Stepper detent torque when half-stepping

jmknapp · 2013-07-02 05:07

I have a basic question about stepper detent torque and half-stepping. Is it the case that there's no detent torque holding the rotor at the half-step points? I.e., when the coils are deenergized, the rotor will tend to slip to the full-step position?

LoopyByteloose · 2013-07-02 06:16

I'd love to know the answer to this question as well.

My intuition tells me that half-step and other micro-step detents are something less than full step detents.

Have you noticed that without any power a stepper motor will seek positions of detent based on the physical magnets? I presumed these are the full step detents.

prof_braino · 2013-07-02 07:12

How do we test this? Do we run it at gull step, nd then de-energize and turn the shaft two steps? Then run it as an even number of half steps and do the same? Would we feel the difference?

Beau Schwabe · 2013-07-02 08:41

To overcome the natural detents of each full step, the stepper needs to always be powered through half stepping or micro stepping.

Reference video:
http://www.youtube.com/watch?v=dmk6zIkj7WM
http://www.youtube.com/watch?v=bngx2dKl5jU

Hal Albach · 2013-07-02 08:51

I tend to think that when a stepper is being half stepped or quarter stepped or whatever fraction and then stepping is stopped, the inertia of the load will probably help push the motor towards the next detent in the absence of reduced holding current. If the final position of the motor absolutely must be where the stepping action has ended then I would think that some provision must be made for supplying sufficient holding current to prevent unwanted / undesirable motor movement. If the load has considerable mass then probably use full current to hold the motor until momentum has stopped and then reduce the current. This will also prevent the motor from stepping erratically when stepping continues since the translator thinks the motor is in the last commanded position when in fact it had moved on its own to the next position.

tonyp12 · 2013-07-02 09:27

>the stepper needs to always be powered through half stepping or micro stepping.

That is why you need to use a stepper driver IC that after a initial allowed inrush current holds it at a fixed level.
You may need to install ~0.2 Ohm current sense resistors etc.

Phil Pilgrim (PhiPi) · 2013-07-02 14:13

The unpowered detents in a stepper motor appear to occur every two full steps, not every full step. I put a knob on a 48-step-per-rev motor and counted the detents in one revolution. There were 24.

-Phil

jmknapp · 2013-07-02 16:49

Thanks for all the replies. I checked a couple of steppers in my bone pile and I think the number of detents felt by turning the rotor matches the number of full steps in their respective specs. Maybe the 48-step-per-rev motor above was specced in half-steps?

Phil Pilgrim (PhiPi) · 2013-07-02 16:59

jmknapp wrote:

Maybe the 48-step-per-rev motor above was specced in half-steps?

Nope. It's a motor I've used a lot, so I know it's 48 full steps per rev.

-Phil

prof_braino · 2013-07-03 06:55

jmknapp wrote: »

... I think the number of detents felt by turning the rotor matches the number of full steps in their respective specs. Maybe the 48-step-per-rev motor above was specced in half-steps?

how many wires are on each motor?

whicker · 2013-07-03 08:16

I think the more precise term for this is "cogging".

The cogging torque on any kind of permanent magnet motor will want to rotate the output shaft when unpowered where it can overcome friction and the other forces present.

This has nothing to do with how much torque is generated while powered. It does effect torque ripple while commutating, however. A motor that has been built not to cog will spin at a more consistent velocity at any speed with a fixed load.

Stepper detent torque when half-stepping

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