Dual PID loops with motor encoder + scale encoder per axis

I couldn't find a good camera angle that shows everything at once, but there's a jackscrew on the back of the motor mount that sets a hard limit.  This physical stop is about 1/4" beyond where my home switch actually triggers, but even when it's parked right at the hard limit, the home/limit switch is still active.  Power was off when I took these photos, so the sensor LED is not on, but you'll get the point.

Hopefully this clears it up.

cheers,
Steve





 

I was lucky to stumble into a DMM DYN4 750W AC servo and drive for free, so this gives me a good opportunity to test a modern setup on one axis. 

I was able to temporarily mount the 80 frame motor into the same mount as the original brushed DC motor, though only two bolt holes align in the motor slots - good enough for testing, but I'll have to modify the mount if this becomes a permanent thing.  For testing, I 3D printed a custom Gates HTD 5M 17T pulley for the AC servo's much larger 19mm shaft, as there's no off-the-shelf availability with exactly the right combination of bore, keyway, and tooth count. 

 

I've got the drive wired up in step/dir mode as I don't have any +/-10V output options on my bench test breadboard at the moment.

 

Using the DMM tuning software, I was able to put a quick auto-tune on it, and I've got it moving around under LinuxCNC control using position control from a stepgen in the Mesa.  It sure seems to move the heavy table with an extra 100lbs on it like it's nothing, at some pretty absurd rapids (350+ inch/minute). 

I need to decide if I'm going to either build or buy a means of getting +/-10V output so I could try running the drive in torque or velocity mode.  With step/dir I'm limited to position mode, or faking velocity mode using the stepgen velocity output mode.

I also have not yet wired up the drive's encoder outputs, but they are available on the 9 pin connector on the drive.  That will be necessary to play with velocity or torque mode.

 
 

TangentAudio wrote:

I need to decide if I'm going to either build or buy a means of getting +/-10V output so I could try running the drive in torque or velocity mode.  With step/dir I'm limited to position mode, or faking velocity mode using the stepgen velocity output mode.


 

1-RPi Pico
2-74HC4050 (level-shifter, 5V to 3.3V)
1-MC3486 (differential line receiver - assuming that the encoder has diff outputs)
1-MC3486 if the pulse/dir is differential
1-MCP4922 DAC
1-OP-amp (TL072 or LM358)
2-LS7366R (one for quad decode and one for pulse/dir)

1- +/- 15V PSU (although I seem to remember that +/- 12V was fine. Bit fuzzy on that one)

Might not need the LS7366Rs because the PIO of the Pico can do it. I am already doing high-speed quad decode
but need to look at the pulse/direction thing.

Craig

Still tinkering on this as time allows.  I did some junk box engineering this week and put together a little isolated PWM+direction to +/-10V circuit.  Now that it's validated I can reconfigure the servo drive from step+direction to analog input.

 

Seems to be nice and linear.

 

.. but it won't win any awards for the neatest prototyping job I've ever done.