steer control - dc motor with potentiometer

The motor shield that was built is now being used to command the steer motor. The base comes with a very small dc motor and a potentiometer. How the original electronic worked with that "servo" was not deeply investigated (what is the rated voltage of the motor?).
The test setup is the motor shield + arduino 2009 + potentiometer + dc motor
The potentiometer is being fed with 3v3 5v output from arduino, to have a stable reading. The input goes to an analogic port of arduino and is represented in a scale of 0 to 1023. There is some noise in this readings, around +/- 2 units in the 1024 scale.
note: maybe the maximum reference is still 5v ??
a 3.3v is provided to the outputs of the l293 on the motor shield, by an external power source (in the future this will come from and independent 3v3 regulator, powered by a 3v7 LiPo battery).
the program running at arduino implements a PID control with the aid of the Arduino - PIDLibrary.