Technical Library

General

A Framework for Smart Transducer Interface Systems
Current progress and concepts of the IEEE P1451 Draft Standards for Smart Transducer Interfacing of Sensors and Actuators will be reviewed. Topics include the Network Capable Application Processor Information Model (P1451.1), Smart Transducer Independent Interface (1451.2), Distributed Multidrop Systems (P1451.3), and Mixed-mode Communication Protocols (P1451.4).

Streamlined Test Setup with TEDS Technology
TEDS (Transducer Electronic Data Sheet) Technology packages manufacturer and user-defined information within the traditional analog ICP® transducer. Using the same simple two-wire electrical connection, these smart sensors are now able to identify themselves over a network, allowing completely self-configurable transducer setup. This paper describes how to efficiently use TEDS technology on large multi-channel structural tests, such as modal analysis. An update on the developing smart sensor IEEE P1451.4 standard that describes TEDS is provided. Also, practical equipment additions (from Pocket PCs and PDAs to a 3D sonic coordinate digitizer) are integrated within the system setup solution.

The Fundamentals of Signal Analysis
The analysis of electrical signals is a fundamental problem for many engineers and scientists. Even if the immediate problem is not electrical, the basic parameters of interest are often changed into electrical signals by means of transducers. Common transducers include accelerometers and load cells in mechanical work, EEG electrodes and blood pressure probes in biology and medicine, and pH and conductivity probes in chemistry. The rewards for transforming physical parameters to electrical signals are great, as many instruments are available for the analysis of electrical signals in the time, frequency and modal domains. The powerful measurement and analysis capabilities of these instruments can lead to rapid understanding of the system under study.

Time Scale Re-Sampling to Improve Transient Event Averaging
As the drive to make automobiles more noise and vibration free continues, it has become necessary to analyze transient events as well as periodic and random phenomena. Averaging of transient events requires a repeatable event as well as an available trigger event. Knowing the exact event time, the data can be postprocessed by re-sampling the time scale to capture the recorded event at the proper instant in time to allow averaging. Accurately obtaining the event time is difficult given the sampling restrictions of current data acquisition hardware. This paper discusses the ideal hardware needed to perform this type of analysis, and provides analytical examples showing the transient averaging improvements using time scale re-sampling. These improvements are applied to noise source identification of a single transient event using an arrayed microphone technique. With this technique, the averaging is performed using time delays between potential sources and microphones in the array. As a result, the relative time information needed is contained within the measured data and a separate trigger event and event time are not required.

Vibration
Fundamentals of Modal Analysis
Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure. This application note emphasizes experimental modal techniques, specifically the method known as frequency response function testing. Other areas are treated in a general sense to introduce their elementary concepts and relationships to one another. Although modal techniques are mathematical in nature, the discussion is inclined toward practical application. Theory is presented as needed to enhance the logical development of ideas. The reader will gain a sound physical understanding of modal analysis and be able to carry out an effective modal survey with confidence. Chapter 1 provides a brief overview of structural dynamics theory. Chapter 2 and 3 which is the bulk of the note – describes the measurement process for acquiring frequency response data. Chapter 4 describes the parameter estimation methods for extracting modal properties. Chapter 5 provides an overview of analytical techniques of structural analysis and their relation to experimental modal testing.

Multi-Tachometer Order Tracking and Operating Shape Extraction
An automobile and a tracked military vehicle were instrumented with multiple tachometers, one for each drive wheel/sprocket and operated with accelerometers mounted at suspension, chassis, and powertrain locations on the vehicles. The TVDFT order tracking method was then used to extract the order tracks from each of the wheels/sprockets and operating shapes estimated based on the order tracks. It is shown that under some conditions a different operating shape is excited by each of the wheels/sprockets simultaneously. This is due to the asymmetries present in the vehicles. The strengths of the TVDFT order tracking method are also shown for this type of analysis which is difficult due to the closeness and crossing of the orders generated by each of the wheels. Benefits of using multiple tachometers and advanced order tracking methods becomes apparent for solving certain types of noise and vibration problems.

The Time Variant Discrete Fourier Transform as an Order Tracking Method
Present order tracking methods for solving noise and vibration problems are reviewed, both FFT and resampling based order tracking methods. The time variant discrete Fourier transform (TVDFT) is developed as an alternative order tracking method. This method contains many advantages which the current order tracking methods do not possess. This method has the advantage of being very computationally efficient as well as the ability to minimize leakage errors. The basic TVDFT method may also be extended to a more complex method through the use of an orthogonality compensation matrix (OCM) which can separate closely spaced orders as well as separate the contributions of crossing orders. The basic TVDFT is a combination of the FFT and the re-sampling based methods. This method can be formulated in several different manners, one of which will give results matching the re-sampling based methods very closely. Both analytical and experimental data are used to establish the behavioral characteristics of this new method.

Calibration
A New Solution for Shock and Vibration Calibration of Accelerometers
Abstract - Shock and vibration phenomena are present around us in everything that moves. The accelerometer is a class of instruments commonly used to measure that motion, producing an electrical output signal related to the applied motion. Accurate accelerometer calibration is a way to provide physical meaning to this electrical output and it is a prerequisite for quality motion measurements. Systems and standards on comparison methods for accelerometer calibration are discussed, providing an overview on current technology available for calibrating and testing accelerometer performance characteristics.

Air Bearing Shaker for Precision Calibration of Accelerometers
This paper presents the design, construction, and performance testing of a new air-bearing shaker for precision accelerometer calibration in a production environment. The new shaker incorporates a number of novel features. A graphite air bearing provides high stiffness to lateral loading. A Lorentz force “electrical spring” minimizes the low frequency waveform distortion that is typically associated with non-linear deformation of metal or elastomer flexures. Finally, a beryllium armature provides high stiffness, low mass, and high resonant frequency.

Shock and Vibration Calibration of Accelerometers
Accurate accelerometer calibration is a way to provide physical meaning to the electrical output an accelerometer produces when subjected to vibration and it is a prerequisite for quality measurements. The most common shock technique, a pneumatic shock exciter, can perform calibration and linearity checks up to 10,000 g and is one of the most versatile anvil shock type devices available for shock calibration (in terms of amplitude range, pulse duration, repeatability, and traceability to primary calibration methodologies).

Accelerometer Calibration Q&A
Why calibrate a vibration transducer? What is driving implementation of automated calibration systems today? What does the typical engineer need to know about accelerometer calibration related to the standards? What is meant by calibration "traceability" and why is it important? Walk us through the typical process for calibrating an accelerometer. What are some specific challenges in calibrating accelerometers? How often should a user calibrate? Is there any way I can make a quick accelerometer sensitivity check without doing a complete calibration? Why are more modern calibration systems moving toward air-bearing shakers? What is The Modal Shop’s role in the calibration and test services market?

Acoustics
PCB Microphone Handbook
Pressure variations, whether in air, water or other mediums, which the human ear can detect, are considered sounds. Acoustics is the science or the study of sound. Sound can be generally pleasing to the ear, as in music, or undesirable, referred to as noise. The typical audible range of a healthy human ear is 20 to 20,000 Hz. A Sound Pressure Level (SPL) beyond the detectable frequencies of the human ear can also be very important to design engineers. Noise, Vibration and Harshness (NVH) is concerned with the study of vibration and audible sounds. Vibrations represent a rapid linear motion of a particle or of an elastic solid about an equilibrium position, or fluctuation of pressure level. Harshness refers to the treatments of transient frequencies or shock. Usually treatments are employed to eliminate noise, but in some cases products are designed to magnify the sound and vibration at particular frequencies. The sound produced or received by a typical object, which may be above and below the frequencies that are detectable by the human ear, or amplitudes concerning its resonant frequencies, are important to designers, in order to characterize the items performance and longevity.

Sound Power Measurements
Sound power level measurements are gaining recognition worldwide as a means of characterizing a product's acoustic signature. The usefulness of sound power is well known among noise control experts. Unlike sound pressure measurements, sound power tests are independent of measurement environment, making possible direct noise level comparisons between different products. Given 1/3-octave spectra of sound power, noise control engineers can determine the resulting sound pressure level in an enclosure or space and choose the quantity and type of acoustic treatment required. With many international import regulations requiring conformance to noise power test standards such as ISO 7779, the measurement has become increasingly important. Growing interest in sound power level is not restricted to noise control professionals. Appliance and office equipment manufacturers - and their customers - are paying attention too. For competitive reasons, many companies now specify sound power levels in product documentation. Their assumption is that consumers will select a quieter appliance. A final reason for growing popularity of sound power measurements is the sharp drop in costs associated with making the measurement. Instrumentation is less expensive and new test techniques are eliminating the need for costly anechoic or reverberant chambers.

Acoustic Source Location in Vehicle Cabins and Free-field with Nearfield Acoustical Holography via Acoustic Arrays
The technique of Nearfield Acoustical Holography (NAH) is used to identify sources in enclosed spaces with coupled structural and acoustical modes. Pressure measurements made on planes within an enclosure are expanded and projected onto the vibrating surfaces of interest, and active and reactive intensities are calculated. This procedure is applied to locate a known source of acoustic excitation in the interior of a small sport utility vehicle exhibiting strong modal characteristics. Acoustic modes somewhat obscure the source location in the measured pressure distributions, while the reactive acoustic intensity clearly indicates the location of the vibrational input. The NAH technique is also applied to an idling engine with unknown sources radiating acoustic noise into a free-field. The active intensity on the engine surface is reconstructed and the located sources at particular frequencies correlated with rotating engine components.

Comparison of Nearfield Acoustic Holography and Dual Microphone Intensity Measurements
The measurement accuracy of nearfield acoustic holography (NAH) and dual microphone intensity measurement techniques are examined in terms of source identification capabilities and sound intensity level estimates. Inherent differences in the data acquisition and post processing methodologies are investigated. The techniques are applied according to their individual limitation to best evaluate the test structure. The variance of their respective results and how those results aid in engineering solutions is thoroughly discussed.

Resonant Acoustic Method NDT
Fundamentals of Resonant Acoustic Method NDT
Rapid conversion of machined parts to powdered metal and cast is driving industries, especially automotive. Due to the high expectations of both primary manufacturers and end consumers, defects cannot be tolerated even in million piece quantities. There is, in effect, a growing requirement for zero defect supply chain commitments. To achieve zero defect output, manufacturers are making the commitment to move to online NDT. This type of online inspection requires accuracy, reliability, and high throughput. Resonant Acoustic Method NDT (RAM NDT) provides a proven technique exhibiting these pivotal performance requirements and automates economically. RAM NDT tests, reports and screens for most common part flaws in a manner similar to the way NASA tests flight hardware and automotive manufacturers validate their new car designs. Utilizing structural dynamics and statistical variation, RAM NDT provides mature, laboratory proven technology in a robust, economical, process-friendly manner.

Physical Basis of the Resonant Acoustic Method for Flaw Detection
Resonant inspection measures the structural response of a part and evaluates it against the statistical variation from a control set of good parts to screen defects. Its volumetric approach tests the whole part, both for external and internal structural flaws or deviations, providing objective and quantitative results. This structural response is a unique and measurable signature, defined by a component’s mechanical resonances. These resonances are a function of part geometry and material properties and are the basis for Resonant Acoustic Method for Flaw Detection. By measuring the resonances of a part, one determines the structural characteristics of that part in a single test. Typical flaws and defects adversely affecting the structural characteristics for powdered metal as would be introduced in the green state are geometry related and typically a result of handling. This paper introduces the physical basis of the technique.

Total Quality with Rapid Through-put of Powdered Metal and Cast Parts for Whole Part Flaw Detection via Resonant Acoustic Method of Inspection
Rapid conversion of machined parts to powdered metal and cast is driving industries, especially automotive. Due to the high expectations of both primary manufacturers and end consumers, defects cannot be tolerated even in million piece quantities. There is, in effect, a growing requirement for zero defect supply chain commitments. To achieve zero defect output, manufacturers are making the commitment to move to online NDT. This type of online inspection requires accuracy, reliability, and high throughput. Resonant Acoustic Method NDT (RAM NDT) provides a proven technique exhibiting these pivotal performance requirements and automates economically. RAM NDT tests, reports and screens for most common part flaws in a manner similar to the way NASA tests flight hardware and automotive manufacturers validate their new car designs. Utilizing structural dynamics and statistical variation, RAM NDT provides mature, laboratory proven technology in a robust, economical, process-friendly manner.

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