It is also assumed that the load is rigidly coupled such that the torsional rigidity moves the natural mechanical resonance point well out beyond the servo controller’s bandwidth. This assumption allows us to model the total system inertia as the sum of the motor and load inertia for the frequencies we can control. Somewhat more complicated models are needed if couple dynamics are incorporated. The actual motor position, q(s) is usually measured by either an encoder or resolver coupled directly to the motor shaft. Again the underlying assumption is that the feedback device is rigidly mounted such that its mechanical resonant frequencies can be safely ignored. External shaft torque disturbances, Td are added to the torque generated by the motor's current to give the torque available to accelerate the total inertia, J.