Fatigue Life of Electronic Power Steering Control Unit (EPCU) under Mechanical Vibration loads. 2019-01-1536
In current scenario, we have challenges in vibration testing on the electronics product considering the cost of testing, time taken for each prototype, pre-conditions and environment effects. Having said this, we have made technical advancement with the use of technology through finite element analysis (FEA) and computer methods. We had a challenging problem for Electronic Power Steering Control Unit (EPCU) to finalize the designs for harmonic, transient and random vibration loads at operating temperature’s (-30C; 120C) in ±X, ±Y and ±Z directions with respect to car co-ordinate systems as per the test requirements.
To solve this complex problem, we proposed finite element analysis and computer methods to finalize the EPCU design where more than 40parts to be validated on the PCB during functional conditions. To start with, we understood the product qualification through test requirement document for the EPCU under mechanical vibrations loads. Based on this, we studied the physics and possible failure modes. Most importantly to idealize mathematical model, we collected all the material information’s for the EPCU assembly. There were four M6 screws on the PCB and a pre-tension load of 1200N is applied on each screws. This will ensure warping on the PCB and its effects on the components during assembly process. In continuation, a modal analysis is performed to identify the natural frequencies in the assembly. A detailed analysis is carried out to understand the natural frequencies, mode shapes and mode types (like bending mode, torsion mode or rotating mode). Hereafter, harmonic loads (Max: 3G’s) are applied in ±X, ±Y and ±Z direction with respect to car co-ordinate system. A similar approach is followed for the EPCU validation under mechanical shock loads (Max: 300Gs) in ±X, ±Y and ±Z direction. For random vibration loads at operating temperature’s we initial solved for screws pre-tension, then a thermal condition (-30C; 120C) is induced with co-efficient of thermal expansion for each material to understand the assembly behavior. This also helped us to validate the assembly during thermal shock testing (Criteria: 2000 Cycles) as part of pre-conditions before the physical testing on the EPCU. With the final stiffness of the model after the thermal conditions, the random vibration analysis is performed in ±X, ±Y and ±Z direction. A Coffin- Manson equation and Basquin’s equation is used to calculate the fatigue life.
At the end of each analysis, we calculated the fatigue life for the PCB and Solder joints. We had silicon sealant in the assembly and strains on the sealants should not be more than 50%. To have a robust design, we also proposed design suggestions based on the simulation results. After the simulation results, we gave recommendations for the placing of the four accelerometers in the assembly. To conclude, we had 85% - 90% simulation results matching with physical test report with respect to natural frequencies, acceleration outputs, strains observed on the critical parts.
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