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

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

3. Consider This Simple Vibration Signal in the Time Domain
20 Hz Time Waveform
60 Hz Time Waveform
Raw Time Waveform
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4. Or in the Frequency Domain
20 Hz
60 Hz
Amplitude
40 Hz
Frequency
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5. 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*
20 Hz
40 Hz High Pass Filter
60 Hz
Amplitude
40 Hz
Frequency
*Remember – If it’s an important concept in the world of Vibration, there must be more than one name for it
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6. 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
20 Hz
40 Hz Low Pass Filter
60 Hz
Amplitude
40 Hz
Frequency
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7. 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
20 Hz
Hz Band Pass Filter
60 Hz
Amplitude
40 Hz
Frequency
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8. 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.

9. 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 to Digital
Converter
Analog Filter
Digital Output
Digital Signal
Processor
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10. 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
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11. 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
Copyright © 2005 Rockwell Automation, Inc. All rights reserved.

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

13. 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 Input
Analog to Digital
Converter
Analog Filter
Digital Output
Digital Signal
Processor
Fast Fourier Transform, Integration and Digital Filtering Occur Here
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14. 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
20 Hz
40 Hz FMAX
Amplitude
40 Hz
60 Hz
Frequency
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15. 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
20 Hz
40 Hz High Pass Filter
Amplitude
40 Hz
60 Hz
Frequency
Copyright © 2005 Rockwell Automation, Inc. All rights reserved.

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
Accelerometer
Eddy Current Probe
bearing
bearing
bearing housing
bearing housing
The key point is that fluid film bearings are generally required on all high load applications, i.e. big, powerful (high horse power) machines that go fast.. Note that with fluid film bearings there is (normally) NO metal to metal contact. Also point out what the sensors actually measure.
Oil Wedge (load zone)
Soft Metal (Babbitt)
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!
Fluid Film: Capable of supporting very high loads, high temperatures, high speed. Expensive and associated rotor dynamics are very complex.
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17. 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
The key point is that fluid film bearings are generally required on all high load applications, i.e. big, powerful (high horse power) machines that go fast.. Note that with fluid film bearings there is (normally) NO metal to metal contact. Also point out what the sensors actually measure.
We feel an impact anytime a ball or roller passes over the defect
This impact energy is typically very low amplitude
Fault or Defect on Outer Race
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18. 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
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19. 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
Band Pass Filter
Amplitude
Frequency
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20. Enveloping and High Pass Filtered Signal
Our Enveloped or High Pass Filtered Signal would look like this:
Amplitude
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21. 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?
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22. 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
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23. 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
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24. 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
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
Peakvue samples at 100KHz looking for stress waves
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25. What Does the Spectrum Look Like?
Amplitude
What are These?
Amplitude
Frequency
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26. Where Do the Sidebands come from?
Amplitude
Sideband or Modulating Frequencies
Carrier Frequency
Amplitude
Frequency
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27. 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
Digital Low Pass Filter (FMAX)
Amplitude
Frequency
The Key Question: What frequencies are left?
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28. Bearing Fault Frequencies!
Ball Pass of the Inner Race (BPIR)
Ball Pass of the Outer Race (BPOR)
Ball Spin Frequency (BSF)
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29. Remember, this was a practical overview!
Any Questions?
Remember, this was a practical overview!
Copyright © 2005 Rockwell Automation, Inc. All rights reserved.