JP2010107527A - Load cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a load cell constituted so that each strain generating part has the same thickness, and two arm parts have a thickness difference so as to reduce an influence of deflection of the strain generating part, in a Roberval type load cell wherein a strain gage is pasted on the strain generating part formed on one arm part. <P>SOLUTION: In this load cell wherein a Roberval mechanism is formed from two parallel arm parts formed between a fixed end and a movable end and four strain generating parts two parts of which are connected to each of the two arm parts respectively, all the four strain generating parts are formed with the same thickness, and since the two arm parts comprise a first arm part wherein each strain gage is pasted on two connected strain generating parts respectively, and a second arm part formed to have a thinner thickness than the first arm part, a strain applied to the strain generating part on which the strain gage is pasted can be balanced by the second arm part. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a load cell in which two parallel arm portions and two strain-generating portions formed on each arm portion constitute a Roberval mechanism.

  As shown in FIG. 19, a conventional load cell having a Robertval mechanism generally used includes a strain body 2 and strain gauges 3a, 3b, 3c and 3d. The strain generating body 2 has a cantilever structure having a fixed end 4 and a movable end 5, and a strain generation for detecting strain by attaching the strain gauge between the fixed end 4 and the movable end 5. A hole 7 for forming portions 6a and 6c, and a hole 8 for forming 6b and 6d, and a gap portion 9 connecting the holes 7 and 8 between the strain generating portions 6a and 6b and 6c and 6d. Are formed by linking two arm portions h1 and h2 in parallel to each other, and the four strain generating portions 6a, 6b, 6c and 6d operate along a locus of four corners of a parallelogram. It is formed as a mechanism, and the amount of strain is detected by forming a Wheatstone bridge circuit with the four strain gauges. At this time, it is assumed that the strain generating portions 6a, 6b, 6c and 6d have the same thickness, and that the arm portions h1 and h2 have the same thickness, and are formed symmetrically with respect to the center of the load cell. To do.

  However, since the Roverval mechanism is constructed by integrating each link and affects each other, the Roverval mechanism does not operate as an ideal Roverval mechanism, especially when the load position is shifted from the center of the load cell. The strain portions 6a, 6b, 6c and 6d are affected by the moment generated by the bending of the strain-generating body 2 having a cantilever structure, and the linearity between the load and the output detected by the strain gauge is deteriorated. It was something that would end up.

  In addition, the output circuit is composed of Wheatstone bridges composed of strain gauges attached to each of the four strain-generating bodies, and the output is obtained by the strain gauges attached to the two strain-generating parts formed on one arm part. In this case, the influence of the moment generated by the bending of the strain body 2 at the time of the eccentric load is reflected more greatly in the output, and the linearity is further deteriorated.

  The following techniques for solving the above-described problems by the shape of the strain-generating body are disclosed.

  First, in the load cell using the strain-generating body having the Roverval mechanism as described above, by adjusting the thickness of each strain-generating portion 6a, 6b, 6c and 6d, that is, the volume forming the strain-generating portion, There is one that adjusts the rigidity of the strained portions 6a, 6b, 6c, and 6d and deforms the strain generating body 2 as an ideal Roverval mechanism, thereby improving the accuracy of linearity. As an example of this, the entire neutral axis of the holes 7 and 8 and the gap 9 that form the arm portions h1 and h2 and the strain-generating portions 6a, 6b, 6c and 6d is shifted from the neutral axis of the load cell. Along with the arm portions h1 and h2, there is shown one formed with a difference in thickness between the strain generating portions 6a and 6b and the strain generating portions 6c and 6d (see, for example, Patent Document 1).

  In addition, for example, when the strain gauges 3a and 3b are affixed only to the strain generating portions 6a and 6b, and the load cell linearity is improved when detecting strain only on one side, the thicknesses of the arm portions h1 and h2 are the same, The strain generating portions 6a, 6b are arranged so as to cancel the influence of the moment generated by the bending of the strain generating body 2 having a cantilever structure while making the thicknesses of all the strain generating portions 6a, 6b, 6c and 6d the same. , 6c and 6d and the thicknesses of the arm portions h1 and h2 are derived, and a load cell manufacturing method is disclosed that makes it possible to manufacture a load cell having a similar shape based on the relationship (for example, Patent Document 2).

JP 2000-214008 A Japanese Patent No. 2666209

  However, as in the load cell disclosed in Japanese Patent Laid-Open No. 2000-214008, one that adjusts the thickness of the strain generating portions 6a, 6b, 6c, and 6d, or the vertical strain generating portions 6a and 6b and 6c and 6d. And the thickness of both the upper and lower arm portions h1 and h2 are adjusted together, for example, in the torsional direction of the strain generating body 2 itself, such as left and right offset load loads with respect to the measurement direction of the load cell. The inventors have found by actual measurement that although the non-linear performance is improved with respect to the load, the span error due to the eccentric load is not improved.

  Further, in the load cell described in Japanese Patent No. 2666209, the upper and lower arm portions h1 and h2 and each of the elastic members are canceled so that the strain generated in the strain-generating portion and the arm portion due to bending of the strain-generating body 2 as a whole is canceled. The strain portions 6a, 6b, 6c and 6d are set to have the same thickness, and the upper and lower arm portions h1 and h2 are both thinner than the total thickness of the strain generating body 2, and the deflection of the entire strain detection portion is large. turn into. As a result, the natural frequency of the load cell is reduced, and it is easy to pick up vibration in the measurement environment, and the output signal may be disturbed. Moreover, since it takes time to stabilize the load, there is a possibility that the measurement time may be extended. Furthermore, if it is going to keep the thickness of an arm part, the strain body itself must be enlarged and it is not suitable for thickness reduction and size reduction.

  Therefore, the present invention solves the above-described problems, and in a loadable load cell in which a strain gauge is attached to a strain-generating portion formed on one arm portion, the strain-generating portion has the same thickness, and the influence of bending of the strain-generating body is reduced. Provided is a load cell configured to have a thickness difference between the two arm portions so as to reduce the thickness.

  In order to solve the above problems, the present invention includes two parallel arm portions formed between a fixed end and a movable end, and four strain generating portions connected to the two arm portions two by two. In the load cell that constitutes the Roverval mechanism, the four strain-generating portions are formed with the same thickness, and the two arm portions are each provided with a strain gauge attached to the two connected strain-generating portions. Provided is a load cell comprising a first arm portion and a second arm portion formed thinner than the first arm portion.

  Further, the load cell is fixed with the strain gauge attaching surface of the first arm portion facing upward in the vertical direction, and the vertically downward direction from above the first arm portion is configured as the load loading direction.

  Further, the second arm portion is based on a load cell having the same thickness of the two arm portions, and the neutral axis of the gap is changed to the neutral axis of the load cell without changing the width of the gap between the two arm portions. It is formed by shifting by a predetermined distance.

  Further, the second arm portion is formed by processing only the thickness of the second arm portion based on a load cell having the same thickness of the two arm portions.

  The load cell according to the present invention is a load cell that forms a Roverval mechanism from two parallel arm portions formed between a fixed end and a movable end, and four strain-generating portions connected to the two arm portions two by two. The four strain-generating portions are formed by forming all the same thicknesses, and the two arm portions include a first arm portion having a strain gauge attached to each of the two connected strain-generating portions. Since the second arm portion is formed thinner than the first arm portion, the two arm portions have a compressive force and a tensile force due to the deflection applied to the entire cantilever during the eccentric load. At least one of force or twist acts and the Roverval mechanism breaks down, but the strain is formed in the first arm portion by receiving more force due to the bending in the second arm portion than in the first arm portion. Balance the strain acting on the , It is possible to maintain the linearity and span performance. In addition, compared with the case where the two arm parts are both thinned based on the thickness of the strain-generating part, there is no extreme change in natural frequency, making it more susceptible to vibrations in the measurement environment and up to load stabilization. The problem of long time is not likely to occur. Furthermore, since it can respond to a small strain body and the strain gauge only needs to be attached to two locations on the first arm side, it can be reduced in size and thickness, and the manufacturing cost can be reduced.

  In addition, the load cell is fixed with the strain gauge sticking surface of the first arm portion facing upward in the vertical direction, and the vertical downward direction from above the first arm portion is configured as the load loading direction. Without any problem, it can be used just like a conventional load cell.

  Further, the second arm portion is based on a load cell having the same thickness of the two arm portions, and the neutral axis of the gap is changed to the neutral axis of the load cell without changing the width of the gap between the two arm portions. It is formed by shifting by a predetermined distance. Thereby, for example, it is possible to deal with a small or thin load cell such as a case where the load cell is originally a small load cell and the thickness of the second arm portion is not sufficient.

  Furthermore, since the second arm portion is formed by processing only the thickness of the second arm portion based on the load cell having the same thickness of the two arm portions, it can be applied to an existing load cell. The thickness of the second arm portion can be formed to a predetermined thickness by simple processing such as cutting.

1 is an external view of a load cell of Example 1. FIG. FIG. 3 is an enlarged view of a main deformation portion of the load cell according to the first embodiment. It is a figure which shows the strain gauge sticking surface of the load cell of Example 1. FIG. It is an enlarged view of a main deformation part of a conventional load cell in which the thickness of the strain generating part and the arm part is symmetric. It is a figure which shows the eccentric load position with respect to a load cell. It is AA sectional drawing of FIG. It is a graph which shows the nonlinear performance of the conventional load cell of FIG. 3 is a graph showing nonlinear performance of the load cell of Example 1. FIG. 6 is an enlarged view of a main deformation portion of a conventional load cell in which a strain generating portion and a thickness of an arm portion are deformed. 10 is a graph showing a deviation error of the load cell of FIG. 9. 4 is a graph showing an offset error of the load cell of Example 1. It is a figure which shows the principal part of the strain gauge affixing surface of a load cell. It is a figure which shows the stress distribution analysis result of the strain generation part at the time of the unbalanced load of the load cell of FIG. It is a figure which shows the stress distribution analysis result of the strain generation part at the time of the unbalanced load of the load cell of Example 1. FIG. It is a side view of the conventional load cell with which the thickness of a strain generation part and an arm part is symmetrical. 6 is a side view of a load cell according to Embodiment 2. FIG. It is a graph which shows the nonlinear performance of the conventional load cell of FIG. 6 is a graph showing the non-linear performance of the load cell of Example 2. FIG. 6 is a side view of a general conventional robust mechanism load cell.

  The load cell according to the present invention is a load cell that forms a Roverval mechanism from two parallel arm portions formed between a fixed end and a movable end, and four strain-generating portions connected to the two arm portions two by two. The four strain-generating portions are formed by forming all the same thicknesses, and the two arm portions include a first arm portion having a strain gauge attached to each of the two connected strain-generating portions. And a second arm part formed thinner than the first arm part.

  Further, the load cell is fixed with the strain gauge attaching surface of the first arm portion facing upward in the vertical direction, and the vertically downward direction from above the first arm portion is configured as the load loading direction.

  Further, the second arm portion is based on a load cell having the same thickness of the two arm portions, and the neutral axis of the gap is changed to the neutral axis of the load cell without changing the width of the gap between the two arm portions. It is formed by shifting by a predetermined distance.

  Furthermore, the second arm portion is formed by processing only the thickness of the second arm portion based on a load cell having the same thickness of the two arm portions.

<Example 1>
The inventors of the present invention, in a robust load cell in which a strain gauge is attached to a strain generating portion formed on one arm portion, the deflection of the entire cantilever strain generating body affects the upper and lower arm portions and the strain generating portion. As a result, the thickness of the lower arm and the upper arm were determined from the measurement results of the load cell formed by changing the thickness of the upper and lower arms. It has been found that by providing an appropriate thickness difference with the thickness of the portion, strain linearity is produced in the strain-generating portion on the arm portion side that is formed thicker.

  Specifically, by compressing and pulling force generated in the upper and lower arm portions due to the bending of the entire strain generating body, the thickness of the lower arm portion is made thinner than that of the upper arm portion, so that the entire strain generating body is The lower arm portion can be easily affected by the bending of the upper arm portion, and an appropriate thickness difference can be provided between the lower arm portion and the upper arm portion that can balance the strain of the strain-generating portion applied to the upper arm portion. is there.

  Hereinafter, specific examples of the load cell of the present invention will be described based on the comparison data of the load cell of the present invention and the conventional load cell with reference to the drawings.

  Example 1 of the present invention is a comparative example in which a load cell is formed by shifting a gap forming an arm portion. The thin load cell of FIG. 1 thru | or FIG. 3 is shown as an example. FIG. 1 is an external view, FIG. 2 is an enlarged view of a main deformation portion of the load cell, and FIG. 3 is a view showing an upper surface of FIG.

  According to FIG. 1, the longitudinal direction of the strain generating body is a thin load cell having a fixed end and a movable end of 80 mm, a thickness of 5 mm, and a distance between the strain generating portions of 10.3 mm. .6 mm.

  According to FIG. 2, each of the strain-generating portions is formed at 0.55 mm by four holes of φ3.9 mm, and the two arm portions are formed by three parallel holes of φ2.6 mm so that each thickness is 1. It is formed as 85 mm and 0.55 mm, and the thicker arm portion is configured as a first arm portion, and the thinner arm portion is configured as a second arm portion. In addition, as shown in FIG. 3, two strain gauges are respectively attached in parallel to the surface of the two strain-generating portions connected to the first arm portion as close to the center of the load cell as possible. Although not shown, in terms of an electric circuit, a known technique for forming a bridge circuit with four strain gauges is used to detect the output of the strain gauge.

  The three parallel holes forming the first and second arm portions are shifted downward from the center of the conventional load cell shown in FIG. 4 so as to form the difference in thickness. The hole is centered at a position of 85 mm.

  Here, the load cell shown in FIG. 4 has the vertically symmetrical conventional structure shown in the above-described prior art, in which the arm portions h1 and h2 have the same thickness and the strain generating portions 6a, 6b, 6c and 6d have the same thickness. A type load cell. That is, the center of the three parallel holes forming the arm part coincides with the center of the load cell, the two arm parts are formed with the same thickness of 1.2 mm, and the others are described above. It is the same structure as the load cell of this invention.

  Comparison of non-linear performance due to the eccentric load of these two load cells will be described with reference to FIGS. 5 and 6 are diagrams showing measurement states such as the load position of the eccentric load, FIG. 7 is a graph showing the non-linear performance of the conventional load cell, and FIG. 8 is the non-linear performance of the load cell of the present invention. It is a graph which shows.

  First, FIG. 5 is a schematic view showing the load position with respect to the load cell from above. In this embodiment, the load cell is fixed with the strain gauge application surface of the first arm portion facing upward in the vertical direction, and the upper portion of the first arm portion is fixed. The load downward direction is defined as the load load direction, and the load position on the load transmission plate with respect to the load cell is shown. The circled X mark portion is the load position. According to this, the load positions at the center of the load cell were set as P1, P3, and P4 at positions P1 and 45 mm apart from each other on the longitudinal extension of the load cell and in the orthogonal direction. Here, P2 is an offset load position on the movable end side of the load cell, and P3 is an offset load position on the fixed end side. The measurement state is shown in FIG. 6, which is a cross-sectional view taken along the line AA of FIG.

  In the actual measurement of the non-linear performance, the step load in increments of 500 (g) was measured at the offset load positions of P1, P2 and P3 with respect to both load cells as the weighing of 2000 (g).

  The non-linear performance of the conventional load cell shown in FIG. 7 is as high as 0.1% RO for the eccentric loads P2 and P3, whereas the non-linear performance of the center load P1 is as high as 0.02% or less. It showed linearity. This indicates that the load cell that is the target of the arm portion and the strain-generating portion has an insufficient mechanism as a Roverval.

  Compared to this, the non-linear performance of the load cell of the present invention shown in FIG. 8 is within 0.02% RO for all of the load positions P1, P2 and P3. It is clear that the two arm portions balance the strain applied to the first arm portion.

  In addition, the load at the load positions P4 and P5 is twisted in the load cell and is not eliminated by the Roverval mechanism. In the conventional load cell, the non-linear performance deteriorates and the span of the eccentric load value with respect to the central load value P1. Since it is clear that the error is large, in the following performance comparison of the deviation error, the arm portions h1 and h2, the vertical strain portions 6a and 6b, 6c and 6d shown in one of the prior arts are shown. As an example of a load cell having a difference in thickness on both sides, a comparison with the load cell shown in FIG. 9 was performed. 9 is formed by shifting the space portion for forming the arm portion and the strain generating portion from the conventional load cell shown in FIG. 4 by 0.1 mm downward from the center of the load cell. Non-linear performance equivalent to the load cell of the invention is realized.

  10 and 11 show the error rate of each load value at the load positions P2, P3, P4 and P5 with respect to the offset error, that is, the load value at the load position P1 at the center of the load cell. 10 shows the deviation error of the load cell in which the arm portion and the strain generating portion shown in FIG. 9 have different thicknesses, and FIG. 11 shows the deviation error of the load cell of the present invention.

  First, according to FIG. 10, the load positions P2 and P3 with respect to the load position P1, that is, the loads before and after the load cell in the longitudinal direction, are within ± 0.2% RO at the weighing 2000 (g). This is a good result. However, at the load positions P4 and P5 where the load cell receives the force in the torsional direction, errors of about + 0.8% RO and −0.8% RO are shown, respectively.

  On the other hand, in the measurement result of the load cell of the present invention shown in FIG. 11, not only the load positions P2 and P3 but also the load positions P4 and P5 where the error is large in the load cell described above is within ± 0.2% RO. And shows good results.

  Thereby, it can be said that the load cell of this invention has eliminated the twist concerning a load cell. The results of analyzing the degree of torsion applied to the load cell when the load position is P5 as stress for both load cells will be described with reference to FIGS. 12 is an enlarged view of the strain gauge affixing surfaces of both load cells, and FIG. 13 is a strain gage affixing surface of the movable side and the fixed side strain generating portion when the load position is P5 in the load cell shown in FIG. FIG. 14 is a diagram showing the analysis result similar to FIG. 13 for the load cell of the present invention.

  In the stress analysis surface of FIG. 12, the stress analysis is performed by dividing the left end of the load cell to 0 mm and the right end of 12.6 mm in 1 mm increments for each of the movable side strained portion and the fixed side strained portion. FIGS. 13 and 14 show approximate curves based on the stress calculation values at each point when a weight of 2000 (g) is applied to the position P5.

What should be noted here is the difference between the maximum value and the minimum value of the stress in each strained portion to which the strain gauge is attached. It can be said that the greater the difference, the stronger the twisting force is applied to the load cell. In the fixed side strain portion of the stress analysis result shown in FIG. 13, the minimum value of the stress is about 3.5 kgf / mm ² around 1.5 mm from the left end of the load cell, and the maximum value is around 11.5 mm from the left end. It is about 11.5 kgf / mm ² and the stress difference is about 3 times. Similarly, also in the movable side strain generating portion, the maximum value is about −3.5 kgf / mm ² near 1.5 mm from the left end of the load cell, and the minimum value is about −10 kgf / mm ² near 12 mm from the left end. The stress difference is about 2.8 times.

On the other hand, according to the stress analysis result of the load cell of the present invention shown in FIG. 14, first, in the fixed side strain portion, the minimum value is about 5 kgf / mm ² near 1.5 mm from the left end of the load cell, and the maximum value is It is about 10 kgf / mm ² near the right end of 12.6 mm, and the stress difference is about twice. Similarly, in the movable side strain generating portion, the maximum value is about 5.5 kgf / mm ² around 1.5 mm from the left end of the load cell, and the minimum value is about 9.5 kgf / mm ² around 10 mm from the left end. The stress difference is about 1.7 times.

  That is, in the conventional load cell in which the thickness of the strain generating portion is adjusted, the stress difference is about 3 times, but in the load cell of the present invention, it is reduced to about 2 times. On the other hand, it is clear that the influence of twisting of the strain generating body is reduced.

<Example 2>
The second embodiment of the present invention is an example in which the load cell can be highly accurate only by adjusting the thickness of only the second arm portion. In the first embodiment, based on a load cell in which a plurality of circular holes are connected in a strain generating body and a space forming a strain generating portion and an arm portion is formed, the width of the space forming the arm portion is constant. The thickness of the upper and lower arm portions is changed by shifting the axis of the gap, but in the second embodiment, only the first arm portion and An example of the load cell which provided the difference in thickness with the 2nd arm part is shown. This is a useful configuration in a relatively large load cell in which both arm portions have a sufficient wall thickness.

  FIG. 15 shows a load cell shape in which the thicknesses of the conventional arm portions h1 and h2 and the strain generating portions 6a, 6b, 6c and 6d are symmetrical, with a spacing of 50 mm at the center of the strain generating body having a length of 130 mm and a height of 22 mm. The load cell is configured by forming a movable-side strain-generating portion and a fixed-side strain-generating portion with a hole of φ12 mm and providing a gap portion connected to the hole so that the arm portion is 4.8 mm. FIG. 16 is based on the load cell shown in FIG. 15, and the thickness of the second arm portion is thinned by 1.8 mm to stabilize the linearity, and the space between the first arm portion and the second arm portion is reduced. The width of the gap is formed wider than that of the conventional load cell. Further, the strain gauge is formed by forming a bridge circuit in the same manner as in the first embodiment.

  In the non-linearity comparison of the load cells shown in FIGS. 15 and 16, the measurement was performed in the same manner as in Example 1, and the results shown in FIGS. 17 and 18 were obtained, respectively. According to this, as shown in FIG. 17, in the conventional load cell, it is about 0.01% RO at the load position P1, whereas at the load positions P2 and P3, 0.02% RO and 0. The non-linearity is large with 04% RO.

  Compared to this, as shown in FIG. 18, the load cell of the present invention has a good result that substantially agrees with about 0.01% RO in all of the load positions P1, P2, and P3, and the thickness of only the second arm portion. It can be said that even if only the thickness is changed, it is necessary to provide an appropriate thickness difference that can balance the strain applied to the strain-generating portion to which the strain gauge is attached.

  In Example 1 and Example 2, the load cell is fixed with the strain gauge affixing surface of the first arm part facing upward in the vertical direction, and the downward load from above the first arm part is set as the load loading direction. It is also possible to reverse the direction and fix the load cell so that the strain gauge attaching surface of the first arm part faces downward in the vertical direction and to apply a load from above the second arm part formed on the upper side.

  In addition, the strain gauges were affixed to two strain gauges on the surface of the two strain-generating portions connected to the first arm portion, respectively, and a bridge circuit was configured with a total of four strain gauges. The same characteristics can be obtained even if one strain gauge is attached to each strain generating part and a bridge circuit is constructed by combining these two strain gauges and two dummy gauges in terms of the electrical circuit configuration. Is possible.

DESCRIPTION OF SYMBOLS 1 Conventional general load cell 2 Strain body 3a, 3b, 3c, 3d Strain gauge 4 Fixed end 5 Movable end 6a, 6b, 6c, 6d Strain generation part 7 Hole 8 Hole 9 Cavity part h1, h2 Arm part

Claims (2)

In a load cell that forms a Roverval mechanism from two parallel arm portions formed between a fixed end and a movable end, and four strain generating portions connected to the two arm portions two by two,
The four strained portions are formed by forming all the thicknesses to be the same,
The two arm portions are more susceptible to the influence of the first arm portion in which a strain gauge is attached to each of the two connected strain-generating portions, and the deflection applied to the entire strain-generating body than the first arm portion. A load cell comprising a second arm portion that is formed to have a thin thickness as described above and balances strain acting on the strain-generating portion to which the strain gauge is attached. 2. The load cell according to claim 1, wherein the load cell is fixed with the strain gauge affixing surface of the first arm portion facing upward in the vertical direction, and the vertically downward direction from above the first arm portion is defined as the load loading direction.

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