RU2593818C1 - Method and device for measuring capacitance - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: present invention relates to measurement of electric values, namely to measurement of electric capacity. Method of measuring electric capacitance consists in measuring the ratio of voltages on series-connected reference and measured capacity, charged from a source of DC voltage. Device for implementation of the proposed method comprises a measured and reference capacity, ADC and a microprocessor, charging and discharge capacity, wherein the differential measuring input of the ADC is connected to outputs of the reference capacity, and the differential input of reference voltage of the ADC is connected to outputs of the measured capacitance.

EFFECT: technical result: improved of accuracy, efficiency, improved linearity characteristics, as well as increased temperature stability, simplicity.

2 cl, 2 dwg

Description

The invention relates to the field of measuring electrical quantities, namely to measuring the electrical capacitance of capacitive sensors - pressure, humidity, position and other sensors in which the electric capacitance is the output characteristic of the measured physical parameter.

A device for measuring electric capacitance [1] is known, which contains two single-vibrators connected according to the scheme of a ring oscillator, containing timing RC circuits with a measured and a reference capacitor, an indication unit with an integrating link at the input, an adder, a voltage reference source and two change-over keys, inputs which are connected to the outputs of single vibrators, and the outputs to the inputs of the integrating link. The device converts the ratio of the capacitance of the reference and measured capacitors into the output voltage. This device does not provide high accuracy for measuring capacitance.

A device and method for measuring the impedance of a capacitor [2]. The device contains a sinusoidal signal generator and two channels for measuring the impedance of the reference resistor and the measured capacitor. Each impedance measurement channel contains an amplifier and an ADC. The ADC samples of each channel are filtered and fed to the slave, which calculates the measured capacitance by calculating the phase difference between the voltage across the reference resistor and the measured capacitor based on the received digital samples from the ADC. The disadvantages of the device and method are a complex electrical circuit, which consists of many components, as well as a complex algorithm for calculating capacitance, requiring a high-speed and high-capacity slave.

A device for measuring pressure with a capacitive sensor in the feedback circuit of the amplifier [3]. The device used an option for measuring electric capacitance based on a charge amplifier, in which the ratio of the measured capacitance of the sensor and the reference capacitance is converted into a voltage signal. The disadvantages of the device are the analog method of generating the output signal, which complicates the linearization, temperature compensation and calibration of the sensor.

A known method and device for measuring capacitance based on a sigma-delta modulator [4]. This method is suitable for measuring only small capacities, and a device for its implementation requires the use of specialized microcircuits.

A known method of measuring capacitance [5], when the capacitor is charged from a constant current source to a specific voltage. Then the charge time determines the measured capacitance. The disadvantages of the analogue are the need to use a precision small current source and the long measurement time required to charge the measured capacitor to a given voltage.

Of the known technical solutions, the closest in purpose and technical essence to the claimed technical solution is the method and device for measuring capacitance [6], selected as a prototype.

In the selected prototype, the method for measuring capacitance includes a preliminary discharge of the measured capacitance, its charge from a constant current source, providing a linear change in voltage on the measured capacitance, measuring the voltage difference and time between two voltage measurements on a linear section of the voltage change on the measured capacitance, calculating the capacitance value as the product of a given charge current and the time difference between two voltage measurements divided by the difference of the measured voltages.

The method has the following disadvantages:

a) non-linear dependence of the output voltage on the measured capacitance;

b) a long measurement time, which is determined by the sum of the times of the charge of the capacitance, the measurement of voltage on the capacitance and its discharge.

In the selected prototype, the capacitance measuring device contains inputs for connecting the measured capacitance with charge and discharge circuits in the form of keys connecting the measured capacitance to a current source and ground, an ADC connected to the input of the measured capacitance, a microprocessor connected to the ADC, and a current source connected to the inputs of the measured capacitance through a charge circuit, an ADC reference voltage source. The ADC is connected to the input of the measured capacitance at the input of the measured voltage, and at the input of the reference voltage it is connected to the source of the reference voltage. The microprocessor control outputs are connected to the keys of the charge and discharge circuits of the measured capacitance.

In addition to the above disadvantages, the device has the following disadvantages:

a) the need to use precision current sources and voltage references;

b) the presence of an error in measuring capacitance from changes in temperature and supply voltage.

The objective of the invention is the measurement of electrical capacitance in a wide range of capacitance values with a linear scale, low error in the entire range of operating temperatures and supply voltages, high speed, and also simplification of the electrical circuit of the device.

The problem is solved due to the fact that in the method of measuring electric capacitance, namely, that the measured capacitance is discharged through the discharge circuit, charged through the charge circuit and the charge voltage of the capacitance is measured, according to the invention, a charge voltage is also measured on a reference capacitance connected in series with measured capacitance, the reference and measured capacitance are discharged by switching them to the ground, and charged with a constant voltage, the capacitance is measured in relation to the voltage ratio and on the reference and measured capacitances using an ADC, the differential measuring input of which supplies voltage from the reference capacitance, and the voltage from the measured capacitance is applied to the input of the reference voltage.

In the device for implementing the proposed method for measuring electric capacitance, containing inputs for connecting the measured capacitance, charge and discharge circuits, an ADC connected to the input of the measured capacitance, a microprocessor connected to the ADC output and charge and discharge circuits of the capacitance, according to the invention, in series with the inputs of the measured capacitance additionally installed reference capacitance connected by a charge circuit to the output port of the microprocessor, the connection point between the reference and the input of the measured capacitance is connected I drink a discharge with a microprocessor input / output port, the differential measuring input of the ADC is connected to the reference capacitance, and the input of the ADC reference voltage to the measured capacitance.

There is a causal relationship between the totality of the essential features of the invention and the technical result achieved, namely, the linearity of the capacitance measurement is ensured by measuring the ratio of voltages on the reference and measured capacities, the low error in measuring capacitances in a wide range of operating temperatures and supply voltages is ensured by that the temperature measurement error is determined only by the TCS of the reference capacitance and the characteristics of the ADC, and from fluctuations in the supply voltage The measurement behavior does not depend at all, the measurement time is determined only by the speed of the ADC, the measurement accuracy is determined by the ADC capacity, a simple circuitry implementation of the device is ensured by the fact that the circuit is constructed without the use of precision current sources and reference voltage.

The technical essence of the proposed solution is illustrated by drawings. In FIG. 1 is a diagram of a method for measuring capacitance; FIG. 2 is a diagram of a device.

The proposed method for measuring capacitance is illustrated by the circuit in FIG. 1, which contains the inputs for connecting the measured capacitance 1.1 and 1.2 and a series-connected reference capacitance 2. There is also a charge circuit with a key 3 supplying the charge voltage to the capacitors, and a discharge circuit with keys 4, 5, switching capacitor leads to the circuit ground. In the circuit Uet, Ux, U are the voltages respectively on the reference capacitance, the measured capacitance and the charge voltage of the capacitors.

Capacitance measurement is performed in the following sequence:

a) discharge the tanks, preparing the measurement cycle, while the key 3 is open, the keys 4, 5 are closed to ground;

b) charge the containers with voltage U, while the key 3 is closed, the keys 4, 5 are open;

c) measure the ratio of voltages Uet and Ux on the reference and measured capacitances.

When the series-connected capacitors are charged from the applied voltage U, in accordance with the law of electrostatic induction, the charges q on their plates will be equal. The charge distribution will occur in such a way that the negative lining of the first capacitance will receive a charge -q, and the positive lining of the second will receive a charge + q.

From the formula for capacity

it follows that the voltage at each of the capacitors connected in series depends on their capacitance and the received charge:

Then the ratio of voltages at the reference and measured capacitance allows you to calculate the value of the measured capacitance by the formula

The ratio of the voltages at the reference and measured capacitances is measured using an ADC (not shown in Fig. 1), the differential measuring input of which is connected to the reference capacitance, and the reference voltage input to the measured capacitance. In this case, the reference capacitance Set is selected more than the measured capacitance Cx.

In this case, the ADC code will be equal to

where N is the resolution of the ADC.

The value of the measured capacitance is determined by the formula

A diagram of a device for implementing the proposed method is shown in FIG. 2, which contains the inputs for connecting the measured capacitance 1.1 and 1.2, the reference capacitance 2, the ADC 6 and the microprocessor 7 connected at the input to the ADC 6, the output of the charge output port of the microprocessor 7 is connected to the capacitance charge circuit, the output of the discharge input and output port of the microprocessor 7 is connected with a discharge circuit of the measured capacitance Cx and the reference capacitance. The conclusions of the reference capacitance 2 are connected to the differential measuring input of the ADC 6, the output of the input of the measured capacitance 1.1 is connected to the input of the reference voltage (Uop) of the ADC 6.

The device operates as follows. When the measured capacitance Cx and the reference capacitance are discharged, a low voltage level is set at the outputs of the ports of the charge and discharge circuits of the microprocessor 7. When measuring, the output of the discharge circuit port of the microprocessor 7 is set to the input state, and at the output of the charge circuit, a high voltage level U is set at which the ratio of the voltages Uet, Ux on the measured capacitance Cx and reference capacitance Set 2 is measured using the ADC 6, after which the microprocessor 4 reads data from ADC 3.

The technical and economic efficiency of the invention is to measure the capacitance of sensors in a wide range of capacitance values with high accuracy, speed, linearity and stability, as well as a simple circuitry implementation of the device.

Information sources

1. RF patent 2308727.

2. US patent 8452557.

3. RF patent 2319124.

4. US patent 7683641.

5. Copyright certificate of the USSR No. 112262.

6. US patent 6275047 - prototype.

Claims (2)

1. The method of measuring electric capacitance, which consists in the fact that the measured capacitance is discharged through the discharge circuit, charged through the charge circuit and the charge voltage of the capacitance is measured, characterized in that the charge voltage is additionally measured at a reference capacitance connected in series with the measured capacitance, a reference and the measured capacitance is discharged by switching them to the ground, and charged with a constant voltage, the capacitance is measured by the ratio of the charge voltages on the reference and measured capacities using an ADC, the differential measuring input of which supplies voltage from the reference capacitance, and the voltage from the measured capacitance is applied to the input of the reference voltage. 2. A device for measuring electric capacitance, containing inputs for connecting the measured capacitance, a charge and discharge circuit of capacitors, an ADC connected to the input of the measured capacitance, a microprocessor connected to the output of the ADC and charge and discharge circuits of the capacitor, characterized in that it is additionally connected in series with the input of the measured capacitance a reference capacitance is installed, connected by a charge circuit to the output port of the microprocessor, the connection point of the reference and input of the measured capacitance is connected by a discharge circuit to the output port of the input-output microprocessor, the differential measuring input of the ADC is connected to the reference capacitance, and the input of the ADC reference voltage is connected to the input of the measured capacitance.

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