Improving The Performance Of Welded Hermetic Load Cells By Laser Processing Techniques And Their Application In On-Board Weighing

The quality of load cells determine the performance of on-board weighing system.A major challenge in on-board weighing is developing and producing high accuracy load cells for harsh environments and long-term condition monitoring.To design and manufacture high-stability,high-durability load cells have been studied in many ways,one of important research domain is to research on welded hermetic load cells.On the other hand,the ability of sensing element to sense strain is the key for the performance of load cells.Laser processing technologies(such as laser welding,laser surface peening and laser surface texturing)have been proved in improving the reliability of sealing,stress transfer efficiency and material working life.Therefore,in this paper,laser welding,laser surface peening and laser surface texturing were used to carry out a series of process and mechanism research on welded hermetic load cells.The main research work and overall findings in this paper are as follows:(1)The influence of diaphragm structure on the output of load cell was studied.The spans of load cells with three different sealing diaphragms were simulated and optimized by the P-type finite element method.The simulation results showed that the optimized cup-type diaphragms had the minimum influence on the span of load cell,which was reduced to less than 2%FS(Full Scale).Through the performance comparative experiment of load cells before and after laser welding,the performance change of the load cells with welding different sealing diaphragms were compared and analyzed.The results showed that the change of the spans of load cells were consistent with the simulation results.However,the preliminary results also indicated that the laser welding sealing process had influence on the stability of the load cells.(2)In order to research the cause of laser welding effect on the stability of hermetic load cells,residual stress produced by laser welding and the mechanism of its influence on the stability of load cells were studied in this paper.A three-dimensional numerical simulation model was established to predict stress field between diaphragms and load cell with different laser welding parameters.The results revealed that the distribution of residual compression stress on gaging surface of load cell was symmetrical and regardless of the laser welding parameters,also there was an inverse correlation between the residual compressive stress amplitude of gaging surface and the residual tension stress amplitude of weld joint surface.According to the above results,the prediction model about the release of the residual stress on the welding surface of the sensor under cyclic loading was established,which revealed the mechanism of the influence of the welding residual stress on the load cell’s stability.(3)Based on the above results,the use of laser surface peening(LSP)on the weld joint surface to modulate the residual stress distribution characteristics of gaging surface zone was investigated by experimental and theoretical method.The stress field simulation model of LSP process on load cell was established to reveal the influence rule of LSP process on the residual stress field in weld joint zone and gaging surface zone of load cell.According to the results of simulation analysis,the experiment of laser surface peening treatment was designed and optimized.The Experimental results showed that the repeatability error of load cells was reduced by 43% after LSP treatment,which proved that LSP treatment could improve the stability of welded load cells.It was also found that the nonlinearity and hysteresis of load cells were improved after LSP treatment.The preliminary analysis indicated that it was caused by LSP treatment improved the local yield strength of the welded diaphragms of the load cells.(4)In order to solve the problem that the bonding durability between load cell body and strain gauges were restricted by traditional gaging technology,a new technology of texturing the gaging surface of load cell by nanosecond pulsed laser was investigated in this paper.Three different texture patterns were designed,then laser processing parameters were optimized for different patterns.The experiment results suggested that hardness of the gaging surface only had little change after LST,however,roughness significantly increased,which indicated LST could meet the gaging surface treatment requirements as traditional processing methods.Further research also showed that the adhesive thickness of samples produced with LST depended on the laser pulse frequency,whereas surface roughness did not,and the best adhesive thickness could be obtained when the laser power was 18 W,the frequency was 70 kHz and the scanning speed was 700mm/s.The mechanism for controlling the adhesive thickness was studied and a theoretical equation was constructed to predict the adhesive thickness,which was consistent with experimental results.Finally,the results of fatigue tests of the load cells indicated that the average life of load cells increased 25% after LST treatment,and its mechanism was preliminarily explained from both a physical and chemical perspective.(5)Taking agricultural locomotive bale weighing system as an example,the running in experiment of the above load cells in on-board weighing was carried out.Firstly,the influence of the installation boundary condition of on-board load cells found in current agricultural locomotive bale weighing system was analyzed.A semi-floating installation mode was developed to solve problems associated with repeatability and static measurement accuracy of current on-board bale weighing system.Then,a method that used multi-sensor data to improve weighing accuracy was studied under a complicated on-board environment.Further,a multi-sensor data fusion method based on fuzzy recognition was developed to determine the status of bales and load cells was constructed for compensating the inertial error in weighing results.According to the compensation results,the fuzzy compensation algorithm improved the bale weighing accuracy.The load cells included laser processing technologies had been applied in practice.