A multifunctional sensor system for brake system applications
Hernandez Aguirre, Rogelio
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The objective of this thesis is to develop a multifunctional MEMS (Micro Electro Mechanical System) sensor system for the simultaneous measurement of pressure and temperature inside a hydraulic system, more specifically a hydraulic brake system for automotive applications. The multifunctional pressure and temperature sensor system presented in this Thesis was designed to be installed in a new brake-by-wire system that requires the simultaneous reading of pressure and temperature per wheel cylinder. This system needs to control and monitor these parameters at each wheel cylinder to adjust the pressure for optimal braking. Current sensing systems installed in regular brake systems use a single pressure sensor that is positioned in the main cylinder and they do not include a temperature sensor. Moreover, while numerous approaches have been taken to control and monitor the pressure in a brake system real-time, no MEMS sensor system has yet been reported that can carry out real-time measurements of the brake system's pressure and temperature. In a representative automobile hydraulic brake system, the pressure and temperature can reach up to about 4 Mpa and 120 °C, respectively. These conditions are developed in an oily environment with a pH ~ 11. The multifunctional sensor system presented here is based on the two sensors, one for pressure and one for temperature, working within the same packaging. These two sensors are glued on the surface of an adequate Transistor Outline (TO) base using a temperature resistance adhesive. The substrate with the two sensors is covered by a parylene layer for dielectric protection, protection from the corrosive medium and protection from the moisture inherent in the brake fluid. The interface of the sensor system to the hydraulic brake system uses a commercial 1/4 18 NPT fitting customized to serve as an interface as well as a metal shell between the sensor and the hydraulic cylinder. The TO base and the metal shell were joined by micro-brazing to minimize heat-affected areas and ensure that critical components are unharmed. A finite element model to understand the effect of the parylene layer on the performance of the sensors was developed using COMSOL Multiphysics®. The model was validated by testing many prototypes of the developed sensor system using a custom made hydraulic hand pump which pressure is monitored by a digital hydraulic pressure gauge. The sample fitting is covered by a coil heater that includes a type -T Thermocouple positioned close to the sample to monitor the temperature. The complete apparatus allowed characterization of the test sensor from room pressure (13 psi) to 500 psi over a temperature range of 25 to 120°C. The test samples were characterized from atmospheric pressure to 450 psi over a temperature range of 25 to 120°C. The experimental data shows a reduction in pressure sensitivity of 18.2 % due to the parylene layer which closely agrees with the model predictions of a reduction of 21%. In summary, a multifunctional sensor system has been developed that can be used to control and monitor the pressure and temperature of a hydraulic cylinder real-time. The sensor system is novel in that it measures both parameters at a single point real-time with good sensitivity and accuracy making it ideal for applications in brake-by-wire systems.