JPH0353181Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a bridge measurement circuit that measures capacitance.

In the field of industrial measurement, capacitance measurement is one of the powerful means of detecting the position, shape, properties, and changes in these of objects to be measured. Incidentally, capacitance measurement in the field of industrial measurement is different from capacitance measurement of circuit elements, and currently there are few circuits that are suitable for a wide range of uses. The requirements for capacitance measurement in the industrial measurement field are, firstly, the ability to perform completely continuous automatic measurements, and secondly, the ability to measure small changes in the measured object, which has a relatively large individual capacitance. Third, it must be possible to detect only
It is required to be able to measure the change over a wide range and to have good linearity.Fourthly, it is necessary to be able to measure even unbalanced capacitance with one end grounded.Fifthly, it is necessary to be able to measure even unbalanced capacitance that is grounded at one end. Because large conductances often overlap in parallel due to the influence of industrial chemicals, it is necessary to detect only the imaginary term of the admittance vector without being affected by these.

The most likely way to satisfy the second requirement is to use a bridge; in fact, if manual adjustment is possible, it is not possible to measure capacitance with a bridge while satisfying the other requirements. Although simple, when limited to automatic measurement, the simple bridge measurement method conflicts with some requirements and requires the provision of a complex servo circuit or the construction of a transformer bridge. However, in industrial measurement, it is desirable to avoid complex servo circuits as much as possible to ensure reliability, and on the other hand, transformer bridges, which require strict electrostatic shielding and winding balance, are unsuitable for mass production. It is.

For these reasons, in the industrial measurement field, apart from cases where it is used on a limited number of objects, capacitance measurement used on a wide range of objects requires either a method that utilizes the characteristics of an LC resonant circuit, or a method that uses constant current charging and discharging. Although it was common to make use of the characteristics, the former cannot satisfy the third requirement because the Q of the circuit cannot be made small, and the latter cannot satisfy the fifth requirement.

This invention was devised in view of the above points and aims to provide a capacitance measuring circuit that is simple, has good performance, and is suitable for a wide range of uses.

This invention will be described below based on embodiments shown in the accompanying drawings.

First, to describe the principle of measurement, as shown in FIG. 1, a load _L1 with an admittance _Y1 is connected to the emitter of the transistor _Q1 . If the current amplification factor hfe of the transistor Q ₁ is sufficiently large, the following relationship holds true between the AC output current i ₁ and the AC input voltage v of the transistor Q ₁ : i ₁ =Y _1V (1). Therefore, this becomes a voltage-current conversion circuit having constant current output characteristics, and if the input voltage v is kept constant, it functions as an admittance _Y1 detection circuit. Note that when using this circuit with an appropriately determined DC operating level, the influence on admittance Y ₁ can be eliminated by using a constant current element as an emitter load for determining the DC output current level.

Moreover, as shown in FIG. 2, if two circuits shown in FIG. 1 are connected in parallel and _the input voltage v is made common, that is, loads _{L 1 and} L ₂ are connected in parallel and an input voltage v is applied to the bases of transistors Q ₁ and Q ₂ , the admittances L ₁ and L ₂ of loads L 1 and L ₂ are
The difference in L ₂ can be detected as the difference in output currents i ₁ and i ₂ .

Next, to explain the processing of the detected AC output current, as shown in FIG. 3a, two transistors Q ₃ and Q ₄ are connected in parallel, the base of transistor Q ₄ is grounded, and If a control pulse P that changes between positive and negative is applied to the base of Q ₃ as a control input signal P, the transistor acts as a switch element when operating in the saturation region, so the switching that is switched by the control pulse P is possible. This becomes a switch circuit, and its equivalent circuit is as shown in FIG. 3b. In addition, as long as the amplitude of the control pulse P is sufficient, a sine wave can have the same effect in practical terms.Also, as shown in Fig. 4, by using two circuits of the circuit shown in Fig. 3a, , 4 transistors
Q ₃ , Q ₄ , Q ₅ , and Q ₆ can constitute a mutual changeover switch circuit that mutually switches and selects output currents i ₃ and i ₄ .

Therefore, by cascading the circuit shown in FIG. 2 and the circuit shown in FIG. 4, a bridge circuit for measuring capacitance can be constructed as shown in FIG. 5. That is, as shown in FIG. 5, a switching circuit 1 consisting of transistors Q ₃ to _{Q 6}
The input terminal of the switching circuit 1 and the output terminal of the voltage-current conversion circuit 2 consisting of transistors Q ₁ and Q ₂ are connected in cascade, and one of the output terminals of the switching circuit 1 is connected to a detection resistor.
The other is connected to _{the detection resistor R 2 ,} and the admittance Y is connected to one of the detection terminals of the voltage-current conversion circuit 2.
The object to be measured 4 is connected thereto, and a reference capacitor C ₀ is provided on the other side. Further, a control signal generation circuit 5 is connected to the control input B of the switching circuit 1, that is, the base circuit of the transistors Q ₃ and Q ₆ , while the detection outputs a and b of the voltage-current conversion circuit 2 are connected to a differential amplifier. A circuit 6 is connected. A compensation circuit 3 is provided on the emitter side of the voltage-current conversion circuit 2 to supply a DC current to determine the DC current level, and the compensation circuit 3 includes three transistors Q ₇ -
Consists of _Q9 . As described above, a constant AC voltage is applied to the voltage-current conversion circuit 2, and the resistance values of the resistors R ₁ and R ₂ are set to be equal.

Therefore, when the AC voltage V is applied to the voltage-current conversion circuit 2, the admittance Y and the reference capacitor
As shown in Fig. 6, the currents i _{and i0} flowing through _C0 have a waveform whose phase is 90° ahead of the AC voltage v.
As the control pulse P, a rectangular pulse whose phase is 90 ahead of the AC voltage v or a sine wave with a sufficiently large amplitude is applied to the control signal generation circuit 5 using a well-known digital or analog phase shift circuit, for example. When generated from the voltage V and applied to the switching circuit 1, the current i ₁ i ₂ flowing through the resistors R ₁ and R ₂ is generated by switching the currents i and i ₀ every half cycle, as shown in Figure 7. It becomes a waveform. Therefore, currents i ₁ and i ₂ are converted into voltages v ₁ and v ₂ by resistors R ₁ and R ₂ of equal resistance value,
Find the difference between voltages v ₁ and v ₂ using a differential amplifier circuit, etc.
By smoothing the difference with an appropriate waveform shaping circuit, it is possible to obtain a DC voltage proportional to the difference in susceptance component between the admittance Y and the reference capacitor C _0. Add the capacitance of the reference capacitor C ₀ to this DC voltage. By doing so, the capacitance of the object to be measured can be detected.

The circuit shown in Fig. 5 is a type of phase-locked detection circuit, so even if there is a conductance component in the admittance Y, it becomes currents i ₁ and i ₂ as shown in Fig. 8, and the final output is Since it does not appear at the DC level, it has good characteristics as an industrial capacitance detector, and linearity can be maintained as shown in equation (1).

Also, the DC currents I ₁ and I ₂ flowing through the transistors Q ₇ and Q _{8 may not be balanced due to unbalance among the transistors Q 1 , Q 2 , Q 7 , and Q 8 or unbalance due to} temperature change. is also switched by switching circuit 1, so the 9th
As shown in the figure, since the currents are evenly distributed to i ₁ and i ₂ and the difference between them is detected, the unbalance caused by the DC currents i ₁ and i ₂ is canceled out, and the final output is not affected and highly accurate detection is possible. I can do it.

In the above embodiments, the transistors Q ₁ to _{Q 6} in the switching circuit 1 and the voltage-current conversion circuit 2 may be other semiconductor devices using operational amplifiers or the like.

As described above, this invention includes a switching circuit that switchably connects one input terminal to one of the output terminals and the other input terminal to the other output terminal;
A voltage-current conversion circuit that has two detection units that convert voltage to current and detect admittance is connected in cascade, and a reference capacitor is connected to one side of the voltage-current conversion circuit, and a device under test is connected to the other. Since we applied an AC voltage to the conversion circuit and switched the switching circuit at a predetermined period, smoothing the difference in the input current of the switching circuit becomes the susceptance component of the measured object. It also has the advantage of being able to measure a wide range of capacitors because it uses a voltage/current conversion circuit.

[Brief explanation of drawings]

Figure 1 is a basic circuit diagram showing the principle of this invention, Figure 2
The figure is an applied circuit diagram of Figure 1, Figure 3a is a basic circuit diagram showing the principle of this invention, Figure 3b is an equivalent circuit diagram of Figure 3a, and Figure 4 is an applied circuit diagram of Figure 3a. , Figure 5 is a circuit diagram showing an example of this invention, Figures 6 and 7 are
The figure shows the operating waveform diagram of Fig. 5 when the object to be measured consists of only a capacitor component, Fig. 8 shows the operating waveform diagram of Fig. 5 when the object to be measured contains a conductive component, and Fig. 9 shows the operation waveform of the transistor Q7, FIG. 7 is an operation waveform diagram illustrating a case where the current of Q8 is unbalanced. 1...Switching circuit, 2...Voltage-current conversion circuit, 4
...Object to be measured, 5...Control signal generation circuit, 6...
Differential amplifier circuit, C ₀ ...Reference capacitor, P ...
Control input signal, V...AC voltage.

Claims (1)

[Scope of utility model registration request] A switching circuit 1 has two input terminals and two output terminals, one input terminal is switchably connected to one of the output terminals, and the other input terminal is switchably connected to the other output terminal. A voltage-current conversion circuit 2 having two detection units for detecting admittance is connected in cascade, and a reference capacitor C ₀ is connected to one side of the voltage-current conversion circuit 2, and an object to be measured 4 is connected to the other side of the voltage-current conversion circuit 2. An AC voltage V is applied to the voltage-current conversion circuit 2, and a control input signal P whose phase is 90° ahead of the applied AC voltage V is applied to the switching circuit 1.
A control signal generating circuit 5 is connected to input the voltage to alternately switch the switching circuit 1, and the reference capacitor C ₀ is connected to the detection outputs a and b of the voltage-current conversion circuit 2.
and a differential amplifier circuit 6 which outputs the output difference proportional to the susceptance component of the object to be measured 4.