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carson wrote:

So would I have any reason to expect better performance for threading operations with the Mesa FPGA board if the spindle synchronization servo is still being done by LinuxCNC? 

Yes. LinuxCNC is not the limiting factor, parallel port is.

carson wrote:

  With my current setup (using the parallel port) threading messes up once every few passes (seems to get slightly out of phase with previous passes

Most probably parallel port exceeded the ability to count encoder pulses.
You can always try a slower feed rate till you find a reliable value.

carson wrote:

Also, for closing the loop with linear encoders and steppers, I've read some people on this forum have had good success, but is it even worth trying for a setup (like mine) that has regular acme threaded lead screws with a fair bit of backlash?  That seems tricky for the feedback controller to handle.

Linear scales would help a lot, the machine will get to where it is supposed to despite backlash.
With simple steppers you can still use backlash compensation, same as with encoders attached to the motors, so not much of improvement, but with encoders it would alert if it skipped steps, and than can be used by LinuxCNC to stop the machine so you can check what went wrong.

So would I have any reason to expect better performance for threading operations with the Mesa FPGA board if the spindle synchronization servo is still being done by LinuxCNC?  Maybe it depends on whether it's the base thread or the servo thread that is limiting performance with my current setup.  With my current setup (using the parallel port) threading messes up once every few passes (seems to get slightly out of phase with previous passes: forum.linuxcnc.org/26-turning/45294-thre...zation-issues#236574).  I never was able to fully diagnose what's going wrong.  I guess it could also be that the software encoder counting is too close to the limits of its performance.  But I'm really hoping things will work better with the Mesa board.

Also, for closing the loop with linear encoders and steppers, I've read some people on this forum have had good success, but is it even worth trying for a setup (like mine) that has regular acme threaded lead screws with a fair bit of backlash?  That seems tricky for the feedback controller to handle.

Yes, a 7I96S basically incorporates a breakout board so is intended
to connect directly to motor drives, limit switches etc. A 7I92T needs
an external breakout so a 7I92T could work with your existing parallel
port breakout.

All higher level motion control is done in LinuxCNC including all
spindle synchronized motion (threading, rigid tapping etc)
Any feedback/PID loops are also done in LinuxCNC.

The nominal 1 KHz servo thread rate is usually sufficient for this
kind of motion control. This is because the controlled device is usually
under velocity control and has a local velocity loop.

There are some threads here ( on the LinuxCNC forum ) that describe
using linear encoders with step/dir drives to increase the accuracy
and which were successful. Unfortunately these were a couple of years ago
and  I could not find them with a quick search.

EDIT: Here is a thread about using linear encoders with step/dir drives:

forum.linuxcnc.org/49-basic-configuratio...le-closed-loop-setup

 

Hi,
I've been pouring over a bunch of threads here on Mesa boards and I'm pretty sure that I want to upgrade my system to use one, but I'm still quite confused about the best one to choose and the overall process of getting it hooked up and any other hardware required.  I'm sure this info is already out there somewhere but I'm having a really hard time piecing it all together.

I have been running LinuxCNC on a dedicated desktop PC using a parallel port interface to control my CNC lathe.  I've had some issues with latency and timing, particularly with threading operations.  I'd like to upgrade my setup to use a Mesa board for (hopefully) better performance and also to get away from using the parallel port.  I think an ideal solution would be to use a Raspberry Pi with the Mesa board so that I don't need to tie up a full PC.  I've read about varying results of this approach on this forum.  Mesa makes the 7c81 board (seems to be out of stock currently) specifically to interface with the GPIO of the RPI or I could get one of the ethernet boards, but some users complain about timing issues with ethernet on the RPI.  An ethernet board also seems nice if I decide to use it with a PC instead of an RPI.  Which boards have people had success with when controlling with an RPI?

My current setup (using the PC with parallel ports) can control 3 open loop steppers, has 6 limit switches, an encoder for the spindle and an MPG with a quadrature encoder, and about 5 other buttons/knobs for input.  So I'd like to get a Mesa board that can handle all of those outputs and inputs.  In the future, I'd also be curious about using some glass scales on the X and Z axes of the lathe to provide feedback, but maybe that's another topic.

Are there any self-contained guides or tutorials on getting the mesa board setup with LinuxCNC?  Will I need to change the firmware on the FPGA to configure it?  Do I even need to run a real-time OS if the FPGA is controlling all the fast timing? 

I really appreciate all help and suggestions!
Carson