Filters and Enveloping - A Practical Discussion - William Tudoroff Condition Monitoring Services Product Manager Rockwell Automation Copyright © 2005 Rockwell Automation, Inc All rights reserved Agenda Sample 1.1.Basic BasicFilter FilterTerminology Terminology 2.2.Basic BasicFilter FilterTheory Theory 3.3.Rolling RollingElement ElementBearing BearingFaults Faults 4.4.Applying ApplyingEnvelopes Envelopes Copyright © 2005 Rockwell Automation, Inc All rights reserved Consider This Simple Vibration Signal in the Time Domain 20 Hz Time Waveform 60 Hz Time Waveform Raw Time Waveform Copyright © 2005 Rockwell Automation, Inc All rights reserved Amplitude Or in the Frequency Domain 20 Hz 40 Hz 60 Hz Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Terminology – High Pass Filter High Pass Filter – Allows Frequencies Higher than the filter value to be retained in the signal – also known as the Low Corner Frequency* Amplitude 40 Hz High Pass Filter 20 Hz 40 Hz 60 Hz Frequency *Remember *Remember –– IfIf it’s it’s an an important important concept concept in in the the world world of of Vibration, Vibration, there there must must be be more more than than one one name name for for itit Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Terminology – Low Pass Filter Low Pass Filter – Allows Frequencies Lower than the filter value to be retained in the signal – also known as the High Corner Frequency or Frequency Maximum Amplitude 40 Hz Low Pass Filter 20 Hz 40 Hz 60 Hz Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Terminology – Band Pass Filter Band Pass Filter – Allows Frequencies in the Band defined by the filter value to be retained in the signal – also known as Enveloping Amplitude 30 - 50 Hz Band Pass Filter 20 Hz 40 Hz 60 Hz Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Theory – When Does the Filtering Occur? • Although we have visualized the filter in the Frequency Domain – the actual signal processing can occur in either the Frequency or Time Domain • Also, when filtering in the Frequency Domain, this can occur in the hardware as the data is being processed, or in the Software, after the data has been processed Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Theory – Analog Filter • Filtering in the Time Domain is also called an Analog Hardware Filter because the filtering occurs during the actual capture of the Time Waveform Analog Input Analog Filter Digital Output Analog to Digital Converter Digital Signal Processor Copyright © 2005 Rockwell Automation, Inc All rights reserved Basic Filter Theory – Analog Filter Raw Time Waveform (Analog Input from Sensor) If the Raw Time Waveform was made up of the 20 Hz and 60 Hz Signals Below 20 Hz Time Waveform 40 Hz Analog High Pass Filter 60 Hz Time Waveform This becomes the Analog Signal Sent to the Analog to Digital Converter Copyright © 2005 Rockwell Automation, Inc All rights reserved 10 Basic Filter Theory – Software Filter • Filtering in the Frequency Domain after the data has been processed in the hardware is called a Software Filter because many vendors just display Zero amplitude in any bins (lines of resolution) that are being filtered out • Thus, an infinite number of filters is available using this method Amplitude 40 Hz High Pass Filter 20 Hz 40 Hz 60 Hz Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved 15 A Word About Bearings The vast majority of bearings are one of two types: Rolling Element, or “Anti-Friction” Bearings and Fluid Film Bearings Accelerometer Eddy Current Probe bearing bearing housing bearing bearing housing Soft Metal (Babbitt) Oil Wedge (load zone) Fluid Film: Capable of supporting very high loads, high temperatures, high speed Expensive and associated rotor dynamics are very complex Rolling Element: Low cost, simple to apply But are capable of only moderate speeds and relatively light loads Rotor dynamics aren’t bad but diagnostics can be complex due to all those spinning balls! Copyright © 2005 Rockwell Automation, Inc All rights reserved 16 Rolling Element Bearing Faults What happens when there is a fault or defect on the inner or outer race of the bearing? Accelerometer bearing bearing housing We feel an impact anytime a ball or roller passes over the defect Fault or Defect on Outer Race This impact energy is typically very low amplitude Copyright © 2005 Rockwell Automation, Inc All rights reserved 17 Rolling Element Bearing Faults • In fact, the vibration energy from a bearing fault is so small sometimes that it gets hidden by all the other machine vibration going on: – Unbalance, Looseness, Misalignment, etc Copyright © 2005 Rockwell Automation, Inc All rights reserved 18 Remember our Band Pass Filter or Envelope • If we pass the signal through the right Envelope or High Pass Filter, we could theoretically leave only the vibration generated by our bearing fault Amplitude Band Pass Filter Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved 19 Enveloping and High Pass Filtered Signal Amplitude • Our Enveloped or High Pass Filtered Signal would look like this: Copyright © 2005 Rockwell Automation, Inc All rights reserved 20 What Envelope or High Pass Filter Should We Use? • We need to know what frequency or frequencies we are trying to isolate • The frequencies are generated by the impact of the ball or roller as it passes over the defect on the race • So what frequency is this? Copyright © 2005 Rockwell Automation, Inc All rights reserved 21 What Envelope or High Pass Filter Should We Use? • If we envelope properly, we should be able to eliminate all the higher amplitude, low frequencies that are present in the signal: Unbalance Frequency (1X) Misalignment Frequencies (1X and 2X) Looseness Frequencies (1X and 2X and possibly more running speed harmonics) Fundamental Bearing Defect Frequencies (Non-harmonics from around 3X to around 40X) Amplitude – – – – • So what frequency is this? Copyright © 2005 Rockwell Automation, Inc All rights reserved 22 Bearing System Natural Frequencies • If we envelope properly, all we should have left is the bearing natural frequency response to the impacts that are occurring as the balls or rollers pass over the defect(s) • Why? – Because this gives us a measure of the energy generated by any impacts or impulses on the system – Since we are measuring the amount of resonance occurring in the system, it will be very sensitive to the severity of the impacts and hence, the severity of the fault – If measured properly, we should see almost all bearing related energy Copyright © 2005 Rockwell Automation, Inc All rights reserved 23 What High Pass or Envelope Filters are Available in the Hardware? • Rockwell Automation (Entek and IRD Brands) – Use Analog High Pass Filters – 100 Hz, 200 Hz, 500 Hz, KHz, KHz and KHz • SKF – Use Analog Envelope Filters – to 100 Hz, 50 to 1000 Hz, 0.5 to 10 kHz, to 40 kHz and 250 to 350 kHz • CSI – Use Analog High Pass Filters – 500 Hz, KHz, KHz and KHz • Both CSI and Rockwell Automation then apply a digital low pass filter to the signal to create the Envelope Copyright © 2005 Rockwell Automation, Inc All rights reserved 24 Amplitude What Does the Spectrum Look Like? Amplitude What are These? Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved 25 Amplitude Where Do the Sidebands come from? Carrier Frequency Amplitude Sideband or Modulating Frequencies Frequency Copyright © 2005 Rockwell Automation, Inc All rights reserved 26 There’s more to Enveloping • If we apply a Digital Low Pass Filter as the upper end of the Envelope, we can then extract the modulating frequencies from the impact waveform Amplitude Digital Low Pass Filter (FMAX) Frequency The The Key Key Question: Question: What What frequencies frequencies are are left? left? Copyright © 2005 Rockwell Automation, Inc All rights reserved 27 Bearing Fault Frequencies! • Ball Pass of the Inner Race (BPIR) • Ball Pass of the Outer Race (BPOR) • Ball Spin Frequency (BSF) Copyright © 2005 Rockwell Automation, Inc All rights reserved 28 Any Questions? Remember, this was a practical overview! Copyright © 2005 Rockwell Automation, Inc All rights reserved 29