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1、优秀精品课件文档资料,The Art ofInstrumentation +-5V input range = +-50 gs Can “Clip” Signal,Signal Integration,Best to Integrate as close to signal 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
2、 the system MAARS Innovation DC Switch that allows AC and DC to work on the same data channel without contaminating phase Allows 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 “false Frequencies
3、” 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
4、 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 kn
5、ow 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 wave Caused by sampling too slowl
6、y Violated 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
7、Bessel Elliptic sharpest rolloff, highest ripple Bessel 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 c
8、onversion. 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 Storage,Analog to Digital Converter Hard disk vs. Flash Memory Physical download vs. Ethe
9、rnet file Transfer FFT Conversion Windowing,ADC Analog-to-Digital Converters,The purpose of the analog to digital converter is to quantize 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 r
10、ange 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 Converters,
11、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 values, 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?,
12、ADC Analog-to-Digital Converters,Wrong! 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 elec
13、trical noise.”,ADC Analog-to-Digital Converters,Dynamic Range For a 12 bit ADC20 log (4095/1) = 72 db Theoretical only, electronic noise 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 Dy
14、namic Range.,ADC Analog-to-Digital Converters,Sampling Rate “Real-Time” Rate in samples/sec 60,000 samples per sec/2.56 = 23,437 Hz Fmax 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 Ham
15、ming Blackman-Harris,Windowing,Why do we use the Hanning Window? Best compromise between frequency resolution and amplitude accuracy 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 s
16、ignal 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.,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 Triggered by tach Shaft and Harmon.,Trending Overalls,Limited Valu
