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Messages - Scott Kelley

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General Mach Discussion / Inaccurate spindle speed reading
« on: June 13, 2011, 08:30:01 PM »
I installed a hall effect sensor on the spindle pully.  At 900 RPM, I get a pulse width of approximately 300 uS.  Up to about 900 RPM the DRO reads correctly, but at any speed much above this, the readout reads erratically.  The Index Debounce is set at 2, but that made little or no improvement from when it was set at 10.  I bumped up the kernel speed (to 45khz), and that improved things (previously, the rpm reading maxed out at something over 600).  

The numbers seem to indicate that this should work up to a much higher speed.  Any thoughts as to why it does not?

I suspect that widening the pulse width would improve results, and if I don't hear of some other solution, I will install a circuit to do that.  


Thank you!



I guess I need to know how to change the kernel speed outside of Mach.  I don't find anything called Kernel in Mach3Mill.xml, so evidently the kernel speed setting is called something else or held in another location...

What's the trick?

Scott Kelley

Right now it seems like that is where the problem lies for me as well.  Need to do some more testing to be sure.

I have neither differential inputs nor outputs.  The point that I was trying to make with my last statement is that I don't think that the problem is noise on those lines, because if that was the problem, it would have at least been improved by the dramatic changes that I have made while trying to run down this problem, and I have actually seen NO CHANGE at all.

Interesting... I will take a look at signals on both sides of the optos tomorrow.

Some more food for thought:

I just ran a test on the servo itself that indicates that the servo is registering fewer pulses in one direction than Mach is supposed to be sending.  (missing approximately 10-30 pulses per 30,000 sent).

The errors occur only in one direction - the count in the other direction is dead on every time.  Seems to me that the step pulse would be the same in either direction, so would expect the same error both directions...

The number of errors per inch decreases as the feed rate is decreased.

Changed step pulse width setting to 1 uS and tested, then to 5 uS and tested.  Did not see a significant difference.  Would expect 1 uS to cause significantly more problem if it was caused by slow circuitry.  But then, I was not looking at the signals, so can't verify  that the change even occurred.  

Since I started this, I have done many things to eliminate any possible noise on the signal lines - None of those changes has made a single bit of difference, and they are now nowhere near anything noisy.  


I would like a simple way to simulate on the parallel port, signals from Mach that would enable the charge pump and exercise one axis. 

The purpose is to have an output signal for testing that is known to be free of errors that may be caused by some kind of setup problem or other issue within Mach. 

Has anyone got anything available like this?

Scott Kelley

if your drive has opto isolation and the BOB also has it on the step/dir outputs it may be  the issue.

Yes, the BOB does have opto-isolation on the step & direction lines, and the servo driver also has opto-isolated inputs.  What is your concern re. this?

I have tried changing the active state.  As I recall, it seemed to make no difference.  But I did not do so while running the test that I described.  I will do that this evening in order to determine exactly what the effect is.

A lengthy description and discussion of this issue is posted here: http://www.cnczone.com/forums/servo_motors_drives/127477-axis_dont_return_zero_-.html#post935541

Basically, the axis' slowly gain counts and creep in the positive direction.  At this point, the remaining possible culprits seem to be the servo drive itself, and Mach3.

I would like to rule out Mach3 by simulating its output.  Is there something available that will provide the pulse train output for the charge pump, and the ability to exercise one axis?

Scott Kelley

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