Laserdave7 Posted April 24, 2013 Share Posted April 24, 2013 I am using a Geobrick and have an axis that is dithering even with the deadband set to -16 gain factor. Only one axis does this. I can see the DAC values fluctuating and hear activity in the motor. I have tried Ixx65 values as high as 48 and it wont stop. How can I diagnose where this stimulation is coming from. My understanding is that Ixx64=-16 should remove all stimulation from the motor... Also if I look at Ixx68 I see it jumping around. I manually removed Ixx30 and it stopped. Seems like it's not entering deadband... Thanks, Dave Link to comment Share on other sites More sharing options...
curtwilson Posted April 24, 2013 Share Posted April 24, 2013 The deadband function modifies action from the proportional and integral terms when the following error is within the band. With a gain factor of -16, they should provide no action. However, the deadband function does not inhibit the action of the Ixx31 derivative gain term, which responds to actual velocity. It looks to me that your Ixx31 value is too high for this motor, causing it to overcorrect continually and dither across the set point. I don't see why Ixx68 would be changing. Make sure you don't have anything writing to it on a continuous basis. Link to comment Share on other sites More sharing options...
Laserdave7 Posted April 25, 2013 Author Share Posted April 25, 2013 Thank you for your reply. I will try reducing Ixx31 and see if that solves the problem. I am confused about something though. According to the PID Diagram in the PMAC User Man. the I and D term are before the P so it seems like a P term of 0 ((-16+16/16) * Ixx30 per the deadband) should block both. Also the Tuning Pro help file states "Because the proportional gain term Kp (Ix30) is outside the brackets in the filter equation (see previous page), it also affects derivative and integral gains". Where am I going wrong in my understanding? Any help you can provide to help me better understand whats going on is greatly appreciated. Thanks, Dave Link to comment Share on other sites More sharing options...
curtwilson Posted April 25, 2013 Share Posted April 25, 2013 If you look at the block diagram of the servo algorithm in the User's Manual, you will see that what the deadband compensation algorithm does is to modify the size of the following error inside the band. If you have it set up to create true deadband (Ixx64 = -16), then if the magnitude of the following error FE is less than Ixx65, the modified following error FE' out of the block is 0. This modifies the effective gain of the position loop in the servo algorithm, but it does not modify Ixx30. Looking again at the block diagram, you can see that it does not affect the inner velocity loop action. Link to comment Share on other sites More sharing options...
Laserdave7 Posted April 25, 2013 Author Share Posted April 25, 2013 Very helpful. The piece I was missing was the placement of the deadband in the diagram. Thanks for the help, Dave Link to comment Share on other sites More sharing options...
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