Displacement measuring apparatus

Displacement measuring apparatus comprising reading means and a member moveable relative to the reading means, said member carrying a position track defined by a sequence of alternative markings made in succession along the member,

• the alternative markings recording different values in a pseudo-random sequence which is the result of repetitive operations on a seed consisting of a small number of elements and is at least a portion of a linear binary sequence obtained by clocking a feedback shift register, the sequence having 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;
• said reading means having 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 occurring 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, said decoding logic comprising:
• first shift register means having a length at least equal to the seed through which logical values are clocked, said values corresponding to successive markings recognized by the reading means,
• second shift register means for reproducing the pseudo-random sequence when the register means are clocked;
• and comparator means for detecting a relationship in the states of the first and second shift register means.