Setup for Feedback Circuit Using Motor

I am trying to set up a feedback loop of sorts that will detect voltage changes in a motor. At the moment I am powering an Arduino Uno and Adafruit V2 motor shield using a 9V battery. The motor in use is a 15.1 VDC motor. I'm trying to set up a circuit that can detect changes in voltage as the motor runs faster or slower. The motor will be running at a set rate, but external forces acting on the shaft should be making it run at different speeds (like running faster inside a wheel). Voltage changes would be detected by the Arduino and result in a faster or slower spin of the motor. Right now I'm having difficulty figuring out a circuit design that would feed a change back into the Arduino. The plan is to use an op-amp for now, but I can't figure it out.

You posed basically the same question i Project Guidance. Why?