GB810478A - Improvements relating to measuring apparatus - Google Patents

Abstract

810,478. Electro-magnetic measuring apparatus. FERRANTI Ltd. May 25, 1956 [May 31, 1955], No. 15563/55. Drawings to Specification. Class 40 (7). [Also in Groups XXXVIII and XL (b)] In an arrangement for determining the extent and sense of the movement of one object relative to another employing the arrangement described in Specification 760,321, [Group XL (b)], in which a cyclic wave pattern is established the movement of which is related to the movement to be determined and in which two detecting devices, fixed relative to one of the objects (and the moving pattern) and so spaced that the respond to quadrature phase conditions of the pattern and produce electrical signals differing in phase by 90 degrees, these signals being supplied to discriminating and counting means which provide the required information, detrimental effects on the accuracy of the measurement caused by variations in the means for setting up the cyclic wave pattern (i.e. brightness variations when this is an optical diffraction pattern) or, in the sensitivity of the detecting devices, are eliminated by employing an additional pair of detecting devices also spaced to respond to quadrature phase conditions of the pattern and spaced relatively to the first pair so as to be individually responsive to counterphase pattern conditions relative to respective ones of the first pair. The output signals of the counter-phase devices (one of each pair) are then combined and the two sets of signals supplied to respective difference amplifiers the outputs of which are proportional to the difference between the D.C. levels of the input signals, no output appearing if, e.g. due to a change in the overall patter, the level of the input signals changes by equal amounts in the same direction. In this manner the difference amplifiers only supply output signals when the input signals vary due to relative movement between the two objects and these signals are then converted into pulses which are employed in the pulse discriminating and counting arrangement described in the above mentioned application to provide the required information. In the described embodiment the cyclic wave pattern is an optical one but it is stated that a recorded magnetic pattern or an electromagnetic or acoustical standing-wave pattern may be employed instead, the detecting devices being appropriate to the kind of energy utilized.