JP4192820B2 - Load cell - Google Patents

Description

  The present invention relates to a load cell using a strain gauge, and more particularly to a load cell suitable for an application requiring accurate load detection, such as a load sensor of an electronic balance.

  In a strain gauge type load cell, generally, a plurality of strain gauges are attached to a strain generating body that is elastically deformed by the action of a load, and each strain gauge constitutes a Wheatstone bridge, and the output of the Wheatstone bridge is strained. Used as a detection output of the magnitude of the load acting on the body.

  FIG. 4 is a perspective view showing a configuration of a conventional load cell. The strain body 20 in this example includes a pair of column portions 20a and 20b, and the column portions 20a and 20b are formed by two upper and lower beam portions 20c and 20d each having a strain generation portion e at both ends. In addition, the strain gauges S1 and S2 are attached to the two strain-generating portions e of the beam portion 20c, and the strain gauges S3 and S4 are attached to the two strain-generating portions e of the beam portion 20d. These strain gauges S1 to S4 are incorporated into a circuit of the Wheatstone bridge 13 as shown in FIG. 5 to constitute a load cell.

  In the above configuration, the output voltage (offset) of the Wheatstone bridge 13 in a state where only one of the column portions 20a and 20b, for example, the column portion 20a is fixed, that is, when no measurement load W is applied. (Voltage) Vo is close to 0V, and on the other hand, when the measurement load W is applied to the column part 20b, each strain generating part e receives a compressive or tensile force depending on its position, and the strain gauges S1, S4 are in the compression direction. The strain gauges S2 and S3 are elastically deformed by receiving a force in the tensile direction. Due to this elastic deformation, the resistance values of the strain gauges S1 to S4 change, and the output voltage Vo of the Wheatstone bridge 13 changes according to the load.

In the load cell as described above, any of the attached strain gauges S1 to S4 is used in either the compression operation region or the tension operation region in the characteristic diagram of the strain gauge shown in FIG. Since the operating area of the gauges S1 to S4 is less than ½ of the operable area, the output change amount is small, the measurement resolution, the S / N ratio, etc. are low, and high accuracy is difficult. Therefore, for example, as a method of increasing the output change amount, another strain generating portion having a different strain amount is provided in the vicinity of each strain generating portion e as shown in FIG. A method has been proposed in which a Wheatstone bridge is constructed by attaching different strain gauges, and the output from the two Wheatstone bridges is switched depending on the magnitude of the measurement load (see, for example, Patent Document 1).
JP-A-11-201839

The conventional load cell is configured as described above, but a strain generating part with a different load load strain amount is connected in series, and a strain gauge with a different measurement range is attached to each to form a Wheatstone bridge. In the method of switching the output according to the size of the output and increasing the amount of change in output, there are problems that the structure becomes complicated and the number of strain gauges increases, thereby increasing the number of manufacturing steps and costs and the shape.
The present invention has been made in view of such a situation, and an object of the present invention is to provide a high-accuracy load cell in which an output change amount when a load is applied is increased.

In order to achieve the above object, the load cell of the present invention is formed by hollowing out a rectangular base material and arranged in parallel, and a pair of lower beam portions and columns connected to both ends of these beam portions. A plurality of strain gauges are affixed to the thin wall portion of the beam portion of the strain body formed by forming a portion, and a Wheatstone bridge is configured with these strain gauges, and output in a state where no measurement load is applied In the load cell that detects the load by converting the output change in the loaded state into a weight value, a strain in the direction opposite to the strain caused by the measurement load is formed in advance in the strain generating section In addition, by deforming the strain gauge in the no-load state, the output in the no-load state is moved in the negative direction to increase the output change amount.
The load cell of the present invention is configured as described above, and an output change amount when a load is applied can be increased using a strain generating body having a simple structure.

  In the load cell of the present invention, the strain generated in the opposite direction to the strain generated when the rated load is applied is formed in advance in each strain generating portion, so that the load can be electrically transmitted by utilizing the entire tension and compression operation region of the strain gauge. Since it can be converted into a signal, a large amount of change in output can be obtained, and the load range that can be measured as a load detector can be widened. As a result, the measurement resolution, S / N ratio, etc. are improved and the accuracy is improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are diagrams showing the structure of an embodiment of the present invention. FIG. 1A is a front view, FIG. 1B is a plan view, and FIG. 1C is a bottom view.

  As shown in FIG. 1, the load cell of the present invention has a rectangular-shaped elastic metal base material seen from the front direction, and arc holes of the same radius facing each other in an oblique direction. By forming a beam portion 6 having strain-generating portions 1, 2 at both ends and a beam portion 7 having strain-generating portions 3, 4 at both ends by being hollowed out in a form in which the holes communicate with each other, A strain body 5 formed by forming a pair of column portions 8 and 9 for connecting both ends of the beam portions 6 and 7, and a metal foil attached by applying an adhesive to the strain portions 1 to 4. Or strain gauges G1 to G4 of a semiconductor type.

As shown in FIG. 2A, the strain generating portions 1 and 4 are formed in a shape in which a part of a circular arc protrudes on a plane, and the strain generating portions 2 and 3 are formed as shown in FIG. Each of the strain gauges G <b> 1 to G <b> 4 is attached to the surface of the strain generating portions 1 to 4 via an adhesive 11. When the radius of the arc of the strain generating portions 1 and 4 is R1, the radius of the strain generating portions 2 and 3 is R2, and the film thickness of the adhesive 11 is t, the strain gauges G1 and G4 are represented by the formula (1). An initial strain ε1 in the tension direction as described above, and an initial strain ε2 in the compression direction as shown in the equation (2) occurs in the strain gauges G2 and G3.
ε1 = (t / 2) / (R1 + t / 2) (1)
ε2 = (t / 2) / (R2 + t / 2) (2)

  As the strain gauges G1 to G4, those having the same characteristics as shown in the strain gauge characteristic diagram of FIG. 6 are used. The radius R1 of the strain generating portions 1 and 4 is a value calculated from the equation (1) when the initial strain ε1 of the strain gauges G1 and G4 is the maximum operating strain in the tensile direction (P-point strain). And the radius R2 of the strain generating portions 2 and 3 is calculated from the equation (2) when the initial strain ε2 of the strain gauges G2 and G3 is the maximum operating strain (strain at the Q point) in the compression direction. Use the value. As a result, an initial strain in the tensile direction corresponding to the point P is generated in the strain gauges G1 and G4, and an initial strain in the compression direction corresponding to the point Q is generated in the strain gauges G2 and G3. And these strain gauges G1-G4 are connected to the Wheatstone bridge 13 with the DC power supply 12 which supplies a reference voltage, as shown in FIG. 5, and the load detection circuit is comprised.

  As shown in FIG. 1, when the column 9 is fixed to the fixed base 10 and the measurement load W is applied to the column 8, the strain gauges G <b> 1 and G <b> 4 of the strain generating units 1 and 4 have a force in the compression direction. However, a tensile force acts on the strain gauges G2 and G3 of the strain generating portions 2 and 3. At this time, in a state where a rated load is applied, an operation strain in the compression direction (strain at the Q point) is generated in the strain generating portions 1 and 4, and an operation strain in the tensile direction is generated in the strain generating portions 2 and 3. The thickness of each thin portion is determined so that (distortion at point P) occurs. The calculation result of the stress distributed in the length direction when the measurement load W is applied to the strain body 5 processed into such a shape is as shown in FIG. 3, which is the result of the stress calculation in the conventional load cell. Is the same.

As described above, the resistance values of the strain gauges G1 to G4 attached to the strain generating portions 1 to 4 are r1 to r4, the resistance value at the maximum operating strain point (P point) in the tensile direction is Rp, and the maximum in the compression direction is When the resistance value at the operating strain point (Q point) is Rq and the proportionality constant is K, the resistance values r1 to r4 are expressed by equations (3) and (4) with respect to the strain ε generated by the measurement load W. Have a relationship.
r1, r4 = −Kε + (Rp + Rq) / 2 (3)
r2, r3 = Kε + (Rp + Rq) / 2 (4)

Further, the output voltage Vo of the Wheatstone bridge 13 shown in FIG. 5 has the relationship of the expression (5).
Vo = E (r2r3-r1r4) / ((r1 + r2) (r3 + r4)) (5)
From the above equations (3) to (5), the output voltage Vo is proportional to the strain ε as shown in the equation (6).
Vo = 2Kε / (Rp + Rq) (6)
When no load is applied, the resistance values r1 and r4 are Rp, and the resistance values r2 and r3 are Rq. Therefore, the output voltage Vo takes the value shown in Equation (7).
Vo = −E (Rp−Rq) / (Rp + Rq) (7)
When the maximum rated load is applied, the resistance values r1 and r4 are Rq, and the resistance values r2 and r3 are Rp. Therefore, the output voltage Vo takes the value shown in the equation (8).
Vo = E (Rp−Rq) / (Rp + Rq) (8)

  Since the output voltage Vo has the relationship as described above, if the output voltage when no load is applied is V1, and the output voltage when the maximum rated load is applied is V2, there is a relationship of V1 = −V2, and the output change amount ΔV (= V2−V1) is 2V2. Therefore, the output change amount in the load cell of the present invention is doubled compared to the output change amount V2 in the conventional load cell, and as a result, the stress generated in the state where the rated load is applied is the conventional load cell. The measurement resolution, S / N ratio, etc. are improved by a factor of 2, starting with the amount of output change.

  As shown in FIG. 2, the load cell according to the present invention includes the strain generating portions 1 to 4 so that the strain gauges G1 to G4 are preliminarily given a strain that is opposite to the strain generated in the strain generating portions 1 to 4 due to a load. By deforming, the strain gauges G1 to G4 attached to the strain generating portions 1 to 4 are given initial strain opposite to the strain due to the load increase, and the operation range is expanded to increase the output change amount. It is characterized by the above, and is not limited to the above-described embodiments. For example, the shape of the strain generating portions 1 to 4 shown in FIG. 2 can be other shapes such as an ellipse instead of an arc shape. Is possible.

BRIEF DESCRIPTION OF THE DRAWINGS It is a structural view of the load cell by the Example of this invention, (a) is a front view, (b) is a top view, (c) is a bottom view. It is a side view which shows the shape of the strain generation part and strain gauge concerning an Example, (a) is a compression strain generation part, (b) is a side view of a tension strain generation part. It is a stress distribution figure in the longitudinal direction of the strain body concerning an Example. It is a perspective view which shows the structure of the conventional load cell. It is a circuit block diagram of the strain gauge used for a load cell. It is an operation | movement area | region figure of a strain gauge.

Explanation of symbols

1-4 Straining part 5 Straining body 6, 7 Beam part 8, 9 Pillar part 10 Fixing base 11 Adhesive 12 DC power source 13 Wheatstone bridge 20 Straining body 20a, 20b Pillar part 20c, 20d Beam part e Straining part G1-G4 Strain gauge R1, R2 Radius S1-S4 Strain gauge Vo Output voltage W Measurement load

Claims (1)

  The beam part of the strain body formed by hollowing out a rectangular base material to form a pair of lower beam parts and column parts connected to both ends of these beam parts. Multiple strain gauges are affixed to the thin-walled part of the steel, and a Wheatstone bridge is configured with these strain gauges, and the output change in the loaded state is the weight value based on the output when the measurement load is not loaded. In a load cell that detects the load in terms of load, a strain in the direction opposite to the strain caused by the measurement load being applied is previously formed in the strain generating portion, and the strain gauge is deformed in an unloaded state. Thus, the load cell is characterized in that the output change amount is increased by moving the output in the no-load state in the negative direction.

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