Slowing down relay/coil activation

Discussion in 'The Projects Forum' started by bman2, Dec 18, 2015.

  1. bman2

    Thread Starter New Member

    Dec 18, 2015
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    Hi, I'm new here.

    Ran into this forum by searches and found good info, but now quite what I need, so thought I could ask. So if anybody has time to share some info I'd be appreciated.

    So thing is, I got a relay. Basically its got coil and moving part, its quite high powered but I dont know all the specs yet, and it'd be good if I'd had idea solution before I get personally to it. Its used for mechanical movement, its not closing any electric circuit so thats important too. Its just moving a mechanical regulation part.
    Problem is, on start of operation, it uses a lot of force and hits other mechanical contact very strongly, too much power. It probably needs all power it has, but problem is its braking mechanism its attached to, hitting it too hard. Solution is supposed to be slowing down initial contact.

    So what I need is a method to slow down activation of that relay, so it doesnt go full power in like 1ms, but slowly getting power and making a softer contact with other mechanical part.

    It needs to be robust and simple, energy consumption is not important. Only thing I thought of so far was adding a capacitor parallel to the coil so that once the circuit is energised capacitor needs to be filled with energy and therefore it slows down activation of relay. Would this work? And if so, how could I calculate or aproximate Farads of capacitor, I will know the voltage, likely its 24 or 72V, and I will probably now know relay details as L or forces.

    Anybody got any ideas?
     
  2. bman2

    Thread Starter New Member

    Dec 18, 2015
    7
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    now=not, typing error, sorry.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Specifications?

    AC?
    DC?
    Voltage?
    Current?
    Pictures?

    Don't make me ask.
     
  4. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    I think you can not use because of the relay. The relay has hysteresis. This operating current more than the current retention. Hysteresis characteristics of a smooth change of the current control turns into a sharp movement of the armature of the relay.
     
  5. bman2

    Thread Starter New Member

    Dec 18, 2015
    7
    0
    Thanks for reply,

    I only need idea solution for now as I'm currently not in proximity of the subject, was told very little details. When I know more I will add here.

    However its definitely DC, it should be fist sized relay, power engineering in a machine, its powered by batteries just dont know yet if its 24 or 72V. For now I'd just like to know what are possible ways of making it activate slower and hit with less power, but later on still all or near its normal force. Could capacitance be a solution, or is there something else?
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,170
    1,797
    What Bordodynov was saying was that he doesn't think using a capacitor will do what you want it to do. A typical relay has one or more sets of contacts with very low inertia. These contacts are normally open or normally closed, and they don't actually move anything. What you have is a solenoid which is activated by a coil. Here's the problem. A solenoid, like the one you described, is not a linear or a proportional device. In order to overcome the inertia of moving part you need to apply enough force to overcome inertia and friction. Once that threshold is reached and the device begins to move the sliding friction is often less than the static friction and it takes less force to keep the solenoid moving. To control the velocity of a solenoid seems like a very challenging control problem. As has been mentioned numerous times details are paramount. Tell us what you know when you know it.
     
  7. bman2

    Thread Starter New Member

    Dec 18, 2015
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    Yeah, selenoid not a relay, my bad. I should have figured it wont be as easy when certain people asked me for help.. I will refresh this topic when I get to see the thing. My suspicion is that it might overcome static friction with less voltage from supply then it gets, it gets full V immediately, thats why I thought capacitor which would slowly allow more and more energy on the coil would at some point trigger movement of moving part, giving it less moving energy then full supply voltage would give it.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    It is current that controls the amount of force (acceleration) applied to the solenoid. Just like in a motor, it is current that controls torque and thus angular acceleration. Also it is not uncommon to see a motor require 3-4 times the running current to start from a dead stop. I suspect from your description that the solenoid might exhibit the same behavior. In which case shutting off the current before it reaches full travel might offer a possible solution.
     
  9. DNA Robotics

    Member

    Jun 13, 2014
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    Maybe use a spring or little shock absorber as the linkage between the solenoid and moving part, and / or a rubber bumper or damper as the stop.
     
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  10. bman2

    Thread Starter New Member

    Dec 18, 2015
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    Thats a good suggestion however I cannot judge on it before I see it myself, mechanism its attached to might require full length of the moving part, something in between would add distance. I got to see the thing for myself.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A motor is different than a solenoid, a motor develops/generates a BEMF in the same polarity as the supply hence lowering the current, in a DC solenoid the current is limited purely by the resistance of the coil.;)
    Max.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    You are right about the back EMF in a motor. The loose analogy was that force is proportional to current, not voltage. Further a motor that is not rotating is a low impedance, just like a solenoid.
     
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a solenoid requires a certain amount of current to start to pull in, but the pull actually increases as the distance traveled incereases. magnetic fields work on a square law basis, the farther apart, the weaker the pull. at half the distance, the pull is twice what it was, at 1/4 the distance, it is 4 times the distance and such. it actually takes much less power to hold the solenoid in than it takes to start the pull. maybe a switch that reduces the current as the core pulls in could help. (like a pinball flipper)
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
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    I don't think playing with the current or voltage will do what you want unless you have a very sophisticated positional feedback/control arrangement. A viscous damper would provide a simpler solution.
     
    Papabravo likes this.
  15. Papabravo

    Expert

    Feb 24, 2006
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    The more I think about it, the more I think you might be right.
     
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