Thread: Torque Problem?
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Old 03-23-2012, 02:45 PM
Brian He Brian He is offline
Join Date: May 2011
Location: Vancouver,BC,Canada
Posts: 44
Re: Torque Problem?

Originally Posted by Vex Mundi View Post
Each wheel will exert the same force. If the wheels are in the standard rectangular configuration and the centre of gravity is centred between them, each one wheel will exert the same torque on the robot because the moment arms will be the same length. If the wheels are at different distances from the centre of gravity, as it looks like yours might be, then the moment arms will be of different lengths so there will be a torque imbalance and the robot may rotate.
Seems like our robot does curved a bit when we test the strafing, haven’t seen it from strafing across the field yet. We have our robot’s base tilted forward, so the supporting beams for the robot’s arm and the arm are leaning forward as well, I guess that reduced the amount of curving by making the distances between each wheel and the center of gravity closer to even.
Originally Posted by Vex Mundi View Post
Another consideration is that while mecanums are designed to exert force at 45 degrees to the orientation of the wheel, roller friction means this isn't always the case. As roller friction increases the direction of the force exerted by the wheel will become closer to the wheel's orientation (i.e. the wheel behaves more like a normal wheel). This doesn't affect the torque balance of a normal mecanum drive, but it will affect yours because your wheels are mounted in different orientations. This is the reason why I said your drive would produce non-straight strafe. It's possible that several conflicting factors will cancel each other out and make the robot strafe straight. If not, it's not a big problem because your driver can adjust for the wonkiness.
This sounds true… So if the robot wants to travel diagonally, the diagonal angle may not even be 45 degree anymore…
Maybe I will upload a driving video soon after we changed the gear ratio.

Originally Posted by Vex Mundi View Post
There is a tendency when building Vex robots to forgo any kind of mathematical analysis and use intuition based on experience because Vex systems are simple enough that it's usually a more effective approach.
This is a bit of a pity from a engineering education point of view. I think the best approach if you're interested in learning more about the engineering (or perhaps if you're a mentor interested in teaching students more about the engineering, but that's not where my experience is) is to see mathematical analysis as complementary to an experience-based approach and to check whether the results of your analysis match up with what works. If not then your analysis is mising something and maybe you can learn from your mistake, if so then maybe you found a good solution quicker than you would have through trial and error.
Okay, I just realized VEX is not rocket science yet, things doesn’t have to be exactly accurate and it is probably more fun to try it with intuition first. I was thinking about some engineering procedures that minimize the mistakes people can make and the robot can come out in its best possible condition, like using the most suitable gear ratio accordingly to the design. =/

Originally Posted by Vex Mundi View Post
Calculating the optimal gear ratio for driving would involve making sure that your motors are giving as much power as possible. Power is torque multiplied by rotational velocity, so the motor gives zero power output while stalled (zero rotational velocity) or while freely spinning (zero torque). The optimum rotational speed for a motor is about halfway between these values, depending on the individual motor's characteristics. For a Vex motor halfway will be a good approximation because Vex motors are permanent magnet DC motors.
The optimal gear ratio will be the one that gives the greatest energy output when driving for a given amount of time, or the one that maximises the integral of power with respect to time over the time travelled. As I mentioned in my previous post the parameters of this calculation would be difficult to get accurate measurements for.
thanks for the help!

Last edited by Brian He; 03-23-2012 at 11:40 PM.
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