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Need a specific kind of motor - does this exist already, and am I on the right track?

  1. Nov 2, 2014 #1
    I want an electric motor that works a bit like a servo motor with a steering horn on it, like so:


    But it needs two specific properties:

    - it creates a controllable amount of force along a given axis, independent of the arm's location, up to the arm's limit of motion

    - it allows the arm to move freely when required

    This is different to a servo motor:

    - servo motors move the arm to a specific location, independent of how much force is applied, up to the stall force

    - servo motors leave a lot of resistance on the arm even when completely unpowered

    The design I'm thinking of involves having a magnet inside a coil, and attaching the arm to either the magnet or the coil and letting it slide freely. This would create linear motion, as opposed to the servo pictured above, which creates circular motion which must then be converted to linear motion.

    Firstly, when a voltage is applied to the coil it should create a force between it & the magnet that is independent of their relative positions, provided the magnet hasn't left the coil. Is this correct?

    Secondly, if I short the coil's circuit so that it has virtually no impedance on it, this should let the coil & magnet move with negligible electromagnetic resistance. Is this correct?

    Thirdly, does this design already exist? No sense in developing it if I can just buy one.
    Last edited: Nov 2, 2014
  2. jcsd
  3. Nov 2, 2014 #2


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    Perhaps something like this rotary-linear actuator?

  4. Nov 2, 2014 #3
    Well, that doesn't look like what I'm after. From what I can google, it looks like linear actuator is the name for a whole range of linear motors, most of them driven by screws. Also, the broken English on that product's description doesn't reveal very much about how it operates :) However, following a few links I discovered the term "direct drive linear motor" which is essentially what I've described above. Most of them are servomotors, which is more complex than what I'm after. Maybe I can modify one.
  5. Nov 2, 2014 #4
    Not sure if youve looked at any companies yet but you can just have the motor made! A place like Maxon Motors Usa they will create any kind of motor as long as you know the specifics! They will even help you with the specifics if needed. Theres a few companies here that can create that motor for you! Iam currently going through the process myself.
  6. Nov 2, 2014 #5
    It's been awhile, but there is a company that specializes in free moving magnets in a long solenoid. That sounds like what your describing in that the for driving the magnet is proportional to the current through the coil. The same mechanism has been used in optical drives to position the lens. This also sounds like the armature of a common speaker.
    As to shorting the wingdings and getting a continual movement, that doesn't happen. Why? Because the magnet is producing a voltage in the windings as it's lines of flux cut through them. If you short the windings, this produces a current that counters the direction that the magnet is moving, and causes it to slow to a stop.

    If you're curious about this phenomena, Youtube has a number demonstrations in which people drop Neodymium Iron Boron magnets down copper tubes to show how slowly they move due to this current.
  7. Nov 2, 2014 #6
    Okay, so in fact opening the circuit and allowing no current flow is the way to make the magnet move freely? That's interesting. I'm obviously missing something fundamental in how electric motors work. I know that if you're driving a lot of work with an electric generator by hand, you can feel the extra load physically. I assumed that meant that low voltage = low physical resistance, but maybe it's low wattage = low physical resistance? Or is it more complex than that?

    Anyway, not having to short the circuit makes it simpler from a control standpoint. I'll look into getting a motor made.
  8. Nov 2, 2014 #7
    Yep, opening the circuit allows the magnet to spin freely, though if you look at the voltage on the generator you'll see it is present and increases with the speed. As you load the generator, the torque you experience increases with current. Thus the shaft watts = angular velocity x torque and electrical watts = voltage x current.
    I used to design motor drives, and it was kinda fun because you could control the current and get a constant torque reaction or control the voltage and get a near constant velocity. Controlling torque was easiest and safest electrically, but introduced an additional low frequency pole into the control loop due to inertia, so it was a pain to stabilize.
  9. Nov 3, 2014 #8
    Okay, thanks for the explanation. So in fact in practice the physical resistance goes down as the voltage goes up, because the current goes down, which would be the opposite of what I said before?

    I'll be driving this from a fixed battery voltage using PWM, so would that be an appropriate way of meting out the force?

    EDIT: Also, Maxon Motors just got back to me - they sound pretty helpful! They've got an Australian branch, which is nice since that's where I live :)
  10. Nov 18, 2014 #9
    So Maxon Motors politely told me to get lost once I explained my requirements. Maybe they were turned off because I wasn't planning to order thousands of them, or something. They didn't answer my questions as to what part of my requirements weren't suitable.

    Anyway I'm now looking for existing linear motors that I could perhaps modify.
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