DSL 3-D Imaging
3-D imaging is the process of capturing the shape of objects in a representation based on x, y, z surface coordinates. 3-D representations have many uses, including measurements to verify dimensions (including depth) of parts, quantify surface defects or damage and reproduce objects using rapid prototyping systems. In the past, 3-D data was obtained with mechanical touch probes (requiring many hours for even simple parts) or from optical systems costing hundreds of thousands of dollars. Southwest Research Institute has recently developed a unique, economical method for generating 3-D images and surface measurements. Dynamic Structured Light (DSL) 3-D imaging is a non-contact, optical measurement technique that offers many advantages over other commercial methods.
Southwest Research Institute can provide 3-D measurement systems for standard applications as well as custom systems for integration into plant and process applications. Southwest Research Institute also offers measurement services for clients who wish to investigate the capabilities of 3-D imaging or have only occasional needs.
DSL Measurement Systems
Dynamic Structured Light 3-D imaging systems consist of an imaging module, a high-performance computer system and custom signal processing software. The imaging module is composed of a digital video camera and a dynamic illumination source, both rigidly mounted on an extruded beam. The computer receives signals from the video camera, and unique signal analysis software interprets the light and dark sequences at each pixel to determine x, y, and z coordinates of the surface in the camera field-of-view.
DSL 3-D imaging systems are based on calculation of quadric surfaces defined by the rotating pattern of projected grating lines. For each pixel in the camera field of view, the value of a specific quadric surface is determined from signal analysis. The surface coordinates (x, y, z) for each pixel are calculated by taking the intersection of the pixel ray from the camera and the quadric surface. This approach offers many advantages including simple calibration, flexibility and scalability for small or large fields-of-view, and the use of relatively inexpensive components. A more detailed discussion of the operation was presented at SPIE Conference 5265.
Verification and Calibration
DSL 3-D systems are calibrated by imaging several reference planes and calculating equations of the projected quadric surfaces by least-squares analysis. The same procedure can be used to verify that a system is performing within specifications. This technique simplifies custom designs with field-of-view and measurement resolution chosen for specific applications. All DSL 3-D systems include calibration software so that they may be verified and recalibrated in the field if necessary.
The object being measured is kept stationary for a short time (several seconds to several minutes, depending on the accuracy desired) while the DSL patterns are captured from the camera. Calculation of coordinates takes about 15 seconds for a complete camera image of 300,000 points, less if fewer points are required. During this time, the object can be removed from the measurement station and the next object brought in for measurement.
3-D "x, y, z" coordinates are normally provided as a comma-delimited file that can be imported into a variety of computer programs including Excel, MatLab, GeoMagic Studio, Rhino 3-D, and others for analysis and processing. Custom data formats and data analysis programs can be provided.
3-D surface geometries of manufactured parts can be compared to design data to determine if parts meet specifications, to identify differences between batches, or after process changes.
Surface damage can result in irregular depressions, dents or gouges that have adverse functional or cosmetic effects. It is very difficult to quantify such damage without complete 3-D measurements.
Digital models of objects are very useful for computer-aided design and for computer graphic representations. In many cases a design or simulation needs to incorporate complex objects when no digital model is available. DSL 3-D images of complex objects can be generated and converted to 3-D graphic objects with commercial software programs, such as Geomagic Studio.
A high-accuracy model of the DSL 3-D measurement system is available for applications requiring precision measurements over smaller areas. This system is configured for a 2" x 2" field-of-view, 1,000,000 measurement points (1000 x 1000 pixel camera) and 0.001" depth accuracy.
Many applications require imaging larger objects at somewhat relaxed accuracy requirements. The DSL 3-D system is available in a large field-of-view (5'x 4') model, with 0.025" depth accuracy and 300,000 measurement points (640 x 480 pixels). This configuration has been used for quantitative analysis of damage to aircraft surfaces, deformation of automobile body panels, or recording depth patterns in outdoor terrain.
The process of reverse engineering requires developing an accurate 3-D representation of a physical part for analysis, modification or reproduction. The DSL 3-D systems provide dense point clouds that can be processed to produce accurate polygon models of surfaces. These polygon models can be further processed by commercial software to generate STL files for controlling rapid-prototyping machines or CAD format (DXF, IGS, etc.) files for importing into CAD systems for modification.
For more information about DSL 3-D Imaging, contact Dr. Ernest Franke, P.E., Automation and Data Systems Division, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-3678, Fax (210) 522-5499.