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Tech Briefs
The automotive industry has responded by setting its own sound-testing requirements for vehicles and demanding the same from their component suppliers. One such company is Meritor Automotive, a supplier of automotive systems and components to the major automotive manufacturers. It worked with Rockwell Automation Drive Systems to install a new, state-of-the-art, hemi anechoic test chamber for automotive component noise and vibration testing. Meritor Automotive tests and manufactures components for medium- and heavy-duty trucks, light vehicles, trailers, off-highway equipment, and specialty vehicles including military, coach, bus, fire, and rescue vehicles. Components manufactured at Meritor include axles, brakes, anti-lock braking systems, transmissions, window regulators, seat recliner mechanisms, clutches, and drivelines. Testing at Meritor focuses on durability and sound and vibration testing of vehicle components. In 1996, Meritor, then a division of Rockwell International, decided to add more advanced testing capabilities to its facility. Meritor's existing testing equipment consisted of dynamometers intended mainly for durability testing, rather than sound and vibration. As a result, the equipment was located in open rooms, allowing outside noise to interfere with sound and vibration measurements. "We used our existing equipment for sound and vibration testing, but it was very inefficient in terms of time and effort," explained Phil Kittredge, Chief Engineer, Experimental Mechanics at Meritor Automotive. "With each test, we spent extra time and equipment separating the outside noise interference from the actual noise being emitted from the component. And even then, we couldn't be sure that our final representation was completely accurate." Meritor worked with Rockwell Automation Drive Systems to increase the capabilities of the current testing system. The result was the addition of a hemi anechoic chamber integrated with the most advanced dynamometer system available. "The new test chamber is more than just a quiet room for testing," said Kittredge. "It's a quiet room uniquely integrated with a dynamometer that supports noise evaluations using both sound pressure and acoustic intensity techniques. That means our tests are both simpler to complete and represent more accurate data." Built by Industrial Acoustic Corp. of Bronx, NY, the hemi anechoic chamber has internal dimensions of 7.9 x 6.7 x 4.6 m (26 x 22 x 15 ft) and incorporates an advanced sound-absorbing perforated metal wedge system that is impact and fire resistant. The wedges absorb sound waves to eliminate sound reflection from the walls or ceiling of the chamber. The unique electric dynamometer control system integrated within the physical chamber uses three 373-kW (500-hp) Reliance Electric ac motor dynamometers and the Reliance Electric AutoMax Distributed Power System for digital drive control. The dynamometer control system provides precise and automatic control of test parameters such as load, speed, and acceleration and deceleration rates, resulting in more efficient data collection. To simplify operation of the integrated dynamometer, the Reliance Electric SIGMA control and management system was installed for parameter control. State-of-the-art MMI graphic displays provide real-time parameter information to operators and minimizes programming time. The complex, three-motor configuration required the coordination provided by a drive system. To simulate all driving environments, one motor powers the simulated engine, while the other two power the simulated wheels. For example, when simulating cornering, one wheel is turning faster than the other, producing different levels of torque that require advanced system coordination. "The dynamometer control system allows us to pre-program test parameters so that the system will automatically run through the test cycle—something the previous equipment did not allow us to do," said Kittredge. "This capability allows us to have better control of the parameters and allows us to repeat tests in a matter of minutes. Previously, it took several hours just to conduct the test and find ways to compensate for background noise." The new chamber now allows Meritor to complete a test that simulates how a transmission would act in an actual vehicle situation, in terms of sound emission. A demanding application requiring a ramp-up speed from 1000 to 2400 rpm in five seconds, the new automated system allows operators to simply load speed, acceleration, and torque conditions and collect and analyze data in a matter of minutes, rather than hours. The data gathered from chamber testing helps Meritor to optimize component designs for noise during and after development. During development, engineers perform specific noise source identification and noise source ranking to accurately determine where noise is originating. As development proceeds, engineers use the system to record sound intensity maps, which help determine how the surface of the component radiates sound. In testing a transmission within a housing or case, recording a sound intensity map pinpoints where on the transmission case the most noise is being emitted. If noise is shown displayed at the top of the case, the maps can further determine whether it was the actual case emitting noise—which would require simple modification to stop the motion—or whether the noise was being generated from a gear or bearing underneath the case. One year after testing began in the chamber at Meritor, a total of 36 different components have been tested for sound and vibration quality. Meritor's previous testing record had only reached 10 to 12 components per year. The company also uses the testing chamber to verify that suppliers' products meet its own sound emission standards. Jean L. Broge AEI May 2000 |
