I haven’t tried out the
I haven’t tried out the whole modified servo thing, but I don’t think you can use it to be able to turn a specific number of turns.
The trick with the modified servo thing is that normally a servo has a potentiometer attached to the output shaft and this makes it able to measure the current angle it is at by simply measuring the resistance of the potentiometer. When you modify it you remove the potentiometer and replace it by two equal valued resistors in a voltage divider setup where you connect the measuring circuitry to the joint between the two resistors. This makes the servo think that it is always centered. There is no way to get back any information on how many turns the servo has done. You should think of the modified servo as a geared motor with a simple control interface where you can set it in forward, reverse or stop. I’m not sure of this - so anyone with better intel please correct me if I’m wrong - but I think that a servo turns faster with greater angles, so if you set the servo to turn to an angle close to center, the servo will not turn as fast as if you set the servo to turn to an angle far from center.
You can add wheel encoders to your project to be able to measure how many turns each wheel has done. You can see a small project for the Tamiya twin motors here.