Automotive components and subassemblies are increasingly subjected to vibration testing using sophisticated vibration control techniques. From instrument panels to seats inside the car and from air-bag sensors to fuel injection pumps in the engine compartment, many automotive components are being tested to precise vibration patterns and levels.
In some cases, the vibration tests prove that the devices will not fail under normal road conditions. In other cases, the tests identify noise-making mechanisms responsible for annoyances such as squeaks and rattles.
The development of digital signal processing techniques in the vibration-control industry has made it possible to produce a more realistic vibration-test environment in the laboratory and the production line. Today, the classical methods of vibration testing using random, sine, and shock waveforms are supplemented by more complex methods such as sine-on-random and waveform replication.
As the names suggest, sine-on-random combines sine and random vibrations, and waveform replication recreates a recorded waveform to simulate an actual vibration environment. Sine-on-random tests combine multiple sinusoids with broadband random noise. The sine tones may be stationary or sweeping, harmonically or nonharmonically related, and have varying levels across a frequency band. Sine-on-random is an excellent tool for simulating a moving vehicle subjected to random vibration due to road variations while the engine speed is increasing or decreasing.
For waveform replication, Data Physics has a digital vibration control system, SignalCalc Replicator, that reproduces recorded time waveforms of unlimited length on a shaker (Figure 1). It uses an advanced adaptive filtering algorithm via continuous convolution to provide online compensation at a real-time rate of 2.5 kHz. This online technique eliminates the costly step of preprocessing to compensate the dynamic characteristics of the loaded shaker.
Automotive testing makes use of sine-on-random and waveform-replication techniques to faithfully recreate the actual vibrations experienced in the operating environment for purposes of design verification as well as quality control.
Several manufacturers, including Visteon and Daimler-Chrysler, test entire instrument panel assemblies to identify squeaks and rattles. These are major causes of dissatisfaction among new car owners and account for the majority of warranty costs.
A specially built shaker that does not use a cooling fan provides an unusually quiet environment in which to identify rattles and squeaks that develop when vibration is applied to the instrument panel. Without cooling, the shaker can operate only for a short period of time before the temperature rises above the operating range and the tests have to pause to allow the equipment to cool down.
Except for the shaker, all instrumentation that emits noise, including the shaker controller, is housed outside the test chamber. A remote-control panel and display hang over the test setup so that an operator may listen to noises and control the test.
The vibration patterns, applied to identify squeaks and rattles, consist of random, swept sine, and waveform segments representing the road. The vibration levels are kept in a range usually experienced in normal usage of the car, avoiding the costs of fixing problems that would occur at more severe levels of vibration.
The operator plays a crucial role in the test. For example, when a swept sinusoid is applied to reproduce the vibration pattern of the accelerating engine, a number of frequency sweeps may be required for objectionable noises to develop.
Since squeaks and rattles are hard to pin down, it is necessary for the operator to stand next to the instrument panel and manually control the vibration frequency and level to dwell at the exact conditions sustaining the unwanted noise. Under these conditions, localization of the noise generating mechanism is possible, and this method has been adopted as a quality-control procedure in many manufacturing facilities.
Squeak and rattle testing sometimes combines vibration with temperature and solar radiation. Daimler-Chrysler recently installed an integrated test system in Detroit where a large temperature chamber houses an electrodynamic shaker and a solar array. An operator at a central command center coordinates the temperature, vibration pattern and level, and solar array position to probe for squeaks and rattles.
Lear Seating, Johnson Controls, and Integram Windsor Seating are three companies that use sophisticated vibration testing to identify squeaky seats. The seats are mounted to a pitch table on an electrohydraulic shaker. In some cases, the detection of noise is automated using a dynamic signal analyzer to make a 1/3-octave spectral measurement. Performing noise detection with an instrument rather than by human observation allows the operator to be summoned only when a preset noise level is exceeded.
Because the seats are more susceptible to road vibration, squeak and rattle testing of seats and seat belts is better performed using the technique of road simulation. In replacing the classical vibration test techniques with recorded vibration waveforms from road tests, a more realistic environment is reproduced assuring better test information.
Road simulation replicates a recorded time history using a shaker. Sometimes a complete rig consisting of four or six shakers is used to replicate the vibration environment experienced by all the wheels of the vehicle simultaneously. The simulation of the precise vibration environment is made possible by recent advances in signal processing techniques and the availability of faster digital signal processors.
Until recently, simulation of even a short segment of road, less than an hour’s duration, required a computational time of a day or so to iteratively arrive at a reasonably accurate replication. Today, a vibration controller can replicate a road history of unlimited time length online without resorting to hours or days of off-line computation. This is possible even when multiple shakers are used to simultaneously replicate the time histories at several test points.
Britax tests complete rear-view mirror assemblies for stability under driving conditions using a shaker to simulate road vibration. A reflected light beam is used to measure the scatter due to mirror vibration and accurately identifies units that would be found objectionable by car buyers.
Since the reflected beam amplifies the motion of the mirror by a factor of two, even a small amount of vibration of the mirror causes blurring of the rear view during driving conditions. The Britax test procedure searches for frequencies that cause blurring of the mirror images using a swept-sine vibration of a controlled acceleration level applied at the input to the mirror assembly.
A laser-imaging system monitors the image produced by the mirror and captures a snapshot when the mirror goes through a resonance. The program that controls the laser-imaging system communicates with the vibration-control system to store the frequency, vibration level, and other relevant parameters such as time and date.
Vibration testing also is used to identify modal properties of the natural frequencies of vibration of mirror assemblies. Instead of a controlled shaker, an instrumented hammer is used to create the vibration. Delbar, a supplier of mirror assemblies to several automotive manufacturers, performs vibration tests on rear-view mirror assemblies using such a tool. The impact testing identifies resonant frequencies of the mirrors and the potential for structural failure due to engine vibrations.
Based on identification of resonant frequencies using the impact method, a quality-control program to ensure consistent manufacturing also has been developed. A dynamic-signal analyzer measures the response to the impact, and the measured resonant frequency is compared against a preset spectrum to provide an automated quality check during manufacture.
Fuel Injection Pumps
Robert Bosch evaluates the operation of fuel injection pumps by subjecting the entire pump assembly to the combined vibration environment of the road and the engine. Sinusoidal waveforms representing engine vibration are superimposed on a random-vibration background to accurately simulate the operating conditions on the road. By adding several harmonically related sinusoids, the simulation of the engine vibration can be made so realistic that when you listen to the noise produced by the shaker you think you are actually listening to the car on the road.
To simulate the actual mounting of the fuel injection pumps on the engine, a fixture is used on the shaker to hold the pump during the vibration test. The fixture adds a further complication because its mechanical resonances introduce excessive levels of cross-axis motion. To limit such cross-axis motion to operational levels, a technique called notching is used (Figure 2).
The maximum allowable levels of sinusoidal vibration and random vibration are defined by limit spectra, one magnitude profile per sine tone, and an additional power spectral density spectrum for random vibration. During test, the acceleration measured in the cross-axis direction is not allowed to exceed the limit spectra. This is accomplished by reducing the drive signal at frequencies where the limit is reached. The resulting control spectrum displays notching at the frequencies where the limit spectrum is active.
Several manufacturers of crash sensors used to deploy air bags, test every sensor by simulating the acceleration transient generated during a crash. The sensors are tested to ensure that they do not fire before the required acceleration levels are reached and that they do fire when the defined acceleration level is reached.
The acceleration transient is simulated by driving a shaker to produce a half-sine acceleration pulse or a recorded acceleration waveform captured from an actual crash. To verify proper operation at each shock level, the sensors are wired to a test rig monitored and controlled by the same computer program that controls the shaker.
With such numerous instances of vibration testing, the automotive industry is on a march toward building better, more reliable vehicles with improved sound quality. The development of shaker controllers that create more realistic vibration test environments in the laboratory is helping to accelerate the march of progress.
About the Author
Sri Welaratna, Ph.D., is president and CEO of Data Physics. He obtained his Ph.D. in signal analysis from the University of Bradford in England, and started his career in noise and vibration 25 years ago at Hewlett-Packard in Europe. Data Physics, 2055 Gateway Place, Suite 300, San Jose, CA 95110, (408) 437-0100.
Copyright 2000 Nelson Publishing Inc.