Design of a MEMS based hydraulic pressure sensor

Göreke, Utku
This dissertation presents a novel technique for detection of hydraulic pressure by using a MEMS resonant sensor. Proposed sensor utilizes a double ended tuning fork (DETF) resonator. In the literature tuning forks are used for measurement of the deflection of a diaphragm. However, in this study, a tuning fork is configured to lay in orthogonal direction with a diaphragm of which center point deflection is being measured. Upon application of pressure, center deflection of the diaphragm induces an axial compressive load on the DETF resonator which induces decrease in natural frequency of the resonator. Since the tip is vulnerable, a roller structure which is simply a guided beam is included in the design to protect tip from transverse components of measured displacement. Additionally, the applied displacement is directed through the roller to a spring element which transmits nearly 1 % of tip displacement to DETF. Although the addition of the spring adversely affects the sensitivity, the spring increases the overall compliance of the tip which increases the assembly yield. Advantage of such design is that by modifying the geometry of the spring or the diaphragm, different pressure ranges for measurement can be attained. The resonator’s tine dimensions are optimized for maximum of sensitivity quality factor product. The device can operate in atmospheric conditions, and hence the design makes use of overlapping comb fingers to avoid squeeze film damping. A pressure port is designed to keep diaphragm and MEMS sensor together in contact. The pressure port combines the deflection performance of aluminum for better sensitivity, and greater strength of steel for larger safety factor. Aluminum and steel parts are fixed together with interference fit for demonstration of the sensor operation. Surface micromachining of MEMS sensor is carried out at METU-MEMS cleanroom facility. Process flow involves 3 photolithography steps and makes use of 1 silicon-on-insulator (SOI) wafer. Pressure port is manufactured with conventional machining. Operation principle and analytical model is validated with FEM simulations. Tests are conducted for both tip displacement for sensor core and hydraulic pressure for the overall assembly. Resonator quality factor and maximum sensitivity measured in tip displacement tests were 238 and 198.49 Hz/µm, respectively, which is equivalent of 9.45 Hz/Bar with an aluminum diaphragm with 6.2 mm diameter and 1 mm thickness. A maximum sensitivity of 34.40 is achieved in hydraulic pressure tests. As a summary, operation of tuning fork resonator in orthogonal configuration with a diaphragm as a hydraulic pressure sensor is demonstrated. Proposed novel configuration promises a high dynamic range hydraulic pressure measurement.
Citation Formats
U. Göreke, “Design of a MEMS based hydraulic pressure sensor,” M.S. - Master of Science, Middle East Technical University, 2016.