METHOD AND CIRCUIT ARRANGEMENT FOR DETERMINING THE INDUCTIVITY OF A MEASURING COIL

Abstract

The invention relates to a method for determining an inductance (L) of a measuring coil (2), comprising the following steps: switching on and switching off a coil current (I) flowing through the measuring coil (2) at times which are predefined with the aid of a clock signal (V3); generating a PWM signal (out) with a duty cycle that is representative for the inductance (L) of the measuring coil (2), in that the PWM signal (out) is generated with rising and falling PWM signal edges which, on the one hand, are triggered simultaneously with the switching on or switching off of the coil current (I) and, on the other hand, are triggered as soon as a value of the coil current (I) reaches a predetermined trigger threshold (TRG), or alternatively both are triggered as soon as a value of the coil current (I) reaches a predetermined trigger threshold (TRG); evaluating the PWM signal (out) for determining the inductance (L). For the purpose of an advantageous temperature compensation, the method further comprises the following steps: detecting a stationary maximum value (Imax) of the coil current (I) after switching on the coil current (I); predetermining the trigger threshold (TRG) as a function of the detected maximum value (Imax). The invention further relates to a circuit arrangement (1) for carrying out the method.

Description

[0001] Description

[0002] Method and circuit arrangement for determining the inductance of a measuring coil

[0003] The present invention relates to a method and a circuit arrangement for determining the inductance of a measuring coil. Furthermore, the invention relates to the use of such a method or such a circuit arrangement.

[0004] Such methods and circuit arrangements are known from the prior art in a variety of designs, for example for determining the inductance of a measuring coil of a position sensor or an angle sensor.

[0005] Inductance is usually determined by integrating the measuring coil as an electrical component into a flip-flop (e.g., monostable) or oscillating circuit in such a way that its inductance defines a time constant, pulse duration, or frequency of a signal generated by the circuit. The inductance can be determined by appropriately evaluating this measurement signal.

[0006] Particularly with a measuring coil made of metallic material (e.g., copper), the problem arises that the measurement signal, or a time constant determined from it, depends not only on the inductance but also on an ohmic resistance component of the impedance of the measuring coil, and this resistance component is often highly temperature-dependent. The temperature dependence of the resistance value leads to corresponding measurement errors in the determination of the inductance when the method is applied in environments with fluctuating temperatures.

[0007] DE 41 20 861 C2 discloses a travel measuring device for measuring two angles, wherein the inductance of a respective measuring coil is determined for each of these angle measurements. To avoid temperature dependence of the measurement result (temperature compensation), this known measuring device further comprises a reference coil, and a ratio between the inductances of the two coils (measuring coil and reference coil) exposed to the same environmental influences is determined. DE 43 04 061 B4 discloses a circuit arrangement for evaluating measuring reactances, in which a reference coil, also exposed to the same environmental influences, is used for a temperature-compensated determination of the inductance of a measuring coil.

[0008] The disadvantages, however, are that a reference coil requires additional space, creates additional weight, and requires additional electrical wiring and connections. Furthermore, it also increases costs.

[0009] With regard to the temperature drift of the ohmic resistance component of the measuring coil that needs to be compensated, it should be noted that due to manufacturing variations, a reference coil usually does not have exactly the same resistance value or the same temperature drift as the actual measuring coil, which can lead to measurement errors.

[0010] Another approach for temperature compensation is to measure the temperature of the measuring coil using a separate temperature sensor and to correct a measured inductance based on the measured temperature.

[0011] When measuring temperature using a temperature sensor, in addition to the associated additional costs, there is the problem of ensuring sufficient synchronization between the temperature measured by the sensor and the temperature of the measuring coil during rapid temperature changes. Since this is often only achieved to a limited extent in practice, this type of temperature compensation also results in measurement errors.

[0012] It is an object of the present invention to show a novel way of enabling a temperature-compensated determination of the inductance of a measuring coil while avoiding the disadvantages of the prior art explained above.

[0013] According to a first aspect of the present invention, this object is achieved by a method for determining an inductance of a measuring coil, comprising the steps of: switching on and off a coil current flowing through the measuring coil at switch-on times and switch-off times which are predetermined by means of a clock signal,

[0014] Generating a PWM signal with a duty cycle representative of the inductance of the measuring coil by generating the PWM signal with rising and falling PWM signal edges, which are triggered simultaneously with the switching on or off of the coil current, and are triggered as soon as a value of the coil current reaches a predetermined trigger threshold, or both are triggered as soon as a value of the coil current reaches a predetermined trigger threshold,

[0015] Evaluating the PWM signal to determine its duty cycle and, therefrom, the inductance of the measuring coil, wherein the method for compensating for a temperature dependence of the duty cycle of the PWM signal due to a temperature dependence of an ohmic resistance of the measuring coil further comprises the steps of:

[0016] Detecting a stationary maximum value of the coil current after switching on and before the subsequent switching off of the coil current,

[0017] Specifying the trigger threshold depending on the recorded stationary maximum value of the coil current.

[0018] The basic idea of ​​the invention is to exploit a temperature dependence of the ohmic resistance component of the measuring coil in order to use the measuring coil itself as a temperature sensor.

[0019] Advantageously, the method of inductance determination and the implementation of temperature compensation are linked in a synergistic manner.

[0020] The invention advantageously eliminates the need for an additional reference coil and an additional temperature sensor. Therefore, the invention saves space and weight. Furthermore, it advantageously simplifies the electrical wiring and contacts between the measuring coil and a circuit arrangement intended for determining the inductance, which is significant for many applications.

[0021] Such a circuit arrangement can, for example, be contained in a separate control device (e.g. control unit) located remotely from the measuring coil and in this case, for example, be "wired" to the measuring coil.

[0022] Alternatively, such a control device or circuit arrangement can be structurally combined with the measuring coil, e.g. arranged together with the measuring coil on a common circuit board.

[0023] The aforementioned generation of a PWM signal with a duty cycle representative of the inductance of the measuring coil is essential for determining the inductance. By appropriately evaluating the PWM signal or its duty cycle, the inductance of the measuring coil can be determined.

[0024] In one embodiment, the evaluation of the PWM signal includes low-pass filtering to obtain a signal (e.g., a voltage signal) representative of the duty cycle or thus the inductance. If desired, the latter signal can be subjected to analog-to-digital conversion (and possibly further digital processing) to obtain, as an alternative or in addition to an analog signal, a digital signal (data signal) representative of the inductance (or a measured value in the case of an inductive sensor).

[0025] In one embodiment, the PWM signal is evaluated using digital signal processing. For example, the durations of pulses and / or pauses of the PWM signal relevant for determining the inductance can be "counted" (see, for example, DE 41 20 861 C2) to determine the duty cycle or thus the inductance of the measuring coil. Based on the result of such a digital count, a digital signal (data signal) representative of the inductance (or measured value of a sensor) can then be generated at the digital level.

[0026] The PWM signal could also be subjected to an analog / digital conversion for the purpose of digital evaluation, for example, in order to subsequently determine the duty cycle and thus the inductance or measured value of the sensor using digital signal processing.

[0027] In all such variants, it may be provided, for example, that the digital evaluation steps are carried out using a program-controlled computer device such as a microcontroller or the like. Alternatively or additionally, the use of an application-specific integrated circuit (ASIC) or the like may also be considered for the evaluation.

[0028] The clock signal can, in particular, be a periodic square wave signal.

[0029] For "switching on" and "switching off" the coil current supply, it can be provided that a predetermined voltage (e.g. a supply voltage of the circuit arrangement used) and a voltage of zero are alternately applied to the measuring coil or to a current path containing the measuring coil.

[0030] Alternatively, for "switching on" and "switching off" the coil current supply, it can be provided that a predetermined first voltage and a different predetermined second voltage, both of which are not equal to zero, are alternately applied to the measuring coil or the current path.

[0031] The switch-on and switch-off times of the coil current supply can, for example, be set to coincide with clock signal edges, for which the respective switching operations can be triggered, for example, by the respective clock signal edges. The frequency of the clock signal can correspond to the frequency of the switch-on (or switch-off) operations of the coil current supply, or, for example, be a multiple of it. In the latter case, the switch-on (or switch-off) operations can then be triggered by the edges of a correspondingly frequency-divided version of the clock signal.

[0032] The PWM signal can be generated, for example, with PWM signal edges that are triggered simultaneously with the coil current being switched on (e.g., rising or falling PWM signal edges), and that are triggered as soon as a value of the coil current, which increases as a result of the switched-on current supply, exceeds a predefined trigger threshold (e.g., falling or rising PWM signal edges). Alternatively, the PWM signal can be generated, for example, with PWM signal edges that are triggered simultaneously with the coil current being switched off (e.g., rising or falling PWM signal edges), and that are triggered as soon as a value of the coil current, which decreases as a result of the switched-off current supply, falls below a predefined trigger threshold (e.g., falling or rising PWM signal edges).

[0033] Alternatively, the PWM signal can be generated, for example, with rising and falling PWM signal edges, which on the one hand (e.g. rising or falling PWM signal edges) are triggered as soon as a value of the coil current increasing as a result of the switched-on current supply exceeds a predetermined trigger threshold, and on the other hand (e.g. falling or rising PWM signal edges) are triggered as soon as a value of the coil current decreasing as a result of the switched-off current supply falls below a predetermined trigger threshold.

[0034] In all three cases, the PWM signal can be generated with a constant period, but with a duty cycle that depends on the inductance of the measuring coil.

[0035] The increase in the value of the coil current after each switching on of the coil current as well as the decrease in the value of the coil current after each switching off generally follows an approximately exponential course with a characteristic time constant that depends on the inductance and the ohmic resistance component of the measuring coil.

[0036] In the case of a preferred use of a "series resistor" in series with the measuring coil within the scope of the invention, which can also be used as a "measuring resistor" to detect the coil current, the above-mentioned exponential curves or thus the characteristic time constants also depend on the value of this series resistor.

[0037] The use of such a series resistor has the advantage, for example, that the aforementioned exponential curves (and thus the representation of the inductance by the PWM duty cycle) are less dependent on the properties of the current source used (e.g., the internal resistance of the current source). Since the series resistor can easily be designed with a very small temperature coefficient compared to the temperature coefficient of the ohmic resistance component of the measuring coil, compensating for the temperature dependence of the duty cycle of the PWM signal solely based on the temperature dependence of the ohmic resistance of the measuring coil is generally completely sufficient in practice.

[0038] For temperature compensation, according to the invention, a steady-state maximum value of the coil current is detected after switching on and before the subsequent switching off of the coil current. Assuming an exponential time profile, the maximum current value would theoretically only be reached after an infinite waiting period (after switching on). Within the scope of the invention, this detection is to be understood from a practical perspective as meaning that a waiting period after which the coil current value would change only insignificantly, in particular, for example, by less than 10%, or for example, by less than 5%.

[0039] The trigger threshold is then specified accordingly depending on the previously detected maximum value of the coil current, ie, for example, in the event of a temperature change of the measuring coil, it is changed in such a way that this at least partially compensates for the influence of the temperature change on the PWM duty cycle (temperature compensation).

[0040] Temperature compensation advantageously reduces or completely eliminates the dependence of the PWM duty cycle on the temperature of the measuring coil. This is because the stated maximum value of the coil current does not depend on the inductance, but only on the ohmic resistance of the measuring coil, which in turn depends only on the temperature of the measuring coil. The maximum value of the coil current, or, for example, a value corresponding to this maximum value (or

[0041] A signal representing the measured value (approximate value) (e.g. voltage drop across the measuring resistor after a sufficiently "long" duty cycle of the current supply) can thus be advantageously used as a measure of the temperature of the measuring coil.

[0042] Based on the respective measured variable, temperature compensation can be achieved by appropriately specifying or adjusting (updating) the trigger threshold in response to temperature changes. In one embodiment of the invention, this is done analogously, for example, by feeding a measurement voltage dependent on the measuring coil temperature via a resistor network to a reference input of a comparator, whose output signal triggers a corresponding PWM signal edge.

[0043] As already mentioned, this measuring voltage can in particular be, for example, a voltage drop across the series resistor mentioned, which in this respect can then also be referred to as a measuring resistor (for detecting the value of the coil current).

[0044] Alternatively, it is possible, for example, to subject the temperature-dependent voltage mentioned to an analog / digital conversion, ie to record it digitally, in order to then realize the temperature compensation by means of digital algorithms of a software running on a digital data processing device (e.g. microcontroller).

[0045] In one embodiment of the invention, it is provided that the steps for compensating the temperature dependence of the duty cycle are carried out after each switching on of the coil current.

[0046] In this case, the reaction time of the temperature compensation is minimal, since after each "measurement cycle", which can be defined, for example, from one switch-on time to the next switch-on time, or from one switch-off time to the next switch-off time, the trigger threshold is redefined (updated) for the purpose of temperature compensation.

[0047] However, a more significant advantage of this embodiment in practice is that (in contrast to some of the alternative embodiments described below) the PWM signal is generated continuously "undisturbed" and can therefore be evaluated with sufficient accuracy, in particular, for example, using simple low-pass filtering.

[0048] In one embodiment of the invention, the steps for compensating for the temperature dependence of the duty cycle are performed after every nth switching on of the coil current, where "n" denotes an integer that can, for example, be in the range from 2 to 100. In this case, special "temperature compensation cycles" can be provided, in which the time period between switching on and the subsequent switching off of the coil current is larger than in "measurement cycles" for determining the inductance.

[0049] The longer time period can ensure that the coil current reaches its stationary maximum value at the end of the temperature compensation cycle or the accuracy of the detection of this stationary maximum value of the coil current can be increased.

[0050] In contrast, in the measuring cycles (for inductance determination), such time periods between switching operations can also be shorter if necessary, since the coil current does not have to reach the stationary maximum value but only the trigger threshold.

[0051] The trigger threshold can be operationally set, i.e., within a temperature range expected in the application situation, or, for example, for a specific reference temperature (e.g., room temperature), for example, in a range of 5% to 95%, preferably 10% to 90%, of the steady-state maximum value of the coil current. In one embodiment, the trigger threshold is either significantly above 50% (e.g., at least 70%) or significantly below 50% (e.g., at most 30%) of the steady-state maximum value of the coil current.

[0052] If the temperature compensation is carried out after every second switching on of the coil current (n = 2), this generally results in the advantage of a PWM signal that can be evaluated with sufficient accuracy in a simple manner, e.g. by means of low-pass filtering.

[0053] If the temperature compensation is only performed after every third, fourth, fifth, ... switching on of the coil current (n > 2), the correspondingly rather sporadically "interspersed" temperature compensation cycles can represent a certain disturbance of the PWM signal, which makes a correspondingly longer time constant useful in the case of an evaluation of the PWM signal by means of low-pass filtering (averaging over many cycles).

[0054] In this case (n > 2), in order to achieve a higher evaluation quality, a digital evaluation of the PWM signal may be appropriate, e.g. by counting the relevant time periods or by analog / digital conversion with subsequent evaluation by a digital data processing device.

[0055] In one embodiment of the invention, it is provided that the execution of the steps for compensating the temperature dependence of the duty cycle is commanded by a control signal as needed.

[0056] In this case, a control device outputting the control signal can, for example, be arranged separately from a circuit arrangement or control device by means of which the remaining steps of the method for detecting the impedance are realized.

[0057] Alternatively, and particularly interesting, for example, when determining the inductance of a measuring coil of a sensor (e.g., position sensor or angle sensor), a single (structurally combined) control device can also be provided in this embodiment, by means of which both the determination of the inductance is carried out, i.e., the actual sensor functionality, and the control signal for commanding a temperature compensation (e.g., a "temperature compensation cycle") is generated.

[0058] When using the method according to the invention for operating a sensor (e.g. position sensor or angle sensor), a control device can be provided, for example, to which the measuring coil is structurally arranged or connected (via an electrical line connection such as cabling) and which, for example, only generates the said control signal before each actual use of the sensor, and optionally also from time to time (e.g. periodically) during such use.

[0059] Finally, with regard to the times at which the temperature compensation is carried out, an embodiment is also possible in which a control device used to carry out the method according to the invention (with means for the current supply, the current detection, the current value comparison with the trigger threshold, etc.) has a digital interface (e.g. CAN interface or the like) via which a control signal can be supplied externally, e.g. via a digital communication bus (e.g. CAN bus), with which the control device can be set to different operating modes, e.g. according to the embodiments explained above with temperature compensation after each switch-on, after every nth switch-on, or as required.

[0060] In one embodiment of the invention, the measuring coil is formed by a metallic material, e.g., by a winding made of metallic material or by a conductor track made of metallic material on a circuit board. At least some of the electrical and / or electronic components of the circuit arrangement used to implement the method according to the invention can also be arranged on this circuit board.

[0061] The metallic material of the measuring coil can be, for example, a copper material (copper or copper alloy).

[0062] The use of such a material to form the measuring coil is preferred in many applications, but usually results in a relatively large linear resistance-temperature coefficient. Technical copper has a linear resistance-temperature coefficient of approximately 4 x 10' 3 K -1, which causes the ohmic resistance value of the measuring coil to vary by about 40% when the temperature fluctuates between room temperature (25°C) and 125°C.

[0063] However, the temperature compensation achieved by the invention can advantageously eliminate the influence of even such large resistance changes on the PWM signal representative of the inductance.

[0064] In this respect, the use of a method and / or a circuit arrangement of the type described here is ideally suited for mobile applications, for example in a vehicle, for example for determining an inductance of a measuring coil of an inductive sensor arranged in a vehicle, such as a position sensor or an angle sensor.

[0065] In one embodiment of the invention, it is provided that the coil current is passed through a measuring resistor (as a "series resistor") arranged in series with the measuring coil and the detection of the value of the coil current is realized by detecting a voltage drop across the measuring resistor.

[0066] The use of such a measuring resistor not only allows the simple generation of a voltage signal representative of the coil current (proportional to the coil current), but also has the advantages explained above with regard to such a series resistor.

[0067] It is understood that the voltage drop across a measuring resistor can be used both to detect the value of the coil current for the purpose of triggering the relevant PWM signal edges (as soon as the value of the coil current reaches the specified trigger threshold) and to detect the stationary maximum value of the coil current as part of the temperature compensation.

[0068] According to a second aspect of the present invention, the object mentioned above is achieved by a circuit arrangement for determining an inductance of a measuring coil, comprising an energizing device for switching on and off a coil current flowing through the measuring coil at switch-on times and switch-off times that are predetermined by means of a clock signal, a PWM signal generating device for generating a PWM signal with a duty cycle representative of the inductance of the measuring coil, in that the PWM signal is generated with rising and falling PWM signal edges, which are triggered on the one hand simultaneously with the switching on or with the switching off of the coil current and on the other hand are triggered as soon as a value of the coil current reaches a predetermined trigger threshold, or alternatively both are triggered as soon as a value of the coil current reaches a predetermined trigger threshold,comprising a trigger specification device for specifying the trigger threshold, a detection and comparison device for detecting a value of the coil current and for comparing this value with the predetermined trigger threshold, an evaluation device for evaluating the PWM signal in order to determine its duty cycle and therefrom the inductance of the measuring coil, wherein the PWM signal generation device is designed for compensation of a,

[0069] Temperature dependence of the duty cycle of the PWM signal due to a temperature dependence of an ohmic resistance of the measuring coil further comprises: a detection and specification update device for detecting a stationary maximum value of the coil current after switching on and before the subsequent switching off of the coil current and for specifying the trigger threshold depending on the detected stationary maximum value of the coil current.

[0070] The embodiments and special configurations described here for the method according to the invention can, individually or in any combination, also be provided in an analogous manner as embodiments or special configurations of the circuit arrangement according to the invention, and vice versa.

[0071] In one embodiment of the circuit arrangement, it is provided that the detection and comparison device comprises: a measuring resistor which is arranged in series with the measuring coil when determining the inductance of the measuring coil and via which the coil current is conducted, a comparator for comparing a voltage drop across the measuring resistor with a trigger threshold predetermined as a trigger voltage.

[0072] In a further development of this embodiment, it is provided that the detection and specification updating device has a sample-and-hold circuit for sampling and holding a voltage drop across the measuring resistor.

[0073] According to a further aspect of the present invention, a use of a method and / or a circuit arrangement of the type described here for determining an inductance of a measuring coil of an inductive sensor is proposed.

[0074] The invention is particularly interesting for mobile applications, for example, in a vehicle. In such an application, the sensor can be, for example, a position sensor or angle sensor arranged in the vehicle, by means of which a position (e.g., adjustment position) or an angle (e.g., adjustment angle) of a displaceable component of the vehicle is determined based on the determined inductance.

[0075] The component can be, for example, a throttle valve of a vehicle's internal combustion engine or a body part such as a door or flap of the vehicle.

[0076] The invention will be further described below using exemplary embodiments with reference to the accompanying drawings. They show:

[0077] Fig. 1 shows a circuit arrangement for determining an inductance of a measuring coil according to an embodiment,

[0078] Fig. 2 shows exemplary time courses of various current and voltage values ​​during operation of the circuit arrangement of Fig. 1,

[0079] Fig. 3 shows a circuit arrangement for determining an inductance of a measuring coil according to a further embodiment, and

[0080] Fig. 4 shows exemplary time courses of various current and voltage values ​​during operation of the circuit arrangement of Fig. 3.

[0081] Fig. 1 shows a circuit arrangement 1 for determining an inductance L of a measuring coil 2. The measuring coil 2 is, for example, a measuring coil of an inductive sensor, by means of which, for example, an adjustment position or an adjustment angle of a displaceable component in a vehicle is measured. This is based on a specific dependence of the inductance L on such a position or angle.

[0082] Before the circuit arrangement 1 shown in Fig. 1 is described in more detail, the steps of the method carried out with the circuit arrangement 1 are first described as follows with reference to Fig. 2: a) Switching on and off a coil current I flowing through the measuring coil 2 at switch-on times and switch-off times that are predetermined with the aid of a clock signal V3 (in Fig. 2 the clock signal is plotted as a function of time t). In the example shown, the clock signal V3 is a periodic rectangular voltage signal (in the example 5V, 2 kHz, 50% duty cycle), whereby, as can be seen in Fig. 2, every third falling clock signal edge in the temporal progression defines a switch-on time for the current supply, and whereby the immediately preceding rising clock signal edges each define a switch-off time.This results in this example that periodic current supply cycles are clocked by a frequency-divided version of the clock signal used (here: V3), and that the on-time phases are longer than the off-time phases. In the example of Fig. 2, the frequency of the on-time processes (or off-time processes) corresponds to half the frequency of the clock signal V3. b) Generation of a PWM signal "out" with rising and falling PWM signal edges, which are triggered on the one hand (rising edges in Fig. 2) as soon as a value of the coil current I, which decreases due to the switched-off current supply, falls below a predetermined trigger threshold "TRG", and on the other hand (falling edges in Fig. 2) as soon as a value of the coil current I, which increases due to the switched-on current supply, exceeds the predetermined trigger threshold TRG.

[0083] In each switch-on phase of the current supply, a value of the coil current I increases, whereas in each switch-off phase of the current supply, the value of the coil current I decreases again, whereby in both phases the value of the coil current I follows an approximately exponential curve with a characteristic time constant (here, approximately: L / (R5+R6)). The trigger threshold TRG is expediently specified in a middle range of the exponential curve of the coil current I. Due to the dependence of the curves of the coil current I and their time constants on the inductance L of the measuring coil 2, the PWM signal out has a duty cycle representative of the inductance L. c) Evaluate the PWM signal out to determine its duty cycle and, from this, the inductance L of the measuring coil 2 and provide it, for example, as a digital data signal "ind(L)", for example on a digital data bus of a vehicle.

[0084] In the example of Figs. 1 and 2, a duty cycle of the PWM signal out, defined, for example, as the ratio of pulse duration to PWM period duration, increases with increasing inductance L and decreases with decreasing inductance L. Using this relationship, which may be calculated in advance or empirically determined in advance, the inductance L of the measuring coil 2 can be determined by evaluating the PWM signal out.

[0085] It should be noted, however, that if a temperature of the measuring coil 2 varies, for example due to a varying ambient temperature in a mobile application, a resistance R of the measuring coil 2 (ie ohmic component of the impedance of the measuring coil 2) also varies with variation of the measuring coil temperature, so that the said duty cycle of the PWM signal out in practice depends not only on the inductance L but also on the temperature of the measuring coil 2.

[0086] Therefore, to compensate for this temperature dependence of the duty cycle of the PWM signal out (due to a temperature dependence of the ohmic resistance R of the measuring coil 2), the method further comprises the steps of: d) detecting a steady-state maximum value "Imax" of the coil current I after switching on (but before the subsequent switching off) of the coil current I, and e) specifying (or updating) the trigger threshold TRG as a function of the detected steady-state maximum value Imax of the coil current I.

[0087] This temperature compensation advantageously eliminates or at least reduces the dependence of the duty cycle of the signal out on temperature, which otherwise exists as explained.

[0088] In the example of the time course shown in Fig. 2, it is assumed that neither the inductance L nor the temperature (and thus the resistance R) varies, so that the duty cycle of the PWM signal out does not change over time.

[0089] However, if, for example, the inductance L were to increase while the temperature remained constant, the duty cycle would increase, and based on this increase in L would be determined during the evaluation (although the determined maximum value Imax of the coil current I remains constant). If, for example, the temperature and, consequently, the resistance R were to increase while the inductance L remained constant, the determined maximum value Imax of the coil current I would decrease. Temperature compensation would then change the trigger threshold TRG depending on the detected value Imax so that the duty cycle remains constant, and thus the constancy of the inductance L is correctly detected by evaluating the duty cycle.

[0090] Regarding the detection of the steady-state maximum value Imax of the coil current I after switching on, Fig. 2 clearly shows that after the "waiting time" selected in the example, corresponding to 1.5 periods of the clock signal V3, the value of the coil current I would change only insignificantly over time t (here, for example, by less than 1%). The value Imax of the current I detected at the end of this waiting time therefore corresponds very precisely to the maximum current "Imax.theo" theoretically resulting only after an infinite waiting time based on an exponential time curve with a time constant "tau" l(t) = Imax.theo x (1 - exp(-t / tau)). Within the scope of the invention, detection preferably occurs after a time period of at least 2 x tau, more preferably at least 4 x tau. The detected value Imax is thus a sufficiently good approximation of the actual maximum current.

[0091] Returning to Fig. 1, the circuit arrangement 1 has an energizing device 10, by means of which the coil current I flowing through the measuring coil 2 can be switched on and off during operation of the circuit arrangement 1, the corresponding switch-on times and switch-off times being specified by means of the clock signal V3.

[0092] As can be seen from Fig. 1, further frequency-divided clock signals or control signals S1 and S2 are formed from the clock signal V3, the time course of which is also shown in Fig. 2.

[0093] The generation of the clock signal V3 is symbolized in Fig. 1 by a corresponding voltage source. Likewise, an electrical supply of the circuit arrangement 1 with, in the example, two supply voltages VCC5 and VCCI .8 is symbolized in Fig. 1 by corresponding voltage sources. The switching on and off of the current supply to the measuring coil 2 (coil current I) takes place, as can be seen in Fig. 1, by means of a controllable switching device SW1 (controlled by S1) and a controllable switching device SW3 (controlled by V3), which alternately connect a first terminal of the measuring coil 2 (at the corresponding switching times) either to the supply voltage VCCI .8 or via a resistor R7 to a ground potential GND.

[0094] A second terminal of the measuring coil 2 is connected to the ground potential GND via a measuring resistor R5, R6 (series connection of individual resistors R5 and R6).

[0095] In the example of Fig. 1, the switching device SW3 is also used to generate an additional (auxiliary) supply voltage Vss that is different from the supply voltages VCC1, VCC8 and VCC5. The additional supply voltage Vss is generated from the supply voltage VCC5 using a voltage converter circuit, which, as shown, is configured with the components SW3, R7, R8, C3, D1, D2, and C4 and which also uses the clock signal V3 for its function.

[0096] The circuit arrangement 1 further comprises a PWM signal generating device 20, by means of which the PWM signal out can be generated with the duty cycle representative of the inductance L of the measuring coil 2 by generating the PWM signal out with correspondingly triggered PWM signal edges (see Fig. 2).

[0097] In the example shown in Fig. 1, as already explained with reference to Fig. 2, the rising PWM signal edges are triggered at the same time as the value of the falling coil current I falls below the predetermined trigger threshold TRG, and the falling PWM signal edges are triggered as soon as the value of the rising coil current I reaches or exceeds the predetermined trigger threshold TRG again.

[0098] In particular, since a value is selected for the trigger threshold TRG that is significantly above 50% of the maximum current value Imax explained above, the triggering of the rising PWM signal edges occurs very shortly after the coil current I is switched off. The coil current I drops exponentially very steeply in the relevant time range, so that the trigger threshold TRG is always reached very quickly, with a variation in the inductance L of the measuring coil 2 having only a very slight influence on this time period until the trigger threshold TRG is reached (deviating from the circuit example according to Fig. 1, it could therefore also be provided, for example, that the rising PWM signal edge is completely rigidly coupled to the rising edge of the clock signal V3, ie each rising PWM signal edge is triggered simultaneously with a respective rising edge of the clock signal V3).

[0099] In contrast, the triggering of the falling PWM signal edges takes place in a time range in which the exponentially increasing curve of the coil current I is already relatively flat, so that a significant time period, which is relatively strongly dependent on the inductance L, elapses between the switching on of the coil current I and the reaching of the trigger threshold TRG.

[0100] The PWM signal generating device 20 has a trigger setting device 22 for setting the trigger threshold TRG and a detection and comparison device 24 for detecting a value of the coil current I and for comparing this value with the predetermined trigger threshold TRG.

[0101] In the example, the detection and comparison device 24 has the measuring resistor R5, R6 provided as a series circuit of individual resistors R5 and R6, which, as can be seen in Fig. 1, is arranged in series with the measuring coil 2 when the measuring coil 2 is connected to the circuit arrangement 1 and via which the coil current I is thus conducted.

[0102] Furthermore, the detection and comparison device 24 in the example comprises a comparator V1 (here, for example, an operational amplifier) ​​for comparing a voltage drop across the measuring resistor R5, R6 with the trigger threshold TRG, specified in this example as the trigger voltage VTRG. In this way, the PWM signal out is provided at the output of the comparator V1.

[0103] Here, a first (inverting) input of the comparator V1 is connected via a resistor R3 to a tap between the measuring coil 2 and the measuring resistor R5, R6 and a second (non-inverting) input of the comparator V1 is supplied with the trigger voltage VTRG.

[0104] Comparator V1 thus compares a time-varying voltage at its inverting input, which is representative of the value of I and is subsequently referred to as the first measurement voltage U1, with the trigger voltage VTRG applied as the "reference voltage" to its non-inverting input. If the latter is higher than the former, comparator V1 outputs a "high" level; otherwise, it outputs a "low" level.

[0105] The circuit arrangement 1 further comprises an evaluation device 30, schematically shown in Fig. 1 as a functional block, by means of which the PWM signal out is evaluated in order to determine its duty cycle and, from this, the inductance L of the measuring coil 2. The information about the value of the inductance L is output by the evaluation device 30, for example, in the form of a digital data signal ind(L).

[0106] A special feature of the circuit arrangement 1 is that the PWM signal generating device 20 further comprises a detection and specification updating device 26, with the aid of which the temperature compensation already explained above is effected, ie a temperature dependence of the duty cycle of the PWM signal out is eliminated or at least reduced, which would otherwise result from a temperature dependence of the ohmic resistance R of the measuring coil 2 and thus falsify the determination result concerning the inductance L.

[0107] By means of the detection and specification update device 26, the stationary maximum value Imax of the coil current I is detected periodically in the example shown in Fig. 1 after switching on, but before the subsequent switching off of the coil current I. Depending on the value Imax thus detected, the trigger threshold TRG (here: trigger voltage VTRG) is then specified or updated by means of the detection and specification update device 26.

[0108] In the example, the acquisition and specification update device 26 comprises a sample-and-hold circuit SH for sampling and holding a voltage drop across the measuring resistor R5, R6, hereinafter also referred to as the second measuring voltage U2, which is proportional and thus representative of the value of the coil current I. Specifically in connection with the measuring resistor (R5, R6), the term "voltage drop across the measuring resistor" also includes the embodiment provided in the example shown in Fig. 1, in which the second measuring voltage U2 in the narrower sense only drops across one of the two individual resistors (R5 and R6), here the individual resistor R6, of the measuring resistor (R5, R6) provided as a series circuit. This measure has the advantage, for example, that by appropriately dimensioning the individual resistors R5 and R6 and sampling the voltage U2 tapped between R5 and R6, a scaling of the relevant (ieby the sample-and-hold circuit SH). In the example, this scaling corresponds to a division of the first measurement voltage U1 in the ratio R6 / (R5 + R6).

[0109] In the example of Fig. 1, the sample-and-hold circuit SH comprises a controllable switching device SW2 (controlled by S2), a capacitor C2 and a voltage follower V2, which in the example is implemented by an operational amplifier wired accordingly for this purpose.

[0110] The switching device SW2 is controlled by means of a logic gate arrangement A1 (flip-flop), A2, A3, which is operated by the clock signal V3, and the control signal S2 generated thereby, such that at the appropriately defined times, the second measuring voltage U2 tapped between R5 and R6 is applied via the switching device SW2 to the capacitor C2, which acts as a "voltage storage device", which holds this voltage until the switching device SW2 next samples the voltage U2 at the tap between R5 and R6.

[0111] In the example shown, the sampled and held voltage U2 is then provided "low-impedance" at the output of the operational amplifier, i.e. output of the sample-and-hold circuit SH, with the aid of the operational amplifier connected as voltage follower V2.

[0112] A further advantageous feature of the detection and specification update device 26, which can be seen in the example of Fig. 1, is that the voltage U2 held by the sample-and-hold circuit SH (at the capacitor C2 or at the output of the voltage follower V2) is not used directly as the said trigger voltage VTRG (for defining the trigger threshold TRG), but is first converted into the trigger voltage VTRG with the aid of a resistor network, whereby a further "scaling" is advantageously realized.

[0113] In the example, the resistor network comprises, as shown, a "KoppeF" resistor R4 and a series circuit of individual resistors R1 and R2 connected to the supply voltage VCCI .8, with the resistor R4 being connected between an output of the sample-and-hold circuit SH (here: output of the voltage follower V2) and a tap of the series circuit (between R1 and R2). The voltage thus provided at the tap of the series circuit R1, R2 is used as the trigger voltage VTRG (and, as already explained, applied to an input of the comparator V1).

[0114] The aforementioned scaling of the trigger voltage VTRG can advantageously be provided, for example, in such a way that at a certain ("reference") temperature such as room temperature (e.g. 25°C), the trigger voltage VTRG at the output of the voltage follower V2 is equal to the voltage resulting from the dimensioning of R1 and R2 at the center tap of the series circuit R1, R2, so that at this temperature the voltage follower V2 does not intervene in the switching process of the comparator V1.

[0115] The aforementioned scaling of the trigger voltage VTRG by applying a voltage dependent on the measuring coil temperature, here the second measuring voltage U2, to the reference input of the comparator V1 via the resistor network R1, R2, R4 is at the same time an advantageous measure in the example shown, by means of which essential parameters of the desired temperature compensation can be set or optimized.

[0116] In the example, the values ​​of the resistors R4 as well as R5, R6 (in conjunction with the values ​​of R1 and R2) are designed in such a way that a reduction / increase in the first measuring voltage U1 caused by a temperature change is compensated as precisely as possible by a subsequent reduction / increase in the reference voltage VTRG, so that the pulse width of the PWM signal out is independent of the temperature.

[0117] This pulse-shaped signal (PWM signal out) output by the detection and comparison device 24 is input to the evaluation device 30, which, based on appropriate evaluation, finally provides the data signal ind(L) representative of the value of the inductance L.

[0118] In the evaluation device 30, which can, for example, represent a functional component of a control device provided for operating the corresponding sensor, a so-called "capture" unit can be provided, by means of which the PWM signal out is counted in order to determine the duty cycle or thus the value of the inductance L. Alternatively, the signal out can be smoothed, for example, using a low-pass filter and output as an analog measurement signal and / or converted by an A / D converter into a digital data signal.

[0119] In the case of a determination of the inductance L during the operation of a sensor, the signal resulting from the evaluation (e.g. data signal ind(L)) can represent the sensor signal representative of the respective sensor measurement variable (e.g. position, angle, etc.).

[0120] In the following description of further exemplary embodiments, the same reference numerals are used for components with equivalent functions. In doing so, only the differences from the previously described exemplary embodiment(s) are discussed, and otherwise, explicit reference is hereby made to the description of previous exemplary embodiments.

[0121] Fig. 3 shows a further embodiment of a circuit arrangement 1 for determining an inductance L of a measuring coil.

[0122] Fig. 4 shows, in a representation corresponding to Fig. 2, the temporal profiles of various current and voltage values ​​resulting from the operation of the circuit arrangement of Fig. 3.

[0123] In contrast to the already described embodiment of Figs. 1 and 2, in the example of Figs. 3 and 4 it is provided that the steps for compensating the temperature dependence of the duty cycle of the PWM signal out are not carried out after each switching on, but only after every eighth switching on of the coil current I.

[0124] After every seven "measurement cycles" to determine the inductance L, a "temperature compensation cycle" follows to update the trigger threshold TRG.

[0125] The circuit arrangement 1 shown in Fig. 3 corresponds in structure and function essentially to the embodiment of Fig. 1 already described.

[0126] However, a necessary circuit difference with regard to the aforementioned functional difference regarding the implementation of the temperature compensation cycles consists in the structure and thus function of a logic gate arrangement A4, A5, A6, A8, A9 of the example in Fig. 3. By means of this logic gate arrangement A4, A5, A6, A8, A9, a clocked control of the detection and specification updating device 26 (there: switching device SW2 controlled by control signal S2) and the current supply device 10 (there: switching device SW1 controlled by control signal S1) is also carried out, but in such a way that, as mentioned above, the temperature compensation steps are only carried out after every eighth switching on of the coil current I.

[0127] In this context, a further difference is that in the example of Fig. 3, the frequency of the switching-on (or switching-off) of the current supply to the measuring coil 2 during the measuring cycles is equal to the frequency of the clock signal V3.

[0128] A further difference lies in the specific manner in which the additional ("auxiliary") supply voltage Vss is generated. In the example of Fig. 3, a voltage generation circuit is designed for this purpose, as shown, using the components C3, D1, D2, C4, which uses the clock signal V3 not only for clocking but also as a "supply voltage source" (whereas in the example of Fig. 1, a corresponding supply is provided by the supply voltage VCC1.8).

[0129] With the invention and the described embodiments, inductive sensors can advantageously be operated with a particularly simple and reliable temperature compensation.

Claims

Patent claims 1. Method for determining an inductance (L) of a measuring coil (2), comprising the steps: Switching on and off of a coil current (I) flowing through the measuring coil (2) at switch-on and switch-off times which are specified by means of a clock signal (V3), Generating a PWM signal (out) with a duty cycle representative of the inductance (L) of the measuring coil (2) by generating the PWM signal (out) with rising and falling PWM signal edges, which are triggered on the one hand simultaneously with the switching on or off of the coil current (I) and on the other hand as soon as a value of the coil current (I) reaches a predetermined trigger threshold (TRG), or which are both triggered as soon as a value of the coil current (I) reaches a predetermined trigger threshold (TRG), Evaluating the PWM signal (out) to determine its duty cycle and from this the inductance (L) of the measuring coil (2), wherein the procedure for compensating a temperature dependence of the duty cycle of the PWM signal (out) due to a temperature dependence of an ohmic resistance (R) of the measuring coil (2) further comprises the following steps: Determining a steady-state maximum value (Imax) of the coil current (I) after switching on and before the subsequent switching off of the coil current (I), Specifying the trigger threshold (TRG) depending on the detected steady-state maximum value (Imax) of the coil current (I).

2. Method according to claim 1, wherein the steps for compensating the temperature dependence of the duty cycle are carried out after each switching on of the coil current (I).

3. Method according to claim 1, wherein the steps for compensating the temperature dependence of the duty cycle are carried out after each nth switching on of the coil current (I), where n is an integer, for example in the range of 2 to 100.

4. Method according to claim 1, wherein the execution of the steps for compensating the temperature dependence of the duty cycle is commanded as required by a control signal.

5. Method according to one of the preceding claims, wherein the measuring coil (2) is formed by a winding of metallic material or by a conductor track of metallic material on a circuit carrier plate.

6. Method according to one of the preceding claims, wherein the coil current (I) is passed through a measuring resistor (R5, R6) arranged in series with the measuring coil (2) and the detection of the value of the coil current (I) is realized by detecting a voltage (U1) across the measuring resistor (R5, R6).

7. Circuit arrangement (1) for determining the inductance (L) of a measuring coil (2), comprising a current-energizing device (10) for switching on and off a coil current (I) flowing through the measuring coil (2) at switch-on and switch-off times specified by means of a clock signal (V3), a PWM signal generation device (20) for generating a PWM signal (out) with a duty cycle representative of the inductance (L) of the measuring coil (2), by generating the PWM signal (out) with rising and falling PWM signal edges, which are triggered simultaneously with the switching on or off of the coil current (I) and are triggered as soon as a value of the coil current (I) reaches a specified trigger threshold (TRG), or both are triggered as soon as a value of the coil current (I) reaches a specified trigger threshold (TRG), comprising a trigger specification device (22) for specifying the trigger threshold (TRG), a detection and comparison device (24) for detecting a value of the coil current (I) and for comparing this value with the specified trigger threshold (TRG), an evaluation device (30) for evaluating the PWM signal (out) in order to determine its duty cycle and from this the inductance (L) of the measuring coil (2), wherein the PWM signal generation device (20) further comprises, for a compensation of a temperature dependence of the duty cycle of the PWM signal (out) due to a temperature dependence of an ohmic resistance (R) of the measuring coil (2): a detection and specification update device (26) for detecting a steady-state maximum value (Imax) of the coil current (I) after switching on and before the subsequent switching off of the coil current (I) and for specifying the trigger threshold (TRG) as a function of the detected steady-state maximum value (Imax) of the coil current (I).

8. Circuit arrangement (1 ) according to claim 7, wherein the detection and comparison device (24) comprises: a measuring resistor (R5, R6) which is arranged in series with the measuring coil (2) when determining the inductance (L) of the measuring coil (2) and through which the coil current (I) is passed, a comparator (V1 ) for comparing a voltage (U1 ) across the measuring resistor (R5, R6) with a trigger threshold (TRG) specified as a trigger voltage (VTRG).

9. Circuit arrangement (1) according to claim 8, wherein the detection and preset update device (26) has a scan-hold circuit (SH) for scanning and holding a voltage (U2) across the measuring resistor (R5, R6).

10. Use of a method according to one of claims 1 to 6 and / or a circuit arrangement (1 ) according to one of claims 7 to 9 for determining an inductance (L) of a measuring coil (2) of an inductive sensor, in particular a position sensor or an angle sensor.