_翻译原文

Fiber Optic Sensors for Predictive Health Monitoring

Jason W. Borinski, Clark D. Boyd, Jason

A.

Dietz

Luna Innovations, Blacksburg, VA

24062-

1704

John

C. Duke, Michael R. Horne

Virginia Tech, Blacksburg,

VA 2406 1-02

19

Abstract

The ability to predict the onset of failure in mechanical systems is key to the reduction in

maintenance costs, downtime, and health hazards in industrial environments. Vibration

monitoring can be used to spot mechanical problems, trigger preventative maintenance

and diagnose the health of rotating machinery. Accelerometers are used in this role

to

detect defects in rotating components through analysis of harmonics in the

power-frequency distribution. Further, high frequency acoustic emission sensors can be

used to determine crack growth in metal components or failures in composite structures.

Currently, a variety of

MEMS

accelerometers and piezoelectric acoustic emission sensors

are used in health monitoring that are limited by their size, cost, and performance. Optical

fiber sensors are rapidly emerging as viable alternatives to these devices as effective

means of health monitoring in harsh environments. These sensors are tolerant to extreme

temperature, EMI, shock and vibration, and offer reduced weight and increased accuracy

over conventional instrumentation.

As

a result, these sensors have begun to replace

conventional sensors in harsh environment applications. Optical fiber sensors offer much

smaller size, reduced weight, ability to operate at temperatures

up

to

2000

,

immunity

to

electromagnetic interference, resistance to corrosive environments, inherent safety within

flammable environments, and the ability to multiplex multiple sensors on

a

single optical

fiber. This paper presents the research and development of both low-profile fiber

optic-based acoustic emissions sensors for use in

nor

destructive evaluation systems and

fiber optic accelerometers for harsh environment health monitoring applications.

Keywords

:

optical fiber sensors, Fabry-Perot, interferometer, nondestructive evaluation,

health monitoring, acoustic emission, accelerometer

1 Introduction

Fiber optic sensors have been established as one of

the best available technologies for

acquiring

measurements

in

harsh

environments.

These

sensors

are

tolerant

to

extreme

temperature, EMI, shock and vibration, and offer reduced weight and increased accuracy

over

conventional

instrumentation.

As

a

result,

these

sensors

have

begun

to

replace

conventional electrical sensors in many applications, and have been proposed for use in

health

monitoring

aboard

high

performance

aircraft.

For

example,

the

immunity

to

electromagnetic

interference

is

especially

advantageous

in

environments

where

electromagnetic interference is unavoidable, such as near radar emitters or on the skin of

spacecraft

and

re-entry

vehicles.

Further,

a

single

optical

fiber

can

support

multiple

sensors, reducing the weight and increasing aircraft efficiency and performance.

Fiber

optic

strain

and

temperature

technology

has

already

found

acceptance

among

commercial users. However, more advanced sensors, such as accelerometers and acoustic

emission

sensors,

are

still

used

primarily

in

R&D

applications

in

the

aerospace

community. The authors developed the fiber optic acceleration sensors discussed here for

use

in

severe

flight

environments,

specifically

for

airborne

measurements

on

board

the

NASA

F-18

Systems

Research

Aircraft

(SRA).

The

performance

of

the

fiber

optic

accelerometer

compares

well

with

that

of

conventional

accelerometers,

with

the

added

gain of reduced weight, increased temperature tolerance and EM1 immunity. The flight

testing and qualification of these sensors on board the

SRA

will validate the all- optical

instrumentation

concept.

The

transition

of

the

technology

to

general

industrial

health

monitoring

will

depend

on

the

system

cost,

usability,

and

performance

gains

of

over

conventional sensing methods.

In addition, this paper reviews the development of both broadband and resonant optical

fiber

sensors

for

monitoring

acoustic

emission

for

NDE

of

composite

and

aluminum

structures.

High

frequency

acoustic

emission

sensors

can

be

used

to

determine

crack

growth in metal components or various damage events in composite structures, such

as

fiber

breaks,

matrix

cracking,

or

delaminations.

Currently,

a

variety

of

piezoelectric

acoustic emission sensors are used in health monitoring that are limited by their tolerance

of

extreme

temperature,

their

size,

cost,

and

performance.

Optical

fiber

sensors

are

rapidly

emerging

as

viable

alternatives

to

these

devices

as

effective

means

of

health

monitoring.

As

a

result,

these

sensors

have

begun

to

replace

conventional

electrical

sensors in many applications, and have been proposed for

use in nondestructive (NDE)

health monitoring in extreme harsh environment conditions. This paper also discusses the

research and development of low-profile fiber optic-based AE sensors for non destructive

evaluation (NDE) systems.

2 Fiber optic accelerometer development

The design of the fiber optic accelerometer (FOA) is focused on creating a thermally

stable, ultra-miniature, low profile sensor through the use of silicon micromachining

technology. This technology obtains exceptional quality, high yields, dimensional

accuracy, and sensor head stability, all of which are required for a reliable low cost

accelerometer. The accelerometer design is based on a suspended silicon mass that is

excited proportionately by acceleration. The design concept is shown in Figure 1.

Acceleration is measured by monitoring an extrinsic Fabry-Perot interferometer (EFPI)

formed by the endface of a fiber and the surface of a moving micromachined silicon

diaphragm. Movement in the mass is then monitored by measuring the distance between

the mass and fiber using interferometric techniques. The device is a version of an offthe-

shelf accelerometer made by a commercial accelerometer manufacturer, modified to

incorporate an optical fiber and the interferometric monitoring techniques. In this manner,

the fiber optic accelerometer retained the performance characteristics of the conventional

sensor while demonstrating increased resolution and temperature tolerance

due to the

optical measurement technique. Additional accelerometer models can be retrofitted

similarly with this technique.

A

picture of the

FOA

used in testing is shown in Figure

2.

A

high frequency dual- wavelength demodulation system' was used to test the

FOA.

The

scope trace in Figure

3

demonstrates the operation of the optical demodulation system

and the

FOA

compared with a piezoelectric accelerometer. Figure

3

shows the two

channel outputs of the photodetectors before demodulation (channels 1 and 2), the output

of the signals after demodulation representing the movements of the mass measured by

the EFPI technique (channel 3), and the output of a conventional reference accelerometer

(channel

A).

This figure demonstrates the correlation between the output of the fiber optic

system and the conventional reference accelerometer. The 10-g low profile accelerometer

was evaluated for frequency response and to determine its transfer function. The high

frequency dual wavelength prototype system was used for demodulation of the optical

signal, and a TrigTek signal conditioner and a model 6810 calibration accelerometer

provided absolute data for comparison, as shown in Figure

4.

The transfer function, or

system input/output response,

is

shown in Figure

5.

The response is linear with a system

constant of 40.8 [ d g , gap movement per g acceleration], giving a minimum detectable

level of 0.025g using the current demodulation system. The frequency response of the

sensor is shown

in

Figure

6.

The response curve showed less than 0.4 dE3 of variation in

response from DC to 1.6

kHz.

The sensor is slightly underdamped with a resonant

frequency of 2

kHz

and a high order roll-off.

The final sensor specifications for the FOA are shown in Figure

7

and a packaged,

flightqualified sensor is shown in the photograph of Figure 8. This sensor, currently

available off-the-shelf, offers increased temperature tolerance, EM1 immunity, reduced

weight, and greater durability over the conventional sensor performance.

3

Fiber optic acoustic emission sensor development

This section reports the results of the development of fiber optic-based acoustic emission

(AE) sensors for non-destructive evaluation

(NDE)

systems. The authors have employed

the extrinsic Fabry-Perot interferometer (EFPI) sensing technology to measure both

in-plane and out-of-plane acoustic stress waves with both resonant and broadband sensor

designs. The sensors have been evaluated for sensitivity and performance compared to

existing conventional technologies such as piezoelectric transducers. Further, feasibility

experiments were conducted, demonstrating EFPI AE sensors in applications such as leak

detection, crack propagation detection and health monitoring of composite pressure

vessels and bridge decks. The authors continue to demonstrate the inherent advantages of

fiber optic AE sensors (over conventional AE sensors) through multiple development

programs both government and commercial.

3.1 EFPI Sensor Design

3.1.1

EFPI AE Broadband In- plane Sensor

The broadband-type optical fiber acoustic emission sensor is based on the extrinsic

Fabry-Perot interferometric (EFPI) displacement sensor. With the appropriate design, the

EFPI sensor can be configured to monitor strain, pressure, temperature, chemical

concentration, acceleration and acoustic waves. The following sections give a background

of the EFPI sensor and proposed modifications to obtain a resonant type AE sensor. The

conventional extrinsic Fabry-Perot interferometric (EFPI) sensor geometry is shown in

Figure 9. Here, a 1300 nm singlemode fiber transmits light from a laser diode to the

sensor element. An interferometric cavity is formed between a signal fiber and reflector

fiber aligned within a silica capillary tube. The entire sensor is attached to the surface

under study with an epoxy or other appropriate adhesive. The surface displacement

caused by the impinging stress wave changes the length

of

the interferometric cavity,

which can be measured extremely accurately through the optical demodulation system.

Figure 9. Conventional EFPI strain sensor element.

Inherent in this broadband sensor design is the primary sensitivity to displacement along

the axis of the sensor. This causes the sensor to be selectively sensitive to in-plane

acoustic energy. This differs from conventional piezoelectric sensors, which are primarily

sensitive to out-of-plane energy. The uniaxial sensitivity can be used to characterize the

nature of the

AE

event using multiple sensors oriented in orthogonal planes. Further, the

sensor can be oriented in the direction of the grain of the material under study, such as in

a unidirectional composite material, to achieve the best sensitivity to failures in the

material, such as fiber breakage or debonding. Finally, the extreme low profile of the

sensor enables placement within layered materials, monitoring

AE

events within each