GB1013847A - Electromagnetic drive arrangement - Google Patents

Abstract

1,013,847. Clutch-brake control; electromagnetic brakes. F. H. SHEPARD. Aug. 20, 1962 [Aug. 21, 1961], No. 31889/62. Headings F2C, F2E and F2L. In an electromagnetically actuated friction clutch-brake arrangement for operation at high speed, in order to reduce the delay in starting or stopping the output shaft when the current to a brake or clutch coil is turned off and the current to the other coil is turned on, a reverse pulse is applied to the first coil to repel the driven clutch disc. The clutch-brake unit may be used for driving the paper shift mechanism of a high speed printer. The unit comprises a clutch electromagnet 16 which is driven by a motor (not shown) and a brake electromagnet 30, between which is positioned a disc 26 fast on a driven shaft 24. The disc 26 is formed of non-magnetic material and has magnetic clutch elements 28 loosely fitted therein. The right-hand face of the clutch electromagnet is formed by two steel rings 18, 20 separated by an annular gap 22, and the left-hand face of the brake electromagnet is similarly formed by rings 32, 34. Each clutch element 28 is formed by a soft iron core 40, the outer faces of which are coated with brake linings 42, and when first placed in the unit the iron core extends at 44 on each side beyond the vertical plane of the brake lining. The portions 44 bridge the gaps between the clutch and brake rings, and after some operation the portions 44 are worn down into the same plane as the friction linings to give maximum engagement. The electromagnets are energized alternately by a circuit including a first output transistor 50 and a second output transistor 52 which are alternately conductive. The collector of transistor 50 is connected through a low resistance 60 to the clutch coil 16 and the collector of transistor 52 is connected through a resistance 72 to the brake coil 30. The base of the transistor 50 is connected to the emitter of an input transistor 82, the collector of which is connected through a silicone diode 90 to the base of the transistor 52, and the input transistor 82 is controlled by a signal from the output of a flip-flop unit 92 which has two inputs 94, 96. When the input 94 is actuated by a momen. tarily trigger pulse, the input transistor 82 becomes conductive, and when input 96 is energized the transistor 82 is blocked. When the input transistor 82 is blocked, the output transistor 50 is blocked and the output transistor 52 is conductive so that the brake coil 30 is energized. When the input transistor 82 is conductive, the base of transistor 50 becomes negative and the transistor conducts. The voltage drop across a resistor 86 in the circuit to the base of the transistor 52 is sufficient in conjunction with the voltage drop across a diode 54 to its emitter to block the transistor 52. When the transistor 52 is blocked, a capacitor 74 which was charged during its conductive phase, is discharged through a capacitor 66, providing a damped resonant discharge circuit causing a very rapid decay of the current and then a reverse pulse in the electromagnet. The clutch elements 28 are therefore rapidly shifted from contact with the brake to the clutch electromagnet. A similar sequence of events occurs when the brake electromagnet is energized. U.S.A. Specification 2,924,314 and U.K. Specification 825,646 are referred to. Reference has been directed by the Comptroller to Specification 803,268.