Inconsistent Rotary Stepper positioning
I added a rotary axis stepper unit to my existing 3 axis all servo machine. The problem I ma having is that the rotary will not accurately repeat its rotational position. If I rotate it 3600 deg (10 turns) is it actually at least as repeatable than if I rotate it 90 or 180 etc. The number of steps calculates properly as verified by the 10 turn test. The error seems to be random and in both an overshoot and an undershoot of anywhere from 0.1 deg to 0.5 deg. The backlash is much less than that so that does not explain the issue. I purchased the shielded cable to replace the current unshielded one hoping it will help. Using Automation Direct Driver and Stepper running on its own 48V power supply. Incidentally, this is occurring during testing without any of the servos or VFD operating so it will likely only get worse. Hoping you can help with any thoughts on what may be causing the inconsistent positioning? Noise seems a likely culprit based on the randomness, but I checked for ground loops, grounded the PS negative to the earth grounds, have all cables as short as possible, and have ordered a shielded cable for the stepper drive that should be here soon(but I don't suspect that is the issue)
Re: Inconsistent Rotary Stepper positioning
Would you expand a little on how to increase the drive gain as I am not sure what parameter that is in the configuration file. I am using a Shopsabre and the following are the settings in the WinCNC.ini file for the rotary axis:
[4th axis stepper Automation Drive With Gear Head]
axisspec=p4 r694.505 a7000 k4 t4 o0
axisvel=r5000 f5000 s5 m50 h1000
maxtrans=10000
From your experience wouldn't the error be multiplied for more revolutions which is not the case for my setup
Or is it in the dip switch settings on the driver. I don't recall reading that term anywhere in my research on the issue.
Also how do I determine what stepper would be superior to the one I have? or do you mean one with encoder feedback?
Thanks for the responses
Re: Inconsistent Rotary Stepper positioning
I don’t think this is a stepper issue. Check the steps per unit again. Do it u til the rotary table moves exactly 360 degrees, then test is again several times. You can also test stepper motor without rotary table, this could be looseness in rotary table
Re: Inconsistent Rotary Stepper positioning
I had a similar problem when I built my stepper driven RT. The worm shaft was binding, so I installed thrust ball bearings on either end of it. Solved the problem.
I started with a 6'' Vertex super spacer that was designed to be hand cranked. It really didn't like being powered, so required a little re-engineering.
The odds of adding steps when moving is pretty much zero unless you have a lot of noise on the step signal line. I have really never seen this. Mechanical binding causing lost steps is very common.
Re: Inconsistent Rotary Stepper positioning
Quote:
Originally Posted by
CNCMAN172
I don’t think this is a stepper issue. Check the steps per unit again. Do it u til the rotary table moves exactly 360 degrees, then test is again several times. You can also test stepper motor without rotary table, this could be looseness in rotary table
If it was the steps I would not be seeing an over run and an under run condition. Also it would be magnified when I command it to go 3600 (10 turns)
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Re: Inconsistent Rotary Stepper positioning
Hi,
I have recently made a fourth axis out of an Atlanta Drives worm drive, <2 arc min lash. Just for context the chuck is 100mm diameter.
Initially I hade a repurposed Vexta 5 phase stepper running it......but it was too slow, or rather when I ran it at a decent speed the stepper was inclined to lose
steps. I'd used these steppers on a previous machine and they have been superb, but the viscous drag of these gearboxes was just too much, so I bought a 750W Delta servo (pictured),
and it works a treat and way WAY faster.
Craig
Re: Inconsistent Rotary Stepper positioning
Quote:
Originally Posted by
Jim Dawson
I had a similar problem when I built my stepper driven RT. The worm shaft was binding, so I installed thrust ball bearings on either end of it. Solved the problem.
I started with a 6'' Vertex super spacer that was designed to be hand cranked. It really didn't like being powered, so required a little re-engineering.
The odds of adding steps when moving is pretty much zero unless you have a lot of noise on the step signal line. I have really never seen this. Mechanical binding causing lost steps is very common.
Since I am seeing an overrun condition at times, it is likely not a binding issue I believe
Re: Inconsistent Rotary Stepper positioning
Quote:
Originally Posted by
servtech
Not necessarily, it may not be a cumulative error and simply a mispositioning error.
I don't have a parameter list for your set up, so how to increase the gain I'm not sure. That's why I suggested swapping or changing the motor as a first step. Some drives have a gain potentiometer some don't and may have a parameter.
I chased a mispositioning stepper rotary axis many years ago, using an oscilloscope and tweaked the drive to stabilise the sine wave, but in the end the motor and drive weren't up to the task.
I will call Automation Direct and see if they can steer me to the drive gain since they sell the driver. It is a I have a scope and can take a look at the sine wave for distortion. Any particular form I should be looking for and which of the servo lines did you monitor? The stepper is a 1288 oz/in 6.3A unit so it seems like the torque is there, I don't have another motor to try
Re: Inconsistent Rotary Stepper positioning
Hi,
if this is a stepper then there is no sinewave. AC servos, yes, they have sinusoidal voltage applied to them, but not steppers.
They have a current limited PWM voltage applied. I would expect to see the full voltage of the supply, whatever that is, 70VDC say, for several microseconds until the current reaches its rated 6.3A thereafter
the voltage with be reduced by PWM down to that level that maintains the 6.3A current. There is no 'gain parameter', a servo drive with a PID loop yes, but not an open loop stepper.
Craig
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Re: Inconsistent Rotary Stepper positioning
Quote:
Originally Posted by
GTOGuy
I will call Automation Direct and see if they can steer me to the drive gain since they sell the driver. It is a I have a scope and can take a look at the sine wave for distortion. Any particular form I should be looking for and which of the servo lines did you monitor? The stepper is a 1288 oz/in 6.3A unit so it seems like the torque is there, I don't have another motor to try
Depending on which drive you have, there is no gain adjustment. I think the 80100 drive has adjustable gain, but the rest only have DIP switches to set the current and steps/rev. I just recieved a couple of 48V EM566S drives that are programmable, but I haven't played with them yet. Those will be used with NEMA 23 motors in a very light duty application.
48V might be a bit light for that motor, that's a large NEMA 34 motor and it would be a lot happier running on 70-80V, but you are limited by the drive input voltage maximum. The torque drops off pretty fast as the RPM increases.
https://www.cnczone.com/forums/attac...d=489868&stc=1
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Re: Inconsistent Rotary Stepper positioning
https://www.cnczone.com/forums/attac...d=489882&stc=1
Quote:
Originally Posted by
joeavaerage
Hi,
if this is a stepper then there is no sinewave. AC servos, yes, they have sinusoidal voltage applied to them, but not steppers.
They have a current limited PWM voltage applied. I would expect to see the full voltage of the supply, whatever that is, 70VDC say, for several microseconds until the current reaches its rated 6.3A thereafter
the voltage with be reduced by PWM down to that level that maintains the 6.3A current. There is no 'gain parameter', a servo drive with a PID loop yes, but not an open loop stepper.
Craig
If I scope the line, with one scope input on voltage and the other on a current probe, I should see the voltage go to full V (48V in my case) then drop as the current line reaches the 6.3A level and hold for the duration of the pulse? Which lines of the stepper should I monitor for V and A? This is my 1st experience with stepper drives so I am not too familiar.
Do you think there is a benefit in reducing the microsteps on the drive? It is currently set to 5,000/rev. (I believe I attached a picture of my available settings on my drive)
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Re: Inconsistent Rotary Stepper positioning
Hi,
using a scope to get useful information about a stepper is not as easy as you might think.
Firstly the voltage on each winding reverses polarity, thus if you have the ground of the scope tied to one of the windings, A+, say then you are inviting the power supply to
clash with the ground of the scope. Thus you need an isolated scope. Many handheld scopes are isolated and it is a useful feature.
More normal AC mains input scopes the scope ground is tied to the electrical system ground and that is where a power supply and scope can fight. In those situations, and many others besides,
I use a differential probe. They are mandatory if you are working on mains powered equipment that derive DC from direct off-line, ie no transformer. The negative side of such a power supply
will get to 0VDC relative to electrical ground, but then fall away to 320VDC negative relative to electrical ground. 320V is the peak voltage of 230VAC. If you hook the ground of your scope to the negative
of the DC power supply your will blow your scope to bits and/or the equipment you are working on.
Before you use a scope on any mains powered equipment you absolutely MUST establish whether the power supply is galvanaiclly isolated. If it is then carry on, if its not then you must use isolated scope or a
isolation transformer on the input side of the equipment, or a differential probe. Further you must be aware that the circuits you are working are not isolated and risk of electrocution is high if you touch
anything.
Just as a matter of interest any modern AC servo drive that is directly fed by incoming 230VAC derives its 320VDC power supply direct offline and thus the output of the servo drive, namely the three phase output
voltages to the servo swing 320V around electrical ground at 50Hz or 60Hz depending on your supply. You'll get a very VERY nasty surprise if you hook the ground of your scope to one of those!
The downside is that these probe are quite expensive, I paid over $1000NZD for the one pictured. It has 1400V isolation and 20:1 or 200:1 voltage reduction. I worked a great deal on welders and many of those
use direct offline power supplies and it proved invaluable, unfortunately I only have one. For a stepper you really want two, one for each phase....but you use what you've got.
Craig
Re: Inconsistent Rotary Stepper positioning
I've had an issue before like this - here are my suggestions to look at:
1. Test at a slow speed - this puts the motor in its peak torque band (eliminates "binding" as a suspect)
The next one is what I had wrong and it was a really weird one
2. Look at the control system and how long are the step pulses that are sent? Try making them longer. If too short they can be missed. What I had happening (with a cheap drive) was that effectively only 3 of the 4 micro steps were being picked up, but during accelerations and deceleration it picked up more of them - so it looked like overshoot, but really was way under, unless driven very slowly. (see #1 above). this can happen when trying to use a driver that may be capable of very fine micro stepping, but it can't handle the resulting high frequency for your desired operating speed. There can also be issues with the cabling etc. with very high step frequencies but I never went down that rabbit hole.
