Patent ID: 7762135

Claim:
An instrument comprising: a) a first component, b) a second component supported by the first component for rotation relative to the first component about a first axis, c) a first drive for rotating the second component about the first axis, d) a first angle sensor to produce a first angular position signal representing angular position (φi, αi) of the second component about the first axis relative to a first reference, e) a second angle sensor to produce a second angular position signal representing one of (A) angular displacement (φ 2 , α 3 ) of the first reference relative to a second reference, and (B) angular position (φ 3 , α 3 ) of the second component about the first axis relative to the second reference, and f) a signal processor to produce a first compensated angular position measurement for the second component relative to the second reference by combining at least portions of the first angular position signal and the second angular position signal.