Abstract: A method of computing imbalance in a motion control system is disclosed. The motion control system includes an object mounting mechanism for holding and rotating the object about a first axis; an object positioning mechanism for displacing the object mounting mechanism; an object positioning drive for driving the object positioning mechanism; and one or more rotation detection devices for generating an output signal indicative of the rotation of the object about the first axis. The method includes the steps of computing an object rotation signal indicative of the speed and phase of the object about the first axis from the rotation detection devices' output signal; demodulating an output signal from the drive with the object rotation signal to derive a demodulated output signal that is synchronous with rotation of the object about the first axis; and computing the magnitude and phase of the object imbalance from the demodulated output signal.