JPH0549949B2 - - Google Patents

Description

Detailed Description of the Invention The present invention uses an impedance element in which at least one of its impedances changes depending on the amount of displacement, etc. corresponding to the process amount, and changes the impedance to obtain an electrical output signal according to the amount of displacement, etc. It is related to the detector.

Methods for obtaining displacement, etc. corresponding to the process amount as an electrical output signal include the capacitance method, which detects displacement as a change in a pair of capacitances, the reactance method, which detects displacement as a reactance change, and the reactance method, which detects displacement as a change in resistance such as a strain gauge. There are various methods such as resistance method. FIG. 1 is a schematic configuration diagram showing an example of a capacitance type impedance change detector that has been widely used in the past. 1 is a sensor equipped with fixed electrodes 11, 13 and a movable electrode 12 sandwiched between them; the electrode 12 moves due to external pressure, etc.;
The capacitance values C ₁ and C ₂ of the first and second impedance elements formed between the electrodes 11-12 and 12-13 are configured to vary differentially. Changes in the capacitance values C ₁ and C ₂ of the first and second impedance elements in the sensor 1 are electrically detected by the output detection section 5 . 2 is an oscillator that generates an AC signal whose amplitude is controlled by a control unit 6, which will be described later, and the AC signal from this oscillator 2 is transmitted to a transformer 3.
is supplied to the primary coil 3a. The number of turns of the secondary coils 3b and 3c of the transformer 3 is equal, one terminal 31 and 32 of each is connected to the electrodes 11 and 13 of the sensor 1, and the other terminal 33 and 34 of each are connected to a rectifying element, respectively. One terminal of 401, 402 and 403, 404 is connected.

The negative electrode side of the rectifying element 401 and the positive electrode side of the rectifying element 402 are connected to the terminal 33 of the secondary coil 3b, and the negative electrode side of the rectifying element 403 and the positive electrode side of the rectifying element 404 are connected to the terminal 34 of the secondary coil 3c. It is connected to the. Positive electrode side of rectifying element 401 and rectifying element 4
The negative electrode side of 04 is connected to the electrode 12 via a parallel circuit including a resistor 41 and a capacitor 42.
Further, the negative electrode side of the rectifying element 402 is connected to the electrode 12 through a parallel circuit of a resistor 43 and a capacitor 44, and the positive electrode side of the rectifying element 403 is connected to the electrode 12 through a parallel circuit of a resistor 45 and a capacitor 46. ing. Further, the output detection section 5 detects the voltage across the parallel circuit of the resistor 41 and the capacitor 42, that is, the voltage between the connection point 49 and the electrode 12 in FIG. The control unit 6 detects the voltage between the connection point 47 between the resistor 43 and the rectifying element 402 and the connection point 48 between the resistor 45 and the rectifying element 403, and detects the voltage between the connection point 47-
The oscillator 2 is controlled so that the voltage across 48 is constant.

Next, the operation of such a conventional impedance change detector will be explained. transformer 3 no 2
AC signals of equal magnitude are generated in the next coils 3b and 3c, but due to the difference between the capacitances _C1 and _C2 of the first and second impedance elements, the rectifying element 4
01, 402 and the currents i ₁ , i ₂ and i ₃ , i ₄ flowing through the rectifying elements 403, 404 change. Rectifying element 40
1 to 404 rectify the alternating current signals applied to the electrodes of the sensor 1, and the rectified currents i ₁ to i ₄ flow as indicated by arrows in FIG. Therefore, currents i ₁ and i ₄ are present across the parallel circuit of resistor 41 and capacitor 42.
A voltage corresponding to the difference value is generated. Also resistance 43
A voltage corresponding to the sum of currents i ₁ and i ₄ is generated across the circuit in which the parallel circuit of capacitor 44 and the parallel circuit of resistor 45 and capacitor 46 are connected in series.

Here, resistors 41, 43, 45 and capacitor 4
If the values of 2, 44, and 46 are made equal, i ₁ = −i ₂ ,
The relationship i ₃ =-i ₄ holds true, and a signal with a value proportional to C ₁ −C ₂ /C ₁ +C ₂ can be obtained from the output detection section 5.

Here, the capacitance C ₁ and the capacitance of the first and second impedance elements between the electrodes 11-12 and 12-13 are
C ₂ is generally around several 100 pF, and the amount of displacement of capacitance C ₁ and C ₂ that changes due to external displacement is several tens of pF.
It is the rank. Therefore, in this method, it is desirable that the resistors, capacitors, diodes, etc. used be ones whose characteristics do not change with temperature. By using high-quality components for resistors and capacitors, it is possible to select resistors and capacitors whose characteristics change little with respect to temperature changes. However, the rectifying elements 401 to 404, which are semiconductor components, have unique temperature characteristics depending on each element. For example, the carrier accumulation effect, interelectrode capacitance, internal resistance, etc. have unique characteristics for each element. ing. Therefore, in conventionally using the four rectifying elements 401 to 404,
Rectifying characteristics of rectifying elements 402 and 403 and rectifying element 4
It is necessary to select so that the rectifying characteristics of the rectifying elements 401 and 404 are the same, so that the detection accuracy does not deteriorate even if the characteristics of the rectifying elements 401 to 404 change due to temperature. This process of selecting rectifying elements with similar characteristics is not desirable from the standpoint of process control or quality control, and is a major impediment to rationalizing production.

Furthermore, FIG. 2 is a schematic configuration diagram showing another example of a conventionally used differential capacitance type impedance detector, and the same elements as in FIG. 1 are given the same symbols and detailed explanations are omitted. However, in this method, one terminal 33 of the secondary winding 3b of the transformer 3 is connected to the electrode 12, and the electrodes 11 and 13 are connected to the other terminal 3 of the secondary winding 3b through a rectifying and smoothing circuit.
Connected to 4. That is, the rectifying elements 401, 4
04 is connected to the electrodes 11 and 13, respectively, and its positive electrode side is connected to the other end 34 of the secondary winding 3b, respectively. Further, the positive electrode sides of the other rectifying elements 402 and 403 are connected to the electrode 1113, and the negative electrode sides are connected to the resistor 41 and the capacitor 4 through parallel circuits of a resistor 43 and a capacitor 44, and a resistor 45 and a capacitor 46, respectively.
The other end of the parallel circuit of resistor 41 and capacitor 42 is connected to terminal 34.

In this way, in this method, the rectifying elements 402, 40
3 are connected in the same polarity direction, a voltage corresponding to the difference between the currents i ₁ ' and i ₂ ' flowing through the rectifying elements 402 and 403 is applied to the connection point 47 between the rectifying element 402 and the resistor 43, and the rectifying element 403 and the connection point 48 of the resistor 45, and a voltage corresponding to the sum of the currents i ₁ ' and i ₂ ' is generated between the connection point 49 of the resistors 43 and 45 and the terminal 34. Connection point 49 and terminal 34
The control unit 6 controls the oscillator 2 so that the voltage according to the sum value between them becomes constant, and the voltage according to the difference value between the currents i ₁ ′ and i ₂ ′ generated between the connection points 47 and 48 The value of C ₁ −C ₂ /C ₁ +C ₂ can be detected.

However, even in the conventional system as shown in FIG. 2, there is a drawback that an error occurs in the output because the characteristics of the rectifying element change depending on the temperature.

That is, in the conventional method shown in FIGS. 1 and 2,
A so-called bridge circuit including first and second impedance elements is constructed between the connection point 49 and the electrode 12. Therefore, it is possible to detect subtle changes in the impedance of the first and second impedance elements based on the voltage between the connection points 47 and 48, but at the same time, it is also possible to detect subtle changes in the characteristics of the rectifying element due to temperature. This will have a large effect on the voltage between 47 and 48. Furthermore, these conventional systems have the disadvantage that the amplitude of the AC signal generated by the oscillator 2 must be controlled by the control section 6, which makes the circuit configuration complicated.

The present invention aims to eliminate these drawbacks of the prior art, and uses a novel configuration to realize an impedance change detector that has an extremely simple circuit configuration and is not affected by temperature. The amplitude of the AC signal generated from the oscillator is always constant, and the AC signal is supplied to a series connection circuit of the first and second impedance elements,
The voltage drop due to the alternating current signal in the first and second impedance elements is measured alternately in a time-division manner by the same detection unit, and the impedance of the impedance element is determined based on the difference value of the voltage drop in each impedance element. The purpose is to detect differences. The present invention will be explained below based on the drawings.

3 and 4 are diagrams for explaining the principle structure and operation of the present invention. That is, in the present invention, the detection circuit 7 first measures the voltage drop in the first impedance element (impedance value Z ₁ ) as shown in FIG.
During this period, the voltage drop across the second impedance element (impedance value Z ₂ ) is measured as shown in FIG. 3b. The series circuit of the first and second impedance elements is alternately connected in opposite directions to the oscillator 2' during the first period and the second period. For example, during the first period, an AC signal is supplied from the oscillator 2'. As shown in Figure a, the AC signal is supplied to the second impedance element and grounded via the first impedance element, and during the second period, an AC signal is supplied from the oscillator 2' to the first impedance element as shown in Figure b. supplied,
Grounded via the second impedance element.
The first period and the second period are selected to have a period larger than the period of the AC signal, and the first and second periods occur periodically and alternately. Therefore, at the connection point 12' between the first impedance element and the second impedance element, assuming that the AC signal voltage is e, the voltage at the ground side impedance element during the first period and the second period is as follows. The voltage e ₁ corresponding to the drop,
e ₂ occurs.

e ₁ =Z ₁ /Z ₁ +Z ₂・e...(1) e ₂ =Z ₂ /Z ₁ +Z ₂・e...
... (2) The AC waveform generated at these connection points 12' is detected by the detection section 7 including rectifying elements 701, 702, a resistor 71, and a capacitor 72, and from this detection section 7, the first waveform is detected as shown in FIG. 3c. period T ₁ to e ₁ ,
A signal with a voltage of e ₂ can be obtained during the second period T ₂ . Therefore, the difference voltage e _d = e ₁ − e ₂ between voltages e ₁ and e ₂ is
The value corresponds to the impedance difference of the impedance elements Z ₁ - Z ₂ /Z ₁ + Z ₂ , and when _the present invention _is implemented in a differential capacitance method, C A signal with a value proportional to ₁ −C ₂ /C ₁ +C ₂ can be obtained. Also the first and second
The AC waveforms at the connection point 12' occurring during the period are detected by the same detection section 7, respectively. Therefore, the characteristics of the rectifying elements 701 and 702 change depending on the temperature,
For example, even if the barrier voltage changes, the voltage waveform obtained from the detection unit 7 will be different from the first and second voltage waveforms as shown in FIG. 3d.
It appears as a voltage shifted by the same potential during periods T ₁ and T ₂ , respectively, and the difference voltage ed remains constant even if the characteristics of the rectifying element in the detector section change. Furthermore, as can be seen from equations (1) and (2), simply by keeping the amplitude of the AC signal e generated from the oscillator constant, it is possible to obtain a voltage corresponding to the impedance difference between the impedance elements, unlike the conventional technology. There is no need to control the amplitude of the AC signal generated from the oscillator.

FIG. 4 is a diagram for explaining the principle structure and operation when the present invention is implemented in a differential impedance type impedance change detector, and the same elements as in FIG. Omitted. An alternating current signal of constant amplitude is alternately applied from the oscillator 2' to the electrodes 11 or 13 via a switch circuit 81 or 82. Therefore, for example, during the first period, an AC signal is applied to the electrode 11 via the switch circuit 81, and the electrode 13 is grounded via the switch circuit 82, and during the second period, the switch circuit 8 is applied to the electrode 11.
An alternating current signal is applied to the electrode 13 via the switch circuit 81, and the electrode 11 is grounded via the switch circuit 81. The switch means 81 and 82 are respectively shown in FIG. 4b.
Signals A and B turn on the switch means 81 and 82 during the first period and the second period, respectively, as shown in FIG.
is given. As a result, during the first period _T1, the AC signal is applied to the electrode 11 via the switch means 81, as shown in waveform a in FIG.
is grounded via switch means 82. Further, during the second period _T2, the AC signal is applied to the electrode 13 via the switch means 82, as shown by waveform b in FIG. 4c, and the electrode 11 is grounded via the switch means 81. In this way, an AC signal with a constant amplitude is generated from the oscillator 2', and this AC signal is applied from opposite directions alternately to a series circuit in which the first and second impedance elements are connected in series, thereby reducing the voltage drop in each impedance element. are detected by the same detector.

FIG. 5 is a diagram showing a specific embodiment in which the present invention is applied to a differential capacitance type impedance detection circuit, and the same elements as in FIGS. Omitted. 21 is an oscillator to which, for example, inverters 201 to 205 are connected in a circular manner. The alternating current signal generated by the oscillator 21 is supplied to one terminal of NAND circuits 81 and 82, respectively, and the alternating current signal is supplied to the flip-flop 8, which is the circuit portion that generates the first and second periods.
01 to 808 are frequency-divided by a step-down counter 800 connected in series, and a frequency-divided output A of the first and second periods having a cycle larger than the cycle of the AC signal and a negative output B of the frequency-divided output A are obtained. but,
The signals are supplied to the other terminals of the NAND circuits 81 and 82, respectively. The output of the NAND circuit 81 is connected to the electrode 11, the output of the NAND circuit 82 is connected to the electrode 13, and the detection section 7 is connected to the electrode 12. The above inverters 201 to 205, flip-flop 8
01 to 808, the NAND circuits 81 and 82 have a constant voltage generated by the output circuit 100, which will be described later, for example.
When driven by Vcc and using C-MOS type elements as the NAND circuits 81 and 82, an AC signal with a constant amplitude is supplied to the series circuit of the first and second impedance elements.

The detection unit 7 includes, for example, rectifying elements 701 and 702, a resistor 71, and a capacitor 72. The positive electrode side of the rectifying element 701 and the negative electrode side of the rectifying element 702 are connected to the electrode 12, and the negative electrode side of the rectifying element 701 is connected to the resistor 7.
1 and a capacitor 72, and the positive electrode side of the rectifying element 702 is grounded. In this manner, the detection unit 7 detects the alternating current potential generated between the electrode 12 and the ground potential.
Therefore, during the first period, an alternating current signal is applied from the electrode 11 by the frequency-divided output A, and the electrode 13 side is grounded, so that the connection point 73 between the rectifying element 701 and the resistor 71 is connected between the electrodes 11 and 13. A DC voltage corresponding to the voltage drop is obtained. In addition, during the second period, an AC signal is applied from the electrode 13 by the frequency-divided output B, and the electrode 11 side is grounded, so that the electrode 11 is connected to the connection point 73.
A voltage corresponding to a voltage drop between .about.12 is obtained.

Here, the voltage generated at the connection point 73 between the rectifying element 701 and the resistor 71 has _a waveform as shown in FIG. The difference value e _d between _{the two} is determined by the measurement unit 9. That is, the connection point 73 is connected to a resistor 93 via a capacitor 91, and the connection point between the capacitor 91 and the resistor 93 is connected to a switching element 9 such as an FET.
2, and the end of the resistor 93 on the side not connected to the capacitor 91 is connected to the capacitor 94.
grounded via. The switching element 92 outputs one output B of the step-down counter 800.
For example, the switching element 92 is turned on during a second period when an AC signal is applied from the electrode 13. Therefore, the voltage e ₂ generated at the connection point 73 during the second period is always stored in the capacitor 91, and as a result, the voltage e ₁ −e ₂ = _ed is stored in the capacitor 94. In this way, capacitor 9
A voltage ed proportional to the value of C ₁ −C ₂ /C ₁ +C ₂ is stored in 4, but this voltage _ed is further converted into a current signal by the output terminal 101, 102 by the output section 100 and sent to a remote location. transmitted to.

The configuration of this output section 100 is conventionally known as a two-wire transmitter, so a detailed explanation will be omitted.
Add a brief explanation. That is, the output terminal 10
1 and 102 supply DC voltage necessary for circuit operation. 103 is an FET that constitutes a constant current device, and 104 is a Zener diode that connects the output section 100.
A constant voltage Vcc is created within the circuit. capacitor 9
The voltage ed stored in the output terminal 4 is connected to the + terminal of the operational amplifier 109 via a resistor 105, and this + terminal and the output terminal 102 are connected via a resistor 106. On the other hand, the - terminal of the operational amplifier 109 is given a divided potential of the reference potential by the variable resistor 107 or another dividing resistor, and the output of the operational amplifier 109 drives the output transistor 110, and the output flows to the output terminals 101 and 102. Control the current. Note that the resistor 108 is a feedback resistor connected between the output transistor 110 and the output terminal 102, and is negatively fed back so that the feedback voltage generated at the feedback resistor 108 is subtracted from the voltage ed stored in the capacitor 94. The output current is controlled by In this way, a current proportional to the voltage ed stored in the capacitor 94 is output from the output terminals 101 and 102.

Note that the present invention is not limited to the above-mentioned embodiments, and can take various configurations without departing from its purpose. For example, as shown in FIG. 6, inverters 206 and 207, a capacitor 208, and a resistor 209 are connected in a ring, and a capacitor 2
08 and the connection point between resistor 209 and inverter 207
Oscillator 2 with a resistor 210 connected between the input of
Oscillators of various configurations can be used, such as 2 oscillators. In addition, the circuit configuration for generating the first and second periods is not a configuration in which the output of an oscillator that generates an AC signal is divided, but as shown in FIG. A signal with a period greater than that of the AC signal
A configuration in which A' and B' are generated may also be used. In the example of FIG. 6, inverters 811, 812, capacitor 8
13, the first and second periods are created by the output of an oscillation circuit including resistors 814 and 815, and the inverted output of that output by an inverter 813.

Furthermore, the configuration of the measurement section 9 also includes a switching element 92.
It is also possible to insert another switching element 95 between the capacitor 94 and the capacitor 94, and to open and close this switching element 95 complementary to the switching element 92.

In the embodiment of the present invention, a case has been described in which a pair of impedance elements are differentially changed, but the above-mentioned effects can be obtained even if only one of the impedance elements is changed.

As described above, the present invention provides an alternating current signal with a constant amplitude to a series connection circuit of first and second impedance elements, and detects voltage drops in the first and second impedance elements by the same detection unit. This is an extremely novel configuration that detects the voltage in parts and obtains a signal according to the difference value of each voltage drop.As the amplitude of the oscillator only needs to be constant, there is no need to perform amplitude control as in the past. . In addition, since the voltage drop in each impedance element is detected by the same detection section, even if the characteristics of the rectifying element included in the detection section change due to the influence of temperature, etc., the voltage drop in each impedance element will be detected during the first and second periods. The fluctuations in the outputs of the two parts become equal, so the difference value is not affected by changes in the characteristics of the rectifying element.

As described above, by implementing the present invention, the circuit configuration becomes simple and many of the drawbacks of the prior art can be solved at once.

[Brief explanation of drawings]

Figures 1 and 2 are schematic diagrams of a conventional impedance change detector; Figures 3a, b, and 4a are diagrams of the principle configuration of the present invention; Figures 3c, d, and 4b. , c are diagrams for explaining the operation of the present invention,
5 and 6 are connection diagrams showing an embodiment of the present invention. 1... Sensor, 11, 12, 13... Electrode, 2
1, 22, 810...Oscillator, 7...Detector, 7
01,702... Rectifying element, 72,91,94...
...Capacitor, 800...Step-down counter, 9...Measurement section, 100...Output section.

Claims (1)

[Claims] 1. An oscillator that generates an alternating current signal whose amplitude is controlled to be constant with respect to a common potential, first and second impedance elements whose impedance changes at least one of them, and a series circuit of the first and second impedance elements. a switch circuit that operates to alternately connect the oscillator and the oscillator in opposite directions at a cycle longer than the cycle of the alternating current signal; a detection unit that measures the potential at the connection point of the first and second impedance elements; The present invention is characterized by comprising a circuit for determining a difference value between signals outputted from a detection section in synchronization with the operating cycle of the switch circuit, and detecting a change in impedance of the first and second impedance elements based on the difference value. impedance change detection circuit.