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Condition Monitoring ArchitectureTo Reduce Total Cost of Ownership
Eric Bechhoefer, NRG SystemsBrogan Morton, NRG Systems
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Barriers to Sales of PHM Systems
• CBM/PHM System are Proven to Work– Low Penetration into Commercial Markets– Example: 3% of Wind Turbines
• Why? - Business Case is Hard to Make– Safety not the primary concern, cost avoidance is– Hard to Quantify Benefit
• Change Architecture to Improve Value– Lower “Costs” and Better Information
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Current System Architecture• System Hardware
– 6 to 8 PZT Accelerometers• 5% Accuracy, .5 to 10,000 Hz
– Tachometer– Signal Conditioning
• 6 to 12 channels• Sample Rate: 60 to 80 KSPS
• Support/Monitoring Services– Human in the loop to turn data into a diagnosis– $1,000 to $1,500 per year per turbine
• IT Infrastructure– Data hosting on local server– Data also shipped to centralized analysis center
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System Layout: Wind Turbines
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From a System Perspective…
• How to Lower Total Ownership Cost– Hardware Considerations
• Costs driven by accelerometer
– Software/Support Considerations• Costs driven by knowledge creation (data to diagnosis)
– IT Infrastructure Considerations• Cost driven by local data storage and associated
maintenance
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Accelerometers – MEMS vs. PZT
MEMS Advantages• Cost
– $6 to $30 vs. $100’s
• Bandwidth– 0 to 32,000 Hz vs. 0.5 to
10,000 Hz
• Accuracy– Typically 1% vs. 5% or 10%
Error
• Self Test– Can Enable BIT vs. No BIT
MEMS Disadvantages• Needs to be Packaged
– No Trivial Task
• Noisier– PDS is 2 to 40x higher
System Issues• 4 Wire?
– Power/Signal
• Local Conversion?– ADC, then Microcontroller
and RT
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Sensor System Considerations
• Low cost target – move to MEMS– Analog vs. Digital Sensing
• If Digital– Local ADC,
• EMI is Reduced
– Microcontroller, RAM, Receiver/Transmitter• If Multi-Drop: RS-485
– If Microcontroller: Local Processing?• Many Smaller, Cheap Processors vs. One Larger Processor
• Low cost packaging– Alternative to Stainless Steel or Titanium– Transfer Function – Has to Be Stiff/Light
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MEMS: A Sensor Solutions
• Noise Was Not An Issue– After Signal Processing,
Noise was Negligible
• Conductive Plastic Package– 40% Mass of Stainless– Similar Stiffness– 12% of Cost of Stainless– 6.5KHz Resonance, Flat
Response to 17 KHz
1000 mv/g vs. 70 mv/gMEMS Accel, 0.25 HzWithin 2% of Low G Accel
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Embedded PHM• Micro with FPU Support
– 32MB RAM– 24 Bit ADC– Sample @ 300-100,000 kbps– R/T > 500 KB/S
• Local Vibe Processing– Time Synchronous Average
(TSA)– FFT/IFFT– Hilbert Transform
• Total Cost: Similar to PZT Accel
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Software & Support Considerations
• Algorithmic– Digital signal processing of the vibration signals for
fault detection• Knowledge Creation
– Goal: Actionable information requiring little interpretation
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Main Shaft
MainBearing
3-stage Gearbox
Generator
• 17 Bearings• 9 Gears• 8 Shaft
Low Speed Shaft
Int. Speed Shaft
High Speed Shaft
Car
rier
Pla
te
Typical Drivetrain Configuration
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Algorithmic
• Process vibration signal into indications of faults– Data reduction without loss of information
• No Spectrums/Order Analysis– Configurable Analysis for Shafts, Gears and
Bearings,– Several Condition Indicators for Each Component
• Use Time Synchronous Average (TSA)
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Why This Approach
• Large Variation in Wind Speeds Cause Large Changes in Rotor Speed
• 3/Rev Torque/Speed Ripple From Tower Shadow/Wind Shear
• Gearbox has many gear meshes; isolate gears of interest
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• Due to Changes in Rotor Speed, Order Analysis or the PSD Cause Smearing of Frequency Content
• Example Main Rotor Shaft
Example of Spectrum Vs. TSA
1st, 2nd, and 3rd Harmonics of Ring Gear Frequency
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The TSA
• Use Tachometer as Phase Reference on Shaft• Reduces Non-Synchronous Noise 1/sqrt(revolutions)• For Each Revolution (From Tach)• Resample length m = 2^Ceiling(log2(number of points in Rev))
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Gear Fault Indicators
• No Single CI Works With All Fault Modes– Surface Disturbance, Scuffing, Deformation,
Surface Fatigue, Cracks, Tooth Breakage, Eccentricity
• Use a Number of Analysis to Cover All Fault Modes– Residual Analysis, Energy Operator, Narrow Band
Analysis, Amplitude Modulation Analysis, Frequency Modulation Analysis.
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Gear Analysis
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Knowledge Creation
• Recall Goal: Create actionable information requiring little interpretation– Convey what to fix and when
• Single Health Indicator for Each Component– Fusion of different condition
indicators– Common scale for every
component (0-1)
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Health as a Function of Distributions
• HI Paradigm: Map the CIs into an HI– HI Ranges from 0 to 1, Where the Probability of
the HI exceeding 0.5 is the PFA– HI in Warning when between 0.75 and 1– HI is Alarm when Greater than 1.0– Continued Operations with HI > 1 could Cause
Collateral Damage
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Controlling Correlation Between CIs
• All CIs have PDFs• Any Operation on the CI to
form an HI is a Function of Distributions– Max of n CI (an Order Statistics)– Sum of n CI– Norm of n CI
• Function of Distribution– PFA Correct if Distribution are
IID– Need to Whiten
ij CI 1 CI 2 CI 3 CI 4 CI 5 CI 6
CI 1 1 0.84 0.79 0.66 -0.47 0.74
CI 2 1 0.46 0.27 -0.59 0.36
CI 3 1 0.96 -0.03 0.97
CI 4 1 0.11 0.98
CI 5 1 0.05
CI 6 1
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CI to HI Mapping
• Six CIs used in HI Calculation– Residual RMS– Energy Operator RMS– FM0– Narrowband Kurtosis– AM Kurtosis– FM RMS
• Statistics Generated from 4 test articles: 100 samples prior to fault propagation
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IT Infrastructure
For Owner/Operator• No Seat License of the CMS
Database• No Local Servers to Host Data• Management of Software
Maintenance. • Allows Pooling of Dataset of
Similar Type/Model Turbines without Risk of Exposing Proprietary Information
For CMS Developer• Simplifies Software Maintenance
Cost– Only one Platform to Develop
and Test to, – Only one Platform to Deploy
Software Updates/Patches to,• Reduces the Cost of Certification
– Configuration Management is Greatly Simplified
• Scalability
Alternate to Local Server: Cloud Computing
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Conclusion
• Significant value can be created by redesigning system architecture– Vibration sensing
• Non-traditional sensor, new packaging and design methods
– Advanced signal processing techniques• Increased sensitivity to faults under dynamic conditions
– Knowledge Creation• Automated fusion of fault modes• Actionable information with diagnostic support
– IT Infrastructure• Economy of scale using cloud services