Stepper linear actuator dropping vertical load

[RyreInc]

Hi All,

We have a machine that includes a stepper-driven linear actuator that moves a large vertical load. Most of the time the system works as intended, but every once in a while the stepper will slip when the brakes are released, causing the load to drop uncontrolled.

A ball screw is used to translate the motion. There are also two holding brakes for safety. By my calculations the torque safety factor is about 5x, and the inertia ratio is about 0.5, so it seems like the motor should be able to control the load.

The stepper driver is a constant-current design. Idle torque is set to 90%. No microstepping. The slip happens without requesting a move from the stepper.

I am aware of one thing set incorrectly: The driver has two inertia settings, 0-4x, and 5-10x. It is currently set to the latter. I am skeptical this is the issue, but I also don't understand how this affects the driver.

What do you think could be the cause of this?

Let me know if you have any questions - I could provide a lot more info if requested; trying to keep the post succinct!

 

[waross]

Check if the stepper has been turned off prior to the brakes being released.
Check if the driver goes directly to holding mode when powered up.
A momentary power failure or someone turning the driver off and back on may be taking the driver out of holding mode.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RyreInc]

Thanks for the quick response waross. I had considered a voltage drop affecting the stepper motor current directly, but not the driver itself - which makes more sense, as the 24 V supply would have to drop to less than 5 V before the driver runs out of headroom, but the driver itself may drop out a lot sooner than that. It IS possible the power supply is undersized, and/or the wiring is too small for the length of the run.

 

[iop95]

Some reasons:
- driver instability in current loop
- driver room voltage too low so when motor winding temperature rise, can't maintain same current
- power supply of driver may enter in current limit or current loop instability