Question on wheel speed sensors

[Dave Scheck]

Originally Posted by Al Skierkiewicz

Crab turns introduce less error than tank turns but can be compensated for in software.

That's not really true Al. Using either encoders or a gyro, you can get some really precise tank turns, especially with a lower gear ratio. Let's take driving through these 4 points as an example

    D
    |
    |
B---C
|
|
A

With a crab system, there is usually a significant delay in turning the wheels 90 degrees. So, unless you come to a complete stop, without coasting, your turns will be skewed. Your B->C transition will cause you to arc instead of making a sharp cut leaving you above C. Then your C->D transition will arc again and leave you to the right of D.

Also, don't forget that there is always concern for wheel alignment with a crab system that may also cause for other inconsistencies in the drive pattern.

With a tank drive with proper gearing, when you get to B and C, you will be able to make sharp, accurate turns.

Yes, you can probably get to point D faster with crab, but you sacrifice some accuracy doing it. Sometimes that matters, sometimes it doesn't. Look back to one of our less used programs in 2006. That was some of the most accurate driving that we've done (although any misalignment was compensated for with the turret). Look at some of the things that teams like 1114, 2056, 469 and countless others have been able to just tank drives, encoders, and gyros.

Crab isn't always the answer, and when it is, it isn't always an accurate one.