Faster loop request

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Faster loop request

Postby Robertspark » Sun Sep 03, 2017 8:39 am

I've been looking at linuxcnc with regards to motion controllers, and trying to understand the loops.

Is there any chance of being able to be given access to a faster loop (1khz or maybe 2khz) with a view to reading encoder inputs and controlling a z axis for plasma torch height control?

Would be nice if the loop setting could be adjustable from say 250hz to 2khz maybe in 500hz steps) that way it would deal with latency.

Or will this be a problem with windows given it does not run a real time kernel as Linux can be set to do?

Thanks
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Re: Faster loop request

Postby cncdrive » Sun Sep 03, 2017 2:56 pm

Hi Rob,

The aux encoders position is read/updated in a 10msec loop and the THC up/down virtual signals and THC speed setting is in the same loop.
You should also take into account that data needs to be transfered to the motion controller which can also take 1msec normally for the ethernet controllers. (data is based on what Microsoft has to say about ethernet latency and in our experience it is mostly true.)
And it has to be processed by the motion controller's CPU which is another 25usec for the ethernet controllers.
So, the best you could get is a bit below 100Hz.
100Hz means that if your Z-axis is moving with infinate acceleration and 1000mm/min feedrate then it still moves about 0.1666mm only per update, I don't think that more latency is really required, but maybe I'm wrong.
The THC control pins on the motion controller are more responsive though, they are in a 4kHz loop in the controller, however I don't think any THC controller electronics are even close to that "fast", because if they are too fast can be a problem to the control loop stability.
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Re: Faster loop request

Postby shad » Sun Sep 03, 2017 6:52 pm

Hello Balazs!
Is it possible to add PWM input for the Ethernet based controllers?
For example 10-12 bit. And up to 4 Khz input frequency. Controller can measure the duty cycle for PWM. I think one timer in GATE mode can make it without problem.
Then add register with this value in the UCCNC software.
It's allow easy to transfer date from PWM sensors (for example valves or arc voltage for plasma).
-- Andrew
UC400ETH
UC300ETH-5LPT
NEURON Lite THC
http://neuroncnc.com/
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