JP2001153735A - Load cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a load cell which is free from variation in detected strain value and the local concentration of strain even if a strain gauge formed on the surface of a strain inducer slightly shifts in position and able to stably take a measurement and can be made small-sized. SOLUTION: The strain inducer 1 in a square shape is formed of an elastic body and has two main surfaces; and two diagonal vertexes 2 and 3 of one main surface of the strain inducer 1 are support points and the other two diagonal vertexes 4 and 5 of the other main surface are load application points. On one or both of the main surfaces of the strain inducer, at least four strain gauges 7a and 7b and 8a and 8b are formed and a Wheatstone bridge circuit detects resistance variation of the strain gauges 7 and 8 due to strain induced by the strain inducer 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a load cell used in a converter of a device for converting the magnitude of displacement or force into an electric signal.

[0002]

2. Description of the Related Art Conventionally, for the purpose of measuring displacement, force, weight, etc., a strain gauge utilizing a resistance change due to distortion of a metal conductor is attached to the surface of an elastic body, or an elastic body having an insulating surface. A load cell in which a resistor pattern is directly formed by a thin film technique or a thick film printing technique is used.

In order to extract a change in resistance of a strain gauge due to a strain as an electric signal, a method of forming a Wheatstone bridge circuit using four strain gauges is generally used. In this method, when two sets of opposing strain gauges on the bridge circuit are paired,
It is known that the detection sensitivity is the highest when resistance changes with the same size and opposite signs appear in the two strain gauges.

[0004] Therefore, many load cells have been proposed which utilize a bending deformation of a plate-shaped strain-generating body and form strain gauges on both sides thereof to detect tensile and compressive strain. 7 to 9 show typical ones. The load cell shown in FIG. 7 has a lower surface serving as a support surface, and connects two plate-like strain bodies 32 and 33 with a load applying unit 31 to form a strain body 3.
The strain gauges 34 and 35 are formed on both sides of the straight gauges 2 and 33, and are called "straight beam sensing type". FIG. 8 shows FIG.
And the case 37 outside the load cell is completely separated, and the case 37 outside the load cell is fixed by screwing or external force.
The deformation caused in the strain gauge does not affect the strain gauge 38, and is called "bending sensitive type".

FIG. 3 shows a cantilever structure using a beam-shaped flexure element 39, which is reduced in size and simplified in structure, and is called a "beam type". In addition, there have been made ones based on the above structure and having improved various characteristics.

[0006]

However, in these conventional low load cells, a narrow portion is provided in the strain gauge formation, and the strain is concentrated locally, so that the displacement when attaching the strain gauge to the strain generating element is displaced. However, there is a problem that the sensitivity characteristics vary.

In order to improve this problem, for example, Japanese Patent Application Laid-Open
In JP-A-191527, a conventional resistor pattern is provided by forming an insulator film on the surface of a beam-like strain body and forming a strain gauge thereon by using thin film technology such as evaporation or sputtering. A structure has been proposed that enables a reduction in the number of manufacturing steps and a high-precision measurement, as compared with a method in which a film is bonded to a strain generating portion of a beam body.

However, even with this method, it is necessary to strictly control the positional accuracy of the resistor pattern, and the capital investment due to the use of the thin film technology has caused a rise in cost.

Japanese Patent Application Laid-Open No. 62-49228 discloses a load cell in which a strain gauge is formed on a beam-type strain body by a thin-film technique. A method has been proposed in which, by forming the strain gauges respectively and then combining the half beam members, the manufacturing is performed in a shorter time as compared with the case where the strain gauges are formed on both surfaces of the integrated beam member which is not divided into half. I have. However, in this method, it is necessary to maintain the assembling accuracy of the half beam body with high accuracy, and as a result, there is a problem that productivity and cost are deteriorated.

As described above, in the conventional load cell structure, the strain is locally concentrated at the strain generating portion of the strain generating element, and the amount of strain changes greatly due to a slight difference in position. There is a problem that the distortion sensitivity is easily changed greatly due to the displacement.

[0011]

Means for Solving the Problems The inventor of the present invention has conducted various studies on the above-mentioned problems, and found that the flexure element has a square shape, one of which has two diagonal points as support points, and the other has two diagonal points. It has been found that a shape having a load application point is preferable. This structure is so-called "saddle-shaped bending" because the deformation of the strain body becomes a saddle shape for riding.

That is, the load cell according to the present invention includes a square-shaped flexure element made of an elastic body and having two main surfaces, and supports two diagonal vertexes in one principal plane of the flexure element. A point, and the other diagonal vertices in the other main surface are set as load application points, and a strain gauge is formed on one or both main surfaces of the strain body, and the strain caused by strain generated in the strain body is formed. It is characterized in that a resistance change of a gauge is detected by a Wheatstone bridge circuit.

In this load cell, at least four strain gauges are formed on a diagonal line of one or both main surfaces of the strain body, and two strain gauges are formed.
The strain in a direction parallel to the diagonal of the book is detected, or two or more strain sensors are formed on one or both principal surfaces of the strain body and a total of two or more strain gauges are used to detect XY.
It is also possible to configure so that distortions in two directions can be detected simultaneously.

According to the present invention, the above-described square-shaped flexure element is generated throughout both principal surfaces of the flexure element as shown in the schematic diagram of the strain state calculated by the finite element method in FIG. It can be seen that the tensile strain and the compressive strain are perpendicular to each other, and the absolute value of the tensile strain and the compressive strain are equal to each other. At this time, the principal axes of strain in tension / compression are in the diagonal direction of the flexure element, and the directions are rotated by 90 degrees on both principal surfaces. Moreover, according to the theory of material mechanics, the magnitude of the strain is equal over the entire surface of both principal surfaces.

Therefore, according to the load cell of the present invention, the distortion is constant on the main surface, and the value of the detected distortion does not change even if the position of the strain gauge is slightly shifted. In addition, since the strain is not concentrated locally, a high strain detection sensitivity can be obtained even if a strain gauge is formed on the entire surface of the strain body, and the load cell can be made compact.

[0016]

2 and 3 show an embodiment of a load cell according to the present invention. FIG. 2 is a schematic perspective view, and FIG. 6 is a schematic plan view.

In the load cell A, two opposing vertices 2 and 3 on the back main surface of the square-shaped strain-generating body 1 made of an elastic body constitute support points supported from below, and the remaining two vertices on the surface side. A load application point to which a load is applied from above is configured for 4 and 5.

In the inner region 6 excluding the outer edges of both main surfaces of the strain generating element 1, a uniform biaxial tensile compression strain state is generated as described above. That is, on the side of the load application surface 6a, the main axis of tension is parallel to the diagonal direction connecting the vertices 4 and 5, which are two load application points, and the main axis of compressive strain is orthogonal to the diagonal direction. On the opposite support surface side 6b, it is pulled in a direction rotated by 90 degrees with respect to this,
The main axis of each strain of compression is oriented. The absolute value of the strain is equal in the inner region except for the outer edges of both principal surfaces of the strain body 1.

According to the present invention, four strain gauges 7a, 7b, 8a, 8b are formed in the area of the inner region 6, and constitute a Wheatstone bridge circuit. The strain gauges 7a, 7b are arranged on four diagonal lines emitted from the center of the strain body 1 toward the corners 2, 3, 4, 5. With such arrangement of the strain gauges, tensile strain is detected in the strain gauges 7a and 7b, and compressive strain is detected in the strain gauges 8a and 8b.

In the arrangement of the strain gauges shown in FIG. 3, four strain gauges are formed only on the load application surface 6a side. However, only the support surface side 6b or both of the load application surface 6a and the support surface 6b are provided. It may be formed on the main surface. In this case, since the directions of the distortion on both the main surfaces are rotated by 90 degrees, when the strain gauges are formed on only one of the main surfaces, the strain gauges differ by two as shown in FIG. When the strain gauges are formed on both main surfaces, (1) two strain gauges are arranged on each main surface on the main diagonal orthogonal to each other on the front and back sides. A configuration that detects tensile strain on the main surface of the device and detects compression strain on the other main surface,
(2) A configuration in which strain gauges are formed on both main surfaces in the same arrangement as in FIG. 3 to detect tensile and compressive strain on each main surface;
(3) Three strain gauges are formed on one main surface,
A configuration in which one strain gauge for detecting tensile strain or compressive strain is formed on the other main surface, or the like is possible.

The strain gauges 7a, 7b, 8
The positions of a and 8b do not necessarily have to be on a diagonal line as shown in FIG. 3, and may be arranged anywhere in the region 6 if the direction of the distortion to be detected is parallel to this.

In this strain gauge, a metal resistor pattern is provided on an insulating film, and a strain gauge utilizing a resistance change due to distortion of a metal conductor is adhered to the surface of the strain generating element, a thin film technique or a thick film printing technique. Thus, the resistor pattern can be formed by directly forming the resistor on an elastic body having an insulating surface.

For bonding the strain gauge to the strain body,
A resin adhesive is used, but in order to prevent such a problem that the resin adhesive swells in a high humidity environment or an organic solvent environment and an error occurs in a detection value, for example, a strain generating body is formed. Using an insulating ceramic as the elastic body, the strain gauge is made of a strain-generating body 1 by thin-film technology or thick-film printing technology.
It is desirable to print and form it on the surface of the substrate.

The inner region 6 of the main surface on which the strain gauge is provided needs to be a region where uniform biaxial tensile strain and compressive strain are generated. Should be removed. This is because the outer edge does not become a biaxial tensile / compression stress state due to the edge effect. Assuming that the distance from the outer sides of the two main surfaces to the support point or the load application point is a, the width w of the outer edge portion outside the region 6 is desirably 2a or more.

FIG. 4 is a schematic plan view showing another embodiment of the load cell of the present invention. In FIG. 4, two strain gauges 9a and 9b are formed on the load application surface 6a. The strain gauges 9a and 9b are formed by laminating two resistor patterns so as to be orthogonal to each other, whereby it is possible to detect strains in two axial directions. A Wheatstone bridge circuit is formed by a total of four resistance changes.

The strain gauges 9a and 9b shown in FIG.
May be arranged anywhere in the internal region 6a. According to such a configuration, it is possible to further reduce the size of the flexure element 1 and finally reduce the size of the load cell itself as compared with the configuration of FIG.

FIG. 5 is a schematic plan view showing still another embodiment of the load cell of the present invention. In FIG. 5, strain gauges 9 used in the load cell of FIG. 4 are formed on both main surfaces of the flexure element 1, and the Wightstone bridge is formed by the two strain gauges 9 formed on both main surfaces. Configure the circuit. This strain gauge 9 may be arranged anywhere in the internal regions 6a and 6b. According to such a configuration, it is possible to further reduce the size as compared with the load cell of FIG.

As the flexure element used in the load cell of the present invention, any one can be used as long as it can be elastically deformed. For example, one or more of ceramics, organic resins, and metals can be used. Any strain may be used, but at least the strain gauge needs to be insulated from the strain generating body.

[0029]

EXAMPLE Stainless steel having a side length of 20 mm and a thickness of 2 mm was used as a strain body. On one main surface, four commercially available resistance wire strain gauges were attached with a cyanoacrylate adhesive. This strain gauge adheres Cu-Ni alloy foil on polyimide film,
A gauge pattern was formed by photoetching. The gauge surface had a length of 1 mm, a width of 0.65 mm, and a resistance value of 120 ohm. These were arranged on four diagonals extending from the center of the strain body toward the corners, and connected as shown in FIG. 3 to form a Wheatstone bridge circuit. This load cell was placed in a jig shown in FIG. As shown in FIG. 6, the vertices 2 and 3 serving as the support points of the load cell A are placed on the support bases 10 and 10, and the working portion of the pressure applying member 11 is moved from the surface opposite to the support surface of the load cell A. 12, a pressure was applied to the center of the flexure element 1 of the load cell, and the output voltage of the Wheatstone bridge circuit by the strain gauge formed on the support surface side was measured (Experiment 1).

Further, of the four strain gauges,
One of the positions was shifted sideways and reattached, the output voltage of the Wheatstone bridge was measured, and the results are shown in Table 1.

[0031]

[Table 1]

According to Table 1, the position of the strain gauge is 2 m.
There was no significant change in the output voltage even if the distance was shifted by m, indicating that the load cell having this structure was extremely insensitive to the displacement of the strain gauge.

A load cell (Experiment 4) shown in FIG. 4 using a strain gauge capable of detecting distortion in two axial directions was manufactured, and measurement was performed in the same manner as in FIG. At this time, a stainless steel body having a side length of 10 mm and a thickness of 2 mm was prepared using stainless steel as the strain body, and the results are shown in Table 2.

[0034]

[Table 2]

As can be seen from Table 2, a sufficient output can be obtained by using the load cell according to the present invention even if the size of the strain body is reduced.

[0036]

As described above, according to the load cell of the present invention, even if the position of the strain gauge formed on the surface of the flexure element is slightly displaced, there is no change in the detected distortion value, and the distortion is not changed. Since the strain is not concentrated locally, a high strain detection sensitivity can be obtained even if a strain gauge is formed on the entire surface of the strain generating element, and the load cell can be made compact.

[Brief description of the drawings]

FIG. 1 is a view showing a state in which strain is generated in a load cell flexure element according to the present invention.

FIG. 2 is a perspective view showing the structure of the load cell according to the first embodiment of the present invention.

FIG. 3 is a plan view showing the structure of the load cell according to the first embodiment of the present invention;

FIG. 4 is a plan view showing a structure of a load cell according to claim 2 of the present invention.

FIG. 5 is a plan view showing a structure of a load cell according to claim 3 of the present invention.

FIG. 6 is a side view showing the structure of a measuring jig used to check the effect of the present invention.

FIG. 7 is a schematic cross-sectional view showing the structure of a conventional linear beam-sensitive load cell.

FIG. 8 is a schematic sectional view showing the structure of a conventional bending-sensitive load cell.

FIG. 9 is a schematic sectional view showing the structure of a conventional beam type load cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Flexure element 2, 3 ... Support point 4, 5 ... Load point 6 ... Inner area 7a, 7b, 8a, 8b, 9 ... Strain gauge

Claims (3)

[Claims] 1. A square strain body made of an elastic body and having two main surfaces, wherein two diagonal vertices in one main surface of the strain body are set as support points, and in another main surface. The other two diagonal vertices are defined as a load application point, a strain gauge is formed on one or both main surfaces of the strain generating element, and a change in resistance of the strain gauge due to strain generated in the strain generating element is detected by a Wheatstone bridge. A load cell detected by a circuit. 2. The strain gauge according to claim 1, wherein at least four strain gauges are formed on one or both principal surfaces of said strain body so as to detect strains in two directions parallel to two diagonal lines of said strain body. The load cell according to claim 1, wherein the load cell is arranged. 3. The load cell according to claim 1, wherein at least two strain gauges are formed on one or both main surfaces of the strain body, and strains in two axial directions can be simultaneously detected by each strain gauge. .