GB953449A - Improvements relating to apparatus for producing signals indicative of relative position - Google Patents

Abstract

953,449. Measuring displacement electrically. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 6,1962 [June 9, 1961], No. 20958/61. Heading G1N. [Also in Division G4] A displacement transducer comprises a member 1 having an electrically conductive toothed profile, which is capacitively coupled to a movable member 2 itself comprising at least three equally spaced conductive strips S 1 , S 2 per tooth pitch which extend transversely to the direction of movement, all strips having a similar positional relationship to each tooth being electrically connected into a set, and the sets being connected to a transformer arrangement T which produces a displacement dependent output. In an arrangement in which there are four strips s to the tooth pitch P, two sets of strips comprising respectively S 1 , S 5 , S 9 &c. and strips S 3 , S 7 &c. are connected to the ends of transformer winding S 2 and strips S 2 , S 6 , S 10 &c. and S 4 , S 3 &c, to transformer winding C 2 . The centre taps of the windings are mutually connected and a supply voltage is connected between lines 7 and 8. In the position shown, the capacities between S 1 , S 3 and the member 1 are equal and hence the voltage in the winding S 1 will be at a Minimum whereas the capacity difference between strips S 2 , S 4 with the member 1 will be at a Maximum and thus result in a voltage Maximum at C 1 . Movement of the member 2 in the directions of the arrows d results in an attenuation of the voltage at C 1 and an increase at S1 so that for any relative position of the members 1, 2, within one tooth pitch, a characterising voltage relationship will exist between the windings S 1 , C 1 . The above arrangement may be used in the reverse i.e. two voltages of a form respectively E cos # and -E sine # may be respectively fed to the windings S 1 , C 1 . The Specification states that if # is the angle proportional to the mechanical displacement of the strips relatively to a given station on a toothed member, then a voltage proportional to E sin (#-#) is obtained at the conductors 7, 8 and may be used to control a positional servo system. In order to obtain a voltage output which is more closely sinusoidal, the strips s may be skewed from otherwise their 90 degrees relative angle to the direction of member movement. The profile of each tooth on the member 1 may be changed from that shown, to one in which arcs of a circle subtend 90 degrees at their centre and the curves intersect 90 degrees. The Specification shows an alternative arrangement in which there are six strips s to the tooth pitch, and also transformer configurations which permit the trimming out or compensation for electrical errors caused by manufacturing tolerances. To avoid unbalancing effects arising from leakage resistance, a guard ring technique may be used by providing a conductive screen connected to the junction 7 and which crosses any leakage path between members 1, 2.