Control Voltage of DC Motor

Discussion in 'General Electronics Chat' started by animartis, Sep 12, 2016.

  1. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    Hello everyone,

    I have a cabinet that raises and lowers with the use of a windshield wiper motor and a cable attached to the top. I have limit switches to stop the movement in both directions, but the problem is that it travels so fast when going down that it often will blast right by the bottom limit switch. I not not very circuit literate, but was hoping there was a way to limit the voltage on the down swing so the cabinet didn't travel so fast and would be stopped by the limit switch. For example, instead of providing 12V to the motor, I want to reduce it to, say, 9V. Any advice would be GREATLY appreciated!

    Here is an example of it in action.



    Thanks!
     
    DGElder likes this.
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
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    What kind of power supply are you using?

    Just replace it with a lower voltage unit?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is there any possibility of moving the LS up so that the overun achieves the correct hight?
    Max.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,103
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    Move the switch? Change the gear ratio to slow it down? Reducing the voltage might help, but you may want to test that before you commit to that strategy.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    If you have less current in the down direction, there will be less torque. Less torque means less acceleration, and thus lower velocity. Depending on the mass involved I could see running the motor in the up direction with so little torque that the cabinet falls gently in the down direction.
     
  6. cornishlad

    Member

    Jul 31, 2013
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    25
    Could you not place the down limit switch higher up so the cab descends to it final position under its own momentum ?
     
  7. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    All great responses and suggestions, thank you. The problem is not that it stops too low, but completely go past the limit switch and the motor keeps running. I've attached some pictures that might help clarify. I am using a two channel wireless relay and believe that if I can reduce the voltage on one side, it should move slow enough to engage the limit switch without going past it. The power supply is a 12V motorcycle battery. As you can see in the pictures, there is just a single spool and no gearing. I could swap that out for a smaller spool but that would impact have fast it goes up, which I don't want. The last picture shows how I engage the existing lower limit switch.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    You still don't get it. Speed depends on torque and torque depends on current -- NOT VOLTAGE!!
    You want to restrict the current when moving down to a value that lets the cabinet descend at an appropriate speed. When you hit the limit switch you want to reverse the motor to decelerate the cabinet for a smooth landing.
     
  9. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    That's why I said that I was circuit illiterate in my initial post. So maybe a better question is, "How do I reduce the CURRENT in my existing design?" Your suggestion doesn't make any sense to me. The torque and current when the cabinet is going up is fine because it is under a load. Its just when its not under a load that it goes too fast.
     
  10. ramancini8

    Member

    Jul 18, 2012
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    118
    Have you tried just using gravity on the way down? Simply use a solenoid to uncouple from the motor for the time it takes for the box to slide down.
     
    cmartinez likes this.
  11. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    Hi ramancini8, I have not, but it is a good suggestion. I fear that weight of the cabinet is too great and it would travel even faster if I were to uncouple the motor. I truly believe that figuring out a way of slowing down the motor in the downward direction would meet my needs, I just don't know how to do it. Would a potentiometer work?
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can't move the switch up?
    Can you lower the voltage to a value that still raises it?
    Max.
     
  13. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    Hi Max (love the name), the problem is not the location of the lower limit switch, but the fact that cabinet is moving so fast that it engages/disengages the switch as it presses it then goes past it. That's why I'm thinking that reducing the current is the right place to start. Do you think something like a rheostat on one leg of the circuit would work?
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As I mentioned I would first look at detecting what the minimum voltage you can go with and achieve both raise and lower as a possibility.
    You reduce the voltage to reduce the rpm on a DC motor.
    Max.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    I'd hang a counterweight over a pulley, so that up and down are roughly the same.
     
  16. wayneh

    Expert

    Sep 9, 2010
    12,103
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    Maybe use a block of wood instead of the metal bar. The wood could extend farther up so that the switch stays engaged. Maybe you don't have the room for an extended piece of wood when the car is at the top of its travel?
     
    cmartinez likes this.
  17. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I was about to suggest exactly the same thing. Design the lever activating the limit switch in such a way that it always engages the switch, even after it goes past it. And then maybe move the switch a bit up, until you get the desired stopping height.
     
  18. Papabravo

    Expert

    Feb 24, 2006
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    Google "Voltage Controlled Current Source". Using this concept you can adjust the current to a value that just holds the load in place. Now decrease the current a little bit and the load will start to descend. We had exactly the same problem with early hard drives. In to out seek times were consistent when laying flat on the table. When the drive was placed on it's side the garvity assist made the in to out seek time much less than we expected.
     
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  19. animartis

    Thread Starter New Member

    Sep 12, 2016
    7
    2
    You are correct, there is limited space on all sides of the cabinet, as well as the top and the bottom.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Have you tried the lower voltage, It must have quite a bit of overrun on that motor, as it is very unusual, if not impossible to back feed a worm and pinion in most cases!
    Max.
     
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