Analysis on the Factors that Affect the Hysteresis of the Load Cell

Use the above graph as a model to introduce the definition of hysteresis: the horizontal axis is the load, and the vertical axis is the sensitivity output. When testing the characteristics of the sensor, the rated load is generally divided into five parts, and the load is gradually loaded from 0 to 100% and read the corresponding Connect the five reading points to form a smooth curve. This curve is the thick solid line in the figure, which is called the loading curve or the progress curve. Use the straight line interpolation method to calculate the corresponding data of the 75% point, draw a straight line with the zero point and the 75% point, the dotted line in the figure, this straight line is called the ideal straight line, and the error between the loading curve and the ideal straight line is called the nonlinear error. Decrease the load in five equal parts from 100% to 0, read the corresponding data, and connect the five groups of data into a smooth curve, as the thick dotted line in the figure, called the load reduction curve, also called the return curve, load reduction The error between the curve and the loading curve is called hysteresis error.

The hysteresis error reflects one of the main indicators of sensor accuracy. The size of the error directly affects the measurement accuracy, so various factors that affect the hysteresis must be strictly controlled in the production process of the sensor. At the same time, it must be ensured that the factors that affect the hysteresis are avoided as much as possible under the different conditions of installing the load cell.

The following details some factors that affect hysteresis.

Load Cell Block: Any kind of metal material, because of the complexity of its internal organizational structure, micro-strain occurs between the tiny grains after being pressurized by external force. After the external force disappears, the micro-strain disappears, but whether it disappears completely Restored to the original state, different materials have completely different performances. In Figure 1, we can see that the strain curve ε1 during the loading process does not coincide with the strain curve ε2 during the unloading process. The difference △=ε2-ε1 is called hysteresis. The magnitude of the difference mainly depends on the material itself. The stability and uniformity of the composition, the metallographic structure after heat treatment, etc. As the key component of the load cell, the requirement for elastomer is even more obvious. Different heat treatment methods can be used to increase the elastic limit to reduce hysteresis. At present, the commonly used material in the domestic market is 40CrNiMoA, which can achieve ideal comprehensive mechanical properties after reasonable heat treatment.

Strain gauge: The typical structure of a metal strain gauge is composed of a sensitive grid, a substrate, a coating layer and a lead. In the application of the sensor, through the resistance strain effect of the sensitive grid, the strain of the elastic body is transformed into the resistance change. According to the hysteresis of the material itself, the strain gauge itself also has the hysteresis. At present, the world-renowned strain gauge factory fully considers the self-compensation of hysteresis in the manufacture of strain gauges, so that its influence in the application of the sensor is minimized. Therefore, this factor should be taken into consideration when selecting a strain gauge.

Sealant: A large amount of sealant is used in the production process of the sensor, and the main function is to fix the circuit and seal. On the surface, it is generally soft after curing, and the relative strength of the elastomer can be almost ignored. But for small-range products, it must be considered. For small-scale products, the deformation zone is relatively weak, and the influence of the thickness of the sealant is significantly increased.