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Dueling PWM speed controllers.

Discussion in 'Sensors and Actuators' started by Doug Simmers, Jun 10, 2018.

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  1. Doug Simmers

    Doug Simmers

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    Jun 10, 2018
    Hi, Doug Simmers here. I am designing a 2-axis solar tracking structure that will hold focusing mirror dishes pointed to a common reciever that will heat up water. I've purchased a 2-axis "shadow tracker" from Home CSP which works well, but my gear reducer drive motors (DC brushed) are too fast, and the controller overshoots, then corrects in a never-ending cycle back and forth. The controller outputs a PWM signal to each motor, but does not have any speed adjustment. I've tried using small third-party PWM adjusters to modify the embedded PWM signal coming out of the controller, but it's not working. I suspect that the embedded PWM and the third-party units are conflicting. Any ideas on how I might resolve this issue?
     
    Last edited by a moderator: Jun 10, 2018
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Welcome to EP, Doug.
    I changed the font of your post to a readable size :rolleyes:

    Couldn't you simply reduce the supply voltage to the motors (or the motor controller, respectively)? Brushed DC motors run slower at lower voltage, that may all that's needed to resolve your issue.
     
  3. Minder

    Minder

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    It is very difficult to position DC motors accurately without some kind of feedback, or extremely high gearing, or quadrature encoder for e.g.
    This is the reason many types of these applications use stepper motors as they step to a angular position without any kind of feedback.
    M.
     
  4. Doug Simmers

    Doug Simmers

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    Jun 10, 2018
    Thanks. The controller will work at some lower voltage level before failing.
    The shadow tracker operates on a feedback loop, as opposed to celestial tracking (time of day and season) . A set of photocells are arranged in a N, S, E ,W array, with a square shading piece to block the sun above. When the sun moves, one or more photocells will become shaded, which triggers the motor(s) through the controller. When the wind blows the tracker off of position, it adjusts back.
    I understand that the stepper motors work incrementally, and also hold position better, but I'm trying to keep the costs as low as possible. I may end up having to accommodate the steppers and new controller.
    Thanks.
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Another option, if possible, is increasing the gear ratio.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Also allowing a window for acceptable positions so that you're not hunting for the exact position.
     
  7. BobK

    BobK

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    Which could be done by reducing the size of the thing casting the shadows, or moving the photocells outwards.

    Bob
     
  8. Doug Simmers

    Doug Simmers

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    Jun 10, 2018
    This is sort of the crux of the problem. For both azimuth and elevation, I only need to travel about 180 degrees once per day. I might have to group two gear motors together to get this fractional rpm. I'll try the reduced voltage that voltage that Harald suggested.
     
  9. BobK

    BobK

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    What speed is your gear motor running at? I would think it should be on the order of 1 RPM. If it is much faster, of course it is going to overshoot.

    Bob
     
  10. Doug Simmers

    Doug Simmers

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    Jun 10, 2018
    The gear motor runs at 3 RPM, but the bevel gears they drive reduce it further to result in 1.5 RPM.
     
  11. BobK

    BobK

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    Hmm, that sound like about the right speed.

    Bob
     
  12. Incony

    Incony

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    Apr 4, 2017
    hmm.. the DC motor is governed by magnetism and brushes determine its position, and they are designed to keep it turning, - always moving from one point to the other out of step. The most dramatic example of that is the uniploar motor ( look on Utube ) which only needs three things... a battery. one piece of wire and a magnet.. one way to stop the DC motor overshoot, would be an electromagnetic brake. ie a disk on the motor shaft that normaly spins with the motor, and a coil thats energised at the stopping point and pulls the disk to to its face along its splines and locks the two together... its a common brake control for motors.. the distance for the disk to move to lock to the coil plate will be tiny... less than a mm probably, with the right kind of construction it could work like a slipping clutch.. the motor becomes disengaged and the brake applied... and the motor can overshoot all it wants then... its not connected to thing one is trying to stop... in theory one could use a dual acting clutch... and the motor could run all the time constantly.... and its the dual clutch that engages to the motor , or the brake... one or the other... it moves or is fixed... one state at a time. What that would mean is one could set the brake to run on a very slow pwm pulse frequency..60 pulses per minute say..and then it would keep time with the sun.. it wouldnt care which way the motor is turning.. it only going to be connected to it once a second.. for a set timed pulse, and for the rest of that second its being braked.. constructing such a dual acting splined clutch would mean you can use a common DC motor, brushed or brushless... since it is no longer governing the distance one wants to move, just the torque and direction.. its the clutch and pwm that determine the distance... not the motor.
     
    Last edited: Jun 18, 2018
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