CN110749340A

CN110749340A - Resistance-capacitance sensor signal measuring circuit

Info

Publication number: CN110749340A
Application number: CN201911209556.0A
Authority: CN
Inventors: 陈希; 刘洋; 蔡建程; 陈赵江; 王晖; 毛和法
Original assignee: Zhejiang Normal University CJNU
Current assignee: Zhejiang Normal University CJNU
Priority date: 2019-12-01
Filing date: 2019-12-01
Publication date: 2020-02-04

Abstract

The invention discloses a circuit for measuring signals of a resistance-capacitance type sensor, and relates to the field of instruments, electronic measurement and sensors. Specifically, the variation of the resistance-capacitance parameters of the resistance-capacitance sensor is converted into high and low level time sequence signals with different durations, and the relationship between the variation of the resistance-capacitance parameters of the sensor and the high and low level time sequence signals with different durations is established. And obtaining the variation of the resistance-capacitance parameters of the sensor by measuring the high and low level time sequence signals. The invention has the beneficial effects that: the measuring range is enlarged, the measuring precision is improved, the circuit is simplified, the interference is reduced, and the cost is reduced.

Description

Resistance-capacitance sensor signal measuring circuit

Technical Field

The invention relates to the field of instruments, electronic measurement and sensors, in particular to a method for realizing signal measurement of a resistance-capacitance sensor.

Background

The output signal form of the sensor is resistance type, voltage type, capacitance type, current type, etc. Some sensors use some form of output, often accompanied by other forms of output. For example, a carbon dioxide sensor with a modified surface is theoretically a resistance output type sensor, and when the environment changes, the resistance changes. But because of the special activity of the surface modificationThe material is such that the change in resistance of the active sensing material with a change in capacitance is accompanied by a change in resistance with a change in chemical environment. Namely, the output resistor has parasitic capacitance. Such sensors can be viewed as a parallel connection of resistance, capacitance. Due to the existence of the capacitance, the measurement accuracy will be affected by simply measuring the change of the sensor resistance value. The common measuring method of the sensor adopts a constant current source, namely, a constant current source circuit supplies constant current to the sensor, and the value of the sensor is obtained according to ohm's law by measuring the voltage at two ends of the sensor. Measuring the sensor resistance in this way avoids the effect of shunt capacitance. However, the resistance of such sensors varies widely, at 10³～10⁹The ohmic range, therefore, only a small current flows, a large voltage variation range is generated, so that the subsequent processing circuit is very complicated, and the complicated and expensive calibration cost is required.

(Zhong Chong, lan Jing Hui, Yan Su Ying, a resistance measurement method for eliminating the influence of distributed capacitance [ J]University of major graduate, 2003, 43 (5): 372 + 376), et al, propose measuring resistance measurements with distributed capacitance using ac excitation. The method needs to provide an alternating current excitation signal source, leads to the complex circuit of a test system, has low measurement precision, and only gives 10⁶The results of the below-ohm tests are not given at 10⁹Test results on the ohmic scale.

Disclosure of Invention

The invention aims to overcome the defects of the existing measuring method and provides a measuring circuit which is simple in circuit structure, wide in measuring range, high in measuring precision, small in interference, low in cost and fast in speed measurement.

In order to achieve the purpose, the invention adopts the following technical scheme:

the equivalent circuit of the sensor is a resistor R_sAnd a capacitor C_sAre connected in parallel. One port of the sensor is connected with the negative input end of the operational amplifier IC3 and is connected with an integrating capacitor C, and the other end of the capacitor C is connected with the output end V of the IC3₃Connected and the positive input of IC3 is grounded. Another port V of the sensor₁Is connected to the input of IC2 with an amplification gain of-a. Two sensor ports andthe digital potentiometer IC5 is connected in parallel.

The output end of the IC2 is connected with a resistor R₁Resistance R₁And a resistance R₂One end connected to a resistor R₂The other end is connected with a voltage VCC. Resistance R₁And a resistance R₂Partial pressure V of₂And the negative end of a comparator CP1 in the singlechip IC4 is connected. Output terminal V of IC3₃And the positive end of a comparator CP1 in the singlechip IC4 is connected.

Output terminal V of IC3₃The positive end of a comparator CP2 in the single chip IC4 is connected, and the negative end of a comparator CP2 is connected with the ground.

Comparator CP1 output terminal V in IC4₄Connecting the positive input end of a voltage follower IC1, the output end of an IC1 and a sensor port V₁Connected and fed back to the negative input of IC 1.

The digital potentiometer IC5 is controlled by a singlechip IC4, and a control end is connected with the IC 4.

Comparator CP1 output terminal V in IC4₄The a input of IC6 and the capture port CAP1 of IC 4. Comparator CP2 output terminal V₅The B input of IC6 is accessed. IC6 XOR inputs A and B and outputs result V₆And a capture port CAP2 connected to the singlechip IC4 is used for realizing waveform pulse width and cycle time measurement.

Further, the output terminal V of the comparator CP1 in IC4₄Has an output signal period of T_xAt T_xWithin a time period, outputting a result V₆Has a pulse width of T_x1、T_x2、T_x3、T_x4。

V₄Output signal period T_xCan be expressed by the following formula:

T_x＝4A(R_s//R_w)C-4(R_s//R_w)C_s

in the formula R_wIs the digital potentiometer IC5 resistance.

Sensor R_sThe value of (c) is calculated using the following formula:

R_s//R_w＝(T_x2+T_x4)/2AC

sensor C_sThe value of (c) is calculated using the following formula:

C_s＝AC(T_x2+T_x4-T_x1-T_x3)/2(T_x2+T_x4)

the invention has the beneficial effects that: the invention can simplify the circuit, enlarge the measuring range, improve the measuring precision, reduce the interference and reduce the cost.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a schematic diagram of a resistance-capacitance sensor signal measurement circuit;

FIG. 2 is a schematic diagram of voltage waveforms of different nodes of a RC sensor signal measuring circuit;

FIG. 3 is C_sAt 15pF, sensor R_sStandard value of (2)_rAnd measured value R_tCompare the figures.

Detailed Description

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, and the scope of the present invention will be more clearly and clearly defined. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and these equivalents also fall within the scope of the present application.

A circuit for measuring a resistance-capacitance sensor signal comprises the following technical scheme:

as shown in FIG. 1, the equivalent circuit of the sensor is a parallel connection of a resistor and a capacitor, R_s、C_sRespectively the resistance and the distributed capacitance of the sensor. One port of the sensor is connected with the negative input end of the operational amplifier IC3 and is connected with an integrating capacitor C, and the capacitance C isThe other end of the input end of the IC3₃Connected and the positive input of IC3 is grounded.

Another port V of the sensor₁Is connected to the input of IC2 with an amplification gain of-a. The two ports of the sensor are connected with a digital potentiometer IC5 in parallel.

The invention is further described below with respect to the circuit operating principle of the rc sensor signal measurement:

assume initial state follower IC1 output voltage V₁At a positive voltage, current flows through the sensor equivalent resistance R_sAnd resistance R of program-controlled potentiometer IC5_wThe capacitor C is charged. Output end voltage V of operational amplifier IC3₃Becomes negative and increases in absolute value and is connected to the output voltage V of the inverting amplifier IC2₂＝-AV₁The comparison is carried out by a comparator CP1 in the singlechip IC4, and the output voltage V is compared₄Is fed back to the follower IC1 to pass current through R_sAnd R_wTo electricityThe capacitor C is charged and discharged continuously to generate an oscillation signal. When the capacitance C is a fixed value, the period of the oscillating signal is controlled by R_sAnd (6) determining. Suppose R is during a charge-discharge cycle_sAs constant values, there are:

in the formula, V₃(0) Is an initial state value, T_xIs a charge-discharge cycle. When V is₃Decreases to V₂＝-AV₁When the comparator is inverted, V is₂＝AV₁Thus, there are:

V₃(T_x/2)＝-V₁(A-(T_x/2)/((R_s//R_w)C))＝AV₁(2)

T_x＝4A(R_s//R_w)C (3)

FIG. 2 is V₄、V₅、V₆Nodal waveform schematic due to sensor parasitic capacitance C_sWhen V is present₄When reversal occurs, C_sAnd the capacitor C, there is a rapid charging or discharging process. Thus V₃At V₄When inversion occurs, there is a sudden change in voltage. Considering C_sEquation (3) can be written as:

T_x＝4A(R_s//R_w)C-4(R_s//R_w)C_s(4)

fig. 1 shows that the chip IC4 internal comparator CP2 will be V₃The signal is divided into two parts, one is when V₄When charging and discharging are carried out in reverse, the charge-discharge unit receives C_sPart of influence, e.g. T in FIG. 2_x1、T_x3(ii) a Second, only with R_sAnd R_wRelevant parts, e.g. T in FIG. 2_x2、T_x4。T_x1、T_x2、T_x3、T_x4From V₄、V₅Obtained by the exclusive-or operation of IC6, then:

R_s//R_w＝(T_x2+T_x4)/2AC (5)

C_s＝AC(T_x2+T_x4-T_x1-T_x3)/2(T_x2+T_x4) (6)

will V₄、V₆The access singlechip IC4 has CAP1 and CAP2 pins with edge capturing function, and T is realized_x1、T_x2、T_x3、T_x4And calculating the sensor R according to the equations (5) and (6)_s、C_sThe value is obtained.

For obtaining a rapid measurement, the resistance R of the program-controlled potentiometer IC5 is preset_wIs a small value, and T is measured_x1According to T_x1Adjusting the resistance R_wOf such that R_sAnd R_wIs at an appropriate value.

FIG. 3 is C_sAt 15pF, sensor R_sStandard value of (2)_rAnd measured value R_tThe test value is consistent with the standard value, and the measurement range is large.

The above is only a specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that are not thought of through the inventive work should be included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope defined by the claims.

Claims

1. A resistance-capacitance sensor signal measurement circuit is characterized in that:

the equivalent circuit of the sensor is a resistor R_sAnd a capacitor C_sAre connected in parallel; one port of the sensor is connected with the negative input end of the operational amplifier IC3 and is connected with an integrating capacitor C, and the other end of the capacitor C is connected with the output end V of the IC3₃Connected, the positive input of IC3 is grounded; another port V of the sensor₁Is connected with the input end of the IC2 with the amplification gain of-A; two ports of the sensor are connected with a digital potentiometer IC5 in parallel;

the output end of the IC2 is connected with a resistor R₁Resistance R₁And a resistance R₂One end connected to a resistor R₂The other end is connected with a voltage VCC; resistance R₁And a resistance R₂Partial pressure V of₂The internal comparator CP1 negative of the access singlechip IC4A terminal; output terminal V of IC3₃The positive end of a comparator CP1 in the single chip microcomputer IC4 is accessed;

output terminal V of IC3₃The positive end of a comparator CP2 in the single chip microcomputer IC4 is accessed, and the negative end of the comparator CP2 is connected with the ground;

comparator CP1 output terminal V in IC4₄Connecting the positive input end of a voltage follower IC1, the output end of an IC1 and a sensor port V₁Connected and fed back to the negative input of IC 1;

the digital potentiometer IC5 is controlled by a singlechip IC4, and a control end is connected with the IC 4;

comparator CP1 output terminal V in IC4₄An A input end of the access IC6 and a capture port CAP1 of the IC 4; comparator CP2 output terminal V₅The B input end of the IC6 is accessed; IC6 XOR inputs A and B and outputs result V₆And a capture port CAP2 connected to the singlechip IC4 is used for realizing waveform pulse width and cycle time measurement.

2. The rc sensor signal measurement circuit of claim 1, wherein: converting the sensor resistance-capacitance parameter change into high-low level timing signals with different durations, wherein the output end V of a comparator CP1 in the IC4₄Has an output signal period of T_x(ii) a At T_xWithin a time period, outputting a result V₆Has a pulse width of T_x1、T_x2、T_x3、T_x4；

Comparator CP1 output terminal V in IC4₄Output signal period T_xCan be expressed by the following formula:

T_x＝4A(R_s//R_w)C-4(R_s//R_w)C_s

in the formula R_wIs the resistance value of the digital potentiometer IC 5;

sensor R_sThe value of (c) is calculated using the following formula:

R_s//R_w＝(T_x2+T_x4)/2AC

sensor C_sThe value of (c) is calculated using the following formula:

C_s＝AC(T_x2+T_x4-T_x1-T_x3)/2(T_x2+T_x4)。