July 27, 2022

Types of faults and solutions for load cells

Sensitive components (elastic components of various structures), conversion components (resistance strain gauges of various structures), and measurement circuits (bridge circuits and various compensation and adjustment circuits) are the three major components of load cells, any of them Part of the failure will affect the overall stability and reliability of the load cell.


The load cell is a knowledge-intensive, technology-intensive and skill-intensive high-tech product, which is mainly reflected in its high technology and high skill.


High technology refers to the large discrete content involved in development and production, high technology intensiveness, and strong edge disciplines. It is the crystallization of multiple disciplines and technologies that intersect and penetrate each other.


High-skilled refers to the complicated and difficult process, high requirements for process equipment, testing methods, and staff quality. It supports the scientific application and integration of process, basic process, core process and special process.


This has caused the multiplicity and complexity of load cell faults and the comprehensiveness and interlacing of fault causes, which has brought greater difficulties to fault diagnosis and failure mechanism analysis. The fault diagnosis and fault analysis process of the load cell is the flexible use of the basic theoretical knowledge of the load cell and the integrated analysis of various process technologies, the use of scientific reasoning methods to diagnose and analyze various faults, find the cause of the fault, and find out the inherent law of the fault. Continuously deepen and enrich the process of understanding the design and manufacturing technology of load cells.


According to the traditional reliability analysis method, the failure of the load cell can be roughly divided into five types:


1. Bridge circuit failure


Each component in the load cell bridge circuit plays a role according to its function. Any slight change in the working characteristics of these components will cause a change in the output signal of the bridge and cause measurement errors. Similarly, the zero point temperature compensation, zero point output adjustment, nonlinear compensation, sensitivity temperature compensation, sensitivity standardization adjustment, and input and output resistance standardization adjustments in the bridge circuit or compensation resistance changes will directly affect the weighing of the load cell. Accuracy and working reliability. Even with a welded and sealed load cell, the quality status of the resistance strain gauges, circuit compensation and adjustment resistance of each bridge arm of the bridge can be judged by measuring the resistance value between the core wires of the cable. The terminal diagram of the measurement of the bridge circuit fault judgment of the cylindrical load cell.


Insulation damage and lightning damage failures of load cells will cause damage to the bridge circuit. Among them, insulation damage or insulation drop is a common fault in the bridge circuit.


2. Conversion element failure


The strain resistance conversion element of the load cell is faulty, that is, the resistance strain gauge, the welding wire terminal, the curing and post-curing failure of the strain adhesive. The working principle and manufacturing process of the resistance strain gauge determine that it will inevitably produce defects. When applied to the load cell, if the manufacturing process lacks scientific and reasonableness, it will bury hidden troubles. The strain adhesive for the resistance strain gauge is a solvent-free adhesive with epoxy resin and epoxy phenolic resin as adhesives. Only at a specified temperature and within a certain period of time, the adhesive itself is bonded and solidified and generated on the surface of the elastic element. Adhesion is not good. Over-curing will make the adhesive layer brittle, reducing the bonding strength, and reducing fatigue life; under-curing will make the adhesive layer’s molecular bonds weaken, causing creep effects and large creep.


3. Technical performance failure


The quality evaluation method of load cell is to establish the quality evaluation concept of the total error band, and the accuracy classification is expressed by the number of divisions. The maximum allowable error includes the influence of non-linearity, hysteresis error and temperature on the output sensitivity. The main quality indicators of accuracy grade assessment, for C-level and D-level load cells, mainly load cell error (EL), repeatability error (ER), temperature influence on minimum static load output (CM), weighing Sensor creep error (CC), minimum static load output recovery value (CVDL()R), etc. The zero point output of the load cell is unstable, the fault with large zero point drift, the non-linear error out-of-tolerance fault, the hysteresis error out-of-tolerance fault, etc. will cause the accuracy of the load cell to be out of tolerance.


4. Protection and sealing failure


Protection and sealing are the main processes in the production process of the load cell. Poor protection and sealing will bury hidden troubles and will completely lose the process results of all processes. Especially the load cell with surface sealing and blind hole potting, due to improper selection or preparation of sealing materials, moisture resistance and aging resistance are reduced. The protective and sealant adhesive layer absorbs moisture in the air and expands and plasticizes, resulting in electrical resistance The bonding strength and rigidity of strain gauges and strain adhesives are drastically reduced, causing various failures such as a drop in insulation resistance and zero drift.


5. Overall structure and failure of durable parts


The failure of the overall structure and accessories is mainly manifested in the unreasonable overall structure design, and the maximum stress or stress concentration outside the strain zone of the elastic element, which buried hidden dangers of performance fluctuations. The choice of the metal material of the elastic element only pursues economy, while ignoring the functionality is an important cause of such failures. For example, the cantilever shear beam type load cell used for the dynamic road vehicle axle load scale, the elastic element material is 40 Cr As a result, steel fractured during dynamic weighing due to insufficient strength reserves. At the same time, improper design and selection of contact surface shape, hardness, and roughness of the metal materials of the load cell pressure pad, indenter and other accessories will also cause malfunctions. Only solid bonding can ensure the quality of the paste. If the curing temperature, heating rate, and holding time are related to the treatment.


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