Science lab engineering: design software aids marine research.
The institute has two remotely operated vehicles (ROVs). One, named Ventana, operates to a depth of 1850 meters, while the other, named Tiburon, operates down to 6000 meters. Both carry out a variety of experiments, one of which includes bearing a Laser Raman Spectrometer (LRS) to a depth of 4000 meters. The LRS--manufactured by Kaiser Optical Systems and modified for deep ocean application by MBARI--shines monochromatic laser light at an object. Although the majority of the photons making up the light scatter at the same frequency as the laser, a small band shifts spectrum to one that is specific to the molecular structure of its target, helping oceanographers identify many deep targets without requiring physical samples.
At the depths involved, the pressure case for the instrument is crucial. Mark Brown, who was a mechanical engineer for design prior to a recent promotion to Manufacturing Group Leader, used COSMOSWorks software from SolidWorks Corp., Concord. MA, to iterate designs for the pressure housing of the Deep Ocean Raman in Situ Spectrometer (DORISS). "COSMOSWorks let me iterate designs to reduce the weight of the housing and stiffen the case so that it did the job properly," Brown reports. Other factors affecting the LRS case design included thermal issues, vibration and movement under load.
The equipment actually fits into three pressure housings for use at depths of around 4000 meters. An electronics housing contains a single board computer, power components, and the 100 mW 532 nm excitation laser. Made of glass-filament-reinforced epoxy, this housing is a little lighter and less costly than a conventional aluminum or titanium design rated for the same depth.
An optical bench, a CCD camera made by Andor Technologies, and associated electronics go into a separate housing made of 7075-T6 grade aluminum. The selection of material was based primarily on thermal capacity for cooling, budget and weight restrictions imposed by the ROV's payload requirements. An off-the-shelf titanium housing holds the holographically filtered probe head. which is connected to the laser and spectrometer by MBARI-built penetrating fiber optic cables.
Brown's work included the design and optimization of the aluminum spectrometer housing. He describes the housing as having a 90-degree angle going into a tube with domed ends. and a circular perforation that enables placement of the camera connected to the optical bench. "The electronic equipment needs to pushed into the tube, and it's a tight fit," he says.
"The largest portion of the housing consists of three robes bolted together," Brown says. "The middle one has the hole that enables containment of the CCD. If we just wanted to hold all the parts in one place, we could have used a big can--but at 6000 psi pressure, in a remotely operated vehicle, the overall weight is as big an issue as it would be for a spacecraft."
MBARI chose aluminum for the housing because corrosion and robustness are very big issues in the harsh operating environment. "The instrument will be handled roughly during launch and recovery from the research vessel that is used for deployments," says Brown. "It then has to undergo an air/sea interface, and potentially high "G" loading in high seas. ! always design for robustness, because of the ocean's harsh environment. In high sea states, it's easy to break instruments and equipment during launch and recovery phases. COSMOS analysis gave us a way to reduce the weight and increase the strength." He aimed for a factor of safety in excess of two, and accounted for buckling issues because the design has a penetration near the middle of the assembly.
The design features a modified ring-stiffened cylinder, for which, says Brown, "we needed to beef up the flange to accommodate for the penetration used by the CCD." He iterated on the radius of the flange to meet his pressure and stiffness requirements, and made a variety of geometric changes, going back and forth between SolidWorks and COSMOSWorks. The overall design went through four geometric iterations, as well as a number of studies of the appropriate thickness of the aluminum.
"The housing design started out as a weldment, but in the end we used a single large aluminum billet to avoid potential inclusions or laminations that can happen with any welding process. The loading at 4000 meters is approximately 6000 psi. Because the potential for stress rises to occur at geometric imperfections is high, we try to design with such possible problems in mind. We had to machine the middle node on a four-axis CNC machine to get some of the contours," Brown describes.
All these considerations had to be met for the device to operate reliably in an environment that precludes maintenance. Brown reports that the scientists want to identify a number of gases, such as carbon dioxide, methane, and higher hydrocarbons that appear to form solid ice at depth because of both temperatures and pressures. Raman spectroscopy enables study of these substances as gases, in dissolved form, or incorporated in clathrates. The tools also analyze minerals and other solid targets, including sulfides, anhydrite, calcium carbonates, silicates, feldspars, magnetite, and hematite.
Circle 231--MBARI, or connect directly at www.rsleads.com/509df-231
Circle 232--SolidWorks Corp., or connect directly at www.rsleads.com/509df-232
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|Date:||Sep 1, 2005|
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