High inertia ratio for stepper motor

[htngwilliam]

Hi

I wonder if the inertia ratio for stepper motor is very high, what is the impact? Will there by mis-step?

 

[tygerdawg]

The usual design rules are to have the rotor inertia match the load inertia. Else you may encounter some resonance and oscillation issues. Also depends on your gear train, how much "frictional dampening" you have with sticky bearings, etc., that may hide any of that oscillation stuff.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[zekeman]

I don't like rules of thumb especially for steppers.
I would say that more often than not I violate that matching criteria for steppers, ostly, and especially for speed.
So I say it is application specific. Name the application and I think you can get a tailored solution.

 

[htngwilliam]

Hi

I am actually sizing a stepper motor for my optical filter wheel indexing. Previously, I have size a suitable motor with a gear ratio. It is working fine until we encounter a problem due to backlash. In order to resolve the backlash issue, I suggest using a larger motor without gearbox. However, my load inertia is > 50 times the rotor inertia. It seems to be overloaded according to the vendor

 

[MikeHalloran]

You can put damping in the electrical side of the stepper control circuits to reduce the ringing you will get with direct drive.

If you're feeling mathematical today, search on "phase plane analysis" applied to stepping motors.

If you've got the budget/time, you might just go to controlled current drivers for the coils, or microstepping.








Mike Halloran
Pembroke Pines, FL, USA

 

[htngwilliam]

Hi Mike

Am I right to say that problems results from large inertia ratio of stepper motor can be overcome by using microstepping?

 

[zekeman]

Before you design the system, you first need to know:
the speed of indexing
amount of allowed overshoot and settling time
load inertia
A geared system is usually satisfactory, despite the backlash, since it gets smaller as the gear ratio is increased.
I would be interested in the problem as you see it with backlash in your case; a 50:1 inertia ratio is too large and will result in excessive transients.

 

[htngwilliam]

Guys

Thanks for all the information. Let me try to explain my design:

1) I am using a 9: 1 gear ratio stepper motor with a backlash of 2deg max

2) I have a timing pulley system with a 2: 1 ratio. Meaning to say I would see a max backlash of 1 deg max at my wheel.

3) If I design the whole system as per the current design, I will not have problem with the load inertia. However, if I remove the gear box at the stepper motor, then I believe the inertia ratio is too high

4) The problem with the backlash is that when the system decelerates, there might be problem with the backlash due to vibration.

 

[zekeman]

What kind of gearing do you have to give such a high backlash?
Where is it measured?
What are you trying to do?
Steppers may not be the best way to go.

 

[htngwilliam]

Hi

The backlash specs is provided by the motor vendor. The gearbox comes attached to the motor. I wonder whether there are seperate gearbox available in the market for stepper motor. Maybe I can source for a better gearbox.

 

[SincoTC]

Sorry, a bit late to this!

Is it possible to move the motor to one side, throw out the troublesome gearbox and drive it from the stepper with a 9:1 ratio timing belt. I assume that your optical filter wheel is a fair diameter, so you could incorporate it into the larger pulley and use a couple of plain idler wheels to get a good "wrap" and tension the belt around the small motor pulley at a minimal centre distance. This would give you a simple, virtually zero-backlash drive.

 

[MikeHalloran]

I.e., 18:1 direct with a timing belt. It will work if you can get a belt that's long enough.

The big wheel doesn't need to be fully toothed. You can make it smooth, at the minor diameter of an equivalent pulley, with a couple of regular teeth inserted 180 degrees apart.





Mike Halloran
Pembroke Pines, FL, USA