JPH0549948B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an impedance change detector that uses an impedance element whose impedance differentially changes depending on the amount of displacement, etc. to obtain an electrical output signal according to the amount of displacement, etc. It is.

Methods for obtaining displacement etc. as electrical output signals include the differential capacitance method, which detects displacement as a change in a pair of capacitances, the differential reactance method, which detects displacement as a reactance change, and the differential 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 differential capacitance type impedance change detector that has been widely used in the past. 1 is equipped with fixed electrodes 11 and 13 and a movable electrode 12 sandwiched between them, and when the electrode 12 moves due to external pressure or the like,
This sensor is configured such that the capacitance values C ₁ and C ₂ of the first and second impedance elements formed between 2 and 13 vary differentially. Capacitance values of the first and second impedance elements in this sensor 1
Changes in C ₁ and C ₂ are electrically detected by the output detection section 5. Reference numeral 2 denotes an oscillator that generates an alternating current signal whose amplitude is controlled by a control section 6, which will be described later. The number of turns of the secondary coils 3b and 3c of the transformer 3 is equal, and one terminal 31 and 32 of each of them is connected to the electrodes 11 and 1 of the sensor 1.
3 and their respective other terminals 33 and 3
4 have rectifying elements 401, 402 and 403, respectively.
One terminal of 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 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. The positive electrode side of the rectifying element 401 and the negative electrode side of the rectifying element 404 are 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 a resistor 43 and a capacitor 4.
The positive electrode side of the rectifying element 403 is connected to the electrode 12 through a parallel circuit with the resistor 45 and the capacitor 4.
6 and is connected to the electrode 12 through a parallel circuit. 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 is connected to the connection point 4 between the resistor 43 and the rectifying element 402.
Connection point 48 between 7 and resistor 45 and rectifying element 403
The oscillator 2 is controlled so that the voltage between the connection points 47 and 48 is constant.

Next, the operation of such a conventional impedance change detector will be explained. transformer 3 no 2
Although AC signals of equal magnitude are generated in the next coils 3b and 3c, the difference between the capacitances _C1 and _C2 of the first and second impedance elements causes the rectifier 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 hundred 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. 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 diodes 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 are connected to the electrodes 11 and 13, respectively, and negative electrodes are connected to the other end 34 of the secondary winding 3b, respectively. Further, the negative electrode sides of the other rectifying elements 402 and 403 are connected to the electrode 1113, and the positive 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.
2, and 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 impedance changes in the first and second impedance elements by the voltage between the connection points 47 and 48, but at the same time, subtle changes in characteristics due to the temperature of the rectifying element can also be detected at the connection point. This will have a large effect on the voltage between 47 and 48.

The present invention aims to eliminate such drawbacks of the prior art, and its structural feature is that the first and second impedance elements are incorporated into a bridge circuit in order to detect changes in impedance. It's in a place where it's not there. Therefore, a change in the characteristics of the rectifying element does not affect the current flowing through the first and second impedance elements, and by carrying out the present invention, it is no longer necessary to perform the conventional process of selecting a rectifying element. Further, the first period and the second period are alternately generated by a timing circuit,
An alternating current signal is supplied to the first impedance element in the first period, an alternating current signal is supplied to the second impedance element in the second period, and two signals are transmitted in the first period and the second period in a so-called time-sharing manner. First and second detection signals of voltages corresponding to the impedance of one of the two impedance elements are generated. The first impedance element corresponds to this pair of impedance elements.
The oscillator is controlled so that the sum or average value of the second detection signal is constant, and the voltage corresponding to the pair of impedance elements is extracted in a time-sharing manner during the first and second periods. The difference value is detected, and the amount of change in impedance of the first and second impedance elements is detected.

The present invention will be explained below based on the drawings.

FIG. 3 is a schematic configuration diagram showing an embodiment of the present invention. 1 is equivalent to the sensor explained in FIGS. 1 and 2 and is given the same reference numeral as in FIGS. 1 and 2, but other sensors such as differential reactance type or resistance type sensor Deyayoi. In the present invention, an alternating current signal is supplied to the sensor from the secondary winding 3b' of the transformer 3', and the alternating current signal is generated by the electrodes 11 and 12 during the first period by opening and closing the switch means 71 and 72. capacitance
The AC signal passes through the first impedance element portion of C ₁ , and during the second period, the AC signal passes through the second impedance element portion of the capacitance C ₂ formed by electrodes 12 and 13 . Therefore, the first and second
An alternating current signal is supplied to the first and second impedance elements, respectively, during the period . The alternating current signal that has passed through each impedance element is double-wave rectified by the rectifying and smoothing section 80, and is detected as a voltage according to the sum and difference values of the impedance values of the pair of impedance elements by a configuration described later.

That is, an alternating current signal is supplied from the oscillator 2 to the primary coil 3a' of the transformer 3'. One terminal 35 of the secondary coil 3b' is connected to switches 71 and 7, respectively.
2 to the electrodes 11 and 13 of the sensor 1, respectively, and the electrode 12 is connected to the secondary terminal 36 of the transformer 3' via a rectifying and smoothing circuit 80.
The rectifying and smoothing circuit 80 includes a rectifying element 801 and a rectifying element 8.
02 are respectively connected to the electrodes 12 in opposite polarity directions,
These rectifying elements 801 and 802 are connected to the other terminal 36 of the secondary terminal 3b' of the transformer 3' through a parallel circuit of a resistor 81 and a capacitor 82 and a parallel circuit of a resistor 83 and a capacitor 84, respectively. . Here, the switches 71 and 72 are alternately opened and closed by the timing circuit 70 during the first period and the second period, respectively. For example, during a first period, switch 71 is closed and switch 72 is open, and during a second period, switch 72 is closed and switch 71 is open. The cycles of the first and second periods created by the timing circuit 70 are set larger than the cycle of the AC signal generated by the oscillator 2.
When an AC signal with a period of about 500KHz is generated,
For example, the first and second periods occur alternately at a period larger than the oscillation period of the AC signal, such as a period of 4KHz.

FIG. 4 is a diagram of the voltage waveform generated between the electrode 12 of the sensor 1 and the other terminal 36 of the secondary coil 3b' of the transformer 3'. This waveform diagram shows a case where the capacitance C ₁ between the first impedance element, that is, the electrodes 11 and 12 is smaller than the capacitance C ₂ between the second impedance element, that is, the electrodes 12 and 13, The voltage drop between electrodes 11-12 during the first period is the same as the voltage drop between electrodes 12-13 during the second period.
greater than the voltage drop between. Therefore, the voltage generated between the electrode 12 and the terminal 36 in the first period has a smaller amplitude than the voltage generated in the second period. Therefore, a voltage having a waveform as shown in FIG. 4 is generated between the electrode 12 and the terminal 36 for the first period T ₁ and the second period T ₂ . Therefore, if the connection point between the rectifying element 801, the resistor 81, and the capacitor 82 of the rectifying and smoothing circuit 80 is 85, and the connection point between the rectifying element 802, the resistor 82, and the capacitor 84 is 86, both the connection points 85
-86, the first period is as shown in Figure 5.
A first detection signal _{e 1 is} generated in a time-sharing manner during T 1 and a second detection signal e ₂ is generated in a second period T ₂ .

Detection of voltage according to the sum value and difference value of each impedance of the first and second impedance elements is performed by
The first and second detection signals e ₁ and e ₂ are passed through a filter 9.
The output detection section 5', which includes the detection section 100 and the output section 110, performs the following processing. That is, the potential difference between the connection points 85 and 86 is smoothed by a time constant larger than the generation cycle of the first and second periods, and the output of the filter circuit 9 is the first and second detection signals _e1 and e. ₂ average value (e ₁ +
e ₂ )/2, that is, a signal corresponding to the sum value of the impedances of the first and second impedance elements is obtained. The control circuit 6' controls the amplitude of the AC signal generated by the oscillator 2 so that the output of the filter circuit 9 is constant.

In this way, the sum value, that is, the average value, of the first and second detection signals e ₁ and e ₂ is controlled to be constant, and the difference value therebetween is detected by the output holding circuit 100. The output holding circuit 100 is configured as follows, for example. That is,
A switch 101 is connected between the terminals 85 and 86, respectively.
Capacitors 103 and 104 are connected through 102, and switches 101 and 102 are opened and closed by a timing circuit 70 in accordance with the opening and closing of switches 71 and 72. Therefore, during the first period when switch 71 is closed and switch 72 is open, switch 101
is closed and switch 102 is opened, and during the second period, switch 101 is open and switch 102 is closed. Therefore, the first detection signal _e1 generated in the first period between the connection points 85 and 86 is held in the capacitor 103, and the second detection signal e1 generated in the second period is
e ₂ is held in the capacitor 104 and output to the output section 110
The difference between these signals e ₁ and e ₂ is calculated and output, and a signal having a value proportional to C ₁ -C ₂ /C ₁ +C ₂ can be obtained from the output detection section 5'.

As explained above, in the present invention, alternating current signals are applied to the first and second impedance elements alternately in a time-division manner, and therefore the first and second detection signals are generated in the smoothing circuit in a time-division manner. . Therefore, since the first and second detection signals are rectified by the same rectifier, errors due to temperature changes will not occur even if any rectifier is used. In contrast, conventionally, voltages corresponding to the sum value and the difference value are rectified by a rectifying element in a bridge circuit including first and second impedance elements. Therefore, if there is a difference in the temperature characteristics of the rectifying elements that create these sum values and difference values, a large error will occur in the detection output, so it has been necessary to match the rectifying characteristics by selecting each rectifying element. However, by implementing the present invention, it becomes possible to omit the work of selecting rectifying elements, which has traditionally been a major problem in process control. Furthermore, it is possible to eliminate errors caused by differences in temperature characteristics of the rectifying elements.

In the above explanation, an example was explained in which the signal generated between the electrode 12 and the terminal 36 is subjected to double-wave rectification, but the object of the present invention can also be achieved by half-wave rectification as shown in FIG. Of course.
That is, the rectifying and smoothing circuit 80' connects the electrode 12 and the terminal 3.
A series circuit of a rectifying element 801 and a parallel circuit of a resistor 81 and a capacitor 82 is connected between the rectifying element 801 and the rectifying element 802 in the opposite polarity direction. With such a configuration, the voltage generated between the electrode 12 and the terminal 36 can be half-wave rectified, and a voltage corresponding to the sum value and difference value of the respective impedance elements can be generated between the connection points 85' and 86'.

Note that the switch 7 shown in FIGS. 3 and 6
1, 72 and 101, 102 are each constituted by a pair of complementary switches, for example, a P channel.
MOSFET switch, N-channel MOSFET switch, complementary MOSFET switch,
Any arbitrary switch such as a janky FET switch or a bipolar transistor switch can be used. Furthermore, the switches 71 and 72 may be constructed from gate circuits instead of the switch elements as described above, and are not limited to the construction of the above embodiment.

[Brief explanation of the drawing]

1 and 2 are schematic configuration diagrams of a conventional impedance change detector, FIG. 3 is a schematic configuration diagram showing an embodiment of the impedance change detector of the present invention, and FIGS. 4 and 5 are schematic diagrams of the present invention. FIG. 6 is a schematic diagram showing another embodiment of the present invention. 1... Sensor, 11, 12, 13... Electrode, 2
...Oscillator, 5, 5'... Output detection section, 6, 6'...
...Control unit, 70...Timing circuit, 71,7
2, 101, 102... switch, 80... rectifying and smoothing circuit, 9... filter, 100... output holding circuit.

Claims (1)

[Claims] 1. First and second impedance elements whose impedances differentially change, an oscillator that generates an alternating current signal with controlled amplitude, and first and second periods with a period larger than the oscillation period of the oscillator are alternately arranged. and a circuit that alternately supplies the alternating current signal to the first impedance element during the first period and to the second impedance element during the second period using the output of the timing circuit; a rectifying and smoothing circuit that rectifies and smoothes alternating current signals that are alternately supplied to the first and second impedance elements to obtain first and second detection signals during the first and second periods; a holding circuit that holds each of the second detection signals; an output detection section that outputs a difference between the first and second detection signals based on the output of the holding circuit; and an average value of the outputs of the rectification and smoothing circuit that is constant. an impedance change detector comprising: a control section for controlling the oscillator;