Since the early 1940s, when BLH and REVERE companies in the United States invented strain gauge load cells, after more than 70 years of improvements and development, their measurement accuracy, working reliability, long-term stability and environmental adaptability have been significantly improved and enhanced, and their application scope has been continuously expanded. They have penetrated into various sectors of the national economy and have become the main means of weighing and measuring in industry, commerce, and households. Today, there are more than 160 strain gauge weighing sensor manufacturers in my country, forming a series of products with a range from a few kilograms to thousands of tons. The accuracy of most products has reached C3 level, and the accuracy of some individual products of some companies is as high as C4 and C5 levels. Such high-accuracy weighing sensors are used in environments with constantly changing temperatures, and they must have very precise sensitivity coefficient temperature compensation. In order to achieve a higher level of accuracy for weighing sensors, all enterprises usually set stricter internal control indicators when performing temperature compensation for sensitivity coefficients. For weighing sensors with higher accuracy levels, the internal control indicator for the temperature error of the sensitivity coefficient is ±0.01%/10°C, which requires a scientific, reasonable and repeatable temperature compensation process for sensitivity coefficients.
2. Temperature error of the sensitivity coefficient of weighing sensors
As early as the advent of strain load sensors, people noticed the influence of temperature on the indication value of mechanical standard force rings made of alloy steel. After repeated experiments and analysis, scholars from the former Soviet Union pointed out that the temperature error of the indication value of the standard force ring is mainly caused by the decrease of the elastic modulus of the metal material of the force ring with the increase of temperature, and measured the magnitude of the influence, giving a more accurate correction coefficient of 0.027%/°C. In the paper "Temperature Coefficient of Standard Force Ring" published in 1946 by American scholar Wilson, the temperature effect correction coefficient of elastic modulus of the same magnitude was given. It is natural to think that strain gauge weighing sensors made of alloy steel will also inevitably produce this kind of temperature error, and its influencing factors are more numerous and more complex than those of standard force rings. In addition to the negative temperature coefficient of the elastic modulus of the metal material of the elastic element of the weighing sensor, there is also the temperature coefficient of the sensitivity coefficient of the resistance strain gauge. It is not difficult to conclude that the influence of temperature on the elastic element mainly produces two physical phenomena: one is that the elastic element produces thermal expansion when the temperature rises, which is expressed by the thermal expansion coefficient of the metal material α L, which causes the weighing sensor to produce zero temperature drift; the other is that the elastic modulus E of the elastic element material decreases when the temperature rises, which is expressed by the temperature coefficient of the elastic modulus E β E, which makes the output of the weighing sensor increase with the increase of temperature, resulting in a temperature error of the sensitivity coefficient. Experiments have shown that within the range of 0 to 50°C, the elastic modulus of high-quality chromium-nickel steel changes by -0.025%/°C, and its influence, that is, the temperature error of the sensitivity coefficient of the weighing sensor, can reach (0.03 to 0.05)%/°C. During the use of weighing sensors, the sensitivity coefficient changes by 0.3% to 0.5% for every 10℃ change in temperature. This is a very considerable error, so temperature compensation of the sensitivity coefficient is necessary.
At present, the elastic elements of weighing sensors that are widely used can be roughly divided into column type (cylinder, square column, rhombus), cylindrical type, curved beam type, parallel beam type, central beam type and cantilever beam type, double-end fixed beam type, spoke type, etc. for normal stress, and there are many kinds of specific structures in each category, even dozens of kinds.
In recent years, foreign weighing sensor manufacturers have broken through some process bottlenecks and developed a variety of self-compensated resistance strain gauges and self-compensated resistance strain gauges for sensitivity coefficient and temperature, which have begun to be used in the temperature compensation of sensitivity coefficients of weighing sensors with higher accuracy levels, and have achieved good test results. The outstanding feature of temperature self-compensation of sensitivity coefficient is that there is no external sensitivity coefficient compensation resistor in the bridge circuit, and the long-term stability of the weighing sensor is good and the fatigue life is high.