A question about microstepping

[stragenmitsuko]

Silly question maybe ,

Example : If I have a small stepper motor , let's say a 100step/rev like those old steppers from harddrives .

Then if i drive it full step i get 100steps / rev or 3,6°/step
Right sofar ?

Now there are a lot microstepping drivers that will divide a step in 4 ,8 sometimes 16 microsteps .
I've understood that these microsteps run smoother , produce better torque etc .
But does this also mean that fi a 16 microsteps driver will produce 1600 steps/rev on the motor mentioned above .
And that each of these 1600 positions can be held in position .
Or is it still thesame 100 steps , with each step taken in 16 microsteps ,
but only each 16th position being stable .

Pat

[pminmo]

Actually microstepping produces less torque than two phase full step. yes you can microstep a 2 phase 3.6 degree stepper, same logic applies. http://www.pminmo.com/wiki/index.php?title=Stepping

[stragenmitsuko]

Thx Phil .

Are there to your knowledge drivers available that wil operate in full or half step , but take the step in microsteps .

This way the motor still need 200/pulses / rev , like in fullstep mode . And the driver would execute each step in 4 8 or 16 microsteps for smooth running .
Just thinking out loud , maybe I'm talking nonsens

Pat

[mugabe]

dont know whether there are such but this could be done if some issues related to next step arrive time prediction will be solved (microsteps could be produced as fast as possible but motor acceleration characteristics must be known a priori ).

of course it is good to have such to overcome step output rate limitations of emc mach3 or turbocnc alike tools while staying away from low frequency resonance.

[Madclicker]

Thx Phil .

Are there to your knowledge drivers available that wil operate in full or half step , but take the step in microsteps .

This way the motor still need 200/pulses / rev , like in fullstep mode . And the driver would execute each step in 4 8 or 16 microsteps for smooth running .
Just thinking out loud , maybe I'm talking nonsens

Pat

This is how the Gecko 201 works when you add the G901 step multiplier. The 201 is a 10 microstep drive and the add-on 901 is just a PLL based step multiplier.

[mugabe]

imho i dont think it is pure pll.

[pminmo]

To do as you describe would still give you the lower torque of microstepping. Other than Gecko's morph to full step from microstepping I know of none.

[kreutz]

If I have a small stepper motor , let's say a 100step/rev like those old steppers from harddrives .

Then if i drive it full step i get 100steps / rev or 3,6°/step
Right sofar ?

Now there are a lot microstepping drivers that will divide a step in 4 ,8 sometimes 16 microsteps .
I've understood that these microsteps run smoother , produce better torque etc .
But does this also mean that fi a 16 microsteps driver will produce 1600 steps/rev on the motor mentioned above .
And that each of these 1600 positions can be held in position .
Or is it still thesame 100 steps , with each step taken in 16 microsteps ,
but only each 16th position being stable .

It will hold those positions, and there are effectively 1600 discrete steps per revolution in your example, but,..... unless you made an specific micro-stepping table for your motor, those micro-steps will not be equally spaced, neither they will be in the same position when reversing the movement. Only those positions corresponding with a mechanical pole of the motor (any of the 100 positions on your hypothetical motor), will be truly statically accurate and reproducible (within manufacturer's tolerance, and depending on the load).

Some Stepper Drive manufacturers will make the fine tuned tables (forward and reverse tables) for your motor, if you need that precise positioning.

[Mariss Freimanis]

OK, a couple of different subjects so let me take them in turn:

1) "Microstepping gives less torque." This is a little fiction and a little truth.

1a) Truth: A 100 in-oz motor with a full-step drive gives 100 in-oz holding torque. A microstepping drive gives 71 in-oz holding torque on the same motor at the same current.

Fiction: You don't buy motors and drives for their holding torque. If you did a bolt with a rusted-on nut would be a better bet for the +20,000 in-oz of torque it would take to loosen it.

You buy motors and drives because they produce torque while turning. Let's see what happens when a full-step drive and a microstep drive begins to turn a step motor:

Full-step drive at 5 full-steps/sec on a 100 in-oz motor gives 65 in-oz.
Microstep drive at 5 full-steps/sec on a 100 in-oz motor gives 71 in-oz.

What happened to the missing 35 in-oz on the full-step drive? Where did it go? Well, it was invested into vibration and resonace to shake, rattle and roll as all of you who have the pleasure of using full-step or half-step drives know. A full-stepping big motor rattles the fillings in your teeth. Score: Microstepping drive 1, full-step 0.

1b) Ok. I'm past the low-speed unpleasantless now. At high speeds my full-step drive beats the pants of off XYZ Inc. microstepping drive. How come?

How come is because XYZ Inc. is still having their drive microstepping at high speeds. That initial 71% torque deficiency comes back to haunt them like a ghost from Chirstmas past. Microstepping loses all benefit at speeds above 3 revs/sec. It becomes a millstone around your neck past 4 revs/sec.

Solution: Simple; stop microstepping above 4 revs/sec. Our drives morph from microstepping below 4 revs/sec to full-stepping by 6 revs/sec. Morphing is completely transparent to the user; you don't notice anything when it happens. You get full-step power at high speeds and microstep smoothness at low speeds. You can have your cake and it it too.

1c) I have to mention one other big power-robbing bad motor behavior. Mid-band resonance, mid-band instability or parametric resonance. You have seen it with XYZ Inc or ABC Inc drives. You get your motor up to 5 to 15 revs/sec when you hear a descending low-frequency pitch sound from your motors and then they stall for no good reason at all a second or two later.

You are the victim of mid-band instability. You try all sorts of things. You add friction, you try to remove it. You add load inertia, you try to remove it. Nothing helps; the motors growl, then they stall for no reason. All you can do is to accelerate past the "death zone speeds" and then everything is OK again.

This is a phenomena we understand completely and all our drives incorporate the 2nd-order compensation circuitry to completely eliminate it. There are no "death zone speeds" with our drives. The motors are well behaved at all speeds from zero to 6,000 RPM.

2) "Full-step is OK, 10 microsteps is better, 1,000,000 microsteps is best."

Fiction: The finer the resolution (the higher the microsteps/step) the better the performance.

Fact: Step motors run open-loop. They are transducers and like all transducers, you depend on the accuracy of the transducer for the accuracy of the final result.

Standard step motors have a non-accumulative error of +/-5% of a full step. This defines their ultimate accuracy. Think of it this way. You have to machine a 1" cube. You have to hold a +/-0.00025" tolerance and the cube you machined would be accurate within 1:2,000. That's how accurate step motor are.

For a step motor to be that accurate, the drive must be accurate as well. It requres a drive accuracy better than 1% (sin/cos distortion) and our drives deliver that. A good motor coupled with a good drive delivers good results.

Audiophiles will understand this analogy. The best NAD stereo amplifier will sound like crap with a pair of $5 computer speakers. The best Axium speakers will sound like crap with a computer audio amplifier. You put a NAD amplifier together with Axium speakes and it's molten sweet smooth honey to your ears.

Same thing here. A 3.6 degree motor from a long forgotten 10MB hard drive is the same as a $5 speaker. It is a NEMA-17 piece of crap nothing can be done with. It was designed to be a full-step motor and no drive will make it be more than that. It is all it was meant to be and it is all it will ever be.

Good motors are NEMA-23 to NEMA-34. Some NEMA-17s (Vexta PK245) and some NEMA-42s (Bridgport) can be included. Stay away from round 23s and 34s, stay away from 'square' 34s with more than 900 in-oz holding torque and you will do OK.

Mariss

[pminmo]

If the controller issues one step pulse to a drive from a stopped position. The motor being driven by a full step drive will produce more torque than a microstepping drive.

I'm not knocking microstepping drives, not at all.

This whole subject is really a system design issue, and Mariss really brought that point home. I would add though to Mariss's answer these questions:
1. For the application is speed an objective?
2. Does the application involve start-stop inertia that wear could be an issue?
3. Is the system mechanically tuned?

It's not really a simple question/answer anymore. Intellegence is so inexpensive to put in drives, that the hybridization of step modes today and drive a vs drive b becomes more than just simple current/voltage/step size comparison. Gecko's speed vs mode is a prime example.