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nano positioner tuning


dchabot
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I have an piezo motor application that is displaying problematic behavior under closed-loop control for some (not all) axes.

 

The system is a Geobrick LV (NSLS-II) outputting step/direction to six MiCos PMC-100, which drive nanopositioner stages (stick-slip piezo). Feedback is via a MicroE incremental linear encoders ( 0.005 um resolution).

 

I myopically tuned all 6 axes of this system for small moves (0.25-5 um), as that is how it is used in practice. On further investigation, 3 of the 6 axes trip on fatal following error for longer moves, but only in one direction (Ixx11 = 32000) . The other direction is fine. The problematic axes trip on this error a fraction of a second after a commanded move (J+/J-, etc), regardless of Ixx22 setting (jog speed).

 

Gravity is not an issue here as one of two of the vertical stages works fine in closed-loop.

 

Is there any way to tune for this type of behavior? I'm kinda at a loss here...

 

Thanks.

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One of the following could be true:

- Friction or sticktion effect in one direction or the other. You may have to implement some gain scheduling (higher PID gains where needed).

- You are exceeding the maximum speed dictated by the DC bus.

- You are exceeding the maximum acceleration dictated by the current.

- The last two are essentially what we call a servo effort. Violating the servo effort, in general in closed loop, will cause a fatal following error.

 

That said, it seems like you are tripping on the acceleration section (Ixx19 for jog moves).

Have you tried slower accelerations?

I don't know how good your tuning is. But you might want to look into adding some friction feed forward (Ixx68).

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One of the following could be true:

- Friction or sticktion effect in one direction or the other. You may have to implement some gain scheduling (higher PID gains where needed).

- You are exceeding the maximum speed dictated by the DC bus.

- You are exceeding the maximum acceleration dictated by the current.

- The last two are essentially what we call a servo effort. Violating the servo effort, in general in closed loop, will cause a fatal following error.

 

That said, it seems like you are tripping on the acceleration section (Ixx19 for jog moves).

Have you tried slower accelerations?

I don't know how good your tuning is. But you might want to look into adding some friction feed forward (Ixx68).

 

I think my tuning is ok. Step and ramp tuning results for one of the problematic axes are here

 

I've tried adjusting velocity and acceleration, neither of which was successful. But, I will look into the adaptive control scheme you suggested (gain scheduling).

 

I've also swapped drives between fully functional and problematic axes, and the problem follows the stage, not the drive.

 

At this point, I'm fairly convinced it is a mechanical problem...

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