DE102015011634B4 - Device for ISO26262-compliant operation of an inductive angle of rotation sensor by detecting asymmetric error states - Google Patents

Abstract

Device for detecting asymmetrical error states (push-pull errors) and for distinguishing these asymmetrical error states (push-pull errors) from symmetrical error states (common-mode errors) when operating and evaluating the output signals of a symmetrical inductive rotation angle sensor head (303)• with m, but at least three, inductive receiving loops ( S1, S2, S3) and- a field-inducing exciter loop (2) and- with a rotor with a rotor loop that distorts the field of the exciter loop (2),- with the star voltage of the receiving loops (S1, S2, S3) of the inductive rotation angle sensor head (303 ) is zero volts in the ideal error-free case, characterized in that • an asymmetric error state is defined in such a way that it does not affect all of the m receiving loops (S1, S2, S3) and/or not all of the signal paths of the m, but at least three signal paths , which followed the respective receiving loops (S1, S2, S3). and• that the device has at least one signal generator (320) which generates at least one excitation signal (322) and• that it has at least one electrical current or voltage source (301) modulated by means of the excitation signal (322) for excitation of the excitation loop ( 2) of the inductive rotation angle sensor head (303) and• that it has at least one device, a star-delta conversion block (307), for performing a star-delta conversion of the output signals of the m, but at least three receiving loops (S1, S2, S3) for formation of m, but at least three, triangular signals (308) and• that it has at least one partial device with two inputs (SIG1, SIG2) for each triangular signal (308), which consists of the excitation signal (322) or a control signal correlating therewith for the Rectification (321) as the first input signal (SIG1) of the sub-device and the relevant triangular signal (308) as the second input signal (SIG2) of the sub-device For each triangular signal (308), a level signal of a total of m, but at least three, level signals (310) assigned to the sub-device and the respective triangular signal (308) is generated by synchronous demodulation and/or scalar product formation, with this sub-device being able to be a synchronous demodulator in particular , and• that it has a summer (324), which forms a summation signal (326) from the m, but at least three, level signals (310) by summation based on a common reference potential, in particular based on an offset, and• that it has a has a first evaluation unit, in particular a comparator (341, 342), which compares the sum signal (326) with a first threshold value (327, 330) and uses it to generate a first result signal (333, 334) for signaling an asymmetrical error state (push-pull error), • where this signaling of this asymmetrical error state (push-pull error) in a purely symmetrical error state (Eq clock error) is omitted.

Description

introduction

Various inductive rotation angle sensors are known in the prior art. An example here is the DE 197 38 839 A1 . Such sensors are also used, for example, in safety-relevant applications. Various standards are known for the requirements of such safety-relevant sensors. Examples would be the ISO26262 standard for the automotive sector or the ISO62508 standard, which generally defines various safety integrity levels, i.e. safety classes.

So that a rotation angle sensor, in particular a rotation angle sensor based on an inductive measuring principle according to the DE 197 38 836 A1 is based, can meet the requirements of such standards, the evaluating electronics must meet certain requirements that result from the difficulties of such rotation angle sensor systems.

For a better introduction to the in the DE 197 38 836 A1 technology used, the basic principle of such an inductive angle of rotation sensor is briefly described again at this point in order to work out the requirements for the evaluation electronics of such an inductive angle of rotation sensor in this context and to present the problems.

one of the DE 197 38 836 A1 In a first approximation, the corresponding inductive angle-of-rotation sensor is rotationally symmetrical with an n-fold rotational symmetry (see 1 ). It typically consists of a carrier, for example made of a conventional circuit board material, to which typically three line loops, the receiver loops (S1, S2, S3), are applied in a meandering pattern. The meander structures of the receiver loops (S1, S2, S3) have an angular periodicity that is the same and constant with regard to the rotation angle. One can speak of an angular wavelength (Δφ) of the meander structures of the receiver loops (S1, S2, S3). The typically three receiver loops (S1, S2, S3) are rotated in relation to each other by typically one third in said angular wavelength (Δφ). If m receiver loops (S1, S2,....Sm) are used instead of three receiver loops (S1, S2, S3), the twisting is only 1/m of the angular wavelength (Δφ) of the meander structures. In the following, three receiver loops are dealt with without limiting the number to these with regard to this disclosure. In addition, a fourth line loop, the excitation loop (2), is applied rotationally symmetrically to the axis of symmetry (Sym), which preferably does not have a meander structure. This whole construction is typically used stationary and is referred to as the stator in the following.

The axis of rotation, which is to be measured in terms of its angle of rotation relative to the stator, penetrates perpendicularly to the image plane 1 the stator in its symmetry point, which is the intersection point of the symmetry axis (Sym) of the system. A rotor is mounted firmly on this axis of rotation, which itself is electrically non-conductive and serves as a carrier for a short-circuit loop, the rotor loop. This rotor loop preferably has the same structure and angular wavelength (Δφ) as the receiving loops (S1, S2, S3), which are preferably all identical to one another, except for the twisting relative to one another.

The excitation loop (2) now sends a primarily magnetic transmission signal that induces a current in the rotor. Since the excitation loop (2) is rotationally symmetrical with respect to the axis of symmetry (Sym) and thus rotationally symmetrical with respect to the axis of rotation, this induction in the rotor loop of the rotor is independent of its angular position and thus of the torsion angle of the rotor relative to the stator that is to be measured. The rotor and thus the rotor loop are typically firmly connected to this axis of rotation, which runs along the axis of symmetry (Sym). The induction therefore depends on the angle of rotation of the axis of rotation relative to the stator.

Likewise, the excitation loop (2) induces a current in each of the three receiver loops (S1, S2, S3), which should be the same for all receiver loops (S1, S2, S3), since they should have an identical geometry and only rotate by one angle (Δφ/m) around the axis of rotation, i.e. the axis of symmetry (Sym). However, due to the rotational symmetry of the excitation loop (2), this is irrelevant.

However, depending on the rotational position of the rotor with its rotor loop in relation to the three receiver loops (S1, S2, S3), the rotor induces a different current in the respective receiver loops (S1, S2, S3). Back induction can be compared to induction in an asynchronous motor, where the Squirrel cage rotor fields are induced in the three stator coils. (see e.g. column 5 lines 45 to 46 of the DE 197 38 836 A1 )

As always, such a system cannot be operated undisturbed and error-free in reality. Since such inductive angle-of-rotation sensors are also to be used in safety-relevant applications, in particular in automobiles, it is necessary to intercept these errors in the electronic post-processing if they cannot already be intercepted in the construction of the sensor element.

For this purpose, in the DE 197 38 836 A1 (see e.g. column 5 lines 45 to 49 of the DE 197 38 836 A1 ) suggested operating the three receiver loops (S1, S2, S3) in a star connection and using the star point to correct interference influences, since this should have a value of 0V due to the construction of the system, without proposing a concrete procedure for this. The excitation loop of the inductive rotation angle sensor head (303) is in the 2 excited with an alternating current by an exciting current source (301) by means of an exciting current which is an alternating current. As a result, when the inductive rotation angle sensor head (303) is functioning correctly, its receiver loops (S1, S2, S3) each show an AC voltage signal. The sum of these AC voltage signals (304) results in the correct functioning of said inductive rotation angle sensor head (303) in the case of star connection zero.

However, the compensation for the disturbances in this star connection is not sufficient. In particular, the star connection is not capable of reliably suppressing common-mode signals that are coupled into the sensor system (300) via the excitation loop (2), for example, in the event of EMC radiation. This limits the amplification of the subsequent amplifiers and thus reduces the robustness of the system.

That in the DE 197 38 836 A1 For this reason, the method disclosed for correcting interference is in particular not able to enable the detection of specific common-mode errors. For example, a lack of excitation of the exciter loop (2) cannot be reliably detected.

In addition, during the preparation of the proposal, it was recognized that the DE 197 38 836 A1 The proposed star connection for interference suppression is also unfavorable because it favors common-mode signals and thus leads to increased sensitivity, for example, to alternating magnetic fields that flow through the receiver loops (S1, S2, S3). This additional common-mode signal, which by definition should have the same amplitude in all three receiver loops (S1, S2, S3), leads to a loss of amplitude bandwidth in the following three input amplifier stages for the output signals of the receiving loops of the inductive rotation angle sensor head (303). If there is not enough amplitude reserve at their inputs, the input amplifier may be overloaded. Since this overload typically does not occur in all three input amplifiers at the same time, the erroneous common-mode signal can ultimately be distorted into an out-of-phase signal and thus lead to an erroneous measurement of the angle of rotation. However, such an incorrect measurement as a result of a common-mode signal is to be regarded as a critical error in safety-critical applications and must therefore be prevented.

A similar device is from DE 197 38 839 A1 known.

Also be here on the scriptures DE 10 2010 031 142 A1 , DE 10 2004 002 629 A1 , U.S. 2002/0 143 450 A1 , DE 12 87 319 A and DE 195 26 723 C1 pointed out. The technical teachings disclosed in these documents show similar circuits for detecting error states as the technical teaching proposed in the document presented here. However, they are all unable to distinguish between symmetrical common-mode error conditions on the one hand and asymmetrical differential-mode error conditions.

task

1. a) Gleichtaktfehler wie eingestreute Felder, Bruch der Erregerschleife, Bruch des Läufers etc. (im Folgenden symmetrische Fehlerzustände genannt)
2. b) Gegentaktfehler wie Bruch einer Empfängerschleife oder Verbindung zu derselben, Schäden an den Verstärkereingängen, Kurzschlüsse auf dem Stator oder Läufer, EMV etc. (im Folgenden asymmetrische Fehlerzustände genannt)

It is therefore the object of the invention to specify a device and a method by which the signals provided by the sensor can be evaluated in such a way that the following errors can be detected:

1. a) Common-mode errors such as interspersed fields, breakage of the excitation loop, breakage of the rotor, etc. (hereinafter referred to as symmetrical error states)
2. b) Normal-mode errors such as a broken receiver loop or connection to the same, damage to the amplifier inputs, short circuits on the stator or rotor, EMC, etc. (hereinafter referred to as asymmetric error states)

At the same time, greater robustness against the irradiation of common-mode signals is to be achieved. This object is solved by a device according to claim 1.

In the prior art, common-mode and differential-mode errors are not detected in such sensors and cannot be distinguished.

suggested solution

The invention of the present document claims a device for detecting asymmetrical error states (push-pull errors) and for distinguishing these asymmetrical error states (push-pull errors) from symmetrical error states (common-mode errors) when operating and evaluating the output signals of a symmetrical inductive rotation angle sensor head (303). According to the invention, the device has m, but at least three, inductive receiving loops (S1, S2, S3), a field-inducing excitation loop (2) and a rotor with a rotor loop that distorts the field of the excitation loop (2). According to the invention, the star voltage of the receiving loops (S1, S2, S3) of the inductive rotation angle sensor head (303) is zero volts in the ideal error-free case. The technical teaching of the invention presented here is characterized in that an asymmetrical error state is defined in such a way that it does not affect all of the m receiving loops (S1, S2, S3) and/or not all of the signal paths of the m, but at least three signal paths. which are respectively downstream of the respective receiving loops (S1, S2, S3). In addition, according to the invention, the device has at least one signal generator (320) which generates at least one excitation signal (322). The device according to the invention has at least one electrical current or voltage source (301) modulated by means of the excitation signal (322) for excitation of the excitation loop (2) of the inductive rotation angle sensor head (303). Furthermore, the device according to the invention has at least one device, a star-delta conversion block (307), for performing a star-delta conversion of the output signals of the m, but at least three receiving loops (S1, S2, S3) to form m, but at least three , triangular signals (308). In addition, the device according to the invention has at least one sub-device with two inputs (SIG1, SIG2) for each triangular signal (308), which consists of the excitation signal (322) or a control signal for the rectification (321) correlating therewith as the first input signal (SIG1) of the sub-device and the relevant triangular signal (308) as a second input signal (SIG2) of the sub-device for each triangular signal (308) a level signal assigned to the sub-device and the respective triangular signal (308) of a total of m, but at least three, level signals (310) by synchronous demodulation and/or dot product formation generated. In this case, this sub-device can in particular be a synchronous demodulator. In addition, the device according to the invention has a summator (324) which forms a sum signal (326) from the m, but at least three, level signals (310) by summation based on a common reference potential, in particular based on an offset. The device according to the invention also has a first evaluation unit, in particular a comparator (341, 342), which compares the sum signal (326) with a first threshold value (327, 330) and uses this to generate a first result signal (333, 334) for signaling an asymmetrical Error condition (push-pull error) generated. According to the invention, this signaling of this asymmetrical error state (push-pull error) is omitted in the case of a purely symmetrical error state (common-mode error).

The sensor system (300) of the proposed device and its technical environment are first using the 2 further described. In the following, all figures are shown in simplified form for better clarity. Only the claims are decisive for the scope claimed. Essential parts of the proposal relate to the disclosure DE 197 38 836 A1 . Their content is therefore part of this disclosure without claiming its disclosure content.

The excitation loop (2) of the inductive rotation angle sensor head (303) is excited as before with an excitation current from the excitation current source (301). Again, if the inductive angle of rotation sensor (303) is functioning correctly, its receiver loops (S1, S2, S3) each show an AC voltage signal in the form of the output signals (304) of the inductive angle of rotation sensor head, with each of the output signals (304) of the inductive angle of rotation sensor head (303) being one Receiver loop (S1, S2, S3) is assigned. The sum of these output signals (304) of the inductive angle of rotation sensor head (303) should be correct in the case of the star interconnection result in zero. Typically, the inductive angle of rotation sensor head (303) is already connected in a star configuration.

The output signals (304) of the rotation angle sensor head (303) are pre-filtered in an EMC filter block (305) to form filtered output signals (306) of the rotation angle sensor head (303), with typically only those signal components being passed through the EMC filter block (305) which Correspond excitation signal (322) with which the excitation current source (301) is controlled, which feeds the excitation current for the excitation loop (2) or is within the bandwidth of the excitation signal (322). A filter is preferably provided for each output signal (304) and/or for each receiving loop (S1, S2, S3) of the inductive rotation angle sensor head (303).

When the proposal was being worked out, however, it was now recognized that it makes more sense to evaluate the delta voltages at the outputs (304) of the inductive angle of rotation sensor (303) instead of the star point voltages for the purpose of fault detection instead of the star voltages. Since the induction of common-mode interference in all three receiver loops (S1, S2, S3) should be the same, the differences in the phase-to-neutral voltages with regard to the common-mode signal components should be zero.

This star-to-delta conversion is performed in a star-to-delta conversion block (307) found in 4 is shown again in more detail. From the filtered output signals (306) of the rotation angle sensor head (303), three differential amplifiers (401 to 403) each form a triangular voltage (308) by forming the respective difference.

The delta signals (308), after being connected to form a delta in the star-delta conversion block (307), no longer contain a DC component if the sensor system (300) is otherwise balanced, as provided for in the error-free case. In this context, symmetrical means that all receiver loops (S1, S2, S3) apart from the aforementioned rotation about the axis of rotation, i.e. the axis of symmetry (Sym), have the same geometry with an n-fold symmetry and are arranged exactly centrically and that this is also the case applies to the rotor loop. In this ideal case, the excitation loop (2) is also attached completely symmetrically with respect to the axis of symmetry (Sym).

größer als die Sternspannungen der Ausgangssignale (304) des induktiven Drehwinkelsensorkopfes (303). Hierdurch wird der Pegelabstand zischen dem Nutzsignal der Empfangsschleifen (S1, S2, S3) und dem eingestrahlten Gleichtaktstörsignal erhöht und zum anderen der Aussteuerbereich der nachfolgenden Verstärkerstufen hinsichtlich der Nutzsignale besser ausgenutzt.The triangular voltages of the triangular signals (308) are also by a factor with respect to their useful signal component $\sqrt{3}$

greater than the star voltages of the output signals (304) of the inductive rotation angle sensor head (303). This increases the level difference between the useful signal of the receiving loops (S1, S2, S3) and the radiated common-mode interference signal and, on the other hand, the dynamic range of the subsequent amplifier stages is better utilized with regard to the useful signals.

Typically, the proposed device in a motor vehicle is supplied with a simple DC voltage that is not symmetrical about the zero point. Therefore, in a preferred version of the proposed device, a DC voltage signal, the offset, is added to each of the three triangular voltages of the triangular signals (308) by the differential amplifiers (401 to 403). 4 is added, which is preferably half the operating voltage (VBD) and/or is in the middle of the control range of the input amplifier in terms of level. This prior art adder circuit is in 4 not shown additionally for the sake of simplicity.

The sum of the amplitudes of the triangular voltages of the triangular signals (308) should be zero volts after rectification by the synchronous demodulator described below. When drafting the proposal, it was recognized that this can be evaluated to evaluate the sensor status. When the proposal was being worked out, it was recognized that error states relevant to the task, which affect the sensor and/or the evaluation in an asymmetric manner, can be detected in this way. Common-mode errors do not affect the sum of the amplitudes of the triangular voltages of the triangular signals (308) after rectification. It is thus possible, according to the proposal, to detect differential mode errors and to differentiate them from common mode errors, in contrast to the prior art. In the device according to the proposal, the operating voltage is therefore subtracted once from the voltage sum or the said offset is subtracted twice, as a result of which the sum signal (326) is again in the middle of the modulation range of subsequent amplifiers.

This formation of the sum signal (326) by summation of the level signals (310) takes place in a summing device (324).

The sum signal (326) formed in this way is compared, for example, by means of a first comparator (341) with a first threshold value (327), for example in the form of a first reference voltage (V _ref1 ), which is above said offset, and for example by means of a second comparator ( 342) is compared with a second threshold value, for example in the form of a second reference voltage (V _ref2 ), which is below the offset. A sixth result signal (334) and a first result signal (333) then typically result. If the voltage of the sum signal (326) is outside the permissible sum voltage range defined by the first reference voltage (Vref1), i.e. the first threshold value (327), and the second reference voltage (V _ref2 ), i.e. the second threshold value, then there is a corresponding Task of the proposal to be detected symmetry error, so a push-pull error before. In addition, the device recognizes that this error is not a common-mode error, ie not a symmetrical error condition.

In the star-delta conversion block (307), the associated delta signals (308) of the output signals (304) of the inductive rotation angle sensor head (303) are formed according to a delta circuit, as previously described. This removes the DC signal interference and overloading of the subsequent amplifier inputs becomes less likely and the headroom increases. The three triangular signals (308) formed in this way are then rectified and filtered in a synchronous demodulator block (309) in order to remove the excitation signal (322) from the excitation current source (301), which means that only a DC signal remains that corresponds to the respective reception level.

A clock generator (320) delivers the excitation signal (322) for the excitation current source (301) and at the same time a control signal (321) for the rectification. Strictly speaking, the triangular signals (308) are now multiplied by the control signal (321) in the form of the excitation signal (322) of the excitation current source (301) and then low-pass filtered. This is a procedure known from synchronous demodulators. In this case, a scalar product is formed between the excitation signal (322) of the excitation current source and the respective triangular signal (308) in the synchronous demodulator block (309) by three separate synchronous demodulators. The low-pass filtering suppresses signal components with higher frequencies. In principle, the use of any linear forms is possible here, provided that the orthogonality of two different signals can be achieved under determinable conditions with regard to the difference between these signals in the form of the disappearance of the result of the linking of these signals using this linear form. 3 shows a simple realization of a scalar product formation unit. Here, a first signal (SIG1) is multiplied by a second signal (SIG2) in a multiplier (M1) and filtered in the subsequent low-pass filter (F1) to form the output signal (OUT). If one of the signals is a binary signal, for example the first signal (SIG1), then the multiplier can be implemented very easily. For example, the multiplier can be an impedance converter or the like, which can be restructured by means of switches actuated by the first signal (SIG1) in such a way that it once has a gain of 1 and another time has a gain of -1. Typically, the said offset is subtracted beforehand and then added up again. If the sign of the second signal (Sig2) then changes synchronously with the first signal (SIG1) after offset subtraction, the result is always a positive signal whose DC component is then eliminated by the subsequent low-pass filter. Other structures for the scalar product formation are of course conceivable.

Any two signals A and B are then referred to as orthogonal with respect to a signal demodulator for the purposes of this disclosure if they would be fed into one of the signal demodulators instead of the first signal (SIG1) and the second signal (SIG2) and the output signal would result in 0. In reality, however, such an amplifier will show system noise and a control error. This system noise and this control error can be clearly determined by feeding the excitation signal (322) into the synchronous demodulator instead of the first signal (SIG1) and keeping the second signal (SIG2) of the synchronous demodulator constant. In the ideal case, the output signal of the synchronous demodulator would then be zero, but in reality it typically shows the said noise and the said control error.

In the example of the synchronous demodulator block (309), for example, each of the triangular signals (308) each having a first signal input (SIG1) corresponds to a respective dot product formation device 3 tied together. All second signals (SIG2) of these dot product devices are typically connected to the rectification control signal (321) from the signal generator (320). Typically, this rectification control signal (321) correlates with the excitation signal (322). All three of these dot product devices together form the synchronous demodulator block (309). Each of the three output signals (OUT) of the three dot product devices is connected to one of the three level signals (310).

In a special form of the proposal, the excitation signal (322), which is formed by the clock or signal generator (320), is preferably a bandwidth-limited signal with a lower limit frequency ω _min and a different upper limit frequency ω _max , the should be different. The lower limit frequency ω _min should be greater than half the frequency bandwidth Δω=ω _max ,- ω _min . The low-pass filter is now preferably designed in such a way that it reliably suppresses signal components with a frequency greater than the lower limit frequency ω _min and/or better than half the frequency bandwidth Δω /2 and allows frequency components below half the frequency bandwidth Δω /2 to pass through as undamped as possible. All statements refer to the frequency values. The result of this scalar product formation are three level signals (310), the voltage value of which is typically a respective measure of the size of the induction of the excitation signal (322) in the receiving loops (S1, S2, S3) of the inductive rotation angle sensor head (303).

In a particularly preferred variant, instead of simple mono-frequency excitation signals (322), spread-code-based excitation signals (322) are generated by the signal generator (320) and/or random signals and/or pseudo-random signals are generated. This is particularly useful in environments with particularly high levels of interference or in the case of special reliability requirements. However, the use of spread codes increases the dead time of the sensor system. It is therefore useful in some cases to switch over the type of excitation signal (322) dynamically during operation and, for example, to switch over the length of the spread code as a function of the permissible dead time and/or the required precision. A longer spreading code is used when higher precision is required, a shorter one, or none at all, when short dead time is required.

Up to this point in the course of the signal processing in the proposed device, the processing has been carried out in spatial multiplex. I.e. up to this point in the signal path of the proposed device there is a signal path for each of the three receiver loops (S1, S2, S3), or there are always three signal paths in parallel up to this point. It has been shown that from this point onwards it is advisable to carry out time-division multiplex processing in the signal path. For this purpose, an analog multiplexer (311) switches cyclically between the three level signals (310) generated up to this point. The multiplexer output signal (312) is fed to an adjustable amplifier (313).

The controllable amplifier (313), whose gain can be adjusted, amplifies the multiplexer output signal (312) to form the amplifier output signal (314). Its amplification is set once by an amplifier control signal (318) for all three level signals (310) at the same level for each measurement run. For the duration of the measurement run, in which all three level signals (310) are evaluated, the amplification of the amplifier (313) remains the same and is only after all three level signals (310) have been evaluated once by a subsequent control logic (317), by this Control logic (317) adjusted at the end of the measurement run. This adjustment is done in such a way that at least one of the three level signals (310) leads to full drive of the downstream analog-to-digital converter (ADC, 315) without overdriving it. For this purpose, the amplifier output signal (314) is fed to the analog-to-digital converter (ADC, 315), which forms the ADC output signal (316) from it. A control logic (317) downstream of the ADC (315) temporarily stores the conversion results of the ADC (315) in the form of the ADC output signal (316) during the measurement run and, at the end of the measurement run, evaluates these conversion results of the ADC (315) in the form of the ADC output signal (316). The control logic (317) adjusts the gain of the amplifier (313) by means of the amplifier control signal (318) for one measurement run of all three level signals (310) up or down at the end of the measurement run, whereby the control loop for the gain of the amplifier (313) is closed. It should be noted here that the information to be measured, namely the angle of rotation, is contained in the height of the amplitudes of the level signals (310) relative to one another and is not changed during a measurement run due to the identical amplification for all level signals (310). The control logic now controls the multiplexer (311) via a multiplexer control signal (319). The control logic (317) now converts the three statuses of the ADC output signal (316) during the measurement of the three different level output signals (310) into the measured value (345) of the angle of rotation sought.

When the proposal was being worked out, it was also recognized that the value of the amplifier control signal (318) of the gain of the amplifier (313) itself is a parameter that is suitable for determining a symmetrical error state, i.e. a common-mode error, of the proposed rotational angle measuring system.

If the value of the amplifier control signal (318) is above a fourth threshold value (335), the permissible maximum amplification value, this can indicate, for example, a lack of excitation due to a lack of excitation current and/or an interrupted excitation loop or a missing rotor.

If the value of the amplifier control signal (318) is below a fifth threshold value (336), the permissible minimum amplification value, this can indicate asymmetric radiation or an interrupted receiver loop (2).

For this purpose, the amplifier control signal (318) is compared with the said fourth threshold value (335) by a third comparator (343), for example, and a third evaluation signal (337) is generated.

The amplifier control signal (318) is compared with said fifth threshold value (336) by a fourth comparator (344), for example, and a fourth evaluation signal (338) is generated.

In addition, a comparison with a second threshold value and a third threshold value can be used to check whether one of the level signals (310) is actually always in the upper control range, a second level signal of the level signals (310) is in the middle control range and a third level signal of the level signals (310) is in the lower control area. This comparison is preferably made by the control logic (317) at the end of a measurement run. Until then, the level values of the individual level signals (310), after amplification in the controllable amplifier (313) and digital conversion in the ADC (315), as described, are temporarily stored by this control logic (317).

This results in the following list of threshold values up to here First threshold (327) for detecting a sum signal (526) of the level signals (310) that is too small Eighth threshold (330) for detecting an excessive sum signal (326) of the level signals (310) Fourth threshold (335) for detection of an excessive amplifier control signal (326) Fifth threshold (336) for detecting an amplifier control signal that is too small (326) Sixth threshold (527) for detecting a second sum signal (526) of the second level signals (510) that is too small (this sixth threshold value will be explained later.) Seventh Threshold (530) for detecting an excessive second sum signal (526) of the second level signals (510) (This seventh threshold value will be explained later.) Second threshold for detection by the control logic (317) whether one of the level signals (310) is in the upper dynamic range, the lower edge of which is defined by this second threshold value. Third threshold for detection by the control logic (317) whether one of the level signals (310) is in the lower dynamic range, the upper edge of which is defined by this third threshold value. Ninth threshold for detection by the control logic (317) whether at least one of the level signals (310) is in the upper dynamic range but not yet in overdrive, the permissible upper edge of which is defined by this ninth threshold value. Other thresholds to evaluate the measurement result (345) (explained further below)

5 shows the exemplary sequence of an exemplary measurement run for measuring exemplary three receiver loops (S1, S2, S3) in time-division multiplex.

In a first step (600), a first of the three level signals (310) is selected by the multiplexer (311) and a first measured value (316) is determined, which is stored by the control logic (317).

In a second step (601), a second of the three level signals (310) is selected by the multiplexer (311) and a second measured value (316) is determined, which is also stored by the control logic (317).

In a third step (602), a third of the three level signals (310) is selected by the multiplexer (311) and a third measured value (316) is determined, which is typically also stored by the control logic (317).

In the fourth step (603), the measured value (345) is determined from these three values by the control logic (317), but is typically not yet output.

In the fifth step (604), the measured value (345) is typically evaluated.

If the measured value is evaluated as OK by the control logic (317), in particular in comparison with other measured values that were measured in parallel or previously, and/or with other threshold values, then in a sixth step (606) this is typically output and the system is typically not reparameterized.

If the measured value is evaluated as not OK by the control logic (317), in particular in comparison with other measured values that were measured in parallel or previously, and/or with other threshold values, then in a fifth step (605), for example, the last one is evaluated as OK evaluated measured value is output and the system is typically reparameterized. In addition, an error message is typically sent to a higher-level system. However, it is also conceivable for this error message to be recorded in a counter and for the control logic to issue an error message only when a certain counter reading is reached. The behavior of this counter can be designed in such a way that a correct measurement resets the counter immediately and, if necessary, a corresponding all-clear signal is sent to a higher-level system. However, it can also be counted down to zero at the same or a different speed as is counted up.

1. a. die die Frequenz des Erregersignals (322, 522) und/oder
2. b. einen für die Erzeugung des Erregersignals (322, 522) verwendeten Spreiz-Code und/oder
3. c. ein für die Erzeugung des Erregersignals verwendetes Pseudozufallssignal, insbesondere das Rückkoppelpolynom eines rückgekoppelten Schieberegisters.

The said re-parameterization of the system typically affects

1. a. the frequency of the excitation signal (322, 522) and / or
2. b. a spreading code used to generate the excitation signal (322, 522) and/or
3. c. a pseudo-random signal used to generate the excitation signal, in particular the feedback polynomial of a feedback shift register.

After the sixth or fifth step, the measurement process in this example starts again with the first step.

In 6 an extension of the proposed device is shown. In addition to the first excitation current source (301) of 2 a second excitation current source (501) is now drawn in, which feeds a second excitation current into the excitation loop (2) of the inductive rotation angle sensor head (303) superimposed on the first excitation current of the first excitation current source (301). This second excitation current source (501) is controlled by a second excitation signal (522). The signal path is initially unchanged downstream of the inductive rotation angle sensor head (303) until the synchronous demodulator block (309) is now connected in parallel with a typically identical second synchronous demodulator block (509). Its second control signal for the rectification is now based on the second excitation signal (322). It can also be identical to this, for example. It is now preferred that the two excitation signals are selected in relation to one another in such a way that the signal components of the output signals (304) of the inductive rotation angle sensor head (303) which can be traced back to the first excitation signal (322) do not produce any signals at the second level signals ( 510), these are the outputs of the second synchronous demodulator (509). Conversely, in this case it is preferably the case that the signal components of the output signals (304) of the inductive rotation angle sensor head (303) that are due to the second excitation signal (522) also have no signals at the first level signals (310), i.e. the outputs of the first synchronous demodulator (309). In that case the excitation signals are said to be orthogonal to each other.

The multiplexer (311) is now simply expanded accordingly. A measurement run preferably relates to the outputs of a synchronous demodulator block (309, 509)

It now makes sense to also check the correct functioning of the sensor system with the additional second level signals (510) which come from the second synchronous demodulator block (509). The sum of the amplitudes of the triangular voltages of the triangular signals (308) after rectification by the synchronous demodulator to form the second level signals (509) must again be zero volts here. This sum of the amplitudes of the triangular voltages of the triangular signals (308) can thus in turn be evaluated to evaluate the sensor state. Error states (push-pull errors) that affect the sensor and/or the evaluation in an asymmetrical manner can also be detected in this way. A second excitation signal (522) that is different from the first excitation signal (322) can do this, for example, at a different frequency. In the proposed device, therefore, the operating voltage is subtracted once from the voltage sum and the said offset is subtracted twice, as a result of which the second sum signal (526) is again in the middle of the modulation range of subsequent amplifiers.

This formation of the second sum signal (526) by summation of the second level signals (510) takes place in a second summing device (524).

The second sum signal (526) formed in this way is compared, for example, by means of a fifth comparator (541) with a sixth threshold value (527), for example also in the form of a first reference voltage (V _ref1 ), which is above said offset, and for example by means of a sixth Comparator (542) with a seventh threshold (530), for example also in the form of a second reference voltage (V _ref2 ) compared, which is below the offset. A fifth result signal (534) and a fourth result signal (533) then typically result. If the voltage of the second sum signal (526) is outside the permissible sum voltage range defined by the first reference voltage (Vref1), i.e. the sixth threshold value (527), and the second reference voltage (V _ref2 ), i.e. the seventh threshold value (530), then there is again a symmetry error to be detected according to the task of the proposal.

Of course, it is also conceivable to relate the measurement run to all synchronous demodulator blocks. However, this is disadvantageous in that the resolution of a block is then reduced.

If necessary, the threshold values specified above, which relates to the fourth and fifth, as well as the second and third and the ninth threshold, can be doubled accordingly.

It is particularly advantageous if at least one of the signal generators (320, 520) uses a random signal as the excitation signal (322, 522). It is also possible to use a pseudo-random signal.

A further advantageous embodiment is the use of a spread code for generating an excitation signal (322, 522) by one of the signal generators (520, 320). With a very long spread code, however, the response time of the sensor system increases. It is therefore useful if, in this case, the other signal generator uses a shorter spread code or no spread code at all. The spread codes or the base clock of the signal generators (520, 320) for generating the excitation signals (322, 522) should preferably be selected in such a way that they result in signals that are orthogonal to one another. This can be guaranteed if the base clock of the first signal generator (322) is 2*m times the base clock of the second signal generator (522) and the spread code length for the first spread code of the first signal generator is a digital spread code is 2*m clocks and the first spreading code contains m 1's and m 0's. The second spread code for the signal generation by the second signal generator should also have an even number of clock pulses and have the same number of 1 values and 0 values.

It is also conceivable that the two signal generators each emit a signal of different frequency, with one signal preferably being an integral multiple of the other signal. Excitation signals of different frequencies are the simplest examples of orthogonal excitation signals.

In this case, it makes sense if the control logic (317) outputs a second measured value (545) for the different excitation signals (322, 522).

• d. die Frequenz des Erregersignals (322, 522) betreffen, was einem Frequency-Hopping-Verfahren entspricht, und/oder
• e. einen für die Erzeugung des Erregersignals (322, 522) verwendeten Spreiz-Code betreffen und/oder
• f. ein für die Erzeugung des Erregersignals verwendetes Pseudozufallssignal, insbesondere das Rückkoppelpolynom eines rückgekoppelten Schieberegisters, betreffen.

It is of course also possible for one or more excitation signals (322, 522), as described, to be changed after each measurement sequence, ie the measurement of all level signals (310, 510). This can

• i.e. relate to the frequency of the excitation signal (322, 522), which corresponds to a frequency hopping method, and/or
• e. relate to a spreading code used to generate the excitation signal (322, 522) and/or
• f. a pseudo-random signal used to generate the excitation signal, in particular the feedback polynomial of a feedback shift register.

In this way, a number of measured values (345, 545) can be obtained by space multiplex and/or time multiplex, which can be compared with one another and with other threshold values, in particular by averaging and calculating the difference in an evaluation unit. This makes it possible, for example, for three measured values measured in parallel with three excitation current sources and three associated excitation currents, which are controlled by three excitation signals, to generate three measured values and thus generate a majority voting for the probably correct measured value. In this case, m level signals are generated by three synchronous demodulator blocks, which are then processed as already described. Each of the three branches is fed by an excitation signal that originates from one of three signal generators. It is of course conceivable that a signal generator generates several of these different excitation signals at the same time.

Depending on the evaluation result, the excitation signals can then be changed by the control logic or another control unit. This makes it possible to detect interference in the event of strong EMC radiation and to minimize it by optimizing the exciter signals, as long as the interference does not affect all exciter signals in the same way, which typically applies to narrow-band interferers.

As in many other inductive sensor systems, it is also the case with a sensor system according to the DE 197 38 836 A1 It makes sense to operate the excitation loop (2) within an LC resonant circuit, for example together with a parallel-connected resonant circuit capacitance (C). However, the advantage of the high resonance amplitude has the disadvantage of a very narrow-band resonance frequency. Parallel to the modulation of the AC source, the modulation of the resonant frequency of the LC resonant circuit itself is therefore also possible and particularly preferred with the aid of a modulatable capacitance (701) and/or a modulatable inductance (801) as excitation loop (2). This is done using the 7 until 10 explained.

7 shows an exemplary system with a modulatable capacity (701).

In the case of a modulatable capacitance (701), this ability to be modulated can take place, for example, by switching on and off partial capacitances (C ₁ , C ₂ ). 8th shows an example of a very simple modulatable capacitance (701). The exemplary modulatable capacitance (701) consists of a first partial capacitance (C ₁ ), which is switched in parallel via the switch (SW ₁ ) depending on the second further excitation signal (722) of the second partial capacitance (C ₁ ), or separated from it will. As a result, the resonant frequency of the LC resonant circuit can be 7 be modulated. Mechanical switches such as relays and in particular micromechanical relays can be used as switches (SW ₁ ), but also MOS and bipolar transistors and similar semiconductor switches that can fulfill the function of such a switch. This can possibly also involve more complex circuits that perform this function of a switch. In this case, the first excitation signal (322) of the first signal generator (320) is typically also switched over synchronously in such a way that the first excitation current source (301) receives an excitation signal (322) that corresponds to the current resonant frequency of the LC resonant circuit from excitation loop (2) and a modulatable capacitance (701) corresponding to the value of the second further excitation signal (722). A variant is particularly preferred in which the second further excitation signal (722) has a period length which is a multiple of the period length of the first excitation signal (322).

The processing is analogous to that of the description of the 6 in a second further synchronous demodulator block (709) which forms second further level signals (710) as a function of a second further control signal (721) for the rectification. The second further control signal (721) for the rectification is typically equal to the second further excitation signal (722). A second further clock or signal generator (720) generates said second further excitation signal (722). This typically occurs synchronously with the generation of the first excitation signal (322) in the signal generator (320).

It is obvious that the second further level signals (710) should be symmetrical and should have equal levels. Deviations can be detected as errors as before.

10 shows an exemplary system with a modulatable inductance (801) and an oscillating circuit capacitance (C).

In the case of a modulatable inductance (801), this ability to be modulated can take place, for example, by connecting and disconnecting partial inductances (L ₁ , L ₂ ). 10 shows an example of a very simple inductance that can be modulated (801). The exemplary inductance (801) that can be modulated consists of a first partial inductance (L ₁ ), which is short-circuited via the switch (SW ₁ ) depending on the second further excitation signal (822) of the second partial inductance (L ₁ ) and is thus removed from the resonant circuit . This allows the resonant frequency of the LC resonant circuit consisting of the excitation loop (2) and the resonant circuit capacitance (C) to be modulated. The exciter loop itself can consist of several such switchable sub-exciter loops, which has the same effect. Here, too, mechanical switches such as relays and in particular micromechanical relays can be used as switches (SW ₁ ), but also MOS and bipolar transistors and similar semiconductor switches etc., which can fulfill the function of such a switch. In this case, the first excitation signal (322) of the first signal generator (320) is typically also switched over synchronously in such a way that the first excitation current source (301) receives an excitation signal (322) that corresponds to the current resonant frequency of the LC resonant circuit from the excitation loop (2nd ) and modulatable inductance (801) and resonant circuit capacitance (C) according to the value of the second further exciter signal (822). A variant is particularly preferred in which the second further excitation signal (822) has a period length which is a multiple of the period length of the first excitation signal (322).

The processing is again analogous to that of the description of the 6 in a second further synchronous demodulator block (809) which forms second further level signals (810) for the rectification as a function of a second further control signal (821). In this case, the second further control signal (821) for the rectification is typically equal to the second further excitation signal (822). A second further clock or signal generator (820) generates said second further excitation signal (822). This typically happens synchronously with the generation of the first excitation signal (322) in the signal generator (320).

It is obvious that the second further level signals (810) should again be symmetrical and should have the same levels. Deviations can be detected as errors as before.

Of course, this modulation technique can also be used in combination by simultaneously using a modulatable capacitance and a modulatable inductance.

advantages

In contrast to the prior art, the proposed device and the proposed method can thus detect the errors in the task with high EMC robustness. It should be particularly emphasized that the device is able to differentiate between common mode and differential mode errors and, in particular in the proposed interaction of claims 1 and 2, also to reliably detect both.

reference list

2

excitation loop

300

sensor system

301

excitation current source

303

inductive rotation angle sensor head

304

Output signals of the inductive rotation angle sensor head (303)

305

EMC filter block

306

filtered output signals (306) of the rotation angle sensor head (303)

307

Star-delta conversion block

308

triangular signals

309

synchronous demodulator block

310

level signals

311

analog multiplexer

312

multiplexer output signal

313

adjustable amplifier

314

amplifier output signal

315

Analog to Digital Converter (ADC)

316

ADC output signal

317

control logic

318

amplifier control signal

319

multiplexer control signal

320

Clock or signal generator

321

Control signal for rectification.

322

excitation signal

324

Summation device for forming the sum signal (326)

326

Sum signal of the level signals (310)

327

first threshold

330

eighth threshold

333

first result signal

334

sixth result signal

335

fourth threshold, the allowed maximum gain value

336

fifth threshold, the allowable minimum gain value

337

second result signal

338

third result signal

341

first comparator for forming the sixth result signal (334) by comparing the first threshold value (327) with the sum signal (326)

342

second comparator for forming the first result signal (333) by comparing the eighth threshold value (330) with the sum signal (326)

343

third comparator for forming the second result signal (337) by comparing the fourth threshold value (335) with the amplifier control signal (318)

344

fourth comparator for forming the third result signal (338) by comparing the fifth threshold value (336) with the amplifier control signal (318)

345

Measured value (345) of the angle of rotation sought

401

first differential amplifier for star-delta conversion of the filtered output signals (306) of the rotation angle sensor head (303) into a triangular signal (308)

402

second differential amplifier for star-delta conversion of the filtered output signals (306) of the angle of rotation sensor head (303) into a triangular signal (308)

403

third differential amplifier for star-delta conversion of the filtered output signals (306) of the rotation angle sensor head (303) into a triangular signal (308)

501

second excitation current source

509

second synchronous demodulator block

510

second level signals

520

second clock or signal generator

521

second control signal for the rectification.

522

second excitation signal

524

second summing device for forming the second sum signal (526)

526

second sum signal of the second level signals (510)

527

sixth threshold

530

seventh threshold

533

fourth result signal

534

fifth result signal

541

fifth comparator for forming the fifth result signal (534) by comparing the sixth threshold value (527) with the second sum signal (526)

542

sixth comparator for forming the fourth result signal (533) by comparing the seventh threshold value (530) with the second sum signal (526)

545

second reading of the angle of rotation

600

first step of an exemplary measurement run (measurement of a first of the exemplary three level signals (310))

601

second step of an exemplary measurement run (measurement of a second of the exemplary three level signals (310))

602

third step of an exemplary measurement run (measurement of a third of the exemplary three level signals (310))

603

Fourth step of an exemplary measurement run (determination of the measurement result (345) before output by the control logic (317))

604

fifth step of an exemplary measurement run (evaluation of the measurement result (345))

605

Sixth step of an exemplary measurement run (execution of actions in the event of a correct measurement result, e.g. output of the measurement result (345))

606

Seventh step of an exemplary measurement run (execution of actions in the event of an incorrect measurement result, e.g. output of a previous measurement result (345))

701

Capacitance that can be modulated, which forms an LC resonant circuit with the excitation loop (2).

709

second additional synchronous demodulator block

710

second further level signals

720

second additional clock or signal generator

721

second further control signal for the rectification.

722

second further excitation signal

801

inductance that can be modulated, which forms an LC resonant circuit with the excitation loop (2) and another capacitance.

809

second additional synchronous demodulator block

810

second further level signals

820

second additional clock or signal generator

821

second further control signal for the rectification.

822

second further excitation signal

Δφ

angular wavelength

Δω

= ω _max - ω _min bandwidth of the excitation signal (322) (amount)

ωmax

upper limit frequency of the excitation signal (322) (amount)

ωmin

lower limit frequency of the excitation signal (322) (amount)

C

resonant circuit capacitance

C1

first partial capacitance of the exemplary modulatable capacitance (701)

C2

second partial capacitance of the exemplary modulatable capacitance (701)

L1

first partial inductance of the exemplary modulatable inductance (801)

L2

second partial inductance of the exemplary modulatable inductance (801)

m

Number of receiver loops

Out

Output of dot product device

S1

first receiver loop

S2

second receiver loop

S3

third receiver loop

SIG1

first input signal of the dot product device

SIG2

second input signal of the dot product device

sm

mth receiver loop

SW1

Switch of the exemplary modulatable capacitance (701) that can be actuated by the second excitation signal (522).

symbol

axis of symmetry

Claims (5)

Device for detecting asymmetrical error states (push-pull errors) and for distinguishing these asymmetrical error states (push-pull errors) from symmetrical error states (common-mode errors) when operating and evaluating the output signals of a symmetrical inductive rotation angle sensor head (303) • with m, but at least three, inductive receiving loops (S1, S2, S3) and - a field-inducing excitation loop (2) and - with a rotor with a rotor loop that distorts the field of the excitation loop (2), - wherein in particular the star voltage of the receiving loops (S1, S2, S3) of the inductive rotation angle sensor head (303) is zero volts in the ideal error-free case, characterized in that • that an asymmetrical error state is defined in such a way that it does not affect all of the m receiving loops (S1, S2, S3) and/or not all of the signal paths of the m, but at least three signal paths that affect the respective receiving loops (S1, S2, S3) each are downstream, and • that the device has at least one signal generator (320) which generates at least one excitation signal (322) and • that it has at least one electrical current or voltage source (301) modulated by means of the excitation signal (322) for excitation of the excitation loop (2) of the inductive rotation angle sensor head (303) and • that they have at least one device, a star-delta conversion block (307), for performing a star-delta conversion of the output signals of the m, but at least three receiving loops (S1, S2, S3) to form m, but at least three, triangle signals ( 308) and • that it has at least one sub-device with two inputs (SIG1, SIG2) for each triangular signal (308), which consists of the excitation signal (322) or a control signal correlating therewith for the rectification (321) as the first input signal (SIG1) of the sub-device and the relevant triangular signal (308) as a second input signal (SIG2) of the sub-device for each triangular signal (308) a level signal assigned to the sub-device and the respective triangular signal (308) of a total of m, but at least three, level signals (310) by synchronous demodulation and/or scalar product -Education generated, this sub-device can be in particular a synchronous demodulator, and • that it has an adder (324) that forms a summation signal (326) from the m, but at least three, level signals (310) by summation based on a common reference potential, in particular based on an offset, and • that it has a first evaluation unit, in particular a comparator (341, 342), which compares the sum signal (326) with a first threshold value (327, 330) and uses it to generate a first result signal (333, 334) for signaling an asymmetrical error state ( push-pull error) generated, • This signaling of this asymmetrical error state (differential mode error) is omitted in the case of a purely symmetrical error state (common mode error). device after claim 1 characterized in that • it has a sub-device for detecting and signaling a symmetrical error state (common-mode error) and • that a symmetrical error state is defined in such a way that it affects all of the m reception loops (S1, S2, S3), but at least all signal paths of the den receiving loops (S1, S2, S3) relates to downstream signal paths in cooperation with the m receiving loops (S1, S2, S3), and • that the device has an amplifier (313) controlled by a gain control signal (318) which amplifies a level signal (310, 312, 510) in a controlled manner to form an amplifier output signal (314) and • that it has a first evaluator, in particular a control logic (317) , which evaluates the amplifier output signal (314) or a variable (316) derived from it by comparison with a second threshold value and/or third threshold value, in particular by forming a difference or an evaluation function, and • that said first evaluator, in particular the control logic (317 ), which emits the amplifier control signal (318) for readjustment of the controllable amplifier (313) in such a way that during a measurement run at least one of the amplifier output signals (314) fully drives the subsequent signal processing chain or sequence without overdriving it, • full drive without overdrive means that the value of at least one amplifier output signal (314) ob above the second threshold value and below the third threshold value, and • that it has a second evaluation unit, in particular a comparator (343, 344), which evaluates the amplifier control signal (318) with a fourth threshold value (335) and/or with a fifth threshold value (336) and • that the second evaluation unit, in particular the comparator (343, 344), sends an error message (337, 338) for a symmetrical error state when the amplifier control signal (318) exceeds or falls below the fourth and/or fifth threshold value a corresponding second result signal (337) and/or third result signal (338). Device according to one or more of the preceding claims, characterized in that • that it has a further excitation current source (501) modulated by means of an associated further excitation signal (522), which excites the excitation loop (2) of the inductive rotation angle sensor head (303) with at least one additional superimposed further excitation current and • that it has at least one sub-device with two inputs (SIG1, SIG2) for each triangular signal (308), which consists of the further excitation signal (522) or a control signal for the rectification (521) correlating therewith as the first input signal (SIG1) of the sub-device and the relevant triangular signal (308) as a second input signal (SIG2) of the sub-device for each triangular signal (308) generates a further level signal assigned to the sub-device of a total of m, but at least three, further level signals (510) by synchronous demodulation and/or scalar product formation, this sub-device being able in particular to be a synchronous demodulator, and • that the first and further excitation signals (322, 522) are orthogonal in relation to the synchronous demodulation and/or the scalar product formation in the synchronous demodulation block (309, 509), orthogonal meaning that when the two excitation signals in question (322, 522) would be combined with one another by a synchronous demodulation block (309, 509) as its first input signal (SIG1) and its second input signal (SIG2), the output signal of this synchronous demodulation block (309, 509) would then be zero except for control errors and system noise, and • that the device has a second adder (524), which forms a second summation signal (526) from the m, but at least three, further level signals (510) by summation based on a common reference potential, in particular based on an offset, and • that it has a third evaluation unit, in particular a comparator (541, 542), which compares the sum signal (526) with a sixth threshold value (527, 330) and from this a fourth result signal (533, 534) for signaling an asymmetric error state ( push-pull error) generated. Device according to one of the preceding claims, characterized in that • the device has an evaluation device, in particular a control logic (317), which calculates a plurality of measured values (345, 545), which are determined one after the other or in parallel, by averaging and/or calculating the difference or compares it with other target values and • that the evaluation device, in particular a control logic (317), at least stores a result of the comparison of the measured values (345) determined in this way. device after claim 4 characterized in that • the evaluation device causes a signal generator (320) to change an excitation signal (322, 522) at least temporarily by means of a control signal as a function of the comparison result.

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