Patent GB-1087293-A (GB)
Title: Improvements in or relating to electrical comparison circuits
Inventor: COX RAYMOND JOHN
IPC: G06G7/14
Filed: 1963-08-15
Abstract: 1,087,293. Measuring temperature electrically measuring radiation flux. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Aug.11, 1964 [Aug.15, 1963 (2)], No. 32389/63. Headings G1A and G1N. [Also in Divisions H3 and G4-6] In a fail to safe system for comparing a D. C. representing for example a radiation or temperature level, with a reference level, the reference level is constituted by a peak level in an A. C. signal and the two are fed to an A. C. amplifier via a gate which is open only during the time the A. C. signal is at the reference level. An ionization detector 1 responsive to the neutron flux in a zero energy reactor is supplied with a D. C. (from 2) upon which is superimposed a sawtooth wave 2B obtained by integrating at 4 the output 2A of a rectangular pulse generator 3. The sawtooth wave is differentiated by the self capacitance of the detector so that upon the output signal of the detector (shown at 2E as 98 micro-amps) is superimposed a rectangular wave 2D having a negative excursion of a 100 micro-amps, this excursion representing the reference level. Portions of the resulting 2 micro-amps excursions are selected by a chopper type gate 5 to produce the signal 2G and this is passed to an A. C. amplifier 6. The amplified signal 2K is applied to a synchronous detector 8 operated by pulses 2H of half the frequency and occurring between the reference pulses D so as to produce the output signal 2L. If the ionization detector output reaches 100 micro-amps or exceeds this value the output signal becomes zero or goes positive, releasing a polarized relay 9 controlling the reactor. Should the chopper 5 fail the whole of the superimposed rectangular wave is applied to the amplifier and this will cause the relay to be released and the circuit will thus fail safe. A signal output from the amplifier is also applied to an amplifier 10 together with rectangular waves from resistor R of opposite phase and larger amplitude than the signal. If the flux decreases the signal increases and the combined signal changes in phase. The resulting output of a synchronous detector 11 operates an alarm 12. The circuit of Fig. 3 may be modified Figs. 3 and 4 (not shown) in that the sawtooth wave from 4 is a symmetrical triangle so that the reference wave resulting from integration in the detector has a unity mark space ratio. In addition in order to further monitor the correct operation of the system a rectangular wave having a zero value during the selected portion of the signal is added to the combined input to the chopper 5 so that when the chopper is operating correctly the output is unchanged. If however the chopper becomes open circuited the remaining part of the wave is passed, the output from the chopper is such that it is of reversed phase and the relay is released. The circuit may be used with other transducers and in Fig. 5 (not shown) the ionization detector is replaced by a thermocouple and in this case as no differentiation takes place, a square wave instead of a sawtooth is added across a resistor in series with the thermocouple.