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


iws

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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.

 

open-loop-30.thumb.png.c120f7b4639583d585cb6a5a236c9c1a.png

 

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

 

open-loop-30-velocity-only.thumb.png.a9f8e80b0b81d7f21400dd71ee8396c5.png

 

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?

 

open-loop-35.thumb.png.cf3baa040c551aef37eb16bd1859907e.png

 

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.

 

open-loop-neg35-velocity-only.thumb.png.1629ef90917cd80c339de3820355c568.png

 

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:

I161=0.014716982841

I162=0

I176=0.3526718616

 

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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