Inconsistant speed on contoured straight line move

[Martin.D]

HI,

I have a linear servo with 0.5um encoder and need to control this very accurately. I am spinning an object while writing spirals on it (via laser), the problem is that the spirals I produce have wobbly lines (when inspected under a microscope). I am using contoured moves for 3 axis all started at the same time, with a interval of 20ms. I guess the problem is due to tuning but I have spent hours trying to improve the accuracy, I was hoping someone out there could help. It's particularly difficult because the stage needs to move 14mm in about 0.8s (straight line) very accurately and then return in about 0.1s (accuracy is not a concern on the return move). I think a critically damped tuning is not required here, or is it?

Thanks for any ideas / Martin

[Brian_Beal]

Please clarify if you are using a standard x-y stage, and that your 3rd axis it a rotary axis. Your move is relatively small, and hence the accel and decel will play an important part in your profile. I have done work on a laser system (with servos), and have found that depending on the type of move, different tuning parameters may be required. For instance, a move at a slow speed requires one set of PID parameters to be accurate and smooth, while a rapid move requires a different set. I am not sure if the controller that you are using can change tuning parameters on the fly, but that may be one thing to try.

[Martin.D]

Brian,

I suppose that all that needs to be considered is two motion stages, the third is an idependant stage that has the same function as one of the others. So there is one rotary stage to spin the object and one linear stage to move the laser beam up & down. I have analysed the actual positions of the linear stage during motion and it does indicate an incosistant speed that seems to match the wobbly lines, although this was using software time position polling. I think with contoured moves the normal accel and decel have no effect, I will need to deal with that myself in the position arrays that I send to the NI card. At the moment there is no accel or decel, I command constant move lengths with a constant time interval throughout the move. The cards I have are PCI-7358's that do support on the fly PID parameter changes (I think), I just hope that they can be changed very quickly if I need to go that way! I guess I'll find out soon, but until I can tune the stage for the slow / accurate move I do not know if there is a requirement for on the fly PID parameter changes. Maybe this is an issue with the mation stage its self, although It was sold to us as a nano-positioning stage.

Thanks / Martin.