JPH05248891A - Evaluation circuit for induction sensor - Google Patents

Abstract

PURPOSE: To obtain an evaluation circuit in which temperature compensation can be performed accurately by supplying a clock signal from an oscillator to a synchronous rectifier and selecting the phase position of the signal with respect to the phase position of an induced voltage such that the resistance and inductance of a coil can be measured individually. CONSTITUTION: A measurement signal UM appearing at the joint of a coil 14 and the output side 17C of a controller is fed through two inverters 27, 28 or only one inverter 28 depending on a clock from an oscillator 10. Consequently, a synchronous rectification operation is brought about. In order to measure only the resistance or inductance, a clock of 90 deg. or 0 deg. phase is selected. Real part of the inductance L and resistance R of the coil 14 depend on the temperature for a specified sensor. Real part of the inductance L and resistance R also depend on the measurement. Consequently, temperature compensation can be performed by adding or subtracting two components each other in an appropriate form.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an evaluation circuit for an inductive sensor having at least one coil, an oscillator and a rectifier, for example, an integrated displacement sensor (Einfederwegsensor) for a suspension system. The inductance of the coil depends on the characteristic amount to be measured, the output signal of the oscillator is supplied to the input side of the coil, and the induced voltage is supplied to the rectifier. Regarding the evaluation circuit.

[0002]

It is well known to use inductive sensors with one or more coils to measure accommodative travel. The electrical characteristics of this coil change depending on the measured amount, for example the amount of movement. A change in the electrical characteristics of the coil is detected using an electronic evaluation circuit and converted into a signal that is easy to post-process.

A measuring circuit used in an inductive type distance sensor, in which a DC voltage is generated depending on a moving distance is DE-.
It is known from OS 3927833. In this circuit, an alternating voltage is applied to the coil. The inductance of this coil changes depending on the moving distance. Furthermore, the amplitude and phase of the voltage dropping in the coil are measured.

Two oscillators are provided in order to increase the measurement accuracy, and these oscillators generate two alternating voltages of the same frequency. The two AC voltages are 180 degrees out of phase with each other. In this case, the second alternating voltage has an adjustable amplitude. However, this amplitude ultimately depends on the inductance of the coil. Therefore, it is possible to accurately detect the position from the relationship between the amplitude and the known inductance of the coil. However, the known measuring circuit does not provide any means for enabling temperature compensation.

A circuit arrangement for an inductive distance measuring unit with an additional temperature measuring circuit is known from DE-OS 3910597. In this known circuit device, the ohmic resistance value of the coil is detected by the temperature measuring circuit. The ohmic resistance value depends on the temperature, and the temperature measuring circuit supplies the ohmic resistance value to the microcomputer. The microcomputer calculates compensation data from the measured resistance value. This compensation data makes it possible to correct the data obtained when the moving distance is detected.

A disadvantage of the known circuit arrangement described above is that the resistance and the inductance of the coil are measured alternately and the measurement results are erroneously calculated between each other. On the other hand, it is impossible to perform the measurement at the same time.

[0007]

The problem to be solved by the present invention is to overcome the drawbacks of conventional circuit arrangements and to provide a particularly simple and very accurate temperature compensation without the use of an additional temperature sensor. The object is to provide an evaluation circuit for an inductive sensor that makes it possible.

[0008]

According to the present invention, the rectifier is a synchronous rectifier, the synchronous rectifier is supplied with a clock signal from an oscillator, and the phase position of the clock signal is the phase of the induced voltage. A solution is constructed and arranged so that the resistance and the inductance of the coil can be separately measured with respect to the position.

The evaluation circuit for an inductive sensor according to the characterizing portion of claim 1 of the present invention has the following advantages. That is, a particularly simple and very accurate temperature compensation is possible without the use of an additional temperature sensor.

This advantage is achieved by providing a controlled output voltage to the synchronous rectifier. The synchronous rectifier itself is controlled by the sinusoidal oscillator clock pulses.

By using the synchronous rectifier, the AC resistance value of the coil is measured and the phase position is detected, so that particularly simple temperature compensation is possible.

Both the inductance of the coil and the real part of the complex resistance value have temperature dependent characteristics, and also depend on the measured quantity. Therefore, the two components are added or subtracted from each other by an appropriate method to compensate for the temperature. It can be achieved.

Particularly advantageously, the addition or subtraction can be achieved in a particularly simple manner by correspondingly selecting the phase position in the control of the synchronous rectifier.

It has also proved to be particularly advantageous to use the evaluation circuit according to the invention in connection with an integrated displacement sensor. This is because the second compensating coil normally required in such a sensor as well as the additional temperature sensor can be omitted.

Advantageous configurations and refinements of the evaluation circuit for inductive sensors according to claim 1 are possible by the measures specified in the subclaims.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In FIG. 1, an oscillator 10, for example a sinusoidal oscillator, is connected via a resistor 12 to a measuring coil, shown as a variable coil 14. The oscillator 10 has outputs 10a, 10b and produces a voltage US. Resistance 12
A connection point 13 is shown between the coil and the coil 14. A capacitor 15 is provided between the connection point 13 and the ground, and the capacitor 15 is further connected to the input side 17 a of the controller 17.

A constant voltage is supplied to the controller 17 via the input side 17b. This constant voltage is 1/2 of UB. In this case UB is the reference voltage of the circuit (eg 5V from the controller).

The output side 17c of the controller 17 is connected to the coil 14 and is connected to the ground via another capacitor 16. Further, the output side 17c of the controller 17 has a capacitor 1
8 and a resistor 19 to connect to the analog switch 21 of the synchronous rectifier 20. This analog switch 21 receives the switching pulse via the output side 10a of the oscillator 10.

The synchronous rectifier 20 has two inverters 27 and 28 in addition to the analog switch 21.
The inverters 27 and 28 are respectively the operational amplifier 2
2, 26, resistors 23, 25 arranged between the output side and the inverting input side of the operational amplifiers 22, 26, and a resistor 24 provided between these resistors 23, 25.

The two inverting input sides of the operational amplifiers 22 and 26 are respectively connected to one terminal of the analog switch 21. Half the reference voltage, 1/2 UB, is applied to the non-inverting input side.

The output side of the operational amplifier 26 of the synchronous rectifier 20 is connected to the filter 29. The measurement signal is taken out at the analog output side 30 of the filter 29.

In the oscillator 10, sinusoidal vibration or vibration containing a relatively small number of harmonic components is generated with a constant amplitude.
In FIG. 1, this sinusoidal vibration is indicated by US. This sinusoidal vibration is supplied to the coil 14 via the resistor 12, and the voltage generated at the connection point 13 is detected. Furthermore, this voltage is supplied to the controller 17 as an actual value. As a target value, the controller 17 is supplied with a constant voltage UB / 2.
The controller controls the output voltage UR such that the input voltage on the input side 17a is likewise kept constant. Connection point 1
3, the constant voltage is adjusted (in this case, the AC voltage is zero, and the output side 10b is applied with the AC voltage US having a constant amplitude). Therefore, the AC voltage having the constant amplitude is applied via the resistor 12. Occurs.

This alternating voltage with a constant amplitude acts such that the current I through the resistor 12 is also an alternating current with a constant amplitude. Since the voltage at the connection point 13 is controlled to a constant value, the current I also does not depend on the inductance of the coil 14.

In some cases, even if the cable has stray capacitance to earth, the capacitor 15 (this capacitor 1
5 does not carry any current, similar to EMC (electromagnetic compatibility) -used as a block capacitor). This is because current can only flow through this capacitor when an alternating voltage is present.

Since the input side of the controller has a high resistance, only a small current can flow to the controller input side 17a.
This ensures that all constant alternating current flows through the coil 14 (the coil 14 is shown as a measuring coil and the electrical properties of the coil are measured). As a result, the AC voltage is adjusted at the output side 17c of the controller 17. This AC voltage is proportional to the so-called reactance of the coil 14. The phase position of this voltage represents the phase position of the reactance. The stray capacitance to the capacitor or ground (which is shown as capacitor 16 in FIG. 1) does not have a negative effect. This is because the flowing current is supplied from the controller 17.

The voltage UR generated at the output 17c of the controller 17 is supplied to the synchronous rectifier 20 via the capacitor 18 and the resistor 19. This synchronous rectifier 20 has an oscillator 1
Switching pulses are supplied via the 0 clock output 10a.

With the devices described so far, measurements can already be made. However, the desired temperature compensation can be guaranteed only by the circuit configuration example of the synchronous rectifier described below.

The measuring signal UM, which occurs at the connection between the coil 14 and the controller output 17c, depends on the clock of the oscillator 10 and either the two inverters 27, 28 or 1
It is supplied via only one inverter 28. As a result, a synchronous rectification action is obtained. A 90 degree phase or a 0 degree phase clock is typically selected for this synchronous rectification to measure resistance or inductance only.
It is also possible to further correct the sampling phase in order to compensate for the phase shift caused by the measuring device itself.

Here, according to the invention, a suitable choice of phase position is made in order to achieve temperature compensation. The real part of the inductance of the coil 14 and the resistance of the coil for a given sensor depends on temperature. Similarly, the real parts of the inductance and resistance may also depend on the measurand. In this case, in many sensors, the real part of the resistance and the inductance are different and depend on the measured quantity. Therefore, temperature compensation can be done by adding or subtracting the two components from each other in a suitable manner.
If the proper choice is made, the temperature control is discontinued, whereas the measurand can still be detected.

No second compensating coil is required for such an evaluation and no separate temperature sensor is required. On the contrary, the measuring coil itself serves as a temperature sensor. The addition or subtraction of the two components can be carried out particularly simply by selecting the phase positions accordingly. Vector diagram shown in Figure 2 (this applies to subtraction)
It is clear from the above that the current and the voltage which are important for the embodiment are always assigned the reference numeral 1.

When the constant current I1 is supplied to the coil 14,
In the actual coil, the series circuit of resistance R and inductance L
A voltage U1 is produced corresponding to an ideal coil with This voltage U1 has a phase difference β with respect to the excitation current I1. The zero degree phase difference synchronous rectifier measures the voltage drop caused by the real part UR1. If, on the other hand, the synchronous rectifier has a phase difference of 90 degrees, the voltage drop UL1 caused by the net inductance is measured.

When the sampling phase is 90 degrees + α, UM1 is generated as the measurement voltage. This voltage is represented in the vector diagram of FIG. 2 by a vertical projection from the coil voltage U1 in the direction of the sampling phase (this direction is labeled A in FIG. 2 and is also indicated by the dotted line). There is. The following equation holds for the measured voltage.

UM1 = cosα × (U1-tanα × UR1) In practice, the real part of the voltage U1 multiplied by a constant coefficient is subtracted from the imaginary part of the voltage U1. At the same time, a further multiplication with a constant coefficient is performed. This multiplication can be compensated again by a constant amplification.

If used in a known integrated displacement sensor, the following angles occur, for example for temperature compensation.

That is, an angle of α = 15 degrees occurs.

Additional filtering and calibration of offset and slope is done in a separate stage from the synchronous rectifier in the embodiment shown in FIG. This stage is shown as filter 29. However, it is also possible to carry out this filtering and calibration with a stage integrated in the second inverter 28.

The suppression of the noise signal, which is already performed relatively well by the low-pass filter in the synchronous rectifier 20, for example, the suppression of low-frequency noise at a frequency lower than the measurement frequency, can be further improved by the following.
This can be further improved by choosing the capacitor 18 and the resistor 19 to act as a high pass filter. This ensures that low-frequency vibrations that are superimposed on the measurement frequency are filtered out.

Since this high-pass filter adjusts the unwanted phase shift, an additional shift of the clock signal is necessary. Overall, the combination of the high-pass filter between the controller 17 and the synchronous rectifier 20 and the low-pass filter 29 between the synchronous rectifier 20 and the output side 30 is effective for all possible frequencies. Noise suppression is performed.

As the high pass filter 31, a relatively high order high pass filter, a band pass filter, or an active high pass filter may be used. Another advantageous effect is that the offset voltage from the controller 17 and the oscillator 10 cannot reach the output 30. It also prevents signal errors.

Both the oscillator 10 and the controller 17 can be implemented in analog or digital form. The control characteristics of the controller 17 can be further improved by using the disturbance amount control circuit. The output voltage of the oscillator 10 in this case is supplied to the controller 17 as additional information.

Another improvement is achieved when the sinusoidal voltage US, which is coupled to the output 10b of the oscillator 10, is fed to the synchronous rectifier 20 via a resistor network and a second analog switch. With this configuration, offset compensation can be performed even for a sensor having only one coil by supplying a DC voltage to the amplification stage. This leads to very stable compensation.

The stability of the oscillator is many times greater than the desired accuracy of the sensor device, since changes in the oscillator voltage and frequency change the overall value of the measured voltage considerably. An improvement in accuracy is obtained via a network with matching transmission characteristics, for example the transmission characteristics of the measuring coil have an average and stable position. In this case, a second analog switch is additionally provided. The voltage is alternately fed to the synchronous rectifier via this analog switch. Offset compensation is possible via this means. This offset compensation has the same characteristics of the oscillator as the original offset. This realization is second in the embodiment according to FIG.
Can be done with an additional high pass filter of the scheme.

FIG. 3 shows the entire circuit device of another embodiment according to the present invention. In this embodiment, the oscillator comprises an analog switch IC4. This analog switch IC4 has two switch elements 3 as twin switches.
It is composed of 3,34. The output side of the analog switch IC4 is connected to the non-inverting input side of the operational amplifier OP1 via a series circuit composed of three resistors R1, R2 and R3. Three capacitors C1, C2 and C3 are provided between the circuit formed by the combination of the plurality of resistors and the ground. A resistor R10 is provided between the output side of the operational amplifier OP1 and the inverting input side of the operational amplifier. The resistor R10 is also connected to ground via a resistor R9.

The operational amplifier O is connected to the non-inverting input side of the comparator K1.
It is connected to the inverting input side of P1. The non-inverting input side of the comparator K1 is connected to the connection point between the analog switch IC4 and the resistor R1 via the resistor R4. A supply voltage is applied to the output side of the comparator K1 via a resistor R5, and the output side of the comparator K1 is connected to the input side of the analog switch IC4.

The controller 17 has an operational amplifier OP3. The inverting input side of this operational amplifier OP3 has a capacitor C
5 and the resistor R13 to ground, and the resistor R15
Is connected to the output side of the operational amplifier OP1 of the oscillator 10 via the resistor R16. Further, the inverting input side of the operational amplifier OP3 is connected to the output side via the capacitor C6 and the resistor R14.

The measuring coil 14 has resistors R15 and R16.
Connected to the connection point between. The other side of this measuring coil 14 is connected to the output side of the operational amplifier OP3 via a resistor R17. Between the two terminals of the measuring coil 14 and the ground, one capacitor C7, C8 is provided.

The non-inverting input side of the operational amplifier OP3 has a resistor R
It is connected to the non-inverting input side of the comparator K1 via 6 and the supply voltage of 5V is applied via the resistor R7. Further, a resistor R is provided between the non-inverting input side of the operational amplifier OP3 and the ground.
8 and a capacitor C4 are provided.

The synchronous rectifier 20 includes two operational amplifiers OP4.
And OP5. The inverting inputs of these operational amplifiers are connected to one terminal of each analog switch IC4. Further, the inverting input side of the operational amplifier OP5 has a resistor R
It is connected to the output side of the operational amplifier OP4 via 26, and is further connected to the inverting input side of the operational amplifier OP4 via the resistor R27.

The non-inverting input sides of the operational amplifiers OP4 and OP5 are connected to each other and further connected to the non-inverting input side of the operational amplifier OP6 of the so-called driver stage 32 and the non-inverting input side of the operational amplifier OP3 of the controller 17. ..

A variable resistor R24 is provided between the inverting input side of the operational amplifier OP5 and ground, and a capacitor C3 and another variable resistor R25 are provided between the inverting input side of the operational amplifier OP5 and its output side. Is provided. In addition, the output side of the operational amplifier OP5 is connected to the resistor R23 of the driver stage 32.

In addition to the operational amplifier OP6, the driver stage 32 further includes resistors R19 to R22 and a capacitor C11.
And C12. In this case, the resistor R20 and the capacitor C11 are provided between the inverting input side and the output side of the operational amplifier OP6, and the inverting input side further has the resistor R2.
3 and R19.

The output side of the operational amplifier OP6 is connected to the output side 30 via the resistors R21 and 22. At this output 30, a measuring signal for temperature compensation is obtained. The output side 30 is further connected to the resistor R19, and the resistors R21, R
A capacitor C12 is arranged between 22 and ground.

Further, a high-pass filter consisting of a resistor R18 and a capacitor C9 is a coil 14 and an analog switch I.
It is located between C4.

The function of the circuit arrangement according to FIG. 3 will now be described.

The analog switch IC4 alternately connects the resistor R1 to the ground and the reference voltage (5V). Therefore, in the capacitor C1, a substantially triangular vibration (e-Funk) is generated.
)) occurs. On the non-inverting input side of the comparator K1, the resistor R4 causes hysteresis. The comparator K1 switches the analog switch IC4 every time the threshold value is reached.

The generated voltage is filtered by the two-stage passive low-pass filters R2, C2 and R3, C3, and further buffered and amplified by the operational amplifier OP1. As a result, a desired vibration with few harmonic components is generated. The coil 14 is powered by this vibration.

Since the switching elements 33 and 34 of the twin switches in the analog switch IC 4 are switched at the same time, the comparator K1 simultaneously supplies the clock to the synchronous rectifier 20. In this case, the switch element 34 is functionally equivalent to the analog switch 21 of the synchronous rectifier 20 according to FIG.
Corresponds to. However, since the switch elements 33, 34 are provided on the same chip, it is ensured that the phase position between the generated vibration, for example the sinusoidal vibration and the vibration of the synchronous rectifier, is absolutely constant. The absolute value of the phase position of the vibration of the synchronous rectifier is determined by two low pass filters R2, C2 and R.
3, C3 can be adapted to the respective required conditions.

The controller 17 controls the voltage supplied to the coil 14 as described in the description of FIG. Therefore, further explanation of the function and action is omitted here.

The voltage produced at the output of the controller or at one of the coil terminals is coupled to the synchronous rectifier 20 via a high pass filter consisting of a resistor R18 and a capacitor C9. By properly selecting or setting the resistor R25, the rise of the sensor signal is adjusted, and the generated offset is adjusted by the resistor R24. Signal filtering is performed by the capacitor C10. The output signal of the entire device is supplied to the output side 30 via the driver 32.

FIG. 4 shows an embodiment for a differential type inductive sensor. In this embodiment, the sensor 11 has two coils 35 and 36. The inductance of these coils changes in the opposite direction depending on the measured quantity.

The coils 35 and 36 receive the sinusoidal excitation signal of the oscillator 10 via the two resistors R29 and R31. Capacitors C13, C14 and resistors R30 and R
The terminal on the non-grounded side of the coil is connected via 32 to the synchronous rectifier 20.

The synchronous rectifier 20 is constructed similar to the embodiment of FIG. 3, for example. However, this synchronous rectifier 20 has twin switches which are switched simultaneously by the clock of the oscillator 10.

A controller, which may likewise be used additionally, is not incorporated in this embodiment.

Next, the function of the embodiment shown in FIG. 4 will be described.

The oscillator 10 produces sinusoidal vibration.
This sinusoidal vibration is supplied to the two coils 35 and 36 via resistors R29 and R31. If the resistance is chosen properly, the alternating current through the coil will be relatively constant in this embodiment as well.

The two induced voltages U21 and U22 are supplied to the synchronous rectifier by the two switching units 35 and 36. In this case it also subtracts at the same time. By narrowing down and selecting the sampling phase, the quantity to be measured can be detected.

The voltages U1 and U2 and the currents Ia, Ib and Ic shown in FIG. 4 are shown in FIG. In this case, the output voltage UA is output once through the additional filter and once without the additional filter. In this figure, the curve shown by the broken line represents the output voltage when it is filtered.

[0069]

The advantage obtained by the evaluation circuit for an inductive sensor according to the characterizing part of claim 1 of the invention is that a particularly simple and very accurate temperature compensation is possible without the use of an additional temperature sensor. That is.

[Brief description of drawings]

FIG. 1 shows a first embodiment of the invention with one coil.

FIG. 2 is a diagram showing a so-called vector diagram.

FIG. 3 is an overall block circuit diagram for an embodiment having one coil.

FIG. 4 is a diagram showing an embodiment for a differential type inductive sensor.

5 is a diagram of the signal curve according to FIG. 4 in terms of time without filtering and the signal curve of the exemplary embodiment in terms of time with filtering.

[Explanation of symbols]

 10 oscillator 10a output side 10b output side 12 resistance 13 connection point 14 measurement coil 15 capacitor 16 capacitor 17 controller 17a input side 17b input side 17c output side 18 capacitor 19 resistance 20 synchronous rectifier 21 analog switch 22 operational amplifier 23 resistance 24 resistance 25 Resistor 26 Operational amplifier 27 Inverter 28 Inverter 29 Filter 30 Analog output side 32 Driver stage 33 Switching element 34 Switching element R Resistance C capacitor IC4 Analog switch OP1 Operational amplifier OP3 Operational amplifier OP4 Operational amplifier OP5 Operational amplifier OP6 Operational amplifier K1 Comparator

Claims (10)

[Claims] 1. An evaluation circuit for an inductive sensor having at least one coil, an oscillator and a rectifier, wherein the inductance of the coil depends on a characteristic quantity to be measured, and the output signal of the oscillator is An evaluation circuit in which an induced voltage is supplied to the input side of a coil, and the induced voltage is supplied to the rectifier, wherein the rectifier is a synchronous rectifier (20), and the synchronous rectifier (20) includes an oscillator (10). ), The phase position of the clock signal is the resistance values (R), (R1), (R1) of the coils (14), (35), (36) with respect to the phase position of the induced voltage. Evaluation circuit for an inductive sensor, characterized in that it is chosen such that R2) and the inductances (L), (L1), (L2) can be measured separately. 2. The temperature compensation of the measurement signal is performed by the coil (1).
The evaluation circuit according to claim 1, which is performed by utilizing the temperature dependency of the resistance value (R) of 4), and in this case, the real part of the AC resistance value is obtained. 3. The phase position of the clock signal is the temperature compensated measured voltage from the difference between the imaginary part of the voltage induced in the coil (14) and the real part multiplied by a constant factor. The evaluation circuit according to claim 2, which is selected to occur. 4. The output signal of at least one of said coils (14) is controlled in a controller (17) before being fed to said synchronous rectifier (20). Evaluation circuit. 5. The synchronous rectifier (20) comprises two inverters (27, 28), the inverter (2)
7. The evaluation circuit according to claim 1, wherein the output signals of the coils (14) are alternately supplied to 7, 28). 6. A filter device (18, 19) is arranged on the input side of the synchronous rectifier (20).
The evaluation circuit according to any one of 1 to 5. 7. The evaluation circuit according to claim 1, further comprising a low-pass filter provided between the one or more coils and the synchronous rectifier. 8. A network for offset compensation is provided, which network is connected via a synchronous rectifier to another analog switch at selectable frequency and phase positions. Evaluation circuit according to item 1. 9. The differential amplifier having a low-pass characteristic is used as the synchronous rectifier (20).
The evaluation circuit according to any one of items 1 to 3. 10. A differential inductive sensor (11) having two coils (35, 36) is provided, the synchronous rectifier (20) having two switching elements, the switching elements being: The two coils (35, 3
Evaluation circuit according to any one of claims 1 to 9, which is connected to each one of 6) and switched in the same clock.