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Time Compression Technologies April 2004
RE Firm Lets Freedom Ring by Recreating the Liberty Bell
A reverse engineering firm combines two laser scanning techniques-resulting in a replica of historic proportions.
By Sherry L. Baranek

Thanks to the advent of laser scanning techniques, history can now be recreated with the reproduction of historical objects. Direct Dimensions, Inc. (Owings Mills, MD)-a provider of 3-D measurement solutions and rapid reverse engineering (RE) technology-is one such company that has scanned the Lincoln Memorial, propellers of the Wright Brothers' airplane and the Monumental Church (a 150-year-old monument in Richmond, VA), to name a few. This time they take it to a whole new level by tackling the digital scanning of the Liberty Bell, which is housed at Independence National Historical Park in Philadelphia. The digitized image will be used by a centuries-old French foundry that will replicate the bell for a 60th anniversary celebration of the Allied invasion of Normandy, otherwise known as D-Day, which resulted in the liberation of Europe in World War II. The new bell-called the Normandy Liberty Bell-will be a centerpiece of the event.

According to Direct Dimensions president and founder Michael Raphael, the project came as a referral from FARO Technologies (Lake Mary, FL)-a technology provider and manufacturer of the portable CMM digitizing arm used by Direct Dimensions. "Luigi and Paul Bergamo-the owners of the Cornille Havard Bell Foundry-realized that in order to recreate the Liberty Bell they needed accurate dimensional information from the actual Liberty Bell," Raphael recalls. "They contacted FARO, who referred them to us."

After discussions with Independence National Historical Park and the Bergamos, the scanning timeline was set. "We are interested in the restoration of art and sculpture and I sensed the Bergamos are interested in preserving history," Raphael states. " I appreciated how important this project was to them and how crucial it was to provide the highest levels of technical competence and authenticity."

Scanning the Surface
Raphael and engineers Sean Frank and Jeff Mechlinski scanned the 1752 Liberty Bell one weeknight in March, after the park closed to the public. "We loaded our van with our FARO arm, a Perceptron laser scanner and a Konica Minolta Vivid 3-D laser scanner," Raphael comments. The chief curator of Independence National Historical Park, Karie Diethorn, and a National Park Service ranger witnessed the scanning process.

Cognizant of the strict time constraints for most projects, Raphael explains that Direct Dimensions has combined a variety of software packages, hardware and techniques into an efficient solution. In this case, the FARO arm was mounted on a mobile stand so it could be positioned next to the Liberty Bell. On the end of the arm-which has a four-foot reach-was the fist-sized laser scanner. The scanner collects thousands of points on the primary X, Y, Z coordinate system for the data and model. Later, this data would be reverse engineered into an accurate surface model of the bell.

"You have to go back and forth and up and down," Raphael emphasizes. "It's as if you were painting. You can tell where you are as you see the computer data appearing onscreen. We had a 3-D point cloud with millions of points of the inside, outside, the clapper, the headstock (wood piece that is above the bell) and the crown.

"Most of the high resolution detail scanning was done with the Perceptron on the FARO arm because it captures high accuracy, high-resolution data that is very reliable," Raphael continues. "We've been doing this for years and have had great success with this system. We were able to record all of the lettering, scratches and imperfections on the Liberty Bell in great detail-including the infamous crack."

The Vivid 3-D laser scanner provided redundancy and overall data coordination. According to Raphael, using both sets of data allows for a check and balance system. "If we missed something from the hand scan, we can pick it up from the Vivid," he states. The Vivid sits on a tripod and passes a three-foot wide red laser beam from top to bottom. In less than five seconds, it captures more than 300,000 points in a three- by four-foot area. As a patch scanner, it is able to accomplish an entire rectangular patch at a time, quickly and reliably, but not quite as accurately, he adds.

Six hours later, the scan was complete. Direct Dimensions walked away with one hundred precise 3-D scanned images from the Minolta and thousands of points from the Perceptron scanner.

Focusing on Details
Within a day, Direct Dimensions performed preliminary processing and sent a basic profile of the bell to the Cornille Havard Bell Foundry so they could begin initial planning work on the mold for the bell. Then, the company spent approximately two weeks performing a more detailed and complete engineering analysis of the data, Raphael says-manipulating the raw data, creating digital models and studying it in great detail.

The bell was recreated without its famous crack.

"We went through the data and analyzed the specific dimensional and geometric characteristics of the bell," he explains, "like its curvature, smoothness, circularity, the concentricity of the inside of the bell compared to the outside of the bell, and its thickness everywhere. All of these characteristics were evaluated mathematically using our computer model. We then engineered the theoretical or design intent shape of the original bell.

"In creating our model, we assumed, for example, a perfectly flat bottom, a perfect axis centerline, that the inside was concentric to the outside and the thickness was consistent around the bell," Raphael adds. Direct Dimensions' engineers then validated their model of the bell by comparing it to the actual Liberty Bell data and quantified the dimensional differences between them using computer-aided-inspection methods developed for other applications.

And-per Luigi and Paul Bergamos' wishes-the company recreated the bell without its famous crack. "We discarded the data from the crack because we knew it was distorted," Raphael says. "The Bergamos want to create a bell that is better than the Liberty Bell-they want to make it perfect-with no cracks or flaws, and perfectly round. Preserving the tonal qualities of the bell is just as important as the dimensional qualities to the Bergamos family. They like to say they are putting the voice back into the Liberty Bell, and we are helping them to do that. They say that much of the sound quality will come from our dimensional analysis and understanding of the Liberty Bell and taking out all of the flaws."

In addition to helping the French celebrate the anniversary of D-Day, Raphael intends to supply the new information about the Liberty Bell to others for scholarly research. "This is one way to fully preserve our rich heritage," he concludes. "I feel privileged to be a part of such a historic endeavor."age," he concludes. "I feel privileged to be a part of such a historic endeavor."