Patent GB-1215774-A (GB)
Title: Improvements in or relating to electrical safety circuits
IPC: F16P3/18
Filed: 1969-02-12
Abstract: 1,215,774. Machinery safety devices. ELMEG ELEKTRO MECHANIK G.m.b.H. 12 Feb., 1969 [12 Feb., 1968], No. 7618/69. Heading F2J. [Also in Division H2] A two-handed safety circuit for starting or stopping a machine comprises switches S1 S2 which, in their inoperative positions, pass blocking signals B1, B2 to gates T1, T3, so that no output signal A2 for controlling the machine is derived from gate T2. When the switches S1 S2 are both actuated, the signals B1, B2 are removed, and a signal E1 opens gate T1 whereby signal A1 opens gate T2 and sets the machine in operation. The machine is stopped when only one of the switches is released, and it is not re-started by re-actuation of the released switch. Under these conditions, the blocking signal B1 is applied to gate T1 so that signal A1 is removed. A switch SG supplies a brief signal E2 to gate T3 which blocks the gate T2 by means of signal A3. If the released switch is re-actuated, the signal B1 is removed and gate T1 is opened by signal E1 The signal A1 passes to gate T2 and to the switch SG, but since the latter has opened the gate T3 at the preceding removal of the signal A1, and this position is unchanged by opening only one of the switches S1 S2, the gate T3 continues to supply the signal A3 to hold the gate T2 closed. Thus, the machine is restarted only after both switches S1, S2 are released. The voltage source may be a pulse generator or may be unidirectional. In the latter case, the gate T1 is omitted as well as the contacts RK3, RK4, Fig. la (not shown). Each gate may consist of a transfluxor with a setting winding receiving the signal E and a blocking winding receiving the opposing signal B. When the transfluxor is set, the resultant reversal of magnetization operates a flip-flop to provide a pulsating signal which renders a transistor conductive so that it supplies a protracted output signal, Fig. 2 (not shown). In another arrangement, the gate is formed by a transistor which is rendered non-conductive by a setting signal so that a further transistor conducts and supplies an output signal through its emitter-collector path, Fig. 3 (not shown). The switch SG comprises a transistor and a capacitor adapted to receive the setting signal. When the setting signal is interrupted, the capacitor is discharged to render the transistor conductive whereby an output signal is provided for a short period. A thyristor is fired to provide the signal A3 until a transistor is rendered conductive by a blocking input to short-circuit the thyristor, Fig. 4 (not shown). In another arrangement, the gate T2 comprises a transfluxor having setting, blocking, and two opposed driving windings for reversing the flux on alternate application of rectified driving pulses. A transistor becomes intermittently conductive to provide a pulsating signal A2. An alternating voltage is applied to a toroidal core for deriving a square-wave pulse to energize the driving windings alternately. The signal A2 thus bears a fixed phase relationship with the voltage applied to the toroidal core, Fig. 5 (not shown). In a modification, the pulse generator G is supplied through phase shifter PS from source ST. Signal E1 is applied to repeater U1 so that, when signal A2 is present, signal A4 is used for phase control of a thyristor associated with the machine control. Otherwise, a uniform signal A2 controls a contactor. Contacts RK3, RK4, close before the contacts AK1, AK2, open. In a circuit arrangement according to this modification, transfluxor gates are employed and the switch SG is a toroidal core element, Fig. 7 (not shown).