3-D Industrial Metrology

June 1, 2000
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POB’s editorial mission states that it is edited “to help the progressive surveying and mapping professional succeed.” How does industrial metrology fit into the realm of progressive surveying and mapping? Better yet, what is industrial metrology anyway?

In order to show how industrial metrology can serve the progressive surveyor and mapping professional, one should start with the definition of metrology. Metrology, simply put, is the science of weights and measures. Its origins are from the Greek words metron (to measure), logy (the science of), and metrologia (the theory of ratios). Industrial metrology is a specific phrase used to denote making spatial dimensional measurements. When you think about it, that is exactly what surveyors and mapping professionals do. They make spatial dimensional measurements, often in three dimensions (3-D). There is indeed a link.

This article will explore the state of the art in the technologies applied to solve today’s industrial measurement problems quickly, accurately, safely and cost effectively. Much benefit can be reaped from applying the right technologies the right ways in the right work environments.

Tools of the Trade

The tools used to perform 3-D industrial measurements continue to evolve as advances in technologies, such as computer computation speed, electronics and optics, improve. Significant developments have occurred in recent years and the benefits are beginning to manifest themselves in tangible ways, such as major cost savings on large and complicated projects. This article will address three such tools: high-accuracy survey instruments, digital photogrammetry and laser scanning (more appropriately called LIDAR, which stands for Light Detection and Ranging).

High-Accuracy Survey Instruments

High-accuracy total stations are used in industrial metrology. Some of the instruments, such as Sokkia’s NET2100 (Sokkia Corp., Overland Park, Kan.), are calibrated and traceable to NIST (National Institute of Standards Technology) measurement standards. They have highly accurate vertical and horizontal encoders as well as highly accurate laser distance measurement hardware. They require that the points of interest to be measured be defined with retro-reflective targets. The stated accuracy of the measurements is about 0.8 mm (±0.032"), but using NIST traceable scale bars (documented to ± 0.0005") measurements on the order of ± 0.002" at 50' can be achieved.

High-accuracy survey instruments can be used for stand-alone measurement projects, such as measuring piping fit up surfaces in nuclear environments, but are more often used to establish high quality control to be used in conjunction with other techniques such as photogrammetry and laser scanning. Why would anyone want to use such an instrument for a control survey? In order to ensure the integrity of the final data set, which is often composed of many different data sets registered together through the control network, it is good practice to establish control with a method that is at least an order of magnitude better than the primary method to be used.

Digital Photogrammetry

Digital photogrammetry, a technique that allows the extraction of measurement information from calibrated photographs, comes in a variety of “flavors” to meet different industrial applications. Some systems, such as Geodetic Services’ VSTARS and INCA products (Geodetic Services, Melbourne, Fla.), are very high-accuracy and yield NIST traceable measurements. These systems require that the objects and features to be measured are targeted appropriately. If it’s not targeted, it’s not measured. These systems are typically used in the automotive, aerospace and nuclear industries where measurement requirements are critical. The camera and optical systems used are highly calibrated to map out measurement errors induced by the optics and image sensor.

Other systems, such as Vexcel’s Foto-G product (Vexcel, Boulder, Colo.), utilize some level of control but allow dimensions to be extracted by identifying the same points in multiple images. This system is typically used to support projects in a plant/process environment. As with the Geodetic Services’ systems, calibration of the camera and optical systems optimizes the accuracy of the final measurements.

Laser Scanning

One of the more interesting new technologies is laser scanning. The concept behind a laser scanner is very similar to that of a total station with a couple of important exceptions. First, no targets are required. As long as the system can get some level of reflection from a surface (typically greater than 10 percent), a measurement can be made. Second, since no targets are required, an operator does not have to site the instrument. As a result, the data collection process can be automated. Data acquisition rates of 100,000 3-D coordinates per second have been achieved with some systems.

Although the end result is the generation of 3-D coordinate points, the methods used to get them can differ. Cyra Technologies’ CYRAX system (Cyra, Oakland, Calif.), for example, bases its measurements on the time of flight of a pulsed laser. Mensi’s SOISIC system (Mensi, Norcross, Ga.) bases its measurements on a triangulation technique. MetricVision’s 100B system (MetricVision, Newington, Va.) makes measurements based on the phase of a frequency modulated laser. Each technique has its advantages and disadvantages.

Real World Applications

To provide a taste for some of the potential applications for these techniques, a few real world examples are provided.

Military Transport Aircraft

A combination of photogrammetric techniques (VSTARS and Vexcel) in conjunction with high-accuracy surveying was performed on three transport aircraft. The purpose was to accurately (better than ± 0.125") as-build the interiors of the aircraft to assist with loading simulations as well as validating the ability to load virtual models of new equipment prior to manufacturing.

The 3-D models were constructed in MicroStation (Bentley Systems Inc., Exton, Pa.) and exported in IGES format for use in Pro/Engineer (Parametric Technology, Waltham, Mass.) and CADSI’s DADS system (LMS International, Leuven, Belgium). The as-built models, which took the place of simple rectangular box models, are used in a variety of loading simulation activities.

Quarry Survey

Laser scanning using a long range system was performed for Clanton Marble (Florence, Ala). The scanner was relatively low accuracy (± 1") but had a range of 1,150' and was capable of scanning a 360-degree field of view. The quarry was 3-D surveyed in approximately 30 minutes with several smaller scans used to supplement areas that were visually obscured by terrain.

The point cloud data obtained with the scanner was read into Carlson Survey (Carlson Software, Maysville, Ky.) where contour maps and wireframe surface models were generated. The wireframe models were exported to other software and converted into solid models for rendering and visualization. Much of the terrain that was scanned would have been very difficult and dangerous to survey by other means. A special viewer was provided to allow a gray-scale encoded 2-D image to be interrogated for picking discrete survey points for applications such as estimating remaining volumes in retention ponds.

Steam Generator Replacement

Using a combination of theodolite networks (Leica ManCAT, Norcross, Ga.), total station survey (Sokkia NET2) and digital photogrammetry (VSTARS), high-accuracy models of existing and replacement steam generators were made for a steam generator replacement project. A custom 3-D simulation program was developed to optimize the fit of the replacement generator to existing facility constraints (thick wall piping, support structure and seismic restraints). Upon completion of the optimization, measurements were made to set automated beveling and welding equipment.

When the generators were installed, the critical piping fit to within ±0.020". The high-accuracy fit allowed automated welding operations to be applied (which reduced personnel radiation exposure) and generation of various shims (usually an iterative trial and error procedure). The overall impact was an outage reduction of approximately 12 shifts valued at about $3.6 million.

3-D industrial metrology is just beginning to make an impact in industry. As the world moves toward 3-D and virtual environments for planning activities and performing simulations, the need for such services and techniques will increase. Many innovations are on the horizon that will make 3-D metrology techniques faster, more accurate and more detailed. How does industrial metrology fit into the realm of progressive surveying and mapping? This overview and these applications may provide some insight into how it can help you.

Sidebar: The Work Process

Before taking 3-D industrial metrology to the field, it behooves one to understand the general work process for creating a useful deliverable. The basic work flow is as follows:
  • A preliminary control survey is performed to establish reference points throughout the work environment.

  • Upon completion of the control survey, the bulk of the field data is collected using one or more measurement techniques.

  • The acquired data is registered (put into a common coordinate system) and transformed (put into a meaningful coordinate system).

  • The volume of data collected can be overwhelming; in fact, the sheer size of the data files can take most CAD systems out of commission. The data collected is typically reduced into geometric primitives and surfaces through specialized intermediate software. This greatly reduces the data set to a manageable size.

  • The reduced data set is exported to a CAD environment for final “clean up,” formatting and deliverable generation.



Sidebar: Advantages of 3-D Industrial Metrology

  • The overall accuracy of a measurement project performed with these types of tools is significantly improved. Cost savings are incurred by significant reductions in rework due to inaccurate measurements.
  • The overall cost associated with acquiring the 3-D data is reduced. In many cases this results in a significant reduction in the project cost. Some hidden savings include getting all of the data the first time out, hence eliminating the need for return trips to the field.

  • Safety can be improved. Laser scanning techniques are non-contact. Thermally and radioactively hot environments can be imaged without sending personnel into hazardous locations. Inaccessible areas, such as side walls in mines and quarries, can be measured without risking survey personnel trying to get targets to areas of interest.

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