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1、优秀精品课件文档资料,The Art of Instrumentation & 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 records
2、Means to show analytical brilliance The answer to every problem!,Back to the Basics,Vibration Simple Harmonic Motion Oscillation about a Reference Point Modeled Mathematically as,Back to the Basics,Back to the Basics,Basic Signal Attributes Static Slowly Changing Temperature,Basic Signal Attributes
3、Dynamic Sensor must respond in fractions of a Second Vibration, Amperage, Pressure,Back to the Basics,Dynamic Signal Fundamentals Amplitude Frequency Timing Shape,Amplitude Proportional to the severity of vibratory motion Expressed as Peak to Peak Zero to Peak RMS,Frequency Determined by the recipro
4、cal of the Period CPS or Hz RPM Orders,Timing, or Phase Represented by the time delay between two signals Leading Lagging,Signal Shape Waveform Simple Complex Pattern Recognition,Peak and RMS Comparison,Relationships of Acceleration, Velocity and Displacement,The Big Picture,Sensor(s),Cables,Signal
5、Conditioning,Data Acquisition & Storage,Communications,Remote Analysis and Diagnostics,Displacement Sensors,Elements Probe, 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 magnet
6、ic 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 drivers DC output. This DC output is usually 200mV/mil or in a similar range.,Displacement Sensors,P
7、ros and Cons Pros Measures Displacement Rugged Cons Limited Frequency Range (0-1000Hz) Susceptible to electrical or mechanical runout Installation Issues,Velocity Sensors,Pros and Cons Pros Measures Velocity Easier Installation than Displacement Cons Limited Frequency Range (0-1000Hz) Susceptible to
8、 Calibration Problems Large Size,Acceleration Sensors,Pros and Cons Pros Measures Accel. Small Size Easily Installed Large Frequency Range (1-10,000 Hz) Cons Measures Acceleration (requires Integration to Vel.) Susceptible to Shock & Requires Power,Machine Speed Sensors,Displacement Probes Active or
9、 Passive Magnetic Probes Optical Permanent Stroboscopes Laser Tach,Voltage or Current?,Current Output Accelerometers 4-20 mA Output Proportional to Dynamic Signal and/or Overall Voltage Output Accelerometers Preferred in U.S. Generally 100mV per g Sensitivity,AC and DC Signal Components,Signals have
10、 both AC and DC AC considered the “Dynamic” Signal DC is the “Static” Signal Displacement Probes Set “Gap” for DC Accelerometers “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
11、 Accelerometers,“Strips off” DC Voltage,Grounds,A Potential Problem Source Ground Loops Caused when two or more grounds are at different potentials Sensors should be grounded only at the sensor, not the monitoring rack!,Sensor Cables,Coaxial with BNC Connectors Long Coaxial can become antennas! Twis
12、ted, Shielded Pair Teflon Shield ground at only one end!,Sensor Cables,Driving Long Cables Under 90 feet, cable capacitance no problem Cable Capacitance specd in Pico-farads per foot of cable length Over 90 feet or so, CCD must supply enough current to charge the cable as well as the sensor amplifie
13、r. May result in amplifier output voltage becoming “Slew Rate Limited”,Sensor Cables,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
14、 being discharged by the cable capacitance.,Sensor Cables,Practical Effect: Signal distortion produces harmonics May lead to vibration signals being misinterpreted. To calculate the maximum frequency for a length of cable:,Signal Conditioning,Gain Integration (Hardware) AC/DC Coupling Anti-Aliasing
15、Filter(s) Sample and Hold Circuit,Signal Gain Circuit,X1 and X10 are Common Gain is simply amplification of a Signal Careful Should know your vibration level and the ADC input range first! 100mV/g accel; +-5V input range = +-50 gs Can “Clip” Signal,Signal Integration,Best to Integrate as close to si
16、gnal source as possible Reduces noise,AC/DC Coupling,Normally, Systems are AC coupled Means that there is a DC blocking Capacitor that only allows AC signal through to the system MAARS Innovation DC Switch that allows AC and DC to work on the same data channel without contaminating phase Allows use
17、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 “false Frequencies” are displayed when Aliasing is present in a system. The maximum frequency component a sampled data system can accurately handle is
18、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 Signals,In old western movies, as a wagon accelerates, the
19、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 hasnt reversed because the rest of the movie action is still taking place. What causes this phenomenon? The answer is th
20、at the shutter frame rate is not high enough to accurately capture the spinning of the wheel.,Aliased Signals,False low-frequency sin wave Caused by sampling too slowly Violated the Nyquist Criterion,Anti-Aliasing Filters,What are they and why do I need them? Generally they are low-pass filters that
21、 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 Elliptic sharpest rolloff, highest ripple Bessel Lowest ripple, fat rolloff. key advantage is that it has a linear phase respo
22、nse,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. The switch connects the capacitor to the signal conditioning circuit once every sample period. The capacitor then holds th
23、e voltage value measured until a new sample is acquired.,Data Acquisition and Storage,Analog to Digital Converter Hard disk vs. Flash Memory Physical download vs. Ethernet file Transfer FFT Conversion Windowing,ADC Analog-to-Digital Converters,The purpose of the analog to digital converter is to qua
24、ntize the input signal from the S&H The 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 convert
25、er 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 Range Usually defined in dB, depends on the number of bits used by the ADC For example, a 12 bit ADC has 212 possible data va
26、lues, or 4,096 “steps” between the lowest and highest values the ADC can see (0 to 5 Volts, typ.) 8-bit is 256 steps 16-bit is 65,536 steps, so more is better, right?,ADC Analog-to-Digital Converters,Wrong! Steve Goldmans Book pp.46-47 “Dynamic Range: The Big Lie” “That the A/D Converter can sense o
27、ne 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 Converters,Dynamic Range For a 12 bit ADC20 log (4095/1) = 72 db Theoretical only, electronic no
28、ise reduces to 65 db For a 16 bit ADC20 log (65536/1) = 96 db Electronic noise may make this only 80 db Massively more data to manipulate w/o much practical gain in Dynamic Range.,ADC Analog-to-Digital Converters,Sampling Rate “Real-Time” Rate in samples/sec 60,000 samples per sec/2.56 = 23,437 Hz F
29、max May also get divided by the number of channels in a multi-channel system,Windowing,Required to solve “Leakage” Several Types Uniform Hanning Most Commonly used Hamming Blackman-Harris,Windowing,Why do we use the Hanning Window? Best compromise between frequency resolution and amplitude accuracy
30、for steady-state machinery analysis Uniform 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 frequencies. The Window or weighting function attenuates the sig
31、nal towards the edge of the window minimizing leakage.,Windowing,Example:,Windowing,Leakage Example:,Windowing,Hanning Window:,Types of Averaging,Linear Most commonly used Peak Hold Coastdown and Impact Exponential Weights most recently acquired data more heavily used for Impact Time Synchronous TSA
32、 Triggered by tach Shaft and Harmon.,Trending Overalls,Limited Value Better than Nothing May miss some types of failures,Spectral Resolution,Common Values 100 to 3200 “Lines” 400 or 800 typical Fmax/Lines = Frequency Resolution 1000 Hz/400 lines = 2.5 Hz Resolution,Spectral Integration,Where does th
33、e “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 cant! Its PHYSICS! What we can do is “Zero”
34、the first 5 or so Spectral Bins!,Spectrum Analysis,Machine Component Condition Identified by Frequency Severity Indicated by Amplitude Rate of Deterioration Indicated by Spectral Comparison over Time,Spectrum Analysis,Waveform Analysis,Pattern Recognition is Key Requires understanding of Machine Com
35、ponents Gearbox Bearings,Waveform Analysis,Orbit Analysis,Transient Analysis,Long-Term Time Waveforms Bode Nyquist Plots RPM vs. Time Waterfall Plots Cascade Plots,Machine Transients,Vibration Severity,When do I make the call? Alarm Levels Fault Levels Do you use GM, API, ISO Guidelines? Risk vs. Re
36、ward,Communications,Area of Greatest Technology Progress Email, FTP, Internet (High Speed) Industrial Ethernet Wireless Phone, Modem, Ethernet Satellite Digital Revolution! (Remote Desktop),Communications,Analysis and Diagnostics,Area of LEAST Progress Not Fundamentally Changed in 20 years Personnel
37、 Downsizing not going to come back, either What is a Vibration Analysts Career Path? In-house are becoming contracted services Constant re-training to solve yesterdays problems!,Analysis and Diagnostics,Will Technology come to the Rescue? Remote, centralized Diagnostics Rapid Service Company Growth Rapid Growth in Wireless Sensor Technology has Cooled Power Supply Problem Spawned new VC-backed Research Companies,