Development of a Precision Underwater Positioner for In Situ Spectrographic Applications

MBARI has developed and deployed a precision underwater positioner for use with its laser Raman spectrometer (DORISS -- Deep Ocean Raman In Situ Spectrometer). The DORISS instrument is capable of obtaining chemical and structural information in situ from gaseous, liquid or solid targets in the deep ocean. However, when analyzing solid, opaque objects, the instrument probe head must be positioned with a precision of ∼0.1 mm. Additionally, the probe head must be free of vibration and other motions induced by the ROV, and stable for tens of minutes for data acquisition. The Precision Underwater Positioner (PUP) was developed as an off-loadable platform to meet the stringent positioning requirements of the DORISS instrument. The functional requirements call for a positioner that can be carried and deployed by an ROV; is off-loadable onto the seafloor to decouple the sensor head from ROV-induced motion or vibration; is stable on various types of seafloor terrain; incorporates a camera, lights, and crossing lasers to help with proper positioning; is controllable by a shipboard scientist via a GUI which provides position feedback; has variable speeds; and has a full range of 15 cm in each direction with a precision of 0.1 mm. While the PUP was designed to meet the needs of the DORISS instrument, it is a stand-alone system that can also be used with other sensors that require similar positioning requirements. The Precision Underwater Positioner is 5 degree of freedom positioner on a tripod platform -- Z, R, θ , and pan and tilt at the sensor head. Adjustable legs allow it to accommodate uneven seafloor. Phase I of the PUP consists of only 2 degrees of freedom (Z and R), and has rigid legs. It has been built, tank tested, and deployed on the seafloor ( ∼175 m depth) for at-sea tests. This testing has verified that the PUP meets the stipulated requirements and is capable of providing the stability and positioning necessary to collect quality Raman spectra of solid and opaque targets on the seafloor (to depths as great as 4000 m).