A Low Cost Optical Sensor to Measure Underwater Flow

PI: Jaffe, Jules (University of California, San Diego)
Start Year: 2019 | Duration: 3 years
Partners: Creare (Hanover, NH)

Project Abstract:

The overall project goal is to develop a portable, low cost, low power, underwater current meter that will estimate current velocity accurately in a large variety of ocean environments when deployed from a variety of fixed or moving platforms using ambient particle imaging and tracking methods. Targeted platforms include both moored and vertically profiling, and moving platforms as AUVs, gliders, and ROVs. Our approach has been piloted via the development of a low cost (< $500) sensor package that uses inexpensive, commercially available, components. Results indicate that the system works well in measuring near bottom currents parallel to the face of the enclosing cylindrical housing compared to an Acoustic Doppler Velocimeter that was simultaneously deployed. We seek funds to further develop an instrument that will have minimal flow disturbance and the necessary sensitivity and processing power to yield identical results to the popular Doppler based products in a large range of environments. To accomplish this we will work with our partner company, Creare (Hanover, NH) that has expertise in Particle Imaging Velocimetry and optical system design. Our plan is to explore several designs that: (1) Use multiple cameras to facilitate larger separation as well as a range of configurations that more closely mimic the geometry of the Doppler systems. (2) Use laser diodes that will provide two orders of magnitude higher irradiance on the small particles to insure system function in a range of environments where the smaller particles dominate. (3) Use frame straddling techniques to circumvent the relatively slow camera frame rate to achieve current estimates in flows as high as 2 m/s, compatible with AUV velocities. Our plan encompasses an initial design phase followed by building several lab prototypes to mimic the new geometry and lighting. Tests with calibrated spheres as well as natural particles in filtered seawater will permit an accurate estimate of system performance in our flume. Improved processing algorithms, tailored to the new geometry, will be developed by Creare. Upon examining the merits of the systems we will decide on a sea-going version that will be packaged and tested in Year 2 via vertical profiles in local waters on the R/V Sproul and on a vertical profiling platform such as the wire walker. Comparisons with the Doppler methods, both ADV and ADCP will be used to validate the estimated in situ flow speed and direction. Upon successful trials, our partner, Creare, will fabricate 10 units in Year 2. Sea trials and calibrations with all units will then take place at the end of Year 2. In year 3, the 10 synchronized units will be deployed in an opportunistic area that exhibits complex flow due to complex bottom terrain. Overall working characteristics will be a unit that can estimate flow from 0 to 2 m/s with reasonable performance errors, consume less than 3W of power, and be adaptable to a variety of platforms. A unique aspect of the optically based velocimeter is that it can measure the 3-dimensional spatial characteristics of flow within the laser sheet allowing observation of small-scale turbulence. Another advantage is that the optical port can be smoothly integrated into the exterior of autonomous vehicles, thereby introducing minimal flow disturbance in their performance.

BAA: N00014-18-S-B007