GB1396423A - Angle-measuring instrument - Google Patents

Abstract

1396423 Photo-electric angle transducers AGA AB 11 July 1972 [23 Aug 1971] 32295/72 Heading G1A In an angle measuring instrument in which the movement of a member carrying two scales is detected by a photo-electric system producing quadrature output signals, each signal is processed by obtaining its mean value which is used to produce an output signal the D.C. level of which is held at a constant value, the mean value being obtained with the assistance of the quadrature signal, and one of the output signals is analyzed to obtain a measure of the movement of the member. The optical system for an angle measuring device Fig. 1 (not shown), comprises a LED source (1) the light from which is collimated (2) passed through a transparent disc carrying two circular scales (4, 5), reflected by means of a series of prisms (6, 8, 10) and lenses (7, 9) and passed through a diametrically opposite point of the disc. The light is then passed through a pair of beam separating prisms 12 to two photo-detectors (15, 16). The scales are arranged in phase quadrature relationship and passing the light beam through the disc twice doubles the number of scale markings scanned in one rotation of the disc. As shown, Fig. 2, the signals from the detectors 15, 16 are processed by means of circuits 22, 23 to produce the mean values which are then subtracted from the respective signals by differential amplifier 20, 21 to produce output signals with a constant (zero) D.C. level. Since the two input signals are in phase quadrature the maxima and minima of one signal occur at the zero crossings of the other and this is used to obtain the mean values. The signals from differential amplifiers are applied to limiters 24, 25 which trigger monostables 26, 27 at the zero crossing points and the output of each monostable is cross coupled to the mean value circuit of the other channel where it samples the input signal to alternately produce the maximum and minimum values which are then averaged to produce the mean value. The limited signals are also applied to a circuit 29 which counts the number of zero crossing points to obtain a measure of the angle of rotation of the disc. Circuit 29 may also store the signals and determine points of equality to effectively double the accuracy of the digital measurement. The signals from the outputs of the differential amplifiers 20, 21 together with the amplitude level signals obtained from the mean value circuits 22, 23 are applied to a circuit 28 which full wave rectifies the signals and compares the voltage levels present when the disc comes to rest with the amplitude levels to interpolate between adjacent digital counts. The outputs of counter 29 and interpolater 28 are applied to a digital display 30. A straight scale member with linear movement may also be used to measure angles.