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Axis 'sticking'


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Hi guys,

I have a system with a mechanism that has 3 linear axes (#1,#2,#3) driven by stepper motors with encoders for position feedback.

This mechanism is physically rotated by another mechanism (#7), that uses a brushless motor and a circular encoder.

All 4 axes are controlled by PMAC. The #7 axis is co-linear with the #3 linear axis, so #1 and 2 rotate in space, with attendant gravity/loading changes. The drive system has harmonic drives attached to the stepper, that then drive leadscrews to get linear movement on a rail system.


When I rotate #7, I get a small offset appearing in #1 (only), even with #1 in closed-loop mode. The strange thing is that the offset value also appears in the following error, count-for-count. When omega stops, the position offset and corresponding following error just stay at a fixed value (50-100 cts).

The offset value seems proportional to #7 velocity. See attached files.

Interestingly, the offset is positive for both directions of #7 rotation.


In this state, a small jog command changes the following error, but not the position. Increasing the jog value just causes (eventually) a fatal following error.

I can clear this fault by issuing "#1o0" then "#1hmz" which sets both position and following error to zero - after this, the axis responds normally to jog commands.


Has anyone seen anything like this before, or can offer any explanation for PMAC not zeroing the error?



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I'm not sure that I understand the physical description of the system, but I deduce the following from the plot.


In the period you are concerned about, the change in #1 actual position and following error are equal and opposite. This means that the commanded position for the motor has not changed. This indicates to me that there is likely a physical interaction pushing the motor away from its desired position during this period.


Now the question is why the Motor 1 servo loop cannot counteract whatever physical effect is offsetting it. The first thing to look at is whether you have enough (or any) integral gain (I133) to overcome the disturbance in a reasonable time. Also check I163 to make sure you haven't limited your integral output. Without an integrator in the servo loop, you can get this kind of behavior, with proportional gain needing an error to provide the countervailing torque/force to balance the disturbance.

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