Well . you should design it with a microcontroller and with some ways to feed back :
1- tako generator
2- encoder and rotary encoder methods .
3- opto counter .
4- .... etc .
At first you'll need a simple PWM that it's duty cycle can change from zero up to 100 percent ( approximately ) . this you'll be able to change the speed as you want .
And about feed back loop , the feed back , response will have effect on duty cycle . to stabilize the RPM.
Good luck
Goldsmith