Servo Wiring and Tuning detailed How To example Mesa 7i77.

I have a machine set up with step/dir servos and encoder feedback via a 7i85s (and a 5i25 FPGA). To take your original open loop step/dir config to closed loop you need to do a few modifications to the hal file.you will need to disconnect the position feedback signals from the hardware stepgen, Originally it was connected to both the motor-pos-fb and the pid.N.feedback. Switching just the motor-pos-fb from the stepgen's position feedback to the encoder's will switch the DRO and f-error, but to truly close the position loop in Linuxcnc you must switch the PID's feedback as well, and then tune the PID. You can not keep the original P=1000 setting, it will be far to aggressive a P setting for a real mechanical system. Read this as bad things will happen, start with a low P of 10 or 1.

Todd and Tommy YOU GUYS ARE GREAT!

Just had to be said, I really appreciate all the time all the board members here have taken to answer all of my questions.

Anyhow that makes perfect sense now (though you had me a bit scared that I was losing it there for a second Tommy lol )

I will give this a whirl when I get home and see how it plays out.

One thing that needs to be correct before you change over to encoder feedback is that the encoder direction and scaling must match the step/dir
scaling and direction.


If the direction is wrong, you will get an immediate runaway. If the scaling is wrong you will be unable to tune for decent performance (because FF1=1 will not be correct)

Thanks PCW i'll make sure to check that.

Before I go to much further here, my drives have an additional option for "Speed control mode" which uses +/-10v analog. Would this be a better option for my mill?

The current mode is "position mode" using step/dir

Just want to make sure I go the the best way possible.

You have a 7i76 so you have to use step/dir.
For +-10V you would need a Mesa 7i77.
But since you have encoders wired back to Linuxcnc, it should not make much difference.

Todd any chance you could provide your .Hal file to refrence the encoder hookups?