Patent ID: 8461830

Claim:
Apparatus for measuring displacements of a rotary displaceable member along a predetermined displacement path by counting periods of displacement thereof along said predetermined displacement path, said apparatus comprising: a first machine-sensible element carried by said displaceable member and occupying a length thereof defining only a portion of a period of displacement of the displaceable member; a first sensor located at a sensing station proximate to said displacement path so as to be capable of sensing the presence or absence of said first machine-sensible element in the sensing station, and thereby of determining the displacement state of the displaceable member at any particular instant during the displacement of said displaceable member; a pulse generator located at a pulse-generation station proximate to said displacement path; a plurality of second machine-sensible elements carried by said displaceable member at spaced intervals along each displacement period of the displacement member, each of said second machine-sensible elements being able to actuate said pulse generator when moving through said pulse-generation station; and an electrical counter for counting the displacement periods of the displaceable member; said second machine-sensible elements being closer to the center of said displaceable member than said first machine-sensible element and dividing the predetermined displacement path into a plurality of fractions each including one of said second machine-sensible elements; said apparatus further comprising an electrical circuit controlled by said pulse generator to actuate said first sensor to sense the displacement state of the displaceable member at the particular instant one of said second machine-sensible elements passes through said pulse-generation station, and to increment the counter only when a transition from one displacement state to another is sensed at that instant.