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

Copyright © 2005 Rockwell Automation, Inc. All rights reserved. 1

Filters and Enveloping - A Practical Discussion -

William TudoroffCondition Monitoring Services Product ManagerRockwell Automation

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

Agenda Sample

1. Basic Filter Terminology 1. Basic Filter Terminology

2. Basic Filter Theory 2. Basic Filter Theory

3. Rolling Element Bearing Faults 3. Rolling Element Bearing Faults

4. Applying Envelopes 4. Applying Envelopes


Consider This Simple Vibration Signal in the Time Domain

20 Hz Time Waveform0 1

60 Hz Time Waveform

Raw Time Waveform


Or in the Frequency Domain

Frequency

60 Hz20 Hz 40 Hz

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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 it’s an important concept in the world of Vibration, *Remember – If it’s an important concept in the world of Vibration, there must be more than one name for itthere must be more than one name for it

*Remember – If it’s an important concept in the world of Vibration, *Remember – If it’s an important concept in the world of Vibration, there must be more than one name for itthere must be more than one name for it

Frequency

60 Hz20 Hz 40 Hz

40 Hz High Pass Filter

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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 Hz20 Hz 40 Hz

40 Hz Low Pass Filter

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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 Hz20 Hz 40 Hz

30 - 50 Hz Band Pass Filter

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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


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 FilterAnalog to Digital

Converter

Digital Signal

Processor

Analog Input

Digital Output


Basic Filter Theory – Analog Filter

60 Hz Time Waveform

Raw Time Waveform (Analog Input from Sensor)

0 1

20 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


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


Basic Filter Theory – Analog Filter is a Roll Off Filter

Frequency

60 Hz20 Hz 40 Hz

40 Hz High Pass FilterRoll-Off

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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 FilterAnalog to Digital

Converter

Digital Signal

Processor

Analog Input

Digital Output

Fast Fourier Transform, Integration and Digital Filtering Occur Here


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 Hz40 Hz

40 Hz FMAX

20 Hz

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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 Hz20 Hz 40 Hz

40 Hz High Pass Filter

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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 ProbeAccelerometer

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!


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


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


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

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Enveloping and High Pass Filtered Signal

• Our Enveloped or High Pass Filtered Signal would look like this:A

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itud

e


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?


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?

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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


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


What Does the Spectrum Look Like?

Frequency

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What are These?


Where Do the Sidebands come from?

Frequency

Carrier FrequencySideband or Modulating Frequencies

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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

Frequency

Digital Low Pass Filter (FMAX)

The Key Question: What frequencies are left?The Key Question: What frequencies are left?The Key Question: What frequencies are left?The Key Question: What frequencies are left?

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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. 29

Any Questions?

Remember, this was a practical overview!

Documents

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