next topic »

[Reply #15 on:]

GRR.  ARG.  The 1M pot I found has a max A-B voltage of 7v, and I'm pretty sure the voltage across the points I need to muck with is 18V.  I may be back to having to think about how to deal with this.  Incredibly annoying and there really should be a better way; I'm just not sure I know how to do it yet.  GRR.  I wish I knew better how this thing was being controlled.

[Reply #16 on:]

Okay, so after thinking about this a little longer, I'm wondering if anyone can throw around some new ideas.

The voltage drop across the pot (which is, again, being used as a rheostat rather than a voltage divider) tends to be 11v-8v or so (the higher the voltage drop, the slower the motor).  The current, if I've measured it right, is actually quite small, about 0.04-0.05 mA, so it sounds like it's just being used to change the voltage to the wiper and not really modifying current much.

So we're not dissipating a ton of power.  If I'm right and it's a voltage sensing issue, is there a way to cleanly provide a voltage drop with the Arduino?  A PWM-controlled transistor or some such?  Any other ideas?

[Reply #17 on:]

I'd try an opto isolator and smoothing capacitor driven by a PWM pin. See attached. The reason for the slightly strange way in which the Arduino drives the opto isolator is so that while it is powering up, the opto is turned on, thereby turning the motor off.

[Reply #18 on:]

Okay, I went the optoisolator-with-two-opamps route.  I now have a cute li'l circuit (HCNR201 for the opto, and 2xLM358 for the opamps) which will take 20V from the motor control for the output supply and 5v from the arduino on the input side, and happily amplify an input signal by 4x.

...I just can't seem to connect it to the motor control board in any way that makes sense.  If I connect it to the high pin of the potentiometer and the "ground", the voltage drops from 20V to virtually nothing as soon as I plug in the VCC to the output opamp.  I did find a good solid 20V source that doesn't drop, but it's a teeny hole in the board and since the "20V" is "mains voltage to mains voltage minus 20V", holding a wire in place is a "chargeable offense."  I still haven't figured out a way to get a signal to the wiper terminal in any way that makes the motor turn.

This is getting really frustrating.  I'm learning a lot, but I have to admit that by now that's starting to be small consolation!

[Reply #19 on:]

Further addendum.  After the "tingle of discovery" where once again I found that both of those terminals, though only 18V apart, were about 100V with respect to case ground... well, I found a good +20 terminal on the board and now I can tweak the input voltage of my optoisolator and make the darn thing do my bidding.  FINALLY!  Now I just need to burn the optoisolator board and get the thing connected so I can tweak the circuit.

It's going to be a little interesting tweaking the PID controller output to set an output voltage, but at least all the parts are known now; I think this is actually going to work!  Hoo, almost certainly not worth the effort, but it's getting there.