Limiting Resistance Range of Potentiometer

Question

Im working on limiting the resistance range of a potentiometer that feeds into a motor control unit. This motor control units runs an electric motor. We want to limit the joystick switch pot range to 40amps in both forward / reverse directions.

• The joystick pot is 1K resistance

• The motor can a pull a maximum of 60amps

• The potentiometer is on a low voltage (5V) circuit that feeds into the motor control unit

We cannot modify the motor control unit circuitry at all

To start ---

We measured the voltages out of the potentiometer at a 40 amp draw to the motor and sized our resistors to put in series with high / low potentiometer legs to limit the measured voltage drop.

Using these voltages we have two 84 ohm resistors in series with the pot to limit the resistance range of the pot. These resistors theoretically limit the allowed voltage range via resistance of the switch pot to pull 40 amps forward / backward.

We simulated the circuit and confirmed that we were getting the correct voltage out of the pot to limit to 40amp draw to the motor.

We physically created the circuit and tested on a breadboard with the motor control unit and motor except we only got a limit of 40amps in the forward direction and no limit in the reverse direction. We also verified the voltage out of the pot was correct and the same as our simulation.

Below is a photo of our circuit.

At this point we think the non-limiting in the reverse direction is internal to the motor control unit and does not have to do with our circuit. We are at a standstill on how to solve this issue, any help would be appreciated.

Answer 1

First you need to know how it works under normal conditions. You could hook it up the way it originally was and plot the motor current vs. control voltage at a number of pot positions. The most important should be fully ccw, centered and fully cw. Then maybe divide each side in half and then those divisions in half again. This would give you nine equally spaced points. Graph the results and you should get a pretty good idea of what’s going on. If you need better resolution divide each division in half again.

Answer 2

I suppose that you have if you turn the potentiometer to the minimum you get max speed in one direction, in the middle position no speed, and at the maximum in the other direction. Correct?

If so, then no speed should be at 2.5V.

If we suppose that current is proportionnal to V_pot-2.5V (it's probably a somewhat wrong assumption, it's quite possible that it's PWM (ie average voltage) that is proportional instead), then if you want 40amps out of 60 amps max, you need V_pot=(2.5V-2.5V*(+-40V / 60V)) = 0.83V or 4.16V

With your 84ohms resistors, you get far more "extreme" voltages : at low end, you get 5V84ohms/(84+1000+84 ohms) = 0.36V, at the low end you get 5V(84+1000 ohms)/(84+1000+84 ohms)=4.64V

So I would advice you try increasing the values of the resistors until you get the current you want (you might choose different values for each resistor, if you need to adjust the current in one direction more than the current in the other direction).

NB : you can of course try to compute the theoretical resistor values, but it will probably not be very reliable because :

• there might be some extra resistors inside the control module
• the potentiometer might not be linear
• there might be some saturation, deadbands or other non linearities, either in the potentiometer itself or in the controller
• we don't know if the motor is controlled in voltage, in PWM, in current, in power or something else

So I would recommend an experimental approach were you increase the value of your resistors until reaching the current you want (or you increase them a lot initially and then decrease them slowly if you want to avoid running your motor above 40A while trying to find the best resistors)

Answer 3

Yes. Plot the motor current vs. pot output voltage, particularly at 0A and 40A in both directions.
That will then show what needs to be done to limit the current both ways.