振動分析和儀器的藝術(shù)達(dá)芬奇課件

1、優(yōu)秀精品課件文檔資料The Art ofInstrumentation & Vibration AnalysisBack to the Basics Forward to the FutureOur ObjectiveThe 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 to

2、 show analytical brilliance The answer to every problem!Back to the BasicsVibrationSimple Harmonic MotionOscillation about a Reference PointModeled Mathematically as Back to the BasicsPeriod, TRMSUnit CirclePeak-to-Peak0 to Peak0Back to the BasicsBasic Signal AttributesStaticSlowly ChangingTemperatu

3、reBasic Signal AttributesDynamicSensor must respond in fractions of a SecondVibration, Amperage, PressureBack to the BasicsDynamic Signal FundamentalsAmplitudeFrequencyTimingShapeAmplitudeProportional to the severity of vibratory motionExpressed asPeak to PeakZero to PeakRMSFrequencyDetermined by th

4、e reciprocal of the PeriodCPS or HzRPMOrdersTiming, or PhaseRepresented by the time delay between two signalsLeading LaggingSignal ShapeWaveformSimpleComplexPattern RecognitionPeak and RMS ComparisonRelationships of Acceleration, Velocity and DisplacementThe Big PictureSensor(s)CablesSignal Conditio

5、ningData Acquisition & StorageCommunicationsRemote Analysis and DiagnosticsDisplacement SensorsElementsProbe, matched extension cable, DriverDisplacement SensorsHow it Works:The tip of the probe contains an encapsulated wire coil which radiates the drivers high frequency as a magnetic field. When a

6、conductive surface comes into 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 SensorsPros and ConsProsMe

7、asures DisplacementRuggedConsLimited Frequency Range (0-1000Hz)Susceptible to electrical or mechanical runoutInstallation IssuesVelocity SensorsPros and ConsProsMeasures VelocityEasier Installation than DisplacementConsLimited Frequency Range (0-1000Hz)Susceptible to Calibration ProblemsLarge SizeAc

8、celeration SensorsPros and ConsProsMeasures Accel.Small SizeEasily InstalledLarge Frequency Range (1-10,000 Hz)ConsMeasures Acceleration (requires Integration to Vel.)Susceptible to Shock & Requires PowerMachine Speed SensorsDisplacement ProbesActive or Passive Magnetic ProbesOptical PermanentStrobo

9、scopesLaser TachVoltage or Current?Current Output Accelerometers4-20 mA Output Proportional to Dynamic Signal and/or OverallVoltage Output AccelerometersPreferred in U.S.Generally 100mV per g SensitivityAC and DC Signal ComponentsSignals have both AC and DCAC considered the “Dynamic” SignalDC is the

10、 “Static” SignalDisplacement Probes Set “Gap” for DCAccelerometers “Bias” voltage is DCAC and DC Signal ComponentsHow 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 VoltageGroundsA Potential Problem So

11、urceGround LoopsCaused when two or more grounds are at different potentialsSensors should be grounded only at the sensor, not the monitoring rack!Sensor CablesCoaxial with BNC ConnectorsLong Coaxial can become antennas!Twisted, Shielded PairTeflon Shield ground at only one end!Sensor CablesDriving L

12、ong CablesUnder 90 feet, cable 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

13、Output of Sinusoid looks like this: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 CablesPractical Effect:Signal distort

14、ion produces harmonicsMay lead to vibration signals being misinterpreted.To calculate the maximum frequency for a length of cable:Signal ConditioningGainIntegration (Hardware)AC/DC CouplingAnti-Aliasing Filter(s)Sample and Hold CircuitSignal Gain CircuitX1 and X10 are CommonGain is simply amplificat

15、ion of a SignalCareful Should know your vibration level and the ADC input range first!100mV/g accel; +-5V input range = +-50 gsCan “Clip” SignalSignal IntegrationBest to Integrate as close to signal source as possibleReduces noiseAC/DC CouplingNormally, Systems are AC coupledMeans that there is a DC

16、 blocking Capacitor that only allows 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 FiltersWhat are the

17、y and why do I need them?Because “false 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

18、input signal. When this rule is violated, unwanted or undesirable signals appear in the frequency band of interest. Aliased SignalsIn 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 w

19、heel appears to turn backwards! In reality, we 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 SignalsFalse low

20、-frequency sin waveCaused by sampling too slowlyViolated the Nyquist CriterionAnti-Aliasing FiltersWhat 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 filterReal Anti-Aliasing FiltersTrade-off

21、s: Elliptic, Chebyshev, Butterworth and BesselElliptic sharpest rolloff, highest rippleBessel Lowest ripple, fat rolloff.key advantage is that it has a linear phase response Sample and Hold CircuitPurpose is to take a snapshot of the sensor signal and hold the value. The ADC must have a stable signa

22、l in order to accurately perform a conversion. The 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 StorageAnalog to Digital ConverterHard disk vs. Flash

23、 MemoryPhysical download vs. Ethernet file TransferFFT ConversionWindowingADC Analog-to-Digital ConvertersThe 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

24、used to set the conversion range 0V input will cause 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 A

25、nalog-to-Digital ConvertersDynamic RangeUsually defined 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

26、more is better, right?ADC Analog-to-Digital ConvertersWrong!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 co

27、ntaminated by electrical noise.”ADC Analog-to-Digital ConvertersDynamic 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

28、 gain in Dynamic Range.ADC Analog-to-Digital ConvertersSampling 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 systemWindowingRequired to solve “Leakage” Several TypesUniformHanning Most Commonly usedHa

29、mmingBlackman-HarrisWindowingWhy do we use the Hanning Window?Best compromise between frequency resolution and amplitude accuracy for steady-state machinery analysisUniform or Flat-Top is the best choice for transient machinery analysis.WindowingWhat is leakage?Caused when the time waveform signal d

30、oes NOT begin and end at the same point, introducing spurious frequencies.The Window or weighting function attenuates the signal towards the edge of the window minimizing leakage.WindowingExample:WindowingLeakage Example:WindowingHanning Window:Types of AveragingLinear Most commonly usedPeak Hold Co

31、astdown and ImpactExponentialWeights most recently acquired data more heavily used for ImpactTime Synchronous TSATriggered by tach Shaft and Harmon.Trending OverallsLimited ValueBetter than NothingMay miss some types of failuresSpectral ResolutionCommon Values100 to 3200 “Lines”400 or 800 typicalFma

32、x/Lines = Frequency Resolution1000 Hz/400 lines = 2.5 Hz ResolutionSpectral IntegrationWhere 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 IntegrationHow do we solve this problem?Spectral IntegrationTruth is we cant!Its PHYSICS!What we can do is“Zero” the first 5 or so Spectral Bins!Spectrum AnalysisMachine Component ConditionIdentified by FrequencySeverity Indicated by AmplitudeRate of Deterioration Indicated by Spectra