Question on closed loop encoder support
26 Apr 2016 17:14 #73922
by TFT
Question on closed loop encoder support was created by TFT
Currently I'm using Mach3 cnc and have no closed loop encoder support.
My question is if I switch to linuxcnc can I set up closed loop support easily. I have a five axis machine, longs stepper motors and drivers, a quad encoder and the parallel port.
Any insight would be great. Thanks.
Tim
My question is if I switch to linuxcnc can I set up closed loop support easily. I have a five axis machine, longs stepper motors and drivers, a quad encoder and the parallel port.
Any insight would be great. Thanks.
Tim
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26 Apr 2016 17:46 #73926
by andypugh
Replied by andypugh on topic Question on closed loop encoder support
You can close the loop, but it is likely to not help very much.
What problem are you trying to overcome?
The problem with trying to close the loop with a stepper is that there is no way to "try harder" if a position error is detected. Steppers make max torque at zero speed, after all. If a position error is detected then stepping faster is counterproductve.
I suppose potentially you could couple adaptive feed to PID error and wind back the cutting speed to try to recover lost steps. I don't know if anyone has made that work.
What problem are you trying to overcome?
The problem with trying to close the loop with a stepper is that there is no way to "try harder" if a position error is detected. Steppers make max torque at zero speed, after all. If a position error is detected then stepping faster is counterproductve.
I suppose potentially you could couple adaptive feed to PID error and wind back the cutting speed to try to recover lost steps. I don't know if anyone has made that work.
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26 Apr 2016 18:32 #73928
by TFT
Replied by TFT on topic Question on closed loop encoder support
For one I want to correct for backlash. Secondly, my setup uses rollers to push an extrusion towards the spindle. I need correction for those rollers slip.
If a stepper and encoder wont help then how do people use encoders?
Thanks
If a stepper and encoder wont help then how do people use encoders?
Thanks
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- Todd Zuercher
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26 Apr 2016 19:15 - 26 Apr 2016 19:16 #73930
by Todd Zuercher
Replied by Todd Zuercher on topic Question on closed loop encoder support
Closed loop normally refer to servo systems often using encoders for feedback.
In the two situations you are referring to, I think the addition of some kind of encoder feedback could be useful and workable. Often people who are asking about adding encoders to steppers are trying to compensate for loosing steps. And in those situations simply adding an encoder usually won't help much, because by the time the problem is detected, the limits of the motor/drive combination have already been exceed.
You have to remember though that backlash compensation is a double edged sword, and is a compromise at best. Far better to minimize it mechanically.
What is your idea to couple the encoder to your extrusion to remove the slip of the rollers? Will the encoder just be connected to a non driven roller?
In the two situations you are referring to, I think the addition of some kind of encoder feedback could be useful and workable. Often people who are asking about adding encoders to steppers are trying to compensate for loosing steps. And in those situations simply adding an encoder usually won't help much, because by the time the problem is detected, the limits of the motor/drive combination have already been exceed.
You have to remember though that backlash compensation is a double edged sword, and is a compromise at best. Far better to minimize it mechanically.
What is your idea to couple the encoder to your extrusion to remove the slip of the rollers? Will the encoder just be connected to a non driven roller?
Last edit: 26 Apr 2016 19:16 by Todd Zuercher.
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26 Apr 2016 19:46 #73933
by andypugh
In that case, yes, it should work.
You would set up the stepgen so that it worked in velocity mode, and then a PID component would be set up to move the stepper until the part was in the right position.
Replied by andypugh on topic Question on closed loop encoder support
For one I want to correct for backlash. Secondly, my setup uses rollers to push an extrusion towards the spindle. I need correction for those rollers slip.
In that case, yes, it should work.
You would set up the stepgen so that it worked in velocity mode, and then a PID component would be set up to move the stepper until the part was in the right position.
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28 Apr 2016 23:54 #74038
by TFT
Yeah exactly. Thanks for the answers guys.
Replied by TFT on topic Question on closed loop encoder support
What is your idea to couple the encoder to your extrusion to remove the slip of the rollers? Will the encoder just be connected to a non driven roller?
Yeah exactly. Thanks for the answers guys.
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