Ame writes “I wanted to accurately control the speed of the motor in a Tamiya 72004 worm gearbox for a robot I am building. To do this you must have some way to measure the current speed. This project shows the evolution of the speed sensor. As you can see in the picture, the motor drives a worm gear directly attached to its output shaft, then a series of three gears to reduce the speed of the final output shaft.”Link.