What effect does microstepping have on torque?

[Stuff-Builder]

Hi All,

I'm using Mach2 to run my Taig mill, and so far things are going pretty well. I would like to be able to run things faster though.

I currently have my drivers (Compumotor A-Series) set up at 10x microstepping. According to my number crunching this gives me a resolution 0.000025" per step. Running Mach2 at 25 kHz, this shoud give me a max speed of 37.5 in/min. To me, this seems to be ludicrous resolution and sorta pokey.

If I switch the drive to half-stepping, that will give me a resolution of 0.000125"/step and max speed of 187.5"/min. This seems both much speedier and still more accurate than my skills can handle.

My question is what effect does microstepping have on output torque? Are there going to be any bugga-boos (technical term) that arise from running half-steps? Would this have any effect on motor heat?

Thanks for the help,
Scott

[ger21]

Microstepping will usually give a little less torque. But the motors run smoother. half step would maybe give you slightly more torque (wer'e talking maybe 5-10%), but the motors may not run as smoothly. It's easier to miss steps with a rougher running motor, though. Usually you'll be better off with microstepping. But every situation is different, and you might be better off with half step. Trial and error is the only way to know for sure.

[eman5oh]

You may intodure resonace to the system when going from 10x to half steps. Like ger21 said trial and error.

[Stuff-Builder]

Gerry, thanks for your reply.

I am finding experimentation is a big part of my learning process right now. The other day I was slotting some aluminum with a 1/4" end mill at 4200 rpm and anything more than 4"/min feed rate was bogging down and missing steps. I dropped the spindle speed down to 2600 rpm and the mill ran 8"/min, no sweat. Amazing what little differences can make.

I think I'll try running it half-step and see what happens. I can always switch it back. It's just a couple of DIP switches and config changes.

Scott

[DDM]

That seems kinda odd about the slower spindle speed and the higher feed rate but who knows there are some endmills out there for high feed rates and meant to go on older CNC mills that don't have the super high spindle speeds. The other thing is getting those chips out of those slots, It'll kill ya and your endmill. After a while sound will tell you what your machine needs.

Carl

Sorry it's a little off topic but the torque and microstepping caught my eye

[Sinistersam]

Thats when chip load/tool resonance comes into play. To me, 2600 still seems a bit high. Depends on the depth your cutting,and quality of material though. Sometimes you can get a better cut from a 2-flute or a 60˚ helix, especially on a slot. Oil based coolant(WD-40, Marvel oil, etc.) will work wonders on tool life and finish surface quality too on aluminum. Once aluminum coats the flutes, everything goes to hell in a handbasket in a hurry. A good oil spritz will help prevent this. Prolly already knew this, and has nothing to do with the original post subject, but thought I'd pass it along anyhow.

[Mariss Freimanis]

It's a give and take kind of situation:

1) For the same peak current, a microstepped motor will have 71% (1/sqrt 2) the holding torque of a full-step drive. This is because motor torque is the vector sum of the phase currents. Advantage: Goes to full-steppers.

2) Most people want motors to turn, not just 'hold'. As soon a full-step driven motor turns, its torque drops to 65% of its holding torque. Where did the missing torque go? To resonating the motor is where. Motor mfgs sometimes specify 'dynamic torque'; this is specified at 5 full steps per second. It is always between 60 to 65% of holding torque. Not mentioned is the horrible racket the motor makes at 5 full steps per second.

Microstepped motors do not resonate at low speeds, so no torque is invested in resonance. Microstepped motors keep all their holding torque while turning slowly. 65% for full-steppers, 71% for microsteppers. Advantage: By a hair (6%), goes to microsteppers.

3) Things get a little dicy as speed increases. Microstepping ceases to have any benefit above 3 to 4 revolutions per second. The motor is now turning fast enough to not respond to the start-stop nature of full steps. You can say the step pulse rate is above the mechanical low-pass frequency limit (100Hz or so) of the motor. Motion becomes smooth either way.

Simple drives persist in microstepping anyway above this speed. This means they still try to make the motor phase currents sine and cosine past this speed. A little problem with that and it's called 'area under the curve'. The area under the sine function (0 to 180 degrees) is only 78% of a square wave (full-step). Advantage: Goes to full-step again.

More sophisticated drives transition from a sine-cosine currents to square-wave quadrature currents about then. Same as full-steppers. Advantage: Draw.

4) As peed increases even more, another really big problem crops up; mid-band resonance. This is the bane of full-steppers and microsteppers alike.

Maybe you have experienced it; the motor is turning 5 to 15 revs per second when you hear a descending growing sound from the motor and then it stalls for no good reason at all. Faster it's OK, slower it's OK, but not OK in that range. All you know is there is a big notch in the speed-torque curve. This mid-band instability, or parametric resonance.

Simple drives have no defense against this except to try not run the motor in this speed range. Better drives have circuitry to suppress this phenomena and it involves rate damping.

This is the equivalent of shock absorbers (rate dampers) on a car, without them a car bounces repeatedly. Imagine a washboard road surface in sync with this bounce; there would be sparks flying from the undercarriage in short order. With rate dampers the 'bounce' is suppressed to a single cycle. Mid-band compensation does the same with steppers.

5) More than any other type of motor, step motor performance is tied to the kind of drive connected to it. More than any other type motor, a stepper can be driven from very simple drives (full-step unipolar L/R) to very complex ones (microstepping full-bridge bipolar synchronous PWM mid-band compensated).

Motor performance will range from "Miserable, give me a servo, I'll never use another stepper again" to "What is the big deal about servos anyway, this is just as good."

It's ALL in the drive.:-)

Mariss

[Ryobiguy]

Thanks for the details, Mariss. I've been passively reading all I can on steppers for the last few years and havn't come across those datapoints yet.

[Xerxes]

Originally Posted by Mariss Freimanis More sophisticated drives transition from a sine-cosine currents to square-wave quadrature currents about then. Same as full-steppers. Advantage: Draw. 4) As peed increases even more, another really big problem crops up; mid-band resonance. This is the bane of full-steppers and microsteppers alike. Maybe you have experienced it; the motor is turning 5 to 15 revs per second when you hear a descending growing sound from the motor and then it stalls for no good reason at all. Faster it's OK, slower it's OK, but not OK in that range. All you know is there is a big notch in the speed-torque curve. This mid-band instability, or parametric resonance.

Hmm. I thought mid-band resonance was caused by the uncontrollable current-to-voltage drive transition. Is it not?

Or is it just mass-spring resonance (torque-inertia)? Wouldn't a perfect vibration free sine drive make this mid-band resonance go away?

I did some experiments with mechanically unconnected stepper using various drives. When increasing stepper speed slowly I can hear multiple resonance points but never had such mid-band stall phenomenon. Disabling or enabling G201 resonance compensation doesn't seem to make much difference. Is there situations where mid-band resonance doesn't cause complete torque loss? I'm curious how this electrical damping works :-)

[Stuff-Builder]

Mariss, thanks for all the info! My drives are surplus Parker Compumotor A-Series drives. I understand they're pretty snazzy. I know there are all kinds of different settings for wave-form and such, but I haven't messed with that.

The other day I did switch from 1/10th step to 1/2 step. Seems to operate about the same overall - only MUCH faster. That first rapid move at 180 ipm was a bit shocking. I ended up at 108 ipm - the motors quit stalling then. Probably had to do with the 20 tpi lead screws. The mill runs great! I was slotting ).050" deep with a 3/16 end mill in aluminum at at 18 ipm and the machine seemed happy as can be. And a 0.015" climb cut finish pass left an awesome surface finish. Fun stuff!

So far I would say the experiment has been worthwhile.

Scott

[Mariss Freimanis]

It's actually system phase shift problem. A step motor is a mass-spring system meaning there is a 90 degree phase lag between torque and velocity. Think of a weight suspended from a coiled spring that has been pulled, then let go. Velocity is maximum when the restoring force is zero and visa versa.

At low speeds a step motor drive is a current source and contributes zero phase shift, making the loop phase lag 90 degrees. As speed increases, the drive has to revert from a current source to a voltage source when motor inductive reactance begins to limit current.

This adds another 90 degrees of phase lag, making the system phase lag 180 degrees. This is when trouble (mid-band resonance) begins. A 180 degree phase lag results in undamped and building oscillation; the motor stalls once the oscillation amplitude reaches +/-1 full step.

To stabilize the loop, a phase lead (derivative) component must be added. This takes the form of a rate of motor load change sense by the drive which is summed to the loop and adds about 70 degrees of phase margin.

Mariss

[Xerxes]

Thank you Mariss!

I think I understand it much better now. :-)

So mid-band resonance doesn't occur when coil current is fully under control. I guess it would be possible to avoid mid-band by increasing drive voltage enough. I have experimentally noticed that resonance frequency increases while I increase supply voltage.