Hey, I was wondering how one would calculate the number of degrees that mecanum wheels would have to rotate if I wanted the robot to turn on its axis, or for it to slide sideways.
[these remarks are on-the-fly and reflect zero previous study… so I could be wrong]
That would vary quite a lot with surface for strafing. Since the rollers on the mecanum wheels can slip along, and roll, while strafing there might be some wasted wheel motion. I have never done the trigonometry to figure out strafing dynamics… and don’t really want to.
For rotation, if the wheels are at the corners of a perfectly square chassis, the rollers in contact with the ground should be at 45 degrees to the chassis sides. This makes them work the same as if you had four hi-grip wheels mounted at 45 degree angles. If there was zero slippage, each full rotation of the wheel should make the robot cut through an arc length equal to the wheel circumference. So size of chassis and diameter of wheels matter. You’d need to do a little geometry here: at the point of contact for the wheel, what is its effective radius from the robot center? Calculate the circumference of the circle made by the wheel radius, and divide that by the wheel circumference. That will give you number of rotations of the wheel to make a full robot rotation… in a perfect world.
I’ll agree with @Zook.
In the real world, this kind of calculation is practically useless. If you want to be able to turn to a specific angle, you’ll need to use a gyro and feedback loop (PID) to control your angle. There’s a lot of good information and examples of how to do that here on CD, on screenstepslive, and in teams’ code.
The question, as asked, is more applicable to swerve drive than to mecanum.
Sliding sideways, with mecanum wheels, is: wheels on opposite diagonal 1 full forwards, wheels on opposite diagonal 2 full reverse. Going the other way sideways is the opposite directions.
Similarly, spinning the robot around its center is side 1 full forwards, side 2 full reverse. At least until you got to the angle you wanted.
Of course, calculating the rate of turn or slide is important–but that’s a good job for a gyro and accelerometers.
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