Manufacturers are under tremendous pressure to produce electronic products in less time and sell them at the lowest possible price. As a result, they need every advantage to get the best product to market in the shortest time frame.
Vibration test is a proven method to help you weed out poor designs and meet critical time-to-market and cost demands. But what type of vibration equipment is most effective?
As basic as it sounds, said Ed Peterson, director of application engineering at MB Dynamics, selecting the right instruments starts with another question: Is the product fit for its intended purposes? To answer this, you must know the service environment or loads acting upon the product, the vibration and shock conditions pervasive in its usual environment, and if the operating loads are periodic, random or transient.
This information will lead you to suitable vibration equipment that supplies the appropriate energy content and levels over the necessary frequency range, added Mr. Peterson. The test system also will be effective for assessing the worthiness of the product design.
Choosing the proper type of vibration equipment depends on the product and its designed purpose, agreed John Raymond, applications engineer at Unholtz-Dickie. In general, vibration test equipment used for product development is selected because of its capability to simulate the service environment.
It is now possible to closely approximate the service environment because of improvements in the vibration simulation techniques, said Susan Brooksbank, marketing manager at Data Physics. For example, the vibration encountered during shipping is often simulated by measuring the average power spectral density and creating a reference spectrum for the simulation.
The acceleration levels and frequency spectrums can be adjusted for maximum effectiveness more easily in a lab than in the actual situation, said Mr. Raymond. Typical service environments that are simulated include flight buffeting, loose cargo and random vibration caused by various road surfaces. The road conditions are also simulated by recording the acceleration history as a time waveform and replaying it on the shaker. However, the controller must compensate for the dynamic properties of the shaker and fixture.
The test equipment used to verify the product design must measure the field environment, characterize the product’s structural dynamics and replicate the field environments in laboratory tests, said Dave Galyardt, product marketing manger at Spectral Dynamics. The in-service vibration is characterized by field measurements using acceleration sensing transducers and recording the data on a tape recorder or spectrum analyzer.
The spectrum analyzer helps characterize the product’s structural dynamics. Transmissibility measurements, the ratio of forces, displacements, velocities or accelerations help show the dynamic response relationships among components of the product and help identify the resonant frequencies. A comparison with field measurements reveals potential problems if the frequency distribution of the vibration environment contains many components close to the product’s resonance frequencies.
Analyzing and fixing the structural dynamic problems may require modal analysis. Some modal analysis software, such as STARModal from Spectral Dynamics, separates mode shapes from the overall structure dynamic response. It allows you to visualize structural deformation by viewing animated mode shapes.
Replication of field vibration in a laboratory test requires a mechanical shaker system, a fixture for mounting the product on the shaker, dynamic measurement transducers and a control system, said Mr. Galyardt. The control system must compensate for shaker/fixture dynamics. It should provide faithful reproduction of the service vibration environment.
Flexibility and functionality are crucial components in a shaker controller, agreed Ms. Brooksbank. When a product is in the design stage, you need to experiment and manipulate the controller.
You also need flexibility of test sequencing, added Ms. Brooksbank. If a system is flexible, you should be able to combine all types of tests, including random, sine and shock, into one testing session and repeat them any number of times. You also need to analyze data, display what is wanted in any format and compare data from multiple tests, simultaneously.
Look for a controller with an up-to-date defaults management system; an integrated data organizer for setup, test result data and summaries; a backup utility and a reporting tool, said Jan Debille, manager of environmental testing products at LMS International. It should feature a sine-dwell option capable of locking onto resonances and produce reports indicating the evolution of the resonance responses as a function of dwell time or number of cycles.
The entire process of measuring and analyzing field data is best performed on the same platform as the vibration control application. This avoids the need for translating the data which often leads to oversimplifications.
Vibration equipment is also used for product screening. In this case, you need to tailor the test to the correct spectrum and run the test repeatedly on the production floor to determine the appropriateness of the screen, said Ms. Brooksbank. Functionality and flexibility are crucial for screening because you must modify the reference spectrum using an iterative process. The best system is easy to use, easy to change and simple to run.
For example, the DP550Win from Data Physics follows the familiar layout of Windows 95 with the most recent tests listed first for quick access. It uses Object Linking and Embedding to transport data to and from other programs and to communicate with other PCs, controllers and equipment. Remote functions are also available for automated testing on the production floor.
Selecting vibration test equipment for production screening focuses attention on defects generated within the manufacturing process, said Mr. Raymond. These environments are not usually similar to the product’s service environment.
Screening environments are typically applied at lower assembly levels and fine- tuned to find defects in the product during the manufacturing cycle where repair costs are low. They tend to have broader frequency bandwidths and higher force levels than service environments.
Choosing the Equipment
Once the vibration environment is selected, the appropriate vibration equipment must be chosen. The shakers used for random and sine vibration and shock tests are electrodynamic, servo-hydraulic and mechanical.
Six major factors should be considered when specifying a shaker, said Mr. Peterson. They include frequency range and waveform, displacement and velocity, the number of axes to test, the payloads and vibration or shock levels, the force rating, and the dynamics of the shaker system.1
You can identify the best test equipment once you have distilled the test needs into requirements for measurement/control capability, frequency range, multipoint acquisition, monitoring and data analysis software, said Mr. Galyardt. Examine the test equipment and verify that it offers the quality and accuracy needed to do the job. Ask these questions about the vibration equipment:
· Does the acquisition hardware provide quality anti-aliasing filters to protect the data from contamination caused by out-of-band signals?
· Do the input channels offer scaleable full-scale voltage ranges so that all the analog-to-digital converter (ADC) bits are used for low-level signals?
· What is the dynamic range of the system? Often it is specified as a theoretical value based on the number of bits of the ADC. For example, the dynamic range for a 16-bit ADC is 96 dB, based on 6 dB per bit. However, the real measurement dynamic range can be significantly less, depending on the quality of the hardware components, anti-aliasing filter design and system noise floor.
There are several criteria you can use to select the appropriate controller for the vibration equipment, according to Ms. Brooksbank. They include:
· Functionality—Does the controller offer matching functions for the job?
· Flexibility—Does the controller offer choices? A controller must adapt to your requirements and be easy to use. For example, can the system be set up to perform the exact test required or do you need to compromise?
· Ease of Use—Is the controller interface quickly learned?
· Capability—Can the slip table be controlled? How long does it take to manipulate parameters before the test can be performed? Can a highly resonant structure be controlled, especially during a resonance search-and- dwell operation?
· Power—How easily can the controller perform tasks? For example, the dynamic range affects the capability to handle a structure with significant resonances. A larger dynamic range enables the controller to manage operations better.
· Versatility—Can the equipment adapt or expand depending on test requirements? For example, some controllers control closed-loop shaker, crash test, seismic test, drop test and FFT analysis using the same hardware.
The test software is another piece of the vibration-equipment puzzle. It should optimize the hardware performance for each application.
Determine if the hardware is designed specifically for vibration testing or if it uses off-the-shelf equipment designed for a different application. Equipment for vibration
testing offers hardware and software features that work in tandem to provide special filtering and processing modes to enhance data quality and to speed up testing.
Is the vibration test equipment expected to reproduce the real-world environment? No, not exactly, said Mr. Raymond. With the use of a sophisticated computer control system, the vibration test system simulates a real-world environment. But due to differences in the dynamic response of the fixturing and shaker, the dynamic response of the product will, at best, be similar to an actual service environment.
Then, you might ask why people use vibration testing if it cannot reproduce the real-world situation. Because when implemented correctly, said Mr. Raymond, it provides results that correlate to dynamic behavior in the field. It produces similar failures and allows you to predict design limitations.
Reference
1. Peterson, E., “Shaker Selection: What You Need to Know,” Evaluation Engineering, May 1996, pp. 140-145.
Vibration Test Products
Vibration Controller Features
Software Modules
The DP550Win Vibration Controller provides modular software for all modes of vibration and shock control. The controller is available in a DOS-based and a Windows 95-based configuration. Both versions offer resolution to 1,600 lines for random, sine-on-random and random-on-random functions. An advanced sine-on-random function offers 10 independent tones with separate profiles, sweep rates and directions. A random-on-random function provides 10 independent narrow bands with separate profiles, sweep rates and directions. Call company for price. Data Physics, (408) 371-7100.
Squeak and Rattle System
Tests Automotive Seals
The EST Model SR-27-H5 Vibration Noise/Wear Test System is a dedicated weather-strip and door-seal test facility. It induces sine or random vibration to simulate the interaction of vehicle suspension with road discontinuities. The acoustic product noise is simultaneously recorded during the vibration tests. The test system consists of the H5 single-axis horizontal vibration system, a Pentium®-based acoustic analyzer and the company’s acoustic test enclosure. $55,100. Environmental Screening Technology, (616) 772-5485.
Vibration Control Option
Synthesizes Test Profiles
The Mission Synthesis option for the CADA-X Data Management Tool Box characterizes the expected loading environment during a product’s lifetime and helps synthesize a design target. It includes validation and correction of selected data and performs automatic sorting and selecting of dynamic phenomena. Fatigue damage and strain characteristics are calculated with user-defined material properties. Random and sine profile target environments are supported. Call company for price. LMS International, (011) 32 16 384 200.
Vibration Controller Permits
Data Sharing Over LAN
Enhancements to the MB Win2001 Vibration and Shock Controller provide a Windows 95 operating system for a Pentium®-based desktop or notebook computer. Test information is shared over the LAN and enables remote monitoring of tests via a LAN or dial-up network. Additional applications help grab and display images displayed on networked monitors. A test editor creates test profiles, supports Windows applications such as data bases and spreadsheets, imports ASCII data, maintains profiles with up to 500 breakpoints and reads industry standard data files created by other vendors. Call company for price. MB Dynamics, (216) 292-5850.
Chamber Has Slide-Out Features
For Triaxial Vibration Table
The QRS™-600V Power Screen System consists of a thermal chamber that features separate heating and cooling compartments and a triaxial quasirandom vibration system. Programming, operation and control are facilitated with the SSC-2000 Controller. The shaker table employs 16 pneumatic actuators. Vibration levels of 30 g rms are attainable and loads up to 600 lb are accommodated. The temperature chamber provides transition rates to 60° C/min. Call company for price. Screening Systems, (714) 855-1751.
System Integrates Acquisition
And Signal Processing
The BOBCAT is a portable modal analysis system that integrates data acquisition and digital signal processing (DSP) with the company’s STARModal® structural analysis software. BOBCAT includes a module with eight 16-bit input channels, a signal generator, a PCMCIA Type III DSP card, data acquisition software and a Pentium®-based notebook computer. The module and card provide multichannel signal conditioning, filtering, digitization and FFT measurements. The frequency ranges from 15 Hz to 20 kHz with up to 1,600 lines. $19,500 to $27,000. Spectral Dynamics, (408) 474-1700.
System Provides
7,500 lbf Force Ratings
The S802 Series Vibration Test System provides 7,500-lbf sine and random force ratings and 12,000 lbf of shock. It features 2″ peak-to-peak displacement, 1,500-lb internal load support, useable frequency to 2,500 Hz, three base configurations and 16″ or 24″ armatures. The system consists of the SA Series Class D power amplifier, an electrodynamic shaker, a DC field supply and a remote cooling blower. The series is used for engineering product development and production screening. $60,000 to $65,000. Unholtz-Dickie, (203) 265-3929.
Card Converts PC Into
Random and Shock Controller
The DRC-I/CSC-1 Card converts your PC into a controller for performing either random vibration or classical shock testing. It consists of an arbitrary waveform generator and data acquisition circuitry. The output signal for the random vibration section is continuous Gaussian with a drive level of 16 V peak-to-peak and a dynamic range of >60 dB. The frequency range is 2.5 Hz to 2 kHz with a spectral resolution of 400 lines. $5,495. Vibration Test Systems, (216) 562-5729.
Shaker Rated at 2,000 lb Force
For Sine and Random Tests
The DS-2000 Shaker is a compact vibration system rated at 2,000 lbf for sine and random tests. It features a 2″ peak-to-peak movement for low-frequency, high-displacement tests. The 12″ dia magnesium armature provides acceleration levels >60 g. The company’s dynamic centering system with optical sensors ensures that the armature remains centered during travel. The system accommodates automotive squeak and rattle testing, military transportation testing and ESS applications. It is air cooled and supports payloads to 700 lb. Call company for price. Thermotron¸ (616) 393-4580.
Copyright 1997 Nelson Publishing Inc.
January 1997