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backlash compensation using load mounted encoder?


brinj
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I have inherited a mechanical build which has a stepper motor driven linear stage and a load mounted renishaw linear encoder. When reversing, the large backlash inherent in the system quickly builds into either a huge servo demand that takes a relatively long time to run out, or else generates a fatal following error. This limits boith the speeds I can achieve and the gains I can apply for low following error at standstill. Does anyone have any methods to over come this. cheers
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Depending on how you are controlling the stepper motor, you might be able to use the Ixx85 to Ixx87 settings in Turbo PMAC to take up the backlash without sacrificing the gains or time. This method is also very useful and easy to setup, given that you have a consistent backlash size through out the travel. If the axis uses a separate velocity feedback (in case of a stepper this can be simulated internal counter) and position feedback (Renishaw load encoder) then Ixx03 and Ixx04 will be different which gives you better stability on your velocity loop. Backlash Take-up Rate(Ixx85), Backlash Size(Ixx86) and Backlash Hysteresis(Ixx87) settings can help you resolve this issue. Let me know if this was helpful.
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The typical "backlash compensation" function in controllers (including the PMAC) is intended to improve the accuracy and repeatability of the load position when only a motor sensor is present. It does this by adding in or taking out an offset to the commanded position based on the direction of travel. This is not your case here, as you have a load sensor that will accurately represent the load position in both directions of travel. Your problem is that during reversals, the motor is unloaded during reversals while the backlash is being taken up. (From your post, it seems that you think it is more heavily loaded.) Unfortunately, dealing with this case is more art than science -- there do not seem to be fixed solution strategies out there. As Sina said, much depends on how you are actually controlling your stepper motor. Without feedback on the motor, you really have no idea what it is doing during reversals. How are you controlling the motor? Curt Wilson VP Engineering Delta Tau Data Systems
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