Single shaft stepper motor vs stepper motor with gearbox

[htngwilliam]

Hello everyone

I am considering between using a 2 phase single shaft stepper motor vs 2 phase stepper motor with gearbox. I know the advantages of the gearbox stepper are as follows: -

1) Torque amplification

2) Speed increase/ reduction ratio

3) Smaller resolution

4) Lower inertia ratio

I wonder whether there is any other differences in terms of accuracy? How accurate is a stepper motor assuming the correct motor is sized? Is the error = 1 step angle?

 

[handleman]

Most gearboxes have some amount of backlash. That could affect your accuracy. As far as stepper "error", there really isn't any unless you "miss" steps. The problem is, unless you add an encoder somewhere your control system has no way to know whether steps have been missed or not. The controller sends out the commands to step, but there is no feedback that verifies the motor was able to execute those steps or not. Whether or not a stepper with no encoder will give you what you need depends on several factors:

1. Are there any conditions you might forsee when the load is such that the stepper could miss? This includes part jamming, mechanism getting gummed up with crud, etc.
2. What is the worst possible thing that could happen if your motor is not in the position that the control system thinks it is in due to missing steps?
3. Can you add a sensor and hard stop somewhere for a "home" position? Can your mechanism return there often (every cycle?) to verify the stepper position?

etc.

-handleman, CSWP (The new, easy test)

 

[benta]

@handleman:

Might interest you that ON Semi now has stepper motor controllers with stall and loss of step detection. This technology came from AMIS and is patented, I believe.

http://www.onsemi.com/PowerSolutions/parametrics.do?id=407

Benta.

 

[MikeHalloran]

Cool chips, AMIS-30623 for example.

Htngwilliam, be careful. The missing step detector is not a substitute for position feedback.

Among other things, it works only at constant speed, maybe only at maximum speed (I'm fuzzy on that), and not at all when ramping speed.

Steppers mostly lose, or even gain, steps, when ramping speed, so the feature may not be all that useful despite its cool factor.



Mike Halloran
Pembroke Pines, FL, USA

 

[tygerdawg]

I suspect that positioning error is something that must be quoted from the manufacturer. Positioning resolution is typically one step on non-geared motors. On geared motors, it would be (step/(gear ratio)). The penalty for this higher resolution is slower speed.

Different manufacturers will have different steps.
Different stepper drives will have different resolutions.

This all assumes, as you say, motor is properly sized. If properly sized, then the assumption is no missed steps.

Then the engineering starts to kick in: if your resolution (for example) is 1/3000th of a revolution, how much does that affect your process?

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com