Position sensing apparatus

A method of determining the position of a movable sensor relative to a datum line comprising the steps of:

• a. establishing the datum line between a pair of radiation generators, each having an associated datum sensor sensing the radiation beam generated by its associated radiation generator and producing a timing signal indicative thereof;
• b. generating a separate radiation beam of radiation from each generator at a constant angular velocity in a common plane and with each radiation beam being generated in an opposite direction of rotation relative to the other radiation beam;
• c. detecting each radiation beam at the movable sensor, and differentiating the radiation beams at the movable sensor by detecting their directions of rotation;
• and
• d. at the movable sensor, monitoring the time delays between the reception of a radiation signal by each datum sensor as indicated by the timing signals therefrom and detection of each radiation beam by the movable sensor to provide a measure of the positional angle defined between position lines between the movable sensor, and each radiation generator and relative to the datum line.