Linear optoisolator for motor control input...?

Discussion in 'The Projects Forum' started by vputz, May 22, 2012.

  1. vputz

    Thread Starter New Member

    May 22, 2012
    Curious problem. I'm trying to make a little control circuit for a potter's wheel (for our meteorology folks; they use it for a spin tank and want to precisely control its rotation rate).

    Good so far. Currently the thing is controlled by a KBLC-240ds motor controller by KB electronics, which SHOULD have been a slam-dunk; you should be able to pass 0-5v dc between two terminals and have voltage following control the motor.

    Unfortunately a call to KB after some false starts revealed that this has been OEM modified so that this no longer works; off to plan B. Right now the beast is controlled by a potentiometer wired as a rheostat between the "high" and "wiper" terminals. I'd like to replace that with something our microcontroller can twiddle, and isolate it.

    Now, if I pull the pot off, the two terminals have a 20VDC difference between them; with the pot in place, voltage drop across the pot is around 8-11 VDC. Voltage between either terminal and ground is stranger (much higher and may be AC). But let's say I've got 20 VDC to deal with. Very low current, on the order of maybe 0.05A? I believe it's a 600k pot, so that makes sense.

    Is there a way to use a linear optoisolator like an HCNR201, loc111, il300, or some such to turn 0-5v input into a 0-20v voltage drop between those two terminals?

    Thanks--I'm more a computer person than an EE type, but I'm learning--it's just that the sample circuits for the linear optoisolators give me loads of information and it's a big step from there to figuring out which op-amps to buy and resistors to use... if this is even an appropriate solution.
  2. mcasale

    Active Member

    Jul 18, 2011
    Can you post a schematic of what you currently have?
    Also, what are the voltage/current specs of the motor.
    I assume it's a DC motor?
    How about a link to the "KBLC-240ds motor controller"?
  3. vputz

    Thread Starter New Member

    May 22, 2012
    I can certainly post a link to the controller manual: here. A relevant picture from the manual is attached.

    The only problem is that it's not as useful as it should be. Instead of a potentiometer being connected between p1/p2/p3 (low/wiper/high), p1 is jumpered via a resistor somewhere, the pot is connected only between p2 and p3 as a rheostat, and the instructions to supply an "isolated analog signal (0-7VDC) to the input terminals P2(+) and F(-)" for a voltage following mode don't work (verified by the manufacturer; this has been modified from the stock and they assure me the instructions for voltage following mode aren't applicable--but there must be some way to do it, since basically the potentiometer is just providing a voltage to P2...).

    The motor is a 1/4 hp 6A max DC motor; getting to its specs plate would involve dissasembling the potter's wheel further, and really rather just address this as a bit of a black box in the sense that the motor controller is working just fine (with the manual potentiometer) and I'd rather supply the right voltage to P2 and use the controller as is rather than basically creating a new controller for the motor.
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  4. vputz

    Thread Starter New Member

    May 22, 2012
    A mite more input:

    I took a DMM to the terminals again just to check. The terminals (P1-P3 and F-) have DC offsets relative to each other, but much larger (and AC) offsets with respect to case ground, which is interesting.

    P3-P2: about 20VDC
    P2-F-: about 0.8vdc
    P3-F-: about 20VDC
    P2-P1: about 0.8vdc ("p1" is where the resistor is jumpered)
    P1-P3: about 20 VDC

    Okay, so that makes some sense with respect to the original manual's voltage following setup. The situation with ground interests me...

    gnd-F-: 67.2vac, 53.8 vdc (just relaying the DMM readings)
    gnd-P2: 66.9vac, 52.4 vdc
    gnd-P3: 66.6vac, 32.35 vdc

    (so it sounds like they're AC with respect to ground but have a constant DC offset between each other?)

    So the key to all this seems to be supplying the right kind of potential to P2. I'm just not sure how to do it in an isolated, microcontroller-based way.
  5. vputz

    Thread Starter New Member

    May 22, 2012
    So... near-complete ignorance of proper solutions can be liberating, maybe. I'm now considering using an HCNR201 opto-isolator with a couple of LM741 op-amps. The relevant unipolar input circuit from the CNR200 datasheet is included. My thinking is that if I hook up the left side to the microcontroller (VIN+ to the PWM pin, VCC to +5v, GND to ground) and the right side to the motor controller (VCC2 to P3, GND2 to F-, VOUT to P2), and use R2=400k, R1=100k, this may generate the right output to P2 to control the thing.

    I honestly have no idea if this would really work, but I tried learning ngspice today and whipping up a comparable circuit (the only thing that didn't work was I had to set gnd2 to ground or spice gave fits, but of course on the real circuit they'd be two separate grounds for isolation). It seemed to give me decent results; varying Vin from 0-5v varied Vout from 0-20, which I *think* would do what I want...

    ...or am I deluding myself?