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1、优秀精品课件文档资料,The Art ofInstrumentation&Vibration Analysis,Back to the Basics Forward to the Future,Our Objective,The objective of Condition Monitoring is to provide information that will keep machinery operating longer at the least overall cost.What it is NOT:Establish new measured point recordsMeans
2、to show analytical brilliance The answer to every problem!,Back to the Basics,VibrationSimple Harmonic MotionOscillation about a Reference PointModeled Mathematically as,Back to the Basics,Back to the Basics,Basic Signal AttributesStaticSlowly ChangingTemperature,Basic Signal AttributesDynamicSensor
3、 must respond in fractions of a SecondVibration,Amperage,Pressure,Back to the Basics,Dynamic Signal FundamentalsAmplitudeFrequencyTimingShape,AmplitudeProportional to the severity of vibratory motionExpressed asPeak to PeakZero to PeakRMS,FrequencyDetermined by the reciprocal of the PeriodCPS or HzR
4、PMOrders,Timing,or PhaseRepresented by the time delay between two signalsLeading Lagging,Signal ShapeWaveformSimpleComplexPattern Recognition,Peak and RMS Comparison,Relationships of Acceleration,Velocity and Displacement,The Big Picture,Sensor(s),Cables,Signal Conditioning,Data Acquisition&Storage,
5、Communications,Remote Analysis and Diagnostics,Displacement Sensors,ElementsProbe,matched extension cable,Driver,Displacement Sensors,How it Works:The tip of the probe contains an encapsulated wire coil which radiates the drivers high frequency as a magnetic field.When a conductive surface comes int
6、o close proximity to the probe tip,eddy currents are generated on the target surface decreasing the magnetic field strength,leading to a decrease in the drivers DC output.This DC output is usually 200mV/mil or in a similar range.,Displacement Sensors,Pros and ConsProsMeasures DisplacementRuggedConsL
7、imited Frequency Range(0-1000Hz)Susceptible to electrical or mechanical runoutInstallation Issues,Velocity Sensors,Pros and ConsProsMeasures VelocityEasier Installation than DisplacementConsLimited Frequency Range(0-1000Hz)Susceptible to Calibration ProblemsLarge Size,Acceleration Sensors,Pros and C
8、onsProsMeasures Accel.Small SizeEasily InstalledLarge Frequency Range(1-10,000 Hz)ConsMeasures Acceleration(requires Integration to Vel.)Susceptible to Shock&Requires Power,Machine Speed Sensors,Displacement ProbesActive or Passive Magnetic ProbesOptical PermanentStroboscopesLaser Tach,Voltage or Cu
9、rrent?,Current Output Accelerometers4-20 mA Output Proportional to Dynamic Signal and/or OverallVoltage Output AccelerometersPreferred in U.S.Generally 100mV per g Sensitivity,AC and DC Signal Components,Signals have both AC and DCAC considered the“Dynamic”SignalDC is the“Static”SignalDisplacement P
10、robes Set“Gap”for DCAccelerometers“Bias”voltage is DC,AC and DC Signal Components,How AC and DC work together:AC signal“rides”the DC bias(VB)Affects the Dynamic Range of the Sensor.,Power Circuit for Accelerometers,“Strips off”DC Voltage,Grounds,A Potential Problem SourceGround LoopsCaused when two
11、or more grounds are at different potentialsSensors should be grounded only at the sensor,not the monitoring rack!,Sensor Cables,Coaxial with BNC ConnectorsLong Coaxial can become antennas!Twisted,Shielded PairTeflon Shield ground at only one end!,Sensor Cables,Driving Long CablesUnder 90 feet,cable
12、capacitance no problem Cable Capacitance specd in Pico-farads per foot of cable lengthOver 90 feet or so,CCD must supply enough current to charge the cable as well as the sensor amplifier.May result in amplifier output voltage becoming“Slew Rate Limited”,Sensor Cables,Output of Sinusoid looks like t
13、his:,Whats Happening?The+part of the signal is being limited by the current available to drive the cable capacitance.In the part of the sin wave,the op-amp must“sink”the current being discharged by the cable capacitance.,Sensor Cables,Practical Effect:Signal distortion produces harmonicsMay lead to
14、vibration signals being misinterpreted.To calculate the maximum frequency for a length of cable:,Signal Conditioning,GainIntegration(Hardware)AC/DC CouplingAnti-Aliasing Filter(s)Sample and Hold Circuit,Signal Gain Circuit,X1 and X10 are CommonGain is simply amplification of a SignalCareful Should k
15、now your vibration level and the ADC input range first!100mV/g accel;+-5V input range=+-50 gsCan“Clip”Signal,Signal Integration,Best to Integrate as close to signal source as possibleReduces noise,AC/DC Coupling,Normally,Systems are AC coupledMeans that there is a DC blocking Capacitor that only all
16、ows AC signal through to the systemMAARS InnovationDC Switch that allows AC and DC to work on the same data channel without contaminating phaseAllows use of same channel to record data for shaft centerline(DC)and Transient data(AC),Anti-Aliasing Filters,What are they and why do I need them?Because“f
17、alse Frequencies”are displayed when Aliasing is present in a system.The maximum frequency component a sampled data system can accurately handle is its Nyquist limit.The sample rate must be greater than or equal to two times the highest frequency component in the input signal.When this rule is violat
18、ed,unwanted or undesirable signals appear in the frequency band of interest.,Aliased Signals,In old western movies,as a wagon accelerates,the wheel picks up speed as expected,and then the wheel seems to slow,then stop.As the wagon further accelerates,the wheel appears to turn backwards!In reality,we
19、 know the wheel hasnt reversed because the rest of the movie action is still taking place.What causes this phenomenon?The answer is that the shutter frame rate is not high enough to accurately capture the spinning of the wheel.,Aliased Signals,False low-frequency sin waveCaused by sampling too slowl
20、yViolated the Nyquist Criterion,Anti-Aliasing Filters,What are they and why do I need them?Generally they are low-pass filters that do not pass frequencies above the ADCs range.Here is a representation of an IDEAL filter,Real Anti-Aliasing Filters,Trade-offs:Elliptic,Chebyshev,Butterworth and Bessel
21、Elliptic sharpest rolloff,highest rippleBessel Lowest ripple,fat rolloff.key advantage is that it has a linear phase response,Sample and Hold Circuit,Purpose is to take a snapshot of the sensor signal and hold the value.The ADC must have a stable signal in order to accurately perform a conversion.Th
22、e switch connects the capacitor to the signal conditioning circuit once every sample period.The capacitor then holds the voltage value measured until a new sample is acquired.,Data Acquisition and Storage,Analog to Digital ConverterHard disk vs.Flash MemoryPhysical download vs.Ethernet file Transfer
23、FFT ConversionWindowing,ADC Analog-to-Digital Converters,The purpose of the analog to digital converter is to quantize the input signal from the S&HThe input voltage can range from 0 to Vref What this means is that the voltage reference of the ADC is used to set the conversion range 0V input will ca
24、use the converter to output all zeros.If the input to the ADC is equal to or larger than Vref,then the converter will output all ones.For inputs between these two voltages,the ADC will output binary numbers corresponding to the signal level.,ADC Analog-to-Digital Converters,Dynamic RangeUsually defi
25、ned in dB,depends on the number of bits used by the ADCFor example,a 12 bit ADC has 212 possible data values,or 4,096“steps”between the lowest and highest values the ADC can see(0 to 5 Volts,typ.)8-bit is 256 steps16-bit is 65,536 steps,so more is better,right?,ADC Analog-to-Digital Converters,Wrong
26、!Steve Goldmans Book pp.46-47“Dynamic Range:The Big Lie”“That the A/D Converter can sense one part in 16 binary bins is no assurance that the analog circuitry is good enough to insure that the information going into the lower bins is not contaminated by electrical noise.”,ADC Analog-to-Digital Conve
27、rters,Dynamic RangeFor a 12 bit ADC20 log(4095/1)=72 dbTheoretical only,electronic noise reduces to 65 dbFor a 16 bit ADC20 log(65536/1)=96 dbElectronic noise may make this only 80 dbMassively more data to manipulate w/o much practical gain in Dynamic Range.,ADC Analog-to-Digital Converters,Sampling
28、 Rate“Real-Time”Rate in samples/sec60,000 samples per sec/2.56=23,437 Hz FmaxMay also get divided by the number of channels in a multi-channel system,Windowing,Required to solve“Leakage”Several TypesUniformHanning Most Commonly usedHammingBlackman-Harris,Windowing,Why do we use the Hanning Window?Be
29、st compromise between frequency resolution and amplitude accuracy for steady-state machinery analysisUniform or Flat-Top is the best choice for transient machinery analysis.,Windowing,What is leakage?Caused when the time waveform signal does NOT begin and end at the same point,introducing spurious f
30、requencies.The Window or weighting function attenuates the signal towards the edge of the window minimizing leakage.,Windowing,Example:,Windowing,Leakage Example:,Windowing,Hanning Window:,Types of Averaging,Linear Most commonly usedPeak Hold Coastdown and ImpactExponentialWeights most recently acqu
31、ired data more heavily used for ImpactTime Synchronous TSATriggered by tach Shaft and Harmon.,Trending Overalls,Limited ValueBetter than NothingMay miss some types of failures,Spectral Resolution,Common Values100 to 3200“Lines”400 or 800 typicalFmax/Lines=Frequency Resolution1000 Hz/400 lines=2.5 Hz
32、 Resolution,Spectral Integration,Where does the“Ski-Slope come from?Integrating Acceleration to get Velocity pops out a constant value,which is manifested as a“DC”component because it has no frequency dependence!,Spectral Integration,How do we solve this problem?,Spectral Integration,Truth is we can
33、t!Its PHYSICS!What we can do is“Zero”the first 5 or so Spectral Bins!,Spectrum Analysis,Machine Component ConditionIdentified by FrequencySeverity Indicated by AmplitudeRate of Deterioration Indicated by Spectral Comparison over Time,Spectrum Analysis,Waveform Analysis,Pattern Recognition is KeyRequ
34、ires understanding of Machine ComponentsGearboxBearings,Waveform Analysis,Orbit Analysis,Transient Analysis,Long-Term Time WaveformsBode Nyquist PlotsRPM vs.TimeWaterfall PlotsCascade Plots,Machine Transients,Vibration Severity,When do I make the call?Alarm LevelsFault LevelsDo you use GM,API,ISO Gu
35、idelines?Risk vs.Reward,Communications,Area of Greatest Technology ProgressEmail,FTP,Internet(High Speed)Industrial EthernetWireless Phone,Modem,EthernetSatelliteDigital Revolution!(Remote Desktop),Communications,Analysis and Diagnostics,Area of LEAST ProgressNot Fundamentally Changed in 20 yearsPer
36、sonnel Downsizing not going to come back,eitherWhat is a Vibration Analysts Career Path?In-house are becoming contracted servicesConstant re-training to solve yesterdays problems!,Analysis and Diagnostics,Will Technology come to the Rescue?Remote,centralized DiagnosticsRapid Service Company GrowthRapid Growth in Wireless Sensor Technology has CooledPower Supply ProblemSpawned new VC-backed Research Companies,
