The Art ofInstrumentation & Vibration AnalysisBack to the Basics Forward to the Future

Our 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 show analytical brilliance The answer to every problem!

Back to the BasicsVibrationSimple Harmonic MotionOscillation about a Reference PointModeled Mathematically as

Back to the Basics

Back to the BasicsBasic Signal AttributesStaticSlowly ChangingTemperature

Basic Signal AttributesDynamicSensor must respond in fractions of a SecondVibration, Amperage, Pressure

Back to the BasicsDynamic Signal FundamentalsAmplitudeFrequencyTimingShape

AmplitudeProportional to the severity of vibratory motionExpressed asPeak to PeakZero to PeakRMS

FrequencyDetermined by the reciprocal of the PeriodCPS or HzRPMOrders

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 PictureSensor(s)CablesSignal ConditioningData Acquisition & StorageCommunicationsRemote Analysis and Diagnostics

Displacement SensorsElementsProbe, matched extension cable, Driver

Displacement SensorsHow it Works:

The tip of the probe contains an encapsulated wire coil which radiates the driver's high frequency as a magnetic field. When a 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 driver's DC output. This DC output is usually 200mV/mil or in a similar range.

Displacement SensorsPros and ConsProsMeasures DisplacementRuggedConsLimited Frequency Range (0-1000Hz)Susceptible to electrical or mechanical runoutInstallation Issues

Velocity SensorsPros and ConsProsMeasures VelocityEasier Installation than DisplacementConsLimited Frequency Range (0-1000Hz)Susceptible to Calibration ProblemsLarge Size

Acceleration SensorsPros and ConsProsMeasures Accel.Small SizeEasily InstalledLarge Frequency Range (1-10,000 Hz)ConsMeasures Acceleration (requires Integration to Vel.)Susceptible to Shock & Requires Power

Machine Speed SensorsDisplacement ProbesActive or Passive Magnetic ProbesOptical PermanentStroboscopesLaser Tach

Voltage or Current?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 ComponentsSignals have both AC and DCAC considered the Dynamic SignalDC is the Static SignalDisplacement Probes Set Gap for DCAccelerometers Bias voltage is DC

AC 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 AccelerometersStrips off DC Voltage

GroundsA Potential Problem SourceGround 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 Long 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 CablesOutput 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 distortion 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 Circuit

Signal Gain CircuitX1 and X10 are CommonGain is simply amplification of a SignalCareful Should know your vibration level and the ADC input range first!100mV/g accel; +-5V input range = +-50 gsCan Clip Signal

Signal IntegrationBest to Integrate as close to signal source as possibleReduces noise

AC/DC CouplingNormally, Systems are AC coupledMeans that there is a DC 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 they 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 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 wheel appears to turn backwards! In reality, we know the wheel hasn't 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-frequency sin waveCaused by sampling too slowlyViolated the Nyquist Criterion

Anti-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 filter

Real Anti-Aliasing FiltersTrade-offs: 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 signal 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 MemoryPhysical download vs. Ethernet file TransferFFT ConversionWindowing

ADC 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 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 Analog-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 more is better, right?

ADC Analog-to-Digital ConvertersWrong!Steve Goldmans Book pp.46-47Dynamic Range: The Big LieThat 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 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 gain in Dynamic Range.

ADC Analog-to-Digital ConvertersSampling RateReal-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

WindowingRequired to solve Leakage Several TypesUniformHanning Most Commonly usedHammingBlackman-Harris

WindowingWhy 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 does 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 Coastdown 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 failures

Spectral ResolutionCommon Values100 to 3200 Lines400 or 800 typicalFmax/Lines = Frequency Resolution1000 Hz/400 lines = 2.5 Hz Resolution

Spectral 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 isZero the first 5 or so Spectral Bins!

Spectrum AnalysisMachine Component ConditionIdentified by FrequencySeverity Indicated by AmplitudeRate of Deterioration Indicated by Spectral Comparison over Time

Spectrum Analysis

Waveform AnalysisPattern Recognition is KeyRequires understanding of Machine ComponentsGearboxBearings

Waveform Analysis

Orbit Analysis

Transient AnalysisLong-Term Time WaveformsBode Nyquist PlotsRPM vs. TimeWaterfall PlotsCascade Plots

Machine Transients

Vibration SeverityWhen do I make the call?Alarm LevelsFault LevelsDo you use GM, API, ISO Guidelines?Risk vs. Reward

CommunicationsArea of Greatest Technology ProgressEmail, FTP, Internet (High Speed)Industrial EthernetWireless Phone, Modem, EthernetSatelliteDigital Revolution! (Remote Desktop)

Communications

Analysis and DiagnosticsArea of LEAST ProgressNot Fundamentally Changed in 20 yearsPersonnel 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 DiagnosticsWill 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