vex LTI DC motor Model

I have estimated parameters of my dc motor.I have tried to create a discrete LTI motor model but the step response is not like a motor.In Mobile Robotics Training library there is a vex motor LTI model and relation between angular velocity and PWM in a lookup table.How the vex motor model was made using system identification toolbox?What is the procedure to make custom LTI motor model like vex?How the experiments are conducted to get relation between angular velocity and PWM value?