JPS6019464B2 - Impedance change detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an impedance change detection circuit that converts impedance changes into DC signals such as voltage or current.

Conventionally, when detecting changes in impedance, an AC voltage is applied to the impedance being measured to convert the change in impedance into an AC signal, and if a DC signal is required, this AC signal is rectified to generate the desired DC signal. I had a signal.

However, such a device requires an independent rectifier circuit, which not only complicates the circuit, but also has the disadvantage that it is difficult to obtain a stable DC signal. The present invention
The purpose is to eliminate the drawbacks of conventional devices such as those mentioned above, and to realize an impedance change detection circuit with a simple configuration that can detect impedance changes directly as a DC signal and obtain a stable DC signal. That is. The impedance change detection circuit of the present invention superimposes a DC voltage on the AC voltage applied to the impedance to be measured, controls the magnitude of this DC voltage to balance the circuit, and obtains a DC signal corresponding to the impedance change. This is how it was done.

Hereinafter, the impedance change detection circuit of the present invention will be explained using the drawings. FIG. 1 is a circuit diagram showing an embodiment of the impedance change detection circuit of the present invention.

In the figure, OSC is an AC power supply, Z and Z are impedances to be measured, and differential inductance is illustrated here. D,, D2 are diodes, R,, R2 are resistors, C,
, C2 are capacitors, and A is a differential input integrator.
The two impedances to be measured Z, , Z are connected in series and between the output terminals 1 and 1' of the AC power source OSC.

The first diode D, whose anode is the AC power supply OSC
The second diode D2 is connected to one output mode 1 of the
whose cathode is the other output terminal 1' of the AC power supply OSC.
It is connected to the. The two resistors R, , R2 are connected in series, and the first and second diodes ○
, , D2 between the output terminals 1 and 1' of the AC power supply OSC.

Two capacitors C, . C2 is connected in series and is connected between the output terminals 1 and 1' of the AC power supply OSC via the first and second diodes D, D2,
Further, the middle point of the contact is grounded. One input terminal (1) of the integrator A is connected to the connection midpoint of the resistors R, , R2, and the other input terminal (10) is grounded. Further, the output terminal is connected to the midpoint between the impedances Z and Z to be measured. The operation of the impedance change detection circuit of the present invention configured as described above is as follows.

Diode D,,D2, Resistor R,,R2, Capacitor C
,,C2 constitute a peak rectifier circuit for the AC power supply OSC, and the capacitor C, is charged with the voltage applied to the anode of the diode D, while the capacitor C2
is charged with the voltage applied to the cathode of diode D2. At this time, the suitability of each charging voltage is as shown in the figure. Here, the frequency of the AC voltage applied to the impedances B and Z to be measured is f, the peak value is Vi (hereinafter referred to as AC voltage Vi), the output voltage of the integrator A is Vo, and the circuit constants are C, R,,C2R2》; Then, the currents i,,i2 flowing through the resistors R,,R2 are expressed as follows.

i. : Toru (2. Branch V.

) '1'i2=R-shape (z sheep; vi-V.

) '21 Next, resistor R. , R2 are equal in size, the voltage appearing at the midpoint of the connection is equal to that of the capacitor C
．． , C2, and this voltage becomes the input voltage of the integrator A. This input voltage is applied to integrator A. It has a negative feedback effect on the output voltage Vo of the integrator A, and the integrator automatically adjusts the voltage so that the charging voltages of the capacitors C and C2 are equal to each other and the input voltage of the integrator A is zero. Perform a balancing operation. That is, the output voltage V of the integrator A,
As o increases, capacitor C increases through diode D,
, increases, and the voltage charged to capacitor C2 via diode D2 decreases. Therefore, the voltage appearing at the midpoint of the series of resistors K,, R2 increases,
It works to reduce the output voltage Vo of the integrator A. In this way, the output voltage yo of the integrator A has the function of adding a DC bias to the voltage drop occurring in the impedances to be measured Z, and equalizing the voltages charged in the capacitors C, , C2. If the measured impedances B and Z are not equal, they change to compensate for the resulting mismatch in the charging voltages of the capacitors C and C2. When this circuit is in equilibrium, the currents i,,i flowing through the resistors R,,R2
2 are equal to each other, and the output voltage Vo of the integrator A at this time is V.

= Ki - Kaname Sanhair / Vi '31.

That is, if the magnitude of the alternating current voltage Vi is constant, the output voltage Vo of the integrator A is equal to the impedance to be measured Z, Z2.
In the impedance change detection circuit of the present invention, the impedance change can be directly converted into a DC voltage. Further, this output voltage Vo is the output voltage of an integrator, and the resistors R, , R2, and the capacitor C.
, C2, the time constant (C, R, , C2R2) can be selected to be large, resulting in a stable DC signal.

Furthermore, in this circuit, the current flowing through the diodes D, D2 causes errors;
, , the time constant of the circuit connected to D2 (C, R, , C2R
2) is large, so when the circuit is balanced, the diodes D,...
Since it can be considered that almost no current flows through D2, there is virtually no problem. FIG. 2 is a circuit diagram showing another embodiment of the impedance change detection circuit of the present invention.

In the figure, parts similar to those in FIG. 1 are designated by the same reference numerals. E is an amplifier for controlling the output voltage of the AC power supply OSC.
s is a reference voltage. One input terminal of the amplifier (10)
The reference voltage Es is applied to the input terminal (1), and the charging voltage of the capacitor C is applied to the other input terminal (1). As mentioned above, when the circuit is balanced, the capacitor C is always charged with a voltage of 1/2 Vi, and the amplifier ~ compares this charging voltage with the reference voltage Es, and adjusts the AC power supply so that they are equal. The amplitude Vi of the output voltage of the OSC is controlled. Therefore, the amplitude of the alternating current voltage Vi is always a constant value, and the output voltage Vo of the integrator A is exactly proportional to the change in the impedances Z to be measured. In addition, in the above explanation, the impedance to be measured Z,
Although the diodes D, D2 are illustrated as rectifying elements that peak-rectify the terminal voltage of the Z (AC power source OSC), this is not limited to the diodes D, D2, and any device having a rectifying effect may be used. For example, as shown in FIG. 3, the same operation can be performed using transistors Tr, Tr2.

In this case, due to the current amplification effect of the transistors Tr, Tr2, the resistors R, . Since the equivalent impedance on the R2 side increases, the AC power supply OSC
It is possible to create a circuit that is not easily affected by the output impedance of the circuit. In addition, although the explanation has been made assuming that the resistors R and R2 are equal, if the magnitude of the resistor R or R2 is changed using a variable resistor etc., the output voltage at equilibrium can be
o can be changed, making it possible to adjust the zero point, etc. Similarly, the impedances to be measured Z, , Z are not limited to differential inductances, and only one of the inductances may change. Further, in the above description, the impedance change detection circuit of the present invention is applied to detecting differential inductance, but it can also be applied to detecting differential capacitance, etc. As explained above, in the variable impedance circuit of the present invention, a DC voltage is superimposed on the AC voltage applied to the impedance to be measured, and the magnitude of this DC voltage is controlled to balance the circuit. Therefore, an impedance change detection circuit that can immediately convert an impedance change into a stable DC signal can be realized with a simple configuration.

[Brief explanation of the drawing]

1 to 3 are circuit diagrams showing embodiments of the impedance change detection circuit of the present invention. OSC... AC power supply, A. ...Integrator, A2...Amplifier. Figure 1 Figure 2 Figure 3

Claims (1)

[Scope of Claims] 1. An AC power supply, two impedances including at least one impedance to be measured and connected in series between the output terminals of the AC power supply, and having different rectification directions and connected to the output terminals of the AC power supply. two peak rectifier circuits connected to each other and whose output ends are commonly connected via a resistor; and a differential input terminal with one input terminal grounded, and the other input terminal connected to the two peak rectifier circuits. an integrator to which a common output terminal of the two impedances is connected, and an integrator whose output terminal is connected to a midpoint between the two impedances. 2. The impedance change detection circuit according to claim 1, wherein the amplitude of the AC power source is controlled so that the output voltage of one of the two peak rectifier circuits is a constant value.