2005 Noisecon Presentation

Vibration Transfer MobilityMeasurements using MLS

Herb Singleton

[email protected]

October 19, 2005

FTA Vibration Impact Guidelines

• FTA/FRA vibration impact

assessment

• Vibration propagation

measurement

www.Cross-Spectrum.com | 2/22

Transfer Mobility Measurement

• Instrumented hammer to generate vibration

• Accelerometers measure ground vibration

• Transfer function calculated using FFT techniques

InstrumentedHammer Surface-mounted Accelerometers

Vibration Transmitted Through Ground


Current Apparatus


Vibration Propagation Data

Vol

ts

Time, secs


It Works, But...

• Physically intensive

– Lift/drop weight 300 times per site

– Move rig 11 times per site

• Custom fabricated

– Expensive to repair

• Costly Process

– 2-3 consultants to operate

– High shipping costs (110 lbs)

• Noise-sensitive

– Vibration from traffic, machinery, etc can corrupt data


Desired improvements:

• Better test signals

• Better equipment


Desired improvements:

• Better test signals

– Maximum Length Sequence (MLS)

• Better equipment

– Tactile Transducer


Maximum Length Sequence

• Pseudo-random binary sequence

Gives impulse under circular auto correlation


MLS Advantages

• High signal-to-noise ratio

→ No averaging of multiple runs

• High noise immunity

→ No repeating of measurements contaminated by cars,

pedestrian traffic, etc

Same data, less effort!


Noise Immunity

Vol

ts

Sample #


Tactile Transducer

• Magnetic coil/moving mass

shaker

• Used in home audio &

specialized training

applications

• Approx 1 lbf/W

• Mounted on load cell to

calibrate force signal


Shaker System

• Car audio amplifier/12V battery

• Laptop computer w/ DAQ card

• 65535-pt MLS signal at 1000 Hz sampling rate

Computer/DAQ

65535-ptMLS

TactileTransducer

Load CelAccelerometers

MLS-Force

GroundVibration

Calibration Signal

PowerAmplifier


Measurement Layout

25 ft

50 ft

75 ft

100 ft

125 ft

150 ft

Accelerometer Array

125-ft slant dist.

168-f

t slan

t dist.

25-ft slant dist.

-7-75f

X X X X X X X X X X X-75 ft -60 ft -45 ft -30 ft -15 ft 0 ft 15 ft 30 ft 45 ft 60 ft 75 ft

Impact Line

X Impact Position

Accelerometer Position


Measurement Comparison (Best Case)


Measurement Comparison (Worst Case)


Coherence

• Used to determine if measured output is related to input

→ Data integrity

• Function of frequency

• Defined for 2-channel measurements

• Coherence value = 0 to 1


Coherence


MLS Coherence

• MLS is a time-domain operation

• Rife & Vanderkooy defined MLS ‘Coherence’:

→ Ratio of energy in analysis window to energy in the

entire impulse response


MLS Coherence

Vol

ts

Sample #


MLS Coherence


The End!

• New method for measurement vibration transfer mobility

– Tactile transfer: inexpensive, lightweight, non-proprietary

– MLS: proven, reliable test stimulus

• Data and error checking comparable to impact/winch

method

• Potential for specialized measurements

• Much easier on the back and the wallet


2005 Noisecon Presentation

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