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

Abstract

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.

Surface micromachined differential piezoelectric shear-stress sensors

“…Sensors based on microelectromechanical systems (MEMS) technologies have been introduced to address this problem, as their small size provides enhanced spatial and temporal measurement resolution [9]. Many MEMS transduction mechanisms for the measurement of wall shear stress have been explored based on detecting the motion of spring-mounted floating elements using optical [10][11][12][13] or capacitive [14,15] methods. The first MEMS embodiment was reported by Schmidt in 1987 [16,17] using capacitive transduction of a surface micromachined floating element.…”