JPH05273068A - Detection circuit of differential pressure sensor - Google Patents

Abstract

PURPOSE:To eliminate a need for adjusting current value of a constant-current source through a signal-processing circuit having simple configuration, and achieve a highly accurate detection in a differential pressure sensor detection circuit. CONSTITUTION:In a differential pressure sensor which is sensitive to two different pressures and measures difference between the two pressures, two strain-generating parts 2 and 3 which receive each of two pressures are formed on a diaphragm substrate 1, two strain gauges 4b, 4d, 5a, and 5c whose resistance changes in positive direction according to the pressure level and two strain gauges 4a, 4c, 5b, and 5d whose resistance changes in negative direction are provided on the pressure-reception surface and the opposite surface of each of two strain-generating parts, a first branch path 8A for connecting a strain gauge whose resistance changes in the positive direction of the first strain-generating part and that whose resistance changes in the negative direction of the second strain-generating part in series is formed, a second branch path 8B for connecting a strain gauge whose resistance changes in the negative direction of the first strain-generating part and that whose resistance changes in the positive direction of the second strain-generating part in series is formed, and then the first and the second branch paths are combined while they are adjacent, thus forming a bridge circuit for detecting differential pressure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a detection circuit of a differential pressure sensor, and more particularly, it is used for detecting the hydraulic state of each part in a hydraulic drive system of a civil engineering construction machine and is sensitive to strain generated in response to pressure. And a detection circuit of a differential pressure sensor configured to detect the pressure.

[0002]

2. Description of the Related Art An electric circuit for extracting pressure in a conventional differential pressure sensor will be described with reference to FIGS.

The conventional detection circuit shown in FIG. 9 includes two Wheatstone bridge circuits 51 and 52 (hereinafter referred to as bridge circuits) each composed of four strain gauges. The bridge circuit 51 is a strain gauge 51.
a to 51d, and the bridge circuit 52 is composed of strain gauges 52a to 52d. Each strain gauge acts as a variable resistance element on the electric circuit.
In each bridge circuit, when pressure is applied to the strain generating part where it is provided and strain occurs, the resistance value of each strain gauge changes, the electrical balance of the bridge circuit is disrupted, and the output voltage proportional to the applied pressure. To occur. The bridge circuits 51 and 52 detect different pressures, and are formed as pressure detecting portions, respectively. In the latter circuit, the bridge circuits 51, 5
It has a configuration for obtaining the difference between the output voltages detected by each of the two. The output voltage of the two bridge circuits 51 and 52 is
The level adjustment is performed by the differential amplifiers 53 and 54 respectively, and then the differential voltage is obtained by the differential pressure amplifier 55. Thus, an electric circuit for detecting the difference between the two pressures is formed.

The input terminal 5 of the bridge circuits 51 and 52
6, 57 are connected to a common power source. Also, terminals 58 and 5
9 is an output terminal of the bridge circuit 51, and terminals 60 and 6
1 is an output terminal of the bridge circuit 52.

FIG. 10 is a longitudinal sectional view of the essential parts of the mechanical components of the differential pressure sensor. Reference numeral 62 denotes a diaphragm base, which forms two recesses 63 and 64 from the lower side, and two thin-walled strain generating portions 65 and 66 are formed. The diaphragm base body is a member for receiving and detecting pressure.
The planar shape of the diaphragm base 62 is, for example, a circle. Reference signs P ₁ and P ₂ indicate two pressures applied to the respective strain generating portions 65 and 66. The pressure is applied to the lower surface of the strain-flexing portion from the side of the recess. The bridge circuits 51 and 5 described above
2 is formed on the upper surfaces of the strain generating portions 65 and 66 based on a film forming technique. In FIG. 10, reference numerals 52a, 52c and the like denote parts of strain gauges. On the upper surface of each flexure,
Each strain gauge is arranged at a position that produces a predetermined responsive state to the strain. Each bridge circuit 51, 5
The output voltage of 2 is supplied to the signal lines 69, 69 through the terminals 67, 68.
It is taken out at 70.

[0006]

In the differential pressure sensor having the circuit configuration described above, when pressures P ₁ and P ₂ are applied to the respective strain generating portions 65 and 66 of the diaphragm base 62, the resistance value of each strain gauge is obtained. The change of is caused as follows based on the relationship of the arrangement position in each strain generating section. In the following equation, S1 is a positive resistance change rate in the strain generating section 65, and S1 is
Reference numeral 1'denotes a negative resistance change rate in the strain generating section 65, S2 indicates a positive resistance change rate in the strain generating section 66, and S2 'indicates a negative resistance change rate in the strain generating section 66. Further, R represents the resistance value of each strain gauge in a state where no strain is generated in each strain generating portion.

[0007]

[Number 1] relates to the bridge circuit 51, the strain gauges 51a: R + S1 · P ₁ strain gauge 51b: R-S1 '· P 1 strain gauge 51c: R + S1 · P ₁ strain gauge 51d: R-S1' · P 1 bridge circuit 52 respect, the strain gauges 52a: R + S2 · P ₂ strain gauges 52b: R-S2 '· P 2 strain gauges 52c: R + S2 · P ₂ strain gauge 52d: R-S2' is applied between the input terminals of · P ₂ bridge circuit 51 If the voltage is V, the potential difference V generated between the terminals 58 and 59 is V
₀₁ is given by the following equation.

[0008]

[Equation 2]

Similarly, the potential difference V ₀₂ generated between the terminals 60 and 61 is given by the following equation.

[0010]

[Equation 3]

When the gain of the differential amplifier 53 is G ₁ , the gain of the differential amplifier 54 is G ₂ , and the gain of the differential amplifier 55 is 1, the potential difference Vd output from the differential amplifier 55 is

[0012]

[Equation 4]

[0013]

In the above equation (4), the denominator of the equation includes a term depending on pressure. According to the above equation, when the differential pressure is detected by the circuit configuration of FIG. 9, the detection output Vd is the pressure difference P ₁ −.
It is not expressed as a linear relationship with P ₂ . That is, the output of the detection circuit has a non-linear characteristic.

Therefore, conventionally, a detecting circuit as shown in FIG. 11 has been proposed as a means for solving the above problem.
In this detection circuit, constant current sources 71 and 72 are connected so that constant currents are supplied to the bridge circuits 51 and 52. According to this circuit, it is possible to eliminate the above-mentioned non-linear characteristic relating to the detection output. The reason is as follows.

For example, in the bridge circuit 51 provided in the strain generating section 65, its power source is a constant current source 71 and its output current is I ₁ . When the constant current source 71 is used, the current flowing through the strain gauges 51a and 51b and the strain gauges 51d,
The current flowing through 51c is always equal, and its magnitude is I _1/2 . Therefore, the output voltage V ₀₁ of the bridge circuit 51 is given by the following equation.

[0017]

Equation 5] _{V 01 = (R + S1 ·} P 1) · I 1 / 2- (R-S1 '· P 1) · I 1/2 = (S1 + S1') · P 1 · I 1/2 apparently by the above formula As described above, since the denominator does not include the pressure element, the non-linearity in the output voltage V ₀₁ is eliminated. This output characteristic is similarly established for the other bridge circuit 52 that uses the constant current source 72 as the current source. This solves the problem of non-linearity.

However, the configuration of the detection circuit having the two constant current sources 71 and 72 shown in FIG. 11 raises the following problem.

It is necessary to exactly match the output current values of the two constant current sources 71 and 72. Therefore, the two constant current sources must be adjusted so that their output current values match each other. Therefore, it takes time and effort to adjust the circuit.

Further, it is necessary to exactly match the gains of the differential amplifiers 53 and 54. Therefore, there is a problem that it takes time to adjust the gain of each amplifier.

As is apparent from the detection circuits shown in FIGS. 9 and 11, the conventional circuit requires three differential amplifiers. Therefore, there is a drawback that the number of circuit elements increases and the circuit becomes large.

An object of the present invention is to provide a highly accurate differential pressure sensor detection circuit which does not require adjustment of the current value of a constant current source and has a signal processing circuit with a simple structure.

[0023]

In order to achieve the above object, the detection circuit of the differential pressure sensor according to the present invention is constructed as follows.

A differential pressure sensor that is sensitive to two different pressures and measures the difference between the two pressures. The pressure detecting member is formed with two strain generating portions for receiving each of the two pressures. Each of the two strain gauges, which have a resistance change in the positive direction and two strain gauges that change the resistance in the negative direction, are provided on the surface opposite to the pressure receiving surface, respectively. Forming a first branch path connecting in series a strain gauge whose resistance changes in the positive direction of the first part and a strain gauge whose resistance change of the second strain generating part changes in the negative direction, and forming a first branch path in the negative direction of the first strain generating part. Forming a second branch path connecting in series a strain gauge whose resistance changes and a strain gauge whose resistance changes in the positive direction of the second strain generating portion, and combining the first and second branches in an adjacent state. To form a differential pressure detecting bridge circuit.

In the above structure, preferably, a constant current source for supplying a constant current is connected as a power source of the bridge circuit.

In the above structure, preferably, the terminal facing the terminal to which the constant current source is connected is kept in a constant potential state.

Further, it is a differential pressure sensor which is sensitive to two different pressures and measures the difference between the two pressures, and the pressure detecting member is provided with two strain generating portions for receiving each of the two pressures.
A strain gauge whose resistance changes in the positive direction depending on the pressure is provided on one of the two strain-generating parts and on the opposite side to the pressure-receiving face. A strain gauge whose resistance changes in the negative direction according to the magnitude of the pressure is provided on the surface of, and a strain gauge whose resistance changes in the positive direction and a strain gauge whose resistance changes in the negative direction are connected in series, and the combined resistance It is configured to determine the differential pressure based on the change.

[0028]

In the detection circuit of the differential pressure sensor according to the present invention, a Wheatstone bridge circuit (or a plurality of strain gauges (or one strain gauge) formed in each of the two strain generating portions are combined and connected in a predetermined connection pattern. A circuit according to this) is formed, and the difference between the two pressures acting on each of the two strain generating portions can be directly taken out at the output of this bridge circuit. Therefore, there is no need for a circuit configuration for obtaining the pressure difference in the subsequent stage of the bridge circuit. Therefore, the differential pressure signal can be extracted by amplifying the output of the bridge circuit with one differential amplifier.

By using a constant current source as a means for supplying a current to the bridge circuit, it is possible to eliminate the non-linearity in the output of the bridge circuit.

Further, in the detection circuit of the differential pressure sensor according to the present invention, since one bridge circuit is sufficient in terms of the circuit configuration, only one constant current source is required, and the process of adjusting the output current of the constant current source as in the conventional case. Is unnecessary.

[0031]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

1 and 2 show a first embodiment. Figure 1
2 is a circuit diagram of a detection circuit of the differential pressure sensor, and FIG. 2 is a plan view of a diaphragm base of the differential pressure sensor.

As shown in FIG. 2, the differential pressure sensor according to the present invention has two strain-generating portions 2 and 3 defined by a circular shape (indicated by a broken line in the figure) on a diaphragm base 1. Four strain gauges 4a to 4d and 5a to 5 having the same resistance change characteristics are provided on one surface of each of the strain generating portions 2 and 3.
d is formed in a predetermined positional relationship. Deflection part 2, 3
The other surface of is formed as a pressure receiving surface that receives pressure.
A pressure medium such as pressure oil is introduced into each pressure receiving surface via a pressure introducing hole (not shown). The cross-sectional shape of the diaphragm base 1 is substantially the same as the conventional one shown in FIG.

The strain gauges formed on the respective surfaces of the two strain generating portions 2 and 3 are formed by a film forming technique or a film forming technique. The strain gauges 4a to 4d in the strain-flexing part 2 and the strain gauges 5a to 5d in the strain-flexing part 3 are changed in the illustrated manner so that they can be clearly distinguished on the circuit of FIG.
The strain gauges 4a to 4d and the strain generating portion 3 formed on the strain generating portion 2
The strain gauges 5a to 5d formed in 1 are simultaneously connected in accordance with a specific circuit pattern during film formation. The circuit pattern is line 6 in FIG. 2 (reference numeral 6 is partially attached).
Indicated by. 7a to 7f are pads for connecting to an external circuit portion. The circuit shown in FIG. 1 shows the connection configuration of each strain gauge and pad. A power supply and an amplifier circuit exist as an external circuit portion. The pads 7a to 7f respectively form terminals in FIG. Hereinafter, the terminals 7a to 7f will be referred to.

The characteristics of the four strain gauges in each of the strain generating portions 2 and 3 will be described. When pressure is applied to each of the strain-flexing portions 2 and 3, the stress distribution in the diameter direction in the strain-generating portion is as shown in FIG. According to this stress distribution characteristic, the resistance value of the strain gauge arranged near the center changes in the positive direction in response to the tensile action, and the strain gauge arranged near the periphery corresponds to the contraction action. It has a characteristic that the resistance value changes in the negative direction. Therefore, the strain gauges 4b, 4e and 5a, 5 arranged near the center of the strain generating portion are
c is a strain gauge that causes a resistance change in the positive direction, and strain gauges 4a, 4c and 5b, 5d arranged in the peripheral portion of the strain generating portion are strain gauges that cause a resistance change in the negative direction. In the electric circuit of FIG. 1, a strain gauge that changes resistance in the positive direction is indicated by an upward arrow, and a strain gauge that changes resistance in the negative direction is indicated by a downward arrow.

Next, the electric circuit shown in FIG. 1 will be described. As shown in FIG. 1, the first and second strain-flexing portions 2,
A Wheatstone bridge circuit is formed by connecting a total of 8 strain gauges arranged in each of 3 in a predetermined combination. This Wheatstone bridge circuit has four branches. The four branches have two types of configurations. The first branch has a strain gauge 4b (4d) whose resistance changes in the positive direction in the first strain generating section 2 and a strain gauge 5b (5d) whose resistance changes in the negative direction in the second strain generating section 3. Connected in series. The second branch has a strain gauge 4a (4c) whose resistance changes in the negative direction in the first strain generating section 2 and a strain gauge 5a (5c) whose resistance changes in the positive direction in the second strain generating section 3. Connected in series. In FIG. 1, 8A is a first branch and 8B is a second branch. The Wheatstone bridge circuit of the differential pressure detection circuit according to the present invention includes the first and second branch lines 8 described above.
It is constituted by connecting A and 8B adjacent to each other and alternately.

A constant current is supplied from the constant current source 9 to the Wheatstone bridge circuit having the above structure through the terminal 7f. The voltage source 10 is connected to the terminal 7c facing the terminal 7f, and the terminal 7c is maintained at a constant voltage. Instead of connecting the voltage source 10, it can be grounded. The terminals 7a and 7b are output terminals of the Wheatstone bridge circuit. The voltage output from the terminals 7a and 7b is input to the differential amplifier 11 and amplified to a predetermined sensor output level. In this circuit, it is sufficient to provide one differential amplifier 11. As is clear from Fig. 1,
On the detection circuit, terminals 7a and 7d, terminals 7b and 7e
Have the same terminal.

In the detection circuit having the above structure, the four strain gauges formed on the first strain-generating portion 2 and the second strain-generating portion 3 are arranged.
By combining the four strain gauges formed on the Wheatstone bridge circuit, the output terminal 7
The output voltage corresponding to the differential pressure can be directly taken out to a and 7b. Therefore, as compared with the conventional detection circuit, the circuit configuration for calculating the voltage corresponding to the differential pressure becomes unnecessary. Therefore, there is an advantage that the signal processing circuit is simplified and it is not necessary to perform the adjustment of gain matching between the two differential amplifiers which has been conventionally required.

Next, it will be shown that the voltage corresponding to the differential pressure can be directly taken out from the output of the Wheatstone bridge circuit. This becomes clear by obtaining an arithmetic expression representing the output of the bridge circuit.

When pressures P ₁ and P ₂ are applied to the first and second strain producing portions 2 and 3 of the diaphragm base 1, the resistance values of the strain gauges change as follows. In the following equation, S1 is a positive resistance change rate in the strain generating section 2, S1 'is a negative resistance change rate in the strain generating section 2, S2 is a positive resistance change rate in the strain generating section 3, and S2' is a strain generating section. 3 is a negative resistance change rate. Further, R represents the resistance value of each strain gauge in a state where no strain is generated in each strain generating portion.

[0041]

[6] strain gauges 4a: R-S1 '· P 1 strain gauges 4b: R + S1 · P ₁ strain gauge 4c: R-S1' · P 1 strain gauge 4d: R + S1 · P ₁ Moreover,

[0042]

Equation 7] strain gauges 5a: R + S2 · P ₂ strain gauges 5b: a bridge circuit by · P ₂ constant current source 9 R-S2 'R-S2 · P 2 strain gauge 5c:: R + S2 · P 2 strain gauges 5d' 2 current supplied
I. In this case, the strain gauges 4a, 5a, 4b, 5
The amount of current flowing in the path of b and the strain gauges 4c, 5c, 4
The amount of current flowing through the paths d and 5d is always I, ignoring the current flowing into the input of the differential amplifier 11. Therefore, the voltage generated between the output terminals 7a and 7b of the bridge circuit is given by the following equation.

[0043]

Equation 8] _{(2R + S2 · P 2 -S1} '· P 1) I - (2R-S2' · P 2 -S1 · P 1) I = (S2 + S2 ') I · P 2 - (S1 + S1') I · P _{1 In the} above formula, each strain gauge is formed under the same conditions,
As a result, if the first and second strain gauges have the same electrical and physical properties, then S1 = S2, S
1 '= S2', and the above equation is

[0044]

Equation 9] becomes (S1 + S1 ') I ( P 2 -P 1). As is clear from this equation, the output voltage of the Wheatstone bridge circuit becomes a signal proportional to the differential pressure.
In order to satisfy the above equation, the above-mentioned "each strain gauge is formed under the same condition, and as a result, the first and second strain gauges have the same electric and physical properties. However, this can be easily achieved by applying the semiconductor film forming technique to the diaphragm substrate 1.

The structure of the detection circuit shown in FIG. 1 can be modified as shown in FIG. In this circuit, the constant current source 9 and the constant voltage source 10 are interchanged and connected. Other configurations are the same as those of the circuit of FIG. Therefore, a similar effect is produced, and a signal proportional to the differential pressure can be output from the output terminal.

The detection circuit of the differential pressure sensor according to the present invention can be further modified as follows.

The detection circuit of the differential pressure sensor according to the present invention is a series circuit of a strain gauge whose resistance changes in the positive direction in the first strain generating section and a strain gauge whose resistance changes in the negative direction in the second strain generating section. It is configured by combining a series circuit of a strain gauge whose resistance changes in the negative direction in the first strain generating section and a strain gauge whose resistance changes in the positive direction in the second strain generating section in a predetermined connection relationship.

Further, as another configuration example, the configurations shown in FIGS. 5 and 6 can be cited. In this embodiment, the strain generating unit 2
It is configured by a series circuit of a strain gauge 4d arranged in the central portion of which changes resistance in the positive direction, and a strain gauge 5d arranged in the peripheral portion of the strain generating portion 3 which changes resistance in the negative direction. A constant current I is applied to the combined resistance formed by the series circuit by the constant current source 9. The potential difference V _OUT generated between the terminals of the series circuit is taken out as a differential pressure signal. In this case, the resistance change of the strain gauge 4d is R + S1 · P ₁ , and the strain gauge 5 is
resistance change d is the R-S2 '· P _2. Therefore, the potential difference V _{OU T} between the terminals I · {2R + S1 · P 1 -S2 '·
P ₂ }. In this embodiment, since the element proportional to the differential pressure is included under the condition that S1 and S2 'are equal, it is possible to extract the differential pressure signal component.

Similarly, the detection circuit of the differential pressure sensor shown in FIGS. 7 and 8 can be considered as another embodiment. In this detection circuit, strain gauges 4b and 4c arranged at the central portion and the peripheral portion of the strain generating portion 2 and strain gauges 5b and 5c arranged at the central portion and the peripheral portion of the strain generating portion 3 are provided.
The circuits shown in FIG. 7 are connected to form a bridge circuit. The resistor 12 is a resistance element connected to form a bridge circuit and has a constant resistance value R. A differential pressure signal can be taken out at the output voltage generated between the output terminals 13 and 14 of the bridge circuit. The constant current source 9 and the constant voltage source 10 are the same as those in the embodiment of FIG.

[0050]

As is apparent from the above description, according to the present invention, two strain generating portions are formed on the diaphragm base body which is a member for detecting pressure, and a predetermined number of strain generating portions are formed on the surface of each strain generating portion. Since the strain gauges are formed and the strain gauges are combined in a predetermined relationship to form a bridge circuit or a circuit equivalent thereto, a differential pressure signal can be directly obtained at the output of the bridge circuit or the like. Therefore, the number of differential amplifiers can be reduced in the subsequent stage such as the bridge circuit, and the configuration of the detection circuit can be simplified. Further, unlike the conventional detection circuit, the adjustment between the differential amplifiers is not required, and the setting of the detection circuit becomes easy. Further, as compared with the conventional detection circuit using the constant current source, the number of the constant current sources is sufficient, and the current value adjustment between the constant current sources is unnecessary.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a detection circuit of the present invention.

FIG. 2 is a plan view showing a circuit pattern formed on the surface of a diaphragm base of the differential pressure sensor.

FIG. 3 is a graph showing characteristics of stress generated in each part of the strain-flexing part.

FIG. 4 is a circuit diagram showing a second embodiment of the detection circuit of the present invention.

FIG. 5 is a circuit diagram showing a third embodiment of the detection circuit of the present invention.

FIG. 6 is a plan view showing a circuit pattern on a diaphragm base of a detection circuit according to a third embodiment.

FIG. 7 is a circuit diagram showing a fourth embodiment of the detection circuit of the present invention.

FIG. 8 is a plan view showing a circuit pattern on a diaphragm base of a detection circuit according to a fourth embodiment.

FIG. 9 is a circuit diagram showing an example of a conventional differential pressure sensor detection circuit.

FIG. 10 is a vertical sectional view of a diaphragm base of the differential pressure sensor.

FIG. 11 is a circuit diagram showing another example of a conventional differential pressure sensor detection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Diaphragm base | substrate 2,3 ... Strain generating part 4a-4d, 5a-5d ... Strain gauge 9 ... Constant current source 10 ... Voltage source 11 ... Differential amplifier

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fujio Sato 650, Kazunachi-cho, Tsuchiura-shi, Ibaraki Hitachi Construction Machinery Co., Ltd. Tsuchiura factory

Claims (4)

[Claims] 1. A differential pressure sensor, which is sensitive to two different pressures and measures the difference between the two pressures, wherein a pressure detecting member is formed with two strain generating portions for receiving each of the two pressures. Two strain gauges are provided on the surface on the side opposite to the pressure receiving surface of each of the strain-flexing portions at locations where it is distorted in the positive direction according to the magnitude of pressure, and two strain gauges are provided at locations that are distorted in the negative direction. Forming a first branch path connecting in series a strain gauge whose resistance changes in the positive direction of the first strain generating section and a strain gauge whose resistance changes in the negative direction of the second strain generating section, Forming a second branch path connecting in series a strain gauge whose resistance changes in the negative direction of the first strain generating section and a strain gauge whose resistance changes in the positive direction of the second strain generating section in series;
A detection circuit for a differential pressure sensor, characterized in that the first and second branches are combined in an adjacent state to form a differential pressure detection bridge circuit. 2. The detection circuit of the differential pressure sensor according to claim 1, wherein a constant current source for supplying a constant current is connected as a power source of the bridge circuit. 3. The differential pressure sensor detection circuit according to claim 2, wherein a terminal facing the terminal to which the constant current source is connected is held in a constant potential state. Detection circuit. 4. A differential pressure sensor which is sensitive to two different pressures and which measures the difference between the two pressures, wherein two strain generating portions for receiving the two pressures are formed in the pressure detecting member. A strain gauge is provided on one of the strain-flexing portions, on a surface opposite to the pressure-receiving surface, at a location where the strain is generated in the positive direction according to the magnitude of pressure, and on the other side of the strain-flexing portion, the opposite side of the pressure-receiving surface On the surface of, a strain gauge is provided in a place where it is distorted in the negative direction according to the magnitude of pressure, and the strain gauge that changes resistance in the positive direction and the strain gauge that changes resistance in the negative direction are connected in series, and a composite thereof is obtained. A detection circuit for a differential pressure sensor, which is configured to obtain a differential pressure based on a change in resistance.