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Open loop mode does not provide constant velocity (or voltage)


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I am using a Geo Brick LV with a Midwest Motion Products LA10 Linear Accelerator motor. This motor is a 12V DC motor. The Geo Brick does not need to commutate the motor for it to work.


I have tested the motor with a bench supply, and it works smoothly at all voltages through the entire range 2V to 12V. If I use the bench supply to send 2V, the motor moves slowly, but at a constant velocity. If I use the bench supply to send 8V the motor moves at a medium speed at a constant velocity. Etc.


However, using the Geo Brick in open loop mode, the motor does not have a constant velocity. I have taken several plots with the PMACPlot32Pro2 utility to prove this.


I cannot tune the PID loop without first having a stable current loop. How can I fix the Geo Brick?


This graph shows the the results of running the command "O30" for the motor. This starts at about 2 seconds. I am unsure why the Geo Brick commanded a negative velocity at the beginning. I certainly did not ask it to do so. You can then see that the velocity remained constant for a while, until about 12 seconds, where it accelerated to maximum velocity. The Geo Brick did this on its own. I issued no commands except "O30".


If I hook up a volt meter on the Geo Brick output to the motor, I can see the voltage increase when the acceleration happens. The open loop commands are not providing a constant voltage.




Here is another instance of the same phenomenon, showing the actual velocity only.




Here is another instance of the same phenomenon, using command "O35" this time. Why did the Geo Brick decide to accelerate at about 2.50 seconds?




In the negative direction, the Geo Brick seems to perform better. This is the results of command "O-35". You can see that other than at the beginning, the motor stayed at a constant velocity.




I am at a loss to explain this behavior. Please help.

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The transient response of an open loop move is governed by the PID loop tuning bandwidth and the current loop tuning band width. What is the bandwidth of these - can you show the plots?


The current loop uses the default parameters from the Geo Brick LV Setup Assistant spreadsheet. The defaults are a Current Loop Bandwidth of 300 Hz and a Current Loop Damping of 0.707.


The current loop gains for this motor are set to:





These numbers were taken directly from the spreadsheet.


Is this what you are asking for? If not, please tell me how to create the plots you want, and I will do so.


Thanks for your help!

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