Rotor speed microsensor for the MIT Microengine

Abstract

This thesis presents the design, fabrication, and testing of a temperature-based sensor for measuring rotor speeds in the MIT MEMS micro gas turbine engine. The MIT microengine is a gas combustion engine made by micromachining and bonding six silicon wafers. The sensor is a boron-doped polysilicon resistor with a serpentine geometry that is thermally isolated from the substrate. The sensor is designed to measure the rotor rpm by responding to the heat flux fluctuations on the wall above the compressor blade tips. This thesis investigates the feasibility of this approach. The sensor development process involved fabricating stand-alone devices (which have only the sensor and contact pads and not integrated with other microengine components) and testing them using a furnace and a shock tube. The furnace test characterized the stability with thermal cycling and annealing. The shock tube test characterized the dynamic response. The temperature coefficient of resistivity (TCR), 0.009/K , and the room temperature resistance, ~9 kohms, measured in the furnace characterization experiments were approximately 50% less and 300% more than the predicted values, respectively. These discrepancies may be due to the fabrication process conditions, such as ion implant dose, polysilicon deposition temperature, and anneal conditions. The time constant, 9-10 [mu] sec, measured from the shock tube experiments matched predicted values to within 20-40% depending on the model used to estimate the convective heat flux into the sensor. However, the sensor's amplitude response was less than predicted values by approximately 10 - 75% perhaps due to the simplicity of the models used to estimate the convective heat flux. The experimental results suggest that this concept is viable as a microengine rpm sensor. Some design changes are suggested which should improve sensor performance.