DE102013206945B3 - Method and device for detecting a faulty tube current detection in an X-ray source - Google Patents

Abstract

The invention provides a method and associated apparatus for detecting a faulty change or failure of a tube current measurement in a high voltage path (2, 3, 6) of an X-ray source (2) electrically connected to an X-ray generator (1), wherein the X-ray source (2) the erroneous change or failure of the tube current measurement after a high-voltage pulse end or after switching off the high-voltage supply is determined from a determined first voltage drop rate (S1) at the X-ray source ( 2) applied high voltage (u (t)), wherein by a deviation of the first voltage drop rate (S1) to a predetermined second voltage drop rate (S2), the faulty change or failure is displayed. The invention offers the advantage of reliably and rapidly detecting the erroneous change or failure of a tube current measurement without changing the hardware.

Description

Field of the invention

The invention relates to a method and a device for detecting a faulty change or failure of a tube current measurement in a high voltage path of an X-ray source electrically connected to an X-ray generator, wherein the X-ray source is supplied with a high voltage generated by the X-ray generator. The invention also relates to the determination of the capacitance of the high-voltage path.

Background of the invention

Typically, in X-ray generators for X-ray sources, control of the tube current takes place to obtain the desired radiation dose for X-ray imaging. Usually, the tube current is detected in the high-voltage circuit, with the voltage drop at mass-near measuring resistors ("shunt") usually being measured for this purpose. In order to properly dissipate strong current pulses and transients that may occur during power surges of the X-ray source, such as during strike-through or "arcing", the components required for the tube current measurement are typically combined with a combination of surge arresters (TVS and / or Transzorb diodes ), Bedämpfungskapazitäten and subsequent filter circuits protected.

If the overvoltage protection elements (eg transzorb diodes become low impedance in the case of a failure), the filter circuit (breakdown of the filter capacitors) or the actual tube current detection (measuring resistors can change their value, for example due to overload), the X-ray generator, if this fails Failure to detect failure can adjust to a wrong tube current. In the individual case, such a malfunction may also occur in the case of external wiring errors of an X-ray system (eg bridging one of the measuring sockets).

In the case of dose-controlled, imaging X-ray systems of medical technology, this misconduct does not pose a risk to the patient, since the existing dose control / deactivation intervenes, but the X-ray emitter can be damaged by the unacceptably high power.

• – Eine Plausibilitätsprüfung der Röhrenstrommessschaltung anhand der Ermittelung der Leistungsabgabe des Wechselrichters in den Resonanzschwingkreis/Hochspannungserzeuger. Die Genauigkeit dieses Verfahrens liegt aber erst oberhalb 100 mA Röhrenstrom in einem brauchbaren Bereich (Erkennung ab 50% Abweichung) und ist für eine gute Abweichungs- oder Fehlererkennung unzureichend. Eine so erkannte Abweichung wird bei bekannten Systemen als „Warning” in das Logfile eingetragen und führt, bedingt durch die Detektionsunsicherheit, zu keiner Abschaltung.
• – Eine Kontrollmessung mit einem externen Strommessgerät oder einem mAs-Messgerät (Strom-Zeitintegral) in Serie zur generatoreigenen Röhrenstromerfassung während der Fertigungsendprüfung des Generators nach einem ersten Burn-In-Test. Dadurch können Fehler an der Röhrenstromerfassung bei Auslieferung des Produkts ausgeschlossen werden. Fehler und unzulässige Veränderungen an der Strommesserfassung können danach nicht mehr (automatisiert) erkannt werden.
• – Bei Vorhandensein einer doppelt ausgeführten Messerfassung kann durch die redundant ausgeführte Wandlung der einzelnen Messerfassungsschaltungen auf Plausibilität geprüft werden. Dies erfordert allerdings einen wesentlich erhöhten Hardware- und Firmwareaufwand. Symmetrische Fehler in der Erfassung können aber nicht erkannt werden.

Well-known possibilities for detecting a faulty behavior of the tube current detection are, for example:

• - A plausibility check of the tube current measuring circuit based on the determination of the power output of the inverter in the resonant circuit / high voltage generator. However, the accuracy of this method is only above 100 mA tube current in a usable range (detection from 50% deviation) and is insufficient for a good deviation or error detection. Such a recognized deviation is entered in known systems as "Warning" in the log file and leads, due to the detection uncertainty, no shutdown.
• - A control measurement with an external current measuring device or a mAs meter (current-time integral) in series with the generator's own tube current detection during the final production test of the generator after a first burn-in test. As a result, errors in the tube current detection at the time of delivery of the product can be excluded. Errors and impermissible changes to the current measurement can not be detected (automatically).
• - In the presence of a duplicate measuring socket can be checked by the redundant conversion of the individual measuring circuits for plausibility. However, this requires a significantly increased hardware and firmware costs. However, symmetrical errors in the detection can not be detected.

As described above, control of the tube current takes place in X-ray generators so as to obtain the desired radiation dose required for X-ray imaging. The tube current is determined in the high-voltage circuit, which is realized by the mass measuring resistors. Generically, the high voltage is discharged in the pulse pauses in the case of primarily pulsed X-ray sources and is built up only for the desired X-ray pulse.

In general, the X-ray source of the X-ray generator with the high voltage generator is spatially separated and connected via high voltage cable with this. The capacitive high-voltage cable must be charged at the beginning of the pulse, which causes a considerable charging current for a length of up to 30 meters. The charging current is measured on the X-ray generator side and first "flows" into the high voltage cables, but at the moment of the voltage build-up does not yet give a dose.

However, since the charging current is still detected by the tube current measuring circuit, the capacitive component of the tube current must be subtracted for correct tube current detection. For such a correction of the tube current measurement, the exact knowledge of all capacitive components of the high-voltage path, ie useful and parasitic capacitances, is necessary.

In addition, for an optimal high-voltage control circuit, the total capacity for the control speed and control quality is crucial. Therefore, the overall capacity of many applications is taken into account by the appropriate choice of controller parameters.

1 shows a simplified block diagram corresponding to the above embodiments of the high-voltage path of an X-ray generator 1 and an X-ray source 2 , The X-ray source 2 includes an anode 4 and an emitter 5 , which is supplied with a heating current i _h . By applying a high voltage u (t) and at a correspondingly high heating current i _h , electrons are emitted from the emitter 5 hurled on the anode 4 impinge and generate an X-ray radiation when decelerating.

The X-ray generator 1 includes a controller 7 with an inverter whose output voltage is a high voltage generator 6 provided. The high voltage u (t) is generated by high-voltage cables up to 30 m long 3 the X-ray source 2 fed. The high voltage cables 3 have parasitic capacitances, the high voltage pulses as well as the capacities in the high voltage generator 6 be charged and discharged in the pauses in the pulse. The high voltage generator 5 , the high voltage cable 3 and the X-ray source 2 together form a high voltage path.

From practice it is known to determine the total capacity of the high voltage path with the aid of a measuring device. Alternatively, a lumped capacity is estimated that takes into account only a small portion of all possible capacitances and is attracted to the tube current correction in addition to controller parameterization purposes. Such correction is insufficient for short x-ray pulses, such as required in angiography, because the tube current measured at the shunt is determined more by transient cable charge currents than by the actual tube current.

Furthermore, the capacity can be disregarded, resulting in significant losses in the high-voltage regulation behavior and in the system performance. This is sufficient for simple applications.

The patent DE 722 867 A discloses a circuit arrangement for accurately switching in advance determinable quantities of electricity for electrical apparatus, in particular X-ray apparatus, with a switch-off delay.

The publication DE 10 2009 051 633 A1 discloses a circuit arrangement for grid-controlled x-ray tubes, wherein a correction voltage is generated in accordance with a correction function. This correction voltage is used to correct a voltage applied between the anode and the cathode voltage in the sense of a constant voltage in the period of control by means of the control electrode. The generated correction voltage corrects the voltage applied between the anode and the cathode.

Summary of the invention

It is an object of the invention to provide a method and a device that accurately and reliably detect a fault in the tube current detection. A further object of the invention is to specify a method and a device which permits the most accurate determination possible of the capacitance of the high voltage path of an X-ray source electrically connected to an X-ray generator.

According to the invention, the stated object is achieved with the method and the device of the independent claims. Advantageous developments are specified in the dependent claims.

The basic idea of the invention is to conclude from the voltage curve of the high voltage applied to an X-ray source after a high-voltage pulse end or after switching off the high-voltage supply by a comparison with a known voltage curve to an erroneous change or failure of a tube current measurement, wherein the voltage profile through capacitances and the currently flowing tube and Messteilerstrom is determined. Another idea of the invention is immediately after an X-ray pulse end, i. H. in a pulse pause to determine the capacitance of the high-voltage path from a measured voltage curve of the high voltage and a measured tube current.

The invention claims a method for detecting a faulty change or failure of a tube current measurement in a high voltage path of an X-ray source electrically connected to an X-ray generator, wherein the X-ray source is supplied with a high voltage generated by the X-ray generator, and wherein the erroneous change or failure of the tube current measurement after a high voltage pulse end or after switching off a high voltage supply is determined from a determined first voltage drop rate of the high voltage applied to the x-ray source, wherein the erroneous change or failure is indicated by a deviation of the first voltage drop rate to a predetermined second voltage drop rate becomes. The invention offers the advantage of reliably and rapidly detecting the erroneous change or failure of a tube current measurement without changing the hardware.

In a development of the method, the second voltage drop rate can be calculated from the capacitance of the high-voltage path and a tube measurement current, wherein the tube measurement current is formed by the sum of a tube current flowing through the X-ray source and a measuring divider current.

In a further embodiment, the second voltage drop rate may be formed from the quotient of the tube measurement current and the capacitance of the high voltage path.

In a further embodiment, the course of the tube current for the formation of the second voltage drop rate may be assumed to be approximately linearly constant.

Preferably, the first voltage drop rate indicates the time change of the high voltage at a particular tube current after a high voltage pulse.

For example, the first voltage drop rate may be determined in a range between 87.5% and 75% of the high voltage drop.

In another embodiment, the capacitance may be determined from the first voltage drop rate of the high voltage applied to the x-ray tube after a high voltage pulse end of a high voltage test pulse.

In addition, the capacitance may be formed from the tube measurement current and the first voltage drop rate, wherein the high voltage may be selected so that no tube current flows after the high voltage pulse end. This makes the capacitance determination independent of the tube current detection since only the known and high precision sense divider current flows.

The invention also claims a method for determining the tube current at a known capacity of a high-voltage path of an X-ray source electrically connected to an X-ray generator, wherein the X-ray source is supplied with a high voltage generated by the X-ray generator. The tube current is determined from a determined first voltage drop rate of the high voltage applied to the X-ray source after a pulse end of the high voltage or after switching off a high voltage supply and the capacitance. As a result, the tube current can be determined even if the tube current measurement is faulty.

The invention also claims an apparatus for detecting a faulty change or failure of a tube current measuring unit in a high voltage path of an X-ray source electrically connected to an X-ray generator, wherein the X-ray source is supplied with a high voltage generated by the X-ray generator. The apparatus includes an electrical circuit configured to determine the erroneous change or failure of the tube current measuring unit from a measured first voltage drop rate of the high voltage applied to the x-ray source after a high voltage pulse end or after switching off a high voltage supply, wherein a deviation of the first voltage drop rate to one predetermined (expected due to the measured tube current) second voltage drop rate indicating the faulty change or failure.

In a development of the device, the electrical circuit may be a digital signal processor, a microprocessor, an FPGA or a combination of such circuits.

Preferably, the electrical circuit can be designed to carry out a method according to the invention.

Other features and advantages of the invention will become apparent from the following explanations of an embodiment with reference to schematic drawings.

Show it:

1 FIG. 2 is a block diagram of the high voltage path of an X-ray generator and an X-ray source according to the prior art;

2 : A block diagram of the tube current measurement and the tube voltage measurement in the high voltage path of an X-ray generator and

3 : a diagram of the time course of tube current and high voltage of an X-ray source.

Detailed description of an embodiment

2 shows a simplified block diagram of the tube current measurement and the tube voltage measurement in the high voltage path of an X-ray source 2 with a high voltage u (t) supplying X-ray generator. From the X-ray generator are just the two High-voltage transformers 8th and the rectifier units 15 of the high voltage generator shown. The high voltage transformers 8th transform the AC voltage coming from an inverter up to 150 kV, using the two rectifier units 15 to be rectified. This rectified high voltage u (t) becomes the X-ray source 2 supplied to the X-ray generation. This will be the positive potential of the anode 4 and the negative potential of the emitter 5 of the X-ray source 2 fed. For damping, a damping resistor R4 is located in the tube circuit.

To be in operation by the X-ray source 2 to determine flowing tube current i _r (t) is a tube current measuring unit 9 provided, which consists essentially of the two measuring resistors R3 and the second operational amplifier 13 consists. The tube measuring current i (t), which flows through the measuring resistors R3, generates a voltage drop in the measuring resistors R3, which is a measure of the tube measuring current i (t). This voltage drop is with the second operational amplifier 13 measured and the reading is an electrical circuit 14 supplied for evaluation according to the invention. The measuring resistor R3, also referred to as "shunt", typically has 5 ohms. The electrical circuit 14 is for example a digital signal processor, a microprocessor or an FPGA.

The at the X-ray source 2 high voltage u (t) is applied by means of a high-voltage measuring unit 10 measured. The high voltage measuring unit 10 essentially comprises a measuring divider 11 with the two series-connected measuring resistors R1 and the two second measuring dividing resistors R2, as well as a first operational amplifier 12 which measures the voltage drop across the second sense resistor R2. The measured value is applied to the electrical circuit 14 transfer.

The first measuring dividing resistors R1 typically have a value of 200 MΩ, the two second measuring dividing resistors R2 a value of 20 kΩ. Due to the finite resistance of the measuring divider 11 flows through the measuring divider a non-negligible measuring divider current i _m (t), which is about 0.312 mA at a high voltage u (t) of 125 kV. The measuring divider current i _m (t) is superimposed in the tube current measuring unit 9 with the tube current i _r (t) to the tube measuring current i (t).

The high voltage generator builds the high voltage u (t) at the X-ray source at a radiation request 2 while charging the high voltage cables 3 and the internal high voltage capacity of the high voltage generator. In addition, via a current flow, not shown, at the emitter 5 of the X-ray source 2 set the required emission temperature. This temperature determines the tube current i _r (t) between the anode and the cathode of the X-ray source 2 and thus the X-ray dose rate. As a result, at a constant high voltage u (t) (= x-ray tube voltage), the dose rate can be set or regulated directly via the tube current i _r (t).

If the radiation request to the high voltage generator is removed, an inverter, not shown, and thus the high voltage transformers 8th no more energy in the high voltage cable 3 and the x-ray tube 2 deliver. Because, however, the emitter 5 is still at emission temperature, the high voltage generator internal capacities and the cable capacity are significantly discharged by the tube current i _r (t).

Will now be at a known in operation capacity C, formed from the capacities of the high voltage cable 3 , the high voltage generator and the X-ray source 2 , determines the first voltage drop rate S1 of the high voltage u (t) in an optimally selected time range after a pulse end through the electrical circuit (eg, the high voltage drop between 87.5% and 75%), this value may be calculated with a second voltage drop rate S2 , calculated from the capacitance C and the last flowing tube measurement current i (t), the actually flowing tube current i _r (t) being compared to that in the measuring divider 11 flowing Meßsteilserstrom i _m (t) is smaller. The first and the second voltage drop rate S1, S2 are indicated in V / s, and represent the temporal voltage change. It is advantageous to carry out this determination shortly after the end of the pulse, since there the voltage drop is approximately linear.

If the measured first voltage drop rate S1 (clearly) is greater than the calculated second voltage drop rate S2, this is an indication that the last tube current i _r (t) flowed was greater than that of the tube current measuring unit 9 captured. This is an indication that a faulty change or failure of the tube current measuring unit 9 occured. If a failure or error is detected, a warning message and / or the X-ray generator ( 1 ) are switched off.

By recalculation on the basis of the first voltage drop rate S1 and the capacitance C, the tube current i _r (t) can nevertheless be determined at the turn-off moment. This relationship can be represented by the following equation (1): i _r (t) = C · du (t) / dt - i _m (t), (1) where du (t) / dt is the first voltage drop rate S1.

• – für alle Arten von Systemen, die eine Hochspannung benötigen,
• – für alle Arten von strahlungserzeugenden Systemen, die eine Hochspannung benötigen,
• – durch eine Berechnung anhand des Spannungsabfalls an Kapazitäten des Hochspannungspfads,
• – als redundanter Röhrenstrom-Kontrollwert,
• – auch unter Berücksichtigung von Röhrenstrom-Nichtlinearitäten,
• – auch unter Berücksichtigung verschiedener thermischer Effekte (Wendel-Elektronenauskühlung etc.),
• – auch unter Berücksichtigung vorhandener weiterer resistiver Lasten (Spannungsmessteiler),
• – als Kontrollwert zur Prüfung eines korrekt funktionierenden Röhrenstrommesseinheit 9,
• – als Vergleichswert für eine Qualitätsermittlung der Röhrenstrommesseinheit 9 und
• – zur Erkennung asymmetrischer Stromflüsse bei Röntgenstrahlern, wenn die Hochspannung differentiell erfasst wird.

With the aid of the invention, it is therefore also possible to use the tube current i _r (t) without the use of the tube current measuring unit provided primarily for this purpose 9 to determine, and indeed

• - for all types of systems that require a high voltage,
• - for all types of radiation-generating systems that require a high voltage,
• By calculation based on the voltage drop across capacitances of the high voltage path,
• As a redundant tube current control value,
• - also considering tube current nonlinearities,
• - taking into account various thermal effects (helical-electron cooling, etc.),
• - taking into account existing additional resistive loads (voltage measuring device),
• - as a control value for testing a properly functioning tube current measuring unit 9 .
• - as a comparison value for a quality determination of the tube current measuring unit 9 and
• - For the detection of asymmetric current flows in X-ray sources, when the high voltage is detected differentially.

Instead of using first and second voltage drop rates S1, S2, the times between predetermined relative voltage levels of the high voltage u (t) may also be compared.

To determine the capacitance C of the high-voltage path can proceed as follows. For a defined emitter turned off 5 of the X-ray source 2 can the entire load of the high voltage path solely by the high voltage measuring in the high voltage generator 6 to be discribed. Other, possible loads can also be taken into account, depending on the design.

If, for a known load, the first voltage drop rate S1 of the high voltage is measured (for example the voltage drop between 87.5% and 75%), this value can be used to calculate the capacitance C. Thus, it is possible to accurately measure the capacity C of the system (cable, tube, high voltage generator) and thus to achieve significant performance improvements and status monitoring during normal operation.

Typically, X-ray generating systems are designed for an operating voltage of up to 150 kV. In order to determine the capacitance C as far as possible without external influences of the radiator (eg cold emission, leakage currents through oil, etc.), the system is charged to its minimum operating voltage (eg 40 kV) when the emitter is switched off and cooled down.

The system is thus only through the measuring divider 11 of the high voltage circuit whose value is highly accurate and known. Over the first voltage drop rate (eg voltage drop of 87.5% to 75%), the capacity C of the system can be determined sufficiently well. This relationship can be represented by the following equation (2): C = i (t) / du (t) / dt, (2) where du (t) / dt is the first voltage drop rate S1.

This procedure is optimally always necessary when components in the high-voltage range are exchanged on the system and can thus be optimally integrated into the already necessary, already existing inverter adjustment. This inverter adjustment, which is necessary to adjust the inverter resonant circuit (for resonant inverters), also uses operating points in the range of 40 kV.

In 3 is a diagram showing the time course of the tube current i _r (t) in a tube current measuring unit 9 to 2 and the timing of the with a high voltage measuring unit 10 to 2 detected high voltage u (t) in pulsed mode shows. At time T1, the inverter of the X-ray generator is turned on. The capacities of the high voltage generator and the high voltage cables are charged so that the tube current i _r (t) and thus the unillustrated tube current and the high voltage u (t) are built up slowly and only reach their maximum values after a few milliseconds at time T2. The high voltage u (t) is established at time T2. The maximum tube current i _r (t) is typically 2 A and the maximum high voltage u (t) is typically 150 kV.

At time T3, the inverter is turned off. The X-ray generator no longer supplies power to the high voltage path. Both the high voltage u (t) and the tube current i _r (t) decay to zero until time T4. In the period between the times T3 and T4, the capacities are discharged. In the period T5, shortly after switching off the pulse, the voltage curve of the high voltage u (t) is linearly decreasing and the course of the tube current i _r (t) is almost linearly constant, so that this range is ideally suited for determining the first voltage drop rate S1 or Determining the capacity C can be used.

LIST OF REFERENCE NUMBERS

1

X-ray generator

2

X-ray

3

High voltage cables

4

anode

5

Emitter / cathode

6

High voltage generator

7

Control and inverter

8th

High Voltage Transformer

9

Tube current measuring unit

10

High voltage measuring unit

11

measuring divider

12

First operational amplifier

13

Second operational amplifier

14

Electrical circuit

15

Rectifier unit

C

Capacity of the high voltage path

i (t)

Tube measuring current

i _h

heating

i _m (t)

Measuring divider current

i _r (t)

Tube current flowing through the X-ray source

R1

first measuring resistor (200 MΩ)

R2

second measuring resistor (20 kΩ)

R3

Measuring resistor / shunt (5 Ω)

R4

damping resistor

t

Time in ms

S1

first voltage drop rate in V / s

S2

second voltage drop rate in V / s

T1

Switching on inverter

T2

High voltage built up

T3

Shut down inverter

T4

High voltage reduced

T5

Period for determining the first voltage drop rate

u (t)

high voltage

Claims (12)

Method for detecting a faulty change or failure of a tube current measurement in a high voltage path ( 2 . 3 . 6 ) one with an X-ray generator ( 1 ) electrically connected X-ray source ( 2 ), wherein the X-ray source ( 2 ) with one of the X-ray generator ( 1 ) is supplied, characterized in that the erroneous change or the failure of the tube current measurement after switching off a Hochspannungsversogung or a Hochspannungspulsende from a determined first voltage drop rate (S1) at the X-ray source ( 2 ) is detected, wherein by a deviation of the first voltage drop rate (S1) to a predetermined second voltage drop rate (S2), the faulty change or failure is displayed. A method according to claim 1, characterized in that the second voltage drop rate (S2) from the capacitance (C) of the high voltage path ( 2 . 3 . 6 ) and a tube measurement current (i (t)), wherein the tube measurement current (i (t)) is given by the sum of a current through the X-ray source ( 2 ) flowing tube current (i _r (t)) and a measuring divider current (i _m (t)) is formed. Method according to Claim 2, characterized in that the second voltage drop rate (S2) is determined from the quotient of the tube measurement current i (t)) and the capacitance (C) of the high-voltage path (S2). 2 . 3 . 6 ) is formed. A method according to claim 3, characterized in that the course of the tube measuring current (i (t)) for the formation of the second voltage drop rate (S2) is assumed to be approximately linearly constant. Method according to one of the preceding claims, characterized in that the first voltage drop rate (S1) indicates the time change of the high voltage (u (t)) after a high voltage pulse. Method according to one of the preceding claims, characterized in that the first voltage drop rate (S1) is determined in a range between 87.5% and 75% of the drop of the high voltage (u (t)). Method according to one of claims 2 to 6, characterized in that the capacitance (C) from the first voltage drop rate (S1) at the X-ray source ( 2 ) high voltage (u (t)) after a high voltage pulse end of a high voltage test pulse is determined. A method according to claim 7, characterized in that the capacitance (C) from the tube measuring current (i (t)) and the first voltage drop rate (S1) is determined, wherein the high voltage (u (t)) is selected so that after the high voltage pulse end no tube current (i _r (t)) flows. Method for determining the tube current (i _r (t)) with known capacitance (C) of the high-voltage path ( 2 . 3 . 6 ) one with an X-ray generator ( 1 ) electrically connected X-ray source ( 2 ), wherein the X-ray source ( 2 ) with one of the X-ray generator ( 1 ) is supplied, characterized in that the tube current (i _r (t)) from a determined first voltage drop rate (S1) at the X-ray source ( 2 ) high voltage (u (t)) after a pulse end of the high voltage or after switching off a high voltage supply and the capacitance (C) is determined. Device for detecting a faulty change or failure of a tube current measuring unit ( 9 ) in a high voltage path ( 2 . 3 . 6 ) one with an X-ray generator ( 1 ) electrically connected X-ray source ( 2 ), wherein the X-ray source ( 2 ) with one of the X-ray generator ( 1 ) generated high voltage (u (t)), characterized by: - an electrical circuit ( 14 ) that is formed, the erroneous change or failure of the tube current measuring unit ( 9 ) from a determined first voltage drop rate (S1) at the X-ray source ( 2 ) after a high voltage pulse end or after switching off a high voltage supply, wherein a deviation of the first voltage drop rate (S1) to a predetermined second voltage drop rate (S2) indicates the erroneous change or failure. Device according to claim 10, characterized in that the electrical circuit ( 14 ) is a digital signal processor, a microprocessor or an FPGA. Device according to claim 10 or 11, characterized in that the electrical circuit ( 14 ) is adapted to perform a method according to any one of claims 2 to 9.

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