Synchronous averging 02 2003

Synchronous Averaging - The Trigger is Everything Gerry Priebe - Pre-B-Tek Representing Data Physics Corporation in North and South Carolina Synchronous Averaging is used to detect a signal in uncorrelated noise Synchronous Averaging – Case Study Case Study – Paper Machine Calender Section Used for changing and/or controlling sheet caliper (thickness) Synchronous Averaging – Case Study Paper Machine Calender – Before Problem Correction 0° 1.0 Vibration Synchronous to Top Roll – 1.64 ips, pk-pk in/s -1.0 in/s 270° RMS: 0.520926 Max: 0.795411 Min: -0.850055 Pk to Pk: 1.64547 Mean: 0.00741009 90° 180° 0° 1.0 Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk in/s -1.0 in/s 270° RMS: 0.0550989 Max: 0.134449 Min: -0.127059 Pk to Pk: 0.261507 Mean: 0.00723551 180° 90° Synchronous Averaging – Case Study Paper Machine Calender – After Problem Correction 0° 1.0 Vibration Synchronous to Top Roll – 0.03 ips, pk-pk in/s -1.0 in/s 270° RMS: 0.00733213 Max: 0.0220629 Min: -0.0133287 Pk to Pk: 0.0353916 Mean: 0.00328174 90° 180° 0° 1.0 Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk in/s -1.0 in/s 270° RMS: 0.0717688 Max: 0.13472 Min: -0.131216 Pk to Pk: 0.265936 Mean: 0.00349375 180° 90° Synchronous Averaging What is it? „ What are the benefits? „ What can go wrong? „ Synchronous Averaging „ Synchronous averaging involves triggering data acquisition with respect to external events such as tachometer pulses „ Triggered acquisitions are averaged to remove the contribution of asynchronous components thereby isolating the contribution of a single synchronous component What is Averaging? „ A data frame of some arbitrary length (N) is gathered into an array (Block 1) Frame What is Averaging? „ A second data frame of the same length (N) is gathered into a second array (Block 2) + Frame Frame What is Averaging? „ These frames are then added together on a line by line basis to get the sum (Result) + Frame = Frame Result Frequency Domain Errors The Odd Harmonics of a Square Waveform 10 Theory (T) Average with ADC trigger (AC) Average with External Trigger (AE) One record with ADC trigger (RC) One record with External Trigger (RE) 0.1 Harmonic Note that even though AE, RC & RE don’t agree with theory (T) they agree with each other How big is the error? % Error of Each Harmonic 0.00% -0.33% -2.57% -7.18% -10.00% -13.87% -20.00% -21.61% -30.00% -30.69% Error -40.00% -40.00% -48.82% -50.00% -56.84% -60.00% -70.00% -80.00% 11 13 15 17 19 How I know? „ Look for evidence of jitter – Examine the average of the tachometer „ Is the average of the tachometer good at the beginning and bad at the end? – Order tracking may be needed „ Is the shape as expected? – Look at the leading and trailing edge for signs of poor averaging – Examine the value of any over- or under-shoot Order Tracking vs RPM-related „ RPM-related for slowly changing machine speed, constant speed within a revolution – Needs only a once per rev tach „ Order tracking for quickly changing machine speed, or speed that varies though the cycle – Need several pulses per revolution Order Tracking How is Triggering done? „ Check the analyzer specifications – How does the trigger input circuit work? – Determine the speed of the trigger sample clock How is Triggering done? Abacus – Data Flow IN TRIG 32 DSP OUT NET recording INPUTS measure voltage TRIGGERS use voltage to measure time How is Triggering done? „ Check the analyzer specifications – Is the trigger threshold adjustable? – Is the Slope (+ or -) selectable? – Is trigger Hysteresis available and adjustable? – Is trigger Hold Off available and adjustable? „ Typically these features are not available for analyzers with poor triggering How is Triggering done? „ DP Abacus (Mobilyzer II) = 25 MHz (i.e + 20 nSec) How is Triggering done? „ Agilent E1432A = 20 MHz (i.e + 25 nSec) How is Triggering done? „ DP Mobilyzer = 10 MHz (i.e + 50 nSec) How is Triggering done? „ DP ACE = 51.2 kHz (i.e + 9,700 nSec) How is Triggering done? DP Abacus (Mobilyzer II) = 25 MHz (i.e + 20 nSec) „ Agilent E1432A = 20 MHz (i.e + 25 nSec) „ DP Mobilyzer = 10 MHz (i.e + 50 nSec) „ DP ACE = 51.2 kHz (i.e + 9,700 nSec) „ DAT Tape (5 kHz) = 12.8 kHz (i.e + 39,000 nSec) „ Input Channel Trigger = 2.6 kHz (i.e + 200,000 nSec) „ YOUR ANALYZER = ? „ Conclusion Jitter effects can be subtle & hidden „ Not all analyzers are created eQuaL „ The same problems arise for digital tape recorders „ Best analysis is “live” and on the scene with a well designed DSA „ End of Presentation ... to Pk: 0.265936 Mean: 0.00349375 180° 90° Synchronous Averaging What is it? „ What are the benefits? „ What can go wrong? „ Synchronous Averaging „ Synchronous averaging involves triggering data... and/or controlling sheet caliper (thickness) Synchronous Averaging – Case Study Paper Machine Calender – Before Problem Correction 0° 1.0 Vibration Synchronous to Top Roll – 1.64 ips, pk-pk in/s... 180° 0° 1.0 Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk in/s -1.0 in/s 270° RMS: 0.0550989 Max: 0.134449 Min: -0.127059 Pk to Pk: 0.261507 Mean: 0.00723551 180° 90° Synchronous Averaging