Abstract:
An analytical circuit for an inductive electromagnetic sensor with external excitation ( 1 ) generates an output signal which is transformed to give an output signal (out), by means of transformation to a reference voltage (Vref) in an inverting low-pass filter ( 12 ), which has no hysteresis delay and is free from multiple triggering. By comparison of the reference voltage with three voltage thresholds in a diagnostic circuit ( 6 ), line interruptions and short-circuits are recognised by battery voltage or reference voltage potentials.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of copending International Application No. PCT/DE02/04088 filed Nov. 4, 2002 which designates the United States, and claims priority to German application no. 101 54 642.4 filed Nov. 7, 2001. 

   TECHNICAL FIELD OF THE INVENTION 
   The invention relates to an analytical circuit for an inductive sensor of a shaft, in particular for a sensor for sensing the rotational behavior of the crankshaft of a motor vehicle internal combustion engine. 
   DESCRIPTION OF THE RELATED ART 
   For operation of a motor vehicle internal combustion engine, precise knowledge of the rotational behavior (angular position, angular velocity-rpm, acceleration) of the crankshaft or camshaft is required. An electronic engine controller can use this information among other things to determine the right instants for fuel injection and firing. These variables are also important for example for controlling automatic transmissions or anti-blocking systems. 
   In the course of the ongoing development of engine technology this data is increasingly used to determine derived variables. An example of this is the emission-relevant detection of misfires required by the legislator. During a combustion process (power cycle) in a cylinder the crankshaft experiences an acceleration, causing an increase in its angular velocity. If no combustion takes place, for example as a result of a misfire, there is no increase in this angular velocity. 
   The observation of the change in angular velocity (acceleration) over the power cycle therefore provides information about a successful combustion. It is important here that not the measured variable but a variable derived therefrom is used. A complicating factor in this case is that the ratio of derived variable to measured variable is extremely small: ˜0.001. 
   These conditions impose demands on the detection precision of the measurement system which can scarcely still be met using known sensors suitable for use in motor vehicles. 
   The exacting requirements in terms of ruggedness, compatibility in respect of pollution and high temperatures have led to widespread use of magnetic sensors. These consist of a permanent magnet with a yoke made of ferromagnetic material (iron) for generating and maintaining a static magnetic field. A toothed gear wheel is mounted on the shaft to be measured in such a way that the teeth project into the static magnetic field. The alternation of tooth and gap in the moving toothed gear wheel produces a change in the magnetic field which is picked up by the sensor. 
   However, a passive magnetic sensor system (permanent magnet with iron yoke for generating and maintaining the static magnetic field and sensor coil in which an alternating voltage signal is induced as a result of changes in the magnetic field) has some disadvantages. 
   For example, the amplitude of the sensor voltage signal is approximately proportional to the frequency (˜0.1V–100V at 20–6000 Hz) at which the teeth of the toothed gear wheel pass the sensor. Precise detection of the zero crossings of the sensor signal in such a wide range is very difficult to perform. Expensive twisted and shielded cables are required. 
   Small minimal amplitudes of the sensor signal necessitate complicated and intensive signal processing and the risk of crosstalk from EMC interference signals increases. 
   This sensor system permits only limited diagnostic options in the event of sensor or line faults. The large amplitude dynamics do not allow simple detection of line breaks by threshold value detection. 
   Moreover, the amplitude of the sensor signal is influenced by the strength of the field generated by the permanent magnet. 
   The magnet must therefore be closely toleranced, which pushes costs up further. 
   An improvement in the sensing accuracy of the sensor signal can be achieved through measurement of the sensor current, since the latter&#39;s amplitude for tooth sequence frequencies above the cutoff frequency (given by the L/R time constant) is—in contrast to the sensor voltage—approximately constant. By this means the contribution to the detection precision error caused by the analytical electronics can be substantially reduced. 
   With sensors with permanent magnet excitation there is however the problem that during current measurement—and the short circuit of the sensor that is necessary for this—the current generated in the sensor itself generates an opposing field which is opposite to the field generated by the permanent magnet. 
   If the sensor is now operated at high temperature and under strong mechanical vibration, there is the risk that the permanent magnet will be slowly demagnetized, as a result of which the sensor ultimately loses its function. Even without this opposing field, a typical crankshaft sensor loses approx. 30% of its magnetization during its lifetime. 
   Alternatively an electromagnetic sensor can also be operated by means of external excitation. In this case the permanent magnet is replaced by soft magnetic material, for example electrical sheet steel, and a constant current is applied to the sensor coil, said current then magnetizing the material. A creeping demagnetization does not take place in this case. In their effect, sensors with permanent excitation and external excitation are identical in terms of function. 
   JP 4-223272 A discloses an analytical circuit for an electromagnetic sensor which is connected by means of a coupling capacitor to the inverting input of an inverting amplifier. At the output of the amplifier there appears a voltage whose direct voltage potential is determined by a reference voltage applied to the non-inverting input. Given suitable sizing of the coupling capacitor, the alternating voltage at the input of the sensor is very small (only several mV peak-to-peak). A problem with this concept is the value of the coupling capacitor that is required in practice. It must be rated at more than 1000 μF in order to be still sufficiently low-resistance at a required minimum tooth sequence of, for example, 20 Hz (start cycle of the internal combustion engine). Too great an impedance of the capacitor would produce an alternating voltage at the sensor, as a result of which the lower cutoff frequency of the sensor would be shifted upward. Furthermore, the capacitor has to be designed for a dielectric strength of approx. 20V to ensure that no damage occurs in the event of a fault, for instance a short circuit to battery voltage. Furthermore, it must also be designed—as is usual in automotive electronics—for temperatures of up to 125° C. in continuous operation. A capacitor of this design format is on the one hand very expensive, and on the other hand unacceptably large. Moreover, this circuit comprises an operational amplifier connected as a Schmitt trigger which converts the analog output signal into a digital signal, for example a rectangular signal, so that it can be processed by a following digital frequency analysis unit. 
   However, the hysteresis associated with the Schmitt trigger causes a time shift in the switchover times during the detection of the voltage zero crossings, said time shift also being dependent on the amplitude of the input signal. This brings no improvement in measurement accuracy and is consequently not acceptable. For the reasons cited the known circuit is unsuitable for motor vehicle applications. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to create a cheaper analytical circuit for an inductive sensor which, while providing increased EMC protection, avoids a time delay caused by hysteresis in the sensor signal and multiple triggerings and allows a simple diagnosis of the sensor with regard to short circuit and line interruption. 
   This object can be achieved according to the invention by an analytical circuit for an inductive sensor, in particular for a sensor for sensing the rotational behavior of the crankshaft of a motor vehicle internal combustion engine, comprising an electromagnetic sensor with external excitation by means of a constant current, a transconductance amplifier to whose inverting input the output signal of the sensor is fed, and whose output signal is converted in an inverting low-pass filter into a reference voltage which is supplied to the non-inverting input of the transconductance amplifier, a digitizing circuit comprising a Schmitt trigger and, in parallel with it, a voltage comparator, to both of which the output signal of the transconductance amplifier and the reference voltage are supplied, and whereby the Schmitt trigger outputs a hysteresis-affected output signal and the voltage comparator outputs a hysteresis-free output signal, and a logic circuit which forms a hysteresis-free output signal of the analytical circuit from the two output signals of the digitizing circuit and makes it available for further processing. 
   The logic circuit may have two inverters and four NAND gates, the input of the inverter and one input of the NAND gate can be connected to the output of the Schmitt trigger, the input of the inverter and the other input of the NAND gate can be connected to the output of the voltage comparator, the output of the inverter can be connected to one input of the NAND gate, and the output of the inverter can be connected to the other input of the inverter, the output of the NAND gate can be connected to an input of the NAND gate and the output of the NAND gate can be connected to an input of the NAND gate, and the two NAND gates may form a transparent RS flip-flop, whereby the output of the NAND gate is connected to the other input of the NAND gate and the output of the NAND gate at which the output signal of the sensor system can be tapped for further processing is connected to the other input of the NAND gate. An upper, a middle and a lower voltage threshold can be predefined, a line break can be detected when the reference voltage exceeds the middle voltage threshold, a short circuit to battery voltage potential can be detected when the reference voltage exceeds the upper voltage threshold, and a short circuit to reference voltage potential can be detected when the reference voltage exceeds the lower voltage threshold. A voltage divider located between a supply voltage and reference voltage potential can be provided for forming the upper, middle and lower voltage threshold, a voltage comparator can be provided in which the reference voltage is compared with the upper voltage threshold, a voltage comparator can be provided in which the reference voltage is compared with the middle voltage threshold, a voltage comparator can be provided in which the reference voltage is compared with the lower voltage threshold, and the levels of the output signals of the voltage comparators which are low when the reference voltage does not exceed the middle and upper voltage threshold or does not fall below the lower voltage threshold can be stored in a holding circuit from which they can be retrieved for further processing and can be cleared by means of a reset signal. 
   The object of the invention can also be achieved by a method for sensing the rotational behavior of the crankshaft of a motor vehicle internal combustion engine, using an analytical circuit for an inductive sensor, comprising the steps of: 
   providing an electromagnetic sensor signal with external excitation by means of a constant current, 
   feeding the sensor signal to an inverting input of a transconductance amplifier, 
   converting an output signal of the transconductance amplifier in an inverting low-pass filter into a reference voltage which is supplied to the non-inverting input of the transconductance amplifier, 
   supplying the output signal of the transconductance amplifier and the reference voltage to a digitizing circuit comprising a Schmitt trigger and, in parallel to a voltage comparator, and whereby the Schmitt trigger outputs a hysteresis-affected output signal and the voltage comparator outputs a hysteresis-free output signal, and 
   forming a hysteresis-free output signal of the analytical circuit from the two output signals of the digitizing circuit and making it available for further processing. 
   An upper, a middle and a lower voltage threshold can be predefined, a line break can be detected when the reference voltage exceeds the middle voltage threshold, a short circuit to battery voltage potential can be detected when the reference voltage exceeds the upper voltage threshold, and a short circuit to reference voltage potential can be detected when the reference voltage exceeds the lower voltage threshold. The method may further comprise the steps of forming the upper, middle and lower voltage threshold by a voltage divider located between a supply voltage and reference voltage potential, comparing the reference voltage with the upper voltage threshold, comparing the reference voltage with the middle voltage threshold, comparing the reference voltage with the lower voltage threshold, and storing the results of the comparison when the reference voltage does not exceed the middle and upper voltage threshold or does not fall below the lower voltage threshold for further processing. Reset means can be provided for resetting the comparison results. Storage means can be provided for storing the comparison results. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment according to the invention will be explained in more detail below with reference to a schematic drawing, in which: 
       FIG. 1  shows the basic design of an analytical circuit according to the invention, and 
       FIG. 2  shows different signal waveforms of this circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  depicts, in a frame with a dashed border, a sensor  1  externally excited by means of direct current as an alternating current source I 1  which simultaneously represents the sensor short-circuit current. The inductor L 1  connected in parallel with it represents the sensor inductance, while the resistor R 1  connected in series with it represent the winding resistance at which a direct voltage used as a reference point for the reference voltage of the analytical circuit drops during external excitation by means of direct current. 
   If the sensor  1  is operated without output load, the alternating voltage increases proportionally to the frequency, as the impedance of the inductor L 1  (=2πL) increases steadily and a bigger and bigger voltage drop is produced due to the alternating current source I 1 . If the sensor is operated in the short circuit, a current divider is produced between sensor inductor L 1  and winding resistor R 1 . Above the cutoff frequency ω 0 =L 1 /R 1  the impedance becomes so great that the sensor current I 1  flows mainly through the winding resistor R 1 . 
   The constant current source  2  for external excitation of the sensor  1  from a supply voltage source V 1  (for example 5V) consists of a current mirror (Q 1 , Q 2 , R 2  and R 3 ) with polarity reversal protection diode D 1  in the output. 
   Two transistors Q 1  and Q 2  whose base terminals are connected to one another and whose emitter terminals are each connected to the supply voltage source V 1  via a resistor R 2 , R 3  form the current mirror. Base and collector of transistor Q 2  are connected to each other, so that the transistor acts as a diode, and are connected to reference voltage potential GND via a resistor R 4 . 
   The current through the series connection comprising R 3 , Q 2  and R 4  is determined by the values of the resistors and the voltage drop at Q 2 . If R 3  is selected with a rating of 50 Ω and R 4  with a rating of 370 Ω, with a supply voltage of 5V a current of approx. 10 mA will flow. If R 2  is also selected with a rating of 50 Ω, 10 mA will likewise flow through transistor Q 1  and diode D 1 —and in fact will do so largely independently of the collector potential of Q 1 . Diode D 1  prevents a polarity reversal of Q 1  in the event of a short circuit of the sensor line to battery and thus avoids destruction of the sensor. The collector current of Q 1  flows into sensor  1  as an excitation current and generates a direct voltage drop of, for example, 2.5V at the winding resistor R 1 , on which voltage the sensor alternating current (signal s 1  in  FIG. 2 ) is then overlaid. 
   The sum of sensor direct and alternating current forms the sensor output signal s 1 , which is supplied to a following transconductance amplifier  3  with regulated reference voltage generation. 
   The transconductance amplifier  3 , amplifier A 1 , is connected by its inverting input via a resistor R 6  to the sensor  1  and the external excitation  2 . Resistor R 6  protects the input of amplifier A 1  in the event of a short circuit to battery voltage. A negative feedback takes place by means of a resistor R 7  which connects the output of A 1  to sensor  1 . The non-inverting input of A 1  is likewise connected to sensor  1  via a resistor R 5 , whereby a series connection of R 5  and a capacitor C 1 , which leads to the reference potential GND, represents a low pass. R 5  also protects the non-inverting input of A 1  in the event of a short circuit to battery voltage. The output of A 1  is connected via a resistor R 8  to the inverting input of an amplifier A 2  which is also connected via a parallel connection of a resistor R 9  and a capacitor C 2  to the output of A 2 . As a result amplifier A 2  acts as an inverting low pass. 
   The output of the amplifier A 2  is connected via a resistor R 10  to the non-inverting inputs of A 2  and A 1 , with the result that the output voltage of A 1 , low-pass filtered and inverted in the amplifier A 2 , arrives at the non-inverting inputs as reference voltage Vref (see signal s 2  in  FIG. 2 ). Its value corresponds to the direct voltage dropping at the sensor. At the output of A 1  there is therefore produced an alternating voltage whose size is determined by the product of the sensor alternating current and the value of the resistor R 7 . Through selection of R 7  it is possible to set it to, for example, 3V (peak-to-peak). On the one hand this avoids a voltage limitation of the output of A 1 , and on the other hand the signal can then simply be processed further. In addition, as a result of the negative feedback with R 7  the sensor alternating current does not lead to any significant alternating voltage at the sensor input. The sensor is therefore short-circuited in terms of alternating voltage. 
   The sensor voltage signal s 2  ( FIG. 2 ) appearing at the output of A 1  is digitized by a digitizing circuit  4 , consisting of a Schmitt trigger K 1  and a voltage comparator K 2  arranged in parallel with it. 
   Signal s 2  arrives directly at the inverting input of the comparator K 2  and, passing via a resistor R 11 , reaches the non-inverting input of the comparator K 1 , which is connected to its output via a further resistor R 12 . The inverting input of K 1  and the non-inverting input of K 2  are connected to the reference voltage Vref. As a result of the connection to R 11  and R 12 , comparator K 1  becomes a Schmitt trigger with hysteresis whose value results from the ratio of R 11 /R 12  and the supply voltage of K 1 . At the output of K 1  there appears the digital output signal k 1  with a time delay caused by the hysteresis (signal k 1  in  FIG. 2 ). 
   In comparator K 2 , signal s 2  is compared with the reference voltage Vref. Since no hysteresis is present here, the output switches exactly at a voltage difference of 0V at the inputs (signal k 2  in  FIG. 2 ). Small, noisy input signals can result in multiple triggering (switchover), however. 
   If the comparators K 1  and K 2  are supplied with a voltage of 5V, as is typically the case, then their output levels are 0V and 5V, which levels are suitable for further processing in logic gates. 
   The two output signals k 1  and k 2  of the comparators K 1  and K 2  are finally supplied to a logic circuit  5  in order to form the actual output signal Out. This logic circuit consists of two inverters N 1  and N 2 , plus four NAND gates U 1  to U 4 . The output of a NAND gate only has low level when both inputs simultaneously have high level. This applies to the NAND gates U 1  to U 4 . 
   The input of the inverter N 1  and an input of the NAND gate U 2  are connected to the output of the Schmitt trigger K 1 . 
   The input of the inverter N 2  and the other input of the NAND gate U 2  are connected to the output of the comparator K 2 . 
   The output of N 1  is connected to one input of U 1 ; similarly, the output of N 2  is connected to the other input of U 1 . The output of U 1  is also connected to an input of U 3 ; similarly, the output of U 2  is connected to an input of U 4 . The output of U 3  is connected to the other input of U 4 ; similarly, the output of U 4  is connected to the other input of U 3 . 
   The two fed-back NAND gates U 3  and U 4  form (according to Tietze/Schenk) a “transparent” RS flip-flop. The output of U 3  represents the output of the logic circuit at which the signal Out is present. The truth table of this RS flip-flop looks as follows: 
   
     
       
             
             
             
           
         
             
                 
             
             
               u1 
               u2 
               Out 
             
             
                 
             
           
           
             
               Low 
               Low 
               Not valid 
             
             
               Low 
               High 
               High 
             
             
               High 
               Low 
               Low 
             
             
               High 
               High 
               Out −1   
             
             
                 
             
           
        
       
     
   
   It can be seen from  FIG. 1  and the signals shown in  FIG. 2  that the inverted output signals k 1  of the Schmitt trigger K 1  and k 2  of the comparator K 2  are supplied to the NAND gate U 1 , at whose output a signal u 1  is produced which is time-delayed due to the hysteresis of the Schmitt trigger. 
   The signals k 1  and k 2  are also supplied—without inversion—to the NAND gate U 2 , the output signal u 2  of which has low level from the rising edge of the signal k 2  to the falling edge of the signal k 1 . 
   In this way two signals u 1  and u 2  are obtained which are then supplied to the “transparent” RS flip-flop. The output signal Out of this flip-flop is high if u 1 =Low and u 2 =High, and is low if u 1 =High and u 2 =Low. If u 1 =u 2 =High, the previous state (Out 1 ) is maintained. Further switching states, caused for example by multiple triggering of the comparator K 2 , therefore have no effect. 
   All in all, in this way a digital signal Out has been produced at the output of the NAND gate U 3 , which digital signal switches in phase synchronism with the rising and falling crossings of the output voltage s 2  of the transconductance amplifier Al through the level of the reference voltage Vref. It has no hysteresis delay, and is free of multiple triggerings. 
   The output signal Out of the analytical circuit can now be supplied to, for example, a microcontroller (not shown) for further processing (frequency sensing, etc.). 
   The diagnostic circuit  6  for the inductive sensor consists of three comparators K 3 , K 4  and K 5 , a voltage divider R 13  to R 16 , and a holding circuit H. 
   The reference voltage Vref is present at the non-inverting inputs of the comparators K 3  and K 4  and also at the inverting input of the comparator K 5 . The inverting inputs of K 3  and K 4  and also the non-inverting input of K 5  are connected to the voltage divider at different tapping points in each case. 
   The outputs of the comparators K 3  to K 5  are connected to the inputs of the holding circuit H, the outputs of which lead to a microcontroller (not shown). A reset line Reset is also connected to this microcontroller. 
   The voltage divider is connected to its supply voltage Vcc (5V) and reference potential GND in such a way that different voltages result at the three tapping points. By suitable selection of the resistor values of R 13  to R 16 , for example, it is possible to create an upper (for example 4.8V), a middle (for example 4.0V) and a lower voltage threshold (for example 0.2V). 
   The outputs of the comparators K 3  to K 5  are pulled low when the reference voltage Vref is around approx. 2.5V, which may fluctuate by ±1V. This corresponds to the normal operating case, in other words the diagnostic state “no error”. 
   If there is a line break, the reference voltage Vref, driven by the current from transistor Q 1  of the current mirror Q 1 -Q 2 , will increase to approx. 4.3V. This causes K 4  to switch to high level and this value is stored in the holding circuit H and the output OW of the holding circuit H likewise goes to high level. 
   In the event of a short circuit to battery voltage potential, the reference voltage Vref is limited to approx. 5.5V by the already described protection circuit. In this case K 3  and K 4  switch to high level, which is stored in the holding circuit H. The outputs SCB and OW assume high level. The switching of OW in the event of a short circuit to battery voltage can be prevented by an additional simple linking logic (not shown). A suppression of OW is also possible by means of the analytical software in the following microcontroller, however. 
   In the event of a short circuit to reference voltage potential, the value of the reference voltage Vref will become very small, as a result of which comparator K 5  switches to high level. This level is likewise stored in the holding circuit H and the output SCG assumes high-level. 
   The already mentioned (but not shown) microcontroller can now interrogate and evaluate the signal levels SCB, OW and SCG present at the outputs of the holding circuit H and from this detect the presence of an error and its type (line break, short circuit to battery voltage potential, short circuit to reference voltage potential). After these signal levels have been read out, the microcontroller can reset the holding circuit H by means of the line Reset. 
   By repeated readout of the signals SCB, OW and SCG with subsequent resetting of the holding circuit in each case and observation of the variation with time of the signal levels it is also possible to distinguish between real, permanent errors and sporadic apparent errors possibly caused by interference voltages.