Vibration and stalling in anti-backlash worm-gear with stepper motor 1

[Mimitte]

Hello all

I have a rotary stage design that was intended as a "simple and cost effective" solution to orient mirrors for a high precision opto-mecanical application. I produced several of these units for using horizontally (output axis is vertical). Now we have to extend its usage to vertical (output axis is horizontal). My problem is that I have vibration, audible noise and stall on specific positions when I use it vertical with its load. The load is 800g mounted offset. Output speed is about 30 rpm.

When the stage vibrates, it resonates altogether and either augments dynamic torque to the stepper enough to stall it, or forces the stepper poles to step out of phase and also stalls the motor.

The construction is an aluminium housing, single block, with precision-machined bearing positions where Center-center distance is well controlled (0.0003 to 0.0006 repeatability).

I use an anti-backlash worm gear - SDP-SI 48 dp 80 teeth AGMA 10 to drive my output shaft, to which is attached a mounting plate. The mirrors for the application are attached to this plate. Worm shaft is single lead. C-C distance is nominal 1.000 inch.

The motor driving this system is a stepper motor, Nema 17 frame, bipolar 0.06 Nm torque.

I have built several units working vertically, and quite frankly several works fine, but about 50% of them , I just cant get them to work without vibration or noise no matter how we assemble them.

WHAT WE TRIED -
- We found out that the original gear set has a natural frequency of 5khz, same as stepper motor. We designed a new gear (same mesh, just thicker and better support) to increase natural frequency. Still vibration, audible noise and stall !
- We reduced C-C to nominal dimension. Still vibrates, noise and stall !
- We augmented C-C to larger than 1.0001 inch. Still vibrates, noise and stall !
- We changed boundary condition on several parts (fit or loose on some assemblies, shaft to output table, etc... Still and stalls !
- We use dampening rubber coupling - It helps but still vibrates !
- We rigorously measure gear height and adjust with 0.001 inch shims if required.

Any help will be greatly appreciated to pinpoint the root cause of my problem / problems !

Thanks in advance,

 

[MikeHalloran]

You ran out of motor.

Try to squeeze in a size 23 motor.

If you can't do that, and maybe even if you can, you may need to change the driver circuits. Steppers are not as simple as they appear; for one thing, they can't be rated in isolation. The torque and speed developed are both highly dependent on the driver circuit used during the test.

You may end up adding some electronic damping, and using a higher voltage power supply to feed the drivers. At this point, you need a good sparky working with you.

Steppers can be simple and cheap for vanishingly small loads that don't change dynamically. The load you have now with the horizontal output shaft and unbalanced mirror bar is neither.

To learn more than you think you should ever need about steppers, search (separately) on authors Leenhouts and Gauthier.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Mimitte,

With your worm gear mesh, "anti-backlash" likely means high friction. I would speculate that the vibration is due to the stick-slip nature in the worm gear (ie. the sudden change in driving torque due to the large difference between break-away and sliding frictions at the gear contact).

Hope that helps.
Terry