David A. Mills

Characterization of an Optical Moiré Wall Shear Stress Sensor for Harsh Environments

The ability to make time-resolved, direct wall shear stress measurements in harsh environments is an important measurement capability for improving understanding of complex fluid flow phenomena. This work presents the design, packaging, and characterization of a third-generation silicon-on-Pyrex moire optical wall shear stress sensor designed for use in environments up to 450◦C. Optical transduction of the floating element microelectromechanical systems (MEMS) sensor is accomplished by interrogation of the geometric moire fringe via a four-channel fiber optic array. Device performance is improved over previous generations by patterning discrete sections of the moire fringe and the use of a silicon nitride anti-reflective coating. Calibration of the sensor system yields a dynamic shear stress sensitivity of 12.6 mrad/Pa at 1.128 kHz, sensor resonance of 4.1 kHz, pressure rejection ratio of 87 dB, minimum detectable shear stress of 0.53 mPa at 1.128 kHz for a 1 Hz bin, and an experimentally verified dynamic range of 79 dB.

Development of a sapphire optical pressure sensor for high-temperature applications

This paper presents the fabrication, packaging, and characterization of a sapphire optical pressure sensor for hightemperature applications. Currently available instrumentation poses significant limitations on the ability to achieve realtime, continuous measurements in high-temperature environments such as those encountered in industrial gas turbines and high-speed aircraft. The fiber-optic lever design utilizes the deflection of a circular platinum-coated sapphire diaphragm to modulate the light reflected back to a single send/receive sapphire optical fiber. The 7 mm diameter, 50 μm thick diaphragm is attached using a novel thermocompression bonding process based on spark plasma sintering technology. Bonds using platinum as an intermediate layer are achieved at a temperature of 1200°C with a hold time of 5 min. Initial characterization of the bond interface using a simple tensile test indicates a bond strength in excess of 12 MPa. Analysis of the buckled diaphragm after bonding is also presented. The packaged sensor enables continuous operation up to 900°C. Room-temperature characterization reveals a first resonance of 18.2 kHz, a flat-band sensitivity of -130 dB re 1 V/Pa (0.32 μV/Pa) from 4-20 kHz, a minimum detectable pressure of 3.8 Pa, and a linear response up to 169 dB at 1.9 kHz.

Fabrication and characterization of a sapphire based fiber optic microphone for harsh environments.

This paper describes the fabrication and characterization of a sapphire based fiber optic microphone designed for the harsh environment of gas turbine engines. The performance requirements are driven by turbine inlet temperatures that have risen in excess of 1500 °C. The harsh environment makes conventional instrumentation unsuitable for time‐accurate, continuous, direct measurements. The use of commercially available sapphire substrates and optical fibers allows for performance in extremely high temperature environments due to the high melting point and matched coefficient of thermal expansion. The sensor is based on a fiber optic lever transduction mechanism with a remote photodiode optical readout allowing for isolation of the electronics from the harsh environment. The microphone’s diaphragm consists of a thin sapphire substrate with a sputtered titanium/platinum reflective surface. The back cavity is formed by macro‐machining of a 1‐mm‐thick sapphire substrate. The diaphragm and back cavity are joine...

Characterization of a Sapphire Optical Wall Shear Stress Sensor for High-temperature Applications

This paper presents the development and initial experimental characterization of the first sapphire micromachined wall shear stress sensor for high-temperature applications utilizing geometric moire optical transduction. The microelectromechanical systems (MEMS) sensor utilizes a folded tether floating element structure to extend the operating range of the sensor. Picosecond pulsed laser micromachining is employed to pattern mechanical structures in sapphire, and a four-channel alumina fiber array with sapphire optical fibers is used to interrogate the moire fringe. Platinum thin-film gratings and a stainless steel package enable a theoretical maximum operating temperature in excess of 800◦C. Calibration of the sensor system in differential mode demonstrates a dynamic shear stress sensitivity of 76.8 μV/Pa at 1.128 kHz, sensor resonance of 3.5 kHz, pressure rejection ratio of 75 dB, minimum detectable shear stress of 4.6 mPa at 1.128 kHz for a 1 Hz bin, and an experimentally verified dynamic range of 52 dB.

Development of a sapphire optical wall shear stress sensor for high-temperature applications

This paper presents the development of the first sapphire micromachined wall shear stress sensor for high-temperature applications utilizing geometric moiré optical transduction. A folded tether floating element structure is employed to extend the linear operating range of the sensor. Picosecond pulsed laser micro-machining processes are developed for patterning of mechanical structures in sapphire, and a four-channel alumina fiber array with sapphire optical fibers is used to interrogate the moiré fringe. Platinum thin-film gratings and a stainless steel package enable a theoretical maximum operating temperature in excess of 800°C, and initial dynamic calibration in differential mode demonstrates a shear stress sensitivity of 76.8 μV/Pa at 1.128 kHz.

A miniaturized optical package for wall shear stress measurements in harsh environments

We report the development of a time-resolved direct wall shear stress senor using an optical moiré transduction technique for harsh environments. The floating-element sensor is a lateral-position sensor that is micromachined to enable sufficient bandwidth and to minimize spatial aliasing. The optical transduction approach offers several advantages over electrical-based floating element techniques including immunity from electromagnetic interference and the ability to operate in a conductive fluid medium. Packaging for optical sensors presents significant challenges. The bulky nature and size of conventional free-space optics often limit their use to an optical test bench, making them unsuitable for harsh environments. The optical package developed in this research utilizes an array of optical fibers mapped over the moiré fringe. The fiber bundle approach results in a robust package that reduces the overall size of the optics, mitigates vibration between the sensor and optoelectronics and enables in situ measurement. The optical package for sampling the amplified moiré fringe is evaluated using bench top test setups. An optical test bench is constructed to simulate the movement of the moiré fringe on the floating element. High-resolution images of the optical fringe and optical fibers are combined in simulation to model the lateral displacement of the fringe. The performance of several fringe estimation algorithms are studied and evaluated. Based on the optical study, the optical package and post-processing algorithms are implemented on an actual device. Initial device characterization using this approach results in a device sensitivity of 12.4 nm/Pa.

A system for vector measurement of aerodynamic wall shear stress

This paper describes a sensor system designed for vector measurement of aerodynamic wall shear stress. The microelectromechanical systems (MEMS) prototype utilizes a serpentine tether flexure for dual-axis in-plane motion. Capacitive gap pairs coupled to a multi-frequency modulation/demodulation system allow for independent transduction across both axes. Sensor performance includes an average dynamic/mean shear stress sensitivity of 0.16 mV/Pa, corresponding to a minimum detectable signal of 0.3 mPa at 1.128 kHz. Pressure rejection is calibrated at 74 dB, with in-plane cross-axis isolation in excess of 24 dB.

Experimental Investigation of Laser Machining of Sapphire for High Temperature Pressure Transducers

Challenges associated with materials for high temperature pressure sensor designs, in excess of 1000◦C, are explored here for future applications such as control of combustion processes and flow control of hypersonic vehicles. Currently, silicon based MEMS technology is primarily used for pressure sensing. However, due to the limited melting point of silicon, such sensors have a limited temperature range of approximately 600◦C which is capable of being pushed towards 1000◦C with active cooling. To overcome thermal limitations, the thermomechanical properties of sapphire are investigated to facilitate the design of an optical based pressure transducer which is designed to operate at temperatures approaching 1600◦C. Due to sapphire’s hardness and chemical inertness, traditional cutting and etching methods used in MEMS technology are not applicable. The proposed methodology for the sapphire based sensing technology is picosecond laser machining. Here we summarize the material property changes that occur from laser machining across temperatures ranging from room temperature to 1300◦C. Both changes in elastic moduli and strength, as functions of laser machining and temperature, are quantified using four-point bending experiments. The results illustrate comparable or improved strength after laser machining while the modulus was reduced after laser machining at room temperature and 1300◦C by a factor of 1.5 to 2.0.

A Sapphire Based Fiber Optic Dynamic Pressure Sensor for Harsh Environments: Fabrication and Characterization

This paper describes the modeling, fabrication, and characterization of a proof-ofconcept dynamic pressure sensor utilizing sapphire materials for the harsh environments of gas turbine engines. The harsh environment makes conventional instrumentation unsuitable for time-resolved, continuous, direct measurements. High temperature sensing is enabled by the use of sapphire materials due to a high melting point and corrosion resistance. The availability of commercial sapphire substrates and optical fibers provides a direct path for optical sapphire sensors with a matched coefficient of thermal expansion. The dynamic pressure sensor uses a fiber optic lever transduction mechanism with a remote photodiode optical readout allowing for isolation of the electronics from the harsh environment. A circular diaphragm is formed by joining two sapphire substrates using an alumina-based high temperature epoxy. The first substrate is 50 μm thick sapphire with a sputtered titanium/platinum reflective surface. The second substrate is a 1 mm thick sapphire substrate that has been machined to form a circular back cavity. Presented along with the sensor fabrication are a systems level dynamic model and a dc exploration of the performance of the fiber optic lever transduction. Finally, initial dynamic characterization of the frequency response function and linearity are presented with a resulting sensitivity of -113 dB re 1 V/Pa.