Author Topic: Unusual stepper issue on lathe  (Read 1144 times)

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Offline DMBGO

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Unusual stepper issue on lathe
« on: August 30, 2017, 10:04:22 PM »
A couple of days ago, I played with my VFD settings, I don't remember what I did, but I didn't think I had altered anything that would upset Mach3 on the lathe.
Today, I decided to bore a pulley, but when I started the spindle, I had no Z axis movement. I stopped the spindle and both axis are moving as per usual, but running a Gcode file generated with the boring wizard, or even starting the spindle with the spindle on button results in no motion from the Z axis. I have tested the X axis, it works as expected.
I am perplexed.
I am expecting it to be something I've done when playing with the spindle, but maybe not.
Thanks for your help
Dave
Cheers

David

Offline DMBGO

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Re: Unusual stepper issue on lathe
« Reply #1 on: August 30, 2017, 10:08:13 PM »
Ironically the reason for boring the pulley, is to replace the VFD with a servo and driver. Perhaps the issue is that the VFD has got its knickers in a twist because it doesn't want to be retired? It has worked well for 10 years.
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David

Offline DMBGO

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Re: Unusual stepper issue on lathe
« Reply #2 on: August 30, 2017, 11:28:36 PM »
A bit more info, since I've no replies yet (hint) The DRO for Z changes to reflect the commanded position, but when the spindle is moving the axis doesn't move. An example - Without the spindle running from the MDI with Z at 50 - I type the command G0Z150 this moves the Z axis to 150mm as expected. I type the command G0Z50 this moves the Z axis back to 50mm as expected. With the spindle running from the MDI with Z at 50 - I type the command G0Z150 this doesn't move the Z axis at all the DRO however updates as though the axis were moving. I type the command G0Z50 doesn't move the Z axis at all the DRO however updates as though the axis were moving.
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David

Offline DMBGO

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Re: Unusual stepper issue on lathe
« Reply #3 on: September 02, 2017, 02:08:12 AM »
I eventually worked out the cause of my problem, rather I should say, the solution to the symptom manifesting itself as the Z axis not moving when the spindle was running. The disable signal to the G203V stepper driver was going high, only when the spindle was on. The solution was to temporary remove the disable wire from the G203V. This isn't really an answer, but since I am in the process of installing servo motors and drives to replace the steppers and the spindle motor, it doesn't really matter any more. I have bored my pulleys, so I can fit my servos.
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David

Offline Tweakie.CNC

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Re: Unusual stepper issue on lathe
« Reply #4 on: September 02, 2017, 04:32:07 AM »
Hi David,

I am pleased to hear that you have solved the mystery (all on your own  :)).

It may be worth checking your Ports and Pins assignments just to make sure you don't have any duplicates.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline DMBGO

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Re: Unusual stepper issue on lathe
« Reply #5 on: September 02, 2017, 07:54:03 AM »
I initially thought along the same lines Tweakie, thanks for the suggestion  anyway. As I said it doesn't really matter now, but I'd like to know what caused it, if only for academic reasons. The strangest thing about the problem was that, although I didn't mention it in my last post, I fully reinstalled a fresh copy of M3 then used an XML file from a backup that I knew was working with the VFD in question. I suspected it was a wiring issue, but I wasn't prepared to fault find it any further.
Another strange thing was that it never reared it's ugly head when running a cycle, it only failed after a cycle had finished.
My theory, for what it's worth, is that the disable line on the Gecko was close to disabling the drive, and did so occasionally, but only kicked in after use, since, during a cycle the load on the power supply was sufficient to keep the voltage below the TTL threshold. We will however, never know.
Dave
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David

Offline joeaverage

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Re: Unusual stepper issue on lathe
« Reply #6 on: September 02, 2017, 06:19:45 PM »
Hi David,
from the way your talking you are of the opinion that this problem while sub-threshold has been there a while?

I have had the occasional noise issue as well and have found the practice of 'defining a load' on any given signal line reduces my problems no end.
By 'defining a load' I mean have the TTL output buffer be required to either source or sink a modest current in either state. Any noise current
impulse would then have to be larger than the standing current before the noise could attempt to change the logic state of the signal.

Take for example your disable input to your Gecko drive. Without looking at the specs its quite probable that the input impedance of the pin is 10kOhm
or more. If your BoB  is logic low and is dragging the input pin low it will have to sink 5V/10k=0.5mA or less. Thus if a noise impulse happens along with an induced
current of more than 0.5mA the pin will change state and cause problems. If the input pin were a lower impedance or deliberately paralleled with a resistor
to make it so, then the BoB would have to sink more current and the pin would be less sensitive to noise. Most Bobs will source or sink 15-20mA, some will
do considerably better than that. While I don't advocate loading a BOB output to the max I try to see to it that it is  loaded to about 5mA, ie an impedance
of about 1kOhm. If a 1kOhm resistor was fitted to your Gecko drive input to 5V then your BoB would have to sink a little over 5mA to pull it low.
Likewise it would take a negative going noise current impulse of at least 5mA to cause the pin to go low when in fact the Bob is inactive ie the signal is
meant to be logic high.

I have fairly recently used a servo and drive for a spindle motor and looking at the Allen Bradley documentation the step/dir inputs to the drive are about
10kOhm. I don't know what your drive is but guess it would be similar. You can see that the step/dir inputs to my drive are reasonably sensitive to induced
noise. I have elected to use a shielded twisted pair cable (one pair each for step and direction) rather than 'load' the servo input with a resistor.  I was concerned
that I may reduce the communication speed by excessive load and so tried to increase noise immunity by shielding.

It may be that other signal lines in your machine could benefit the idea of being suitably loaded.

Craig
My wife left with my best friend...
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Offline DMBGO

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Re: Unusual stepper issue on lathe
« Reply #7 on: September 03, 2017, 07:11:17 PM »
Many thanks for your input Craig
Many years ago, when I set the lathe up, I had problems with electrical noise. I was using a USB Smoothstepper and I think from memory (always a dangerous thing), that the option to alter the noise filtering of inputs was not in the driver at that stage. Time passed and I moved to an Ethernet Smoothstepper which had the "noise filtering of inputs" option in the driver, and in conjunction with the anti-interference measures that I had already put in place for the Usb Smoothstepper (shielded cat 5 cables for motor and VFD connections etc), the lathe worked well for years.
This was a new problem that only reared its head at the last moment when I started boring the pulleys for the servo motors.
Although I may have sounded rational in my posts, a little voice inside my head was saying " you know that it is malignancy on the part of the equipment that is causing this."
Cheers

David

Offline joeaverage

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Re: Unusual stepper issue on lathe
« Reply #8 on: September 03, 2017, 07:50:09 PM »
Hi,
the filtering settings are good but by then the noise has already 'got in', best to arrange it that either it doesn't
'get in' the first place or reduce a signal lines sensitivity if it does.

Craig
My wife left with my best friend...
     and I miss him!