Back-EMF

Back-EMF refers to using the voltage generated by a spinning motor (EMF) to conclude the speed of the motor's rotation. This can be used in motion control algorithms to modulate the velocity or to compute the angular distance the motor has traveled over time. This article attempts to describe this form of motion control feedback in more detail.

Typically a motor takes power in the form of voltage and current and converts the energy into mechanical energy in the form of rotation. With a generator, this process is simply reversed. A generator takes mechanical energy and converts it into both electrical energy with a voltage and current. Most motors can be generators by just spinning the motor and looking for a voltage/current on the motor windings.



http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html


The DC motor that is driven, however is also a dynamo, so in theory it can be used to measure its own speed. All we need to do for that is to measure the Back-EMF voltage, which we denoted with Vg previously and called generator-voltage.