[Rickertsen2]

Whats wrong with encoders or hall effect sensors? Since you know what direction the thing is spinning, if you use encoders you only need to use of of the two lines. This makes life alot easier because you can use one of the PIC's built in CCP modules to compute the speed automagically. you only need and interrupt each time a measurement is complete. These measurements will complete in a fixed time, and can be even used as the timebase for your speed control loop