US2379125A - X-ray tube protective system - Google Patents

Description

Patented June 26, 1945 UNITED STATES PATENT OFFICE X-RAY TUBE PROTECTIVE SSTEM Louis L. Weisglass, New York, N. Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 4, 1942, Serial No. 449,814 6 Claims. (Cl. 250-95) The invention relates to X-ray apparatus and concerns electric measuring systems, in. connection with such apparatus, for indicating or preventing X-ray exposures under overload conditions.

The permissible duration of an exposure to be carried out with an X-ray tube without damage operation of mechanical interlocking devices and cam gears.

Another object, in conjunction with the foregoing, isto provide a measuring instrument for ends, is limited by the maximum temperature which the targetcanwithstandwithout damage to its metal (tungsten) body or to other parts of the tube structure. At higher temperatures, the target metal may crack ormelt, or gas may be liberated therefrom.

For a given high-tension generator (full-wave, half-wave, self-rectified or three-phase current) and a selected focal spot size (projected area of the target), there is adefinite. relationship between anode current..(ma.),.anode voltage -(kv. peak value), and permissible maximum exposure time. The tube rating charts, available to the operator, contain these relationships expressed by families of parameter curves. By properly reading these charts, and based upon a selected current, voltage, and focal spot, the X-ray technician can determine the permissible maximum exposure time. method requires relatively reat skill and attention and'hence may easily incur errors to the detriment olthe X-ray tube.

It has been suggested to simplify the operation of X-ray apparatus and to protect themfrom overload adjustments by interconnecting the volt eand current controlling rheostats and the timing device of the apparatus so as to indicate injurious adjustment combinations or prevent an exposure with such combinations. These known devices contain mechanical blocking devices or cam gears of complicated design and are of rather limitecfaccuracy. V

' It is an object of, the invention to provide Xeray apparatus with means for accurately determining the safe maximumexposure load of the X-ray tube under a great variety and within a large range of selective. operating conditions. It 'is al'soan object to achieve this result with theaidv dielectric devices or. high reliability, and more. particularly, .by' means of an electrical measuring circuit whose function is based on. a measuref 99s sis e s leeg ii d w t ee n "According. tdlithe directly indicating the permissible maximunrexposure time'for any chosen X-ray technique (focalsp'o't',millismpslkilovolts).

An object of the invention is also to provide X-ray apparatus with-a protective relay'which automatically preventslenergizing the X-ray tube underconditions which would incur overloading the-tiibe.{l I Other objects of the, invention'wfll be'flph rent from Qth effdllowihg; in particular iromthe 'd' tailed. description oren bq nfl latepplace. f' I r invention,"

-' sthument has, its'voltage-responsiveoil connected with the energizing network of the X-rayfapparatus, to be operated, while thecurrent coil ot the instrument energizedin proportion to the anode current of .the apparatus. In'one of the aspects of this invention, .thewattmetrio instrument is designed as an indicatin'gf or recording device having its wattage-responsive armature movable in relation to a timescale calibrated in permissible maximum exposure periods. In an.- other aspect the wattmetri'c instrument forms a relay which causes an interruption of the energizing network when the selected combination of current, voltage and time adjustments would resuit in an exposure energy whose wattage exceeds a given limit value.

The invention is based on the following consideration: Between the various 'curves o'i'the above-mentioned rating charts of anX-ray tube exists a definite relationship, namely, that'ih all possible cases of exposures the permissible max- .of the exposure time t) a. definite functioni of the wattagelkv. ma) applied to the target spot:

Instead of measuring in -tube current; directly, anothericurrent be su V lijedtbjth'e wattmetric' instrument 'ahdlilikept sumciently. proportional to he .t beicurr s result. is a; wattage l .111. mm.

measurement proportional to the permissible time maximum.

Other and more specific features of the invention relate, among other things, to the provision of correcting, calibrating and compensating means within or in connection with the circuits of the voltage and current coils of the wattmetric instrument and will be understood from the following description of the embodiments shown in the drawings.

Fig. 1 of the drawings is a circuit diagram of an X-ray apparatus embodying a wattmetric measuring system in accordance with the invention.

Fig. 2 is an exterior front view of the wattmetric instrument used in the system of Fig. l and shows details of the scale calibration of the instrument.

Fig. 3 is explanatory and exemplifies a rating chart of an X-ray tube applicable in connection with the invention.

Fig. 4 illustrates diagrammatically another embodiment containing a wattmetric control relay for automatic overload protection of an X-ray apparatus.

Figs. 5, 6 and 7 are explanatory, referring to the embodiment of Fig. 4, and serve to elucidate some voltage relations occurring in the wattmetric relay circuit.

Referring to the embodiment shown in Fig. 1,

I I is the feeding A. C. line and I02 the mainswitch connecting the line with a regulating transformer I03 which, in the example, consists of an autotransformer with two series connected windings I04 and I05. A line voltage regulator I03 in the primary transformer connection permits ad- Justing the proper number of transformer turns (line voltage selector). Two selector switches I01 and f03-have their contact rows connected with transformer taps and serve for the coarse (in terms of tens) and fine (in terms of units) regulation respectively of the secondary voltage in order to adjust the kilovoltage (kv.) of the X-ray tube. The winding I in conjunction with an additional selector switch I 09, permits compensation of load losses occurring within the elements of the network as to their eflect on the high-tension voltage actually supplied to the X-ray tube.

The secondary circuit III! of the autotransformer I03 leads to the primary I I I of a high-tension transformerfllover the control contacts I I 3 of a timer relay fl'kactuated' by a relay coil I I5. The secondary H3 of the high-tension transformer I I2 has its midpoint grounded and its terminals III and H3 connected with a rectifier bridge H3 whose D. C. output diagonal includes the anode circuit of the X-ray tube I20.

The above-described portion of the apparatus forms the energizing network of the apparatus. When the main switch I82 and the relay contacts I I3 are closed, the X-ray tube I20 is energized by a high-voltage current whose kilovoltage depends on the adjustment of the selector switches of the regulating transformer I03. The relay coil I I5 is controlled by a timing; device I2 I. The operating circuit 133 of this device is connected with the main switch. I02 over a starting switch I23, for instance, of the push button type. When actuating the starting switch, the relay coil I I 5 is enersized and starts the X-ray exposure by closing the circuit IIII. At the same time the timing mechanism (not shown) of the device I2I is set in motion and after a preadjusted period interrupts the coil circuit of H5 thereby opening the tube circuit III and terminating the exposure.

Combined with the above-mentioned excitation and timing means of the apparatus is a metering system to be described presently.

A wattmetric measuring instrument I30 is shown which is provided with stationary current coils I3I and I32 and a movable voltage coil I33 for actuating a pointer I34 over a scale I35 of the instrument. The voltage coil I33 is excited in accordance with the kilovoltage of the X-ray tube and, to this end, is connected with the selector switches I01 and I08 over a series resistor I36. By virtue of the correction effected by coil I05 and switch I09, the voltage acting on the instrument coil I33 is in fact equal to the primary voltage of the high-tension transformer H2. The current coils I3I and I32 of the wattmetric instrument are connected with a transformer I31 energized with line current of constant voltage. A tap switch I38 controlling the effective resistance of a resistor set I39 is disposed in the current coil circuit and serves for adjusting the coil current in proportion to the milliamperage of the X-ray tube current since the scale of resistor set I3! is calibrated in milliamperes and is initially set prior to each exposure to correspond tothe milliamperage desired for the X-ray tube as set by the oathode heating control (not shown). Current and voltage effective in the wattmetric instrument are supposed to be in phase with each other.

The scale I35 of the wattmetric instrument is calibrated in seconds ranging, for instance, from ,4, of a second up to several seconds. when in operation, the pointer of the instrument indicates the maximum exposure time for which the timer III can be set under the chosenenergy conditions without overloading the X-ray tube. That is. when the. X-ray apparatus isadiustedfor a given technique, 1. e., a selected focal spot size oi the target, a selected kilo-voltage and milliamperage, the pointer shows a corresponding time value in seconds; and the timer should not be settoan exposure period exceeding the limit thus indicated, to prevent overheating the target.

The scale of the wattmetric instrument I33 may be provided with two or more scales for different focal spot sizes. An example of a double scale instrument is apparent from Fig. 2. The scale I35 contains two rows of indicia for a large and a small focal spot. The division of eachrow corresponds to the actual exposure and circuit conditions. The 54 second divisions of the two rows coincide. This is obtained by changing the calibration of the meter when changing from one to the other focal spot. The change in calibration is effected by varying the resistance value of resistor I36 in Fig. 1 in accordance with the selected setting of the focal spot control (not shown). Hence, if the operator varies any of the voltage, current, or spot determining settings and adjusts the resistors I 33 and I36 in accordance withthe selected current and spot data, the permissible exposure time is indicated by the pointer [34' of the instrument I 30.

The operation of the wattmetri'c system in dependence upon the determinants of a chosen technique will be more full; understood from the following. As mentioned, the permissible exposure time of an X-ray tube is a function of the target spot wattage of the tube. Thi wattage is determined by the product of the primary voltage of the high-tension transformer and thetube current. The deflection of the wattmetric instrument I30 is proportional to the product the primary voltage of the high-tension transfbrmer and a cm'rent which, by means of the variable resistance set I33, is kept sumciently proportional to the load current of the tube network. Hence, the indication or the instrument is in tact proportional to the wattage supplied to the target spot and can be taken as a measure of the maximum exposure time permissible with any given energizing conditions of the tube. For instance, the permissible maximum exposure time as a function of the wattage for a chosen focal spot is represented by the rating chart shown in Fig. 3. Point P, representing a chosen wattage of somewhat less than 30,000 (milliamps times kilovolts), corresponds to a permissible maximum time t. Since the deflection of the meter is proportional to the wattage, its scale can be correctly calibrated in exposure seconds in accordance with the rating chart of the tube.

While the wattmetric instrument in the embodiment of Fig. 1 serves merely as an indicator in order to obviate the study of rating charts or tables by the operator, a similar wattmetric principle can also be applied for controlling a protective relay so as to prevent overload exposures. An example of such a protective system is illustrated in Fig. 4.

The energizing circuit proper of the system according to Fig. 4 is largely similar to that of Fig. l, the similarity being made apparent by the use of the same reference numeral for parts identical in both figures. The energizing current from line IOI passes through the main switch I02 to the regulating autotransformer I03 having two windings I04 and I05 provided with taps and connected with selector or dial switches. I 06, I01, I03 and I03 whence the secondary circuit H0: of the autotransformer leads to the primary III of ahlghtension transformer H2 through contacts H3 of a timer relay H4 whose control coil is denoted by H5. The secondary H6 of the high-tension transformer is grounded at its midpoint and has its terminals H1 and H8 connected with a rectifier and tube circuit (not shown in Fig. 4) similar to the circuit denoted by I I3 and I in Fig. 1.

The control coil H5 of the relay H4 (Fig. 4) is connected with a timing device 22I fed through leads 222 from the main switch I02 and provided with a push button starter switch 223. Similar to the timing means in the system of Fig. 1, the exposure is started by pushing the button 223 ther by nergizing the relay coil H5 to-close the circuit H0. At the same time the timer mecha-- nism is released so as to interrupt-thecircuit of coil I I5 when reaching the time limit set by means of a manually adjustable pointer 224.-

The operation of the above-mentioned elements is substantially similarto that of the corresponding portion of the system described in connection with. Fig. 1. However, a protective relay 244 is added having its contacts 243 disposed in the energizing circuit I I0. The contacts are normally closed and hence do not interfere with the regular operation of the circuit. The purpose of the protective relay is to prevent an exposure when the combination of settings chosen by: the operator represents a target spot load injurious to the tube. The means for controlling the relay 244 in dependence upon thfii load are 01' wattmetric-nature and will be described presentlyt A wattmetric relay 230 with an armature 234 consisting of a light-weight contact arm has its movable and stationary contacts 234 and 235, respectively, connected. with the control coil 246 of the protective relay 244 and with the feed line -222 so as to excite the relay coil 246 when the contacts are closed, thereby causing the relay- 244 to Open the ayapperatuscircuit m.

The relay armature is controlled by a wattmetric coil assembly comprising a set of current coils HI and 232 and a voltage coil 233. The-current coils are excited from a transformers-231 through the resistance coils 233 and the movable selector contact 233 of a variable resistance -device. The primary oi transformer 231 receives constant voltage through leads 222 from the main switch I02.

The voltage coil 233 of the wattmetric relay is connected with the regulating selector switches I01 and I08 over a variable resistor 236 and a potentiometer 250. The circuit of voltage coil 233' extends from switch I03 over lead 246 to resistor 236, thence through the coil 233 and lead 241 to a terminal of the potentiometer resistor 25 I, and from the movable contact 252 of thepotentiometer through lead 243 to switch I01. The potentiometer resistor 25I is connected to the tapped secondary 26I of a transformer 260 whose primary 262 is excited by a constant voltage over leads 222 from the main switch I02. A voltagecontrolling selective switch 263 is. inserted be.- tween the transformer secondary 26I and the potentiometer. Consequently, the wattmetric voltage coil 233 is subjected to the resultant effect of two voltage components, one being supplied from the autotransformer I03 and proportional to the kilovoltage oi the X-ray tube, the other being supplied from transformer 26I and potentiometer 260. The voltagecomponent controlled by the potentiometer 260 ls.-- out of phase (bucking voltage) with respect to the component supplied from the kilovoltage selectors andserves to compensate the latter down;to zero.v ,At zero compensation the armature 2-34 of the wattmetric zero receivesno torque and assumes the contact position shown in broken lines. In this position the relay contacts 234 and 236 are closed and cause the coil 246 to open the contacts 243 of the protective relay 244 in order to preventvthe exposure. Y

The just-mentioned voltage compensation and interruption of the exposure network is to occur when the target spot wattage exceeds the safe limit. To ensure this dependency the bucking voltage supplied by the potentiometer 250-.must be indirectly proportional tothe wattage. This condition is satisfied if the output voltage of this transformer 260, adjusted by switch 263, isindirectly proportional to the milliamperage. In order to secure this relationship, the movable member of switch 263 is mechanically coupled, for instance placed on the same shaft 264, with the movable contact'ot switch 233, and the ,tape of the transformer secondary 261 are so rated that each current adjustment or switch 233- corresponds to a proper magnitude or the output voltage of transformer 260-Ieeding the potentiometer 250. Similarly, the movable contact 252. of the potentiometer 250 is mechanically connected with the setting mechanism or pointer 252' of the timing device 22I, as is indicated at 254, andthe resistance steps of the potentiometer resistor 26| are graduated in accordance with the gradation of the timer scale so as to satisfy the relationship t=l (wattage) corresponding to-the time divisions of the timer dial.

When, according to the diagram of Fig.. 5,.the current coil 233 of the wattmetric relay is enersized by a coil current corresponding to theladjustment of the current selector switch 233, while the voltage coils HI and 232 are not energized,.

since with the chosen combination ot milliamp'erage,' "kilovoltage and exposure time the-woltage DGHI'CH H00! supplied from the. kin-step switches lit-and .108 istnlly compensated bythe bucking voktagesnperimposed by the potentiometer, no torque is enertcdton the relay armature. so that theabovementtmed protective operation. takes place. Vltheri, according to the diagram. of Fig. 6, the chosen; combination of adjustments corresponds to a. bucking voltage from. the potentiometer smaller than the kv.-step voltage, a' resultant voltage 1' remains eflective on the voltage coil 233 of the wattmetric relay so that a deflecting torque is produced and the relay contact opened. Hence, the protective nelay 2- remains closed and the X-ray apparaws. is now ready for operation. when selectingthe determinants of an exposure so that, tor instance, a. considerable overload would occur, the potentiometer voltage, according to Fig. 7, is correspondingly larger than that supplied from-the kv.-step switches. .As. a result, the resultant voltage 1" impressed on the voltage coil 23: has. a. direction opposite to. that of the voltage 1- in-Fig. 6. The relay armature is now under a torque of unsuitable direction so that its contactsremain closed and the protective relay 2 energized. Consequently, when using the X-ray apparatus, the operator has merely'to set his dial switches in accordancewith the kilovoltage, milliamperage and time of the contemplated exposure. If the load corresponding to. the selected combination involves a target spot wattage beyond. the safe limit, the exposure is automatically prevented by the wattmetric re.- lay system.

It will be apparent. to one. skilled in the art, that many modifications ot the above-described systems are applicable within the scope of: this invention. For instance, the transformers 23-1 and 260 shown in Fig. 4 may consist of a single unit having a common core andprimary; the resistance potentiometer 250 may be replaced by another regulatabl'e' voltage source, the current selector 238 can be mechanically connected with the milliamperage adjusting meansof the regulating transformer oi the energizing tube network, or the wattmetric current-coil current can otherwise be made dependent on the anode current of the X-ray' tube, for instance by coupling it electrically with one of the circuitsof the tube network. Such modiflcationsdc not afiect the basic operation of wattmetric measuring systems ac;- cording to the I'- claim:

1". An X-ray apparatus, comprising an energizing network having regulating c'lrcuitmeans for adjusting the exposn'eenergy of. the apparatus,

a selective timing device connected with saidnet;

worlt'for limiting the fiowzpenlod of said energy, a-wattmetrio relay having ammvable member and voltage and current coils respectively for controtling said member, circuit means connected between said: networks-1rd said voltage coil ion sup piying' tothe latter a voltage: dependent on. that of the exposure energy current supply'meansof variableoutput voltage connected with saidzcurrent coir for energizin': the: latter in: accordance wild: the current magnitude of. said energy, whereby movable relay-member iscontrolled in accordance with the wattage of said-energy, and l a. protective; relay. disposed. in said. network and-connected withsaid relay-member for comsaid networkscas .to'prevcntaa exposure te -be started when said wattage exceedsa given limit'valuez.

r it InzwarbinatiommE-mvtube. a resuming exposure. energy thereto, a. selective: timing: dcvicealso connected with aidnetworlr tor limiting the flow periodot. said energy, an indicating instrument having a. multiple time scale tor-idliierent focal spot sizes and a wattmetric system for eflecting an indication on said scale, said scale being calibrated in permissible maximum exposure times, said system containing a current coil and a voltage coil, said voltage coil being connected with said network and excitable by a voltage dependent on that of said exposure energy, a variable resistor inserted between said voltage coil and said network for changing the calibration oi said instrument in accordance with the selected focal spot size, and variable current supply means connected with said current coil for. energizing the latter in accordance with the current, magnitude of said energy, whereby said instrument. is caused to indicate the permissible maximum exposure time as a motion of the wattage-of said energy.

3. An Km apparatus comprising in combination, an X-ray tube, a regulating transformer having a tapped main winding and a tapped auxiliary arrangedin series for supplyingt exposure energy to said; tube, main selective switch means interposed between the taps of said main. winding andsaid tube for: adjusting the voltage of said energy, and auxiliary selectise switch means interposed between the tape of said auio'liary-winding'and: said tube torr'providing a corrective voltage for compensating losses. within: said. network a. wattmetric' instrument having an operatingmovnblse member and voltage and current coils respectively for controlling saidmember, said voltage coil being connected with. said network and subjected to a voltage dependent upon the voltageof said energy adjusted by said main switch means and effective: in saidtube, and: variable current suppb means connected with said cm-rent coil for energizing the latter'in proportion to the current magnitude of said energy, whereby said movable member is controlled in accordance with the wattage oi said energy and its operation is indicative of the permissible maximum exposure time.

4. The combination with an X-ray apparatus having. regulating means for providing exposure energy of adjustable current and voltage magnitudes and a selective timing. device for limiting the flow periodsof said energy, of a measuring system comprising a wattmetric instrument having a movable member and voltage and current coils-respectively for controlling said member,

variableourrent supply means connected witlrsaid current coil for energizing. it in accordance with the selected current magnitude or said exposure energy, circuit means connecting said voltage coil with said regulating means to provide a voltage component dependent upon that of said exposure energy, a voltage source connected with said circuit means for providinga booking voltage component in opposition to said first component so-that said voltage coil is energized by the resultant or said two voltaeecomponents, adiusting means associated with said voltage source for varying the magnitude of said bucking component inaccordance with saidselected current magnitude;. and sdiusting means also associated with said;,voltage source tor .superposing a variation 0.! laixiibuchingvoltagecomponent in accordance witnthe selected. timeradfitment 01 said timing deviee-smthat the efiectire total magnitude-oi saidibucking voltage component is inaccordance network-connected said. whens-supplying tt with a given time tunctionotthewettageoi said energy, to cause said movable member to eflect a control operation indicative of the maximum permissible exposure time when said bucking voltage component reaches a total magnitude equal to that of said first voltage component.

5. The combination with an X-ray apparatus having regulating means for providing exposure energy of adjustable current and voltage magnitudes and a selective timing device for limitin the flow periods of said energy, of a wattmetric relay having a movable contact member, a stationary current coil and a movable voltage coil for controlling said member, a protective relay controlled by said contact member and connected with said regulating means for preventing the flow of said exposure energy when the selected adjustments of said regulating means and said timer involve overload conditions, variable current supply means connected with said current coil for energizing it in accordance with the selected current magnitude of said exposure energy, circuit means connecting said voltage coil with said regulating means to provide a voltage component dependent upon that of said exposure energy, a voltage source connected with said circuit means for providing a bucking voltage component in opposition to said first component to cause said voltage coil to be energized by the resultant of said two voltage components, adjusting means associated with said voltage source for varying the magnitude of said bucking component in accordance with said selected current magnitude, and adjusting means also associated with said voltage source for superposing a variation of said bucking voltage component in act.-

cordance with the selected time adjustment of said timing device so that theeflective total magnitude of said bucking voltage component is in accordance with a given time function of the wattage of said energy, whereby said movhaving regulating means for providing exposure energy of adjustable current and voltage magnitudes and a selective timing device for limiting the flow periods of said energy, of a wattmetric relay having a movable contact member, a stationary current coil and a movable voltage coil for controlling said member, a protective relay controlled by said contact member and connected with said regulating means for preventing the fiow of said exposure energy when the selected adjustments of said regulating means and said timer involve overload conditions, variable current supply means connected with said current coil for energizing it in accordance with the selected current magnitude of said exposure energy, circuit means connecting said voltage coil with said regulating means to provide a voltage component dependent upon that of said exposure energy, a potentiometric arrangement containing a voltage source and being connected with said circuit means for providing a bucking voltage component in opposition to said first component to cause said voltage coil to be energized by a resultant voltage determined by said two voltage components, said potentiometric arrangement comprising voltage regulating means connected with said variable currentsupply means for varying said bucking component in dependence upon said selected current magnitude, further voltage regulating means associated with said arrangement and connected with said timing device for varying said bucking component in dependence upon the selected exposure time adjusted by said timing device in order to obtain a total magnitude of said bucking component in accordance with the wattage of said exposure energy as a given function of the exposure time, whereby said movable member is caused to control said protective relay for preventing an exposure when the wattage of said energy obtained with the selected adjustments of said regulating means and said timing device would exceed a safe maximum value.

LOUIS L. WEISGLASS.

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