Application of stress distribution improvement in the production of high-performance bridge sensors

Introduction

Among the various structural sensors produced by China's weighing sensor manufacturing industry, the steel bridge sensor, with its unique structure of supporting both ends and bearing force in the middle, has enabled large-tonnage sensors to enter the field of high-accuracy and high-reliability measurement. At the same time, the force transmission component adopts a spherical pressure head, which gives full play to the advantages of the automatic reset and centering of the steel ball, good resistance to lateral force and impact, easy installation, good interchangeability and other advantages in China's automobile weighing field. However, the range of the sensor is mostly limited to more than 10 tons, and there are very few ranges below 10 tons. In order to meet user needs, we have developed a small-range 3-ton sensor.

1. Design of the bridge 3-ton sensor

In order to maintain the simplicity and versatility of installation and use, the 3t sensor adopts the installation size of the 10t sensor, and requires that the rest of the sensor components except the elastic body are universal, which requires that we can only change the size of the strain zone during the design of the sensor and cannot affect the installation size of the sensor.

1.1 Design flaws in the small-range bridge sensor structure

The structure of the bridge sensor is that both ends are fixed, and the shear force is applied to the I-shaped section (Figure 1):

Figure 1 Figure 2

Therefore, the shear force on the patch is ½ F.

The shear stress is obtained from the stress analysis diagram of the elastic body section at the strain position (Figure 2):

According to formula (1), τ is maximum when y = 0:

Where: Poisson's ratio: ν; elastic modulus of the material: E.

According to formulas (2) and (4), the quadratic equation of the sensor strain beam t is obtained:

It is known that when the sensitivity is 2mV/V

, ε=1200με, ν=0.3, E=2.1×10 ⁴ kg/mm2;

Substitute the dimensions of the elastic body of the 10t sensor (c × d × h=56mm × 52mm × 35mm) into the formula, and we get:

t1=1.78mm, t2=-25.81mm (discarded).

At the same time, ½(dh)=½(52-35)=8.5mm 》1.78mm. The dimensions of the upper and lower beams in the I-beam in Figure 2 are much larger than the dimensions of the vertical beam (strain beam). In the process of strain beam loading by the sensor, the strength of the upper and lower beams is much higher than that of the strain beam, and the shear stress generated by the loading does not play its due role, so that the strain beam fails to produce similar deformation, and sufficient micro-strain cannot occur, making the output sensitivity of the sensor far lower than the design requirement of 2mV/V.

1.2 Improvement plan

Scheme 1:

In order to make the shear stress play a sufficient role, adjust A, d and h to make the size of the upper and lower beams in Figure 2 as close as possible to the size of the strain beam. According to the design requirements, the dimensions c and A cannot be changed. It is difficult to meet our requirements by changing d. Reducing h can increase t. Due to the relationship between c, too small h will add a lot of difficulties to the production of the sensor. Therefore, the solution of reducing the local size of the strain area of ​​the elastic body, changing the size of d and h, but keeping the installation size unchanged is adopted (see Figure 3).

Figure 3

According to the elastic body size scheme in Figure 3, we put five BM-LS-3 sensors into the test to verify the sensitivity index of the sensor. The primary test index of the sensor is shown in Table 1:

From the test results of the 5 sensors in the table, the average sensitivity is 1.2718mV/V, which is consistent, but far from the sensitivity of 2mV/V required by the design calculation; and the linear and hysteresis indicators have good consistency.

Scheme 2:

On the basis of Scheme 1, referring to the force diagram 2 of the strain part of the sensor, the shear force on the elastic body does not produce enough micro-strain on the strain beam, which is the main reason for the low sensitivity. Therefore, we can improve the ability to generate micro-strain by reducing the size of the strain beam t. The stress distribution is the same as that of Scheme 1. Through our experiments, the effect of this scheme is not obvious.

Scheme 3:

From the analysis of Figure (2), we know that where y is close to "0", the closer the generated shear stress τ is to its maximum value τmax, can we only need the local area where the shear stress peak is generated and remove the rest? Considering the required size of the strain gauge patch and the convenience of elastomer processing, a through hole 2-ΦB is drilled along the load direction of the sensor (Figure 4). After a simple stress distribution analysis (Figure 4, where the dotted line is the stress curve in Scheme 2), it is found that not only the maximum stress value τ2max has increased significantly compared to τ1max, but also it is more concentrated.

1.3 Test results

Three sensors were selected from the five sensors in Table 1 for the 2-ΦB through-hole drilling test. The sensitivity test results are shown in Table 2. The linearity and hysteresis indicators did not change much and were close to the values ​​in Table 1.

Comparing Table 1 and Table 2, the sensitivity of the same sensor varies greatly, all increasing by about 1.2mV/V. On this basis, we continue to adjust the thickness of the strain beam to make the sensitivity closer to the target value of 2mV/V. The sensitivity results of the small batch sensor test after adjustment are shown in Table 3.

The maximum sensitivity of the sensor is 2.251mV/V, and the minimum is 2.00243 mV/V, with a large dispersion. The reason for this phenomenon is that when drilling the through hole 2-ΦB in the sensor elastic body, manual marking and drilling are used, which affects the center position, symmetry and size consistency of the two holes. As long as we use high-precision drilling tools to ensure the processing accuracy of the two through holes in the future processing and manufacturing of the elastic body, the consistency of the sensor sensitivity will be well guaranteed.

2. Conclusion

The test results of the 3t sensor produced this time all meet the requirements of Class C3 in GB/T7551-1997 "Weighing Sensor". To summarize the design and production process this time, it is to make full use of the concentrated area of ​​the shear stress distribution on the elastic body. On the basis of theoretical design calculations, by adjusting the structural form of the elastic body strain beam, the shear stress of the elastic body is concentrated as much as possible in the patch area of ​​the strain gauge, which plays a great role in adjusting the sensitivity of the sensor. This will be of great help to our future design and production of sensors with smaller ranges.