Filters enveloping 09 2005

Basic Filter Terminology – High Pass Filter... Basic Filter Terminology – Low Pass Filter... Basic Filter Terminology – Band Pass Filter... Basic Filter Theory – Analog Filterbecause the

Filters and Enveloping

Agenda Sample

1 Basic Filter Terminology

2 Basic Filter Theory

3 Rolling Element Bearing Faults

4 Applying Envelopes

Consider This Simple Vibration Signal in the Time Domain

Or in the Frequency Domain

Basic Filter Terminology – High Pass Filter

Basic Filter Terminology – Low Pass Filter

Basic Filter Terminology – Band Pass Filter

Basic Filter Theory – When Does the Filtering Occur?

signal processing can occur in either the Frequency or Time Domain

hardware as the data is being processed, or in the Software, after the data has been processed

Basic Filter Theory – Analog 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

Basic Filter Theory – Analog Filter

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

Basic Filter Theory – Digital 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

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

Basic Filter Theory – Software Filter

Zero amplitude in any bins (lines of resolution) that are being filtered out

A Word About Bearings

The vast majority of bearings are one of two types:

Rolling Element , or “ Anti-Friction ” Bearings and Fluid Film Bearings

Oil Wedge (load zone)

Soft Metal (Babbitt)

Fluid Film : Capable of supporting very high loads, high temperatures,

high speed Expensive and associated rotor dynamics are very complex.

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

Rolling Element Bearing Faults

it gets hidden by all the other machine vibration going on:

Remember our Band Pass Filter or Envelope

could theoretically leave only the vibration generated by our bearing fault

Enveloping and High Pass Filtered Signal

What Envelope or High Pass Filter Should

We Use?

passes over the defect on the race

What Envelope or High Pass Filter Should

We Use?

amplitude, low frequencies that are present in the signal:

Bearing System Natural Frequencies

frequency response to the impacts that are occurring as the balls or rollers pass over the defect(s)

on the system

very sensitive to the severity of the impacts and hence, the severity of the fault

What High Pass or Envelope Filters are Available in the Hardware?

signal to create the Envelope

What Does the Spectrum Look Like?

Where Do the Sidebands come from?

There’s more to Enveloping

can then extract the modulating frequencies from the impact waveform

Bearing Fault Frequencies!

Any Questions?

Remember, this was a practical overview!