This article describes use of photoacoustics by automakers to track and tackle leakage problems. Photoacoustics uses momentary heat from a light source to excite acoustic waves. Laser Imaging Systems of Punta Gorda, FL, developed the idea of photoacoustic leak detection. Dowling worked with Ford in developing leak localization as part of a team that was put together by the National Center for Manufacturing Sciences. Once a leak is sensed, a technique originally developed for a US Navy underwater acoustics program deploys to pinpoint its place. Vertical location is known already. Vacuum Instrument and Laser Imaging Systems are building the alpha prototype now.
Dave gravel, a senior technical specialist at Ford Advanced Manufacturing in Detroit, has it in for leaks. Not big leaks so much as small ones—very small. “We’re talking gas leaks that take a week to fill a thimble,” he said.
Tracking down low-level leaks in auto air conditioners is a matter of customer satisfaction, Gravel said. But given the tight leakage allowance that the company places on air conditioner components—about an ounce or less a year—even a leaky condenser, evaporator, or compressor that passes muster would keep drivers and passengers comfy for five years before anyone noticed a difference.
Current helium-spectroscopy leak detection at Ford tests helium-charged parts in a vacuum chamber, Gravel said. A mass spectrometer detects any helium molecules escaping from the part.
Ford sought a way to take some cost out of its current helium-spectroscopy leak detection, and possibly rid the process of cantankerous vacuum pumps. According to Gravel, it looks as if Ford will be getting that, and will not only detect leaks, but also be able to pinpoint where they are as part of the bargain. But, first, the company had to call upon the work of Alexander Graham Bell himself and his 1880 discovery of photoacoustics.
Photoacoustics, explained mechanical engineering professor David Dowling of the University of Michigan, uses momentary heat from a light source to excite acoustic waves. When light from a C02 laser is shined momentarily on a cloud of sulfur hexafluoride gas, the gas is heated instantaneously, Dowling said. Expanding rapidly from the heating, the gas emits an acoustic wave.
The gas and laser interact in this way because of a coincidental match between the quantum mechanical vibration states of the large SF6 molecules and the output line of the C02 laser, Dowling said. “A photon at 10.6 microns matches the vibration states of SF6 perfectly.”
Molecular activity in SF6 is increased by shining a C02 laser on the gas.
Putting Together a Team
Laser Imaging Systems of Punta Gorda, Fla., developed the idea of photoacoustic leak detection. Dowling worked with Ford in developing leak localization as part of a team that was put together by the National Center for Manufacturing Sciences.
According to Constance Philips, a program manager at the center, other companies are participating in the consortium along with Ford. Consortium members include Ford, DaimlerChrysler Corp., Vacuum Instrument Corp., and Laser Imaging Systems. Contracting partners to the consortium include Honeywell Federal Manufacturing & Technologies, Argonne National Laboratory, and the University of Michigan. Industry partners and the Department of Defense fund the project.
Dowling said the first step in the process is to charge the part under test with SF6 gas. Then a laser, directed through a rotating prismatic mirror, scans its narrow beam across the part.
“The laser beam scans from left to right and we move the parts from the floor to the ceiling,” Dowling said.
The invisible C02 laser beam, combined with a He-Ne laser for visibility, bounces off the 20 sides of a polygonal mirror, which spins at 18,750 rpm. The beam scans the part 6,250 times a second, while a stepper motor moves the part vertically.
“We paint the part completely,” he said. “There’s no way for a leak to escape us unless it finds a way to be on an interior surface that is not illuminated by the laser.”
The process is one of leak detection followed by leak location. When the laser beam illuminates a tracer-gas cloud formed near a leak, the rapid heating of the gas sends off an acoustic wave at the scan-rate frequency and its harmonics. Sensitive microphones measure the sound wave.
“We know what frequency the laser scans over the part,” Dowling said. “The sound appears at frequencies of 6.25, 12.5, 18.75 kHz, and higher multiples. We might measure harmonics up past 10 or 12, depending on how loud the leak is.”
Fast Fourier Transform analysis divides the wave into its constituent frequencies, which then can be compared against a measure of background noise taken at the fundamental scan-rate frequency (6.25 kHz) and its harmonics. Any test harmonic that has an amplitude two or more times the baseline harmonic flags a leak, Dowling explained.
Measuring background noise is an important step in the process, Dowling said. It can include noise from the equipment. The prismatic mirror, for instance, hisses as it spins within its housing.
Merely shining the laser on the test part, even one having no leaks, makes noise, Dowling said. “Run the laser over anything and it either catches fire or makes a little sound,” he said.
“We’re testing mainly brushed aluminum. The hot C02 laser passing over brushed aluminum makes a bit of photoacoustic sound. We have to quantify the sound before we test for SF6,” Dowling said.
Once a leak is sensed, a technique originally developed for a U.S. Navy underwater acoustics program deploys to pinpoint its place. Vertical location is known already, Dowling said, based on the whereabouts of the part as the stepper motor pushes it past the laser. But knowing horizontal location—where the leak is along a single line of the laser’s sweep—takes matched field processing.
Determining Leak Location
“Matched field processing allows you to determine the location of a leak just from the sound that you hear,” Dowling said. Four microphones listen to the acoustic wave from the expanding gas. Knowing the location of the microphones and test piece, a computer can simulate the acoustic environment in software that Dowling and his student Serdar Yönak developed.
“The computer takes the sound that the microphones hear and plays it backward into a computational model of the acoustic environment,” Dowling said. “By playing the sound backward, we reverse time and calculate an acoustic retrofocus at the location of the leak.”
There is virtually no limit to the size of a part that might be tested in this way, Dowling said. “We once got an e-mail message from the people who service the Goodyear blimp. Could we scale up the technology to detect leaks on their craft?”
Size was not the limitation there, he explained, but rather the fact that SF6, being heavier than air, could not support a lighter-than-air craft.
In addition to testing for leaks on a flat aluminum plate, they’ve looked at other shapes, Dowling said. Cylinders, for instance. “We even placed a calibrated leak in an automotive heat exchanger,” lending an air of practicality to the test.
According to Ford’s Gravel, leak location was an important goal of the project. So was lowering cycle time. “I’m going to achieve the same sensitivity to leaks that helium mass spectrometry provides, for about one-third the cost, without troublesome vacuum pumps to maintain,” he said. “And I get leak location out of the deal.” Cycle rates that now last as much as a minute will drop to about 10 seconds, he added.
Gravel explained why the new test would be an improvement over the mass spectrometry Ford is using today. Mass spectrometry needs a vacuum chamber, he said, with attendant vacuum pumps. The pumps demand regular servicing.
“The big problem with the way auto manufacturers test air conditioning systems is that we test the subcomponents and get go/no-go information from them,” he said. “Then we connect them all together without testing the worthiness of the connections.” Once the system is dropped into the car, the problem gets worse.
“You can’t easily stick a whole car inside a mass spectrometry chamber and look for leaks on your air conditioner,” Gravel said, “because of weird things that you wouldn’t think of. When you put tires into a vacuum, they out-gas. At those low pressures, things that normally wouldn’t turn into a vapor, do.”
So atop the list of advantages for photoacoustic leak detection goes testing in ambient conditions.
Pinpointing leaks on components makes repair possible, Gravel said. “A mass spectrometer is go/no-go. With leak location, you can go back to processes that could possibly influence the leak’s existence and try to correct them upstream.” Knowing the location of a leak makes repairing the component possible, too.
Taking the Next Step
Vacuum Instrument and Laser Imaging Systems are building the alpha prototype now, Gravel said. Although the prototype cannot test whole systems, Gravel said that is a future goal. If trials succeed, the next step would be to build a production prototype.
The prototype will be tested at the Detroit location where Gravel works. DaimlerChrysler and Honeywell FM&T will test it at their labs as well, he said. “After that, I think if it survives our scrutiny, we’ll pilot it at one of our plants.” From there, he thought the machine would go to work at Ford’s Connersville, Ind., plant.
By the way, leak testing probably wasn’t the application that Alexander Graham Bell had in mind for photoacoustics. “We almost had photoacoustic telephones at the beginning,” Dowling said.