The quality of the load cell determines the accuracy and reliability of the scale. So the load cell is the heart of the weighing system. Weighing instrument
In particular, weighing instruments used for trade are all measuring instruments subject to compulsory calibration by the state. Therefore, no matter what principle the weighing sensor is used to manufacture, it must meet national standards or international standards. But so far, except for strain gauge load cells, which have standards and verification procedures formulated in accordance with the rules for non-automatic weighing instruments, there are no other types of weighing sensors that have similar standards and verification procedures. Fortunately, about 90% of the weighing instruments in use today use strain gauge sensors (except balances). This brings great convenience to our use of strain sensors to design and manufacture weighing instruments. This article mainly discusses issues related to the technical parameters of strain gauge sensors.
In 1938, Simmons and Ruge invented the strain gauge to measure strain, mainly used in military industry and engineering. By 1966, PTB published an article on the use of strain sensors in electromechanical weighing devices. At this time, the accuracy of the sensor had reached 0.1% and it had passed product certification. In 1976, PTB published an article on the first experimental verification of strain gauge sensors, which was based on the IR3 OIML rules for weighing machines and the European Directive 71/316/EEC. However, at this time, the test is based on the technical specifications of the strain gauge force sensor. There are differences and different requirements between it and the measurement requirements and technical indicators of weighing instruments.
In particular, with the rapid development of electronic weighing instruments, there are fundamental differences between the individual technical indicators of strain gauge force sensors and the maximum allowable error limits of segmented ladders for non-automatic weighing instruments. There is an urgent need to formulate standards and metrological verification procedures for load cells that are compatible with the measurement requirements and technical requirements of non-automatic weighing instruments. Since the early 1980s, the OIML organization has begun to compile international recommendations for load cells suitable for non-automatic weighing instruments and international recommendations for non-automatic weighing instruments suitable for electronic weighing instruments, namely international recommendations R60 and R76. The basic rules of the 1992 version of Recommendation R76 (including the 1994 revision) are the closest to the requirements for weighing on non-automatic scales. R60 published recommendations in 1991 and 1993 including calculation procedures and test reports. This recommendation is an international recommendation specifically formulated for sensors applicable to weighing instruments based on the metrological requirements and technical requirements of Recommendation R76. It was revised for the first time at the "Weighing Towards the Year 2000" meeting held in Paris in 1995, and the content of R60 was revised by B.MEIBNER of PTB in an article titled "Reflections on NAMIs Module 'Load Cell'" Explain, this article helps us understand
The R60 and relationship to the R76 is helpful. Subsequent revised versions have not changed the basic principles.
The trapezoidal error envelope limit of non-automatic scales is determined by the following test
Certainly:
Nonlinear + Hysteresis + Static Temperature Test
Note that this error does not include the effects of creep and zero temperature.
The error limit of the sensor is basically determined based on the test procedure of the weighing instrument. In principle, based on this correspondence, using C3 level sensors to design level three scales (scales for trade use) can fully meet the requirements. It brings great convenience to designers.
Due to technological progress and the application of microprocessors in weighing systems, the following four new aspects have been brought to sensors and scales:
develop:
Single point load cells for platform scales
digital sensor
Multi-index weighing device
Multi-range (range) weighing device.
So in the new R60 (2000 edition) a test for digital sensors and two optional parameters were added: relative DR or Z and relative Vmin or Y. Although these two parameters are not mandatory, they are very important for us to design the weighing instrument. Therefore, the sensor is required to provide a supplementary certification calibration certificate for the following parameters:
The maximum weighing capacity of the sensor: Emax (kg)
Maximum number of calibration divisions: nLC Max
Minimum calibration graduation value: Vmin (kg)
The CMDLOR value obtained according to OIML R60 is in V, and the DR value needs to be calculated according to the following formula. DR=（CMDLOR×Emax）/nMax
Sensitivity: mv/v
Input resistance: Ω
Supply voltage: V
Protection level
Wire length
These parameters are the basic parameters for designing a weighing instrument. First, select the maximum weighing capacity of the sensor according to the measuring range of the weighing instrument.
Sensor calibration graduation number: nLC=Emax/V
Calibration scale of the weighing instrument: n=Max/e
Sensor rated range: Emax, Emin
Weighing range of the scale: Max, Min=20e
Sensor calibration graduation value: V, Vmin
Weighing instrument calibration graduation value: e, emin
The scales commonly used are required to meet trade n requirements, that is, the requirements for Class III scales. Class C3 sensors should be used for this purpose. For C3 level sensors, when the graduation n=3000, the maximum allowable error is PLC times the maximum allowable error of the weighing instrument, where mpe is the maximum allowable error of the weighing instrument, PLC is the component coefficient, usually PLC=0.7. It must be remembered that the 3000 divisions for a Class C3 sensor are not the number of divisions at which its maximum error limit cannot be exceeded. Its maximum graduation number nMax ≥3000, determined by DR or Z value, Z=Emax/(2DR) and Z≥MaxMax/emin for C3 level sensor. The nMax value is generally 3000 to 10000. It determines the measurement range in which the sensor's measurement results do not exceed the maximum allowable error (mpe), and the maximum number of divisions nMax that can be divided into. It also determines the minimum graduation value emin for scales made with it. The minimum calibration graduation Vmin or Y of the sensor is also an important parameter in designing the weighing instrument. Vmin is the drift value of the sensor zero point for every 5°C change in temperature, Y=Emax/Vmin. It determines the maximum number of divisions within the range of use of the sensor that does not exceed the maximum allowable error. Typically Y values are two to three times larger than Z values. It should be noted that Y has nothing to do with nLC
From the above theoretical analysis, it can be concluded that using a C3 level sensor, a level III scale with n=4000 divisions can be produced. However, general sensor manufacturers do not have the conditions to measure the DR value of large-tonnage sensors. Brings uncertainty to designers. It is worth noting that for outdoor scales such as truck scales, due to the influence of environmental conditions such as wind, even a scale with n=3000 divisions has certain problems in ensuring measurement accuracy, so n=4000 divisions are used outdoors. It is more difficult to ensure the measurement accuracy of weighing instruments. The experts who formulate the calibration procedures and standards for weighing instruments in China do not know much about weighing instruments, and conversely, the experts who formulate the verification procedures and standards for weighing instruments also do not know much about sensors. This is especially reflected in the fact that when formulating standards for weighing instruments, the specific measurement requirements and requirements for using sensors must be met. Technical requirements are not clearly defined. This reflects the lack of basic knowledge and theory of weighing instruments among Chinese weighing instrument workers. This is an important aspect that China needs to work hard to become a powerful weighing instrument country. So far, there is no load cell that can shake the position of strain sensors in the field of weighing. However, how strain sensors should be developed is always a research issue. Many manufacturers have also tried to sputter "strain gauges" directly onto elastomers. But so far no official batch of products has been seen. We have also done research in this area and found that not only the elastomer needs to be specially designed, but the physical properties of the sputtered "strain gauge" and the original foil also change. The original foil was an alloy body, but now it is "layered" due to sputtering. Overall, the nature of strain sensors has not changed much in recent decades. Its improvements are mainly reflected in the process and the design of elastomers that can adapt to various measurement needs. Standards and test methods for measurement requirements and technical requirements for weighing instruments have been formulated. In order to simplify the design of the weighing instrument. It is worth mentioning that the emergence of component sensors can simplify and facilitate the design, installation and debugging of weighing instruments. This is also an obvious progress in sensors, but the progress of sensors is always the prerequisite for the development of weighing instruments. Therefore, it is said that "the load sensor is the heart of the weighing system."