Photogrammetry Collects Precision 3D Data In Unlikely Places

Remember when computing technology was changing so fast that your new computer was almost antiquated in just a few months? Digital photography and imaging technology is following a similar pace, with cutting-edge developments rolling off the assembly line in record time. What does this have to do with industrial measurement, you ask? Plenty.

Article From: 2/23/2006 Modern Machine Shop

Remember when computing technology was changing so fast that your new computer was almost antiquated in just a few months? Digital photography and imaging technology is following a similar pace, with cutting-edge developments rolling off the assembly line in record time. What does this have to do with industrial measurement, you ask? Plenty.

Photogrammetry Basics

Metrologists, engineers, surveyors, manufacturers, scientists and quality control technicians use all types of measurement technologies for their own particular disciplines. Photogrammetry is a three-dimensional coordinate measuring technique that uses photographs as the fundamental medium for metrology. For more than 30 years, civil engineers and surveyors have used photogrammetry for measuring and creating large 3D topological mapping systems. Industrial engineers use close-range photogrammetry for in-place measurement, tooling inspection and adjustment, test-retest measurement, deformation studies, part inspection, tamoana (disassembly and reassembly) studies and more. The technology is used worldwide by diverse industries such as aerospace, automotive, shipbuilding, power generation, construction and more.

Triangulation is the fundamental principle used by photogrammetry. By taking photographs from at least two different locations, so-called "lines of sight" can be developed from each camera to points on the object. These lines of sight, sometimes called rays because of their optical nature, are mathematically intersected to produce 3D coordinates of the points of interest.

Not Your Father's Camera

Photogrammetry has made the transition from film to digital imaging-and that is where the innovation starts. One new product is the Inca3 digital camera from Geodetic Systems, Inc. (Melbourne, Florida), which forms the foundation of the V-Stars photogrammetry system. This 8-megapixel camera combines a high-resolution CCD sensor with a compact industrial PC-based computer to capture field measurements with accuracy of better than 0.001 inch at 160 inches in most industrial settings. The portable device integrates numerous innovations, including an integrated strobe, automatic exposure, advanced calibration features and (WIFI) wireless capabilities for unfettered operation with immediate, online connectivity.

The V-Stars industrial measurement system is comprised of a digital camera, a laptop or desktop computer system, interface hardware, software for connecting the camera to the computer, and software for measuring and processing digital images. The entire pack-and-go system (camera and notebook computer) weighs less than 13 pounds.

The camera can operate independently of the computer via hand-held operation or remote control. Digital pictures can be taken and saved to a Compact Flash (CF) memory card, then downloaded to the computer at a later time. The camera can also be connected directly to a computer wirelessly or via a thin, combined power/network cable. In this mode, each photograph taken can be measured immediately after it is taken. Multiple cameras can be operated simultaneously online for real-time measurement of dynamic or static objects. Whether the camera is off-line or online with the computer, its internal PC processes images immediately after each picture is taken, so problems can be detected instantly during the session. The embedded PC can rapidly compress images (typically > 10:1) for higher disk capacity and faster data transfer.

Once photographs are taken, sophisticated image processing software automatically inspects the collected digital pictures and extracts the required three-dimensional data. The data can be aligned into an object 3D coordinate system, used for surface inspection operations, compared to previous measurements or examined further. The multifaceted software has built-in analysis, visualization and statistical tools. The software's graphical interface allows the operator to view points, camera stations, intersection angles, surfaces and more. The user can also view text showing point XYZ coordinate information and analysis results. 3D coordinate data can be exported in a variety of CAD file formats.

Primetime Applications for Photogrammetry

Photogrammetry is best suited for in-place measurement and inspection of large objects in formidable, busy industrial settings where environmental factors such as vibration, movement and extreme temperatures are often found. With digital photogrammetric systems, precise measurements can be captured while the operator is standing on a wavering or unstable floor, or from a lift or scaffolding on the plant floor. Also immune to harsh conditions in outdoor environments such as shipyards and remote space antenna fields, a photogrammetric camera can gather 3D data with laboratory quality and accuracy without the laboratory conditions required by other metrology instruments.

Measuring and recording data from large assemblies (aircraft, automobiles, space hardware) and processes in a production environment is not an easy task. A shop floor or assembly line presents many obstacles for quality control personnel. A typical environment might include workers, tools, equipment, cabling channels and many other obstructions to impede the line of sight. Because of the portability of a camera, operators can shoot field measurements with minimal downtime to others working concurrently on an assembly of an aircraft, automobile or other large assembled object.

To put this process in perspective, picture a real-world photography situation. A photographer arrives on the scene and visually sizes up the subject to be photographed and the environment surrounding the subject. The photographer asks questions such as: will there be line of sight issues? Will trees, buildings or power lines obstruct the view? What location provides the best viewpoint to the subject? Are there areas of the subject that are completely hidden that may require a few shots taken from a lift? Given the visual issues that exist in any given setting, the photographer can always move the camera to the best line of sight and get the optimum photograph. The portability and ease-of-use of a point-and-shoot camera provides endless possibilities for photography, and that is why virtually every household owns and uses a camera to capture snippets of their private lives.

In the same way, photogrammetry transforms the plant floor into a place where metrology adapts to the production line in practical way. With little impact to others working in the same environment, the QC department can use a photogrammetric camera to gather critical coordinate data, perform analysis, break down the assessment and make adjustments . . . within the confines of a real working day. In doing so, QC personnel can bypass all the roadblocks presented by a busy manufacturing environment.

Mobility, speed, CMM accuracy and adaptation to local space restrictions are the strong points of photogrammetry. The entire V-Stars system (camera and notebook computer) can be carried onboard an airplane, or checked as standard baggage to go anywhere in the world. Photogrammetric measurements have been conducted on land, sea (and undersea), air and even in outer space, on objects both larger and smaller than a football field.

Measuring With Photogrammetry

Measuring industrial objects with photogrammetry usually consists of the following steps: 1) planning the measurement; 2) targeting the object; 3) taking pictures; 4) measuring pictures; 5) processing pictures to obtain 3D coordinate data; and 6) analyzing the results. This list is a general guide. Because every measurement project is unique, the content and even the order of the steps given above may be changed to meet project requirements and user preference. For some jobs, the cameraperson will shoot all the photographs first to minimize time on site, then measure them later. Other QC tasks require the operator to shoot and measure each picture immediately after it is taken. An operator may also choose to take and measure a few photographs and process them instantly to get preliminary results that will make measuring the remaining pictures easier. Nonetheless, all the steps listed above are carried out in some fashion on every project.

There are a variety of ways to capture points of interest on an object. Targeting is just one method used in close-range photogrammetry. The photogrammetry system measures special targets made of a 0.1-mm thick (0.004 inch), flat, grayish colored retro-reflective material. This material has several advantages over conventional targets, which are typically a white circle on a black background. The retro-reflective material returns light efficiently to the light source, and it is typically 100 to 1,000 times more efficient at returning light than a white target. The retro-reflective targets resemble highway reflectors, but they are far more efficient.

The photogrammetric camera does not measure the object directly; rather, it measures the center of the retro-reflective target. By placing the target on or in a known relationship to the point of interest, the point can be measured. In some circumstances, it may be difficult to target the point you want to measure directly. Fortunately, many different types of retro-reflective targets have been developed to solve this problem.

When a manufacturer plans ahead and implements strategic target points into a product design, targeting becomes even more efficient. Target points that are specifically designed into the CAD model, then machined directly into the tooling, can be easily tracked by the QC expert. These precisely-made tooling targets are located in bushed holes, or they may be defined by features on the measured object, such as part edges or hole locations that are targeted in some way.

Once an object is targeted, then the camera's low-powered flash illuminates the targets and makes exposure of the targets independent of the ambient light level. This means that the object can be photographed in bright light or in total darkness, and the target exposure will be the same. Therefore, the target and object exposure are largely independent, with target exposure provided by the strobe and object exposure provided by the ambient light. By properly setting the shutter exposure time, the cameraperson can expose the object to the desired level.

Those who are new to photogrammetry often ask how many photographs are needed for an accurate measurement. Because photogrammetry measures by triangulation, in theory, only two photographs are required for a measurement. However, a minimum of four to six photographs is recommended, because that range allows the operator to self-calibrate the camera.

Capturing The Object

While targeting has long been the mainstay in photogrammetry, new developments are giving the quality control professional alternatives for capturing points of interest on an object. A breakthrough was Geodetic's Pro-Spot, a projector-like device that casts thousands of dots upon the measurement surface. Designed with large objects in mind, this new system generates fast, accurate, dense, non-contact measurements on large surfaces such as molds, master models, panels, antennas and other components.

The target projector casts a whole new light on the subject of 3D data collection. Pro-Spot measures thousands of points in less than 1 minute. When used with a single camera, it can be used to measure static objects by taking photos from various locations. When the projector is used in conjunction with two cameras, 3D data collection begins to get interesting. By virtue of the dual-camera synchronization, this system can actually "freeze" the dots on the object and accurately measure moving objects or objects undergoing deformation. The process and expense of targeting is eliminated, as is the need for probes or reflectors. The setup is simple and fast, and the system can be configured for fully automatic measurement.

Another advancement in photogrammetric data collection is the I-Can, a completely self-contained camera canister produced to operate specifically in thermal vacuum environments. The I-Can wraps around the Inca3/V-Stars camera system and keeps it cool with dry nitrogen purge. The instrument's sleek design includes a rigid, optically flat quartz window created to maintain accuracy. This camera configuration consists of built-in heating and flash systems, internal camera roll and a single, thin cable for power and data connections. The system's controller can handle up to three cameras/canisters. With notebook computer operation, two or more I-Cans can obtain immediate, dynamic, real-time measurements.

How Accurate Is Photogrammetry?

The short answer is that photogrammetry is precise and provides accuracies comparable to those achieved by other large-volume, high-accuracy coordinate measurement systems. Typical accuracies are 25 to 50 microns (0.001 inch to 0.002 inch) on a 3-meter (10-foot) object. The long answer is a bit more elaborate, because photogrammetric accuracy depends on several interdependent factors: 1) camera resolution, 2) the size of the measured object, 3) the number of photographs taken and 4) the geometric layout of the camera stations (camera locations) relative to the object and to each other.

Based on these factors, accuracies can vary accordingly. To get higher accuracy, you need some combination of higher resolution, smaller size, more photos and/or wider geometry. However, as a rule of thumb, if you use the Inca3 camera and take six to eight photographs with good geometry, and each photograph sees the entire object, you should obtain accuracies of 25 to 50 microns (0.001 inch to 0.002 inch) on a 15-foot object.

Automated Repeatability Measurements

The V-Stars system features Automated Repeatability Measurement (ARM) software. Geodetic developed this technology with customer BMW Manufacturing Co. (Spartanburg, South Carolina). ARM is composed of advanced 3D data collection and analysis functionality, including automatic data processing. According to the company, the technology has enabled BMW to quickly and accurately study everything from a headlight assembly to the effects of welding on the body structure.

The test retest method used by ARM allows a metrologist to quickly test a tool or even a hand-applied process for repeatability. Tests that were not feasible using other types of CMMs are easily conducted with photogrammetry. The ARM module transforms "targeting" into a huge advantage. Unlike a CMM measurement point, a photogrammetry target stays on the part in the exact same location. This allows the engineer or QC expert to track planar movement and not just hard features such as holes, edges and slots, because the points stay on the part when it is removed and replaced. While ARM is useful for automotive repeatability applications, the system can also be used in a variety of industries and applications such as deformation analysis, part loading studies and assembly operation.

On The Horizon: Feature Targeting

Several manufacturers are currently working on "feature target" benchmarks for specific industrial measurement applications. These studies use a new type of targeted adapter called a feature target, and several configurations are available today. Feature targets work similarly to a regular target adapter, but they are identified via the use of a "coded target". A coded target is comprised of a central dot and a pattern of squares. The patterns are automatically found, identified and measured via photogrammetric image processing techniques. Automated approaches to feature targeting are evolving to fit routine inspection needs in the production line environment, such as the auditing of tooling. Feature targets can be used to eliminate line-of-sight problems and for measuring features that cannot be directly targeted.

Photogrammetry has aptly earned its respected position in the field of metrology. As a result of photogrammetry's portability, QC departments can perform dimensional maintenance and inspection in places where no other metrology instruments can be used. As metrologists and engineers continue to create and expand their current inspection routines, photogrammetry continues to evolve around those customer needs.

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