RE: Rullit #6 - The software (update)

robogary — Thu, 08 Sep 2022 13:57:07 GMT

sorry, i forgot to mention why soft start doesnt limit instantaneous current. The PWM voltage is battery volts, always. Chopping the pwm waveform to 10% duty reduces the average volts, reducing average current , but not the peaks.

RE: Rullit #6 - The software (update)

robogary — Thu, 08 Sep 2022 13:39:16 GMT

Nicely written blog. If the failure mode is truely instantaneous current, a soft start probably wont help. The soft start doesnt limit current of the stalled motor, but it does limit the time spent in instantaneous current while stalled. A short term patch could be to simply add a series resistor with the motor. Measure the motor's resistance at the motor terminals, take several readings, and turn the shaft a bit for each reading so get an average resistance measurement. Use your battery voltage's highest value, and divide by 2 amps to get a desired circuit resistance value. Leave some margin in the peak amps rating. Subtract the measured motor resistance, the remaining resistance is your stall current protection resistance.

Interestingly enough, that same circuit was used in industry back in the analog control system/dc motor days. It is called a droopy speed regulator. For example, if a conveyor has alot of driven rolls for something heavy like a steel girder, if every roll is a hard speed regulator, each will go to limit to push that girder. Each roll wants to set the girder speed, but nothing is perfect, so they will all fight each other. Using a droop resistor in the motor circuits, the roll will slow down because the resistor has more voltage drop as load increases. All the motors were fed from really big adjustable volts dc power supply. Modern day motor controls have current feedback for every motor, so that feature can be put in software.

Your motor will slow down at high loads, maybe that is a desireable trait in your application.