Electric Boat Corporation, Groton, CT, is evaluating laser scanning to capture shipboard as-built conditions – the company hopes to use laser scanning on the SSGN program to validate the redesign of Trident-class submarines, among other applications. In last week’s article we read about how EB used laser scanning on several field projects to capture and document as-built conditions using 3D laser scanning technology to perform repair work. This article follows the company’s activities in the Ship Check Data Capture Project, a cost-share project awarded by the National Shipbuilding Research Program (NSRP) to deliver a process to capture ship check data in digital format, process the digital data, and create and/or validate 3D CAD models from the data captured in digital format, cost-effectively.
The project goals are to evaluate two key hardware technologies – laser scanning and digital photogrammetry; evaluate various data processing software products for accuracy of the data collected, costs to create models, ease of use, and time studies; determine potential synergies between the two technologies that would produce cost-effective solutions; and prototype a process for ship check data capture to be used by the U.S. shipbuilding industry.
The project was awarded January 28, 2005. The POP (period of performance) runs through January 31, 2006. Ken Peters, Integrated Product Development Environment (IPDE) Operations Manager at Electric Boat, is the Project Manager. Raj Thiyagarajan, engineering specialist with EB, is the Project Technical Lead. Program technical representative (PTR) is Ron Wood of Northrop Grumman Ship Systems. Project participants include Electric Boat (lead); Intergraph Corporation; Gulf Coast Region Maritime Technology Center (University of New Orleans); Direct Dimensions, Inc., supported by Photo Measurements Solutions, Inc.; Woolpert, Inc., supported by Leica Geosystems HDS, LLC and Visi Image, Inc./BitWyse Solutions, Inc.; and Ship Shaper, LLP, supported by FARO/iQvolution. geovisit();
TWR ship check
The first ship check conducted for the project was of a TWR, or torpedo weapons retriever – a ship used to locate and retrieve torpedoes and missile drones. The ship check was conducted on board the TWR-841 berthed in Newport, RI on May 2-5, 2005. Data capture was completed for the ship’s engine room, main deck aft, and pilot house.
Phase-based scanners used in the TWR ship check included the FARO iQsun880, Leica HDS4500, and Visi Image 3Dguru. Time per scan ranged from 4.5 minutes with iQsun and HDS4500, to 8 minutes with 3Dguru. Each scanner collected 200,000 to 500,000 points/second (approximately 28 million points per scan). All are portable, weighing between 20 and 35 pounds. Several scans were needed from different locations to get line-of-sight coverage of scene details and a complete 3D model of the space. Coverage was 360deg horizontal and ranged from 270deg to 320deg vertical (per scan) depending on scanner. A total station was used to survey and to register the scans to global coordinates.
“You can see how congested this space is,” Thiyagarajan notes of the engine room. “We didn’t use control points for capturing the data. We wanted to create 3D models using feature-based registration.” On the main deck aft, by contrast, “we used targets and total stations to survey the scene and used control points to create models in this space.”
The figure above shows the ease and safety of being able to take measurements from the scan data. Thiyagarajan notes, “It’s going to be very difficult for a human being to go there and take measurements.”
TWR pilot house scans
A Minolta Vivid9i Digitizer was used “only in the pilot house, just to check data capture” using this technology inside ship’s space. This non-contact 3D digitizer requires less than one minute per scan, Thiyagarajan reports, but the field of view is small – 36in. x 36in. (640 x 480 pixels of 3D data/color). Because of this, approximately 100 scans were needed to capture the inside of the pilot house – this took about 2-1/2 hours.
Data analysis is in progress, according to Thiyagarajan. Data processing and modeling is being done with various software applications including PolyWorks, Geomagic Studio, RapidForm, ImageWare, BitWyse Solutions – LASERGen, and Cyclone 5.2. The resulting models will be tested in various CAD systems – CATIA, AutoCAD, Tribon, UG NX and Pro/ENGINEER. Of special interest to EB is the amount of time required for post-processing – EB is conducting time studies to quantify this, Thiyagarajan reports, and will include this information in its final project report. geovisit();
Close-range digital photogrammetry – evaluating synergy with laser scanning
Close-range digital photogrammetry was used in order to evaluate its synergy with 3D laser scanning. This process uses a 14mp digital camera (hand-held), a National Institute of Standards and Technology (NIST) traceable scale crossbar (1 meter), automatically recognizable targets, FotoG software from Vexcel, and a laptop PC. Data post-processing begins by linking the photos using FotoG. AutoCAD is then used to create a 3D model from the linked FotoG photos. The process has several benefits, according to Thiyagarajan. “The camera is small, compact and inexpensive. Accuracy of point locations is within thousandths of an inch. Data collection occurs in a matter of minutes.” Image processing is rapid. And the data can be translated into a CAD wireframe model – digitizing images in the CAD system, then overlaying the CAD model on photos to ensure completeness. Data processing and creation of the AutoCAD wireframe model required about a week, according to Thiyagarajan.
What were Electric Boat’s findings in its TWR investigation? First, 3D laser scanning technology “requires optimum lighting conditions – sunlight needs to be blocked and fluorescent lights need to be covered.” Also, “shiny objects do not provide accurate point data.” On the other hand, platform movement was not a problem – “laser scans are not affected when the ship is in still water with minimum roll.” Digital photogrammetry, for its part, needs “proper lighting for good picture quality, a steady hand and skill in taking sequential photos, and an experienced 3D AutoCAD designer (to create 3D models).” Efforts required and benefits gained from each system from the TWR ship check will be published in a report to NSRP later this month.
EB’s preliminary conclusion is that 3D laser scanning offers potential benefits in several major areas of shipyard operations – overhaul and repair, ship alterations, new construction, and facilities redesign. According to Thiyagarajan, potential benefits include:
- Reduction of costly design changes and improved design capability.
- Reduced construction rework, thanks in part to the ability to accurately factory-fabricate in lieu of field-fabricate.
- Reduced ship check costs – “fewer days, fewer personnel, and only one ship check.”
- Elimination of return visits to the ship for missed measurements.
- Ability to easily and safely obtain measurements that are difficult or unsafe for a human to reach.
- Creation of as-built data models and validation of as-built models to design.
But laser scanning is not a panacea. “Traditional ship checks are still needed for measurements which cannot be obtained by these technologies,” says Thiyagarajan. Also, “large scan data files need extensive manual effort using several software applications to create CAD models.” Needed is a more automated way to merge multiple scans and create 3D CAD models based on scan data.
What about close-range photogrammetry? “Potential synergy exists between laser scanning technology and the FotoG process during ship check data capture,” Thiyagarajan reports. In particular, if laser scan data does not yield precise information about the edges of captured objects, a wireframe model produced from photogrammetric data may be able to be used to resolve the question. In addition, photogrammetry may be able to be used to capture areas where laser scanning can not be used due to the area being more congested.
Now EB’s evaluation has progressed to a Trident submarine (SSGN 729) ship check. Laser scanning was carried out June 13-17, 2005 in Norfolk, VA. The SSGN 729 ship check data was captured using FARO/iQvolution, Leica Geosystems and Visi Image 3Dguru laser scanners, and Minolta digitizer. Stay tuned – we’ll report EB’s findings as soon as they become available. geovisit();