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Help me choose a linear solenoid

  1. Nov 27, 2012 #1
    Hi all,

    I'm building a tennis ball launcher for my dogs, and was successful in building a spring loaded one with a "whacker bar" that rotated on a pin. A wiper motor turned the bar until the bar got to the half way point around the pivot, then the spring would pull the bar, hit the ball, and the switch would turn off and turn the motor off.

    Anyway, it works OK, but it's pretty big, and takes a couple of seconds to execute (at which time the dog anticipates the launch) so I wanted to try to make one with a linear solenoid.

    The problem is, I don't know what size I would need. Figure I want to toss the ball 20 or 30 feet. A tennis ball weighs approx 58 grams.

    I'm sure the calculation isn't going to be dead nuts accurate, but I need a starting point.

    Some of these things are rated in Newtons, others in Kg. For example:

    http://www.sourcingmap.com/12v-15a-...ush-type-solenoid-electromagnet-p-288060.html

    thanks for the help!
     
  2. jcsd
  3. Sep 5, 2014 #2
    The distance for an object (assuming no drag) is d = v*v/g*sin(2*angle). You can assume an angle of, say 30 degrees (hence, sin(60) is .866 to make calculations easy) or d/.866 *32.2 = v*v. If you want to throw your ball 40 feet, then v is sqrt(40/.866*32.2) (about 39 ft/sec). Hence, the energy needed to launch your ball is its mass (plus solenoid plunger mass) times v*v * stroke time (guessing .05 sec) or approximately 1600*1/8 * .05 or 10 ft-lbs and your solenoid need to be able to handle that amount of energy for, say, .05 second.

    Ultimately, all you need is a solenoid with sufficient stroke (maybe 1"?) and a pulse source to the solenoid (so it won't burn out). I suggest using a capacitor and SCR to fire the solenoid with a battery or power supply of about 20 volts or so (just guessing). You can estimate the voltage required by measuring the solenoid coil resistance and computing the current it can handle at a given voltage. The SCR needs to be able to handle the maximum current -- no heat sink is required. You can control the distance by varying the charge on the capacitor. A mechanical switch can replace the SCR for initial testing -- it will eventually burn out its contacts if heavily used.

    If you want more details, look at the various rail gun web sites. In their case, the projectile is the solenoid plunger but the ideas are the same. I just thought: You have both the solenoid plunger and the tennis ball to accelerate -- some idea of the weight of the plunger would be helpful.
     
    Last edited: Sep 5, 2014
  4. Sep 5, 2014 #3

    Mech_Engineer

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    I would recommend one with more than 1" of stroke, my feeling is you'll be better off with something that has 4" or 6" of travel. Remember the solenoid has to impart all of the energy on the ball over it stroke, and the ball will deform too.

    If we assume your ball has to shoot at about 40 ft/s, then the kinetic energy you'll need is about 4.3 J not including the weight of the solenoid's plunger which will be significant. The stored energy tells you what you need in your capacitor, 1/2*C*V^2 if memory serves, and you'll need to impart that energy over the solenoid's stroke. In the case of a 6" travel solenoid that means you need a force of 28.28 newtons over that travel. 4 inches of travel needs 42 newtons, and so on.
     
  5. Sep 5, 2014 #4
    The longer the stroke, the vastly more expensive a solenoid. Even a 1 inch stroke is about $100 or more. That said, you could use a lever to multiply the stroke and then use a "pull" solenoid which may be cheaper. While the ball will deform, a lot of the energy may be recovered or, if the stroke is fast, the energy transferred before transformation.

    Thanks for adding the math for the energy.
     
    Last edited: Sep 5, 2014
  6. Sep 5, 2014 #5

    Mech_Engineer

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    A lever could be a good idea if you're limited in the stroke solenoid available at your price point, but also keep in mind it will significantly increase the required solenoid force and hence electrical power/current needed (addition of the lever's inertia, plus the lever arm force reduction).

    Have you considered an air cannon? ;-) You could get a solenoid air valve with some pipe and hook it up to a small pancake compressor...
     
  7. Sep 7, 2014 #6

    Mech_Engineer

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  8. Sep 9, 2014 #7
    Geez, I forgot about this post. Thanks for the input!
     
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