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by Mike Richman

The successful use of 3-D technology in working with large objects in a manufacturing environment often has less to do with “measuring” and more to do with the location and alignment of parts and fixtures. A case in point is the recent implementation of GPS technology into 3-D measurement.

Mention the term “GPS” to the average person and most will associate it with small devices that read satellite signals and can provide surveillance or driving directions. However, an exciting new system known as indoor GPS is now being used by many manufacturers to perform large-scale measuring and alignment tasks. Indoor GPS doesn’t use satellites; instead, it works by installing an array of infrared laser-pulse transmitters around the object to be measured. These objects are generally large manufactured items such as parts and assemblies for aircraft, automobiles or ships. Sensors pick up the signals from the transmitters, and calculate angle and position based on the timing of the arriving light pulses. An amplifier converts the analog signals into digital pulses, and a receiver converts the pulses into angle data. Indoor GPS network software processes the angle data into highly accurate position information and then makes this information available to the client network, whether on the shop floor or off-site. Because the number of transmitters that can be used is practically unlimited, the size and shape of the work area can be fully defined by the user. For large assemblies, more transmitters means that measurement, inspection and alignment can be performed accurately and without having to move or reset equipment. This flexibility is the key to the utility of this technology.

One of the pioneers of this developing technology is ArcSecond Inc. of Dulles, Virginia. ArcSecond began developing positioning systems in 1990, with the intention of using satellite-based GPS for outdoor construction and civil engineering applications. When that concept, based on radio signals, proved problematic (due to general inaccuracies as well as line-of-sight issues), ArcSecond began to experiment with signals based on laser light, which were optimally employed in interior factory environments. Today, ArcSecond’s indoor GPS system is used in a variety of heavy industrial markets, including automotive, shipbuilding, robotics and, particularly, aerospace.

“There isn’t a new airplane program that is not either using, planning on using or investigating using indoor GPS,” says ArcSecond President Edward R. Barrientos.

Indoor GPS in action
Aerospace manufacturers spend an enormous amount of money on tooling, especially on the jigs that are necessary for their unique manufacturing requirements. One major aerospace manufacturer is reputed to spend upwards of $1 billion on jig construction and maintenance annually. The manufacturers are trying to reduce the costs for these gigantic and precisely built jigs, which enable the accurate alignment of parts such as wings and fuselages. In addition to their fantastic expense, these jigs are inflexible. Once a jig is built for a specific process, there isn’t much that can be done to alter the jig or the process that relies upon it--other than creating an entirely new jig.

Indoor GPS can provide cost savings in the millions of dollars for large manufacturers by helping them move away from jigs and toward a virtual alignment process in which parts are precisely located and mated within the factory. Sensors can be placed on a wing, for example, and the movement of that wing can be precisely tracked as it’s slowly being aligned with the fuselage that it will be riveted to.

“Much like you can track your car moving outdoors on the planet’s surface using satellite-based GPS, now you can track the motion of that wing in real time, to very high levels of accuracy within the factory,” says Barrientos. “At the same time, you’re doing the exact same thing with the fuselage.”

The next step is to track these two moving bodies using a real-time CAD representation. The two parts are graphically joined together on the computer screen, and adjustments are made throughout the process to ensure accuracy. The manufacturer still requires a platform to perform the actual joining and riveting, but the indoor GPS system will alert the user if parts are out of alignment to even the slightest degree before the first connection is completed. It’s a subtle but revolutionary change in the process that enables manufacturers to significantly reduce their tooling investment, while ensuring increased production quality.

Another big benefit is having the ability to make modifications. The indoor GPS system doesn’t care if the wing is a different shape or an alternative model--you simply place the sensors on the wing and you’re ready to do the actual manufacturing, using robotic tooling systems or autonomously guided vehicles to bring parts together. Increasingly, the fantastic expense associated with precise tooling elements such as jigs is being eliminated from the process. Best of all, indoor GPS works for practically any manufacturing application in which parts are measured, aligned and/or joined.

Improving 3-D measurement and manufacturing technology
3-D measurement devices such as laser trackers and mechanical arms, or systems based on photogrammetry, can benefit when equipped with indoor GPS sensors. By providing a universal coordinate system for these devices, indoor GPS can minimize the risk of errors from “leapfrogging”--the process whereby measurement equipment is moved around a stationary target. Indoor GPS provides the same universal coordinate system to production equipment such as robots, laser projectors and tooling fixtures--and it can report the position of work pieces in the same coordinate system. The user can also measure multiple reference frames at the same time, meaning that key features of a part can be measured in the part coordinate system, while also reporting the location of the part in the universal coordinate system of the entire factory floor. This allows the user to know the location of items in the factory, while also determining whether measurements of a specific part conform to a given tolerance.

Indoor GPS has certain advantages over other 3-D measurement technologies. For example, for certain manufacturing environments, it is more cost-effective and rugged. It also offers superior operating ranges, with accuracies in the range of 70-100 µm. “In working with volumes larger than those found in a 10-meter box, we consider indoor GPS to be the most accurate measurement system available anywhere,” states Barrientos. “Another key advantage of indoor GPS is our 360-degree coverage. We have the ability to measure around an object such as a car with only one setup, without needing to move the instruments like laser trackers or photogrammetry systems need to after they measure one side of the car. This 360-degree measurement capability reduces or eliminates the need to leapfrog.”

In comparison to indoor GPS, laser trackers are superior at close range, with accuracies of better than 10 µm (typically inside of a 3 x 3 m work volume) in some cases. There is no question that for 3-D measurement on a small scale, laser trackers, mechanical arms and photogrammetry systems are outstanding choices for most projects. However, indoor GPS is an option to consider as well, particularly if the technology can be incorporated with other 3-D measurement systems in a “best of both worlds” scenario.

What’s next?
The future of GPS technology is bursting with metrological possibilities. Later this year, ArcSecond will introduce a new generation of indoor GPS systems, providing even greater accuracy, ease of use and functionality. Further down the line, expect the miniaturization of sensors and electronics, as well as the seamless integration of wireless capability at the sensor level. The integration of GPS technology into devices such as robots, augmented reality headsets and autonomously guided vehicles will have positive repercussions for the manufacturing world in reducing costs, increasing efficiency and improving quality.

According to Barrientos, within 20 years indoor GPS has the potential to become a ubiquitous technology, not only in manufacturing, but also in the office and home. It may sound like science fiction, but by 2025 tiny, advanced, humanoid-type robots, both industrial and consumer, will always “know” where they are and will easily interface with people, their environment and other machines. Humans using augmented or virtual reality headsets with GPS sensors will interact with the cyberworld in new and unimaginable ways. Services such as today’s On-Star system from General Motors will emerge that combine position information, the Internet and wireless communication. Indoor GPS will be equally well-connected and bring new and exciting hybrid services and products to the manufacturing, assembly and robotics industries. The revolutionary developments may just be beginning.

About the author
Mike Richman is Quality Digest’s managing editor.