Controlling micro stepper motor

AllyCat

Senior Member
Hi,

The specification is incomplete (e.g the coil resistance) and somehow the 18 degrees/step (which I believe is correct) changes to 1.8 degrees/step in the listing, but (if they are the same as the ones that look very similar) then IMHO they are moderately "easy" to drive (for a low-voltage stepper).

I gave all the information that I could in this recent thread (which is waiting for some more input from that OP), but I do have plans to update my related PAM8403 thread in due course.

Cheers, Alan.
 

AllyCat

Senior Member
Hi,

As usual, we really need to know the application. But why not use a classic (model-maker's) Servo (or the "continuous rotation" type) ?
With ebay listings I always look at the last predicted arrival date (at which you can "complain" if it hasn't arrived - December ?).

Cheers, Alan.
 

Gramps

Senior Member
But why not use a classic (model-maker's) Servo (or the "continuous rotation" type) ?
Because of the shape and size of a standard servo. The space they need to go is very limited.
Four of these small motors would lie in the palm of shadow bot's mechanical hand.
The driveshafts would face the fingers, and be connected to threaded rods. Nuts on the threaded rods would connect to the cords that pull the fingers closed. All the electronics would fit inside the wrist area.
The feedback pots are slider type and would also be connected to the nuts on the shaft.
The stepper Motors in the first post are connected to a really nice assembly already constructed and have a place to bolt them down.
We might even attempt to remove a stepper motor and connect a DC motor in its place.
From what we read about stepper Motors they are low torque low amps and not really constructed to handle this kind of a load.
 
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AllyCat

Senior Member
Hi,
We might even attempt to remove a stepper motor and connect a DC motor in its place.
From what we read about stepper Motors they are low torque low amps and not really constructed to handle this kind of a load.
The leadscrew in that stepper looks rather integral with the motor and it's not clear if the "nut" is made of plastic or metal. The leadscrew has a 2 mm diameter which would normally have a pitch of 0.4 mm, so about 80 revolutions from end to end. A rough count on the photo doesn't look too far from that. I got about 8 revs/second from a similar stepper, so maybe 10 seconds end-end. I measured the shaft torque up to about 2 gm.cms and estimate the leadscrew mechanical advantage around 100 : 1, so perhaps you'd be able to lift up to around 100 grams. However, the linked dc motor gives only around 1.5 revs/sec so it would be the best part of a minute to travel end to end for the same leadscrew. And its torque so high that it could probably destroy the "nut" even if it's made of brass or copper.

A micro servo like the HXT500 has a similar thickness (11 mm) to both of those motors, but is about twice as wide so you would need to stagger them somehow, and I'm not sure what linkage you'd use (or whether as a "real" servo or continuous rotation). Their torque is around 600 gm.cms and speed about 2 revs/sec., with no need for any additional electronic components (at least in their closed-loop form).

It really depends if your "fingers" are just required to "move" (so the stepper solution might be fine) or to perform "manipulation" (where a Servo is likely to be far more satisfactory).

Cheers, Alan.
 

Gramps

Senior Member
Thank you Alan for the detailed performance data!
And its torque so high
With the DC motors (and servo guts), we planned the add 10k slider pots to the length of travel arm to replace the feedback pot in the servo.
Is it ok to "stall" steppers at the end of travel, without damaging them?
"fingers" are just required to "move"
Yes, able to tighten upon an object.
Like hold the drill and pull the trigger!;)
 
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AllyCat

Senior Member
Hi,
Is it ok to "stall" steppers at the end of travel, without damaging them?
Yes absolutely, a traditional way to "calibrate" a stepper is to drive it to one of its endstops. An "advantage" of a stepper (although in practice it's usually an enormous DISadvantage) is that the load (or drive) current is the same whether it's moving or not (stalled). Strictly, the inductance of the coils causes the average current to be reduced each time a coil is switched (on), so the current is a little smaller when being driven fast, but that seems to be a very small effect with these micro steppers. The great advantage of dc motors is that as the load increases, their speed drops and the "back emf" (self-generated, reverse voltage) falls. That causes the current to rise which increases the torque and automatically helps to maintain the motor's speed. The only "problem" with that is, if the load is so high that the motor stops (stalls), then the current can rise enormously (perhaps by a factor of 10) which can overheat the motor and/or damage the drive circuit.

Yes, able to tighten upon an object.
To me "move" is move (and no more), so IMHO a stepper is unlikely to be satisfactory. I'd call your requirement "manipulation" and for that any serious robot would need pressure sensors. That might be a physical switch or strain gauge, but with a dc motor (or servo) could simply involve monitoring its current drain to detect its speed.

Cheers, Alan.
 

AllyCat

Senior Member
Hi,

Then IMHO forget steppers. The "N20" dc motor you linked is also available from other suppliers with a large range of gear ratios, typically between 15 and 2000 rpm (and with shorter shipping times). For example, you might use a faster output shaft and attach a lead screw, either directly onto its end, (e.g. a 3 mm Allen-keyed cap-head screw) or with a coupler (plenty on ebay for various shaft diameters). Perhaps use m3 or m4 threaded rod, or an m6 nut has a 1 mm pitch and is 9.9 mm across flats, so you could glue a flat strip (of metal, plastic or even Teflon) onto one face of the nut and the strip could run along the side of the motor to prevent the nut rotating.

Or there are plenty of other possibilities with a geared dc motor.

Cheers, Alan.
 
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