JPS5937916Y2 - capacitive displacement transducer - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a capacitive displacement transducer that extracts a signal in accordance with the amount of displacement by differentially changing the capacitance value of a pair of capacitors in accordance with the amount of displacement.

This type of capacitive displacement transducer is known from Japanese Utility Model Publication No. 15398/1983 entitled "Displacement Transducer Using Capacitance Change".

First, this will be briefly explained with reference to FIG.

In the figure, numerals 1 and 2 indicate capacitors whose capacitances differentially change depending on the room capacity, and one end of each of these capacitors 1 and 2 is connected to one end of an AC signal source 3.

The AC signal from these three AC signal sources is maintained at a constant current,
This current is distributed according to the respective impedances of capacitors 1 and 2, and the AC signals flowing through these capacitors 1 and 2 are half-wave rectified by first and second half-wave rectifier circuits 4 and 5, respectively.

That is, the other end of the capacitor 1 is connected to the other end of the AC signal source 3 through a rectifier 6 and further through a resistor 7-8, and a smoothing capacitor 9 is connected in parallel with the resistor 1-8 to form a rectifier circuit 4. Ru.

The other end of the capacitor 2 is connected to the other end of the AC signal source 3 through a rectifier 10 and further through a resistor 11-8, and a smoothing capacitor 12 is connected in parallel with the resistor 11-8 to form a rectifier circuit 5. be done.

In order to pass the other half-wave of the alternating current signal source 3, the connection points of the capacitors 1 and 2 with each rectifier 6, 10 are connected through a rectifier 13, 14 of opposite polarity to this rectifier, respectively, and a resistor 15.
and a capacitor 16 connected to the other end of the AC signal source 3 through a parallel circuit.

The rectified outputs of rectifier circuits 4 and 5 correspond to the respective capacitance values of capacitors 1 and 2. Therefore, when the displacement to be converted is zero, the capacitance values of both capacitors 1 and 2 are equal, and the outputs of both rectifier circuits are also equal. When the capacitance of one of the capacitors 1 and 2 increases due to the displacement, and the capacitance of the other decreases, the output of the rectifier circuit connected to the capacitor whose capacitance has increased increases, while the output of the other capacitor decreases.

By taking out the difference between the outputs of these rectifying circuits 4 and 5, the amount of displacement can be detected.

In order to detect this output difference, the connection point between rectifier 6 and resistor 1 and the connection point between rectifier 10 and resistor 11 are connected to two input terminals of differential amplifier 19 through resistors 17 and 18, respectively.

The differential voltage obtained at the output of the differential amplifier 19 is supplied to the base of the transistor 200 and converted into a current.

A current corresponding to the displacement obtained in the emitter of the transistor 20 flows through the emitter resistor 21. Furthermore, it is supplied to one output terminal 23 through a feedback resistor 22.

The collector of transistor 20 is connected to the other output terminal 24.

These terminals 23 and 24 are connected to a power source on the receiving side through, for example, a two-wire transmission line.

1st. The sum of the rectified outputs of the second rectifying circuits 4 and 5 is connected to the resistor 8.
The connection point of this resistor γ, 8 is connected to the inverting input terminal of the comparison circuit 260 of the control circuit 25 for the AC signal source 3.

On the other hand, the terminal 24 is connected to one end of the Zener diode 27 through the constant current circuit 36, and the Zener diode 2T
Voltage dividing resistors 28 and 29 are connected in parallel with the voltage dividing point thereof, and the voltage dividing point thereof is connected to the non-inverting input terminal of the comparator circuit 26 as a reference voltage.

The other end of the Zener diode 2T is connected to the connection point between the resistor 8 and the capacitor 9 to form a common potential point 35, and this common potential point is connected to the connection point between the resistors 21 and 22.

so that the sum of rectified outputs obtained by resistor 8 is constant.
The AC signal source 3 is controlled by the output of the comparison circuit 26, and as a result, the output of the AC signal source 3 is maintained at a constant current.

The resistor 22 is a variable resistor, and its mover is connected to the inverting input terminal of the differential circuit 190 through the resistor 30 to apply feedback, and the non-inverting input terminal of the differential circuit 19 is connected to the terminal 31.
A fixed bias is provided through resistor 32.

As mentioned above, in this type of capacitive displacement transducer, it is necessary to maintain the AC signal current applied to the capacitors 1 and 2 constant. The voltage drop is compared with the reference voltage, and the comparison output is used to control the AC signal source 3.
was under control.

Generally, it is considered difficult to control the output amplitude of an AC oscillator to be constant.

Conventionally, this has had the disadvantage that the configuration of the AC signal source 3 is complicated.

This idea is based on this point. With a relatively simple configuration, a DC constant current circuit is inserted in series in the path through which the sum current of each output of the second rectifier circuit flows, and AC flows through a pair of capacitors whose capacitance differentially changes according to displacement. Ensures that the sum of the signals remains constant.

For example, as shown in FIG. 2 with the same reference numerals assigned to parts corresponding to those in FIG.
is connected to the common potential point 35 through the collector of the transistor 3T, the terminal □, and further through the resistor 38.

The connection point between the emitter of the transistor 3γ and the resistor 3B is connected to the inverting input terminal of the comparison amplifier circuit 390, and the non-inverting input terminal of the comparison amplifier circuit 39 is connected to the voltage dividing resistor 28,
The output terminal of the amplifier circuit 39 is connected to the base of the transistor 3γ.

No feedback control is performed on the AC signal source 3, and the terminal 24
is connected to the AC signal source 3 through the amplifier 40, and the AC signal source 3 operates.

Since it is a two-wire converter, the current flowing between the terminals 24 and 23 changes depending on the converted signal, so the amplitude of the output AC signal from the AC signal source 3 also changes.

However, the sum of the respective outputs of the rectifier circuits 4 and 5 is held constant by a constant current circuit 41 including a transistor 37, a resistor 38, an amplifier 39, and the like.

Now, the capacitances of capacitors 1 and 2 are C1 and C2, the resistance values of resistors 7 and 11 are R1, the output signal amplitude of AC signal source 3 is E, and its angular frequency is ω, and capacitors 1 and 2 are
If each impedance is sufficiently thicker than each resistance value of the resistors 7 and 11, the difference output voltage V between the outputs of both rectifier circuits becomes V = ωE (C2C1)R(1).

Since the sum of the currents 11 and 12 flowing through the resistors 7 and 11 is equal to the output current 1 of the constant current circuit 41, ■=ωE(C2+
CI, + (2).

Therefore, from equations (1) and (2), 2-C1 V=IR C2+C1 is obtained, and the influence of the temperature of each capacitance of capacitors 1 and 2 on the conversion output is removed, and at the same time, the AC signal source 30
It is not affected by fluctuations in the angular frequency ω.Furthermore, the AC signal source 3 does not require a complicated circuit configuration to keep the output amplitude constant.

In particular, since DC constant current circuits generally have a simple structure and a good stabilization rate can be easily obtained, the circuit structure can be simplified.

In particular, a simple combination of a field effect transistor and a resistive element is commercially available as a single element, and if this is used, the overall configuration becomes extremely simple.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing a conventional capacitive displacement converter, and FIG. 2 is a connection diagram showing an example of a capacitive displacement converter according to this invention. 1 and 2: Capacitor whose capacitance changes differentially depending on displacement, 3: AC signal source, 4 and 5: Half-wave rectifier circuit, 19:
Differential circuit, 20: voltage-current conversion transistor, 41: constant current circuit.

Claims (1)

[Scope of utility model registration request] A pair of capacitors whose capacitance value changes differentially according to the amount of displacement, and one end of which is connected in common; one AC signal source that applies an AC signal to these capacitors; and the AC signal source that applies an AC signal to each of the capacitors. first and second rectifier circuits that half-wave rectify an alternating current signal; a difference circuit that differentially extracts the rectified outputs of these rectifier circuits; a voltage-current conversion circuit that converts the output into a current; A capacitive displacement converter comprising a direct current constant current circuit connected in series to a path through which a current equal to the sum of the outputs of the second rectifying circuit flows.