[fsgond]

We have not used shifters in a few years, but when we did it was a manual shift. Mainly we had problems when we would shift under stress, such as driving at high speeds or in a pushing match. We started to round the gears down. Basically we had to do what you would do in a manual transmission car if you shift without a clutch. As long as everything was moving at about the same speed everything would shift great, but if we were really on the sticks and shifted, it would make a horrible sound due to the grinding gears.

If you are set on doing it though, it should not be too hard. You would need to figure out at what speed you want to shift. Then you would need to have an encoder on the wheels determine when you reach that speed. As for pushing, you may look at monitoring how fast you are going VS current pull. If you have high current an little acceleration, you can think that you are pushing someone or some thing. This is all of course if you are on a flat floor.