Accelerometers for Automotive Modal Analysis

Understanding Dynamic Structural Behaviors

Classical and operating modal analyses are important tools for understanding and optimizing dynamic automotive structural behaviors, leading to stronger and safer automobiles; lighter construction yield; improved fuel consumption and performance, ride quality, handling, and NVH. From the measured vibration data and modal analysis, engineers are able to construct dynamic models of vehicles and substructures.  The dynamic models predict resonant frequencies, damping values and deflection patterns for each mode of vibration.  Frequency ranges of interest may be sub-one hertz to a few hertz in terms of ride handling and from 10 hertz to 500 hertz for full vehicle operating data and body-in-white modal tests.

To provide better understanding of structural dynamic behavior and visualization of mode shapes, in classical modal analysis dynamic characteristics are extracted from transfer functions of measured force inputs and vibratory responses on the structure under test. Typically starting on a body-in-white, an engineer experimentally obtains a mathematical model describing a test article’s structural behavior.

Measurement complexity can range from simple point mobility tests, using instrumented impact hammers, to multi-shaker testing of large and complex structures, using hundreds of ICP® accelerometers and through-hole armature design modal shakers. For optimal results, it is critical for an engineer to understand techniques, goals and restrictions. Classical modal analysis relies upon understanding the primary assumptions of observability; linearity; time invariance; and reciprocity, as well as the desired out of the test results.

To observe specific modes on a test structure, chosen measurement points must have adequate spatial resolution and proper excitation techniques. Modal array accelerometers support this assumption, by offering convenient mounting and large channel management. Choose ICP® accelerometers that are lightweight, highly sensitive and have good resolution within a cost-effective, compact package. Packages can range from cubic shape for convenient adhesive mounting on any of 5 sides to simple press fit designs with low cost cabling solutions. Many ICP® signal conditioning options are available from stand alone to built-in to the multichannel data acquisition. Choosing Modally Tuned® impact hammers to be utilized alongside accelerometers in tests, helps locate stiff areas of a test structure away from modal nodes, which are good locations for a modal shaker to adequately excite modes of interest.

An important assumption of classical modal analysis is that a test structure exhibits time invariance or stationarity. However, practically this is often untrue, with structural changes in material properties over time/temperature, and other transient phenomena. Thus, it is best to obtain all measurement data in one time “snapshot”.  This has led to a trend toward high channel count use of economical modal array accelerometers.  The use of innovative cabling systems, consolidation patch panels, modular signal conditioning and computer control was pioneered at the University of Cincinnati Structural Dynamics Research Laboratory and has proven its effectiveness at major users for over almost two decades. A key is to consider the move to ribbon cables and multi-pin connectors and away from single channel BNC connectors as test configurations cross the 64 channel count.  A number of papers (titlelink bono/Dillon + titlelink ATA) have documented the bottom line benefits in terms of reduced test time and measurement reliability.

To satisfy precise requirements of modal analysis, consider efficient mounting and connection systems, as well as incorporation of TEDS self-identifying technology. Automated TEDS memory is available in a variety of multi-channel vibration sensing systems to expedite test setup, decrease total test time and reduce human errors. The benefits of TEDS on-board memory, which enables identification of sensor and sensitivity in test, are ideal for larger channel count NVH tests.

In situations where smaller channel count data acquisition must be used, a cost-effective solution is to choose bank switching. By using ICP® modal array accelerometers with modular bank switching signal conditioners, groups of response signals can be electronically switched into a smaller channel count data acquisition unit thus providing rapid automatic cycling through measurement sets to approximate a time “snapshot” while meeting an optimum in the measurement system price/performance curve.

While setup takes slightly longer, shaker operation along side sensors and hammer data sets can help verify system linearity. By controlling input levels to the test structure, a series of input forces (low, medium, and high) are applied, to determine input range over which the test object structural response will remain linear in output vibration. A tenet of modal analysis states that measured or estimated complex test structure frequency response is composed of a linear combination of single degree-of-freedom frequency response functions (FRF); hence, it is desirable to provide test excitation in the linear range of output behavior.

In a multiple input multiple output (MIMO) modal testing, verification of identical response between any pair of input points is critical. To validate reciprocity, PCB® developed a unique ICP® impedance head sensor, consisting of a co-located driving point accelerometer and input force sensor, to allow exact comparison of driving point FRF’s, ensuring that reciprocity is met.

For smaller, more delicate objects like brake pads or rear view mirrors, a very lightweight accelerometer is preferred to prevent mass loading. Generally, the optimal accelerometer used for classical modal testing should have high sensitivity, good resolution, robustness, and small mass. Higher sensitivity usually dictates larger mass. For body-in-white or a full vehicle modal test, where mass loading is less of a factor, high sensitivity accelerometers are preferred.  To address this, consider compact yet high sensitivity accelerometers with sensitivities as large as 1 V/g.

Classical modal analysis is invaluable to automotive structural design, to identify weaknesses and provide direction on dynamic improvements.  Though advances in Finite Element Analysis (link to STI) have increased reliability of today’s analytical modal model, and in some cases, reduced the role of classical modal analysis requirements, especially with legacy structures. Despite this, classical modal analysis remains an automotive engineering requirement for providing added objective, empirical data, with the ever-increasing drive for new automotive products. Common applications include analytical model correlation; troubleshooting; design studies; structural optimization and damage detection; force response simulation; and cascade target setting.

Though many engineers choose operating over classical modal analysis, due to timing and budget, both are needed to solve complex structural dynamics challenges. With operating modal analysis, structural behavior is extracted in its operating environment from vibratory responses alone, relying solely on measured structural response, and often used in concert with classical modal analysis. Both have advantages and limitations, in designing automotive structures, related systems and components.

The operating modal analysis method is employed where modal characteristics are needed under actual operating conditions; where it is difficult to quantify force input(s) to a structure; or when timing and cost outweigh benefits of a classical modal analysis test.  In the automotive market, the trend has been toward operating modal analysis, due to timing, cost and increased confidence in the fidelity of today’s analytical modal models.  Benefits include ensuring response represents real-world boundary conditions; quicker set-up; and, in many cases, measurements may be made simultaneously to other vehicle development tests. But unlike classical modal analysis, where time invariance is not always certain, responses must be collected simultaneously requiring large-channel data acquisition systems.

An optimum operating modal analysis accelerometer is one with good sensitivity, adequate range to allow for significant transient inputs, rugged construction, and small mass. These needs are satisfied by 10 or 100 mV/g compact triaxial accelerometers in a titanium package, hermetically sealed to withstand more severe outdoor environments found in operating modal tests.

For more information on classical and operational modal analysis, please contact PCB Automotive Sensors at 1-716-684-0001 or by selecting "Contact an Expert."