DISPLACEMENT MEASURING APPARATUS

摘要

PCT No. PCT/US86/02557 Sec. 371 Date Jan. 9, 1991 Sec. 102(e) Date Jan. 9, 1991 PCT Filed Nov. 26, 1986 PCT Pub. No. WO87/03361 PCT Pub. Date Jun. 4, 1987.A tape rule or other displacement measuring apparatus comprises a reader means and a member that are relatively movable. The member carries a position track defined by a sequence of alternative markings made in succession along the member. The alternative markings record different values in a pseudo-random sequence which is the result of repetitive operations on a seed consisting of a small number of elements. The sequence has the property that a group of successive markings of length at least equal to that of the seed occurs at a single position in the sequence so that a succession of markings read from the track and of length at least equal to the seed defines an absolute position of the reading means and the member. The reading means has at least three sensors arranged to respond to different locations within each marking with the pattern of the markings and the location of the sensors arranged so that only a single sensor changes state each time that the member moves through an interval of displacement relatively to the sensing means. The states occur consecutively in alternative sequences along the member that are recognizable by decoding logic fed with successive states as a logical value corresponding to one of the alternative markings depending upon which sequence has been followed.