FR2685474A1 - OPERATING CIRCUIT FOR INDUCTIVE SENSOR WHOSE INDUCTANCE DEPENDS ON THE SIZE TO BE MEASURED. - Google Patents

Abstract

a) Operating circuit for inductive sensor whose inductance depends on the quantity to be measured b) Operating circuit characterized in that the rectifier is a synchronous rectifier (20) which receives the cadence signals from the oscillator (10) ), the phase of which is chosen with respect to the phase of the induced voltage to allow a separate measurement of the resistance and the inductance (L) of the coil (14).

Description

"Operating circuit for an inductive sensor, on which the inductance depends

  of the quantity to be measured "The present invention relates to an operating circuit for an inductive sensor, in particular an inductive sensor comprising at least one coil whose inductance depends on the quantity to be measured, an output signal oscillator is applied to a input of the coil and a rectifier which receives the voltage It is known to measure the adjustment strokes by inductive sensors with one or more coils The electrical characteristics of the coils change according to the measured variables, for example according to the translation Electronic operating circuit detects variations in the electrical properties of the coils and transforms them

into a simple signal to process.

  Document DE-OS 39 27 833 discloses a measurement circuit which can be used for stroke sensors, inductive, and generating a DC voltage dependent on the stroke. In this arrangement, an alternating voltage is applied to the coil, the inductance of which varies as a function of the stroke and the amplitude and phase of the voltage across the terminals of the

coil.

  To increase the accuracy of the measurement, two oscillators are provided which generate alternating voltages of the same frequency, but phase shifted. The second alternating voltage has an adjustable amplitude which ultimately depends on the inductance of the coils so that the ratio of the amplitude and known inductances of the coils allows a precise detection of the position In the known measurement circuit, no means is nevertheless provided to allow temperature compensation. A circuit inductive measurement of a stroke and which also has a temperature compensation circuit is known according to document DE-OS 39 10 597 In this known circuit, the temperature circuit detects the ohmic resistance of the coil which depends

  temperature and apply this signal to a micro-

  computer This calculates compensation data

  tion based on the resistance measured to allow

  the correction of the data obtained upon detection

of the race.

  A drawback of the known circuit is that the resistance of the coil and the inductance of the coil are measured alternately and that the results of the measurements are combined, without it being possible to

make simultaneous measurements.

  Advantages of the invention The aim of the present invention is to remedy the drawbacks of known solutions and relates to

  an operating circuit of the type defined above

  above, characterized in that the rectifier is a synchronous rectifier which receives the cadence signals from the oscillator whose phase is chosen relative to the phase of the induced voltage to allow a separate measurement of the resistance and the inductance of

the coil.

  The operating circuit according to the invention with an inductive sensor having the characteristics

  above, has the advantage of allowing compensation

  particularly simple and nevertheless very precise temperature without having to use a temperature sensor

additional temperature.

  This advantage is obtained from the fact that the regulated output voltage is applied to a synchronous rectifier itself controlled by the

  cadence pulses of a sinusoidal oscillator.

  The synchronous rectifier measures the reactance of the coil and determines the phase, we can compensate

  in a particularly simple way the temperature.

  As both the inductance and the real part of the reactance of the coil depend on the temperature, and since in addition both the inductance and the real part of the reactance depend on the measured quantity, we can arrive at temperature compensation if we add or

  appropriately subtract the two components.

  It is particularly advantageous that the addition or subtraction is carried out in a particularly simple manner by an appropriate choice of the

  phase of the synchronous rectifier control.

  It is particularly advantageous to use

  ser the operating circuit according to the invention in connection with sensors at a spring stroke, since this eliminates the second compensation coil usually required for such sensors, as well

  than an additional temperature sensor.

  According to another characteristic of the

  Note, the temperature compensation of the measurement signal is made using the dependence of the resistance R of the coil as a function of the temperature, by determining the real part of

  resistance in alternating current.

  According to another characteristic of the

  Note, the phase of the cadence signals is chosen so that the temperature compensated measurement voltage results from the difference between the imaginary part and the real part multiplied by a coefficient.

  constant of the voltage induced in the coil.

  According to another characteristic of the

  vention, the output signal of at least one coil is regulated in a regulator before being applied to the

synchronous rectifier.

  According to another characteristic of the

  vention, the synchronous rectifier includes two inverters which alternately receive the voltage of

coil output.

  According to another characteristic of the

  vention, it includes a frequency and selective phase offset compensation network, which is connected by another analog switch to the

synchronous rectifier.

  According to another characteristic of the

  vention, the synchronous rectifier is a differential amplifier having a filter characteristic

low pass.

  According to another characteristic of the

  vention, the rectifier comprises a differential inductive sensor with two coils and the synchronous rectifier comprises two switching elements each in relation to one of the coils and being switched synchronously. The present invention will be described below in more detail with the aid of the appended drawings in which: FIG. 1 shows a first example of

  embodiment of the invention with a coil.

  Figure 2 shows a vector diagram.

  Figure 3 shows the complete circuit of a

example with a coil.

  Figure 4 shows an exemplary embodiment

  a differential inductive sensor.

  FIG. 5 shows the two curves of the signals obtained in the embodiment of FIG. 4, without filtering and with filtering, as a function

time.

  FIG. 1 shows an oscillator 10 for example a sinusoidal oscillator with outputs l Oa, l Ob generating a voltage US by being connected by a resistor 12 to the variable coil 14 constituting the measurement coil Between the resistor 12 and the coil 14, we have defined a point 13 connected to ground, for a capacitor 15 Point 13 is also

  connected to input 17 a of a regulator 17.

  The regulator 17 receives by its input 17 b, a constant voltage corresponding to a 1/2 UB, UB being the reference voltage of the circuit; it is for example a voltage of 5 V supplied by -the

control device.

  The output 17 c of the regulator 17 is connected to the coil 14; another capacitor 16 connects this

  output to ground In addition, output 17 c of the regulator

  tor 17 is connected via a condenser

  18 and 19 resistance, to the analog switch

  logic 21 of the synchronous rectifier 20 The analog switch 21 receives switching pulses from

the output l Oa from oscillator 10.

  The synchronous rectifier 16 comprises, in addition to the analog switch 21, two inverters 27, 28

  which each consist of an operation amplifier

  nel 22, 26 as well as resistors 23, 25 provided each time between the output and the reverse input of the operational amplifiers 22, 26 A resistance 24

  is provided between resistors 23, 25.

  The two inverted amplifier inputs

  operational timers 22 and 26 are each connected to a terminal of the analog switch 21; the non-inverted inputs are at half the voltage of

reference is 1/2 UB.

  A link connects the output of the amplifier

  operational sensor 26 from synchronous rectifier 20 to filter 29 and analog output 30 from this filter

provides the measurement signal.

  Oscillator 10 generates a sinusoidal oscillation or an oscillation containing relatively

  few harmonics, and this oscillation has an ampli

  constant In FIG. 1, this oscillation bears the reference US It is applied by the resistor 12 to the coil 14 and the voltage generated at point 13 is detected to be applied as a real value to the regulator 17 As a set value, the regulator 17 receives the constant voltage 1/2 UB; the regulator regulates the output voltage UR so that the input voltage applied to input 17 a is also constant. As a constant voltage is established at point 13, the alternating voltage is then zero, and at the output l Ob we have an AC voltage of constant amplitude, so that resistor 12 gives a

  alternating voltage of constant amplitude.

  This alternating voltage of constant amplitude means that the intensity I in the resistor 12 is also an alternating current of constant amplitude As the voltage at point 13 is regulated to a constant value, the intensity I is also

  independent of the inductance of the coil 14.

  Possible parasitic capacitances of the cable with respect to ground do not allow the passage of any current, such as the capacitor 15 which serves as a DC blocking capacitor because only an alternating current can pass through the capacitors. As the input of the regulator is strongly ohmic, only a negligible current can pass by the input of regulation 17 a what ensures that the whole of the constant alternating current, crosses the coil 14 which corresponds to the measurement coil which one wants measure the electrical properties Thus, at the output 17 c of the regulator 17, an alternating voltage is established proportional to the reactance of the coil 14 The phase of this voltage represents the phase of the reactance The capacitors or parasitic capacitances with respect to the mass, which are shown schematically

  in Figure 1 by the capacitor 16, does not

  not try in a negative way because the current which

pass, comes from regulator 17.

  The voltage UR at the output 17 c of the regulator 17 arrives via the capacitor 18 and the resistor 19 to the synchronous rectifier 20 which receives the switching pulses by the cadence or clock output

i Qa of oscillator 10.

  The assembly described above allows measurements, but the desired temperature compensation is carried out only by the embodiment described.

below of the synchronous rectifier.

  The measurement signal UM which is collected at the link between the coil 14 and the regulation output 17 c is transmitted to the two inverters 27, 28 or to a single inverter 28 depending on the rate of the oscillator 10 This corresponds to a turnaround

  synchronous Usually, we choose to straighten

  synchronously, a cadence of a phase of 90 or a phase of O to measure either only the resistance or only the inductance It is also possible to correct the detection phase to compensate for

  phase shifts specific to the measurement arrangement.

  According to the invention, an appropriate choice of phase is made to arrive at temperature compensation. For certain sensors, the inductance of the coil 14 and the real part of the resistance of the coil depend on the temperature. In the same way, the inductance as well as the real part of the resistance can depend on the measured quantity For most sensors, the real part of the resistance and the inductance depend differently on the measured variables We can compensate in temperature by addition or appropriately subtract the two components A choice

  adequate, exactly compensates for the action of temperature

  while allowing the measurement quantity to be determined. In the case of such an operating circuit, neither a second compensation coil nor a separate temperature sensor is required. Furthermore, the measurement coil itself constitutes the temperature sensor. The addition or subtraction, described for the two components is carried out in a particularly simple manner by an appropriate choice of the phase. The vector diagram represented in FIG. 2 corresponds to a subtraction; it shows the relationship between intensities and tensions which, in this

  example, always bear the reference 1.

  When the coil 14 is supplied with a constant current Il, for a real coil, corresponding to a series connection of a resistor R and of an ideal coil of inductance L, there is obtained a voltage U 1 phase-shifted 8 with respect to the excitation current II A synchronous rectifier with zero phase shift measures the voltage drop corresponding to the real part U Rl; if on the other hand the rectifier with a phase shift of 90, corresponding to a pure inductance, the voltage drop UL 1 is measured. For a detection phase equal to 900 + ar, the measurement voltage UM 1 is obtained timing diagram of FIG. 5, as vertical projection of the coil voltage UM 1 in the direction of the detection phase This direction

  bears the reference A in FIG. 2; she is represented there

felt in broken lines.

  For the measurement voltage we then have the relation

UM 1 = cosa * (Ul tana * UR 1).

  We thus effectively subtract a real part of the voltage Ul multiplied by a constant factor from the imaginary part of the voltage Ul At the same time, we also make a multiplication with a constant coefficient; this multiplication can again be compensated by a constant amplification. Applied to a sensor with a better known integrated spring stroke, one obtains for example for temperature compensation an angle equal to a = 150 In the embodiment shown in Figure 1, we keep in a separate stage of the rectifier synchronous, that is to say the filter 29, additional filtering and calibration of the offset and slope are retained. It is however also possible to carry out this filtering and this calibration at

  using a stage integrated into the second inverter 28.

  The interference signal compensation which is already relatively good in the case of a synchronous rectifier 20 with a low-pass filter, can

  still be improved especially compared to para-

  low frequency sites having a frequency lower than the measurement frequency in that the capacitor 18 and the resistor 19 are dimensioned so that it functions as a high pass filter This guarantees the elimination by filtering of the low frequency oscillations, which combine with the measurement frequency. But as this high-pass filter causes annoying phase shifts, it is necessary to provide an additional phase shift of the cadence signal. Overall, the combination of the high-pass filter between the regulator

  17 and the synchronous rectifier 20 and the pass filter

  bottom 29 between synchronous rectifier 20 and output 30 ensures effective suppression of noise for

  all predictable frequencies.

  From a high-pass filter 31, it is also possible to use a high-pass filter or a higher-order band-pass filter or even a high-pass filter, active. A secondary effect, advantageous is to ensure that the offset voltages of the regulator 17 and oscillator 10, cannot reach output 30

and thus do not distort the signal.

  Although the oscillator 10 and the regulator 17 can be produced in analog or digital form, the behavior in regulation of the regulator 17 can be improved even more by a circuit with parasitic quantities; for this the output voltage of oscillator 10 is applied as information

additional to regulator 17.

  It is possible to further improve the circuit if at the output 10 b of oscillator 10, the decoupled sinusoidal voltage US is injected into the synchronous rectifier via a resistor network and a second analog switch. possible to compensate for the offset with a single coil for the oscillators, for example by injecting a DC voltage into an amplification stage,

  which results in very stable compensation.

  As variations in eyebrow tension-

  lation and frequency of the total value of the measurement voltage vary considerably, it is necessary that the stability of the oscillator corresponds to a multiple of the desired accuracy of the sensor We improve the accuracy by providing the voltage

  sinusoidal US by a network with trans-

  appropriate fert, for example with the same transfer behavior as that of the measuring coil with a central and stable position, and in addition having a second analog switch by which the voltage is injected in the opposite direction into the synchronous rectifier These means allow compensation for the offset having the same properties vis-à-vis the oscillator as the actual offset The realization can be done in the example of Figure 1 using a high-pass filter, additional

second order.

  FIG. 3 shows the complete diagram of another embodiment of the invention In this exemplary embodiment, the oscillator consists of an analog switch IC 4 in the form of a double

  switch with two switching elements 33, 34.

  An output of this analog switch IC 4 leads by a series connection formed of three resistors Ri, R 2, R 3, to the non-inverted input of the operational amplifier O Pi Between this combination of resistance and the ground, we have three capacitors Cl, C 2, C 3 The resistor R 10 is provided between the output of the operational amplifier O Pl and its inverted input; this input is also connected to ground

by resistance R 9.

  A Ki comparator is connected by its

  input not inverted at the inverted amplifier input

  operational designer O Pl; the non-inverted input of comparator Kl is connected by a resistor R 4 to the junction of an output of the analog switch IC 4 and of the resistor Rl The output of comparator Kl is

  connected by a resistor R 5 to the supply voltage

  tion as well as an input of the analog switch

IC 4.

  The regulator 17 consists of an amplifier

  OP 3 operational unit, whose inverted input is

  connected to ground via a condensa-

  tor C 5 and a resistance R 13; this input is connected by a resistor R 15 and a resistor R 16 to the output of the operational amplifier O Pl of the oscillator 10 and by a capacitor C 6 and a

  resistance R 14, this input is connected to the output.

  The measuring coil 14 is connected to the junction of resistors R 15 and R 16; the other terminal of the measuring coil is connected by a resistor R 17 to the output of the operational amplifier OP 3; between the two terminals of the measuring coil 14 and the

  ground, there is each time a capacitor C 7, C 8.

  The non-inverted input of the operational amplifier OP 3 is connected by a resistor R 6 to the non-inverted input of the comparator K 1 and by a

  resistance R 7 at supply voltage of 5 Volt.

  In addition, between the non-inverted input of the amplifier

  OP 3 operational mass and ground, we have a resistance

R 8 and a capacitor C 4.

  The synchronous rectifier 20 comprises two operational amplifiers OP 4, OP 5, the inverted inputs of which are connected each time to a terminal of the analog switch IC 4. In addition, the inverted input of the operational amplifier OP 5 is connected by a resistor. R 26 at the output of the operational amplifier OP 4 and by a resistor R 27 at the input

  from the operational amplifier OP 4.

  The non-inverted inputs of the operational amplifiers OP 4, OP 5 are connected to each other and also lead to the non-inverted input of

  the operational amplifier OP 6 of the drive stage

  32 as well as the non-inverted input of the amplifier

  operational ficitor OP 3 of regulator 17.

  Between the inverted input of the operational amplifier OP 5 and ground there is a resistance

  variable R 24; between the inverted amplifier input

  OP 5 op erator and its output, there is a condensa-

  C 3 as well as another variable resistance

  R 25; additionally the output of the operation amplifier

  nel OP 5 leads to a resistance R 23 of the circuit

32.

  In addition to the operational amplifier OP 6 already mentioned, the drive circuit 32 includes resistors R 19 R 22 as well as capacitors Cli, C 12; the resistor R 20 and the capacitor Cil are mounted between the inverted input of the operational amplifier OP 6 and its output; the reverse input is

  further connected to resistors R 23 and R 19.

  The output of the operational amplifier leads by the resistors R 21, R 22 to the output 30 which gives the temperature compensated measurement signal. The output 30 is further connected to the resistor R 19; a capacitor C 12 connect the

  resistors R 21 and R 22 and ground.

  Finally, between the coil 14 and an input of the analog switch IC 4, there is a high-pass filter

  formed of resistance R 18 and capacitor C 9.

  The circuit of Figure 3 works as follows: The analog switch IC 4 applies the resistance Rl alternately to ground and to the reference voltage (+ 5 V) It thus establishes a triangular oscillation on the capacitor Cl (exponential function ) At the inverted input of comparator K 1, the resistor R 4 generates a hysteresis The comparator Kl switches the analog switch IC 4 each time

  that the comparator reaches the threshold value.

  The voltage obtained is filtered by the passive low-pass filter, with two stages R 2, C 2 and R 3, C 3; the signal thus filtered is retained and amplified by the operational amplifier OP 1 One thus obtains an oscillation poor in harmonics which feeds the

coil 14.

  The comparator K1 simultaneously supplies the rate to the synchronous rectifier 20, since the switching of the switching elements 33, 34 by double

  analog switch IC 4 is done simultaneously.

  The switching element 34 corresponds by its function to the analog switch 21 of the synchronous rectifier 20 of FIG. 1 As the switching elements 33, 34 are nevertheless on the same wafer, this guarantees that the phase of the oscillations generated for example by the sinusoidal oscillation and the oscillation of the synchronous rectifier is absolutely constant The amplitude of the phase shift of the oscillation of the synchronous rectifier can be adapted to all conditions by the two filters

low pass R 2, C 2 and R 3, C 3.

  The regulator 17 regulates the voltage applied to the coil 14 as described in Figure 1; this is why, it is useless to describe here its

  operation in more detail.

  The voltage at the output of the regulator or at one of the terminals of the coil is injected by the high-pass filter formed by the resistor R 18 and the capacitor C 9 into the synchronous rectifier 20 By a choice or an appropriate adjustment of the resistance R 25, the slope of the sensor signal is adjusted and using

  resistance R 24, we set the offset The condensa-

  C 10 filtering the signal The circuit

  drive 32 provides the output signal from the

seems from the montage at exit 30.

  FIG. 4 shows an exemplary embodiment of a differential inductive sensor This sensor 11 comprises two coils 35, 36 whose inductance varies in opposite directions, depending on the amplitude of the measurement. The coils 35, 36 receive the excitation signal, sinusoidal, from the oscillator 10 via the two resistors R 29, R 31 The terminals opposite the ground of the coils are connected to the synchronous rectifier 20 by the capacitors C 13 , C 14

and by resistors 30, 32.

  Synchronous rectifier 20 is made of

  similar to the example in Figure 3; he behaves

  te however a double switch which is switched

  simultaneously by the cadences of oscillator 10.

  In this exemplary embodiment, no regulator is provided which could be implemented if necessary. The oscillator 10 generates sinusoidal oscillations applied to the two coils 33, 34 by the resistors R 29, R 31 The resistors are dimensioned appropriately so that in this embodiment, the alternating current

  crossing the coils is relatively constant.

  The two induced voltages U 21 and U 22 are injected into the synchronous rectifier by the two circuits 35, 36 and the rectifier ensures at the same time the subtraction By an appropriate choice of the phase of

  detection, the quantity to be measured can be determined.

  The voltages Ul and U 2 represented at the

  Figure 4 as well as the intensities Ia, Ib, correspon-

  dant to the curves of Figure 5; the output voltage UA is indicated once with filter and once without additional filter; the dashed curve

  corresponds to the output voltage UA with filter.

Claims (6)

R E V E N D I C A T I O N S   1) Operating circuit for an inductive sensor comprising at least one coil whose inductance depends on the quantity to be measured, an output signal oscillator is applied to a coil input and a rectifier which receives the induced voltage, circuit characterized in that the rectifier is a synchronous rectifier (20) which receives the cadence signals from the oscillator (10) whose phase is chosen with respect to the phase of the induced voltage to allow a separate measurement of the resistance (R), (Ri), (R 2) and inductance (L), (Ll), (L 2) of the coil (14), (35), (36).   2) Operating circuit as claimed   cation 1, characterized in that the temperature compensation of the measurement signal is made using the resistance (R) of the coil (14) as a function of temperature, by determining the   actual part of the AC resistance.   3) Operating circuit as claimed   cation 2, characterized in that the phase of the cadence signals is chosen so that the temperature compensated measurement voltage results from the difference of the imaginary part and the real part multiplied by a constant coefficient of the induced voltage in the coil (14).   4) Operating circuit as claimed   cations 1, 2, 3, characterized in that the output signal of at least one coil (14) is regulated in a regulator (17) before being applied to the rectifier synchronous (20).   ) Operating circuit according to one of   previous claims, characterized in that the   synchronous rectifier (20) comprises two inverters (27), (28) which alternately receive the voltage of coil output (14).   6) Operating circuit according to one of   previous claims, characterized by a filter   (18), (19) at the output of the synchronous rectifier (20). 7) Operating circuit according to one of the   previous claims, characterized by a filter   low pass between the coil (s) and the synchronous rectifier. 8) Operating circuit according to one of the   previous claims, characterized in that it   includes a frequency and selective phase offset compensation network, which is connected by a   another analog switch to the synchronous rectifier.   9) Operating circuit according to one of the   previous claims, characterized in that the   synchronous rectifier (20) is a differential amplifier having a low pass filter characteristic. 10) Operating circuit according to one of the   previous claims, characterized in that it   comprises a differential inductive sensor (11) with two coils (35), (36) and the synchronous rectifier (20) comprises two switching elements each in relation to one of the coils (35), (36) and being switched in synchronism.