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Author Topic: Slaving closed loop stepper motors  (Read 765 times)

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Slaving closed loop stepper motors
« on: April 19, 2019, 08:12:41 PM »
I have bought a couple of closed loop stepper motors off StepperOnline. Chinese company. This is to replace the Chinese steppers I have been using for many years.

Very strange issue though, and thought I might post here to see if anyone else has had a problem with closed loop slaved steppers losing steps. Yep, but not very many, lets say 1mm over a span of 500mm increasing exponentially.   (ooh did I spell that right? Ha)

System has the A axis slaved off the Y - and I think, this is the problem. Although I cant confirm that unless I put one on my X axis which I am reluctant to do.

But I have troubleshot this system to death (apart from the above). Infact to the point of hooking the drivers up to their stolen software (off Leadshine) to double check the resolution settings of the encoder and also the driver. To no avail, I get exactly the same fault. The closed loop steppers are cool, if I overcome them on standstill, they will drive right back to where they were. So thought I would post here prior to pulling them off.

Boyd
Re: Slaving closed loop stepper motors
« Reply #1 on: April 19, 2019, 08:32:52 PM »
Hi,
I am intrigued that you should replace open loop steppers that work with closed loop steppers that don't.

As you have discovered close loop steppers can lose steps just like open loop steppers and for exactly the same reasons.
While it is true that if unloaded a closed loop stepper will try to correct itself but is loaded it will try and probably fail,
I mean if it lost a step because of overload at speed then why should it respond any more favourably to another pulse
inserted in the pulse stream to correct it?

If you are going to replace steppers you should go for AC servos....no shagging around with steppers. Good AC servos
have come down in price and will wipe the floor with any stepper open or closed loop.

http://www.dmm-tech.com/

Does the stepper driver have a programmatic 'following error window'?. That is if the required position deviates from the
actual position it triggers an error. Some latitude between the two ( actual vs commanded) is required. If the demand is within
a few steps then the drive is highly likely to fault 'following error' whereas if the window of latitude is 50 to 100 steps then
it probably wont fault but at the expense of accuracy.

It may be that the window is set too wide and therefore accumulated error is causing the drift in position but still be less than
the error window.

Craig
My wife left with my best friend...
     and I miss him!
Re: Slaving closed loop stepper motors
« Reply #2 on: April 19, 2019, 09:11:30 PM »
Hi Craig. Well, I am the guy with the hair of a wire inserting 230vac into my system. So popped everything.

The machine was working but I wanted more speed, so when purchasing more drivers etc I upgraded. The closed loop steppers gave that speed without losing steps, its just they are inaccurate, but inaccurate with the same difference exactly whether its over 500mm or in the case of a job that went bad a hour operation (wasnt watching).

I guess I need to reclarify, these steppers are not losing steps. This I say because they are exactly the same, every time. Almost like the resolution settings are out. But I have set the resolution to what it should be. And even changed it to get the same incorrect results. 2000 steps, 5mm pitch, 400 steps per mm. Or, 1600 steps, 5mm pitch, 320 steps per mm. But I am getting something like 320 steps per 0.99998mm (guess) or in the case of 2000steps, 400 steps per 0.99998mm.

Now I could work out the proper maths to change the settings to get this setup perfect, but it just seems wrong to change the parameters from factory to suit the needs if you know what I mean. Something is not happy. And thought someone here may have the solution. If I need to I will go back to open loop steppers.
Re: Slaving closed loop stepper motors
« Reply #3 on: April 19, 2019, 09:20:01 PM »
Hi,
you are right that certainly sounds weird and I don't have any bright ideas.

If you were to consider buying something new then GET AC SERVOS. Those DMM's that I linked to earlier will blow
your mind. Steppers are Ford Model t's, servos are  2019 model sports cars.

Craig
My wife left with my best friend...
     and I miss him!
Re: Slaving closed loop stepper motors
« Reply #4 on: April 19, 2019, 11:44:04 PM »
Yep. I actually think there was a mistake, very small in the mechanical production on these motors. A shame really, they perform well. Except for the inaccuracy haha.

Thanks for the link. I have pulled off the rouges. And will look at the upgrade when I feel like working on my router again.
Re: Slaving closed loop stepper motors
« Reply #5 on: April 20, 2019, 02:03:57 AM »
Hi,
I don't now a huge amount about AC servos.

I wanted a high torque (5-6Nm) spindle at round 3000-5000 rpm for milling steel. My 24000 rpm spindle is very nice but
has low torque and very low power at low rpm, an impossible combination for milling steel.

I ended up buying an Allen Bradley 1.8kW 3500 rpm AC servo and matching servo drive second hand.

Talk about 'open my eyes' to the reality of modern AC servos. Its about 10 years old so its far from state of the art anyway
but the speed, accuracy and power just humble any stepper. That does not account for the multitude of control strategies
modern servos offer, torque, velocity and positions modes or a mix, indexing, in-drive oscilloscope tuning.....the list goes on.

Had I realized just how capable AC servos are I would have hung out and got them rather than steppers. Don't get me
wrong, steppers are good, but good doesn't compare to f*********king fantastic!

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