethos

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Re: I seem to be gaining steps...
« Reply #5 on: March 11, 2010, 12:13:46 PM »
Yes, South Wales.

Offline Hood

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Re: I seem to be gaining steps...
« Reply #6 on: March 11, 2010, 01:49:37 PM »
Could send you a loan of a G201 to try out if you wished?
Hood

Offline ethos

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Re: I seem to be gaining steps...
« Reply #7 on: March 11, 2010, 04:56:19 PM »
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

Offline Hood

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Re: I seem to be gaining steps...
« Reply #8 on: March 11, 2010, 05:20:47 PM »
Sort of sounds like the microstepping is not working properly, you wil maybe pick that up with the scope.
Hood

Offline ethos

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Re: I seem to be gaining steps...
« Reply #9 on: March 14, 2010, 03:04:40 PM »
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