Reality of Closed loop steppers vs standard steppers for Milling Machine?

Theoretically closed loop steppers should be better for direct drive of milling machine axis but..

Has any-one got experience of both and what differences did you notice?

False...
A closed loop stepper of the same rating (i.e. 1200 ozin) will provide the exact same motion or propulsion of the machine. That said, most closed loop systems will exert 100% torque of the motor once a threshold (usually one full step) has been crossed. Depending on the actual torque being used at that moment, this may get the axis baxk on position, but not often enough. Should the closed loop stepper exceed a user set threshold, the drive will "alarm". If connected properly, that alarm will stop motion for all axes.

Other than ability to stop motion and allow the operator to make a correction, the cutting ability between the two are insignificant if the toolpath parameters are within the torque limits of the motors. The reality is, if you muff the toolpath parameters enough to loose steps, you will loose steps. The closed loop system should stop, not so much for the plain stepper.

Hi,
if you have an overload situation an open loop stepper will stall.....a closed loop stepper will stall and for exactly the same reason....overload.

A closed loop driver does not make a stepper faster or more powerful, it will try to introduce more steps if it needs to 'catch up' but 'catch up' means
that the stepper has been overloaded.

A closed loop stepper does have better resolution. Open loop steppers seldom achieve greater resolution than half stepping no matter how much microstepping
you employ. Closed loop steppers can however stop and hold a position between steps by virtue of the encoder feedback.

Increased resolution is the only genuine advantage you get with closed loop steppers and for the premium you pay I don't think they are worth it.
For the same money you could buy even more capable open loop steppers and drivers or with a little more money you could get a genuine AC servo.

Craig

If an open loop stepper looses position - say 8 microsteps does that error continue to exist until the machine is re-set?

I guess a closed loop (hybrid) stepper can only start to correct things if it in out 1/1000 of a rev with a 1000 line encoder. For 5mm pitch ballscrews that is 0.05mm - quite a lot really so maybe it is best to spend the money on open loop steppers with more torque and higher voltage PSU's so the torque does not drop off as fast...?

Is there any way to tell how much torque a stepper is using (other than it will loose steps if it runs out of torque and I don't know how you find that out?)

You might want to check your math, I think that should be 0.005 mm with a 1000 line encoder. And in fact if the encoder is full quadrature and the drive can read that, then the pulses / rev would be 4000, so 0.00125mm resolution with a 5mm pitch lead screw. Please note, resolution and accuracy are not the same thing. Resolution is a best case possibility, normally there is some position error, a few pulses, but normally not something you would be able to measure in the home shop.

There are two possible scenarios here.

When microstepping, even with open loop drives, the motor can lag behind some number of microsteps and still catch up at half or full steps.
This is different then losing steps.

If you do indeed lose steps, you don't get them back, and don't regain position until you re-home the machine.

If an open loop stepper looses position - say 8 microsteps does that error continue to exist until the machine is re-set?

Yes

I guess a closed loop (hybrid) stepper can only start to correct things if it in out 1/1000 of a rev with a 1000 line encoder.

In most cases it takes a full step (1/200th of a rev or 5 encoder counts) to trigger the torque increase, if available. The motor may have been using full torque at the time of loss
For 5mm pitch ballscrews that is 0.05mm - quite a lot really so maybe it is best to spend the money on open loop steppers with more torque and higher voltage PSU's so the torque does not drop off as fast...?

I would do both

Is there any way to tell how much torque a stepper is using (other than it will loose steps if it runs out of torque and I don't know how you find that out?)

In the real world, no. But based on the torque curve of the motor and the published reductions, you should be able to approximate. The real answer would only be relevant if you knew exactly what torque your cutting process required, which would be rare

Quote:

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Originally Posted by Jim Dawson

You might want to check your math, I think that should be 0.005 mm with a 1000 line encoder. And in fact if the encoder is full quadrature and the drive can read that, then the pulses / rev would be 4000, so 0.00125mm resolution with a 5mm pitch lead screw. Please note, resolution and accuracy are not the same thing. Resolution is a best case possibility, normally there is some position error, a few pulses, but normally not something you would be able to measure in the home shop.

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I think this was based on:
5mm per rev
1/1000 accuracy
So 5/1000
Yup you're right 0.005
Which is a lot better

I think you may be conflating the features of servos that have encoder feedback to the controller with the system that is used on stepper motors. There are some similar actions and terminology, but for the most part very different. Which makes sense as they are in very different price categories.