DC servo - mathematical model

Hi everybody, I need to derive a mathematical description for non-linear torque-speed curve for DC servo motor.
I'm not sure how to approach the problem and I was wondering if anybody
can recommend some literature (books, articles, etc.). What do I need to take into consideration? How to model such a system? Does it involve modelling the control (PID loop and PWM).
I understand it's not an easy task but it would help me a lot if someone could push me into the right direction.
Regards, Pawel
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Pawel Chmielarczyk wrote:

...
For cars, they use a tool called a dynamometer. Run the motor at various fixed speeds. Observe the output power. You can back out the torque/velocity curve from there.
Later, Daniel
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