GB1106457A - Guarding of machine tools - Google Patents

Abstract

1,106,457. Transistor switching circuits. WILMOT BREEDEN Ltd. 28 Oct., 1965 [3 Sept., 1964], No. 36071/64. Heading H3T. [Also in Divisions F2 and G1] The circuit controls power-actuated guards for the danger zone of a power press in accordance with the positions of switches 59, 53, 49 respectively controlled by, a device such as a capacity loop or a light curtain which detects an obstruction in the danger zone, a cam indicating the position of the guards, and a cam indicating the position of the press ram. An " AND " circuit including PNP transistors 65, 70, 74 controls the potential at 76 in response to switches 53, 59, the effect of switch 59 being over-ruled when the switch 53 contacts 53c. Initially, with the guards open, switch 53 contacts 53b and switch 59 is closed. Insertion of a hand with a workpiece into the danger zone causes switch 59 to open, and withdrawal of the hand re-closes the switch. The successive opening and closing of switch 59 acts via a differentiating circuit including a capacitor 86 and a diode 85 to produce a pulse which switches the bi-stable circuit comprising transistors 77, 78 to a state in which transistor 77 is non- conductive and transistor 78 is conductive, to provide an output to an amplifier including transistors 89, 93, 97 causing energization of a solenoid 61. This initiates closure of the guards, which when fully closed initiate press operation. Also when the guards are closed, switch 53 contacts 53c. At the end of the press ram working stroke the switch 49 is momentarily closed to reset the bi-stable circuit to its initial state, de-energizing solenoid 61. At the end of the press-ram return stroke, the guards are released, and open, allowing switch 53 to return to 53b. A capacitor 66 in the " AND " circuit causes a time delay between the return of switch 53 to contact 53b and the change of transistor 65 to its conductive state, which allows switch 59 to become effective to initiate a new cycle.