Heat-sensing Circuit

Abstract

An analog voltage multiplier in combination with an operational integrator. The overall circuit measures the energy input to an X-ray generator tube and compensates for its thermal dissipation. If safe bounds are exceeded, the tube is shut down. These circuits are independent of the normal control circuits so that the tube and patient are protected.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

The present invention may be utilized with other circuitry for controlling an X-ray generator, for example, as described and claimed in copending Pat. application Ser. No. 742,463 entitled "An RMS Current Regulator" by Melvin P. Siedband and Jack L. James; copending Pat. application Ser. No. 860,603 entitled "X-ray Tube Control Circuitry" by Melvin P. Siedband, Philip A. Duffy and Jack L. James; copending Pat. application Ser. No. 860,686 entitled "Starting Voltage Suppressor Circuitry For An X-ray Generator" by Fred J. Euler and Jack L. James; copending Pat. application Ser. No. 860,687 entitled "A Brightness Stabilizer With Improved Image Quality" by Melvin P. Siedband and Philip A. Duffy; and copending Pat. application Ser. No. 28,664 entitled "MAS Meter Circuit" by Melvin P. Siedband and Jack L. James; all being assigned to the present assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to X-ray tube control circuitry and specifically to circuitry which monitors the amount of heat being built up on the anode of the X-ray tube caused by the current and voltage applied to it.

2. Description of the Prior Art

The prior art contains a large number of devices for determining the heat built up in an X-ray tube.

One device placed an optical hole in the tube so that the anode could be viewed by a photodetector to determine when the anode got hot. The determination would be made based upon the color of the anode. An indication of too much color would mean that the tube was too hot. Obviously, such a method was impractical due to the difficulty of determining the inception of "too much color" and because it was very difficult to insert the necessary circuitry into the tube.

Another device measured heat by controlling the integration of charge on a capacitor. In order to compensate for the various ma. levels, a variable resistance was placed in series with the capacitor and was varied an amount which was proportional to the change in ma. The choice in the magnitude of the resistor and the choice of kv. would charge the capacitor at various rates. However, this, too, did not give an accurate indication because the resistor was fixed and could not compensate for deterioration of various system elements such as the X-ray tube.

In most present day X-ray apparatus, "high-set" circuits protect the generator tube by preventing the operator from making any exposure which would exceed the ratings of that for a single shot. High-set circuits consist of a simple analog detector which senses the control settings and determines if the settings would result in usage beyond the ratings of the generator tube. However, these circuits are unable to determine if a single exposure, which is ordinarily well within safe limits but which is part of a series of closely spaced exposures, will cause overheating of the tube if performed at a given time after the previous exposure or exposures.

BRIEF SUMMARY OF INVENTION

The present heat-sensing circuit protects the anode of an X-ray tube from burning out by preventing it from getting too hot due to the currents and voltages applied to it during use. This is accomplished by monitoring the energy fed to the anode of the X-ray generator tube as a function of the heat capacity of the anode and the thermal dissipation rate so that an estimate may be made of the anode temperature. In this way, it is possible to operate the X-ray tube for single exposures or a multiplicity of exposures with assurance that the tube rating is not being exceeded. By making the heat-sensing circuit independent of certain other circuits used in X-ray apparatus, independent protection of the X-ray generator tube and the safety of the patient may be secured.

The circuit accomplishes its functions by calculating the anode temperature as a function of the actual energy applied to the X-ray tube anode for single shots or a series of shots such as would be taken in an angiography procedure. Obviously, then, it is possible for any single exposure to be taken at factors which are higher than a series of exposures. It is also possible for one series of exposures to be taken and followed by a second series of exposures. In either case, this invention will terminate the subsequent exposure or series of exposures if it becomes apparent that the heat capacity of the anode is being exceeded.

In order to determine the actual energy applied to the X-ray tube, a first signal in the form of a number of pulses is sent to a sensing means in the form of an integrating circuit. The frequency and magnitude of the pulses are related to the ma. flowing through the tube and the kv. applied to it, respectively. The integrator provides a second signal by adding the factor of time to add up the amount of energy applied to the anode during the total time of exposure. In order to compensate for the cooling of the X-ray tube between exposures, a bleed off resistor is connected across the integrator to decrease the heat indication provided by the integrator at a predetermined rate to approximate the cooling of the anode.

The invention also provides a meter which can be switched in at any time to give an indication of the percentage amount of rated heat to which the anode has been subject. In addition, it has a voltage detector which energizes a lamp when the maximum temperature is reached and a DC trip to prevent further energization of the tube when the maximum temperature is reached.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings in which:

FIG. 1 shows the basic X-ray tube circuit including the appropriate tap points for ma. and kv.;

FIG. 2 shows a block diagram of the main elements of the invention;

FIG. 3 shows part of a train of pulses which are produced by a pulse generator which is part of the invention;

FIG. 4 shows a detailed circuit diagram of the invention; and

FIG. 5 shows two approximations of the rate of discharge of the integrator capacitor.

DETAILED DESCRIPTION OF THE INVENTION

This invention monitors the temperature of the anode of an X-ray tube by accurately approximating the amount of energy being applied to the anode. This objective is accomplished because the power fed into the anode can be measured in terms of current and voltage.

Power = IV = (ma.) (kv.)

where ma. is current flowing in the anode circuit and kv. is the voltage applied to the anode.

Furthermore, the total amount of energy fed into the anode over a particular period of time (and consequently the amount of energy which must leave the anode) is determined by measuring the amount of power fed into the system over that period of time. Therefore,

heat energy .apprxeq. (power) (time) = (ma.) (kv.) (sec).

This invention directly measures the ma., the kv., and the time over which each is applied to the anode. It uses the pulse generator 30 of FIGS. 2 and 4 to multiply ma. and kv. together.

FIG. 3 shows a portion of the pulse train which is generated by pulse generator 30. The average voltage of the pulse train as measured between the leading edges of two successive pulses can be expressed as

V.sub.Av =V t.sub.1 /t.sub.1 +t.sub.2

If t.sub.2 >> t.sub.1, this expression becomes

V.sub.Av .apprxeq.V t.sub.1 /t.sub.2

Because the width of each pulse is constant, we can set t.sub.1 = k. Therefore,

V.sub.Av .apprxeq.K.sub.1 (V /t.sub.2)

One overall purpose of the invention is to indicate a larger temperature when a larger current is flowing. Therefore, the period between pulses must decrease as current increases. Stated another way, we want t.sub.2 to be inversely proportional to ma. Accordingly, we can set t.sub.2 = K.sub.2 (1/ma.).

Then,

V.sub.Av .apprxeq.K.sub.1 K.sub.2 V(ma.)

But V .apprxeq. K.sub.3 (kv.) where V is the anode voltage. Therefore, V.sub.Av =K.sub.4 (ma.) (kv.)

From the above derivation, it is clear that we can get an accurate indication of ma. and kv. by varying the frequency an amplitude respectively of the series of pulses generated by pulse generator 30.

FIG. 1 shows X-ray tube 1 being energized by the secondary windings 3B of transformer 3 through bridge rectifier 4. In series with each of the windings 3B, rectifier 4, and tube 1 is a second bridge rectifier 4A to which is connected a resistor 5 by line 6. Resistor means 5 includes a "viewing" resistor for sensing the ma. which is flowing through the tube. As the current through the tube changes, the voltage drop across resistor means 5 is changed in direct proportion. The voltage drop across resistor means 5 is transferred to the heat-sensing circuit shown in FIGS. 2 and 4 by line 6.

The anode voltage is provided by AC source 7 and primary winding 3A of transformer 3. When switch 8 is closed, a preselected kv. is applied to primary 3A and is simultaneously directed to the heat-sensing circuit by line 9.

In all subsequent drawings, the same reference numbers will be used to indicate the same elements in each figure.

Referring to FIG. 2, the voltage across resistor 5 is directed to operation amplifier 20 by line 6. The operational amplifier 20 includes a differential amplifier for reasons which will be more fully developed in the discussion of FIG. 4. For now, suffice it to say that the differential amplifier compensates for any additional signals which are incidental to the present invention but which are very necessary for proper control of an X-ray tube.

After leaving operational amplifier 20, the ma. signal is directed to pulse generator 30. Pulse generator 30 sends out a train of pulses. As was explained above, the width of each pulse is very small compared to the time between pulses. However, the frequency of the pulses is regulated by the amount of ma. For high values of ma., a large number of pulses will be generated. For smaller values of ma., a smaller number of pulses will be generated. Therefore, the period of the pulses generated by pulse generator 30 is inversely proportional to the magnitude of the ma. as sensed across resistor means 5 and becomes a signal representative of the ma.

After leaving pulse generator 30, the pulses act as one of two controlling signals on voltage clamp circuit 40. The second controlling signal is a voltage proportional to actual kv. which is fed by line 9 to claim 40. In response to the kv. signal along line 9, clamp 40 will control the amplitude of the pulses directed to it by pulse generator 30. As a result, clamp 40 acts as a multiplier circuit to multiply ma. and kv. together. Therefore, the average voltage output of voltage clamp 40 is V.sub.Av = (ma.) (kv.) which is a measure of the power being dissipated in the X-ray tube.

In order to determine the heat energy being developed, the circuit now integrates the (ma.) (kv.) product over time to get (ma.) (kv.) (sec). The integration is performed by an integrating circuit 50 which includes a capacitor 51 which has been separately shown. The charge on capacitor 51 of integrator 50 provides a second signal which increases as long as an exposure is being made-- i.e., as long as there is an ma. and a kv. being applied to the X-ray tube. The rate of the increase in charge on capacitor 51 is related to, and depends upon the amplitude and frequency of the pulses coming out of voltage clamp 40. As explained above, the clamp 40 is controlled by the ma. and the kv.

When there is no current flowing through the X-ray tube, the tube is not being heated. In fact, it is cooling down. In order to recreate and approximate the cooling state, a bleeder resistance R is placed across capacitor 51. When capacitor 51 is not being charged, it is discharging through resistance R. The discharge rate is designed to closely approximate the rate of cooling of the anode of the X-ray tube. As a result, after a predetermined time the tube will have entirely cooled down and, concurrently, the capacitor 51 will have entirely discharged.

A number of different operations are performed with the voltage which is present at the output of integrator 50. The first operation is a visual reading provided by heat meter 90 which is a voltmeter with a scale which reads 0-100 percent of total allowable heat. Because integrator 50 maintains its charge for a prolonged period of time, heat meter 50 will, if desired, provide a continuous indication of the amount of heat which has been build up in the anode of the X-ray tube. In an alternative embodiment, a switch can be placed in series with the meter to provide selective energization for the meter. Such an indication is very much unlike normal meter operation which provides a reading only while power is delivered to the system. In this invention, however, the reading on the meter will be retained as long as there is a charge stored in the integrator-- i.e., as long as the tube is hot. Connection of the meter to the output of integrator 50 can be made optional by inserting a switch between the output of the integrator and the input of the meter.

Also connected to the output of integrator 50 is a voltage detector 70. When the tube reaches its maximum allowable temperature and the integrator is storing its maximum charge, the voltage detector will energize AC switch 80 and DC trip 95. When AC switch 80 is closed, a lamp is energized to provide a visual indication that the maximum allowable temperature has been reached. Simultaneously, DC trip 95 is actuated to preclude any further energization of the X-ray tube. The voltage detector does not release the switch and the trip until the tube has cooled down to 75 percent of its maximum rated temperature-- i.e., until the integrator 50 has dissipated 25 percent of its charge.

FIG. 4 shows the anode current flowing through a portion of resistor means 5 by way of line 6. Resistor means 5 comprises resistors 5A, 5B, 5C and 5D. when an exposure is being made and anode current is flowing, a floating voltage is developed across resistor 5A of resistor means 5 which is proportional to the current flowing through it. The floating voltage is obtained by making the magnitude of sensing resistor 5A much less than the magnitude of resistors 5B, 5C, 22 and 24. The magnitude of resistor 5D is likewise much less than the magnitude of resistors 5B, 5C, 22 and 24. Therefore, anode current will flow in line 6 and through resistors 5A and 5D to ground.

Operational amplifier 20, consisting of four normally conducting transistors, three of which are NPN-transistors Q1, Q2 and Q3 and one PNP-transistor Q4, senses the voltage developed across resistor 5A at the bases of transistors Q1 and Q2 which transistors form a differential amplifier. A differential amplifier is used in order to obtain a signal which is directly related to the voltage, and therefore the current, through resistor 5A.

Specifically, variable resistor 5D represents circuitry external to the heat-sensing circuit. During the course of operating the X-ray equipment, resistor 5D may be varied. If the voltage of resistor 5A were measured from line 6 to ground, the effect of varying resistor 5D under constant current conditions is apparent; the voltage, and therefore the current measurement would vary as 5D were varied. However, by using a differential amplifier as shown, only the voltage across, and therefore, the current through, resistor 5A is measured.

The path of current flow for transistors Q1 and Q2 begins at the positive supply terminal Vcc and continues through line 25 to Q1 and Q2, then along line 10 to Q3, and then to the negative supply terminal -V.sub.E along the line 12. When there is no current flowing in resistor 5A, the conduction of transistors Q1-Q4, along with the value of Vcc and -V.sub.E and resistors 21 and 14 are so chosen that transistor Q5 is biased just below the point of conduction.

When current does flow in resistor 5A, the amplified signal from the differential amplifier appears as a negative going voltage at the collector of transistor Q2. The voltage at the collector of transistor Q2 is connected to the base of transistor Q4 causing it to be amplified. This negative voltage, proportional to the current through resistor 5 is connected to the base of transistor Q5 through resistors 13 and 14. Thus, as the current through resistor 5A increases, the bias on the base of transistor Q5 becomes increasingly negative causing it to become more conductive.

Transistor Q5 is part of pulse generator 30. The voltage at the base of transistor Q5 causes current flow through resistor 25.1, the emitter-collector circuit of transistor Q5, capacitor 23 and ground resulting in capacitor 23 becoming charged. When the voltage across capacitor 23 reaches the firing point of unijunction transistor Q6, capacitor 23 discharges through unijunction transistor Q6 and resistor 24, producing a narrow positive-going pulse across resistor 24 as the transistor goes into a brief state of conduction causing current to flow from line 25, through resistor 25.2, transistor Q6 and resistor 24 to ground.

Then, capacitor 23 is recharged until it again reaches the firing point of unijunction transistor Q6. This cycle keeps repeating as long as there is collector current flowing in transistor Q5. The time between successive positive-going pulses across resistor 24 is inversely proportional to the collector current of transistor Q5 which is in turn proportional to ma.

The output of transistor Q6 is applied to pulse-width shapers Q7 and Q8, capacitor 27 and the various resistors and diodes associated with them. The narrow positive-going pulses across resistor 24 causes transistor Q7 to conduct current from line 25, through resistor 25.3, transistor Q7 to ground through diode 28 thereby reducing the voltage impressed by line 25 upon the collector of transistor Q7 by a substantial amount. This negative-going pulse is coupled to the base of transistor Q8 through resistor 22 causing Q8 to be turned off and causing its collector voltage to go from ground potential to the voltage of line 25-- the positive supply voltage. The resulting positive-going pulse on the collector of transistor Q8 is coupled through capacitor 27 and resistor 26 back to the base of transistor Q7, thereby keeping transistor Q7 turned on until the capacitor discharges to a predetermined level. Therefore, the output of transistor Q8 is a positive-going pulse the width (t.sub.1 as shown in FIG. 3) of which is determined by the values of capacitor 27 and resistor 24 (t.sub.1 and t.sub.2 as shown in FIG. 3). The period of these shaped pulses is inversely proportional to the current through viewing resistor 5.

The output of the transistor Q8 is applied to voltage clamp 40 consisting of transistor Q9 and associated resistors and diodes. The collector of transistor Q9 is connected to the negative kv. input over line 9. The kv. voltage is caused to be negative by placing a diode (not shown) in line 9 in the reverse direction. When the collector voltage of transistor Q8 is at ground potential (when Q8 is conducting), the base of transistor Q9 is biased sufficiently negative to saturate transistor Q9 and to place its collector voltage at approximately ground. Diodes 41 prevent any current from flowing into the next stage when transistor Q9 is saturated. Later, when the voltage at the collector of transistor Q8 goes positive, the base of transistor Q9 is driven to a positive voltage causing transistor Q9 to turn off. The voltage at the collector of transistor Q9 is then a negative voltage proportional to kv.

The output of the voltage clamp is negative-going pulses of constant width whose amplitude is proportional to kv. and whose period is approximately inversely proportional to the current through "viewing" resistor means 5. The product of the average value coming out of transistor Q9 and time is directly proportional to the heat energy supplied to the X-ray tube.

The negative-going pulses from the voltage clamp are applied to the operational integrator 50 which consists of transistors Q10, Q11, Q12 and associated resistors, capacitors and diodes. The operation integrator is able to sense the voltage output from voltage clamp 40 because the negative voltage at the collector of transistor Q9 is coupled through diodes 41 and resistor 42 to the input terminal 60 of the operational integrator 50 and then to the gate G2 of FET Q10B through resistor 52. The voltage at G2 is inverted and amplified and appears at D2 of transistor Q10B. Here, the voltage is amplified by transistors Q11 and Q12 and appears as a positive-going voltage at the collector of transistor Q12-- the output of the integrator.

Capacitor 51 is connected between the output terminal 61 of the integrator at the collector of transistor Q12 and the input terminal 60 at one side of resistor 52. Since the operational amplifier portion of the integrator has a very high gain and very high impedance, all of the current flowing into terminal 60 tends to flow through capacitor 51 leaving input terminal 60 at virtual ground-- i.e., 0 volts. Consequently, the current flow out of capacitor 51 is determined by the voltage at the collector of transistor Q9 and resistor 42. The charge and therefore the voltage stored across capacitor 51 is directly proportional to the summation of the products of time and voltage across capacitor 51. Because one side of capacitor 51 is at virtual ground and the other side is connected to the collector of transistor Q12, the voltage across the capacitor 51 is taken at the collector of transistor Q12.

Diode 54 prevents capacitor 51 from charging in the reverse direction. When the negative kv. (and the other bias voltages) is first applied to the system, but before switch 8 of FIG. 1 is closed to start an exposure, the operational integrator will begin to develop a negative output voltage at output terminal 61. If such a situation were permitted to continue, the capacitor 51 would be charged to a couple of volts in the negative direction and the X-ray tube would get quite warm once actual exposures began before the capacitor 51 would return to 0 volts and begin its positive excursion. Therefore, as soon as a very small negative voltage appears on output terminal 61, diode 54 goes into conduction to shunt the negative voltage around capacitor 51 to ground through diodes 55. Furthermore, when the power is removed from the entire system, the kv. input falls off slower than the other voltages. Therefore, diodes 55 insure that capacitor 51 does not continue to charge after power is removed from the system.

Resistors 25.7 and 57 provide a discharge path for capacitor 51 when voltage at the collector of transistor Q9 decreases to ground potential. This gradual discharging of capacitor 51 through resistors 25.7 and 57 simulates the cooling of the X-ray tube. Due to the high gain of the operational amplifier, the current flow through resistor 25.7 is constant. As a result, capacitor 51 discharges through resistor 25.7 at a constant rate approximately following curve A of FIG. 5. The discharge path through resistor 25.7 can be traced from capacitor 51, line 53, capacitor 56, line 59, resistor 25.8, line 25, resistor 25.7 and back to capacitor 51. The discharge rate through resistor 57 follows an exponential rate as approximated by curve B of FIG. 5. The two discharge rates, A and B, taken together, more closely approximate the actual cooling rate of the X-ray tube than either rate could have done alone.

Voltage detector 70 consists of transistors Q13, Q14 and diode 71 and associated resistors and capacitors. When the collector of transistor Q12 reaches a predetermined voltage as determined by the X-ray tube reaching its maximum rated temperature, the voltage at the base of transistor Q13 causes it to turn on which also causes transistor Q14 to turn on. Energizing of transistor Q13 causes the DC trip circuit shown in FIG. 2 to be grounded through diode 71, line 72 and transistor Q13. The trip circuit 95 is connected to the voltage detector at terminal 99. With this latter circuit closed, a circuit is completed to a relay or other switching device (not shown) and a voltage source (not shown) to energize the switch which will prevent any further exposures from being made by opening the circuit which is in series with switch 8 of FIG. 1. Energization of transistor Q14 causes a positive voltage to appear across resistor 73 in the AC switch circuit 80 turning on traic Q15. Energizing traic Q15 closes an AC circuit (not shown) to light lamp 88 (FIG. 2) to provide a visual indication that the anode temperature has reached 100 percent of rated temperature. The lamp circuit is connected to the AC switch circuit by terminal 100.

Furthermore, when transistor Q14 is turned on, a positive voltage is supplied to the base of transistor Q13 through resistor 74. Therefore, in order for transistor Q13 to turn off, the collector voltage of transistor Q12 must fall to a lower level than that required to turn Q13 "on." In practice, this means that the charge on the capacitor 51 will have to decrease by 25 percent. That is, the circuit will not permit another exposure by releasing the DC trip until the X-ray tube has cooled down to a point where its temperature is not more than 75 percent of rated temperature.

Connected by resistor 91 and line 92 to the output terminal 61 of the integrator is a terminal 98. Terminal 98 provides an input to heat meter 90 (FIG. 2). By means of line 92 and terminal 98, a continuous indication can be had of the percentage of rated heat (0-100 percent) to which the tube is being subjected.

It will be clear, therefore, that this specification has disclosed a system which can monitor the temperature of the anode of an X-ray tube by providing an accurate measurement of the amount of energy delivered to the tube. It will also be clear that the apparatus described herein can be used to monitor the heat developed across any load device.

Claims

We claim:

1. Apparatus for providing a continuous approximation of heat developed in a load device, said heat being caused by current and voltage applied to said load, comprising first means including pulse generator means for providing a first signal related to the current through said load, said first signal comprising a plurality of pulses having a period inversely proportional to the amount of current flowing through said load, second means for controlling said first signal in response to the voltage which is applied to said load, means for sensing said controlled first signal for a predetermined period of time and for storing a second signal, said second signal being related to the value of said controlled first signal and the time over which it is available for storage, and means for decreasing said second signal at a predetermined rate.

2. The apparatus of claim 1, wherein said second means determines the amplitude of each of said plurality of pulses.

3. The apparatus of claim 2, wherein said sensing and storing means includes an operational integrator means for providing an output signal responsive to said plurality of pulses and further includes a capacitor connected across the output and input of said operational integrator means.

4. The apparatus of claim 3, wherein said decreasing means includes resistance means operatively connected to said operational integrator means and said capacitor.

5. The apparatus of claim 4, including means connected to said operational integrator means for enabling a continuous indication of the amount of heat remaining in said load service.