DE102009036940A1 - Aging condition determining method for cathode of x-ray tube of x-ray system, involves outputting value representing aging condition of cathode, where value is numbered around determined dependency that deviates from output dependency - Google Patents

Abstract

The method involves determining dependency of electric current flowing through carbon nanotube (CNT) cathodes (112-1 to 112-n) of an x-ray tube (110) on electric voltage applied at the cathodes. A value representing an aging condition of the cathodes is outputted, where the value is numbered around the determined dependency that deviates from an output dependency between the electric current and the electric voltage, and the output dependency is determined at a defined early time period. The number of electric sparkover in the x-ray tube is determined at the defined time period. An independent claim is also included for an x-ray system comprising a programmable controller for determining dependency of cathode current on cathode voltage.

Description

The The present invention relates to a method by which the state of aging a cathode of an x-ray tube is detected and monitored can be used to detect impending failures.

It is known in x-ray tubes the classic thermal cathodes through so-called carbon nanotubes, also called Carbon Nanotubes (CNT), to replace. Carbon nanotubes can be designed so that they electron by field emission and as a powerful electron emitter for flat and self-illuminating field emission displays or as well Cathodes serve in x-ray tubes.

Out the article "Stationary scanning x-ray source based on carbon nanotube field emitters ", published in 2005 in Applied Physics Letters 86, 184104 is a particularly interesting Design of an X-ray tube known. at These are arranged in a tube several CNT cathodes. A Such a multi-cathode tube allows a spatial Resolution, what with conventional single-cathode tubes only by mechanical displacement of the X-ray tube can be achieved.

Either in classical X-ray tubes as well as in X-ray tubes With CNT cathodes, the lifespan is mainly determined from the emissivity of the cathode. Especially with the mentioned field emission controlled cathodes occurs over time a deterioration of the electron-emitting material. exceeds the degree of deterioration a certain threshold, is the end of life reaches the tube, and it must be extremely short term be exchanged to the availability of the entire X-ray system to ensure.

It is therefore an object of the present invention, a method and to provide a system with which the state of aging of cathodes can be determined by X-ray tubes.

• – Ermitteln der Abhängigkeit eines durch die Kathode fließenden elektrischen Stroms von einer an der Kathode angelegten elektrischen Spannung; und
• – Ausgeben eines den Alterungszustand repräsentierenden Wertes, der um so größer ist, je stärker die ermittelte Abhängigkeit von einer zu einem definierten, früheren Zeitpunkt ermittelten Ausgangsabhängigkeit zwischen dem durch die Kathode fließenden elektrischen Strom und der an der Kathode angelegten elektrischen Spannung abweicht.

This object is achieved by a method for determining the state of aging of a cathode of an x-ray tube, comprising the following steps:

• Determining the dependence of an electrical current flowing through the cathode from an electrical voltage applied to the cathode; and
• - Outputting a value representing the aging state, the greater the greater the determined dependence on a determined at a defined, earlier time output dependence between the current flowing through the cathode electric current and the voltage applied to the cathode voltage.

In a development of the present invention may additionally the number of electrical flashovers in the x-ray tube be determined since the defined time, in which case the output value representing the state of aging the bigger, the bigger the number of electrical flashovers in the x-ray tube is.

to Determination of the number of electrical flashovers in the x-ray tube can be the tube internal pressure be measured.

Preferably the tube internal pressure is measured indirectly by a Pump current of a vacuum ion pump, with which the tube internal pressure is maintained, is measured.

In an advantageous embodiment of the invention is the temporal Course of the value representing the state of aging used to end of life of the x-ray tube predict. It can be a defined time before reaching the predicted end of life generates an alarm message and on sent a service center.

In a further development of the invention Method is the determined dependence of the cathode current from the cathode voltage in a nonvolatile memory filed and to determine the for a desired Cathode current necessary cathode voltage used.

object The invention further comprises an X-ray system comprising a X-ray tube and a controller, which at least a memory, a programmable controller and measuring means for a cathode current and a cathode voltage of a cathode of X-ray tube, wherein the programmable Controller the dependence of the cathode current of the Determined cathode voltage and representing the aging state The higher the value, the greater the stronger the determined dependence of one to one defined, previously determined output dependency differs between cathode current and cathode voltage.

in the Following are embodiments of the present invention Invention explained in more detail with reference to three figures. Show it:

1 a schematic representation of a portion of an X-ray system with an X-ray tube having a plurality of cathodes;

2 the sequence of a method for determining the aging state of the x-ray tube according to an embodiment of the invention; and

3 the sequence of a method for controlling the cathode current based on the by the method of 2 determined characteristic (s).

In 1 shows a schematic representation of an x-ray tube 110 with a variety n of CNT cathodes 1121 ... 112n in the evacuated area 111 , Each of the CNT cathodes 1121 ... 112n is through a separate cathode line 1131 ... 113n provided. Further, in the evacuated area 111 a grid 115 and an anode 116 arranged. Outside the evacuated area 111 There are other components of the X-ray system, in which the X-ray tube 110 is embedded: one with the grid 115 electrically conductive grid voltage supply 120 , one with the anode 116 electrically conductively connected anode voltage supply 130 , and cathode voltage actuators 1401 ... 140n , Typical grid voltages are 5 kV; typical anode voltages are between 20 kV and 180 kV.

Every CNT cathode 1121 ... 112n has a parasitic capacity 1191 ... 119n on, the state of charge also changes with a change in the anode voltage.

An important quantity for the precise control of the cathode current is the voltage U _GK between grids 115 and each active cathode 112 , This is done, for example, by subtraction in means 150 to determine U _GK determined. These are the means 150 both with all cathodes 112 as well as with the grid 115 connected. In addition, the funds received 150 the information which of the cathodes 112 is currently active or active (not shown).

The means 150 give the value of the voltage U _GK between grids 115 and each active cathode 112 to a voltage regulator 160 out. This receives from a central control 170 of the system as a second input value, the current desired value U _{S of} the voltage between the grid and the cathode. voltage regulators 160 compares the actual value U _GK with the desired value U _S and generates a correction signal K, which is applied to one of the respectively active cathode 112 associated actuator 140 is issued. The respective actuator 1401 ... 140n modifies the cathode voltage in response to the correction signal. With this modified value, the process starts again and continues continuously until the value of U _GK corresponds to the setpoint U _S.

The value of the voltage U _GK between grids 115 and each active cathode 112 will also be sent to the controller 170 delivered with the in 1 illustrated embodiment, this size is regulated in order to achieve an indirect control of the cathode current I _K. Further details of this scheme can the patent application with the file number DE 10 2009 017 649.7 taken from the same applicant.

1 also shows a rollover detector 180 to which the grid voltage U _{G is} supplied as the first input value. The rollover detector 180 receives as a second input value the measured value I _K for the cathode current, for example, by measuring voltage across a shunt 175 is determined. From the two input values U _G and I _K can be deduced whether a rollover has occurred. If so, this is sent to the central controller 170 signals which the flashovers per cathode or in total for the tube 110 counts.

In addition, funds can 190 be provided for measuring the tube internal pressure, which is the current internal pressure of the tube 110 to the controller 170 signal. After a rollover, the internal tube pressure increases abruptly. This may be in combination with or as an alternative to the rollover detector 180 can be used to calculate the flashovers per cathode or in total for the tube 110 to detect and count. The knowledge of the number of flashovers is of particular importance, since each flashover leads to particularly heavy wear of the respective cathode and thus to accelerated aging.

Further, provision may be made for the tube to be turned off when the internal tube pressure reading exceeds a critical value (eg, 10 ^-6 Torr, corresponding to 10 μA pump current of a vacuum ion pump, preferably used to maintain the vacuum). If the internal pressure was too high, further flashovers would occur which would further increase wear and aging. Only when the tube internal pressure falls below the critical value, the operation of the tube is allowed again.

2 shows an example of the in the controller 170 expiring procedure. In a first step 210 is for every cathode 112 a reference characteristic is determined, ie it is selected a defined time, to which for each cathode, the dependence between the by the cathode 112 flowing electric current I _K and the respective voltage actuator 140 at the cathode 112 applied electrical voltage is determined. In particular, the time of initial start-up of the tube, the time of completion of any calibration measures of the system or the time of installation of the system at the place of use are considered as defined times. The for each cathode 112 determined reference characteristic is not in a volatile memory of the controller 170 filed (step 220 ). steps 210 and 220 may be considered as the "initialization phase" of the process; they become for each x-ray tube 110 Usually run only once and then only when a new X-ray tube is installed in the X-ray system.

An "operating phase" of the process begins with step 230 , which at given times for the current characteristic each cathode 112 determined. Predetermined times may be, for example, once a day or once a week when the system is booted or before / after each use of the tube. This current characteristic curve can optionally also be stored in non-volatile memory in order to record the time course of the aging-related characteristic shift. On the one hand, this allows a more accurate lifetime prediction, on the other hand, along with other data, the history can provide important information to the tube manufacturer for improving the tube.

In one step 240 First, a characteristic displacement is determined from the reference characteristic curve and the current characteristic curve, ie the deviation of the current characteristic curve from the reference characteristic curve. This will be in step 240 calculates a value representing the aging state, which is the greater, the stronger the characteristic shift is. As the value representing the state of aging, for example, the difference between the cathode voltages necessary for the reference characteristic and the actual characteristic, respectively, can be selected to achieve a certain, typical cathode current. Other, in particular statistical, characteristics are of course also conceivable, for example the mean difference between the cathode voltages across all cathode current values.

Optionally be in step 240 to determine the value representing the state of aging, the number of flashovers used by the rollover detector 180 and / or the pressure measuring means 190 detected and from the controller 170 be counted. The consideration is such that the value representing the state of aging is increased with each rollover.

In step 250 Finally, it is evaluated whether the aging condition exceeds a critical value. If this is the case, the end of life of the tube 110 Close and an alarm message is generated and sent to a service center to ensure timely replacement of the tube before its failure.

The state of aging can be determined in the method according to 2 are first determined and processed per cathode, in which case a common value for the aging state of the tube is formed from these individual values. In the formation of the common value is considered in particular whether the failure of individual cathodes makes the tube already useless. In this case, ultimately, the cathode with the most advanced state of aging determines the overall aging state of the tube. On the other hand, if individual failures can be compensated by neighboring cathodes, this is considered accordingly in the calculation of the value representing the overall aging state.

As mentioned earlier, provided the controller 170 enough non-volatile memory is available, also a history of characteristics are stored. Then, instead of comparing the current characteristic curve with the reference characteristic curve, it is also possible to evaluate the time profile of the characteristic change, in order to better predict the end of life of the tube 110 to get.

In 3 Finally, it is shown how the detected characteristics can be used to control the cathode current I _{K of} a cathode. The prerequisite for this is that the respectively most recent characteristic curve in the memory of the controller 170 is present.

In one step 310 becomes the voltage regulator 140 the respective cathode 112 to be set cathode voltage setpoint from the characteristic using the desired cathode current and set. In step 320 the actual cathode current I _{K is} measured. Results in one step 330 Carried out comparison between the setpoint and the actual value of the cathode current is a difference exceeding the allowable tolerance, is in one step 340 Based on the difference, a new voltage setpoint is determined from the characteristic curve. In this case, the increase in the current characteristic in the range of the voltage setpoint (or a higher derivative) can be taken into account in order to arrive at the most accurate determination of the new voltage setpoint. In step 350 the new voltage setpoint is set and the procedure starts again with step 320 ,

Results in one step 330 Carried out comparison between the setpoint and the actual value of the cathode current is not exceeding the allowable tolerance difference, the process is terminated (not shown).

It should be noted that instead of cathode voltage command values applied directly to the voltage actuators 140 be directed (not shown) in the steps 310 and 340 also, as in 1 shown corresponding setpoints U _S for the Voltage between grid and cathode U _GK can be determined, which is then connected to the voltage regulator 160 be transmitted.

Of course, the measurement of the cathode current I _K not only by shunt 175 but done in any way, for example by means of current transformers or current sensors based on Hall element. The current measurement can also take place at any point in the cathode circuit.

control 170 preferably includes a programmable controller that can access internal and / or external memory (not shown). Furthermore, the controller 170 have a setpoint memory for radiation parameters (not shown).

The present invention has several advantages. On the one hand, tube status is monitored and a fairly reliable prediction can be made of slow-paced failures. This allows timely information to an operator or service center in good time before the system fails. Furthermore, by the improved cathode current control according to 3 an extension of the life can be achieved.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 102009017649 [0021]

Claims (11)

Method for determining the state of aging of a cathode ( 112 ) of an x-ray tube ( 110 ) comprising the steps of: determining the dependence of an electrical current flowing through the cathode on an electrical voltage applied to the cathode; and outputting a value representing the aging state, which value is the greater, the greater the determined dependence on a determined, at a defined, earlier output dependence between the current flowing through the cathode and the electric voltage applied to the cathode. Method according to Claim 1, in which additionally the number of electrical flashovers in the x-ray tube ( 110 ) is determined since the defined time and in which the output, the aging state representing value is greater, the greater the number of electrical flashovers in the X-ray tube. Method according to Claim 2, in which the number of electrical flashovers in the x-ray tube ( 110 ) the tube internal pressure is measured. The method of claim 3, wherein the tube internal pressure is indirectly measured by a pump current of a vacuum ion pump, with which the tube internal pressure is maintained, measured becomes. Method according to one of the preceding claims, in which the time profile of the value representing the state of aging is used to calculate a lifetime end of the x-ray tube ( 110 ) to predict. Method according to claim 5, wherein a defined Time before reaching the predicted end of life alarm message is generated and sent to a service center. Method according to one of the preceding claims, wherein the determined dependence of the through the cathode ( 112 ) stored electrical current from the voltage applied to the cathode voltage in a non-volatile memory and is used to determine the necessary cathode current for a desired cathode current. X-ray system comprising an X-ray tube ( 110 ) and a controller ( 170 ), which has at least one memory, a programmable controller and measured values of measuring means for a cathode current and a cathode voltage of a cathode of the x-ray tube ( 110 ), wherein the programmable controller determines the dependence of the cathode current on the cathode voltage and outputs a value representing the aging state, the greater, the stronger the determined dependence on a determined at a defined, earlier time output dependence between the cathode current and the cathode voltage differs. X-ray system according to claim 8, whose X-ray tube ( 110 ) has a vacuum ion pump, with which the tube internal pressure is maintained and the pump current is evaluated by the programmable controller to determine a number of electrical flashovers in the X-ray tube and output a value representing the aging state, the greater the number of electrical Flashovers in the x-ray tube is. X-ray system according to one of the preceding claims, whose programmable controller from the time course of the value representing the aging state, the end of life of the X-ray tube ( 110 ) predicts. X-ray system according to claim 10, whose programmable Controller a defined time before reaching the predicted Lifetime generates an alarm message and sent to a service center dispatches.

Images