Weird behaviour of DC servo motor controlled by arduino

Discussion in 'The Projects Forum' started by ptriantd, Oct 6, 2015.

  1. ptriantd

    Thread Starter New Member

    Oct 13, 2014
    Ok, so I have the DC servo motor shown in powered at +-24 V and I am trying to control it with an arduino.
    I have an intermediate circuit on a PCB where I do the necessary voltage transformations in order to have a velocity and position input to the arduino between 0 and 5V.
    Then I use 2 PWM pins of the arduino in order to produce my control signal again with the help of my intermediate circuit. One of them is connected to a non-inverting amplifier and the other one is connected to an inverting-amplifier. This way I can produce both a positive and a negative control signal. This signal is then passed to a power amplifier in order to control the motor.
    The problem is the following : When I produce a 6.6 V signal in the way described above, the motor starts spinning too fast. However, when instead of a PWM arduino pin I use the 3.3 V pin in order to produce the 6.6 V signal the motor is moving as fast as expected. The input current to the power amplifier is in both cases 70 uA. Any idea what is going wrong?

    I have attached the schematic of the intermediate circuit.

    The power amp is on a different board and it apparently works fine because it has no problem working with the feedback console that comes along with the mechanical unit.

    I don't know if it matters, but this experiment was working while my circuit was still on the raster and it also worked the first time I tried it with the PCB, but it suddenly stopped working ever since.
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Welcome to AAC!
    What are the symptoms of 'stopped working'? Are voltages at all points in your circuit as expected?
  3. ptriantd

    Thread Starter New Member

    Oct 13, 2014
    Intermediate voltages are all as expected (checked with a voltometer), but for small output voltages (below 5V) the motor does not move and for higher voltages the motor spins at full velocity.
    I figured that if I put an RC filter between my output and the power amp, kind of fixes the problem, but this does not explain why the circuit was working fine all that time and suddenly needs the RC.