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Choosing the right motors (torque)
  • 1nvenib0
    Hey,

    I'm trying to build a gimbal for a Canon E0S 5D3. To do so I need to choose motors that can handle this kind of weight, but since the gimbal is intended for professional work I don't want to rely on the product description stating "for 5D sized cameras". So I found a company which includes the nominal torque (in Nm) in the product description. My question is how do I calculate the torque I need?

    My thought was:
    1. Determine the angular velocity that the motor should reach in a given period of time (e.g. 360° in 1s).
    2. Calculate the velocity (in m/s) using 2π*r (r being the maximal distance from the axis) and acceleration respectively.
    3. Calculate the force using F = m*a (m -> mass of the moved part).
    4. M = F * r (same r as in 2.) should give me the torque needed.

    With r = 10cm and m = 2kg this gives 0,125Nm.

    Is my approach correct? Do I use the right values and is 360°/s set too high or too low?

    Thanks in advance!
    Josua

  • Garug
    On normal stabilization camera does not move. When the gimbal is well balanced and mechanically good, it almost works without controller.

    When using follow mode or joystick, then motors are moving the camera. If you want to pan the gimbal 360 degrees in 1 second, then you really need much power and you need to take into account not only the camera inertia, but also the gimbal inertia.

    So the determining factor would be how fast you want to pan or tilt the camera.

    I have been using GB85 for long time with 5D II , this was a year ago.



    GB54 with GH4 and without encoders it is a bit under powered. stabilization works really nice, but I need to be all the time careful not disturbing it, and it takes only small lenses.



    GB90 gimbal with encoders has really a lot power. it could easily take RED with big lens. Here used with GH4 and Canon 24-105 F4



    The same gimbal as above at about 200 mm focal length (cropped)



    And it can take some crazy ride too this one wit Canon 8-15 mm



    My advise is, go with big motors, they make things much easier. You can always tune the power down, if there is too much of it, but that is usually not needed.

    About motor pole count. High pole count motors are strong, but they can be a bit jittery at certain pan speeds. This normally is not a problem ,but something to be aware of. low pole count motors are weaker, but smoother.

    Encoders complicate the build somewhat but bring many advantage, including much more tongue, so smaller and low pole count motors can be used.


  • 1nvenib0
    I have considered using the GB85 and GB90 since rctiger is generally renowned as a reputable company (compared to other almost no-name products).
    However, it seems a bit strange to me to chose the motors based only on experience an the vague statement of the manufacturer "for 5D sized cameras" given the effort I put into calculating and testing the best designs and materials for the frame. Furthermore, I will potentially use the gimbal for a scientific project and then I have to answer why the chosen parts are the best in their field of application.
    There are for example motors with 60mm and almost 300mNm torque. According to the example in my original post, this would mean I don't need 90mm motors.
  • Garug
    The needed tongue depends of the maximum acceleration for each axis you want to archive. That depends much of your gimbal geometry and weight and also camera + lens of-course. Also if there is wind, high speed etc. the aerodynamical forces need counter force, that also much depends of your gimbal geometry.

    There is certain tongue needed for the stabilization, but compared to above mentioned it almost always is less than needed for above mentioned.

    If you want to take the scientific approach for defining the needed tongue, all possible reasons requiring torque should be taken into account. But I think it is also scientific approach if you select bigger motors than needed, build an encoder gimbal and observe the used power in different situations. Encoder gimbal makes that possible.

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