Patent GB-1340072-A (GB)
Title: Apparatus for signalling the angular displacement of a  body about an axis
IPC: H03M1/00
Filed: 1971-04-19
Abstract: 1340072 Angular displacement indicator BRITISH AIRCRAFT CORP Ltd 19 April 1971 [8 Jan 1970] 1032/70 Heading G4H The angular displacement (shaft angle) of a synchro is determined by generating from the output signals of the synchro signals representing a number of tangent and cotangent functions of the shaft angle, selecting the most linear function for a range in which the instantaneous shaft angle lies, and supplementing the selected signal by a further signal corresponding to the function which has been chosen. General.-Three output signals # 1 , # 2 , and # 3  from the synchro are fed to a Scott type transformer network 1 which generates on a number of leads a number of tangent and cotangent function signals one of which is selected by circuitry 2 as being the most linear for the range of shaft angles within which the instantaneous shaft angle lies. The selection is made in accordance with the contents of a counter 4 (see below). The selected function is digitized by the method of successive approximations (see below) and is fed to an adder and to a read only memory where an appropriate correction factor is accessed. The digitized function and the correction factor are added to give a more linear representation and the result is taken as the least significant part of a digital output indicative of the shaft angle. Three more significant output bits are provided by the counter 4 in accordance with the chosen function and hence the range in which the shaft angle lies and the most significant bit, which resolves the ambiguity as to whether the shaft angle is greater than or less than 180 degrees, is obtained by comparing the phase of the synchro input and output signals. Function selection.-The output leads from the Scott transformer are connected to corresponding FET switches. Two groups of transformer outputs are provided carrying, signals representing sine, cosine and their sums and differences. One function is selected from each group and a division is performed to produce the appropriate tan and cot functions (see below). The functions are selected in response to the state of a three bit counter 4 driven by a 200 KHz clock 5. The three bit parallel output is converted into a one-out-of seven signal, which via a gating circuit selects one function from each group. Analogue-Digital conversion.-The signals representing the selected functions are passed to the non-inverting input of a comparator and to the inverting input via a ladder network of resistors. The ladder network consists of a network of series-parallel resistances each controlled by a pair of semi-conductor switches. The comparator output is passed to a level translator and a gate circuit. When the correct function has been selected the output of the gate changes from zero to one and the supply of clock pulses to the counter 4 is stopped and the clock pulses are admitted to the A-D converter where they increment the contents of a shift register which causes selected semi-conductor switches to open and close and provide by means of the ladder network, a series of different signals to the comparator inverting input. When the comparator indicates equality the clock is stopped and the contents of the shift register represent the digital output required.