GB1219262A - Safety apparatus for controlling circuits - Google Patents

Abstract

1,219,262. Logic circuits. C. R. EDWARDS, and J. R. MADLEY. 19 Jan., 1968 [20 Oct., 1966; 1 March, 1967], Nos. 47070/66 and 9646/67. Headings H3P and H3T. [Also in Divisions G4 and H2] A fail-safe logic circuit comprises a plurality of inputs each having two alternative states, an output B having two alternative states, a switch device connected between the inputs and the output and arranged to provide an output in a predetermined state only when each input is in a predetermined state, and a checking or further circuit 24, 25 connected to all the inputs and to the output and arranged to prevent the output taking up the predetermined state unless each input is in the correct state to result in that output state. In Fig. 1 the switch device is an AND gate formed by a transistor 13 and diodes 1, 2, and has its inputs connected to changeover microswitches which may be limit switches on a machine or on process control valves. When the contacts 4, 8 and 3, 6 are closed the diodes 1, 2 are reverse-biased and the transistor 13 conducts to energize a relay 19, the current through the relay coil being insufficient to light the lamp 21. Should either or both switches be operated to open contacts 4, 8 and/or 3, 6 and close contacts 4, 7 and 3, 5, the transistor should cease to conduct as the relay coil must not be energized. To ensure that the relay is not energized, the closing of the contacts 4, 7 and 3, 5 connects diodes 25, 24 across the relay coil short-circuiting it so that the relay releases and the lamp 21 is lit if the transistor still conducts. The transistor 13, the diodes 1, 2 and resistors 11, 14, 15 may be part of a microcircuit, or the microswitches may be replaced by transistors, Figs. 2 and 3 of the drawings accompanying Provisional Specification 47070/66, (not shown). Fig. 1 of the drawings accompanying the Complete Specification (not shown) illustrates the fail-safe logic and checking circuit included as part of a dairy plant. Diodes are plugged into points of the matrix corresponding to the correct initial state of valves V 1 , V 2 &c. Incorrect setting of the valves thus causes a relay RR 1 &c. to be operated so that a command issued by a relay bi-stable circuit F is not transmitted to the corresponding valve operating line X 2 , X 4 &c. The valve contacts VS 1 &c. operate corresponding reed relays A 1 , B 1 &c. so that contacts AS 1 , BS 1 &c. operate in inverse manner to provide a fail-safe logic operation according to the invention. In Fig. 3 (Comp.) (not shown) forward and back contacts respectively operate the output circuit directly and through a transistor relay circuit and in Fig. 2 (Comp.) (not shown) somewhat similar circuits operate a common output circuit through a combining circuit.