Joseph N. Mitchell

3D precision surface measurement by dynamic structured light

This paper describes a 3-D imaging technique developed as an internal research project at Southwest Research Institute. The technique is based on an extension of structured light methods in which a projected pattern of parallel lines is rotated over the surface to be measured. A sequence of images is captured and the surface elevation at any location can then be determined from measurements of the temporal pattern, at any point, without considering any other points on the surface. The paper describes techniques for system calibration and surface measurement based on the method of projected quadric shells. Algorithms were developed for image and signal analysis and computer programs were written to calibrate the system and to calculate 3-D coordinates of points on a measured surface. A prototype of the Dynamic Structured Light (DSL) 3-D imaging system was assembled and typical parts were measured. The design procedure was verified and used to implement several different configurations with different measurement volumes and measurement accuracy. A small-parts measurement accuracy of 32 micrometers (.0012”) RMS was verified by measuring the surface of a precision-machined plane. Large aircraft control surfaces were measured with a prototype setup that provided .02” depth resolution over a 4’ by 8’ field of view. Measurement times are typically less than three minutes for 300,000 points. A patent application has been filed.

System-level solar module optimization

Concentrated photovoltaic (CPV) systems achieve the highest level of solar conversion efficiency of all photovoltaic (PV) technologies by combining solar concentration, sun tracking, and high-efficiency multi-junction PV cells. Although these design features increase the overall efficiency of the device, they also dramatically increase the cost and physical volume of the system and make the system fragile and unwieldy. In this paper, we present recent progress towards the development of a robust, reduced form-factor CPV system. The CPV system is designed for portable applications and utilizes a series of low profile optical and optomechanical components to concentrate the solar spectrum, enhance energy absorption, and track the sun throughout the diurnal cycle. Based on commercial off-the-shelf (COTS) single-junction PV cells, the system exploits the efficiency gains associated with tuning the wavelength of the incoming light to the band-gap of a PV material. This is accomplished by spectrally splitting the concentrated incident beam into multiple wavelength bands via a series of custom optical elements. Additional energy is harvested by the system through the use of scavenger PV cells, thermoelectric generators, and biologically inspired anti-reflective materials. The system’s compact, low-profile active solar tracking module minimizes the effects of wind-induced loads and reduces the overall size of the system, thus enabling future ruggedization of the system for defense applications. Designed from a systems engineering approach, the CPV system has been optimized to maximize efficiency while reducing system size and cost per kilowatt-hour. Results from system tests will be presented and design trade-offs will be discussed.

A prototype mass spectrometer for in situ analysis of cave atmospheres

Research in cave environments has many applications: studying local hydrogeologic activity, paleoclimate studies, analyzing white nose syndrome in bat populations, analogs for underground atmospheres in mining facilities, carbon sequestration efforts, and terrestrial analogs for planetary caves. The atmospheres of many caves contain tracers of current geological and biological activity, but up to this point, in situ studies have been limited to sensors that monitor individual components of the cave atmosphere. A prototype cave mass spectrometer system was assembled from commercial off-the-shelf parts to conduct surveys of atmospheric compositions inside four local Texas caves and to perform atmospheric analysis of two aquifer wellheads to a depth of 60 m. We found increased levels of CO(2) in all caves and, surprisingly, increased levels of O(2) in Bracken Bat Cave. Aquifer wellhead measurements showed indications of methane, other hydrocarbons, and other constituents not anticipated.