Understanding Steppers

Does a stepper motor have any greater torque potential when microstepping versus full stepping?

Also, should there be any change in potential "top speed" by changing to a half, quarter or eighth step?

The tests that I've done so far seem to suggest no for both of these questions, so I'm wondering if the only difference in microstepping is just greater resolution and higher power consumption.

Thanks,
JHoff

The torque for micro stepping is about 0.7 that of full step but you won't notice with a driver like the A3977 because it does not apply full current in the full step mode, so the torque is the same. I.e. it applies 0.7 times the current to each coil.

In half step mode it will apply the full current when one coil is on and 0.7 when two are on. That keeps the torque constant as it rotates. Simple constant voltage drives always apply full current so the steps with two coils on have more torque. So even in half step mode, a micro-stepping driver is already smoother.

The reason for the 0.7 factor is as follows: If you consider the holding torque when one coil is on then the pole piece of the rotor is lined up with the stator so you have the maximum force resisting movement. When you displace it away from that position the magnetic gap widens and the torque falls off roughly sinusoidally. When you get halfway to the next single coil position the torque from one coil is sin(45) which 2^0.5 = ~ 0.7. In full step mode with both coils energised the holding torque is twice that, i.e. 1.4 times the single coil value. Dropping the current by 0.7 makes the two coil torque equal the one coil value.

For further sub divisions the currents in the two coils are sinusoidal, 90 degrees phase shifted.

The power consumption is not higher unless you compare it to wave mode when only one coil is ever on.

I think top speed is similar if your micro can keep up with the step rate. Maybe full step has the edge.

The smoothness of micro stepping is a big advantage even if you don't need the resolution. Resonance effects are far less.

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[www.hydraraptor.blogspot.com]

... i learned that with microstepping i can get much higher clock-frequencies at smaller steps.

With my 1/256-microstepping drivers and max 10kHz clock frequency i have an 'optimum' around 1/16 microstepping at 10kHz versus max. fequency below 2kHz at fullstep (depending of resonancy and mass-loads) or 6kHz at half-stepping with my Isel-drivers.

Recently i've got four motors/drivers with 1/50-microstepping and 200kHz max. Clock-rate - here i have a complete new speed-profile, and will make some tests for defining the practical limits ...

Viktor

Microstepping doesn't technically provide more torque, but it does give you smoother movement an less likelihood of slipping when torque is required, so below the motor's regular stall torque, it *behaves* like it has more torque.

there's also mid-band resonance to take into account. This is where, at a particular speed, your torque drops to almost zero due to various resonance effects. I read that microstepping all but eliminates this issue.

btw, 1 amp into one coil should provide the same holding torque as 0.7 amps (actually sqrt(2)/2=0.707...) into each of two coils.

Excellent. Thanks for the help guys.