Mehmet,

If you are willing to close the control loop in a '52, Orlando has a good idea.

If you want to close the loop via analog, consider applying the encoder pulse train to a 4000 series one shot. The fairly repeatable output voltage of the one shot can then be applied to a low pass filter to obtain a dc voltage proportional to speed. Don't forget the low pass filter adds lag to the measurement and the lower the puls train frequency, the lower the break frequency on the LPF to minimize ripple.

Good Luck,

Bruce