Patent GB-872152-A (GB)
Title: Improvements in or relating to position sensing devices
Inventor: MYNALL DENNIS JAMES
IPC: G01D5/20
Filed: 1958-06-04
Abstract: 872,152. Capacitive transducers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 14, 1959 [June 4, 1958], No. 17879/58. Class 40 (1).  A position-sensing device comprises two members having surfaces adjacent to one another, one surface having a helical conductive path and the other having at least three conductive paths conforming in shape and spacing to those on the first surface so that capacitance exists between them and will change if relative longitudinal movement of the two members occurs. Normally the two members are coaxial cylinders with the second one outside, but the latter may comprise only part of a cylinder, in which case the conductive path on the first member is helical only on the part adjacent to the second member. The cross-section of the members may be oval or rectangular instead of circular, and their axes may be curved or annular if they are to sense angular motion instead of linear motion. The first member may have more than one conductive path 3a, 3b, 3c (Fig. 4) or may have only a single one (Fig. 1, not shown). The second member may have three paths (Fig. 2, not shown) or more than three paths 4a, 4b, 4c, 4d (Fig. 5), and is normally wider and shorter than the first so that it moves longitudinally over it. The relative sizes and positions may, however, be interchanged. The various terminals 6a, 6b, 6c, 6d are supplied with alternating voltages whose instantaneous magnitudes are proportional to sin #, sin (#+2#/n), sin (#+4#/n) . . ., where n is the number of conducting paths (four in Fig. 5). There is thus set up in the member an electric field which is of the same form for any value of # but shifts longitudinally by a distance equal to the lead of the helical system if # is changed from 0 to 2# radians. Voltages are induced at terminals 5a, 5b, 5c of the first member which vary cyclically with a change of # or with relative longitudinal movement of the elements, and two nulls will be obtained in a distance equal to the lead of the helical system. The positions of the nulls can be set by the value of #. In the system shown (with three helical paths on the first member) the difference between the voltages between any two of these can be used to determine the relative position of the members, and compared with the voltage of the third to determine and control the overall sensitivity. In an alternative method of working, the second member is supplied with voltages of the same instantaneous magnitudes but of different phasing, so that the electric field moves continuously in an axial direction. The relative position of the members is then determined by the phase relationships of the voltages picked up by the first member. If the second member does not extend the full distance round the first the conductive paths on the latter are helical only on the portion adjacent to the second, the parts of the helices being completed by axial bridges 4a<SP>11</SP>, 4c<SP>11</SP> (Fig. 6). The two members can be arranged to have relative angular motion about their common axis as well as linear motion, either to measure angular displacement (or control small rectilinear movements therefrom) or to modify the relationship between # and the linear displacement. The conductive paths may be printed on the insulating material of the members or may comprise strips wrapped round their surfaces or laid in grooves. Alternatively they may be machined from a continuous conducting covering. The device is useful in indicating and/or controlling the relative position of two parts of a machine-tool, in which case some " coarse " indicating means may be necessary.