EP1377137A2 - Circuit and procedure for generating a X-ray tube voltage - Google Patents

Abstract

The circuit (1) has a HF voltage stage (Gsi) coupled to a voltage converter (Gsu) for providing the HV for operation of the X-ray tube, with a voltage regulator (G RU) comparing the actual X-ray tube voltage (V U(t)) with a required X-ray tube voltage (W U(t)), for providing a first setting value (Y U(t)) for the HF voltage stage. A measuring circuit (7) determines the oscillation current (V I(t)) at the output of the HF voltage stage for comparison with a maximum value (W Imax), for providing a second setting value (Y I(t)) for the HF voltage stage. The setting values are compared for controlling a switching device (8) supplying the smaller setting value to the HF voltage stage. Also included are Independent claims for the following: (a) an X-ray generator; and (b) a method for generating an X-ray tube voltage.

Description

The invention relates to a circuit arrangement for generating an X-ray tube voltage with an inverter circuit for generating a high-frequency AC voltage, with a high-voltage generator for converting the high-frequency AC voltage into a high voltage for the X-ray tube and with a voltage regulating device which is based on the deviation of an actual X-ray tube voltage from a target voltage. X-ray tube voltage generates a first manipulated variable value for a manipulated variable for the inverter circuit in order to achieve an adaptation of the actual X-ray tube voltage to the desired X-ray tube voltage. Such a circuit arrangement is known from DE 29 43 816 C2.

Furthermore, the invention relates to an x-ray generator with such a circuit arrangement, an x-ray device with such an x-ray generator and a corresponding method for generating an x-ray tube voltage.

Modern X-ray generators often have circuit arrangements of the type mentioned at the outset for generating an X-ray tube voltage. Since the mains frequency is first rectified and then converted back into a high-frequency AC voltage, which is finally transformed to the desired voltage, such generators are also referred to as high-frequency generators. The voltage regulating device serves to regulate the high voltage on the x-ray tube to the diagnostically required value as optimally as possible and to maintain it there with the required accuracy. Compared to conventional generators, in which the high voltage transforms with the existing mains frequency, then rectified and finally Such a circuit arrangement has the advantage that it can in principle be made almost independent of changes in both the mains voltage and the tube current by means of a relatively fast control loop and therefore the tube voltage is very well reproducible and can be kept constant. Compared to the so-called direct voltage generators, which are also known and in which a high voltage transformed and rectified at the mains frequency is finely regulated with the aid of triodes, the high frequency generators have the advantage of a relatively small construction volume and lower production costs. These advantages are the reason for the preferred use of such circuit arrangements in today's X-ray generators.

In the case of the conventional circuit arrangements of the type mentioned at the outset, however, difficulties arise from the fact that the parameters of the control system consisting of the inverter and the high-voltage circuit encompass a large range of values depending on the selected operating point of the X-ray tube and that the inverter in particular is caused by resonance phenomena in the inverter - represents a highly non-linear control loop element. Furthermore, the oscillating current of the inverter must not exceed a predetermined maximum value in order to avoid damage to the power semiconductors. In the case of a conventional, single-track X-ray tube voltage control circuit, its control speed must therefore be set at least so slowly that the resonant circuit current does not exceed the maximum permissible value even when starting up. As a result, however, the small signal behavior of the control loop is inevitably slowed down, which results in slower correction of disturbance variables than would be possible in itself. In addition, the oscillating current is only indirectly limited in such a one-way control. Therefore, when the inverter is re-dimensioned, the control parameters of the control with regard to the oscillating current must also be adapted accordingly. An easy one Voltage control device can therefore only solve the requirements placed on them unsatisfactorily.

It is therefore an object of the present invention to provide an alternative to the known state of the art, which allows regulation at high speed without the maximum permissible oscillation current being exceeded.

This object is achieved by a circuit arrangement according to claim 1 and by a method according to claim 9.

According to the invention, the circuit arrangement additionally has a measuring circuit for measuring an oscillating current of the high-frequency alternating voltage present at an output of the inverter circuit. By means of an oscillating current control device, a second manipulated variable value for the specified manipulated variable for the inverter circuit is then generated on the basis of the deviation of a determined current actual oscillating current value from a predetermined oscillating current maximum value. The voltage regulating device and the oscillating current regulating device are then followed by a switching device which compares the first manipulated variable value and the second manipulated variable value and only forwards the respectively smaller manipulated variable value as the resulting manipulated variable value to the inverter circuit.

The method according to the invention, using a vibration current control device, to separately determine a second manipulated variable value on the basis of the deviations of an actual vibration current value from a predetermined maximum oscillation current value and to compare it with the first manipulated variable value of the voltage control device and to supply only the respectively smaller manipulated variable value to the inverter circuit, that, in the normal case, the voltage regulating device regulates very quickly, which is only in the limit cases when a critical range with regard to the oscillating current is reached, is replaced by the oscillating current control device. This means that with this _" release control", the manipulated variable of the voltage control device is passed on to the controlled system as long as the voltage control device operates "normally" and only _" loads" an oscillation current that is smaller than the maximum permissible oscillation current. Only if the maximum permissible oscillation current was reached or exceeded, which z. B. will usually be the case when starting up, the oscillation current control device intervenes and limits the oscillation current to its maximum permissible value.

The dependent claims each contain particularly advantageous refinements and developments of the invention.

Preferably at least one PI controller (proportional / integral controller) is used for at least one of the two control devices, particularly preferably for both control devices. The integral part of the controller in question has the task of determining the stationary control error. H. to force the control error in the steady state to zero. This reliably avoids a permanent control deviation. The control devices preferably consist of successive proportional parts and integral parts. The advantage over a parallel PI controller structure is that the controller parameters regarding gain and reset time can be set separately. A PID controller can also be used instead of a PI controller.

In a particularly preferred exemplary embodiment, the output of the switching device is connected to an input of the voltage control device and / or the oscillation current control device in order to return the resulting manipulated variable value. The voltage regulating device and / or the oscillating current regulating device are designed in such a way that they are carried along with the resulting manipulated variable value when the one generated by the relevant regulating device Manipulated variable value itself is not passed on as the resulting manipulated variable value. For this purpose, the respective control device compares the resulting manipulated variable with its own manipulated variable value, which is also traced internally. This variant prevents additional settling processes due to jumps when switching between the two control devices.

The switching device is preferably designed such that it forwards at least one predetermined minimum manipulated variable value to the inverter circuit as the resulting manipulated variable value. In addition, a maximum of a predetermined manipulated variable maximum value is preferably also forwarded to the inverter circuit as the resulting manipulated variable value. As a result, the resulting manipulated variable is actively limited to a range between the minimum value and the maximum value.

Since the controller parameters, i.e. H. the controller gain and the reset time, which are generally dependent on the working point, the voltage control device and / or the oscillating current control device preferably each have means for depending on a set x-ray tube voltage and / or depending on a set x-ray tube current at least one parameter (= controller parameter) of the relevant one Control device to vary. This means that a value for the set x-ray tube voltage and preferably also for the set x-ray tube current is given to corresponding inputs of the respective control device, as a result of which the parameters of the relevant control devices are set appropriately internally.

Such a circuit arrangement according to the invention for generating an X-ray tube voltage can in principle be used in any conventional X-ray generator, regardless of how the X-ray generator relates to its other components, such as the various measuring devices or the heating power supply is set up. Likewise, the invention can be used largely independently of the specific design of the inverter circuit and the high-voltage generator.

Figur 1a

eine Prinzipskizze einer Schaltungsanordnung nach dem Stand der Technik mit einer Wechselrichterschaltung und einem dahinter geschalteten Hochspannungserzeuger zu Erzeugung einer Hochspannung für eine Röntgenröhre,

Figur 1b

eine Modelldarstellung eines Regelkreises für eine Schaltungsanordnung nach dem Stand der Technik gemäß Figur 1a,

Figur 2

eine Modelldarstellung des Regelkreises einer erfindungsgemäßen Schaltungsanordnung, und

Figur 3

eine detailliertere Modelldarstellung des Regelkreises einer besonders vorteilhaften Variante der erfindungsgemäßen Schaltungsanordnung.

The invention is explained in more detail below with reference to the attached figures using exemplary embodiments. Further advantages, features and details of the invention result from the examples described and the drawings. Show it:

Figure 1a

1 shows a schematic diagram of a circuit arrangement according to the prior art with an inverter circuit and a high-voltage generator connected behind it for generating a high voltage for an X-ray tube,

Figure 1b

1 shows a model representation of a control circuit for a circuit arrangement according to the prior art according to FIG. 1a,

Figure 2

a model representation of the control loop of a circuit arrangement according to the invention, and

Figure 3

a more detailed model representation of the control loop of a particularly advantageous variant of the circuit arrangement according to the invention.

The typical components of an X-ray generator are shown in FIG. 1a, which represent the controlled system with respect to the regulation of the X-ray tube voltage U _Rö . These initially include a resonant circuit inverter G _si, a high voltage generator G _su and an X-ray tube 6.

The inverter circuit G _si has a plurality of power semiconductors 3, which are switched accordingly in such a way that a DC link voltage V _z becomes a high-frequency voltage is converted. The inverter circuit G _si also has a voltage frequency converter 2, which converts a voltage value Y (t) into a drive frequency f _a with which the power semiconductors 3 of the inverter G _si are driven. The input voltage thus forms the manipulated variable Y (t) of the controlled system.
The G _si inverter circuit is a resonant circuit inverter (inverter). However, other inverter circuits, for example a rectangular inverter or any series or multi-resonance inverter, can also be used.

The high voltage generator G _su consists on the one hand of a transformer 4 with a transmission factor ü and a rectifying and smoothing device 5 connected downstream of the transformer. The x-ray tube voltage U _Rö present at the output of the rectifying and smoothing device 5 is supplied to the x-ray tube 6.

Figure 1b shows a structure diagram for a control loop according to the prior art. The inverter circuit G _si is shown here as a block and can be described in the control-technical sense by the proportional transmission factor K _si and a time constant T _si , whereby in particular the proportional transmission factor K _si is strongly non-linear due to resonance phenomena in the inverter G _si , i.e. from the operating point of the inverter G _si depends.

The high voltage generator G _su is also shown as a block. It can be described by the proportional transmission factor K _su and the time constant T _su , both variables being directly dependent on the X-ray tube voltage U _Rö and the X-ray tube current I _Rö , ie encompassing a large range of values depending on the _{operating point} . i _sw (t) is the oscillating current of the inverter G _si , which supplies the primary winding of the high-voltage transformer 4 of the high-voltage generator G _su . To damage the power semiconductor 3 in of the inverter circuit G _si , the oscillating current i _sw (t) must not exceed a maximum value.

According to the prior art, in order to regulate the output voltage at the high-voltage generator G _su, the actual voltage V _u (t) present there at a specific time t is compared with a desired value W _U (t) which corresponds to the desired X-ray tube voltage U _Rö , ie the Difference is fed to a voltage control G _RU , which is also shown here in the form of a block.

This voltage control device G _RU is conventionally a simple PI controller which, depending on the deviation of the actual value V _U (t) from the setpoint W _U (t), generates the manipulated variable Y (t), which then responds to the input of the Voltage frequency converter 2 of the inverter circuit G _{si is} given.

In such a conventional control circuit according to FIG. 1b, the control speed of the voltage control device G _RU must be set so slowly that the oscillating current i _sw (t) does not exceed the maximum permissible value even when starting up. This means that rapid regulation with the G _RU voltage regulator is not possible, and thus malfunctions can only be regulated slowly. When the inverter circuit G _{si is} re-dimensioned, the regulator parameters of the voltage regulator G _RU must also be adapted accordingly, since the oscillating current i _sw (t) is only limited indirectly here.

In comparison to FIG. 1b, FIG. 2 clearly shows the change in the structure of the control loop according to the invention. With this detachment control system, according to the invention, a switch is made between two control circuit structures which are in principle guided in parallel.

As in the prior art according to FIG. 1b, the X-ray tube voltage regulating _device G _{RU also forms} from the difference between the desired x-ray tube voltage, ie the target voltage W _U (t), and the actual x-ray tube voltage, ie the actual x-ray tube voltage V _U (t), a manipulated variable Y _U (t).

In addition, however, the oscillating current i _sw (t) is measured by means of a smoothing element 7. This smoothing element 7 is described in terms of control technology by the additional time constant T _MI . The actual vibration _current value V _I (t) determined in this way is compared with a maximum permissible vibration _current value W _{I_max} (= setpoint), ie the difference between these values is formed and fed to a further control device, the vibration _current control device G _RI , which also has a manipulated variable value Y _I (t) for the manipulated variable for the inverter circuit G _si .

Both the first manipulated variable value Y _U (t), which is formed by the voltage control device G _RU , and the second manipulated variable value Y _I (t), which is formed by the oscillating current control device G _RI , are fed to a switching device 8. This switching device 8 selects between the two manipulated variable values Y _U (t) and Y _I (t) that manipulated variable value Y _U (t), Y _I (t) that is smaller at the current time t, and routes this manipulated variable value Y _U (t ), Y _I (t) as the resulting manipulated variable value Y (t) to the inverter circuit G _si .

Both control devices G _RI , G _RU each contain a PI controller. A permanent control deviation is avoided by the integral part of the PI controller.

This separation control according to FIG. 2 has the advantage that, in the _" normal case", the voltage regulating device G _{RU is} responsible for regulating the x-ray tube voltage. Only in cases in which the current manipulated variable value Y _U (t) generated by the voltage regulating _device G _RU would lead to the oscillating current i _sw (t) being a permitted maximum value would exceed, the current manipulated variable value Y _I (t) generated by the oscillating current control device G _RI is smaller than the manipulated variable value Y _U (t) generated by the voltage control device G _RU . Therefore, in these cases the voltage regulating _device G _RU is effectively overridden and only the oscillating current regulating device G _RI acts. This has the advantage that the voltage regulating device G _RU can be set considerably faster than in a control circuit according to the prior art and accordingly disturbance variables can be corrected quickly. The detachment in extreme cases nevertheless reliably prevents the oscillating current i _sw (t) from exceeding the permissible maximum value.

The x-ray tube voltage control itself is not normally slowed down by the measuring time constant T _{MI of} the oscillating current i _sw (t) in the structure according to the invention, since the smoothing element 7 is not in the control loop of the x-ray tube voltage.

Since the parameters of the two partial control systems each depend on the current working point of the X-ray tube 6, the dimensioning of the two control devices G _RU , G _{RI can be made} considerably easier if their parameters, ie the controller gains and the reset times, are controlled depending on the working point. For this purpose, as is shown schematically in FIG. 2, the two control devices G _RI , G _{RU are} each supplied with the values of the set X-ray tube voltage U _Rö and the set X-ray tube current I _Rö .

FIG. 3 shows a more detailed structural diagram of the control loop according to FIG. 2, the control loops here having additional, particularly advantageous features.

An additional feature is that here the switching device 8 has further inputs via which the switching device 8 is given a _maximum manipulated variable value Y _max and a _minimum manipulated variable value Y _min . The switching device 8 is constructed in such a way that at least the manipulated variable _minimum value Y _min and at most the manipulated variable maximum value Y _{max are} output. This means that a manipulated variable range is specified dynamically, within which the manipulated variable Y (t) currently passed on to the inverter circuit G _si moves. The manipulated variable _maximum value Y _max and the manipulated variable _minimum value Y _min are usually set at the factory. In this respect, they can already be specified by appropriate design of the switching device 8 itself.

FIG. 3 also shows a more precise structure of the voltage regulating _device G _RU and the oscillating current regulating device G _RI . These are PI controllers with a proportional component 12, 15 and an integral component 13, 14 connected behind them. In terms of control technology, the proportional components 12, 15 are again determined by the transmission factors K _PRI and K _PRU and the integral components 13, 14 by the Time constants T _NI or T _NU .

This structure shown in FIG. 3, with proportional components 12, 15 and integral components 13, 14 connected in series, has the advantage over a parallel PI controller structure that the controller gains K _PRI , K _PRU and the reset times T _NI , T _NU are set separately from each other can.

As a further feature, the resulting manipulated variable value Y (t) is fed back in this exemplary embodiment by connecting the output 9 of the switching device 8 to additional inputs 10, 11 of the voltage control device G _RU or the oscillating current control device G _RI . Internally, the respective manipulated variable value Y _U (t), Y _I (t) generated by the control device G _RU , G _RI is also fed back before the integral component 13, 14 and the difference between the feedback, resulting manipulated variable value Y (t) and the respective own manipulated variable value Y _U (t), Y _I (t) are formed.

This means that both control devices G _RU , G _RI each have limit monitors which are coupled such that the integral part 13, 14 of the respectively inactive control device G _RU , G _RI from the integral part 13, 14 of the active control device - ie the control device G _RU , G _RI , whose manipulated variable value Y _U (t), Y _I (t) forms the resulting manipulated variable value Y (t) - is carried along. In this way, malfunctions when switching between the control devices G _RU, G _{RI are} avoided. Otherwise there would be a risk that the control devices G _RU , G _{RI would run} into the stop, which would result in the integral parts 13, 14 being overloaded. This would in turn lead to a deterioration in the transient response when switching over (wind-up effect).

It is pointed out once again that the circuit arrangements shown in the figures are only exemplary embodiments and that there are numerous possible variations for the person skilled in the art to implement a circuit arrangement according to the invention. So z. B. Adaptive control of the voltage regulator is also carried out in such a way that the reset time is set as a function of the actual tube voltage value in the course of the tube voltage.

Claims (14)

Circuit arrangement (1) for generating an X-ray tube voltage
with an inverter circuit (G _si ) for generating a high-frequency AC voltage,
with a high-voltage generator (G _su ) for converting the high-frequency AC voltage into a high voltage for the X-ray tube (6)
and with a voltage control device (G _RU ) which, based on the deviation of an actual x-ray tube voltage (V _U (t)) from a desired x-ray tube voltage (W _U (t)), provides a first manipulated variable value (Y _U (t)) for a manipulated variable generated for the inverter circuit (G _si ) to achieve an adaptation of the actual x-ray tube voltage (V _U (t)) to the desired x-ray tube voltage (W _U (t)),
marked by
a measuring circuit (7) for measuring an oscillating current (i _sw (t)) of the high-frequency alternating voltage present at an output of the inverter circuit (G _si ),
a vibration _{current control device} (G _RI ) to generate a second manipulated variable value (Y _I (t)) for said manipulated variable on the basis of the deviation of an actual vibration _current value (V _I (t)) determined from a predetermined maximum vibration _current value (W _{I_max} ) .
and a switching device (8) connected downstream of the voltage control device (G _RU ) and the oscillating current control device (G _RI ), which is designed such that it compares the first manipulated variable value (Y _U (t)) and the second manipulated variable value (Y _I (t)) and only forwards the respectively smaller manipulated variable value (Y _U (t), Y _I (t)) as the resulting manipulated variable value (Y (t)) to the inverter circuit (G _si ). Circuit arrangement according to claim 1, characterized in that the voltage control device (G _RU ) and / or the oscillating current control device (G _RI ) comprise a PI controller. Circuit arrangement according to claim 1 or 2, characterized in that an output (9) of the switching device (8) for feedback of the resulting manipulated variable value (Y (t)) with an input (10, 11) of the voltage regulating device (G _RU ) and / or Vibration current control device (G _RI ) is connected and that the voltage control device (G _RU ) and / or the vibration current control device (G _RI ) are designed such that they are carried along with the resulting manipulated variable value (Y (t)) when the one generated by the relevant control device Command value (Y _U (t), Y _I (t)) is not passed on as the resulting command value (Y (t)). Circuit arrangement according to one of Claims 1 to 3, characterized in that the switching device (8) is designed in such a way that it forwards at least one predetermined minimum value (Y _min ) as the resulting value (Y (t)) to the inverter circuit (G _si ). Circuit arrangement according to one of claims 1 to 4, characterized in that the switching device (8) is designed such that it forwards a maximum of a predetermined maximum value (Y _max ) as the resulting value (Y (t)) to the inverter circuit (G _si ). Circuit arrangement according to one of claims 1 to 5, characterized in that the voltage regulating device (G _RU ) and / or the oscillating current regulating device (G _RI ) each have means for depending on a set X-ray tube voltage (U _Rö ) and / or depending on one set X-ray tube current (I _Rö ) to vary at least one parameter of the relevant control device (G _RU , G _RI ). X-ray generator with a circuit arrangement (1) according to one of Claims 1 to 6. X-ray device with an X-ray generator according to claim 7. Method for generating an X-ray tube voltage,
in which a high-frequency AC voltage is first generated by means of an inverter circuit (G _si ), which is then converted into a high voltage for the X-ray tube (6),
wherein on the basis of the deviation of an actual voltage value (V _U (t)) from a target voltage value (W _U (t)) by means of a voltage control device (G _RU ) a first manipulated variable value (Y _U (t)) for a manipulated variable for the Inverter circuit (G _si ) for adapting the actual voltage value (V _U (t)) to the target voltage value (W _U (t)) is generated,
characterized in that
on the basis of the deviation of an actual oscillating current value (V _I (t)) of the high-frequency alternating voltage determined at an output of the inverter circuit (G _si ) from a predetermined maximum oscillating current value (W _{I_max} ) by means of an oscillating _{current control device} (G _SI ), a second manipulated variable value (Y _I (t)) is generated for the specified manipulated variable,
and then the first manipulated variable value (Y _U (t)) and the second manipulated variable value (Y _I (t)) are compared and the respectively smaller manipulated variable value (Y _U (t), Y _I (t)) as the resulting manipulated variable value (Y (t )) of the inverter circuit (G _si ) is supplied. A method according to claim 9, characterized in that a PI controller is used as the voltage control device (G _RU ) and / or as the oscillation current control device (G _SI ). The method of claim 9 or 10, characterized in that the resulting control variable value (Y (t)) is fed back as an input to the voltage control means (G _RU) and / or the resonant current control means (G _RI) and the regulating device in question (G _RU, G _RI ) with the resulting manipulated variable value (Y (t)), if that of the generated manipulated variable value (Y _u (t), Y _I (t)) is not supplied to the inverter circuit (G _SI ) as the resulting manipulated variable value (Y (t)). Method according to one of claims 9 to 11, characterized in that at least one predetermined manipulated variable _minimum value (Y _min ) is fed to the inverter circuit (G _si ) as the resulting manipulated variable value (Y (t)). Method according to one of claims 9 to 12, characterized in that a maximum of a predetermined manipulated variable maximum value (Y _max ) is fed to the inverter circuit (G _si ) as the resulting manipulated variable value (Y (t)). Method according to one of claims 9 to 13, characterized in that at least one parameter of the voltage control device (G _RU ) and / or the oscillating current control device (G _RI ) as a function of a set x-ray tube voltage (U _Rö ) and / or as a function of a set x-ray tube current (I _Rö ) is varied.

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