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Vertical Axis Tuning


Laserdave7
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What is the proper way to tune a vertically oriented axis. When I tune the step response in tuning pro I can't seem to get the upward motion to pull all the way into position in the dwell period without resonance occurring. I have tried high Kp and low Ki and low Kp and High Ki. I can get it close but when it pulls in tight it starts to resonate. Is there a proper way to bias the drive to compensate for the effects of gravity on the stage?

 

Thanks,

Dave

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In this case tune the motor disconnected from the load for good step response. Then re-attach the load and use parabolic trajectory to fine tune to the load. Note that without a true mechanical counterbalance there will always be a higher following error on the gravity side. There is no feature to “offset” the drive for this.
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One thing you may try is to use the deadband to reduce you gain near the in position. Set the Deadband Size (Ix65) to about half your In Position Zone (Ix28) so you will not reduce the gain until you are within your position tolerance. Use Ix64 in the range of -1 to -16 to reduce the gain until your position is stable.

The mechanical fix is to use gas springs such as the Suspa units to counterbalance the load.

 

What is the proper way to tune a vertically oriented axis. When I tune the step response in tuning pro I can't seem to get the upward motion to pull all the way into position in the dwell period without resonance occurring. I have tried high Kp and low Ki and low Kp and High Ki. I can get it close but when it pulls in tight it starts to resonate. Is there a proper way to bias the drive to compensate for the effects of gravity on the stage?

 

Thanks,

Dave

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When I tune an axis with a net load like gravity on a vertical axis, I like to put a very small, but non-zero, integral gain term on early. It can be 1/1000 of the likely value you will end up with, and take take 30 seconds to reduce the steady state error to a minimal value. Once the error is minimal, I can do step responses to tune Kp and Kd for a good step response with minimal overshoot. At this point Ki is too low to affect the dynamic response of the step significantly.

 

Once I have set Kp and Kd this way, I now start increasing Ki. This will increase the overshoot. Personally, I don't mind getting about 15% overshoot (others don't like this), but I find as long as the overshoot is not followed by some undershoot (this may be the "resonant" behavior you see), you will have good performance.

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