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Ground Vibration Testing
Noise, Vibration and Harshness
Experimental Modal Analysis
Material and Fatigue Testing
Aerospace Vibration Testing
Laser-based Vibration Measurement Technology Helps toIncrease Performance, Improve Time-to-market and Lower Costsin Aerospace Development
Aerospace Vibration Testing
Todays market pressure for new,affordable high performance aerospaceproducts is increasing the number ofproduct variants and the complexity of tested structures. Product develop-ment and design refinement teams are requesting more efficient modaltesting to increase throughput, whilemaintaining accuracy adequate to correlate with FE analysis models (i.e.load analysis, acoustic radiation, etc.).
In addition, these new structuresrequire a substantial number of spatialdata points. The combination of morestructures and and more measurementpoints is rapidly increasing the costs ofdoing a traditional modal test with itslabor intensive approach of instrumen-ting structures with accelerometers andmulti-channel data acquisition systems.
Noise, Vibration and Harshness
The fight for commercial aircraft ordershas lead aerospace manufacturers toseek competitive advantages in twosignificant areas: fuel economy andincreased passenger comfort. Conse-quently, todays aerospace engineersare more concerned with noise mea-
surements than their predecessorswere. By improving interior soundquality, aircraft engineers increase passenger comfort and desire to fly in a next generation commercial jet. In addition, by reducing exterior noise,the designer can improve the aircraftsacceptance in urban settings where airtraffic is growing rapidly.
Polytec Vibrometers are a requirementfor leading aerospace companies eagerto make NVH measurements on theirnewest aircraft.
Vibration Measurements in Aerospace Development
Structural testing is an integrated part of aero-
space product design, development and manufacture.
It is an essential step to ensure performance, quality,
safety and reliability in the final product.
Material delamination detected by non-linear laser vibrometry. Photo courtesy:IKP-ZFP, University of Stuttgart.
At the Air Force Institute of Technology(AFIT), Polytecs PSV-400-3D ScanningVibrometer technology is used tomeasure vibration characteristics ofUnmanned Aerial Vehicles (UAV) andother intricate aerospace and vehicularstructures. Its use has improved theresults of flutter analysis for airplane FE and reduced significantly test timeand post processinganalysis.
The demand for environ-mentally friendly and morepowerful jet engines ispushing engine designto new limits.
The correct interpretationof lifetime-relevant vibration
phenomena is one of the most chal-lenging and important tasks which canbe successfully solved by laser vibro-metry. Read the article on page 12.
Predator UAV photo courtesy ofGeneral Atomics.
Measurement of turbine blade vibrations.Photo courtesy: Greg Roberts, Pratt & Whitney.
Flying the BestScanning laser vibrometry features rapid, full-field, non-contact (no massloading) vibration measurement with high spatial and frequency resolution.By using Polytecs Scanning Vibrometers, aerospace development engineersand scientists can reduce both the time and complexity of vibration testing.Polytec vibrometers are the gold standard for non contact vibration meas-urement for aerospace development, quality control and aircraft healthmonitoring. Find more detailed information on page 15 or visit www.polytec.com/usa/aerospace.
MEMS and PCB Testing
Laser vibrometry is the first choice for vibration testing printed circuit boards and micro-electro-mechanical sensors and actuators (www.mems-analysis.com).
Ground Vibration Testing
Ground Vibration Testing (GVT) is acostly requirement for new aircraft andaerospace structures. Data taken canbe used for modal analysis and finiteelement (FE) model correlation, forloads analysis to prevent structural failure and flutter certification.
A vibration analysis of an aircraft com-ponent can characterize the structuraldynamics, determine the fundamentalfrequencies and define a completemodal model of the component.
For instance, aircraft tires are criticalcompo-
very high qualitystandards. Read more about
experimental 3-D scanning vibrometermeasurements on an A320 aircraftwheel (page 7).
Material delamination and crackingare common defects that can signifi-cantly degrade the performance ofaerospace products. To find localizeddefects, both nonlinear laser vibrometry(www.polytec.com/usa/aerospace)and lamb wave detection (article onpage 9) are successfully used as a meansof non-destructive testing (NDT).
Find a comprehensive article aboutground-based, dynamic testing ofsolar sails at NASA on page 4.
Operational deflection shape of an aircraft wheel.
NASA has been developing Gossamerspace structures for many years to reducelaunch costs and to exploit the uniquecapabilities of particular concepts. For instance, dish antennas (Figure 1)are currently being pursued becausethey can be inflated in space to sizes aslarge as 30 meters and then rigidizedto enable high data rate communica-tions. Another example of a Gossamerstructure is a solar sail that provides acost effective source of propellantlesspropulsion. Solar sails span very largeareas to capture momentum energyfrom photons and to use it to propel a spacecraft. The thrust of a solar sail,though small, is continuous and actsfor the life of the mission without theneed for propellant. Recent advancesin materials and ultra-lightweightGossamer structures have enabled ahost of useful space exploration mis-sions utilizing solar sail propulsion.
The team of ATK Space Systems, SRS Technologies, and NASA LangleyResearch Center, under the direction ofthe NASA In-Space Propulsion Office (ISP),has developed and evaluated a scalablesolar sail configuration (Figure 2) toaddress NASAs future space propulsionneeds. Testing of solar sails on theground presented engineers withthree major challenges:
Measurements on large area surfacesthinner than paper
Air mass loading under ambientconditions was significant thusrequiring in-vacuum tests
High modal density required partitioning of the surface intomanageable areas.
This article will focus on the uniquechallenges with vacuum chamber,dynamic testing of a 20-meter solarsail concept at the NASA Glenn PlumBrook Facility (Figure 3).
A Polytec Scanning Laser Vibrometersystem (PSV-400) was the main in-strument used to measure the vibra-tion modes. The laser scan head wasplaced inside a pressurized canister toprotect it from the vacuum environment(Figure 4). The canister had a windowport from which the laser exited, anda forced air cooling system preventedoverheating. A Scanning Mirror System(SMS) was developed and implemented,that allowed full-field measurements of the sail from distances in excess of60 meters within the vacuum chamber.
The SMS (Figure 5) was mounted nearthe top of the vacuum chamber facilityand centered over the test article, whilethe vibrometer head was mountedabove the door frame of one of thelarge chamber doors. The SMS con-tained a stationary mirror that reflectedthe Polytec laser beam to a system oftwo orthogonal active mirrors.
Figure 1: Inflatable 4x6-meter communications antenna concept.
Figure 2: Deployed 20-meter solar sail on vacuum chamber floor.
These mirrors were used to scan thesurface of the sail to find retro-reflectivetargets previously attached to the sailsurface. These targets were essential to getting a good return signal andovercoming the specular nature of the reflective sail surface.
Fully Automated Test Procedure
A specially developed target trackingalgorithm enabled automatic centeringof the laser beam on each retro-reflectivetarget. The initial laser system alignment,
Sail AwayLaser Vibrometry Helps to Validate Gossamer Space Structures
NASA is pursuing the development of large ultra-lightweight structures commonly referred to as Gossamer
space structures. These structures have large areas and small aerial densities, which complicates ground
testing significantly as the ground operations interfaces and gravity loading can become cumbersome. Laser
vibrometry has proven to be a critical sensing technology for validating the dynamical characteristics of
these Gossamer structures, due to its precision, range, and non-contacting (zero-mass loading) nature.
Figure 3: Vacuum chamber facility. Figure 4: PSV-400 Scanning Vibrometer inpressurized canister.
Figure 5: Scanning Mirror System inside vacuum chamber
target tracking process, and entiredata acquisition procedure was auto-mated using the Microsoft Visual Basic (VB) programming language. Polytecs VB Engine and PolyFileAccessallowed the program to control all the functional capability of the Polytecsystem. The alignment of the vibro-meter laser to the SMS steering mirrorswas accomplished by software thatused the vibrometer scan mirrors totrace out a square grid across a retro-reflective target ring on the SMS. The strength of the laser return signalwas measured during the scan. Thesoftware finds the angular location ofthe center of the target by calculatingthe centroid of this array of signalstrength values and the correspondingmirror angles.
Once the laser was aligned to theSMS, a second program aligned thelaser to the targets on the solar