Patent GB-738778-A (GB)
Title: Improvements in or relating to radiographic apparatus
IPC: H05G1/46
Filed: 1953-02-13
Abstract: 738,778. X-ray apparatus. GENERAL ELECTRIC CO., Ltd., and LAMBERT, R. A. Feb. 15, 1954 [Feb. 13, 1953], No. 4167/53. Class 98 (1). Radiographic apparatus comprises independent controls for setting for each exposure the voltage applied to the X-ray tube, the anode current, and the exposure time; means for generating a current which depends on the combination of the settings of the controls in such a manner that it lies on one side or the other of a predetermined value according to whether the combination is respectively unsafe or safe; an electromagnetic relay adapted to occupy two states, in the first of which an exposure is prevented and in the second of which an exposure can take place; together with means for passing the current through an operating winding of the relay, means for biasing the relay with a first value of bias before an exposure is initiated, and means for initiating the exposure including means to change the bias to a second value before the exposure is initiated, the difference between the first and second values of the bias (one of which may be zero) being such that the change in current through the operating coil of the relay is not less than the differential of the relay, and the values of the bias being such that when the bias has its second value the relay will occupy the first or second state according to whether the value of the current is on the one side or the other of the predetermined value. The differential of the relay means the minimum amount by which the current through an operating coil of the relay must change in order to change the state of the relay after the relay has been altered electromagnetically in the opposite direction by a justsufficient current. The voltage applied to the X-ray tube is also applied by a wire 1 to an auto-transformer 3. The anode current to the X-ray tube is controlled by means of a variable impedance, not shown, in the filament circuit. The variable impedance is controlled by one bank (not shown) of a current selector switch which is provided with two further banks CS1 and CS2, some of whose contacts are connected to tappings 630S, 500S, 400S, &c. on the autotransformer 3. These tappings are chosen so that for any combination of the settings of the voltage applied to the X-ray tube and the anode current flowing through the X-ray tube the output voltage of the auto-transformer 3 will be proportional to the product of these two quantities. This output voltage is applied across a chain of resistors 5, a series of tappings, some of which are shown at 6, being provided on the resistor chain 5 to determine the part of the chain across which the voltage is applied. The particular tapping which is utilized is determined by means of a time selector switch (which covers the range 0.02 seconds to 8 seconds in twenty-four steps) having two banks TS1 and TS2 and ganged to the exposure-time control, and by a three-pole switch selecting the focal spot to be used. The focal spot switch has two contacts F1 and F2 which according to the focal spot in use connect one of the banks CS1, CS2 to one of the banks TS1, TS2. The contacts of the bank TS1, TS2 are connected to the tappings 6 so as to apply the voltage from the autotransformer through banks CS1, CS2 and banks TS1 and TS2 to the resistor chain. The tappings are so chosen that for any value of the products of the anode voltage and the anode current the current flowing in the resistor chain is proportional to the product. This product is twice as great in the case of the larger focal spot. The current in the resistor chain is rectified by rectifier bridge 11 and is measured by a meter 15 which may be calibrated in percentages of the maximum bias current. The rectified current passes through a winding 7 of a differential relay RL1/1 which has a second winding 8 through which is passed substantially constant current derived by full-wave rectification at 9 from a constant source of alternating current. The relay RL1/1 controls a contact in the energizing circuit of the main contactor which may remain either open or closed when the apparatus is disconnected from the supply. An exposure may be initiated by the operator by means of a switch, not shown, having three positions, " off," " prepare " and " expose." Movement of the switch to the " prepare " position causes the apparatus to be prepared for an exposure (which takes about one second) and at the same time closes a contact 12 which short-circuits part 13 of the second winding 8 of the differential relay RL1/1. The value of the current passed through this winding 8 is chosen so that when the three-position switch is in its " off " position the contact of the relay RL1/1 will close (assuming it is initially open) if the value of the current flowing through the first winding 7 of the relay is less than about 104 per cent of the critical value corresponding to the transition between the safe and unsafe combinations of the settings of the three controls; if the contact of the relay RL1/1 is initially closed it will not open unless the value of the current flowing through the first winding 7 of the relay exceeds about 130 per cent of the critical value. The part 13 of the second winding 8 which is short-circuited when the switch is moved to the " prepare " position is chosen so that if the contact of the relay RL/1 is closed when the switch is in the "off" " position the relay RL1/1 will open if the value of the current flowing through the first winding 7 is greater than the critical value and will remain closed if the value of the current is less than the critical value. If the current flowing through the first winding 7 is so high that the contact is open when the switch is in the " off " position it will remain open when the switch is moved to the " prepare " position. This will prevent an exposure if an unsafe combination of the settings of the controls have been selected while an exposure will take place if a safe combination has been selected. If an unsafe combination has been selected the operator will then release the switch to the "off" " position after which he can reset the controls immediately to a combination which is just within the safe region. After the part 13 of the second winding 8 of the relay RL/1 has been short-circuited and before the end of the preparatory period, the current passed through the second winding 8 of the relay RL1/1 may be slightly increased in order to ensure that if the combination of the settings of the controls is just within the safe region the contact of the relay RL1/1 will be held firmly in the closed position during the exposure. In such a case it will be possible for the operator to select a safe combination of the settings of the controls while the switch is in the " off " position and then to readjust the controls after the current has been increased to a combination which is just within the unsafe region, without thereby preventing an exposure. However, this can be tolerated in practice, since it will normally be the case that the transition between the safe and unsafe regions is chosen so as to leave some safety margin with respect to the maximum ratings specified for the X-ray tube. Firm holding of the contact of the relay in the closed position during an exposure will also be assisted by the fact that the value of the current flowing in the first winding 7 will tend to drop during the exposure due to a drop in the output voltage of the main auto-transformer.