16.81016.810Engineering Design and Rapid PrototypingEngineering Design and Rapid PrototypingInstructor(s)Structural TestingJanuary 18, 2005Prof. Olivier de WeckProf. Olivier de WeckLecture 916.8102Outline!Structural Testing!Why testing is important!Types of Sensors, Procedures ….!Mass, Static Displacement, Dynamics!Test Protocol for 16.810 (Discussion)!Application of distributed load!Wing trailing edge displacement measurement!First natural frequency testing16.8103(1) Sensor Overview: Accelerometers, Laser sensors , Strain Gages ,Force Transducers and Load Cells, Gyroscopes(2) Sensor Characteristics & Dynamics: FRF of sensors, bandwidth, resolution, placement issues (3) Data Acquistion Process: Excitation Sources, Non-linearity, Anti-Alias Filtering, Signal Conditioning (4) Data Post-Processing: FFT, DFT, Computing PSD's and amplitude spectra, statistical values of a signal such as RMS, covariance etc. (5) Introduction to System IdentificationETFE, DynaMod Measurement ModelsData Acquisition and Processing Data Acquisition and Processing for Structural Testingfor Structural Testing16.8104Why is Structural Testing Important?! Product Qualification Testing ! Performance Assessment! System Identification! Design Verification! Damage Assessment! Aerodynamic Flutter Testing! Operational Monitoring ! Material Fatigue TestingFF--22 Raptor #01 during ground 22 Raptor #01 during ground vibration tests at Edwards Air vibration tests at Edwards Air Force Base, Calif., in April 1999Force Base, Calif., in April 1999Example: Ground Vibration TestingExample: Ground Vibration TestingStructuralSystemstimulusu(t)DAQ DSPresponseDAQ = data acquisitionDSP = digital signal processingx(t)16.8105Type Linear Rotational Bandwidth Low Medium High Derivative Position Rate AccelerationReference Absolute Relative Quantity Force/Torque Displacement Impedance Low High I. Sensor OverviewThis Sensor morphology is useful for classification of typical sensors used in structural dynamics. Example: uniaxial strain gageNeed units of measurement: [m], [Nm],[µstrain],[rad] etc…Sensor Morphology TableSensor Morphology Table16.8106Sensor Examples for Structural Dynamicsstrain gagesshakerlaser sensorsaccelerometersgyroscopesload cellsgroundinductivesensorsExample: fixed-fixedbeam with center loadexcitationFirst flexible mode frequency:()3140.375nbEIlm mω=+mbmlGoal: Explain what they measure and how they work16.8107Strain Gages Current Nominal length lo: inooVIlρ=Strain:ollε∆=GNDV+loWith applied strain:()inoVIllερ=+∆Strain gages measure strain (differentialdisplacement) over a finite area via achange in electrical resistance R=lρ [Ω]strain gages feature polyimide-encapsulatedconstantan grids with copper-coated solder tabs.bond to testarticleImplementation:Wheatstone bridgecircuitMfg:16.8108AccelerometersSingle-AxisAccelerometer must bealigned with sensing axis.Accelerometers measure linearacceleration in one, two or threeaxes. We distinguish:• single vs. multi axis accelerometers• DC versus non-DC accelerometersExample: Kistler Piezobeam(not responsive at DC)2(0)() () (0)dxxt sX s sxdt→−−!!Recorded voltage0() ()out aVt KxtV=+!!Can measure: linear, centrifugal and gravitational accelerationUse caution when double-integrating acceleration to get position (drift)(generally neglect initial conditions)Manufacturers:Kistler, Vibrometer, Summit,... magωωωωAccelerometerresponse tobase motion+40dB/decrolloffExample: Summit capacitiveaccelerometer (DC capable)C1C2a16.8109Laser Displacement SensorsRecords displacement directlydisplacement directlyvia slant rangevia slant range measurement.Advantages:contact-free measurementDisadvantages:need reflective, flat targetlimited resolution ~ 1µmtarget() ()xt X s→Voutx [m][VDC]Distance x is recorded via triangulation between the laser diode (emitter), thetarget and the receiver (position sensitive device - PSD).xsensor0-7+7CLrangeManufacturers: Keyence, MTI Instruments,...Vibrometersinclude advanced processing and scanning capabilities.Typical SettingsI: 2µm-60 msII: 15µm-2msIII: 50µm-0.15msResolution tradeoffspatial vs. temporal16.81010Force Transducers / Load CellsForce Transducers/Load Cells are capable of measuringUp to 6 DOF of force on three orthogonal axes, and the moment (torque) about each axis, tocompletely define the loading at the sensor's locationManufacturers: JR3, Transducer Techniques Inc. ...The high stiffness also results in a high resonant frequency, allowing accurate sensor response to rapid force changes. FxFzFyMxMzMyLoad cells are electro-mechanical transducers that translate force or weight into voltage. They usually contain strain gages internally.16.81011Other Sensors!Fiber Optic strain sensors (Bragg Gratings)• Ring Laser Gyroscopes (Sagnac Effect)• PVDF or PZT sensorsIλinputreflectiontransmittedoptical fiberBroad spectrum input light s reflected only at aspecific wavelength determined by the grating spacing which varies with strain.16.81012II. Sensor Characteristics & DynamicsWhen choosing a sensor for aparticular application we mustspecify the following requirements:Sensor Performance Requirements:Sensor Performance Requirements:• Dynamic Range and Span• Accuracy and Resolution• Absolute or Relative measurement• Sensor Time Constant• Bandwidth• Linearity• Impedance• Reliability (MTBF)Constraints:Power: 28VDC, 400 Hz AC, 60 Hz ACCost, Weight, Volume, EMI, HeatXSsaturation limitdynamicrangelinearnon-linearCalibration is the process ofobtaining the S(X) relationshipfor an actual sensor. In the physicalworld S depends on things other than X.Consider modifying input Y (e.g. Temp)E.g. Load cell calibration data:X= mass (0.1 , 0.5 1.0 kg…)S= voltage (111.3 , 563.2, 1043.2 mV)Goal: Explain performance characteristics (attributes of real sensors)S(X)Y16.81013Sensor Frequency Response Functionmckx(t)xb(t)Example: Accelerometerm = 4.5 gk = 7.1e+05 N/mc= 400 Ns/m222()()()absXs cs kGssXs ms csk+==++100101102103104105-18-16-14-12-10-8-6-4-202Frequency [Hz]Magnitude [dBlIdeal Accelerometer FRF from Base MotionTypically specify bandwidth as follows:Frequency Response +/-5%: 0.5-2000 HzbandwidthNote: Bandwidth of sensor shouldbe at least 10 times higher thanhighest frequency of signal s(t)Example: Kistler 8630B Accelerometer16.81014Sensor Time ConstantFirstFirst--Order InstrumentsOrder Instruments1 oodyaaybudt+=Dividing by aogives:""10oobadyyuadt aKτ+=In s-domain:()() 1Ys KUs sτ=+ττττ : time constantK: