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DC motor mounted on a glider which slides linearly on a frictionless surface

  1. Mar 25, 2012 #1
    Hi guys,

    if i mount a dc motor on a glider that slides in 2 directions without friction, AND, instead of having it drive a mechanism, have a circular plate with an eccentrically drilled hole in it (not centre) mounted on the motor shaft though this hole, i should observe the glider sliding back and forth in a frequency, shouldnt i? In a vibrational manner?

    Ok, so im going to apply this to my final year project as an experiment which can be conducted by students, and the students can attach circular plates of different mass, to figure out the vibrations that it causes! The distance that the glider travels back and forth will be experimentally measured with a keyence laser i will mount one the end of the airtrack, but the students will also have to theoretically calculate the vibrations too.

    to clarify,its my final year mech eng project to create an airtrack for various experiments to be carried out on.

    can you guys point me in the right direction in terms of equations to explain this? I need show the theory behind this obviously and calculate it myself to put in my report.

    i shall select a motor tomorrow, and post the specs to aid our calulations here. bed time right now!

    any help appreciated.
  2. jcsd
  3. Mar 25, 2012 #2
    If I'm understanding your description correctly, yes you should. What you're effectively doing is having it shift the center of mass of the system. It will "try to correct itself" by exhibiting reaction forces on the rest of the system to keep the center of mass constant.

    Based on what geometry you're using it might be somewhat difficult for the students to theoretically calculate the vibrations, so I'd say try to keep it relatively simple. But it'd definitely be a good idea to mock-up a solid model in Inventor or SolidWorks to show us better what you're talking about.
  4. Mar 26, 2012 #3


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    I strongly suggest that you make some calculations before the fact. You may be disappointed with the small amplitude of motion at any but the very lowest frequencies. You can save your self a lot of trouble by making some design calculations ahead of time on this.
  5. Mar 27, 2012 #4
    OldEngr63 this is exactly my point. My knowledge in that particular area of dynamics is quite rusty though, and im looking for a bit of help on what equations to start using. Do you have any ideas about this? Im selecting a motor as we speak to start with.

    Schliere ill quickly cad up a motor and a glider and pop it onto my assembly. Solidworks step file do you? i would sent the parts but im using 2011 and not many people have upgraded yet, even though 2012 is out!
  6. Mar 27, 2012 #5
    Surely though id imagine that the further off centre i drill the hole in the plate the more motion i see?
  7. Mar 27, 2012 #6


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    Adamxrt: The reason I suggested you make calculations before you choose a motor is that the calculations should guide your choice of motor. What you will find is that motor rotor and frame weight have a major impace on the motions you will see.

    You need to write F = m a for the motor frame and for the unbalanced rotor. You can include a variable frequency excitation on the motor and see where that leads you.

    You will also have to give some thought to the matter of how you will react the motor torque on the frame. What will stop the frame from simply spinning in the opposite direction?
  8. Mar 27, 2012 #7
    Forgive me for being so ridiculously stupid sounding, but not having dealt much with dc motors before, what exactly do you mean by motor frame? is this what the rotor spins in?

    or are you talking about what i mount the motor to?

    Am i naive in thinking i could have simply used the tapped mounting holes that come already tapped and drilled on most motors off of RS.co.uk to mount it to a flat plate on my glider?

    So i find the weight of the motor and is frame?

    Im also not sure what a variable frequency excitation is either, though i can find out, sitting in the university library here.

    Well, to react the force, the glider that the motor will be mounted to will be sitting on a thin film of air exiting the holes of my airtrack at a pressure great enough to support the weight of the glider. if the downward force of the glider onto the airtrack surpasses this pressure at any time, the glider will tip down and hit the surface of the airtrack. i dont believe it could spin around the motor shaft....but then again i may have misunderstood the question......

    EDIT: (to help aid understanding , the airtrack is a triangle shape, the glider travels linearly, and the motor will be mounted with the shaft horizontal and also perpendicular to the direction of motion)

    Note: i dont have a lot of experience with motors etc, hence why i wanted to add this little bit into my project, to gain more knowledge.
  9. Mar 27, 2012 #8


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    Specifically, the motor frame is the non-rotating exterior part of the motor. For the purposes of this experiment, you should perhaps think in terms of adding to that any additional mass that you mount to the motor. That applies to both AC and DC motors.

    The holes that are already there, drilled and tapped are provided for mounting purposes.

    Think about conservation of angular momentum. The electric current generates a torque on the rotor, but there is an equal and opposite torque on the stator (in the frame) by Newton's third law. What are you providing to react this reaction torque? I don't know because you have not given a very clear image of what your set up will be. From the mental image I have, the motor frame could spin one way while the rotor spins the other way, at least until the power cord tangles completely. That's why I asked the question.
  10. Mar 27, 2012 #9
    Thanks for the help so far man. Take a look at the attached image. This is a basic mock of the setup i envisage.

    Attached Files:

  11. Mar 27, 2012 #10


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    That makes it clear that there will be no tendency for the motor frame to spin. Carry on, and good luck.
  12. Mar 27, 2012 #11
    Still stuck as hell! have no idea where to start calculating this. Surely i have to have selected a motor first before i can start calculating? becaus eill nee to know its weight and the rpms and torque?

    My goal is to calculate the maximum linear displacement of the centre of mass of the whole glider plus motor system through one revolution of the shaft.

    Would the first step be calculating the rotational inertia of the mass?
  13. Mar 28, 2012 #12


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    Write down whatever you know to be true and go from there.
  14. Mar 28, 2012 #13
    I would suggest that you consider using a model airplane electric motor. They are quite light for their output, built to spin a relatively heavy device (the propeller), and use a pulse type electronic drive drive that controls the speed nicely.

    You might also want to think about mounting just the rotating disc on the support and try to design some type of flexible drive; that would eliminate the motor weight. It might work because the movement of the disc assembly will be small?

    Sounds like you need help from two people, a electromechanical person with wide experience with hobby and industrial equipment, and a physicist/mathematician for the calculations.

    You can get information on the model airplane motor at a hobby shop and likely online.

  15. Apr 9, 2012 #14
    Sorry guys, this just got brushed under the carpet because i got it to work in solid edge dynamics motion simulator. but i still have to show the calculation either way.

    Ive settled on a small vibration motor commonly found in dualshock controllers and, forgive me for saying it, womens vibrators. it provided a data sheet which im going to link below.


    go to the calculation section and it gives the vibration formula im going to use. I just have one problem with that formula....k, the sytsem stiffness....

    having just ploughed through my engineering dynamics textbooks i see a similar set of problems to mine, but they all involve fecking springs or something of the like for which it is easy to give a value for k.

    What do i have in mine? Should i put k to 0? or 1? what!?!
  16. Apr 9, 2012 #15


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    Wouldn't it be a good idea for you to try writing the equations of motion for your system yourself, rather than simply assuming that you should use something you found on a data sheet? The system described on the data sheet does not really look like your system, so why would you expect that equation to apply to your case?
  17. Apr 9, 2012 #16
    Its because i tried writing them and failed horribly. I went back to my porject supervisors and they wanted to see the motor id selected. I went back, selected a motor and saw that on the data sheet. brought it back to them and said, is this by any chance the equation i should be using? they said it was very similar. they then directed me to a question in my textbook which is supposed to be nearly identical to mine, and said try look at how this question is solved.....
    they sent me the solution and it uses the same formula.

    I completely appreciate your suggestions though man, its just i aint got nothin. Vibrational dynamics has never been my strong point. That equation gives the displacement from the centre of rotation. The k value is whats stopping me. In the textbook, that question had k a k value, but they managed to pull that from a spring.

    If you look at my other posts for help on this forum, you see that i have been writing my own stuff, particularly on solving the airflow in side the track.

    i can fill every other variable in that equation with a number, except for maybe Wn, im not so sure how to figure that out either.

    i should also say that this is classified as a "forced vibrations" problem, and that this equation fits that category.

    EDIT: Just to clarify aswell, i think that system on the data sheet does apply, because they have fixed their motor onto a test right with and accelerometer and they are measuring its amplitude. Its very similar.

    i have mine fixe don a glider on a frictionless surface, and i think, i want to figure out its amplitude in the 1 degree of freedom (travel along the track).

    I managed to get it simulated in solid edge! ill try and upload the animation.

    EDIT 2:motor animation

    Last edited: Apr 9, 2012
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