OpenPnP with LinuxCNC via linuxcncrsh?

10 May 2023 14:33 #271107 by Donb9261
One of the issues with homing using limit switches and steppers is there is no feedback from the motor to know the exact reference to the rotation of the motor. ie. an encoder Z marker. You can buy steppers with encoders but the cost is 3~4 times a standard stepper.

One way that could possibly help with you issue is to use a switch like this

Metrol has been a round for a long time and I have worked with them hundreds of times. This switch is expensive @ $81 ea. But it has a 5um repeatability and a 2.8mm stroke. If you have a reasonable motor and driver you could allow a .02 or better home repeat position consistently.

There are just some things that hobby machines lack due to the expectation of solid accuracy for basically very small spend. You can cut corners in some things but cutting corners in accuracy is not a good idea.

The switch can act as both an end stop and a limit. You could avoid having a second opposing end stop by having this accurate switch and then setting soft limits once homed. As long as the reference is good and your code is set so that if the reference is lost the axis can ONLY move in the home direction you can save some money. Typically I do not use end stop switches in any builds. The clutter the machine and I would never use a $2 switch to use as my datum for the machine.

Although I recommend using the switch above, they offer small and slightly less accurate versions for much less.

Hope that helps.

PS - Control the speed at reference and acc/dec to ensure that when the reversal happens no steps are lost. As long as your overall backlash is less than .025mm or .001" you should be able to have solid control over your net accuracy.

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16 May 2023 05:20 #271440 by iforce2d
I have been working on this recently.

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19 May 2023 04:49 #271650 by blazini36
Anyone looking to build a PnP and use LinuxCNC via OpenPnP should giveĀ  iforce2d's repo a try. Once I get my controller sorted I'll be giving that a go for sure.

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