Actuator and broken potentiometer

Discussion in 'General Electronics Chat' started by vaka85, Aug 21, 2014.

  1. vaka85

    Thread Starter Member

    Aug 21, 2014
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    Hi everyone,

    I'm working with a linear actuator on a bigger mechanical project, and since I've never worked with them I'd like to have some explanation on them.

    It has a potentiometer inside, to sense it's position.
    How the potentiometer actually works inside? And how it is connected? It has power supply +5V, gnd, and then the output with the read value.
    I have two of them and after few minutes of tests the potentiometers stopped working.
    I explain better:

    I pushed the actuator all the way out, and let him rotate freely for some seconds. It could be possible that this caused the potentiometer run out of scale?
    This is the datasheet
    http://www.thomsonlinear.com/downloads/actuators/Max_Jac_bruk.pdf

    Any other idea of any cause for the potentiometer stop?

    Thanks
     
  2. RichardO

    Well-Known Member

    May 4, 2013
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    Which is the part number for your particular actuator and how did you connect it?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What type of controller are you using and are you integrating it yourself?
    What do you meant the potentiometer stopped working?
    Max.
     
  4. vaka85

    Thread Starter Member

    Aug 21, 2014
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    Thank you for your reply!
    The PN is MX12-B8M05P0.
    It's connected to an electronic control unit that controls other parts of my machine. However the problem is not on the Electronic control unit, that's for sure.

    Sorry, I needed to be more clear. The potentiometer doesn't show any variation of its value anymore. Even if I change the actuator position, the read value remains the same.
    That's why I thought that by letting him rotating freely, I moved it out of scale..

    I connected it as it says on the datasheet:
    1 and 2 potentiometer supply voltage
    3 potentiometer output
    4 and 5 motor supply voltage.

    I don't exactly get why a potentiometer needs a supply voltage to work...Sorry if this is a stupid question..
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The potentiometer requires a supply across the outer ends and the wiper is read as a position usually.
    IOW a simple feedback device.
    If you have a voltage across the pot and nothing out of the wiper then this point to maybe a defect somewhere.
    Max.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Hey, I have the same problem, but with a different brand of actuator.

    It quit working after only a few weeks and a few hundred cycles. I took it apart, and it uses one of these pots. There is a nylon gear pressed onto the shaft. My take is that they screwed up the pot when they pressed the gear onto the shaft...

    In my application, I was drawing only uA of current from the wiper. Could that be a problem??? Only the input bias current of the opamp flows in the pot wiper... (see attachment)
     
    Last edited: Aug 21, 2014
  7. RichardO

    Well-Known Member

    May 4, 2013
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    I am no expert but I think the low current is good.

    I remember seeing servo mount precision potentiometers that had a tag on them saying that you should NEVER test the pot with an ohmmeter. They said that just the current from the meter into the wiper could damage the plastic resistance element. These pots had something like 0.25% maximum linearity error and were hundreds of dollars each in the 1970's!
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Back in the 70's the Ohmmeter could well have been analogue, passing many mA. I find it hard to believe that a modern DMM would pass enough test current to damage the pot.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Also If in this case the pot were that sensitive, the +5v across it would harm it?
    Max.
     
  10. MikeML

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    Oct 2, 2009
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    The one I took out of the actuator is rated at 10K 1W, so I doubt that...
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The one I took out of the actuator is rated at 10turns 10K 1W, so I doubt that...
     
  12. vaka85

    Thread Starter Member

    Aug 21, 2014
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    so, in your opinion, it's impossible to move the pot out of its range?

    I'd love to see how the pot is fisically connected to the wiper.. I mean, the output of the pot is directly the resistance or a circuit reads the voltage across the + and the wiper and produces the output?
     
    Last edited: Aug 22, 2014
  13. MikeML

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    Oct 2, 2009
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    Yes. The wiper travels less than half of the ten turns as the actuator goes from limit to limit. In mine, there was 2.63KΩ at each end of the travel that could not be reached. See the schematic I posted. I connected the ends of the pot to +5V and Gnd. I wanted a signal that was normalized 0 to 5V as the actuator moved, so I amplified the wiper signal with a gain slightly less than 2.

    The actuator simply moves the wiper. If the pot is working properly, the voltage on the wiper is proportional to actuator position, and ratiometric with respect to the voltage across the pot, with the aforementioned dead zones at the two ends...

    The failure mode was that the wiper lost contact with the resistor element (became open). The 10KΩ end-to-end resistance remained intact.
     
  14. vaka85

    Thread Starter Member

    Aug 21, 2014
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    Thank you for the detailed explanation Mike!

    I have to make some tests..
     
  15. Alec_t

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    Sep 17, 2013
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    I have no experience of actuators, but if it's possible to drive one beyond its normal limits then could the gears moving the wiper become unmeshed? The OP did state "I pushed the actuator all the way out, and let him rotate freely for some seconds."
     
  16. MikeML

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    Oct 2, 2009
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    The one I am using has built-in limit switches so that if you drive it all the way one way or the other, the motor current drops to zero, and the motor stops. The wiper only travels the middle five turns of a ten turn pot, so there are approximately 2.5 turns on each end where the wiper never goes...

    The company I bought mine from sells the same actuator with or without the pot, but they all the limit switches...
     
  17. RichardO

    Well-Known Member

    May 4, 2013
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    I agree. However, I would not want to do the test with that expensive a pot.:eek:
     
  18. RichardO

    Well-Known Member

    May 4, 2013
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    I was referring to wiper current. If I remember correctly, the manufacturer said that the resistance at the end of the rotation could be very small. If so, the high current would _only_ damage part of the pot resistance element.

    Another data point -- for what it is worth. I don't believe that these pots have end stops so you can't tell where you are in the rotation by rotating them.
     
  19. MikeML

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    The pot I took out of my actuator (which I linked above) is a standard 10K 10turn 1W pot. A replacement can be bought off FleaBay for $4 (also linked above)
     
  20. vaka85

    Thread Starter Member

    Aug 21, 2014
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    this is exactly what I meant!
    I have to ask with the actuator manufacturer, because there's nothing on the datasheet..
     
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