Patent GB-1178819-A (GB)
Title: X-Ray Apparatus
IPC: H02P7/292
Filed: 1968-08-16
Abstract: 1,178,819. Control of D.C. motors. PICKER CORP. 16 Aug., 1968 [30 Aug., 1967], No. 39294/68. Heading H2J. [Also in Divisions G3 and H5] X-ray apparatus, in which a carriage 312 for supporting X-ray devices is mounted for movement in opposite non-horizontal directions by a D.C. motor 18, see also Fig. 1 (not shown), has a switching control circuit 14 controlling the motor speed and a dynamic brake 22 to prevent the motor running away under gravitational force when the carriage is moving downwards, a resistance 220 then being connected across the armature 18a to reduce the motor speed. The input device 16 for a signal representing the desired speed of rotation of armature 18a includes an adjustable potentiometer 54 with an arm 54b coupled to a handle 335, Fig. 1 (not shown), controlling both motor speed and direction, any movement of arm 54b from its centre position closing a microswitch 56 or 57 and energizing relay coils 60a or 61a to actuate contacts 60b; 60c; controlling the direction of rotation of the motor 18. The motor control system includes differential amplifiers 10, 12, the amplifier 10 receiving the signal from device 16 and a feedback signal from switching circuit 14 indicative of the actual speed of rotation of motor armature 18a, and the amplifier 12 receiving the error signal and a sawtooth voltage from a generator 20. The output signal of amplifier 12 controls the switch circuit 14, which in turn controls the D.C. voltage supplied to armature 18a, a reduction in load 19 and consequent increase in motor speed resulting in amplifier 12 applying a signal to circuit 14 at a later time in each cycle of the sawtooth voltage. Armature 18a is energized from a full-wave bridge rectifier 68 comprising S.C.R.'s 72a, 72b, and diodes 70a, 70b, a smoothing capacitor 80 being connected across the output thereof. On moving potentiometer arm 54b to stop the motor, the insertion of a resistor 84 across armature 18a provides rapid stopping, due to actuation of contacts 82b by a relay coil 82a energized by contacts 60d, 61b controlled by relay coils 60a, 61a. End of travel limit switches 58, 59, also control stopping of the motor 18. Differential amplifier 10 includes NPN transistors 86, 88, the base of transistor 88 being connected to potentiometer arm 54b and its collector being connected to the base of a PNP transistor 116 connected in turn to a transistor 124 of differential amplifier 12. A capacitor 122 has a charging current which decreases as arm 54b moves from its centre position. Sawtooth generator 20 comprises PNP transistor 134 and NPN transistors 136, 138. Transistor 126 conducts during that portion of the sawtooth voltage cycle when the voltage exceeds that on the base of transistor 124, resulting in turning off of transistors 166 and 172, and turning on of transistor 178 in switch circuit 14 to develop potentials across resistors 194, 196, and cause S.C.R.'s 72a, 72b, to conduct to energize motor armature 18a. Dynamic brake 22 includes PNP transistors 206, 208, 210 and NPN transistor 212, the transistors 210, 212, acting as switches to turn transistors 206, 208 on and off, and thereby place resistor 220 in or out of shunt with armature 18a.