Column weighing sensor
Advantages: compact structure, strong overload capacity, high natural frequency, fast dynamic response, convenient installation, low manufacturing cost.
Disadvantages: poor lateral and eccentric load resistance (double diaphragm structure is better),
poor inherent linearity, weighing sensor is not easy to fix and easy to rotate. Column weighing sensor is currently used more on truck scales due to the above advantages, but its disadvantages cannot be avoided.
The length change of the load cell exceeds the allowable displacement given by the unit of the weighing sensor device, thus affecting the metering performance of the product. There is also a situation that the weighing sensor will continue to rotate slightly during use, and even break the cable. Why does the column weighing sensor installed on the truck scale rotate? It has been used on the automatic track scale for a long time, but the rotation phenomenon rarely occurs at the installation site? This is because the load cell of the automatic track scale is generally only 3.7m~4m long, and its limit adopts a tensioning structure, which makes the load cell basically in a non-moving state in the horizontal state, while ensuring that it does not affect the effect of the vertical force. The load-bearing device of the truck scale is relatively long, and the limit device used allows a certain displacement of the load-bearing device in the horizontal state, so that the weighing sensor may also shake. That is, in this continuous shaking, the cable is constantly wrapped around the weighing sensor until it is broken.
The problem of eccentric load component, why is the step difference (that is: eccentric load error) of the truck scale using the column weighing sensor relatively large? This is certainly related to the influence of the "boundary conditions" brought about by the processing of the load-bearing device, but from a theoretical point of view, due to the influence of thermal expansion and contraction, the column weighing sensors at both ends of the load-bearing device have a large inclination, which brings about eccentric load component. The larger the weighing value, the greater the eccentric load error. At the same time, this component force is also related to the height of the weighing sensor. The relative influence of the weighing sensor with a large height is smaller, and the relative influence of the sensor with a low height is larger.
2. Bridge weighing sensor
Advantages: insensitive to changes in the loading point, good anti-eccentric load performance, good inherent linearity, easy installation, the weighing sensor is fixed and does not rotate, and the manufacturing cost is low.
Disadvantages: high center of gravity, poor overload capacity, and large-scale weighing sensors are difficult to achieve high accuracy levels. For long load-bearing scales using bridge-type weighing sensors, the spherical pressure head changes with the length of the load-bearing device during use, and the pressure head also rolls in the ball socket of the elastic body along the length direction, causing the force loading direction to shift, thereby generating eccentric loads. Although this type of weighing sensor has good anti-eccentric load performance, it will also affect certain metering performance when the displacement is large. For this reason, in 1991, the author applied for a utility model patent for the relatively long load-bearing device of the digital indicating track scale product, considering the influence of thermal expansion and contraction of multiple weighing sensors, and designed the pressure head on the weighing sensor into two forms. One is to retain the spherical shape of the original design, which is used for the four middle weighing sensors to play a positioning role; the other is designed to be a plane shape according to the highest point of the sphere, which is used for the weighing sensors at the end. No matter how the length of the load-bearing device changes during thermal expansion and contraction, it can ensure that the force on the load-bearing device always acts vertically on the weighing sensor.
3. Double shear beam ring weighing sensor
Advantages: good anti-eccentric load performance, weighing sensor is fixed and does not rotate.
Disadvantages: complex structure, troublesome installation, high manufacturing cost, poor overload capacity, large space occupation, and this type of product is not suitable for manufacturing large weighing structures. In use, this type of weighing sensor is the same as the previous mechanical lever scale ring swing structure. The point of action always swings around the weighing sensor, and the force is always applied vertically to the weighing sensor. Even if the size change of the load-bearing device due to thermal expansion and contraction is large, it will not affect the measurement performance. The key point of this structure is that the force is always on the weighing sensor surface, thereby ensuring the stability of the system. Therefore, this type of weighing sensor is widely used in large scales in the United States.

4. Axisymmetric torsion ring weighing sensor (as shown in Figure 6)
Advantages: compact structure, small size, low height, light weight, good inherent linearity, high accuracy, no deformation or very small deformation of the bottom ring after loading, almost no hysteresis, insensitive to eccentric loads and lateral loads, large input and output impedance, and the weighing sensor is fixed and does not rotate.
Disadvantages: installation is slightly complicated, and the manufacturing cost of accessories is relatively high. It is precisely because of the large impedance of this type of product that the output signal is large and the power consumption is relatively low that many European countries are accustomed to using this type of weighing sensor for large weighing instruments such as truck scales. The author has seen that many companies use only four such weighing sensors to support an 18m long, 42t reinforced concrete load cell on a concrete structure truck scale. If used in an environment with impact loads, the upper pressure head can be a specially made structure with a buffer layer; if used for a load cell with large thermal expansion and contraction, the height of the pressure head can be designed to be higher. 5. Advantages of spoke-type weighing sensors: strong anti-eccentric load and lateral load capacity, strong overload capacity, not affected by loading points and support boundaries, uniform stress release, good linearity, and the weighing sensor is fixed and does not rotate. Disadvantages: difficult machining, high manufacturing cost, large inherent hysteresis and difficult to control.
This type of weighing sensor was widely used in large-scale weighing instruments such as automobile scales in my country in the early and mid-1980s. However, due to its own mechanical processing problems and hysteresis errors, it was later replaced by cantilever beam weighing sensors and bridge weighing sensors.
III. Conclusion
From the above five types of weighing sensor structures, it can be clearly seen that only the elastic body of the column weighing sensor moves with the shaking of the load-bearing device during use, that is, the vertical column of the weighing sensor during installation, due to the thermal expansion and contraction of the length of the load-bearing device, the column of the weighing sensor is offset, which also causes the force to produce a component force, so that the weighing sensor has a weighing error. As for the other four types of weighing sensors, since the elastic body is fixed, when the thermal expansion and contraction affect the change of the length of the load-bearing device and the shaking of the load-bearing device, only the pressure head is offset. The elastic body that transmits the force will not move with the change of the load-bearing device, so that the force on the weighing sensor can still be transmitted vertically, and the impact on the weighing performance of the automobile scale is relatively small; at the same time, since the elastic body does not shake with the load-bearing device, the elastic body will not rotate, and the cable will not be entangled with the elastic body.