Can I use PWM and count pulses for feedback?

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  1. #1
    Join Date
    Jun 2011
    Indian Harbour Nova Scotia

    Default Can I use PWM and count pulses for feedback?

    I have a car with a split hood that opens with 12VDC linear actuators, using momentary switches. I want to automate this using the remote control I use for my car doors, since I have two spots available. The hood has to open in a particular sequence, drivers side first, because this part overlaps the other part. I worked out the sequence with wire logic but it is horrendous. I want to use PWM to open and close the hood(s). I've already worked out the flowchart and details regarding sequencing,etc.

    I have no feedback from these actuators, no pot to tell me where they are, and if I go past the extents I could wreck my hood or the actuators. My question: if I use PWM (or something else you might think of), can I count the pulses I send to the actuator and use that number for my "soft-stop"? Could I just send 3440 pulses (for example) to the actuator to fully open the hood? or 1720 pulses to open it halfway? How repeatable would this action be?

    It would be very difficult to install hardware feedback devices.

    For backup protection I would/could/will put in limit switches on both ends to perform an emergency stop.

  2. #2
    Join Date
    Oct 2005

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    Default Re: Can I use PWM and count pulses for feedback?

    I don't think that's going to work very well, you might as well just run the motor for a certain amount of time. In theory that'll work but probably not in real life where things wear, grease dries, battery voltage (and therefor motor velocity) changes and so on. So, there's no 'hard' connection between time (or number of pulses) and the true position of the actuator.

    If you really can't fit any limitswitches or other feedback devices then perhaps measuring the motorcurrent and then trying to calculate the position based on the I*R drop of the armature, motor Kv and time.

    Then again, if the actuators are really sturdy perhaps time and battery voltage is enough to calculate position, just make sure you incorporate some way to "manually" run the actuator in case a fuse blows or whatever in the middle of a 'cycle'.


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