Patent GB-1315453-A (GB)
Title: Apparatus for deriving electric signals
IPC: G06G7/22
Filed: 1971-04-19
Abstract: 1315453 Voltage measuring BROWN BOVERI & CO Ltd 19 April 1971 [22 Jan 1970] 20010/71 Heading G1U A system for producing a voltage proportional to a positive function # (t) from a voltage proportional to sin #(t), includes the step of comparing the voltage proportional to sin #(t) Fig. 2, with a reference signal Sinwt, having a sufficiently high frequency that the value of sin#(t) remains substantially constant over each cycle of the reference signal, in order to produce a series of pulses each of a duration t for which the reference signal exceeds the voltage proportional to sin #(t), which pulses are each linearly proportional to #(t). From Fig. 2 it can be seen that if t 1  and t 2  are the instants at which the reference signal intersects the signal sin#(t), then sinwt 1  = sin #(t 1 ), and if #t = t 2 -t 1  then wt 1  = ¢ (# - w#t), and therefore #(t 1 ) = ¢ (# Œ w#t) Œ 2k# where k is any integer. Thus, subject to the the indeterminacies resulting from the inversion of a sine function, the duration t of each pulse is linearly proportional to # (t), so that by smoothing the pulses a voltage proportional to #(t) can be produced. The ambiguities in the value of #(t) are resolved in principle by detecting and counting when the value of #(t) passes through the values #/2, #, 3# 2 , etc and adding an appropriate potential to that produced by smoothing the pulses of duration #t. In the preferred embodiment it is assumed that the signal 1 + sin#(t) is available, rather than the signal sin#(t), and that the signal 1 + cos#(t) is also available. Each of these signals is compared in comparators C1, C2, Fig. 4, with a reference signal 1 + sinwt to produce pulse trains S and C, Fig.3, the pulses being of durations #t and #t<SP>1</SP>, respectively, where #t is as defined above #t<SP>1</SP> is defined similarly for the cosine signal. The inverses S and C of these pulse trains are also produced, and the specification shows that within four regions 11, 12, 13, and 14, Fig.1, the values of a function #(t) derived from #(t) by subtracting multiples of 2# until the result lies in the range -#/4 to 7#/4 are as follows: - region value of 2# (t) 11 wS t  - # 12 wC t  13 wS t  + # 14 WC t  + 2# where the suffix 't' indicates that the signal has been integrated over a period of the reference signal sinwt. These results are utilised by a logic circuit LG which, by receiving from comparators C3, C4, C5 and C6 the results of comparing the signals 1 + sin#(t) and 1 + cos#(t) with signals representing unity and the values 1 Œ ¢#2, identifies firstly which of the four regions 11, 12, 13, 14, the function #(t) lies in, and secondly its direction of change when it passes from region to region. As a result of this, the logic circuit LG causes the appropriate one of the four signals S, S, C, and C to be passed by a gate TK to an adder A2, where a signal from a digitalto-analogue converter D/A and a counter VRZ, representing the appropriate number of #'s in accordance with the above table, is added to it in the form of a d.c. bias, and the result is smoothed TP and displayed DP. In practice, when #(t) is initially zero, the counter is initially set to -1, and as #(t) increases each time it passes from one region to the next the counter is stepped forward. When #(t) advances outside region 14, #(t) reverts to region 11, but rather than resetting the counter to the value -1, (from its setting of +2 for region 14) which would be necessary for the display device DP to indicate #(t), the logic circuit LG merely steps the counter back by unity so that it holds a count of 1, and hence the signal produced by the smoothing circuit TP represents #(t) rather than #(t). The construction of the logic circuit LG is described Fig. 5, (not shown) together with the construction of a gate, Fig. 6, (not shown) designed to respond to transitions of one direction only.