1. Structure of the force sensor
The design of the elastic element, housing, diaphragm, upper pressure head, and lower pressure pad of a force sensor must ensure that its performance does not fluctuate or fluctuates very little under load. Therefore, the design of a force sensor should strive to ensure that the strain zone experiences a single, uniform stress; the contact area should ideally be planar; the structure should have a certain capacity to resist eccentric and lateral loads; the installation force should be far from the strain zone; and displacement of the load support point should be avoided during measurement. Although a force sensor is an assembled product, it should be designed as a single, integrated structure to ensure optimal technical performance and long-term stability.
2. Metallic materials of elastic elements
The metallic material of the elastic element plays a crucial role in the overall performance and long-term stability of the force sensor. Materials with high ultimate tensile strength and elastic limit, good time and temperature stability of the elastic modulus, low elastic hysteresis, and low residual stress generated by machining and heat treatment should be selected. Data shows that as long as the material has good plasticity after quenching, its residual stress after machining and heat treatment will be low. Special attention should also be paid to the stability of the elastic modulus over time, requiring that the elastic modulus of the material remain unchanged throughout the service life of the force sensor.
3. Machining and heat treatment processes
During machining, elastic elements generate significant residual stress due to uneven surface deformation. The greater the cutting parameters, the greater the residual stress, with grinding producing the highest residual stress. Therefore, a reasonable machining process and appropriate cutting parameters should be established. During heat treatment, uneven cooling and phase transformations in the metal cause residual stresses in the core and surface layers, with tensile stress in the core and compressive stress in the surface. A tempering process is necessary to generate stresses in opposite directions within the element, canceling out the residual stresses and reducing their impact.
4. Resistance strain gauge and strain adhesive
The resistance strain gauge should possess optimal performance, requiring good sensitivity stability, low thermal output, low mechanical hysteresis and creep, a fatigue life of up to 10⁸ when the strain is 1000 × 10⁻⁶, small resistance value deviation, and good batch quality uniformity. The strain gauge adhesive should possess high bonding strength, high shear strength, a large and stable elastic modulus, good electrical insulation properties, a coefficient of thermal expansion similar to or the same as the elastic element, low creep and hysteresis, and minimal volume shrinkage during curing. Strict control of the adhesive layer thickness is crucial when bonding resistance strain gauges, as bonding strength decreases with increasing adhesive layer thickness.
This is because a thin adhesive layer requires greater stress to deform, making it less prone to flow and creep. The internal stress at the interface is very small, reducing the chance of bubbles and defects. It also has good strain transfer performance, and as long as the protective sealing is reasonable, a high level of stability can be achieved.
5. Manufacturing process flow
The working principle and overall structure of strain gauge force sensors dictate that some steps in the production process must be performed manually, and human factors have a significant impact on the quality of the force sensors. Therefore, it is essential to develop a scientific, reasonable, and repeatable manufacturing process, incorporating automated or semi-automated steps controlled by electronic computers to minimize the impact of human factors on product quality.
6. Circuit compensation and adjustment
Strain gauge force sensors are manufactured through assembly; after patch panel assembly and bridge bonding, the product is formed. Due to unavoidable internal defects and the influence of external environmental conditions, some performance indicators of the force sensor may not meet design requirements. Therefore, various circuit compensations and adjustments must be performed to improve the stability of the force sensor itself and its stability to external environmental conditions. A complete and precise circuit compensation process is a crucial step in improving the stability of force sensors.
7. Protection and Sealing
Protection and sealing are crucial processes in the manufacturing of force sensors, fundamentally ensuring their stable and reliable operation in the face of environmental influences. Poor protection and sealing will cause the strain gauge and adhesive layer bonded to the elastic element to absorb moisture from the air, resulting in plasticization, decreased bond strength and stiffness, zero-point drift, and irregular output changes, ultimately leading to force sensor failure.
Therefore, effective protective sealing is the fundamental guarantee for the long-term stable operation of force sensors; otherwise, all the technological achievements will be wasted.
8. Stability treatment (artificial aging test)
In addition to addressing the influence of the aforementioned factors, the most important way to improve the stability of force sensors is to adopt various technical measures and processes to conduct effective artificial aging tests that simulate usage conditions, thereby releasing as much residual stress as possible and minimizing performance fluctuations.
