Differentiator as feedback to stabilise rate?

Discussion in 'The Projects Forum' started by Actuator, Dec 16, 2007.

  1. Actuator

    Thread Starter New Member

    Dec 16, 2007
    I just read (with interest) the Rate-of-change experiment:

    I'm considering a project which involves a DC motor which needs to have its speed modified over time so that a mechanical arm maintains a constant rate of angular change. The differentiator circuit sounds like it might be a feasible way to achieve this but I'm an electronics newb so I don't know whether it's the best option for me to investigate.

    The project involves 2 arms connected with a hinge and driven apart by linear actuator controlled by a PWM circuit (the actuator is the 3rd side of the triangle). However, if the linear actuator is driven apart at a constant (linear) rate, then the rate of *angular change* between the arms increases slowly (ie. it isn't a linear rate). I was considering attaching a potentiometer at the hinge to reduce the motor speed as the angle between the arms increases, but wasn't sure how to achieve this.

    When I saw the differentiator idea I thought maybe it could somehow measure the rate of change of the potentiometer at the hinge and, if the rate is increasing, then reduce the speed of the motor to bring the rate of change back to constant. BTW: this would all be running very slowly, only 15 degrees/hour - would that be too slow to register?

    Does this make sense? Is it feasible? Or are there already gadgets/circuits that are more appropriate?

  2. thingmaker3

    Retired Moderator

    May 16, 2005
  3. Actuator

    Thread Starter New Member

    Dec 16, 2007
    No, I hadn't. The process change part sounds very similar to what I would like to achieve. Thanks, I'll do some further research.

    Would an off-the-shelf potentiometer having its knob rotated by only 15 degrees over an hour cause enough change to register in one of these circuits?

  4. gootee

    Senior Member

    Apr 24, 2007
    All of that might work. But maybe you should calculate the kinematics of what the actuator would need to do, to give a constant angular rate, and try to design a drive circuit to do that. Then, if necessary, you could add the angular rate feedback to get better precision.

    - Tom Gootee