1. Overview
The spoke type weighing sensor, as the name implies, looks like a wheel, with spokes symmetrically distributed between the hub and the wheel hoop, mostly 4, 6, or 8. It has low height and good stability; strong resistance to eccentric loads and lateral loads; good linearity and repeatability; strong overload capacity, which can reach more than 300% by controlling the gap between the hub and the bottom surface of the support; the elastic element is a symmetrical overall structure, with good thermal expansion consistency in all directions and low temperature coefficient; the elastic element structure is convenient for welding and sealing of the upper and lower circular diaphragms, and the sealing level can reach IP68.
The main disadvantage of the spoke type weighing sensor is the large hysteresis error, which needs to be improved through structural design and manufacturing process.
1The hysteresis error of the weighing sensor is usually defined as: for the same external load value, the difference between the output measurement values of the weighing sensor during the forward stroke (input increase) and the return stroke (input decrease). In other words, for the same input signal, the output signals of the weighing sensor in the forward and reverse strokes are not equal, which is the hysteresis error.
The hysteresis error loop of most weighing sensors is usually cigar-shaped, and the width of the loop is determined by the amplitude of the applied load cycle. When measuring hysteresis, there are always some effects caused by creep and creep recovery, so the hysteresis test should be completed in a very short time.
The hysteresis error of the weighing sensor is closely related to the structure and heat treatment process of the elastic element, the thickness of the resistance strain gauge base and the strain adhesive layer. In summary, the main factors affecting the hysteresis error are: (1) The design of the contact surface between the elastic element bearing surface and the lower pressure pad is unreasonable, mainly due to the large contact area and the large friction coefficient of the lower pressure pad material. When the elastic element is loaded, the bottom surface will inevitably produce an outward moving moment. When the load is unloaded, due to the friction torque of the bottom surface, the moment of the bottom surface moving back is smaller than the moment of the outward moving during deformation, which hinders the recovery of the deformation of the elastic element and produces hysteresis error.
(2) The design of the strain zone and the support boundary of the elastic element is unreasonable, and the inherent hysteresis is large. For example, the sliding of the contact surface between the double shear beam elastic element and the base is an important cause of hysteresis error. During loading and unloading, the sliding direction of the double shear beam elastic element is opposite to that of the base, so the direction of the friction force acting on the elastic element is also opposite. It is this friction force that causes the shear stress change in the strain zone. The contact surface friction coefficient is large, and the absolute value of the hysteresis changes from small to large as the load increases. The distance from the center of the blind hole to the end face of the elastic element is too small, and the bottom friction force has a greater impact on the strain zone. (3) The influence of machining geometric tolerances is prominently manifested on the circular ring and plate ring elastic elements. When the loading threads at both ends are not concentric, the load value actually passing through the elastic element deviates from the measuring axis at an angle of α. As the load increases, the angle α gradually decreases, and the effective measuring load continues to increase, making the output present an increasing parabola. After unloading, the connectors do not return to their initial positions, resulting in hysteresis errors. (4) The protective shell and sealing diaphragm on the elastic element of the weighing sensor are not designed properly. If there is improper contact or a certain stress, a force or torque that blocks the deformation recovery of the elastic element will be generated, resulting in hysteresis errors. (5) Improper selection of metal materials or heat treatment processes of elastic elements results in large elastic hysteresis of the metal material itself. For example, the most widely used medium carbon alloy steel 40CrNiM0A has elastic hysteresis related to its microstructure. Different tempering temperatures result in different metallographic structures and different elastic hysteresis, with a maximum value of 0.11%.
(6) The principle and manufacturing process of strain type weighing sensor determine that its sensing element resistance strain gauge is pasted on the elastic element with strain adhesive such as epoxy resin. Although the adhesive layer is very thin in order to improve the shear strength of the strain adhesive, the elastic element will still produce creep and hysteresis errors after being loaded. The reason is that when the surface strain of the elastic element is transmitted to the strain adhesive layer and the base of the resistance strain gauge, a large shear stress is generated, which weakens its viscoelasticity and causes viscous flow (sliding between the base of the resistance strain gauge and the adhesive layer), which is called the effect of viscoelastic aftereffect. The hysteresis error of the spoke-type weighing sensor basically conforms to the above law. Due to the unique elastic element structure formed by the hub, spokes and hoop, the hysteresis error has its particularity. This is the main issue discussed in this article.