Limiting Resolution

Please explain what limits the resolution in LinuxCNC. I am looking at getting encoders with a resolution of .0001mm. Will i be able to set a configuration that will utilize this resolution? Thanks

Encoders of 1um resolution are normally only found in sensitive measuring machines.

For any kind of normal machining, that degree of resolution would be of no advantage.
Tool run out, machine vibration etc would render it unusable.

It would likely only be achieved at the expense of velocity in a stepper setup, requiring huge micro-stepping figures.
Since these degrees of micro-stepping are purely interpolated, the accuracy is questionable between physical rotor detents anyway, plus the effect of holding torque per step reducing with micro-stepping, this all adds to accuracy problems.

You need to say what you will be using the encoders on, before their usefulness can be commented upon properly.

regards

Nothing limits your resolution. You configure how many pulses you have per machine unit. (pulses/mm or pulses/inch for exempel).
What makes the limit is the combination of resolution and speed. High resolution at high speed will generate a high frequency and somewhere your hardware will set a limit.

edit: This is theoretically. In real Life there are other things that sets the limit. ArcEye wrote a better explanation.

--Per

.0001 mm resolution is not all that high ~= 250,000 counts per inch
Many 1980s vintage machine tools have encoder/resolver resolutions in this range.

Current machine tools often have much higher resolution, for example the current Fanuc
encoders are 16M-32M counts per turn. This, direct coupled to a 5MM lead pitch screw
gives a resolution of ~.0000034 or .0000017 mm !

Note that the main object of these high resolutions is getting good velocity feedback at
low speeds ( a decent number of counts per "servo thread" ) , not positional accuracy.

I have configured my servos so they give 10000 lines (or 40000 pulses) per turn to my Mesa 7i77 board for a 5mm screw. That is a resolution of 0.000125mm. This will produce a 2Mhz signal at 3000rpm wich is maximum speed in my setup.
I have only dry run this since I have not got my mill yet due to some delays, but without any load it works fantastic well.
After some adjustment of the PID there is about 0.005mm following error at 2000 mm/min if i have interpred halscope correct.
It shall be interesting to se if I can use this in reality as well.

Thanks for the feedback. Speed is not my main concern so losing speed to achieve this resolution is fine. I am using servo motors with linear encoders so I am not worried about the problems with microstepping. Tool run out and machine vibrations will all be issues that will need to be dealt with, but I really was just checking that their is no inherit limit of LinuxCNC on resolution before attempting.

Currently the encoders in the mill I am using have a resolution of .0005 mm ~= .00002 in. But the Axis GUI only displays 4 decimals in inches so I am limiting my resolution to .0001 in. Is there a way to change Axis to show 5 decimals? If so, will they be used in by the gcode interpreter or is there any rounding that occurs there?

I'm pretty sure what Axis displays, has little or no effect on the actual resolution used by Linuxcnc (all the numbers behind the scenes are double precision floats). So if your hardware (including your physical machine) can support such resolutions, Linuxcnc will do it whether or not it is displayed on the gui. I must say I've never seen a machine that could achieve such precision, in reality most are lucky to be +-.01mm. (Most people couldn't measure something with that accuracy even if you gave them the tools to do it.)

It makes sense that Axis will have no effect on the actual resolution. I am thinking it might be easier to test and verify though, if I can get the GUI to read out more decimals. I will try what is recommended here:
forum.linuxcnc.org/forum/49-basic-config...the-default-axis-gui

Currently, the .0001 in ~= .0025 mm resolution is pretty reliable so having that extra decimal should help in getting closer to the encoders resolution limits.

I thought about doing that, but I have not made any panels yet so I don't know how difficult that would be. I did edit the glcanon.py file and now have 6 decimals displayed in axis with inches as the units.