Let's Make Robots!

Difference in motor speed

I have build a robot out of old LEGO bricks, since my gearbox and tank tracks haven't arrived yet. I have used the standard one-motor-per-wheel-in-the-front-and-one-small-wheel-in-the-back-design. My problem is, that the two motors aren't running with the same speed, so my 'bot is turning to the right all the time. I have considered putting a resistor on the fast motor to make it slower, but would like to ask, if any of you have any other solution to this problem? The best solution would be some sort of sensor on the motor to tell the speed and let the CPU adjust the speed.

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then how can you turn the rotation sensor with the differential? i don't get how it can start turning if the axles are rotating freely

EXCATLY. The rotation sensor will not turn if the axles are going the same speed inthe same direction! 

Differential gears are so called because the output of the main gear is the difference between the other two. Mechanically, they can, of course be configured as adders.

ah ok i got it now, thanks for the infos mate