The load cell in the industry will be more understood, our load cell manufacturers or take a look at the elements of our force measurement stability, there are many factors that affect the stability of the load cell, which can be summarized as follows:
1. The structure of the load cell
The planning of the elastic elements, shells, diaphragms and upper pressure heads and lower pressure pads of the load cell is necessary to ensure that the structure is not firm or the function is not firm after loading. The patch part should be flat; Structurally, it is guaranteed to have a certain talent for resisting eccentric loads and lateral loads; The force of the device is far away from the strain zone, and the displacement of the load bearing point should be avoided when measuring. Although the load cell is manufactured as a device, it is planned as a single structure as much as possible in order to guarantee the best technical function and long-term stability.
2. Metal material of elastic element
The metal material of the elastic element plays a key role in the inductive function and long-term stability of the load cell. Materials with high strength limit and elastic limit, good time and temperature stability of elastic modulus, small elastic hysteresis, and small residual stress caused by machining and heat treatment should be selected. Some materials show that as long as the plasticity of the material after quenching is good, its residual stress after machining and heat treatment is small. Special attention should also be paid to the stability of the elastic modulus over time, and the elastic mold of the material should not be changed during the service life of the load cell.
3. Machining and heat treatment process
In the process of machining, the elastic element produces a large residual stress due to the uneven surface deformation, and the larger the amount of cutting, the greater the residual stress, and the residual stress generated by grinding is the largest. Therefore, reasonable machining technology and rules should be formulated, and the appropriate cutting amount should be used. In the heat treatment process of elastic elements, due to the uneven cooling temperature and phase transformation of metal materials, the residual stress in different directions is generated on the core and the surface, and the core is tensile stress and the surface layer is compressive stress. It is necessary to use the tempering process to generate stress in the opposite direction inside it, canceling each other out with the remaining stress, and reducing the effect of the remaining stress.
4. Resistance strain gauge and strain binder
The resistance strain gauge should have the best function, requiring good stability of sensitivity coefficient, small heat output, small mechanical hysteresis and creep, fatigue life of up to 108 when the strain variable is 1000×10-6, small resistance value error, good batch quality uniformity, etc. The strain binder should have high bond strength and high shear strength; The modulus of elasticity is large and stable; The electrical insulation function is good; Have the same or adjacent coefficient of thermal expansion as the elastic element; Low creep and hysteresis; When curing, the volume of the adhesive line shrinks small, etc. The thickness of the adhesive layer must be strictly controlled when posting the resistance strain gauge, because the bond strength decreases with the increase of the thickness of the adhesive layer.
This is due to the fact that the thin adhesive layer requires greater stress and deformation, is not easy to produce activity and creep, the internal stress on the interface is very small, the probability of producing bubbles and defects is also relatively small, the strain transmission function is good, and a higher level of stability can be reached as long as the protective seal is reasonable.
5. Manufacturing process
The operating principle and overall structure of the strain gauge load cell have been decided, and some processes in the production process are necessary to be operated manually, and the human elements have a great impact on the quality of the load cell. Therefore, it is necessary to develop a scientific, reasonable and repeatable manufacturing process, and increase the computer-controlled automatic or semi-automatic process to minimize the impact of human factors on product quality.
6. Circuit compensation and adjustment
Due to the inevitable internal defects and the influence of external environmental conditions, some functional indicators of the load cell can not meet the planning requirements, so it is necessary to carry out various circuit compensation and adjustment, and advance the stability of the load cell itself and the stability of the external environmental conditions. The perfect and fine circuit compensation process is an important part of the stability of the forward load cell.
7. Protection and sealing
Protection and sealing are the key processes in the manufacturing process of load cells, and they are the fundamental guarantee for the load cells to withstand the influence of the objective environment and the sensing environment and operate safely and reliably. If the protective seal is poor, the resistance strain gauge and strain binder adhesive layer attached to the elastic element will absorb the moisture in the air and produce plasticization, resulting in a decrease in bond strength and stiffness, causing zero drift and irregular changes in output until the load cell fails.
Therefore, a useful protective seal is the fundamental guarantee for the long-term and stable operation of the load cell, otherwise the process effect will be wasted.
8. Stability treatment (artificial aging test)
In addition to dealing with the influence of the above-mentioned factors, the most important way is to adopt various technical measures and process techniques, imitate the application conditions to carry out useful artificial aging tests, and release the remaining stress as much as possible to minimize its function.
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