Patent GB-771675-A (GB)
Title: Improvements in or relating to radiographic apparatus
IPC: H05G1/46
Filed: 1953-12-23
Abstract: 771,675. X-ray apparatus. GENERAL ELECTRIC CO., Ltd., and LAMBERT, R. A. Dec. 1, 1954 [Dec. 23, 1953], No. 35696/53. Class 98(1) Radiographic apparatus comprises means operable for independently selecting the value or values of one or more exposure determining factors and further means operable for selecting a value of a combination of two or more exposure determining factors, means for scanning the whole or a part of the provided values of said two or more factors at which the apparatus may be set, and means for seizing a suitable pair or combination of values of said two or more factors which produces substantially the selected combination and which would result in a sate loading of an X-ray tube controlled by the apparatus with the apparatus also set at the independently selected value or values of, said one or more exposure determining factors. The voltage applied to the X-ray tube is variable by the operator and is also applied to an auto transformer 3. A current selector switch CS has levels CS1, CS2, CS3, CS4, CS5 and a sixth level (not shown) controlling an impedance in the filament circuit of the X-ray tube to control the tube current. A seventh level, (not shown) may be provided to compensate for the fact that the tube current depends not only on the voltage supplied but also on the tube current. A time selector switch TS has levels TS1, TS2, TS3 and a fourth level (not shown) connected to mechanism controlling the exposure time. Tappings 630S, 500S, 400S &c. on autotransformer 3 are selected by level CS1 and connected through switch' F2, when closed, to tappings 6 on a resistor chain 6 selected by level TS1. Similarly, tappings 630L, 500L, 400L on the auto-transformer are connected to tappings on the resistor chain through levels CS2, TS2 and switch F1 when closed. Switches F1, F2 are ganged with a switch controlling the size of the focal spot, so that' when a small focal spot is used, F1 is open and F2 is closed, and vice versa when a large focal spot is used. The current flowing through resistor chain 5 is proportional to the tube loading at any setting of the controls. Part of this current is rectified by a bridge rectifier 11 and passes through a meter 15 and a winding 7 of a differential relay RL1/1. The other winding 8 of this relay is energized by a current from a constant A.C. source rectified by a bridge rectifier 9, which current is proportional to the maximum safe load. When the tube control settings would result in a load exceeding the maximum safe load, the current in winding 7 exceeds the current in winding 8 and relay RL1/1 operates contacts RL1 in the circuit controlling the uniselector magnets CSM. TSM which operate the switches CS, TS respectively. Setting up. The milliampere-seconds value is set by a multicontact switch having six banks A, B, C, D, E, F, the wipers being connected to contacts on level CS4. Thus in each position of the multicontact switch, each wiper is connected to a contact on level CS4 corresponding to a given value of tube current, and hence each contact on the multicontact switch corresponds to a certain exposure time. The apparatus only deals with a fixed number of exposure times, and those contacts of the multicontact switch which correspond to times in this range are connected to corresponding contacts on level TS3, the wiper of which is connected to a relay RL2/2. The remaining, or " blank," contacts are connected through a high resistance 12 to the rectifying bridge 11. The number of blank contacts is made as small as possible by choosing the values of milliampere-seconds and of anode current as geometrical progressions of the same common ratio. Operation. The operator, having set the tube voltage control and focal spot switch, and having set the milliampere-seconds switch to the required value, moves a switch 16 to its " select position. At this stage switches CS, TS are both in .their 25th. and last, positions as shown, so that movement of switch 16 completes a circuit energizing a relay RL2/2. In consequence. contacts RL2B open to ensure that magnet TSM is not energized, and contacts RL2A close, allowing current to flow through magnet CSM and a half-wave rectifier 13 which is connected in series with both magnets. The next cycle of A.C. energizes the magnet CSM so that the wipers of all levels of switch CS are moved to their first contacts. The resistor 14 bridging rectifier 13 passes enough current to de-energize. magnet CSM in the next half-cycle. The movement of switch CS breaks the circuit energizing relay RL2/2, so that contacts RL2A, RL2B open and close respectively, thus breaking the circuit energizing magnet CSM and closing a circuit energizing magnet TSM. At this stage the wiper of level CS4 is on the contact representing the highest tube current, which is connected to bank A of the multi-contact. switch. The wiper of bank A energizes a contact which may correspond to one of the given time values or a blank. If it corresponds to a time value, magnet TSM is energized through the first contact of level CS3 and contacts RL1, RL2B by alternate half-cycles until the wiper of level TS3 reaches a contact corresponding to this time value. Relay RL2/2 is then energized, opening contact RL2B so that magnet TSM is no longer energized. The values of tube current and exposure time have been set, and the exposure may be initiated automatically by known means. Switch 16 is then moved to " reset " position, whereupon switches CS, TS are stepped to their 25th position. Movement of switch 16 may be effected automatically on termination of the exposure. It has been assumed that the tube load set at no time exceeds the safe load, so that relay RL1/1 is not operated. However, the current flowing in resistor chain 5 is always proportional to the tube load, and if it exceeds the current in coil 8 the relay RL1/1 operates contact RL1, energizing magnet CSM and stepping switch CS to the second contacts. In the apparatus shown, the stepping switches are of the bridging wiper type and the even-numbered contacts are reconnected. The contacts 2-14 of level CS3 are, however, connected together to magnet CSM so that the switch is always stepped from an even-numbered contact to the next contact. When the load set is safe, relay RL1/1 is de-energized and switch TS is again operated until the appropriate exposure time is reached. If the set load again becomes unsafe, the tube current set is again reduced. If no combination giving the required milliampere-seconds value for a safe load can be reached, the 13th contacts of switch CS will eventually be complete, a circuit to energize relay RL2/2, and open contacts RL2B to prevent energization of magnet TSM. At level CS5, a buzzer 38 is energized, indicating to the operator that all available combinations of tube current and exposure time have been scanned without finding a combination resulting in a safe load. The buzzer sounds until the switch 16 is moved to " reset position, causing both switches CS, TS to be stepped to their 25th positions. If during the .scanning process, one of the " blank " contacts of the milliamapere-seconds switch is brought into circuit by the wiper of level CS4, current flows through resistance 12 to energize relay RL1/1 and to operate contacts RL1, causing magnet CSM to step switch CS to its next position. Current limiting and indicating circuits. A series of resistors 23-28 are connected in series with neon lamps 29-34 respectively across the main A.C. supply, and the junctions of each resistors and each neon lamp is connected to a contact on level CS5. The resistor values are such that normally each lamp glows dimly, but the switch CS5, by short-circuiting each resistor 23-28 in turn, causes the corresponding lamp to glowbrightly, thus indicating the tube current value to which the switch CS is set. A current limiting switch 4 has a sectorshaped, moving contact, a fixed contact 17 connected through resistor 12 and rectifier 11 to coil 7 of relay RL1/1, and fixed contacts 18-22 connected respectively to the junctions of resistors 23-27 and neon lamps 29-33. To limit the tube current which can be set, with the switch CS in its starting position, switch 4 is adjusted to connect one or more of the contacts 18-22 to contact 17, extinguishing the associated neon lamps. Current will flow through coil 7, but will not be sufficient to operate relay RL1/1 until the wiper of level CS5 reaches positions where connection is made to those contacts of switch 4 which are connected to contact 17. When these positions are reached, relay RL1/1 operates contacts RL1 and causes energization of magnet CSM, until switch CS is stepped to a position corresponding to a lower tube current than that to which switch 4 has been set. Movement of controls after setting. If, after a setting has been reached, the voltage control or the focal spot switch are moved, then if current in resistor chain 5 is increased the apparatus operates to reduct'the tube load to a safe value. Movement of the current limiter switch 4 has no effect, but if the milliampere-seconds control is moved, scanning may recommence to reduice the tube load to a safe value. Modification. In a modification, no separate switch is provided for selecting the focal spot, the apparatus automatically setting to a small focal spot if it can be safely used. In this case levels CS1, CS2 may be combined so that tube current values for the small spot are scanned first, followed if necessary by values for the large spot. Specification 738,778 is referred to.