Adaptation from PI output to PWM???

[nesppi@gmail.com]

hi everybody!
i am implementing a control loop of speed for a DC motor. The motor is
controller throught PWM signal and the feedback is obtained with an
optical encoder. Has anybody solved a similar system?
Anyway, when the PI controller generates the signal from the speed
error which is the way to pass this signal to the PWM? I mean, does
somebody know which is the transformation between this signal and the
compare value input to the PWM generator?
Thanks in advance