At present, China’s electronic weighing instruments are still based on the principle of resistance strain measurement, and a large number of resistance strain type load cells are used. The load cell is the forefront of information technology. If there is no load cell to reliably collect and convert the original information, reliable measurement is difficult to achieve. Based on my many years of experience in the inspection of load cells and related phenomena in the user’s use of the site, I will talk about the use of resistance strain type load cells in electronic weighing instruments in the field.
- Use characteristics of resistance strain gauge load cell
Since the sensor is a product that combines multiple disciplines such as mechanical mechanics, electricity, chemistry, etc., various technical processes will affect the use characteristics of the sensor. The following points are mainly the influence of the field use conditions on the sensor characteristics.
(1) Zero drift of resistance strain type load cell
For sensors of different accuracy levels, the technical indicators of the zero temperature coefficient are correspondingly different. The ambient temperature has different effects on sensors of different grades. The environmental temperature condition of the load cell at the use site is very different from the ambient temperature conditions during the compensation and verification operations. The “temperature drift” of the load cell at the use site is mostly caused by the humidity gradient. Therefore, when users find that the load cell has excessive temperature drift during use, they should first test whether the temperature of the use environment is relatively constant, and measures should be taken to create a local relatively constant temperature environment around it.
(2) Overload, eccentric load, lateral force impact and zero point change caused by collision during use, if the force application plane is not parallel to the supporting base surface, the eccentric load situation will be bad. Eccentric load or overload will cause greater plastic deformation in a certain local section of the elastomer, and the zero point will suddenly become larger. If the local stress caused by the eccentric load exceeds the strength limit, the load cell will be damaged.
The influence of lateral force has been considered in the design and installation of electronic weighing instruments. Various load-bearing mechanisms and limit devices are designed for the lateral force in use. The lateral force causes a sudden change in the zero point, which is often an accidental pressure switch.
(3) Zero point change caused by electromagnetic field interference on site
If the space electromagnetic field on the use site suddenly impacts or changes, causing a considerable amount of induced potential on the bridge network or signal transmission line, it will cause the sensor output to fluctuate. In view of the above use characteristics, the user should take measures in terms of electricity, ambient temperature, mechanical structure and mechanical operation to reduce or eliminate the influence of the load cell.
- Fault diagnosis and analysis in use
The fault phenomenon of electronic weighing instrument is always manifested in the terminal display instrument of the system. After the fault phenomenon manifests itself, do not blindly dismantle the load cell, and use corresponding methods to find the fault point. Whether it is a complex electronic weighing instrument and force measurement system used in engineering, or a test device used in a laboratory, they are composed of one or more sensors, mechanical devices, system intermediate junction boxes, transmission cables and detection instruments. When field measurement and control personnel diagnose a load cell failure, they should first determine which part of the measurement system the failure comes from. The following methods can be used:
(1) Check the installation status of mechanical devices, find and eliminate obvious mechanical failures;
(2) Use a small high-accuracy load cell of the same type with different ranges as a standard signal generator to measure and judge whether the display is malfunctioning;
(3) Open the middle junction box and use a normal instrument to detect the working status of each sensor separately to judge whether a certain load cell is faulty;
(4) Use an electrician’s three-meter or shaker to check whether the cable is injured. Use the above method to narrow down the fault range and find the faulty load cell. Further testing and analysis should be done on the faulty load cell, and the operating conditions should be understood in time when the fault occurs. This is very important, because the failure of the load cell is often related to factors such as illegal operation or abnormal mechanical devices or changes in the use environment.
- Precautions for on-site use
3.1 Grounding problem of load cell
(1) The ground potential is grounded. In fact, the ground potential in some areas changes frequently. The changes in the ground potential of industrial sites mainly come from the leakage of electrical equipment, the opening and closing of power lines, the load changes of power equipment, and the effects of lightning.
(2) The shielding sleeve and the shell are grounded. The grounding pile position of the shielding sleeve should be selected in an area where the ground potential is constant and zero, and the pile position should be solved as close as possible. The load cell has a fully enclosed metal shell, which protects the strain gauge and measuring circuit that fits on the elastic body from moisture, protects the internal components from electromagnetic field interference and prevents thermal radiation. In order to ensure that the load cell itself has a stable zero potential and is not affected by the potential fluctuations of the equipment, in addition to the safety grounding of the equipment, the load cell housing should also be grounded separately.
(3) Set up a high-current return cable for the load cell to prevent the influence of the current on the load cell during lightning strikes or welding operations on the load carrier.
3.2 The problem of mechanical installation
(1) The load cell should be handled with care. Shocks, collisions, and drops may damage the measurement performance of the sensor.
(2) The mounting surface of the base where the sensor is installed should be flat and clean, and there should be no oil film or glue film, and should be adjusted horizontally, especially for weighing systems with more than 3 sensors. Pay more attention to adjusting each load cell during use. The level is consistent.
(3) The loading direction of the load cell is determined, and it must be loaded in this direction when using it to avoid the generation of lateral force and additional bending moment torque; long-term placement of heavy objects on the weighing platform is prohibited to prevent the sensor from creeping. Change.
(4) Check the system for irregularities in movement frequently. It can be used to add or subtract about one-thousandth of the rated load on the weighing platform, and observe whether the display instrument has a reflection method to check whether the movable parts are “contaminated” and jammed.
4 Conclusion
Resistance strain type load cell has achieved rapid development and successful application in recent years. It is making the domestic weighing field undergo a profound change, and various electronic weighing instruments have become trusted weighing equipment. As a key component of electronic weighing instruments, the load cell has been deeply concerned by people. Because of the interleaving of disciplines and the interleaving of components involved in the working mechanism of load cells, the interleaving of causes of faults and the difficulty and complexity of fault diagnosis are also caused. The failure causes of load cells can be divided into the following three categories:
(1) Failure caused by purely mechanical components;
(2) Failure caused by the electric bridge network line;
(3) Failure caused by the sticking part.
In order to reduce or eliminate the failure of the load cell, in addition to the manufacturer’s effort to study the influence of strain gauges, strain glue, sealant, compensation components and elastomers on the load cell index, improve the manufacturing process and raise the manufacturing level, users should modify the on-site environment It is also very important to improve operation methods and pay attention to maintenance.
