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  • VEX IQ Wheel Diameter/Circumference

    https://www.vexrobotics.com/vexiq/pr.../228-3505.html

    So, I don't get how to get the diameter or circumference from the given values. Can anyone walk me through it please? I know it must be real basic, but I'm just not getting it.

    Here is my attempted work:

    200 * 3.14 = 628mm which equates to 24.7244in which doesn't look right, then I tried
    100 * 3.14 = 314mm which equates to 12.3622in which looks a bit better than the one above.

    Since afterwards I'd do (12.3622 / 360) = Encoder Ticks per second.

    PS: I'm following this guide http://www.robotc.net/wikiarchive/Tu..._some_distance
    Last edited by RepeaterCreeper; 02-05-2018, 06:33 PM.

  • #2
    You are on the right track. However the VEX IQ product page lists the travel for the different wheels in millimeters so you don't really need to calculate it unless of course you want to,

    https://www.vexrobotics.com/vexiq/products/view-all

    The standard tire is a 200mm "travel" so that is going to be the same as the circumference. You just need to convert to inches if you are working in inches.

    Your reference is a good one.

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    • #3
      I think VEX made a very smart decision to use circumference instead of diameters to mark the wheels. It makes it so much easier for the kids to calculate the number of revs, travel angle, and the speed related to the wheels.

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      • #4
        I have mixed feelings about it. On one hand the wheels being labeled "xxx Travel" makes it simple to figure how far it will go. On the other hand it's the loss of a teachable thing that the diameter of the wheel times 3.1 (a close value of Pi) will get you the travel. That's a skill that works forever for anything that is round. (and you can use 3 for a value for Pi for third graders, it's close enough(*).

        So in the above example, the 200mm Travel wheels are about 65 MM in diameter. Which makes sense since the hub in the middle is 44mm in diameter and the tire is about 10mm thick.

        And it's not encoder ticks per second, it's encoder ticks per revolution.

        (*) Most often the use of wheel diameter is the desire to "drive straight for some distance" and roboteers soon find that they can't drive straight by "drive for so many seconds" or "so many ticks". It does create another teachable moment about friction, field floors not being flat, manufacturing tolerances, etc.

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