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

Description

[0001] The present invention relates to a method and a circuit arrangement for determining the inductance of a measuring coil. The invention further relates to the use of such a method or circuit arrangement.

[0002] Such methods and circuit arrangements are known in various designs from the prior art, for example for determining the inductance of a measuring coil of a position sensor or an angle sensor.

[0003] In most cases, determining inductance involves integrating the measuring coil as an electrical component into a flip-flop (e.g., monostable multivibrator) or oscillator 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 then be determined by appropriately evaluating this measurement signal.

[0004] Particularly with a measuring coil made of metallic material (e.g., copper), the problem arises that the measurement signal, or a time constant derived from it, depends not only on the inductance but also on an ohmic resistance component of the coil's impedance, and this resistance component is often strongly temperature-dependent. When using this method in environments with fluctuating temperatures, the temperature dependence of the resistance value leads to corresponding measurement errors in determining the inductance.

[0005] From DE 41 20 861 C2, a displacement measuring device for measuring two angles is known, wherein the inductance of a respective measuring coil is determined for each of these angle measurements. In order to avoid a 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 exposed to the same environmental influences (measuring coil and reference coil) is determined.

[0006] From DE 43 04 061 B4 a circuit arrangement for evaluating measurement reactances is known, in which a reference coil exposed to the same environmental influences is also used for a temperature-compensated determination of the inductance of a measuring coil.

[0007] However, a reference coil requires additional installation space, adds weight, and necessitates additional electrical wiring and connections. It also results in higher costs.

[0008] Regarding the temperature drift of the ohmic resistance component of the measuring coil that needs to be compensated for, it should be noted that due to manufacturing variations, a reference coil usually does not have the exact same resistance value or temperature drift as the actual measuring coil, which can lead to measurement errors.

[0009] Another approach to temperature compensation is to detect the temperature of the measuring coil using a separate temperature sensor and to correct a detected inductance based on the detected temperature.

[0010] When measuring temperature using a temperature sensor, aside from the associated additional costs, problems arise, such as 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 partially successful in practice, this type of temperature compensation also introduces measurement errors.

[0011] Furthermore, DE 42 04 060 A1 discloses a method for controlling the pressure in containers, in particular in metering and low-pressure furnaces as well as metal pumps, in which the pressure in the furnace interior is measured and controlled via a control loop. A pressure cascade control is implemented, whereby the actual pressure value and the setpoint pressure for the interior of the container are fed to a first control loop, and the resulting manipulated variable is added to the setpoint pressure, and the result is fed as a reference variable to a second control loop comprising a proportional valve.

[0012] It is an object of the present invention to demonstrate a novel method by which a temperature-compensated determination of the inductance of a measuring coil is made possible while avoiding the disadvantages of the prior art explained above.

[0013] According to a first aspect of the present invention, this problem is solved by a method for determining the inductance of a measuring coil, comprising the steps of: Switching on and off of a coil current flowing through the measuring coil at switch-on and switch-off times specified by a clock signal; 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 either simultaneously with the switch-on or switch-off of the coil current and, on the other hand, as soon as a value of the coil current reaches a specified trigger threshold, or both are triggered as soon as a value of the coil current reaches a specified trigger threshold; evaluating the PWM signal to determine its duty cycle and from this 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 following steps: Detecting a steady-state maximum value of the coil current after switching on and before the subsequent switching off of the coil current, and specifying the trigger threshold depending on the detected steady-state maximum value of the coil current.

[0014] 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.

[0015] Advantageously, the method of determining inductance and the achievement of temperature compensation are synergistically linked.

[0016] The invention advantageously eliminates the need for an additional reference coil as well as an additional temperature sensor. Therefore, the invention saves space and weight. Furthermore, it advantageously simplifies the electrical wiring and connections between the measuring coil and the circuit arrangement used to determine the inductance, a significant simplification for many applications.

[0017] Such a circuit arrangement can be contained, for example, in a separate control device (e.g., control unit) located away from the measuring coil, and in this case, for example, be "wired" to the measuring coil.

[0018] Alternatively, such a control device or circuit arrangement can be structurally integrated with the measuring coil, e.g. arranged together with the measuring coil on a common circuit carrier plate.

[0019] Determining the inductance requires generating a PWM signal with a duty cycle representative of the measuring coil's inductance, as mentioned above. By appropriately evaluating the PWM signal and its duty cycle, the inductance of the measuring coil can then be determined.

[0020] In one implementation variant, the evaluation of the PWM signal includes low-pass filtering to obtain a signal (e.g., a voltage signal) representative of the duty cycle and thus the inductance. If desired, this signal can be subjected to analog-to-digital conversion (and possibly further digital processing) to obtain, as an alternative or additional to an analog signal, a digital signal (data signal) representative of the inductance (or a measured quantity in the case of an inductive sensor).

[0021] In one implementation variant, the PWM signal is evaluated using digital signal processing. For example, the relevant durations of pulses and / or pauses of the PWM signal for determining inductance can be "counted" (see, e.g., DE 41 20 861 C2) in order to determine the duty cycle and 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 the measured quantity of a sensor) can then be generated at the digital level.

[0022] The PWM signal could also be subjected to analog / digital conversion for digital evaluation, for example, in order to subsequently determine the duty cycle and thus the inductance or measured quantity of the sensor using digital signal processing.

[0023] In all such variants, it may be possible, for example, to perform the digital evaluation steps using a program-controlled computer device such as a microcontroller or similar. Alternatively or additionally, the evaluation process may also involve the use of an application-specific integrated circuit (ASIC) or similar device.

[0024] The clock signal can, for example, be a periodic square wave signal.

[0025] To "switch on" and "switch off" the coil current, 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.

[0026] Alternatively, for "switching on" and "switching off" the coil current, 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.

[0027] The switching-on and switching-off times of the coil current can, for example, be scheduled to coincide with clock signal edges, for which the respective switching operations can be triggered by the respective clock signal edges. The frequency of the clock signal can correspond to the frequency of the switching-on (or switching-off) operations of the coil current, or it can be a multiple thereof, in which case the switching-on (or switching-off) operations can then be triggered by the edges of a correspondingly frequency-divided version of the clock signal.

[0028] The PWM signal can be generated, for example, with PWM signal edges that are triggered simultaneously with the switching on of the coil current (e.g., rising or falling PWM signal edges) and, on the other hand, are triggered as soon as a value of the coil current increasing as a result of the switched-on current exceeds a predetermined trigger threshold (e.g., falling or rising PWM signal edges).

[0029] Alternatively, the PWM signal can be generated, for example, with PWM signal edges that are triggered simultaneously with the switching off of the coil current (e.g., rising or falling PWM signal edges) and, on the other hand, are triggered as soon as a value of the coil current, which decreases as a result of the switched-off current, falls below a predetermined trigger threshold (e.g., falling or rising PWM signal edges).

[0030] Alternatively, the PWM signal can be generated, for example, with rising and falling PWM signal edges, which are triggered on the one hand (e.g., rising or falling PWM signal edges) as soon as a value of the coil current increasing as a result of the switched-on current exceeds a predetermined trigger threshold, and on the other hand (e.g., falling or rising PWM signal edges) as soon as a value of the coil current decreasing as a result of the switched-off current falls below a predetermined trigger threshold.

[0031] 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.

[0032] 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 curve with a characteristic time constant that depends on the inductance and the ohmic resistance component of the measuring coil.

[0033] In the case of a preferred use within the scope of the invention of a "series resistor" in series with the measuring coil, which can also be used as a "measuring resistor" to detect the coil current, the aforementioned exponential curves or thus the characteristic time constants also depend on the value of this series resistor.

[0034] 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 properties of the current source used (e.g., internal resistance of the current source).

[0035] Since the series resistor can easily be provided with a very small temperature coefficient compared to the temperature coefficient of the ohmic resistance component of the measuring coil, the compensation of the temperature dependence of the duty cycle of the PWM signal based solely on a temperature dependence of the ohmic resistance of the measuring coil is usually completely sufficient in practice.

[0036] For temperature compensation, according to the invention, a steady-state maximum value of the coil current is recorded after switching on and before the subsequent switching off of the coil current. Based on 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 recording is understood from a practical point of view to mean that a waiting period is sufficient after which the value of the coil current would only change negligibly, in particular, for example, by less than 10%, or, for example, by less than 5%.

[0037] The trigger threshold is then set according to the previously recorded maximum value of the coil current, i.e., for example, in the event of a temperature change of the measuring coil, it is changed so that this at least partially compensates for the influence of the temperature change on the PWM duty cycle (temperature compensation).

[0038] Temperature compensation advantageously reduces or eliminates the dependence of the PWM duty cycle on the temperature of the measuring coil. This is because the maximum coil current depends not 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 coil current, or, for example, a signal representing this maximum value (or an approximate value thereof) (e.g., the voltage drop across the measuring resistor after a sufficiently long period of energization), can thus be advantageously used as a measure of the measuring coil's temperature.

[0039] Based on the relevant measured variable, temperature compensation can be achieved by appropriately setting or updating the trigger threshold in response to temperature changes.

[0040] In one embodiment of the invention, this is done analogously, for example by supplying a measuring voltage dependent on the measuring coil temperature to a reference input of a comparator via a resistor network, with whose output signal a corresponding PWM signal edge is triggered.

[0041] As already mentioned, this measuring voltage can, for example, be a voltage drop across the aforementioned series resistor, which in this respect can also be described as a measuring resistor (for measuring the value of the coil current).

[0042] Alternatively, it is possible, for example, to subject the aforementioned temperature-dependent voltage to an analog-to-digital conversion, i.e., to digitally capture it, in order to then implement temperature compensation using digital algorithms of software running on a digital data processing device (e.g., microcontroller).

[0043] 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.

[0044] In this case, the reaction time of the temperature compensation is minimal, since after each "measurement cycle", which can be defined, for example, as the cycle 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 for temperature compensation is reset (updated).

[0045] However, a more significant advantage of this embodiment in practice is that (in contrast to some of the alternative designs described below) the PWM signal is generated continuously "undisturbed" and can therefore be evaluated with sufficient accuracy, especially, for example, using a simple low-pass filter.

[0046] 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 nth switching on of the coil current, where "n" denotes an integer, which may, for example, be in the range of 2 to 100.

[0047] In this case, special "temperature compensation cycles" may be provided in which the time interval between switching on and subsequently switching off the coil current is larger than in "measurement cycles" for determining the inductance.

[0048] The longer time period ensures that the coil current reaches its steady-state maximum value at the end of the temperature compensation cycle, or increases the accuracy of detecting this steady-state maximum value of the coil current.

[0049] In contrast, for measurement cycles (for determining inductance), such time intervals between switching operations may be shorter, since the coil current does not need to reach the steady-state maximum value but only the trigger threshold.

[0050] The trigger threshold can be operational, i.e., within a temperature range expected in the application, or, for example, for a specific reference temperature (e.g., room temperature), for instance, 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 provided to be 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.

[0051] If temperature compensation is performed after every second switching on of the coil current (n = 2), the advantage of a PWM signal that can be evaluated with sufficient accuracy in a simple way, e.g. by means of a low-pass filter, usually results again.

[0052] If temperature compensation is only performed after every third, fourth, fifth, ... switching on of the coil current (n > 2), the correspondingly rather sporadic 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).

[0053] For the sake of higher evaluation quality, in this case (n > 2) a digital evaluation of the PWM signal may alternatively be useful, e.g. by counting the relevant time durations or by analog / digital conversion with subsequent evaluation by a digital data processing device.

[0054] 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 as required by a control signal.

[0055] In this case, a control device that outputs 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 procedure for detecting the impedance are implemented.

[0056] Alternatively, and particularly interesting for example when determining the inductance of a measuring coil of a sensor (e.g., position sensor or angle sensor), in this embodiment a single (structurally combined) control unit can be provided, 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.

[0057] When using the inventive method for operating a sensor (e.g. position sensor or angle sensor), a control device can be provided, for example, on which the measuring coil is structurally arranged together or connected (via an electrical 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.

[0058] Finally, with regard to the timing of the temperature compensation, an embodiment is also possible in which a control device used to carry out the method according to the invention (with means for current supply, current detection, 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 from an external source, e.g. via a digital communication bus (e.g. CAN bus), with which the control device can be put into different operating modes, e.g. according to the embodiments explained above with temperature compensation after each switch-on, after each nth switch-on, or as required.

[0059] In one embodiment of the invention, the measuring coil is formed by a metallic material, for example, by a winding of metallic material or by a conductor track of metallic material on a circuit board. At least some of the electrical and / or electronic components of the circuit arrangement used to carry out the method according to the invention can also be arranged on this circuit board.

[0060] The metallic material of the measuring coil can be, for example, a copper material (copper or copper alloy).

[0061] The use of such materials for the construction of the measuring coil is preferred in many applications, but it generally results in a relatively large linear resistance-temperature coefficient. Technical-grade copper has a linear resistance-temperature coefficient of approximately 4 x 10⁻³ K⁻¹, which causes the ohmic resistance of the measuring coil to vary by about 40% when temperatures fluctuate between room temperature (25°C) and 125°C.

[0062] However, the temperature compensation achieved with the invention is advantageously able to eliminate the influence of even such large changes in resistance on the PWM signal representative of the inductance.

[0063] Therefore, 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 to determine the inductance of a measuring coil of an inductive sensor installed in a vehicle, such as a position sensor or an angle sensor.

[0064] In one embodiment of the invention, it is provided that the coil current is guided via a measuring resistor (as a "series resistor") arranged in series with the measuring coil and that the detection of the value of the coil current is realized by detecting a voltage drop across the measuring resistor.

[0065] 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.

[0066] It is understood that the voltage drop across a measuring resistor can be used both for detecting the value of the coil current for triggering the relevant PWM signal edges (as soon as the value of the coil current reaches the specified trigger threshold) and for detecting the steady-state maximum value of the coil current within the framework of temperature compensation.

[0067] According to a second aspect of the present invention, the aforementioned problem is solved by a circuit arrangement for determining the inductance of a measuring coil, comprising a current-energizing device for switching on and off a coil current flowing through the measuring coil at switch-on and switch-off times specified by means of a clock signal, a PWM signal generation device for 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 on the one hand simultaneously with the switching on or off of the coil current and on the other hand as soon as a value of the coil current reaches a specified trigger threshold, or alternatively both are triggered as soon as a value of the coil current reaches a specified trigger threshold, comprising a trigger setting device for specifying the trigger threshold,a detection and comparison device for detecting a value of the coil current and comparing this value with the specified trigger threshold, an evaluation device for evaluating the PWM signal in order to determine its duty cycle and from this the inductance of the measuring coil, , wherein the PWM signal generation device further comprises for compensation of a temperature dependence of the duty cycle of the PWM signal due to a temperature dependence of an ohmic resistance of the measuring coil: 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.

[0068] 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.

[0069] In one embodiment of the circuit arrangement, the detection and comparison device is provided to have: a measuring resistor, which is arranged in series with the measuring coil when determining the inductance of the measuring coil and through which the coil current is passed, a comparator for comparing a voltage drop across the measuring resistor with a trigger threshold specified as a trigger voltage.

[0070] In a further development of this embodiment, it is provided that the detection and specification update device has a scan-and-hold circuit for scanning and holding a voltage drop across the measuring resistor.

[0071] According to a further aspect of the present invention, the use of a method and / or a circuit arrangement of the type described herein for determining the inductance of a measuring coil of an inductive sensor is proposed.

[0072] 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 movable component of the vehicle is determined based on the measured inductance.

[0073] The component could be, for example, a throttle valve of an internal combustion engine of the vehicle, or, for example, a body part such as a door or flap of the vehicle.

[0074] The invention is further described below with reference to exemplary embodiments and the accompanying drawings. These depict: Fig. 1 a circuit arrangement for determining the inductance of a measuring coil according to an exemplary embodiment, Fig. 2 Exemplary time profiles of various current and voltage values ​​during the operation of the circuit arrangement of Fig. 1 , Fig. 3 a circuit arrangement for determining the inductance of a measuring coil according to a further embodiment, and Fig. 4 Exemplary time profiles of various current and voltage values ​​during the operation of the circuit arrangement of Fig. 3 .

[0075] Fig. 1 Figure 1 shows a circuit arrangement for determining the inductance L of a measuring coil 2. The measuring coil 2 is, for example, a measuring coil of an inductive sensor, which is used to measure, for example, the adjustment position or angle of a movable component in a vehicle. This is based on a specific dependence of the inductance L on such a position or angle.

[0076] Before the in Fig. 1 The circuit arrangement shown in 1 is described in more detail, with reference to the following: Fig. 2 The steps of the procedure carried out with circuit arrangement 1 are described as follows: a) Switching on and off of a coil current I flowing through the measuring coil 2 at switch-on and switch-off times that are specified by means 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), where, as in Fig. 2 It is evident that every third falling clock signal edge in the time sequence defines a switch-on time for the current, and the immediately preceding rising clock signal edges define a switch-off time. In this example, this means that periodic current cycles are clocked by a frequency-divided version of the clock signal used (here: V3) and that the switch-on phases are longer than the switch-off phases. In the example of Fig. 2 The frequency of the switch-on (or switch-off) processes corresponds to half the frequency of the clock signal V3. b) Generating a PWM signal "out" with rising and falling PWM signal edges, which on the one hand (in Fig. 2 (rising edges) are triggered as soon as a value of the coil current I, which decreases as a result of the switched-off current, falls below a predefined trigger threshold "TRG", and on the other hand (in Fig. 2 The PWM signal (falling edges) is triggered as soon as a value of the coil current I, which increases as a result of the switched-on current, exceeds the predefined trigger threshold TRG. During each switch-on phase of the current, the coil current I increases, whereas during each switch-off phase, the value of the coil current I decreases again. In both phases, the value of the coil current I follows an approximately exponential curve with a characteristic time constant (here, for example: L / (R5+R6)). The trigger threshold TRG is expediently set in the 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) Evaluating the PWM signal out to determine its duty cycle and, from this, the inductance L of the measuring coil 2, and, for example,to provide a digital data signal "ind(L)", for example on a digital data bus of a vehicle.

[0077] In the example of the Fig. 1 and 2 The duty cycle of the PWM signal `out`, defined, for example, as the ratio of pulse duration to PWM period, increases with increasing inductance L and decreases with decreasing inductance L. Using this relationship, which may be calculated or empirically determined beforehand, the inductance L of the measuring coil 2 can be determined by evaluating the PWM signal `out`.

[0078] However, it should be noted that if the temperature of the measuring coil 2 varies, for example due to a varying ambient temperature in a mobile application, the resistance R of the measuring coil 2 (i.e., the ohmic component of the impedance of the measuring coil 2) also varies with the variation of the measuring coil temperature, so that the 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.

[0079] The procedure therefore further includes the following steps 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): d) Detect a stationary maximum value "Imax" of the coil current I after switching on (but before the subsequent switching off) the coil current I, and e) Specify (or update) the trigger threshold TRG depending on the detected stationary maximum value Imax of the coil current I.

[0080] This temperature compensation advantageously eliminates or at least reduces the dependence of the duty cycle of the signal out on the temperature, as otherwise explained.

[0081] In the example of the in Fig. 2 In the time course shown, 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.

[0082] However, if, for example, the inductance L were to increase while the temperature remained constant, the duty cycle would increase and this increase in L would be determined during the evaluation (although the determined maximum value Imax of the coil current I remains constant).

[0083] For example, if the temperature and consequently the resistance R were to increase with a constant inductance L, the determined maximum value Imax of the coil current I would decrease, whereupon the trigger threshold TRG is changed by temperature compensation depending on the detected value Imax so that the duty cycle remains constant, and thus the constancy of the inductance L is correctly recognized by evaluating the duty cycle.

[0084] Regarding the determination of the steady-state maximum value Imax of the coil current I after a switch-on, in Fig. 2 It is easy to see that after the "waiting time" chosen in the example, corresponding to 1.5 periods of the clock signal V3, the value of the coil current I would only change negligibly over time t (here, for example, by less than 1%). The value Imax of the current I recorded at the end of this waiting time therefore corresponds very closely to that calculated using an exponential time profile with a time constant "tau". I t = Imax , theo x 1 − exp − t / tau Theoretically, the maximum current "Imax,theo" would only occur after an infinite waiting period. Within the scope of the invention, the measurement preferably takes place after a time interval of at least 2 x τ, and more preferably at least 4 x τ. Thus, the measured value Imax is a sufficiently good approximation of the actual maximum current.

[0085] Returning to Fig. 1 The circuit arrangement 1 has a current supply 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, whereby the corresponding switch-on and switch-off times are specified by means of the clock signal V3.

[0086] As from Fig. 1 It can be seen that further frequency-divided clock signals or control signals S1 and S2 are generated from the clock signal V3, the temporal progression of which is also shown in Fig. 2 is shown.

[0087] The generation of the clock signal V3 is in Fig. 1 symbolized by a corresponding voltage source. Likewise, an electrical supply for circuit arrangement 1 with, in this example, two supply voltages VCC5 and VCC1.8 is shown in Fig. 1 symbolized by corresponding voltage sources.

[0088] The switching on and off of the current to measuring coil 2 (coil current I) is carried out as in Fig. 1 evident 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 VCC1.8 or via a resistor R7 to a ground potential GND.

[0089] 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).

[0090] In the example of Fig. 1 The switching device SW3 is also used to generate an additional (auxiliary) supply voltage Vss, different from the supply voltages VCC1.8 and VCC5. This 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 operation.

[0091] The circuit arrangement 1 further includes a PWM signal generation 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 triggering the PWM signal out with appropriately triggered PWM signal edges (see Fig. 2 ) is generated.

[0092] In the example shown from Fig. 1 This will be done, as with reference to Fig. 2 As already explained, the rising PWM signal edges are triggered simultaneously with the falling coil current I falling below the specified 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 specified trigger threshold TRG again.

[0093] In particular, since a value is chosen 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 decreases exponentially very steeply in the relevant time range, so that the trigger threshold TRG is always reached very quickly, whereby a variation in the inductance L of the measuring coil 2 has only a very small influence on this time interval until the trigger threshold TRG is reached (deviating from the circuit example according to Fig. 1 It could therefore be intended, for example, that the rising PWM signal edge is rigidly coupled to the rising edge of the clock signal V3, i.e., every rising PWM signal edge is triggered simultaneously with a respective rising edge of the clock signal V3).

[0094] In contrast, the triggering of the falling PWM signal edges occurs in a time range in which the exponentially increasing curve of the coil current I is already relatively flat, so that a significant time interval, which is relatively strongly dependent on the inductance L, elapses between the switching on of the coil current I and reaching the trigger threshold TRG.

[0095] The PWM signal generation 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 comparing this value with the specified trigger threshold TRG.

[0096] In the example, the detection and comparison device 24 has the measuring resistor R5, R6, which is designed as a series circuit of individual resistors R5 and R6, as in Fig. 1 This can be seen when measuring coil 2 is connected to the circuit arrangement 1 and is arranged in series with measuring coil 2, and the coil current I is thus passed through it.

[0097] Furthermore, the detection and comparison device 24 in the example includes 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.

[0098] 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.

[0099] The comparator V1 thus compares a time-varying voltage at its inverting input, which is representative of the value of I and is subsequently also referred to as the first measurement voltage U1, with the trigger voltage VTRG applied as the "reference voltage" at its non-inverting input. If the latter is higher than the former, the comparator V1 outputs a "high level", otherwise a "low level".

[0100] Circuit arrangement 1 further features a Fig. 1 The evaluation unit 30, schematically depicted as a functional block, is used to evaluate the PWM signal out in order to determine its duty cycle and, consequently, the inductance L of the measuring coil 2. The information about the value of the inductance L is output by the evaluation unit 30, for example, in the form of a digital data signal ind(L).

[0101] A special feature of the circuit arrangement 1 is that the PWM signal generation device 20 also has a detection and specification update device 26, with the help of which the temperature compensation already explained above is accomplished, i.e. 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 would thus distort the determination result concerning the inductance L.

[0102] Using the data acquisition and specification update device 26, in the example shown, Fig. 1 The stationary maximum value Imax of the coil current I is periodically recorded after switching on, but before the subsequent switching off of the coil current I. Depending on the value Imax thus recorded, the trigger threshold TRG (here: trigger voltage VTRG) is then set or updated by means of the detection and specification update device 26.

[0103] In the example, the acquisition and specification update device 26 includes 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 to 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 voltage in the example shown. Fig. 1 The intended embodiment, in which the second measuring voltage U2, strictly speaking, drops only across one of the two individual resistors (R5 and R6), here the individual resistor R6, of the series-connected measuring resistor (R5, R6), 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 simple scaling of the relevant voltage U2 (i.e., the voltage to be held by the sampling-hold circuit SH) is achieved. In the example, this scaling corresponds to a division of the first measuring voltage U1 in the ratio R6 / (R5 + R6).

[0104] The sample-and-hold circuit SH includes, in the example of Fig. 1 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 configured accordingly.

[0105] The switching device SW2 is controlled by means of a logic gate arrangement A1 (flip-flop), A2, A3 operated by the clock signal V3 and the control signal S2 generated therein, 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", which holds this voltage until the switching device SW2 next samples the voltage U2 at the tap between R5 and R6.

[0106] In the example shown, the sampled and held voltage U2 is then provided "low impedance" at the output of the operational amplifier, i.e., the output of the sample-and-hold circuit SH, using the operational amplifier V2 configured as a voltage follower.

[0107] One example of Fig. 1 A further evident advantage of the acquisition and specification update device 26 is that the voltage U2 held by the sample-and-hold circuit SH (at capacitor C2 or at the output of the voltage follower V2) is not used directly as the aforementioned trigger voltage VTRG (for defining the trigger threshold TRG), but is first converted into the trigger voltage VTRG with the help of a resistor network, thereby advantageously realizing a further "scaling".

[0108] In this example, the resistor network comprises a coupling resistor R4 and a series circuit of individual resistors R1 and R2 connected to the supply voltage VCC1.8. Resistor R4 is connected between an output of the sample-and-hold circuit SH (here: the output of the voltage follower V2) and a tap in the series circuit (between R1 and R2). The voltage provided at the tap of the series circuit R1, R2 in this way is used as the trigger voltage VTRG (and, as already explained, applied to an input of the comparator V1).

[0109] The aforementioned scaling of the trigger voltage VTRG can advantageously be provided, for example, such 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 interfere with the switching process of the comparator V1.

[0110] 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 also an advantageous measure in the example shown, by means of which essential parameters of the desired temperature compensation can be set or optimized.

[0111] In this example, the values ​​of resistors R4 and R5, R6 (in conjunction with the values ​​of R1 and R2) are designed so that a decrease / increase of the first measurement voltage U1 caused by temperature changes is compensated as accurately as possible by a subsequent decrease / increase of the reference voltage VTRG, so that the pulse width of the PWM signal out is independent of the temperature.

[0112] This pulse-shaped signal (PWM signal out) output by the acquisition and comparison unit 24 is input to the evaluation unit 30, which, based on appropriate evaluation, finally provides the data signal ind(L) representative of the value of the inductance L.

[0113] The evaluation unit 30, which can, for example, be a functional component of a control unit intended for operating the corresponding sensor, can, for example, include a so-called "capture" unit by means of which the PWM signal output is counted in order to determine the duty cycle and thus the value of the inductance L. Alternatively, the signal output can be smoothed, for example, by means of a low-pass filter and output as an analog measurement signal and / or converted by means of an A / D converter as a digital data signal.

[0114] In the case of determining 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 relevant sensor measurement quantity (e.g. position, angle, etc.).

[0115] In the following description of further embodiments, the same reference numerals are used for components with the same effect. Essentially, only the differences from the embodiment(s) already described are addressed, and otherwise, reference is expressly made to the description of the preceding embodiments.

[0116] Fig. 3 Figure 1 shows a further embodiment of a circuit arrangement 1 for determining the inductance L of a measuring coil.

[0117] Fig. 4 shows in one of the Fig. 2 corresponding representation in the operation of the circuit arrangement of Fig. 3 Resulting time profiles of various current and voltage values.

[0118] In contrast to the previously described embodiment of the Fig. 1 and 2 In the example of the Fig. 3 and 4It is intended that the steps to compensate for the temperature dependence of the duty cycle of the PWM signal out are not performed after every switch-on, but only after every eighth switch-on of the coil current I.

[0119] After each of seven "measurement cycles" to determine the inductance L, a "temperature compensation cycle" follows to update the trigger threshold TRG.

[0120] The in Fig. 3 The circuit arrangement shown (1) corresponds in structure and function essentially to the previously described version of Fig. 1 .

[0121] However, a necessary circuit-technical difference with regard to the aforementioned functional difference concerning the implementation of the temperature compensation cycles lies in the structure and thus the function of a logic gate arrangement A4, A5, A6, A8, A9 of the example of Fig. 3 .

[0122] By means of this logic gate arrangement A4, A5, A6, A8, A9, a clocked control of the detection and specification update 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.

[0123] In this context, a further difference lies in the fact that in the example of Fig. 3 The frequency of the switching-on (or switching-off) processes of the current supply to the measuring coil 2 during the measuring cycles is equal to the frequency of the clock signal V3.

[0124] Another difference lies in the specific way in which the additional ("auxiliary") supply voltage Vss is generated. In the example of Fig. 3 For this purpose, a voltage generation circuit is designed 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 via the supply voltage VCC1.8).

[0125] With the invention and the described embodiments, inductive sensors can advantageously be operated with a particularly simple and reliable temperature compensation.

Claims

1. Method for determining an inductance (L) of a measuring coil (2), comprising the steps of: - switching on and switching off a coil current (I) flowing through the measuring coil (2) at switch-on times and switch-off times that are specified by means of a clock signal (V3), - generating a PWM signal (out) with a duty cycle that is representative of the inductance (L) of the measuring coil (2) by generating the PWM signal (out) with rising and falling PWM signal edges, - which on the one hand are triggered at the same time as 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 specified trigger threshold (TRG), or - both of which are triggered as soon as a value of the coil current (I) reaches a specified trigger threshold (TRG), - evaluating the PWM signal (out) in order to determine the duty cycle thereof and from this the inductance (L) of the measuring coil (2), wherein, for compensation of a temperature dependency of the duty cycle of the PWM signal (out) due to a temperature dependency of an ohmic resistance (R) of the measuring coil (2), the method further comprises the steps of: - detecting a steady-state maximum value (Imax) of the coil current (I) after switching on and before subsequently switching off 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 to compensate for the temperature dependency of the duty cycle are carried out after each time the coil current (I) is switched on.

3. Method according to Claim 1, wherein the steps to compensate for the temperature dependency of the duty cycle are carried out after each nth time the coil current (I) is switched on, 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 to compensate for the temperature dependency of the duty cycle is commanded by a control signal as required.

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

6. Method according to any one of the preceding claims, wherein the coil current (I) is conducted via 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) dropping across the measuring resistor (R5, R6).

7. Circuit arrangement (1) for determining an inductance (L) of a measuring coil (2), comprising - a current feed device (10) for switching on and switching off a coil current (I) flowing through the measuring coil (2) at switch-on times and switch-off times that are specified by means of a clock signal (V3), - a PWM signal generating device (20) for generating a PWM signal (out) with a duty cycle that is representative of the inductance (L) of the measuring coil (2) by generating the PWM signal (out) with rising and falling PWM signal edges, - which on the one hand are triggered at the same time as 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 specified trigger threshold (TRG), or - both are triggered as soon as a value of the coil current (I) reaches a specified trigger threshold (TRG), having - 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 evaluating device (30) for evaluating the PWM signal (out) in order to determine the duty cycle thereof and from this the inductance (L) of the measuring coil (2), wherein, for compensation of a temperature dependency of the duty cycle of the PWM signal (out) due to a temperature dependency of an ohmic resistance (R) of the measuring coil (2), the PWM signal generating device (20) further comprises: - 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 subsequently switching off the coil current (I) and for specifying the trigger threshold (TRG) depending on 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) arranged in series with the measuring coil (2) when determining the inductance (L) of the measuring coil (2) and via which the coil current (I) is conducted, - a comparator (V1) for comparing a voltage (U1) dropping across the measuring resistor (R5, R6) with a trigger threshold (TRG) specified as trigger voltage (VTRG) .

9. Circuit arrangement (1) according to Claim 8, wherein the detection and specification update device (26) comprises a sample-and-hold circuit (SH) for sampling and holding a voltage (U2) dropping across the measuring resistor (R5, R6).

10. Use of a method according to any one of Claims 1 to 6 and / or a circuit arrangement (1) according to any 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.