Replace potentiometer in Dart Controls 90vdc motor controller with raspberry pi

Question

My existing setup:

• Rae Corporation gear motor (90vdc, 4.5amps, 120rpm, 150 in/lb torque, Part number 6040042). I can't find a datasheet, but it looks a lot like this
• Dart Controls motor controller 15DV1A, (datasheet) which takes in AC main power and runs the motor. Pots on the board control max and min rpm as well as implementing IR compensation.
• the main speedpot shown attached to the 15DV1A relocated to the end of a long cable with connecting it back to the board. I've measured the voltage across this speedpot - it's about 6.5V from Pot-Low to Pot-High, with the Pot-wiper being somewhere in between depending on the pot position. The site says it's a 5k ohm pot.

Here's the question: I would like to control the motor with a raspberry pi - how?

I see a couple of approaches:

#1 Replace the pot:

• there are variable resistor ICs, but reading around it seems like they would not handle the current that passes through when the resistance is low.

• I've also read that mosfets can be used to create variable voltages, but they are very sensitive and only operate this way over narrow range. Also, it seems like hooking this up is complicated.

• digital potentiometer, like this or this. Would this be able to handle the current as a replacement for the pot? If so, this seems like the best approach but how can I wire it so the 6.5v motor control signal is isolated from the RPi?

#2 Control the existing pot:

• I've seen folks use a stepper motor to control a knob, sort of like this. While I appreciate that approach wouldn't modify the existing arrangement at all, I want something smaller and more direct ideally.

#3 Replace the entire Dart Board

• I really don't want to do this, since the board provides some nice features. But if this would be cheaper than either of the above two, I'm listening.

Answer 1

I suspect that injecting a voltage into the wiper wire from a DAC and a buffer will work. What won't work is a digipot of the type you linked - they are only rated for 5.5V max despite the digital inputs being capable of 12V+.

This also needs to be an isolated solution but you may find that there is enough power from the outer two pot wires to power the DAC and op-amp. This just leaves digital control of the DAC via a digital isolation chip like an ADuM3401: -

It will need a little bit of testing first to ensure that adding say 1k across the pot doesn't significantly reduce the 6.5 volts. Then try injecting a dc control signal from a power supply between most negative pot position and wiper. If this all seems to work you should be OK.

Answer 2

Normally the control pots don't run any particularly large currents.

Have you measured the control pot current? Per your numbers it should be all of 1.3 mA (end to end) and the wiper current in most controls is virtually none (since a large wiper current makes the voltage on the wiper non-proportional to position.)