JPH0658358U - Capacitive sensor processing circuit - Google Patents

Abstract

(57) [Summary] [Purpose] The internal resistance included in the detection signal of the capacitance sensor is removed, and the capacitance of the fluid to be measured is detected with high precision. [Arrangement] An amplifier circuit 4 is connected to the output side of the electrostatic capacity sensor 1, and the electrostatic capacity sensor 1 is connected to the alternating voltage V from the oscillation circuit 3.
Enter 0. Then, the phase detection circuit 11 detects the phase difference from the AC voltage V0 from the oscillation circuit 3 and the output voltage V1 from the amplification circuit 4. Then, the control unit 12 outputs the DC voltage VOUT 'from which the internal resistance RC included in the output voltage V1 is removed by the phase difference.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a processing circuit of an electrostatic capacity sensor, which is preferably used in an alcohol concentration measuring device for measuring the concentration of alcohol contained in a liquid such as alcohol-blended gasoline or dampening water for a printing machine.

[0002]

[Prior art]

 Here, a processing circuit of a conventional capacitance sensor is shown in FIG. 2 and will be described.

In the figure, reference numeral 1 denotes a capacitance sensor, which is composed of parallel plate type or concentric cylindrical type electrode plates 2A and 2B, and between the electrode plates 2A and 2B. For example, it has a structure in which a fluid to be measured such as a fountain solution for a printing machine containing isopropyl alcohol (hereinafter referred to as “IPA”) flows.

Reference numeral 3 denotes an oscillating circuit, and the oscillating circuit 3 is adapted to apply an AC voltage V 0 (frequency f) to the capacitance sensor 1. When this AC voltage V0 is applied between the electrode plates 2A and 2B, the peak value due to the capacitance C due to the relative permittivity in the dampening water changes and is output to the amplifier circuit 4 described later. It

Reference numeral 4 denotes an amplifier circuit, which comprises an operational amplifier 5 as an operational amplifier and a feedback resistor 6 having a resistance value R connected between an inverting terminal and an output terminal of the operational amplifier 5. Configured, the non-inverting terminal of operational amplifier 5 is shorted to ground. Then, the output voltage V1 as an output signal amplified by the amplification factor corresponding to the resistance value R of the feedback resistor 6 is output from the amplifier circuit 4 to the full-wave rectifier circuit 7 described later.

Reference numeral 7 denotes a full-wave rectifier circuit. The full-wave rectifier circuit 7 full-wave rectifies the output voltage V1 from the amplifier circuit 4, converts it into a DC voltage VOUT, and outputs it to an arithmetic circuit (not shown). .

In the processing circuit of the electrostatic capacity sensor thus configured, the electrostatic capacity C corresponding to the concentration of IPA in the dampening water is detected by the electrostatic capacity sensor 1, and based on this detection signal. Then, the alternating current applied from the oscillation circuit 3 is changed, and this current change is output as the direct current voltage VOUT through the amplification circuit 4 and the full-wave rectification circuit 7. It is possible to detect the alcohol concentration in sushi water.

[0008]

[Problems to be solved by the device]

 By the way, in the dampening water, there is an internal resistance RC as shown in FIG. 2 between the electrode plates 2A and 2B of the capacitance sensor 1 due to the influence of the conductivity due to ions and the like.

Therefore, in the above-described processing circuit of the electrostatic capacity sensor according to the related art, not only the electrostatic capacity C in the dampening water but also the internal resistance between the electrode plates 2A and 2B of the electrostatic capacity sensor 1 is reduced. RC will also be detected.

Therefore, when the internal resistance RC is almost zero, the amplitude | V1 | of the output voltage V1 as shown in the following equation 1 can be obtained from the amplifier circuit 4.

[0011]

## EQU1 ## | V1 | = ωCR | V0 | where ω = 2πf

However, when the internal resistance RC exists, the following equation 2 is obtained.

[0013]

[Equation 2]

That is, since the internal resistance RC changes due to the conductivity of ions or the like in the dampening water, not only the electrostatic capacitance C but also the internal resistance RC is involved in the detection signal from the electrostatic capacitance sensor 1. The value of the DC voltage VOUT (VOUT ∝│V1│) becomes an output due to changes in the electrostatic capacitance C and the internal resistance RC. As a result, there is a problem in that the processing circuit of the capacitance sensor according to the related art cannot accurately detect the alcohol concentration.

The present invention has been made in view of the above-mentioned problems of the prior art, and the present invention provides a processing circuit of an electrostatic capacitance sensor capable of eliminating the influence of the internal resistance included in the detection signal of the electrostatic capacitance sensor. It is intended to be provided.

[0016]

[Means for Solving the Problems]

 In order to solve the above-mentioned problems, the features of the configuration adopted by the present invention are that a phase detection circuit that detects the phase difference between the AC voltage from the oscillation circuit and the output signal from the amplification circuit is provided and The calculation means is provided to cancel the internal resistance component contained in the detection signal of the capacitance sensor based on the phase detection signal from the circuit.

[0017]

[Action]

 With the above configuration, the signal output from the computing means can be an output corresponding to only the change in capacitance, which is free from the influence of the internal resistance of the capacitance sensor, and can be a signal that does not depend on the change in internal resistance.

[0018]

【Example】

 An embodiment of the present invention will be described below with reference to FIG. In the embodiment, the same components as those of the conventional technique shown in FIG. 2 described above are designated by the same reference numerals, and the description thereof will be omitted.

In the figure, 11 indicates a phase detection circuit according to the present embodiment. The phase detection circuit 11 includes an AC voltage V 0 from the oscillation circuit 3 via a signal line 11 A and an amplification circuit via a signal line 11 B. The output voltage V1 from 4 is input, the phase difference α between the two voltages V0 and V1 is detected in the phase detection circuit 11, and the reference voltage Vf as a phase detection signal is output to the control unit 12 described later. It is like this.

Reference numeral 12 denotes a control unit as an arithmetic means. The control unit 12 receives the DC voltage VOUT from the full-wave rectification circuit 7 and the reference voltage Vf from the phase detection circuit 11, and controls the control unit 12. In the unit 12, a calculation as will be described later is performed and a DC voltage VOUT 'is output.

The processing circuit of the capacitance sensor according to the present embodiment has the above-mentioned configuration. Next, the calculation processing in the control unit 12 will be described.

First, the phase difference α of the output voltage V1 is expressed by the following Expression 3.

[0023]

(3) α = tan ^-1 (ωCRC)

Further, the relationship between the DC voltage VOUT from the full-wave rectification circuit 7, the amplitude | V1 | of the output voltage V1, and the reference voltage Vf and the phase difference α is as shown in Equation 4.

[0025]

[Equation 4]

Further, from the relation of the equation 4, the DC voltage VOUT and the reference voltage Vf can be expressed as follows.

[0027]

[Equation 5] However, a, b: constant

As a result, the internal resistance RC becomes Equation 6 according to Equations 1 and 3,

[0029]

[Equation 6]

Substituting this equation 6 into the equation 3 to obtain the electrostatic capacitance C,

[0031]

[Equation 7]

Further, by substituting the equation 5,

[0033]

[Equation 8] Therefore, the electrostatic capacitance C is only the DC voltage V OUT ′ from the full-wave rectification circuit 7 and the reference voltage Vf from the phase detection circuit 11 which do not include the internal resistance RC.

Therefore, the phase detection circuit 11 detects the phase difference α, and the DC voltage VOUT from the full-wave rectification circuit 7 is applied to this phase difference α in the control unit 12 by the internal resistance RC of the capacitance sensor 1. It is possible to obtain a DC voltage VOUT 'based on only the electrostatic capacitance C not including, and it is possible to output a DC voltage VOUT' that does not depend on the change of the internal resistance RC.

Thus, in the processing circuit of the electrostatic capacity sensor according to the present embodiment, the internal resistance RC included in the detection signal of the electrostatic capacity sensor 1 is calculated by the phase difference α to obtain the internal resistance RC Can be removed from the output voltage V1. Even if the conductivity of the ions contained in the dampening water changes, its influence is eliminated, so that the DC voltage VOUT 'corresponding to the relative permittivity of IPA can be output from the control unit 12. The IPA concentration in the dampening water can be detected with high accuracy.

The fluid to be detected in the above embodiment is not limited to the IPA of the dampening water of the printing machine, but may be used in a concentration measuring device or the like for measuring the alcohol concentration of an alcohol mixed gas in an automobile or the like. Of course.

[0037]

[Effect of device]

 As described above in detail, according to the present invention, the phase detection circuit for detecting the phase difference between the alternating voltage from the oscillation circuit and the output signal from the amplification circuit is provided, and based on the phase detection signal from the phase detection circuit. Since an arithmetic means for canceling the internal resistance component included in the detection signal of the electrostatic capacity sensor is provided, the internal resistance included in the detection signal from the electrostatic capacity sensor can be removed from the detection signal, and the detection signal can be removed. Only the capacitance corresponding to the relative permittivity in the measurement fluid can be changed, and highly accurate capacitance detection can be performed.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing a processing circuit of a capacitance sensor according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing a processing circuit of a capacitance sensor according to a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Capacitance sensor 2A, 2B Electrode plate 3 Oscillation circuit 4 Amplification circuit 7 Full-wave rectification circuit 11 Phase detection circuit 12 Control unit (arithmetic means)

Claims (1)

[Scope of utility model registration request] 1. An oscillating circuit for applying an AC voltage to a capacitance sensor for detecting the dielectric constant of a fluid to be measured as a capacitance, and an operational amplifier for amplifying a detection signal from the capacitance sensor as an output signal. In the processing circuit of the electrostatic capacity sensor, which comprises an amplifier circuit consisting of: and a full-wave rectifier circuit for converting the output signal from the amplifier circuit into a full-wave rectifier, an alternating voltage from the oscillator circuit and an output signal from the amplifier circuit. Is provided with a phase detection circuit for detecting a phase difference from the phase detection circuit and an arithmetic means for canceling an internal resistance component included in the detection signal of the capacitance sensor based on the phase detection signal from the phase detection circuit. Capacitance sensor processing circuit.