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Synchronous Averaging Synchronous Averaging -- The The Trigger is Everything...Trigger is Everything...
Gerry Priebe Gerry Priebe -- PrePre--BB--TekTekRepresenting Data Physics Corporation in North and Representing Data Physics Corporation in North and
South CarolinaSouth Carolina
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Synchronous Averaging is used to Synchronous Averaging is used to detect a signal in uncorrelated noisedetect a signal in uncorrelated noise
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Synchronous Averaging Synchronous Averaging –– Case StudyCase StudyCase Study – Paper Machine Calender Section
Used for changing and/or controlling sheet caliper (thickness)
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Synchronous Averaging Synchronous Averaging –– Case Study Case Study Paper Machine Calender Paper Machine Calender –– Before Problem Before Problem
CorrectionCorrection
0°
180°
90°270°
1.0 in/s
-1.0 in/s
RMS: 0.0550989Max: 0.134449Min: -0.127059Pk to Pk: 0.261507Mean: 0.00723551
0°
180°
90°270°
1.0 in/s
-1.0 in/s
RMS: 0.520926Max: 0.795411Min: -0.850055Pk to Pk: 1.64547Mean: 0.00741009
Vibration Synchronous to Top Roll – 1.64 ips, pk-pk
Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk
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Synchronous Averaging Synchronous Averaging –– Case Study Case Study Paper Machine Calender Paper Machine Calender –– After Problem After Problem
CorrectionCorrection
0°
180°
90°270°
1.0 in/s
-1.0 in/s
RMS: 0.0717688Max: 0.13472Min: -0.131216Pk to Pk: 0.265936Mean: 0.00349375
0°
180°
90°270°
1.0 in/s
-1.0 in/s
RMS: 0.00733213Max: 0.0220629Min: -0.0133287Pk to Pk: 0.0353916Mean: 0.00328174
Vibration Synchronous to Top Roll – 0.03 ips, pk-pk
Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk
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Synchronous AveragingSynchronous Averaging
What is it?What is it?What are the benefits?What are the benefits?What can go wrong?What can go wrong?
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Synchronous AveragingSynchronous Averaging
Synchronous averaging Synchronous averaging involves triggering data involves triggering data acquisition with respect to acquisition with respect to external events such as external events such as tachometer pulsestachometer pulses
Triggered acquisitions are Triggered acquisitions are averaged to remove the averaged to remove the contribution of contribution of asynchronous components asynchronous components thereby isolating the thereby isolating the contribution of a single contribution of a single synchronous componentsynchronous component
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What is Averaging?What is Averaging?
A data frame of some arbitrary length (N) is gathered A data frame of some arbitrary length (N) is gathered into an array (Block 1)into an array (Block 1)
Frame 1
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What is Averaging?What is Averaging?
A second data frame of the same length (N) is gathered A second data frame of the same length (N) is gathered into a second array (Block 2)into a second array (Block 2)
Frame 1
+
Frame 2
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What is Averaging?What is Averaging?
These frames are then added together on a line by line These frames are then added together on a line by line basis to get the sum (Result)basis to get the sum (Result)
Frame 1
+
Frame 2
=
Result
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What is Averaging?What is Averaging?
If more averaging is desired each new data frame is If more averaging is desired each new data frame is added to the previous result to yield a New Resultadded to the previous result to yield a New Result
Result
+
New Frame
=
New Result
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What is Averaging?What is Averaging?
When the averaging is done the “right answer” is When the averaging is done the “right answer” is obtained by dividing by the number of averages (K)obtained by dividing by the number of averages (K)
Result
/
Number of Averages
=
Average
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What is Averaging?What is Averaging?
Eight records with a signal buried in noiseEight records with a signal buried in noise
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10R1 -0.12323 -1.01524 1.381033 -1.92251 0.456146 0.392856 1.874923 -1.96605 -0.25703 -1.08908R2 0.369698 0.761429 0.920689 1.441884 -0.3041 1.598214 2.187615 -0.49151 0.514061 0.544538R3 -0.49293 0.25381 1.841378 0.961256 -0.45615 3.205358 2.125077 0.491513 -0.77109 -0.81681R4 -0.24647 -0.50762 -1.38103 0.480628 0.152049 2.803572 2.250154 1.966052 -1.02812 -0.54454R5 0.492931 -0.25381 -1.84138 -0.96126 -0.15205 3.607144 1.937462 0.983026 0.257031 0.272269R6 -0.3697 -0.76143 0.460344 -1.44188 0.608194 0.794642 1.749846 1.474539 0.771092 0.816807R7 0.123233 1.015239 -0.92069 1.922513 0.304097 2.401786 1.812385 -1.47454 -0.51406 -0.27227R8 0.246465 0.507619 -0.46034 -0.48063 -0.60819 1.196428 2.062538 -0.98303 1.028122 1.089076
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What is Averaging?What is Averaging?
Eight records with a signal buried in noiseEight records with a signal buried in noise
-3
-2
-1
0
1
2
3
4
1 2 3 4 5 6 7 8 9 10
R1R2R3R4R5R6R7R8
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What is Averaging?What is Averaging?
Eight records with a signal buried in noiseEight records with a signal buried in noiseTwo Averages:Two Averages:
A2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
1 2 3 4 5 6 7 8 9 10
A2
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What is Averaging?What is Averaging?
Eight records with a signal buried in noiseEight records with a signal buried in noiseFourFour Averages:Averages: A4
-1
-0.5
0
0.5
1
1.5
2
2.5
1 2 3 4 5 6 7 8 9 10
A4
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What is Averaging?What is Averaging?
Eight records with a signal buried in noiseEight records with a signal buried in noiseEightEight Averages:Averages: A8
-0.5
0
0.5
1
1.5
2
2.5
1 2 3 4 5 6 7 8 9 10
A8
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Some History?Some History?
The first Digital Signal Analyzer were The first Digital Signal Analyzer were Waveform Averagers (HP 5480A Circa Waveform Averagers (HP 5480A Circa 1967)1967)
External Trigger
Analog-to-Digital ConverterInput Signals
Digital Processing Circuits
Digital Data
Operator Controls
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What are the benefits of What are the benefits of Synchronous Averaging?Synchronous Averaging?
Reduce noiseReduce noise–– Noise Reduction is Noise Reduction is
proportional to the proportional to the square root of the square root of the number of averages number of averages 11
Records Averaged
Noise Reduction
(dB)2 34 68 916 1232 1564 18
128 21256 24512 271024 302048 334096 368192 39
16384 4232768 45
1. “Synchronous Waveform Averaging: Magic Bullet for Noise”, Bob Masta, www.chipcenter.com
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What are the benefits of What are the benefits of Synchronous Averaging?Synchronous Averaging?Isolates your analysis to only the rotor Isolates your analysis to only the rotor you are evaluatingyou are evaluating2 2 (maybe)(maybe)Will allow balancing in the presence of Will allow balancing in the presence of many nearby machinesmany nearby machines22
Allows analysis of speed varying Allows analysis of speed varying machines (maybe)machines (maybe)Time domain signals hidden in noise Time domain signals hidden in noise can be viewedcan be viewed
2. “Synchronous Time Averaging Applications”, Technical Associates of Charlotte, P.C., www.technicalassociates.net
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When Synchronous Averaging When Synchronous Averaging won’t work?won’t work?
When the Signal being averaged does When the Signal being averaged does not appear at the same place not appear at the same place eacheach and and everyevery frameframe
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When Synchronous Averaging When Synchronous Averaging won’t work?won’t work?
When the Signal being averaged does When the Signal being averaged does not appear at the same place not appear at the same place eacheach and and everyevery frameframe–– Trigger jitterTrigger jitter–– Signal jitterSignal jitter–– Poor trigger signal shapePoor trigger signal shape–– Noise on the trigger signalNoise on the trigger signal–– Synchronous noise Synchronous noise -- can’t be removedcan’t be removed
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Trigger JitterTrigger Jitter
0 1.0 2.0 3.0 4.0 5.0 6.0-2.0
-1.5
-1.0
-500.0m
0
500.0m
1.0
1.5
2.0
sec
Rea
l, g
Trigger Signal - Single Record
0 1.0 2.0 3.0 4.0 5.0 6.0-1.0
-500.0m
0
500.0m
1.0
secR
eal,
g
Trigger Signal - 18 Averages
Detected by examining the average of the trigger signal
Examination of the first trigger pulse in the average shows that the pulse is not always in the exact same place. Therefore, the entire average is bad and it deteriorates the farther
you go into the data block
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Signal JitterSignal Jitter
Trigger Signal - Single Record Trigger Signal - 18 Averages
Detected by examining the average of the trigger signal
Examination of the first trigger pulse in the average shows that the pulse is the correct shape. Therefore, the average appears to be starting at the same place each record.
However, the fact that the average of the trigger deteriorates as time increases suggests that even though the trigger is stable the time between pulses is not stable.
0 1.0 2.0 3.0 4.0 5.0 6.0-2.0
-1.5
-1.0
-500.0m
0
500.0m
1.0
1.5
2.0
sec
Rea
l, g
0 1.0 2.0 3.0 4.0 5.0 6.0-2.0
-1.5
-1.0
-500.0m
0
500.0m
1.0
1.5
2.0
secR
eal,
g
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What Causes Jitter?What Causes Jitter?
Relative motion between the machine Relative motion between the machine and the photoand the photo--tachometertachometer22
Laser Tachometer
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Poor Trigger Signal ShapePoor Trigger Signal Shape
-1.0 0 1.0 2.0 3.0 4.0 5.0-800.0m
-600.0m
-400.0m
-200.0m
0
200.0m
400.0m
600.0m
800.0m
sec
Rea
l, V
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Noise on the Trigger SignalNoise on the Trigger Signal
0 1.0 2.0 3.0 4.0 5.0 6.0-400.0m
-300.0m
-200.0m
-100.0m
0
100.0m
200.0m
300.0m
400.0m
sec
Rea
l, V
Single Capture
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Synchronous noiseSynchronous noise
0 1.0 2.0 3.0 4.0 5.0 6.0-4.0
-3.0
-2.0
-1.0
0
1.0
2.0
3.0
4.0
sec
Rea
l, g
10+ Averages
RMS: 0.3960780 1.0 2.0 3.0 4.0 5.0 6.0
-4.0
-3.0
-2.0
-1.0
0
1.0
2.0
3.0
4.0
sec
Rea
l, g
One Average
RMS: 0.400377
Identified by the absence of a Signal to Noise Ratio (SNR) improvement
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What Causes Jitter?What Causes Jitter?
When the signal being averaged does When the signal being averaged does not appear at the same place not appear at the same place eacheach and and everyevery frameframe–– Poor trigger processing in the analyzerPoor trigger processing in the analyzer
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What Causes Jitter?What Causes Jitter?
Poor trigger processing in analyzerPoor trigger processing in analyzerDP Mobilyzer DSA
DPMentorTraining
Aid
Signal
Trigger
Square waveformof frequency F1/4
Square waveformof frequency F1
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What Causes Jitter?What Causes Jitter?
Test One: Trigger on Input Channel Test One: Trigger on Input Channel DP Mobilyzer DSA
DPMentorTraining
Aid
Signal
Trigger
Square waveformof frequency F1/4
Square waveformof frequency F1
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What Causes Jitter?What Causes Jitter?
Test One: ResultsTest One: Results
750.0m700.0m 750.0m-8.0
-6.0
-4.0
-2.0
0
2.0
4.0
6.0
8.0
sec
Rea
l, V
Signal - Channel Two Trigger
Ch 1
Ch 2
Signal
Trigger
Square waveformof frequency F1/4
Square waveformof frequency F1
Display of 10+ records from T0 + 0.7 seconds superimposed in a single display
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What Causes Jitter?What Causes Jitter?
Test Two: External Trigger InputTest Two: External Trigger InputDP Mobilyzer DSA
DPMentorTraining
Aid
Signal
Trigger
Square waveformof frequency F1/4
Square waveformof frequency F1
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What Causes Jitter?What Causes Jitter?
Test Two: ResultsTest Two: Results
750.0m700.0m 750.0m-8.0
-6.0
-4.0
-2.0
0
2.0
4.0
6.0
8.0
sec
Rea
l, V
Signal - External Trigger
Ch 1
ExtTrigger
Signal
Trigger
Square waveformof frequency F1/4
Square waveformof frequency F1
Display of 10+ records from T0 + 0.7 seconds superimposed in a single display
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What Causes Jitter?What Causes Jitter?
–– What is different between triggering on an What is different between triggering on an input channelinput channel and the and the external trigger external trigger inputinput??
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What Causes Jitter?What Causes Jitter?
The Sampling FrequencyThe Sampling Frequency–– When the trigger is connected to the When the trigger is connected to the input input
channelchannel, it is sampled at the same rate as , it is sampled at the same rate as the the signal channelsignal channel
In the case of our example the Sampling In the case of our example the Sampling Frequency FFrequency Fss = ~ 2,600 Hz= ~ 2,600 HzTherefore, Delta T = 400,000 Therefore, Delta T = 400,000 nSnSThe trigger jitter = The trigger jitter = ++ 200,000 200,000 nSnS
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What Causes Jitter?What Causes Jitter?
The Sampling FrequencyThe Sampling Frequency–– When the trigger is connected to the When the trigger is connected to the
external trigger inputexternal trigger input it is sampled at a it is sampled at a much higher rate to minimize the jittermuch higher rate to minimize the jitter
In the case of the DP In the case of the DP MobilyzerMobilyzer the external the external trigger is sampled at a frequency of trigger is sampled at a frequency of 10 MHz10 MHzTherefore, Delta T = 100 Therefore, Delta T = 100 nSecnSecThe trigger jitter = The trigger jitter = ++ 50 50 nSecnSec
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Frequency Domain ErrorsFrequency Domain Errors
Consider the analysis of a Square WaveformConsider the analysis of a Square Waveform–– If a perfect square wave is analyzed:If a perfect square wave is analyzed:
Fundamental (HFundamental (H11) = 1) = 1Third Harmonic (HThird Harmonic (H33) = 1/3) = 1/3Fifth Harmonic (HFifth Harmonic (H55) = 1/5) = 1/5Seventh Harmonic (HSeventh Harmonic (H77) = 1/7) = 1/7Ninth Harmonic (HNinth Harmonic (H99) = 1/9) = 1/9......
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Frequency Domain ErrorsFrequency Domain Errors
0.1
1
10
Harmonic
The Odd Harmonics of a Square Waveform
Theory (T)
Average with ADC trigger (AC)
Average with External Trigger (AE)
One record with ADC trigger (RC)
One record with External Trigger (RE)
Note that even though AE, RC & RE don’t agree with theory (T) they do agree with each other
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How big is the error?How big is the error?% Error of Each Harmonic
-0.33%-2.57%
-7.18%
-13.87%
-21.61%
-30.69%
-40.00%
-48.82%
-56.84%
-80.00%
-70.00%
-60.00%
-50.00%
-40.00%
-30.00%
-20.00%
-10.00%
0.00%
1 3 5 7 9 11 13 15 17 19
Error
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How do I know?How do I know?
Look for evidence of jitterLook for evidence of jitter–– Examine the average of the tachometerExamine the average of the tachometer
Is the average of the tachometer good at the Is the average of the tachometer good at the beginning and bad at the end?beginning and bad at the end?
–– Order tracking may be neededOrder tracking may be needed
Is the shape as expected?Is the shape as expected?–– Look at the leading and trailing edge for signs of poor Look at the leading and trailing edge for signs of poor
averagingaveraging–– Examine the value of any overExamine the value of any over-- or underor under--shootshoot
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Order TrackingOrder Tracking vsvs RPMRPM--relatedrelated
RPMRPM--related for slowly changing machine speed, constant related for slowly changing machine speed, constant speed within a revolutionspeed within a revolution
–– Needs only a once per revNeeds only a once per rev tachtach
Order tracking for quickly changing machine speed, or Order tracking for quickly changing machine speed, or speed that varies though the cyclespeed that varies though the cycle
–– Need several pulses per revolutionNeed several pulses per revolution
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Order TrackingOrder Tracking
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How is Triggering done?How is Triggering done?
Check the analyzer specificationsCheck the analyzer specifications–– How does the trigger input circuit work?How does the trigger input circuit work?–– Determine the speed of the trigger sample clockDetermine the speed of the trigger sample clock
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Abacus –– Data Flow
DSP32
8
8
NET
recording
IN
TRIG
OUT
INPUTS measure voltage
TRIGGERS use voltage to measure time
How is Triggering done?How is Triggering done?
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How is Triggering done?How is Triggering done?
Check the analyzer specificationsCheck the analyzer specifications–– Is the Is the trigger thresholdtrigger threshold adjustable?adjustable?–– Is the Is the SlopeSlope (+ or (+ or --) selectable?) selectable?–– Is trigger Is trigger HysteresisHysteresis available and adjustable?available and adjustable?–– Is trigger Is trigger Hold OffHold Off available and adjustable?available and adjustable?
Typically these features are not available for Typically these features are not available for analyzers with analyzers with poor triggeringpoor triggering
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How is Triggering done?How is Triggering done?
DP Abacus DP Abacus ((MobilyzerMobilyzer II) = 25 II) = 25 MHz (i.e. MHz (i.e. ++ 2020 nSecnSec) )
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How is Triggering done?How is Triggering done?
AgilentAgilent E1432A = 20 MHz E1432A = 20 MHz (i.e. (i.e. ++ 2525 nSecnSec))
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How is Triggering done?How is Triggering done?
DPDP MobilyzerMobilyzer = 10 MHz (i.e. = 10 MHz (i.e. ++ 5050 nSecnSec))
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How is Triggering done?How is Triggering done?
DP ACE = 51.2 kHz (i.e. DP ACE = 51.2 kHz (i.e. ++ 9,7009,700 nSecnSec))
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How is Triggering done?How is Triggering done?
DP Abacus (DP Abacus (MobilyzerMobilyzer II) = 25 MHz (i.e. II) = 25 MHz (i.e. ++ 2020 nSecnSec))AgilentAgilent E1432A = 20 MHz (i.e. E1432A = 20 MHz (i.e. ++ 2525 nSecnSec))DP DP Mobilyzer Mobilyzer = 10 MHz (i.e. = 10 MHz (i.e. ++ 50 50 nSecnSec))DP ACE = 51.2 kHz (i.e. DP ACE = 51.2 kHz (i.e. ++ 9,700 9,700 nSecnSec))DAT Tape (5 kHz) = 12.8 kHz (i.e. DAT Tape (5 kHz) = 12.8 kHz (i.e. ++ 39,000 39,000 nSecnSec))Input Channel Trigger = 2.6 kHz Input Channel Trigger = 2.6 kHz (i.e. (i.e. ++ 200,000 200,000 nSecnSec))YOUR ANALYZER = ?YOUR ANALYZER = ?
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ConclusionConclusion
JitterJitter effects can be effects can be subtle & subtle & hiddenhiddenhiddenNot all analyzers are created Not all analyzers are created eQuaLeQuaLThe same problems arise for digital The same problems arise for digital tape recorderstape recordersBest analysis is “live” and on the scene Best analysis is “live” and on the scene with a with a well designed DSAwell designed DSA
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End of PresentationEnd of Presentation