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Solenoid Piston help

Discussion in 'Sensors and Actuators' started by Jason Barker, May 28, 2018.

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  1. Jason Barker

    Jason Barker

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    May 28, 2018
    Hi, thanks for reading. I have started a project to create a solenoid piston, i hope im calling it right im not the greatest at electronics but trying to learn. I will be using a 3d printer to create a housing and other parts but my issue is with the electronics.

    I will use a row of disk neodymium magnets as the core and surround this with solenoid, so when electricity is applied the solenoid shoots forward approximately 3.5-4 cm and then return.
    i intend to test several methods of making the piston return from a spring to electrical methods.

    I am however very unsure of the type of circuit i should be using, and since i am inexperienced with electronics i am having trouble finding the information on google.

    I have included a picture of my gen1 design so the cylinder on the left are the magnets which will measure 15mm diameter and 50mm in length
    the middle cylinder is the solenoid and will have copper wire around it there is approximately 2mm gap between the magnets and the piston with an additional 1mm for the thickness of the material which i may make thinner testing is required.
    The cylinder on the right holds everything in place and has holes for wires.

    Thanks for reading! I hope someone can help me out :)
     

    Attached Files:

  2. Externet

    Externet

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    Aug 24, 2009
  3. Jason Barker

    Jason Barker

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    May 28, 2018
    thank you for the reply however i did first of all look at google extensively, none of those are what i am trying to do. If it was that easy for me to find i wouldn't have posted here.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Is it your intention that the solenoid (with wires attached) be the moving piston? This is not a good idea because, if the piston must cycle numerous times, the wires will eventually break from fatigue stress. A better solution might be to use the neodymium magnets (perhaps super-glued together?) to act as the moving piston constrained by the inner diameter of the solenoid. This would also allow you to choose two alternate positions for the "piston" depending on direction of current in the solenoid, which would remain fixed and attached to the outer support cylinder. Of course with no current in the solenoid, the piston position would be indeterminate since there is no provision (yet) for a restoring force.

    More details, please, on what you are trying to do and fewer preconceived notions of how to do it. Are you aware of the highly non-linear relationship between solenoid current and the force on a solenoid armature? What is the purpose of the neodymium magnets? Why would you want the solenoid to act as a piston?
     
  5. Jason Barker

    Jason Barker

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    May 28, 2018
    It is my intention that the solenoid be the piston as acceleration and speed is important and being much lighter than the magnets is the best part to be moved.
    the mechanics involved is not a concern mechanics im good at.

    the device will have a piston head attached to it and will compress air in a cylinder out through a hole. i made a small animation to show this



    Red are the neodymium magnets, blue is the solenoid, yellow is the piston head which will be attached to the solenoid which compresses air in the dark grey cylinder against the green cylinder head and forces it under pressure through the red nozzle. speed and acceleration is needed and since the solenoid is lighter than the magnets it will accelerate quicker which is important.
    I am open to suggestions as i said electronics is not my area of expertise ideally a low tech solution would be preferable with a spring in between the piston head and cylinder head but i am unsure if i could get the force required that way.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Okay, not much we can offer you since you seem convinced that the moving coil (solenoid) is the best and/or only solution to your problem. Your construction is reminiscent of a loudspeaker whose voice-coil (your solenoid) is suspended in a magnetic field (your neodymium magnets) by a flexible "spider" mechanism and whose coil wires are attached (soldered) to braided flexible wires to avoid metal fatigue effects. Perhaps your outer piston cylinder serves a similar "centering" and guiding purpose to that of a loudspeaker spider. In a loudspeaker, the spider also serves the important function of restoring the voice coil to a known position when there is no current in the coil, thus maximizing the back and forth motion possible.

    Depending on size, required frequency response, and power handling capability, the linear travel of a loudspeaker voice coil can be quite large, but it is eventually limited by the compliance of the spider mechanism and the edge compliance of the speaker cone. Your design has no such limitation since the solenoid is free to slide any distance within the outer cylinder. You may have a problem with friction between the outer diameter of the solenoid and the inner diameter of the enclosing cylinder.

    You may be correct in assuming that your freely sliding solenoid will have less inertia and will respond more quickly than a moving armature located within the solenoid. On the other hand, given sufficient current in the solenoid coil, and a sufficiently strong neodymium magnet sliding within its core, you can probably get equal or better performance from the neodymium magnet (attached to the rear face of the piston) without worrying about how to conduct current reliably to a moving solenoid. That design would also lend itself to a linear bearing to support the piston during its back and forth travel. A stationary solenoid is also much easier to cool since you will likely want to run the piston as fast as possible, meaning maximum current through the solenoid. Note that with either design approach, the piston position and direction of motion is determined by the direction of current through the solenoid. No need for a restoration spring and the mechanical resonances it would introduce. Just reverse the current in the solenoid at each end of the travel of the piston. Of course there will be some "springyness" associated with compressing the air in the cylinder, but presumably you have taken that mechanical consideration into account.

    As far as electrically exciting the solenoid is concerned, most folks use a power MOSFET for this purpose. Since you will probably want to reverse the current in the solenoid at the end of the compression stroke, and restore the original current direction on the next compression stroke, a so-called "H"-bridge configuration is normally recommended.

    Care must be taken in the design of the "H"-bridge circuit to allow for dissipation of the electrical energy stored in the magnetic field of the solenoid. This must be done to avoid damage to the MOSFETs which are acting as very fast on/off switches. Most MOSFETs have a so-called "body diode" that conducts when the magnetic field of the solenoid load collapses or reverses, thereby protecting the MOSFET from damage but limiting how fast the circuit can cycle. You must also carefully design the gate drive circuits for the MOSFETs to avoid "shoot thru" where two MOSFETs connected between the two power supply rails simultaneously conduct because of gate capacitance effects and poor timing design. There are commercial integrated circuits that implement "H"-bridges with significant current capacity and other features, such as pulse-width modulation (PWM) of the current, with negative-feedback current sensing to control the PWM duty cycle.

    About the middle of the last century I had the dubious opportunity to examine a "one of a kind" proportional-flow hydraulic servo valve based on a light-weight aluminum voice-coil actuator, about two inches in diameter, and wound with a single layer of enamel-insulated copper wire. Rare earth (neodymium) magnets hadn't been invented yet, so this puppy used a huge Alnico permanent magnet to create a magnetic field for the voice coil to react against.

    IIRC, the voice coil was just the first stage of the valve, controlling flow with a small spool valve attached to the voice coil actuator. That small spool in turn operated a much larger, high flow-rate, spool valve. This "servo valve" was supposed to control a 1,000,000 Lbf (static load) dynamic fatigue testing machine capable of producing dynamic tension and compression loads of ±500,000 Lbf with sixty hertz sinusoidal motion. IIRC, the piston had a diameter of about twelve inches, a stroke of about eighteen inches, and operated with 3000 psig hydraulic pressure. With this much flow and pressure, most engineers would opt for a variable-displacement pump. The contractor opted for a less expensive fixed-displacement pump equpped with a dump valve to modulate the pressure down to 3000 psig.

    This California company that won the low bid for the contract to build the system went bankrupt trying to get their valve and actuator piston/cylinder to work to Air Force contract specifications. The actuator piston and cylinder were actually quite competently designed and built, but they decided to save some money by designing their own version of a high-performance hydraulic servo valve. An existing off-the-shelf Moog high-performance hydraulic servo valve eventually got the machine working to the satisfaction of the Air Force... years after the original principals went bankrupt and defaulted on their contract. Let us hope you never place yourself in such a precarious position because you underestimate the complexity of a job.

    Good luck with your "flying solenoid" thingamabob.
     
  7. Jason Barker

    Jason Barker

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    May 28, 2018
    Thank you for the reply its very helpful, I did say i was open to ideas, i am well aware of my lack of knowledge in this are and this was just the idea i had. as long as the piston head moves as in the video i would be happy with shrinking hamsters and making little wheels and have them do it.
    I will also run experiments as you suggest with a stationary solenoid and magnets on the piston head.
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    In most engineering fields the KISS principle is applied. In your animation, the piston head, attached to the solenoid coil, compresses air in a cylinder while allowing the air to escape through the small red orifice. There is no indication that any sort of valving is applied at any time. So, on the downward stroke, there will be a partial vacuum formed in the cylinder as air tries to rush back into the cylinder through the red tube. All the more reason to reverse the current in the coil to instantiate the downward stroke. It would probably also be desirable to implement some form of position feedback so the electronics "knows" when to reverse the solenoid current.

    When using solenoids to move things magnetically, it is important to understand the importance of the magnetic circuit through which the components move. For example, the outer cylinder should be a soft-iron material that forms a magnetic path with one pole of the neodymium magnets, coaxially with the magnetic field produce by current in the solenoid winding. The magnetic path is always a closed loop of course, and it is completed in air at the top of the solenoid, unless you extend the neodymium magnets to pass through the solenoid and connect magnetically with the outer magnetic cylinder at the cylinder head. This would require a clearance hole in the piston head for the magnets, and an offset hole for the gas escape tube, but it would be a more efficient use of the magnetic fields.

    If you think electronics is difficult, the knowledge of genetics and genetic engineering associated with shrinking hamsters is much, much, worse. Plus you now have to add mechanisms to feed and water the hamsters and remove their poop and urine lest it "gum up" the mechanism. The moving solenoid doesn't sound quite so bad compared to the complexity of a bio-friendly solution.:D
     
    Jason Barker likes this.
  9. Hopup

    Hopup

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    Jul 5, 2015
    You should probably look how high power car audio subwoofers are constructed. They can travel over 40mm without much problem and still maintain good control. Using flexible spiders would probably be good idea. Your biggest problem is the heating of the coil/solenoid if it has to run long time.
     
    hevans1944 likes this.
  10. Jason Barker

    Jason Barker

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    May 28, 2018
    thanks for the sugestions, once my magnets arrive i will test everything XD
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    Which way are your magnets polarised? Across a diameter, or orthogonal to the major faces? From your sketch it looks as though you are assuming the latter.
     
  12. Jason Barker

    Jason Barker

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    May 28, 2018
    That is correct, and they are :)
     
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