Autonomous Winding And Pay-out System

Hello everybody, my name is Andrei, and I'm a third year student. My bachelor thesis is to design an autonomus winding and pay out device. The device will be mounted on a moving robot platform. The device consists of a spool or drum on which the cable is wound, the other edge of a cable will be fixed. So when the robot moves away from the cable fixing point the spool must rotate and unwind the cable, when the robot moves towards the cable fixing point the spool must wind the cable. The faster the robot platform moves the faster must the spool rotate. As a sensor I use potentiometer, which measures the angle of the cable, and its output is analog signal. As a spool drive i've decided to use a dc motor. In other words i have to design a dc motor speed/direction control system and take into account the length of a cable since the more cable is wound around the spool the greater the motor's torque is. My question is whether it is possible to control motor's speed and direction with Atmega AVR microcontroller with analog input signal from potentiometer. If it is how exactly can it be done? What sensor should I use to measure the the length of an unwound part of the cable and how the output of this sensor can be used to set the proper speed for the motor? I would really appreciate if somebody could help me.