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Re: fine control of robot with mecanum wheels
I'll echo Jared341 on all points. We don't have so much experience with the holonomic platform as they do, but our experience matches exactly with respect to what has to be considered, and our solution ended up remarkably similar.
We understood the importance of keeping uniform load on all wheels at all times and chose the simplest way to do that: use three wheels and keep the CG in the middle. Low-tech, and very effective.
Unless all your drivetrains are perfectly linear the thing won't drive straight, since the motors are almost always running different speeds, and hence at different spots on the input vs speed curve.. In principle, it should be possible to get linearity with individual feedback loops on each wheel, but we weren't able to make that work. It helped, but it wasn't sufficient. Possible or not, it isn't easy. We ended up taking it out entirely for other reasons.
Gyro-based correction, in the other hand, is simple. Our feedback code looks almost exactly like the posted snippet. For driver control, rotation feedback isn't terribly important except when trying to go straight,, since the driver can't distinguish, for instance, between rotating 47 degrees per second and rotating 52 d/s. But there's a big difference between driving straight and drifting 5 d/s.
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