S & V OBSERVER
Acoustic Testing Facility Raises Plant Capability
John F Impeduglia, BMW Manufacturing Company, Greer, South Carolina
In April 1999, BMW Manufacturing Company in Greer, SC, officially opened its Engineering Analysis Centre. Plant 10.1 (as it is referred to at BMW) is currently building the Z3 Roadster, the Z3 Coupe, the M Roadster, the M Coupe and at the end of this year Plant 10 will Launch the new X5 Sport Activity Vehicle.
The Analysis Centre supports new product integration within our manufacturing processes and confirms, through testing and problem analysis, that the vehicle adheres to the design requirements. The crown jewel of the Analysis Centre and the Structural Dynamics and Acoustics group is the state of the art AWD (all wheel drive) chassis roll dynamometer used for total vehicle noise and vibration testing. The dyno is housed in an "over achieving" hemi-anechoic environment.
The chamber utilises Eckel Industries EMW high transparency, perforated metallic anechoic wedge lining. The chamber's performance exceeded the design criteria low frequency cutoff of 100Hz by a significant margin. This allows test engineers to make precision measurements to below 63 Hz at the low end of the spectrum and to 10,000Hz at the high end. Qualification of the chamber was done in accordance with ISO 3745. The "inverse square law" performance of the chamber (variance from 6dB per doubling of distance) for frequencies from 50 to 10,000 Hz are shown in the accompanying graph.
The interior dimensions (free field) of the chamber are 37ft x 23.5ft x 14ft. The chamber walls and ceiling were constructed of concrete and rest on a 6in. thick steel reinforced isolated floor. Integrated into the floor is the Siemens, AG AWD chassis roll dyno.
The Eckel metallic wedge (EMW) utilises classic "Harvard design" with two 12in. peaks on a 24 x 24in. base frame with an overall depth of 34in. Incorporated into the wedge construction is a "white" acoustically transparent matting between the perforated facing and acoustic fill for positive fibre containment along with uniform colour and appearance. The wedge units are factory finished in white enamel and were mounted to the walls and ceiling using Eckel's track mounting system. For vehicle access to the chamber, two in-swinging wedge "cage doors" and two out swinging sound doors were provided with a clear opening of 10 x 10ft. The "cage doors" incorporate curved radius wedges at their leading edges that mate when the doors are closed providing full depth of treatment coverage across the entire entryway.
A specially designed silenced ventilation system was incorporated into the dihedral corners of the chamber utilising vertical slot wedge concept. The room also uses "pop-up" vents in front and behind the vehicle to provide an adjustable air stream for vehicle cooling; silenced spot cooling and exhaust extraction are also integrated into the floor of the room. The building management control system allows the user to set room temperatures, air stream temperatures and ventilation flow rates. The ability to control room temperature combined with the white finish of the Eckel metallic wedges help make the test chamber comfortable for people working in the chamber.
AWD Chassis Dyno.
As mentioned earlier, the AWD chassis dyno is a Siemens, AG product which utilises 4 independent DC (direct current) machines. The entire chassis roll system is mounted on a seismic mass for isolation from the environment. The system is controlled by the NT-based CATS (Computed Aided Testing and Simulation) software and is capable of individual wheel velocity control (left to right and front to rear differential speed control), total vehicle velocity control (250km/hr max), torque control and road load simulation which utilises vehicle co-efficients and slope control to simulate on-road conditions. The system allows the user to implement feedback which is user defined into the dyno control loop should there be a desire.
The data acquisition hardware is a mix of Mueller BBM and Hewlett Packard VXI. The signal conditioning is from Mueller BBM and consists of charge (AC713), ICP/AC (EL101) and universal (AE781) input cards. The HP VXI is set up for 32 channels (4-1433 cards) plus 16 "slow channels" for temperature or other quasi DC signals The software is the Mueller BBM PAK which also is used for processing the data. This combination of hardware and software has proven to be extremely robust and user friendly. The User Interface. When stepping up to the control console of the acoustic test chamber one sees approximately the same system that is used in Munich (slightly updated). Controls for the three basic systems (building management, road load simulator and data acquisition) are at the fingertips of the operator. There are two closed circuit TV cameras showing the wide view of the hemi-anechoic room and an under vehicle zoom in camera which can be aimed with a joy stick. Large digital displays show engine RPM, coolant temperature engine oil temperature, and 2 exhaust temperatures (more sensors could be added). A 40 channel BNC connection panel and an HP54602B oscilloscope are flush mounted on the console - this allows monitoring of the analogue signals exiting the signal conditioning box. In short, test stand operators are able to produce accurate, repeatable sound pressure and acceleration measurements on BMW vehicles.
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