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Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 1 Filters and Enveloping - A Practical Discussion - William Tudoroff Condition Monitoring.

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Presentation on theme: "Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 1 Filters and Enveloping - A Practical Discussion - William Tudoroff Condition Monitoring."— Presentation transcript:

1 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 1 Filters and Enveloping - A Practical Discussion - William Tudoroff Condition Monitoring Services Product Manager Rockwell Automation

2 2 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Agenda Sample 1. Basic Filter Terminology 2. Basic Filter Theory 3. Rolling Element Bearing Faults 4. Applying Envelopes

3 3 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Consider This Simple Vibration Signal in the Time Domain 20 Hz Time Waveform Hz Time Waveform Raw Time Waveform

4 4 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Or in the Frequency Domain Frequency 60 Hz 20 Hz 40 Hz Amplitude

5 5 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* *Remember – If its an important concept in the world of Vibration, there must be more than one name for it Frequency 60 Hz 20 Hz 40 Hz 40 Hz High Pass Filter Amplitude

6 6 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 Frequency 60 Hz 20 Hz 40 Hz 40 Hz Low Pass Filter Amplitude

7 7 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 Frequency 60 Hz 20 Hz 40 Hz Hz Band Pass Filter Amplitude

8 8 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

9 9 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 Filter Analog to Digital Converter Digital Signal Processor Analog Input Digital Output

10 10 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Basic Filter Theory – Analog Filter 60 Hz Time Waveform Raw Time Waveform (Analog Input from Sensor) Hz Time Waveform If the Raw Time Waveform was made up of the 20 Hz and 60 Hz Signals Below 40 Hz Analog High Pass Filter This becomes the Analog Signal Sent to the Analog to Digital Converter

11 11 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Basic Filter Theory – Analog Filter is a Roll Off Filter An Analog Filter is considered a Roll-Off Filter – in other words the filter is not a brick wall but allows some of the signal beyond the filter value to pass through The Filter Attenuates or reduces the amplitudes of the frequencies below the filter value The filter values will be defined by the hardware manufacturer as they are an actual Analog Chip Lets visualize it in the Frequency Domain – even though the filtering is occurring in the Time Domain

12 12 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Basic Filter Theory – Analog Filter is a Roll Off Filter Frequency 60 Hz 20 Hz 40 Hz 40 Hz High Pass Filter Roll-Off Amplitude

13 13 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Basic Filter Theory – Digital Filter Filtering in the Frequency Domain is often called a Digital Hardware Filter because the filtering occurs during the actual Digital Signal Processing of the Time Waveform into a Frequency Spectrum Analog Filter Analog to Digital Converter Digital Signal Processor Analog Input Digital Output Fast Fourier Transform, Integration and Digital Filtering Occur Here

14 14 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Basic Filter Theory – Digital Filter A Digital Filter is considered an Absolute Filter – in other words the filter is essentially a brick wall, allowing none of the signal beyond the filter value to pass through The filters available are dependent on the hardware being used Most common example is the Frequency Maximum (FMAX) setting we are all familiar with Frequency 60 Hz 40 Hz 40 Hz FMAX 20 Hz Amplitude

15 15 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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 Frequency 60 Hz 20 Hz 40 Hz 40 Hz High Pass Filter Amplitude

16 16 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. A Word About Bearings The vast majority of bearings are one of two types: Rolling Element, or Anti-Friction Bearings and Fluid Film Bearings bearing bearing housing bearing bearing housing Oil Wedge (load zone) Soft Metal (Babbitt) Eddy Current Probe Accelerometer 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 arent bad but diagnostics can be complex due to all those spinning balls!

17 17 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Rolling Element Bearing Faults bearing bearing housing Accelerometer What happens when there is a fault or defect on the inner or outer race of the bearing? Fault or Defect on Outer Race We feel an impact anytime a ball or roller passes over the defect This impact energy is typically very low amplitude

18 18 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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

19 19 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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 Frequency Band Pass Filter Amplitude

20 20 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Enveloping and High Pass Filtered Signal Our Enveloped or High Pass Filtered Signal would look like this: Amplitude

21 21 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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?

22 22 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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) So what frequency is this? Amplitude

23 23 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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

24 24 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 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, 1 KHz, 2 KHz and 5 KHz SKF –Use Analog Envelope Filters –5 to 100 Hz, 50 to 1000 Hz, 0.5 to 10 kHz, 5 to 40 kHz and 250 to 350 kHz CSI –Use Analog High Pass Filters –500 Hz, 1 KHz, 2 KHz and 5 KHz Both CSI and Rockwell Automation then apply a digital low pass filter to the signal to create the Envelope

25 25 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. What Does the Spectrum Look Like? Frequency Amplitude What are These?

26 26 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Where Do the Sidebands come from? Frequency Carrier Frequency Sideband or Modulating Frequencies Amplitude

27 27 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Theres 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 Frequency Digital Low Pass Filter (FMAX) The Key Question: What frequencies are left? Amplitude

28 28 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. Bearing Fault Frequencies! Ball Pass of the Inner Race (BPIR) Ball Pass of the Outer Race (BPOR) Ball Spin Frequency (BSF)

29 Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 29 Any Questions? Remember, this was a practical overview!


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