-by M. French and M. Jay

AN INTRODUCTION TO

his article is the first in a series of four describing : ers (suppliers selling directly to the manufacturers are dynamic structural and acoustic testing in the : called tier 1 suppliers, while companies selling to the tier 1 automotive industry. The automotive industry is suppliers are tier 2 suppliers and so on). T one in which vibration and acoustic testing is per- :

formed on a large scale during product development. In : The size of the industry has a significant effect on how it contrast to the aerospace industry, where a ground vibra- : operates. About 15 million vehicles were sold in the US in tion test is a major event involving one or two airframes, : 1997. That’s about 288,000 vehicles sold per week. With automotive dvnamic testing: occupies dozens of test labs on : an average price of approximatelv $20.000. that’s about Y

a full time basis. Dynam- ics, modal analysis, acous- tic testing and the like are usually lumped under the broad category of Noise, Vi- bration and Harshness, or just NVH. The field of sound quality has devel- oped to describe human perceptions of noise and the field of structural vi- brations is ubiquitous. As far as we know, nobody has a good mathematical de- scription of harshness. It is, however, a permanent part of the common ver- nacular.

Wind

~

Engine

Exhaust

Drivetrain

Road

Qure I -Typical noise sources

In this first article, we will try to describe the larger envi- ronment of automotive NVH testing, why it is of such im- portance now and how the industry is balancing the re- quirements for improved NVH performance and for de- creased testing costs. The second article describes some of the details of NVH testing including the types of tests and the facilities built around them. The third article focuses on instrumentation and data processing requirements. Fi- nally, the fourth article describes the lab operations and the business environment.

STATE OFTHE INDUSTRY

“ 1

$300 billion in domestic sales last year. Large manufacturers have an- nual revenues of more than $100 billion and there are many tier 1 suppliers with annual sales of over $1 billion.

The roles of the differ- en t companies have changed greatly in the last ten years or so. The typical automobile manufacturer now looks much more like an assembler. I t is com- mon for manufacturers to retain control over

styling and to manufacture the body and the powertrain, while outsourcing many of the remaining components. The sophistication of purchased components is increasing and will eventually include things like complete modular inte- riors. In an effort to reduce their own fixed costs, manu- facturers are increasingly assigning engineering responsi- bility to their tier 1 suppliers. Thus, tier 1 companies are quickly building engineering capabilities including test fa- cilities. We work in a test lab for a tier 1 supplier of interi- ors and, as of this writing, are involved in a major lab con- struction project.

‘ Cost and quality concerns dominate every aspect of the automotive industry. Continuing quality improvements Perhaps the primary feature of the automotive business

structure is the relationship between the auto manufac- : . from offshore manufacturers, combined with excess global

River Rouge plant, he made a self-contained factory in cies create a very demanding business environment. Dras- which raw materials went into one end of the plant and : tic improvements in the noise and vibration characteristic completed cars came out the other. At the time, Ford needed : of almost all vehicles Over the past decade have added to control the whole manufacturing process in order to con- : greatly to the customers’ perception of quality and have

made NVH testing a hot field. trol the quality of his cars. The situation today, however, : is nearly the opposite. The US manufacturers assemble :

turers and their When Henry Ford the manufacturing capacity, and now, devalued Asian curren-

cars using many-components purchased from their suppli- : . The automotive industry is the archetypal mature indus- : try, being built around high sales volumes and low profit : margins. Profit margins of a few percent are common : among successful companies. In addition, there is great

M. French (SEMMember) is Manager of Testing and Techliology pressure to maintain or reduce prices a t all levels of the Development - Vibration and Acoustics, and M. Jay is Senior En- : industry. Thus NVH testing must be inexpensive, fast and gineer at Lear Corporation, Southfield, MZ. : cost-effective. Fixing a problem while adding $0.50 to the

. cost of a high volume component is not acceptable. Fixing Editor’s Note: An Introduction to Automotive NVH Testing is . the first in a series of Feature articlesprepared by Mark French, : the same problem Chair of s E M s ModalAnalysis/Dynamic Systems Technical Di. for great celebration. The fix must be Simple to mmufac- vision. Please contact SEM ifyou have any questioiu or comments ture, SO the design, test and manufacturing engineers must regarding this, or any other, ETarticle. PB . cooperate.

32 EXPERIMENTALTECHNIQUES JulylAugust 1998

decreasing the $Oa50 is

STANDARDSAND PROCEDURES

Some facets of the automotive NVH environment may be unfamiliar to engineers working in other industries or a university environment. One of the most significant is the extent to which both the tests and reporting is subject to formal, written procedures. Many, perhaps most compa- nies in the industry, are subject to IS09000 or QS9000 re- quirements. The idea is that no process can be improved without first being made absolutely consistent. Along this line of thinking, most test facilities have explicit procedures for all the tests they perform. In addition, IS0 9000 re- quires a standard reporting format for all tests and formal control of sensor calibration traceable to NIST standards.

While this might sound unnecessarily bureaucratic, we find the benefits well worth the initial overhead. By writing and maintaining procedures, process improvements are easy to document and the resulting gains are then cap- tured almost automatically. It also makes cross training much easier. Few organizations can afford not to make the most of their employees’ time and skills, and formal test procedures reduce the time required for an inexperienced person to be productive. They also save organizations from the chaos that results from having knowledge kept only in the personal experience of a few senior employees. The goal is never to hear anything like, (‘GO ask Joe how to run this test. He’s our modal guy”.

Finally, many organizations are contractually required to test according to formal procedures. For instance, a sup- plier who fails to test properly will, at the very least, be required to repeat the test properly and for free. If the problem recurs, manufacturers are reluctant to award fu- ture business. The automotive industry in the late 1990’s is extremely competitive, with extreme pressure to improve products while decreasing costs. Poor performance from a testing organization will never be tolerated for long.

PROBLEMS UNIQUETO AUTOMOTIVETESTING

Automotive NVH testing, while extremely complex in prac- tice, is conceptually pretty simple. Excitation from either the engine, power-train, the road surface or the air pass- ing over the vehicle excites the structure and makes noise.

Not all noises and vibrations are undesirable as they often provide useful information, In the past the goal was often to minimize noise and vibration. Now, though, the NVH characteristics of a vehicle are often used as a way to rein- force the image of the vehicle. Of course, many noise sources interact with each other. So, attenuating andlor shaping one noise source may make another (and previously masked) one more objectionable than before. A sports car enthusiast may actually expect certain characteristic noises and vibrations that the luxury car buyer would find objec- tionable. Thus, sound and vibration engineering must ad- dress the various noise and vibration sources and be tai- lored to the product to convey the proper image to poten- tial customers.

NVH testing can be divided up into a couple of broad areas: Power-trains, Exhaust, Accessory Components, Suspen- sions, Bodies in White and Interiors. Power-trains include the engine, transmission and drive shafts. Exhausts in- clude manifolds, catalysts, and silencers. Accessory Com- ponents include air conditioning compressors, alternator, and cooling fans. A body in white is the load carrying struc- ture of the vehicle as it exists before anything is attached to it. A typical body in white is made of welded sheet metal stampings. Interiors (our field) includes everything inside the sheet metal (See Fig. 1).

Perhaps the most fundamental problem in automotive NVH testing is that the industry regularly builds products it can’t analyze. The limitations may be either technical (nobody knows how) or economic (we know how, but it is too expen- sive). The result is that testing is currently an inseparable part of the product development process.

For instance, we spend a lot of time performing modal tests on car seats even though they violate many of the assump- tions inherent in the underlying mathematics. They have non-linear stiffness curves with both freeplay and hyster- esis, friction and multiple contacts. The analysis tools that work fairly well for static problems on seat structures are of limited use for dynamic problems, so we spend time test- ing and modifying during the development process.

Another source of difficulty in testing is that many of the criteria for acceptance or rejection of a product are subjec- tive, based on the customer’s perceptions. Many of the for- mal criteria for acceptance of a new product have subjec- tive components. In addition to object specifications like minimum natural frequency, maximum sound level, etc., there is usually something like “no objectionable noises or vibrations.” Thus, there is often some testing involving juries of representative customers or of the product engi- neers. Fortunately, new tools such as juror-based sound quality analysis and ride comfort metrics are being devel- oped to help quantify people’s reactions.

THE ROLE OF TESTING IN PRODUCT DEVELOPMENT

NVH testing is an integral part of automotive product de- velopment. Although manufacturers are working hard to reduce the need for expensive and time-consuming proto- types, it is unlikely that analysis tools or design practices will eliminate the need for testing soon. One result is pres- sure to improve understanding between the design, test and manufacturing organizations within a company. The days of “throwing a design over the wall” to the test or manufacturing people are long gone.

The business environment demands that any test performed must be accurate, repeatable and result in practical sug- gestions for designers. This, in turn, demands well-trained personnel, good communication between organizations and continually improving test practices. Companies that can- not do these things will not survive long..

July/August 1998 EXPERIMENTALTECHNIQUES 33