| 摘要 |
<p>A tape rule or displacement measuring apparatus, comprises a reader means (30) and a member (1) that is relatively movable. The member (1) carries a position track defined by a sequence of alternative markings (3, 4) made in succession along the member (1). The markings (3, 4) record different values in a pseudo-random sequence resulting from repetitive operations on a seed consisting of a small number of elements. The sequence has a group of successive markings (3, 4) of length at least equal to that of the seed which occurs at a single position in the sequence so that a succession of markings read from the track defines an absolute position of the reading means (30) and the member (1). The reading means (30) has at least three sensors arranged to respond to different locations within the pattern of each marking (3, 4) and only a single sensor (30a) changes state each time the member (1) moves through a displacement interval. The state changes are decoded by logic (10).</p> |
