The major features are: the measurement is non-contact; the circular path radius can be varied continuously from less than 1 mm to 150 mm; all linear accuracy are traceable to NIST; the feed rate is up to 4 m/sec; the data rate is up to 1000 Hz; and the actual feed rate, velocity, and acceleration profiles can also be determined. This information are important to determine the motion dynamics of the machine and the servo performance.