KR800002012Y1 - Capacity type convertiing device - Google Patents

Abstract

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Description

Capacitive inverter

1 is a connection diagram showing an embodiment of the present invention device,

2 is a characteristic line showing the operation thereof;

3 and 4 are connection diagrams showing another embodiment of the present invention device.

The present invention relates to a capacitive converter for converting a measurand into an electrical signal by using a pair of variable capacitors whose capacity varies differentially according to the displacement corresponding to the measurand such as pressure and differential pressure.

In general, a capacitive converter has an electrical signal corresponding to the sum of capacitances that are differentially changed according to the displacement amount, as shown in Japanese Utility Model Publication No. 44-15398. The oscillator is controlled so as to be constant, and an electrical signal corresponding to the difference in capacitance is outputted to convert the measured amount into an electrical signal. By the way, in this type of capacitive converter, since the stray capacitance exists in parallel with the variable capacitor, the linearity was poor due to the influence.

For example, when a differential pressure is changed to a metal diaphragm displacement like a differential pressure transducer, and the capacity of a pair of variable capacitors is differentially converted to convert the measured amount (differential pressure) into an electrical signal, the displacement of the metal diaphragm is changed. If it becomes large, it will not be proportional to the car arm, and therefore, the measured amount and the output will be nonlinear.

An object of the present invention is to realize a capacitive converter which can be configured to obtain an output having a desired relationship with respect to the displacement amount, and which can easily correct the nonlinearity as described above.

1 is a connection diagram showing an embodiment of the present invention device. In the drawing, C ₁ and C ₂ are a pair of variable capacitors whose capacitance varies differentially according to the displacement amount. In the drawing, the movable electrode 10 and the movable electrode 10 are disposed between the movable electrode 10 and the displacement electrode. It consists of the fixed electrodes 11 and 12 arrange | positioned opposingly. C ₃ is a fixed capacitor for compensation, A is a high-gain differential amplifier, O is an oscillator, T is a transformer, and the primary coil n ₁ and three secondary coils n ₂ , n ₃ , n ₄ Have. Vr is a stabilized DC power supply, D ₁ to D ₆ are rectifier diodes, R ₉ to R ₅ are resistors, Cf ₀ to Cf ₃ are smoothing capacitors, and C ₄ is a DC blocking capacitor.

In the apparatus of the present invention configured as described above, since the oscillating output of the oscillator O is given to the pair of variable capacitors C ₁ and C ₂ , AC currents i ₁ and i ₂ flow according to their respective capacities. The alternating currents i ₁ , i ₂ are rectified and smoothed by diodes D ₁ to D ₄ and smoothing capacitors Cf ₀ to Cf ₂ . Therefore, the average value current I ₀ corresponding to the difference between the currents i ₁ and i ₂ flows through the resistor R ₀ , and the output voltage E ₀ is generated at both ends thereof as shown in the following equation.

E ₀ = Rok {(C ₁ + Cs ₁ )-(C ₂ + Cs ₂ )} (1)

Here, K is a function of the amplitude and frequency of the oscillator output of the oscillator O, and Cs ₁ and Cs ₂ are stray capacitances present in parallel in the variable capacitors C ₁ and C ₂ .

In addition, the resistors R ₁ and R ₂ flow in the polarity of the average current I ₁ I ₂ corresponding to the currents i ₁ and i ₂ , respectively.

The resistors R ₁ and R ₂ are connected in series with the resistors R ₄ and R ₅ to the DC power supply Vr, and a constant DC current Ir is supplied in the opposite direction to the above average value currents I ₁ and I ₂ . Therefore, the feedback voltage Ef is generated at both ends of the series circuit of the resistors R ₁ and R ₂ as shown in the following equation.

Ef = R {(C ₁ + C ₂ + Cs ₁ + Cs ₂ ) -2Zr)} (2)

Where R = R ₁ = R ₂

On the other hand, since the oscillating output of the oscillator O is given to the compensating fixed capacitor C ₃ through the transformer T, the AC current i ₃ according to its capacity flows. The alternating current i ₃ is the diode D _5, D _6, and so the smoothing capacitor Cf be rectifying and smoothing at ₃ flows to the resistor R _3, the compensation voltage Ec such as that the two ends of the resistor R ₃ is displayed on the food is obtained.

EC = KR ₃ C ₃ (3)

The difference between the compensation voltage Ec and the feedback voltage Ef is applied to the differential amplifier A. The differential amplifier A controls, for example, the power supply voltage of the oscillator O such that the difference voltage becomes zero. Therefore, the output voltage E ₀ is given by

The capacitance of the pair of variable capacitors C ₁ and C ₂ is determined with respect to the displacement amount X (the amount according to the measured amount) of the movable electrode 10.

C ₀ : initial capacity

b is the distance between the fixed electrodes 11 and 12 And a pair of variable capacitors C ₁ and C ₂ are made symmetrically. Assuming that the stray capacitance is Cs ₁ = Cs ₂ = Cs, the output voltage E ₀ is given by the following equation.

Therefore, the resistance value of the resistor R ₃ ,

When adjusted so that the output voltage E ₀ is

In this way, it is not affected by the stray capacitance Cs, and is exactly proportional to the displacement amount X as shown in FIG. 2A. Experiments have shown that linearity errors can be drastically reduced from -0.98% to ± 0.03% without compensating the effects of stray capacity.

The resistance of the resistor R ₃ If it is adjusted to be, the increase rate increases as the displacement amount X increases, as shown in FIG. 2B. As shown in FIG. 2C, the increase rate decreases as the displacement amount X increases, and the input / output relation can be made nonlinear.

In the non-linearity it may be set by the size of changing the value of the resistor R _3. Thus, for example, the nonlinearity of the measured amount (differential pressure) and the displacement amount X of the movable transmission 10, as in the case of converting the differential pressure into a displacement using a metal diaphragm, a mechanical arrangement or electrical component of a pair of variable capacitors The nonlinearity of the input / output relationship according to various conditions such as the unbalance can be corrected by properly calculating the value of the resistor R ₃ .

3 is a connection diagram showing another embodiment of the device of the present invention, which is different from the embodiment of FIG. 1 in a two-wire configuration. In FIG. 3, R ₀ is a variable resistor for span adjustment, A ₁ is a high gain differential amplifier, and the input terminal (+) has a potential E ₁ and a resistance R at the connection points of the resistors R ₁ and R ₂ . the zero point to have a constant voltage-adjusting-the sum of the both-end voltage E ₀ of ₀ is applied via a resistor R ₆ both ends of the feedback resistor Rf at the same time and load voltage E ₂ that is applied through a resistor R _7, the other terminal of which () The voltage E ₃ divided by the voltage dividing resistor R ₁₀ is applied via the voltage dividing circuit of the resistors R ₈ and R ₉ .

In addition, the resistors R ₀ -R ₉ are selected sufficiently large compared to the signal source resistance. Q is driven from the output transistor to the output of the amplifier A ₁ and supplied to the load R _L arranged on the receiving side via the feedback resistor Rf and the pair of transmission lines L to the output current I ₀ . The voltage drop generated across the resistor Rf by the output current I ₀ becomes the above-described voltage E ₂ . J is a constant current circuit, for example, as shown in the figure, and a transistor and a field effect transistor. A constant current is supplied to the zener diode Dz to obtain a stabilized DC voltage Vr at both ends thereof.

The stabilized DC voltage Vr is applied to the series circuits of resistors R ₄ , R ₁ , R ₂ , and R _{5 and} the voltage divider R ₁₀ for zero adjustment, respectively, and is applied to the power terminals of the amplifiers A and A ₁ via the transistors. All. E is a direct current power source, which is the only power source of this apparatus, is connected in series to the load R _L , and is disposed on the receiving side.

In the inventive device configured as described above, the output current I ₀ transmitted to the load R _L on the receiving side via the pair of transmission lines L is

Provided that R ₆ = R ₇ = R ₈ = R ₉

β = partial voltage ratio of voltage divider R ₁₀

This corresponds to the displacement amount X. As apparent from the equation (10), when the voltage dividing ratio β of the voltage dividing resistor R ₁₀ is adjusted, the value of the output current I ₀ at 0% of the displacement amount X can be set as a desired value (for example, 4 mA). Can be adjusted. If the value of the resistor R ₀ is adjusted, the value of the output current I ₀ at 100% of the displacement amount X can be set to a desired value (for example, 20 mA), and the span can be adjusted. Also, changing the value of the variable resistor R ₀ has no effect on the zero point, and changing the partial pressure ratio β of the voltage divider resistor R ₁₀ has no effect on the span.

In other words, in the two-wire changer, the zero point adjustment and the span adjustment can be performed independently of each other. In the above-described embodiment, the feedback voltage Ef and the compensation voltage Ec are added in series, but as shown in FIG. 4, the operational resistors R ₁₀ to R ₁₃ are used to add in parallel as the differential amplifier A. Also good. In the above-described case, the oscillation output of the oscillator is given to the pair of variable capacitors via two sets of secondary windings of the transformer T, but may be configured to be provided through one set of secondary windings. However, when configured as in the embodiment, there is an advantage in that one end of the variable capacitor can be grounded. In the same manner, a switching transistor connected between collector bases instead of a rectifying diode is used. In addition, the relationship of the expression (8) may be satisfied by making the compensating capacitor C ₃ variable. In the case of being affected by the ambient temperature of the stray capacitance, the compensating capacitor C ₃ may be arranged near a pair of the variable capacitors C ₁ and C ₂ so as to be affected by the ambient temperature.

As described above, according to the present invention, an output electric signal having a desired relationship with the displacement amount can be obtained with a simple configuration, and a capacitive converter which can easily correct nonlinearity is obtained.

Claims (1)

A pair of variable capacitors (C ₁ , C ₂ ) whose capacitance varies differentially in accordance with the measured amount, an oscillator (O) for applying an oscillation output to the pair of variable capacitors (C ₁ , C ₂ ), and Means for flowing the current corresponding to the sum of the capacities of the pair of variable capacitors C ₁ and C ₂ and the reference current to the resistor in the opposite direction to obtain a feedback voltage Ef, and oscillation of the oscillator O described above Means for obtaining a compensation capacitor C _{3 to} which an output is applied, a compensation voltage Ec corresponding to a current flowing in accordance with the capacitance of the compensation capacitor C ₃ , and the feedback voltage Ef described above. Means for controlling the oscillator (O) so as to be equal to one compensation voltage (Ec), and means for obtaining an output signal related to a current corresponding to a difference in capacitance of the pair of variable capacitors (C ₁ , C ₃ ). With capacitive inverter.