Automotive NVH Series: Part 2 by M . French and M. Jay
AUTOMOTIVE TEST METHODS
T
his article, the second of a four-part series, addresses some of the practical issues surrounding Automotive Noise, Vibration, and Harshness (NVH) testing and the facilities required for interior development. We discuss rough costs and infrastructure requirements for full-vehicle and component-level testing. Finally, we address some of the practical concerns of NVH testing as they relate to data acquisition and simulation.
A typical NVH lab is equipped with both component-level and full-vehicle test rigs. The goal of each test rig is to provide excitation and or load to the test article with known, repeatable boundary conditions. Test accuracy depends on good data acquisition equipment, faithful reproduction of boundary conditions, and accurate transducers. However, given the cost sensitivity of the industry, test capability must always be tempered by capital constraints. In addition to the purchase and installation costs, most of the equipment described herein requires some pretty serious infrastructure. Four-posters, chassis dynos and shaker tables must be mounted on large reaction masses. A small shaker table can easily require a 50,0001b. reaction mass while a large fourposter may require a 750,000 lb. reaction mass. We are currently installing a 4wD chassis dyno with a reaction mass of about 1,000,000lb. Most shaker tables operate from either dedicated or backbone hydraulic power supplies. Typical operating pressures for such hydraulic supplies are 3,000 psi and a typical piece of hydraulically powered equipment requires 50-150 gpm. Remember also that most pumps are loud and may produce objectionable vibration. This requires that they be well isolated from sensitive test areas within a typical NVH laboratory. There can also be significant W A C requirements. A typical chassis dyno can require a fresh air supply of 25,000 cfm. Our machine has ducts 8 ft. in diameter supplying air to the vehicle while running. Also, the dyno can either absorb or generate power depending on the test. The electrical supply and dynamometer control system must be designed so that power can either be fed back into the grid or dissipated.
FACILITIES AND CAPITAL ISSUES
NVH testing can be divided into full-vehicle or component level testing. It can take place either in controlled, laboratory settings, or on the road. Component level tests include simple modal tests, road simulation on a shaker table and M. French @EM Member) is Manager of Testing and Technology Development Vibration and acoustics, and M. Jay (SEM Member) is Senior Engineer at Lear Corporation, Southfield, MI. Editor’s Note: Automotive Test Methods is the second in a series ofFeature articles prepared by Mark French, Chair of SEWS Modal Analysis I Dynamic Systems Technical Division. Please contact SEM i f you have any questions or comments regarding this, or any other, ET article. PB
material acoustics tests. Component tests tend to be quick and, of course, don’t require access to a full vehicle. Full vehicle tests are much more involved, but capture the interactions between components and the vehicle structure. Examples of full-vehicle test rigs are chassis dynamometers, four-post shakers, anechoic (free of acoustic reflections providing free-field conditions above room cutoff frequencies), hemi-anechoic(similar to anechoic but with one acoustically reflective boundary -generally the floor), and reverberant chambers (all boundaries acoustically reflective). What follows is a brief discussion of the roles of these different pieces of equipment as they relate to vehicle interior development. Prices offered are order of magnitude and only intended to illustrate the commitment required to build an automotive NVH test capability.
BENCH-TOP TESTS AND EQUIPMENT Bench-top equipment such as an impedance tube measures the absorption of materials as a function of frequency. The absorption properties of materials are very important in determining the acoustic performance of a vehicle’s interior. Leather, for instance, is very popular, but performs poorly as it is highly reflective over a wide frequency range. Changing to a suitable fabric as suggested by bench-top material tests can greatly reduce interior noise levels. Bench equipment including control computers usually cost a few tens of thousands of dollars and, properly chosen, offer a lot of utility for a relatively small capital investment. Another useful bench-top test is the Flow Resistivity Measurement apparatus. This is commonly used to test a porous material’s ability to attenuate sound. Porous materials are utilized in various areas of the cabin interior to attenuate high frequency noise. The test entails placing a sample of the material in the apparatus and measuring the sample’s resistance to steady state airflow. The resulting pressure drop is primarily a function of pore tortuosity (the pore’s insample deviation from a straight line) as well as viscous drag effects in the material. Modal analysis is a very useful tool in NVH testing. It is important that component natural frequencies be separated from those of the structure on which the component is attached. Experimental boundary conditions are always a problem, but paying careful attention to modal interactions can greatly improve vehicle noise and vibration performance. Modal test hardware can range from a small two channel analyzer with a hammer and accelerometer up through a high channel count system built around a dedicated front end and control computer. Costs can range from less than $10,000 to more than $250,000 Binaural heads are widely used for interior sound measurements. These systems can be either portable or fixed and typically range in price from $35,000 to $250,000. They are generally comprised of one or more binaural mannequins SeptemberlOctober 1998 EXPERIMENTALTECHNIQUES
39
(torso only) acquisition and analysis software, and ancillary electronics. The mannequin, equipped with high quality microphones in either ear, is designed to allow signals to be recorded in a manner which mimics human hearing.
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sis dyno has rollers on which the vehicle’s wheel turn. Elec-
: tric motors attached to the rollers either drive the wheels or are driven as the drive train spins the rollers. Dynos de,
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signed to measure power output or run quick checks on an assembly line sometimes use two small rollers for each Binaural data acquisition can take place on a chassis dyno, wheel. However, these are not very useful for NVH work; the four-post shaker, on the road, with the vehicle parked in a . tire contact patch is a significant source of noise and large test space, or under any other suitable conditions.These bin- . roller diameters more closely simulate the flat road. Rollers aural signals are then available for playback analysis or filshould be at least four feet in diameter and at least one tering. Quite often we assemble a collection of sounds into a machine with 10 ft rollers has been built. One major manplay-list and present them to a jury for evaluation. The ju- . ufacturer has even developed a system in which the tires rors then listen to and rate these, usually in a paired- . roll on a flat surface formed by large steel bands akin to that comparison or semantic differential test. Assuming the jury . on a bandsaw. population is statistically adequate, the attributes of ‘good’ . sounds may be quickly and repeatedly identified in an un- * During a dyno test, dynamic inputs due to the tire contact * patch and the drive train and other sources are reproduced. biased setting. * Some chassis dynos have rough road shells, so they can also ’ Digital filtering allows ‘virtual signatures’ to be derived in reproduce body response due to road inputs. This allows acoustical and vibratory responses at varying speed and load real time from those of physical prototypes, minimizing the number of follow-on prototypes required. These signatures , conditions to be acquired with good repeatability.Many chascan then be listened to, with filters being switched into and . sis dynes are mounted in fully Or hemi-anechoic chambers out of the analysis in real time. . (see Fig. 2) to provide a repeatable and faithful environment . for acoustic measurements. However, additional runs on the Shaker tables are quite useful for dynamic tests on vehicle * test track are often required to generate accurate wind noise components. A typical shaker table has one or more adua- * inputs to the vehicle. tors driving a rigid (for the purposes of this discussion)table to simulate vehicle dynamic response on the road. While a These are among the most expensive pieces of test shaker table can have from 1-6 degrees of freedom, the maequipment. A typical four-wheel drive dyne can easily cost jority are either single or 6 DOF. The economics of table conmore than $1.5 million. Installation costs can easily top struction make any other configuration disproportionately . $250,000. expensive. A small six DOF table (see Fig. 11, suitable for . component level tests, would cost $350,000-$500,000, inclu- . Four-post shakers are comprised of four independent closedsive of the controller. Installation costs vary widely, but . loop servo-hydraulic actuators, one at each wheel of the ve* $100,000 is an optimistic estimate. hicle. This allows reproduction of road-load or other userdefined excitations to-be imparted to each wheel. There is no FULL-VEHICLE TESTS AND EQUIPMENT input . - from the drivetrain or noise from the tires. Thus, the Chassis dynamometers provide load to the vehicle drive . four-post is largely used for BSR (BUZZ,Squeak, and Rattle) train as well as offeringa repeatable driving surface. A &as- . tests to help locate components that are sensitive to road . input. Some four-post chambers have environmental caps. bility to expand the conditions under which a vehicle may * be tested. +
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Again, the purchase price varies widely. The machine in Fig. 3 was assembled very inexpensively from readily available components and is used for light BSR testing in an open test bay. There is no dedicated reaction mass and the actuators can be moved out of the way when not being used. A more
Fig. I: Six DOF Shaker Table
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EXPERIMENTAL TECHNIQUES SeptemberlOctoberI998
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Fig. 2: Chassis Dynamometer in Hemi-Anechoic Chamber
nantly free-field conditions above the reflective plane of the road, i.e. hemi-anechoic) while measurements are made. These chambers need to be large, as the walls should be far enough from the test specimen not to compromisethe boundary conditions. Chambers large enough for a full vehicle test actually have enough floor space to park several vehicles. Reverberant chambers may be sized to accommodate a full vehicle or portions thereof. The goal of the reverberant chamber is to ensure that over the measurement period, the same average energy is present uniformly throughout the measurement space. Because these chambers cannot be Fig. 3: Small Four-Post Simulator made completely reflective, they must be sized accordingly. This is due to the fact that, even if the surfaces could be conventional, permanently placed NVH four post rig will . made completely reflective, the air itself absorbs energy. cost around $450,000 with controller. Installation costs sim- . Eventually, this effect can no longer be neglected. Thus, those chambers which are useful for full-vehicle measureilar to those for a large shaker table. ments may not be the optimal choice when the need arises Anechoic and hemi-anechoic chambers may be stand-alone . for material absorption property measurements. or combined with the other rigs such as the chassis dyna- . ' mometer. Their function is to approximate the acoustical ' Another desirable full-vehicle 'test rig' is the vehicle test boundary conditions that the vehicle will see during normal ' track, which may be used for acoustic or vibration testing and data acquisition. The test track allows a known, repeatoperation (i.e. operated on a smooth surface with predomi-
Table I -Test Equipment Summary TEST CELLlEQUlPMENT
SAMPLE TYPE
FUNCTI0N
DATA
PURPOSE
Anechoic Chamber
Full-vehicle or component depending on room volume
Provides free-field boundary conditions above room cutoff frequency
Component o r full-vehicle properties
Development
Hemi-Anechoic Chamber
As Above
Similar t o anechoic, but one reflective planezenerallv the floor
As Above
Development
Reverberation Room
As Above
Provides uniform (diffuse) sound field
Acoustic Absorption
Development
Dual-axle Chassis Dynamometer (may be placed in fully o r hemi anechoic space)
Full Vehicle
Provides load o r motor action of powertrain. Allows dynamic o r static speed and load conditions
Overall interior noise, Vibration. NVH diagnostics, Acoustic Intensity (steady state), Sound Qualiry, etc.
Development; Accurate conditions during measurements.
Binaural Acquisition and Analysis System (Sound Quality)
Full-vehicle o r components
Binaural data acquisition, post processing, metrics. filtration, virtual signature generation
Data generally acquired in full vehicle, occasionally in other test cells
Development Specification Development, Preference Correlation (Jury), Spectral Shaping
Impedance Tube
Small Fabric Samples
Measure Material Properties
Acoustic Absorption
Development; Measure Acoustic Absorption
Flow Rig
As Above
Measure flow resistance of materials
Pressure delta
Development
Four-post Shaker (environmental)
Full-vehicle
Provides independent simultaneous excitation at each wheel
Reproduce data from RLDA. perform forced response tests
Development: BSR. Forced Response. Durability.
Modal Analysis hardware/ software
Components and Full Vehicle
Measurement system for analysis
Mode shapes, damping Values.
Development; Mode shapes etc.
Multi-DOF shaker (manrated)
Components (seats, Instrument Panels etc.)
Reproduces body-side inputs t o components
Uses data from RLDA sessions
Development: BSR, Ride Comfort. Durability
Multi-DOF shaker (environmental)
As above
As above
As above
Development and Durability. Testing above o r below ambient
Road Load data Acquisition (RLDA)
Full vehicle
Dynamic response of vehicle at discrete locations versus time
Acquired on road or test track. Shared with MultiDOF and Four-post rigs.
Development; BSR, Specification Development
SeptemberlOctober 1998 EXPERIMENTAL TECHNIQUES
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able surface to be used, as well as provide other inputs such as wind noise. NVH data acquired on the road is done so for . the purpose of analysis, simulation or both in the test lab. . The idea behind simulation is to compress test time by re- . producing only the most damaging portions of the time his- . tories. While vehicle prototype costs are reduced using this * approach, a greater benefit is reduced time-to-production for * the OEMs.
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COMMON TESTS AND TEST EQUIPMENT Table 1 outlines the types of test rigs required for interior development, the stimulus they provide, and their roles. This list is by no means exhaustive.
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Calibration and Equipment Costs In a n environment where time is always at a premium, “get- . ting it right the first time” becomes paramount. A test which . must be re-run due to equipment failure or “bad data” (ei- . ther hard failures or assumed transducer parameters) adds significant cost to an already expensive process. These costs . can and must be avoided. Moreover, this “bad data” is often ** discovered only after the test article has either been returned to the customer or destroyed, in either case no longer being available for testing.
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However, sensors that are “in calibration” are dropped, driven over, stepped upon, soaked or otherwise maligned. Further, many test personnel within the lab often draw from a common equipment and transducer pool. While this pooling maximizes investment, it also increases the likelihood of equipment damage. Hence the importance of performing a channel-by-channel interrogative system calibration (complete with transducers and electronics), prior to, and after, completion of data acquisition cannot be overlooked. While annual transducer calibration is required to maintain the general health of the transducer inventory, it is no substitute for pre- and post-acquisition system calibration performed on a per-test basis.
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Hardware costs are also significant in an NVH laboratory. A *’ typical accelerometer or microphone costs $1,000-$1,500. A typical lab may have any number of accelerometers. Excluding microphone arrays and binaural mannequins, that same . lab may have any number of microphones. Data acquisition . gear (front ends, DAT recorders etc.) typically cost in the . vicinity of $ 1,000 per channel, and a typical NVH lab may . be equipped with several multi-channel acquisition systems (16-128 channels each) with complementary workstations * and software.
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Of course, none of the equipment discussed here is useful without well-trained people to run it. That training should . include a solid background in the fundamental principles at . work, training specific to the task at hand and practical ex- . perience, preferably with the oversight of a n experienced en- . gineer. Good people will make the most of average equip- . ment.
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EXPERIMENTALTECHNIQUES SeptemberlOctober I998
strong to permit me to respond informally to his friendship : more than willing to accept the manuscripts written by hand. and help. : Through the years he turned down many offers to become : a n administrator at the department level and above. His Recognized as a n outstanding elastician at an early age, : successful stint a s head of a group that developed the Ray was also a highly respected experimentalist, a pioneer : proximity fuse during World War I1 led to the award of the in two and three dimensional photoelasticity. His first : Presidential Medal of Merit from President Truman and the publication proposed and analyzed a reflection polariscope : firm resolve to avoid administration from then on, to return for photoelastic analysis. In those pre World War I1 days, : to and remain in research and teaching (which included most university experimentalists did their own model : appropriate relevant consulting). preparation and instrumentation. Ray was a superb craftsman, who machined his own photoelastic specimens : He shuddered grimly all during each of my subsequent and loading devices of considerable complexity, as well as : administrative activities at Brown and smiled broadly during an innovative applied mathematician with great physical those longer periods when I reverted to his ideal of teaching insight into very difficult problems in the theory of elasticity i and research, represented so well by his good friend and that had long remained unsolved. In his Ph.D. thesis he : mine, Eli Sternberg. He reacted with horror when I left a solved the “Mindlin problem” of a force at a point in the : comfortable named professorship at Brown to become Dean interior of a semi-infinite solid with a free surface. In later : of Engineering at the University of Illinois in Urbana. He years he put his stamp on the bearing problem with friction : told my wife Ann that had he suspected that I was ever to and slip on the contact surface of two bodies pressed together. : become a Dean he never would have bothered to be my His pioneering work for Bell Labs on the vibration of plates : mentor and thesis advisor a t Columbia.. and his reduction of theory to practice was motivated by the need to design stable and reliable delay lines and filters. : Ray was a track star at Columbia in his undergraduate days When the Japanese picked up his work on anisotropic crystal : and then led an active adult life symbolized well by the sports plates, the results were the highly accurate and yet : car he drove to and from work. Approaching retirement he inexpensive quartz crystal watches and clocks that have : developed heart difficulties and his health deteriorated. His : physicians recommended that he take it very easy if he altered the meaning to all of us of being on time. : wished to survive and above all not subject himself to the His significant contributions continued after his retirement : rigors of northern winters. Winters therefore were spent in in 1975. Most were published in the International Journal : the south of Portugal but, year by year, he became noticeably of Solids and Structures because George Herrmann was : weaker. His wife, Pat, trundled him about to international Continued on Page 44
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