TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport R R L

Contractor Report 217

Optical fibre axle sensors

by S U Ahmed, G R Jones (Department of Electrical Engineering and Electronics, University of Liverpool) and J R Spindlow (TRRL)

The work reported herein was carried out under a contract placed on the University of Liverpool by the Transport and Road Research Laboratory. The research customer for this work is Statistics C Division, DTp.

This report, like others in the series, is reproduced with the authors' own text and illustrations. No attempt has been made to prepare a standardised format or style of presentation.

Copyright Controller of HMSO 1990. The views expressed in this Report are not necessarily those of the Department of Transport. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

Traffic Operations Division Traffic Group Transport and Road Research Laboratory Old Wokingham Road Crowthorne, Berkshire RG11 6AU

1990

ISSN 0266-7045

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation o n 1 st

April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

1 INTRODUCTION. ...... ..................................................................................................... 2 2. AVAILABLE SYSTEMS ............................................................................................... 3

2.1 The Concepts of Microbending and Speckle Patterns ............................... 3 2.2 Operation of Herga sensor and electronics ................................................. 4 2.3. Operation of Pilkington sensor and electronics ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4 Summary of Available Systems ...................................................................... 7

3. AXI F. DETECTION EXPERIMENTS ............................................................................... 8 3.1 Performance Criteria ....................................................................................... 8 3.2 Encapsulation Techniques .............................................................................. 8 3.3 The Pilkington System ..................................................................................... 9

a) Long Term Road Tests With Encapsulated Pilkington Sensor ................. 10 3.4 The Herga System ............................................................................................. 13

3.4.1 HS1 Type System .......................................................................................... 13 a) Uniformity of Sensitivity ........................................................................... 13 b) Variation of Outpu t with Load ................................................................... 15 c) Stability of System ....................................................................................... 15 d) Effect of Temperature Variation on System ............................................ 16 e) Effect of Encapsulation of Sensing Fibre ................................................ 18 f) Temperature .................................................................................................. 18 g) Sensitivity and Uniformity ........................................................................ 19 h) Tests using Improved Road Axle Simulation .......................................... 21

3.4.2 HS2 Type System .......................................................................................... 22 a) Load Tests Using Herga Electronics .......................................................... 23 b) Uniformity of Sensor .......... . ....................................................................... 24 c) Temperature Effect on Sensor .................................................................. 25

3.5 Summary of Axle Detection Experiments .................................................. 26 4 VEHICLE WEIGHING CONSIDERATIONS .................................................................... 28

4.1 Performance Criteria ....................................................................................... 28 4.2 Chromatic Modulation Techniques ................................................................ 29

4.2.1 Principles of Chromatic Modulat ion ....................................................... 29 4.2.2 Spectral Responses of the Candidate sensors ........................................ 30 4.2.3 Tests using the Pilkington system ........................................................... 33 4.2.4 Experimentation using the Herga Sensors ............................................ 33

i) HSI Tests ......................................................................................................... 33 a) Load tests ..................................................................................................... 33 b) Drift Tests ................................................................................................... 35 c) Temperature Effects ................................................................................. 38

ii) HS2 Tests ........................................................................................................ 39 a) Sensitivity to Load ..................................................................................... 39 b) Uniformity Along Sensor Length ......................................................... 40 c) Temperature Effects ................................................................................. 41

4.2.5 Discussion ..................................................................................................... 42 5 CONCLUSIONS ............................................................................................................... 43 6 RECOMMENDATIONS ................................................................................................... 46 7 REFERENCES ................................................................................................................ .47

2

1 INTRODUCTION

The aim of the project was to investigate the feasibility of detecting vehicle

axles us ing available opt ica l fibre sensing systems embedded in the

roadway. Both moving and static vehicles should be detectable. Two systems

which are c o m m e r c i a l l y avai lable for purposes other than vehicle

detect ion were identif ied as possible candidates and their application for

vehicle detection was to be established. The first system is manufactured by

Herga and is used as a safety system in industrial environments e.g. on

robotic machines. The second system is produced by Pilkington and is used

as part of an underground intruder alarm.

The reason for investigating the possibilities of optical fibre based methods

was to examine their potential for overcoming deficiencies with existing

road embedded systems which rely upon e lect romagnet ic , capacitive and

pneumatic effects. The improvements being sought are in sensor lifetime,

rel iabil i ty, reduced servic ing and ease of installation. However optical

fibre systems have other inherent advantages which include the immunity

f rom e l ec t romagne t i c in te r ference .

A secondary but equal ly impor tant object ive of the project was to

invest igate the potential of the two systems for vehicle weighing both

statically and dynamical ly .

The repor t provides an insight into the operat ion of the Herga and

Pi lkington systems. Results are given regarding the performance, stability,

sens i t iv i ty and t empera tu re suscept ib i l i ty of the two systems. Their

potential for vehicle weighing has been investigated and possible methods

for improving or extending their performance are discussed.

The first section of the report deals with the principles of operation of the

two sys tems provided . The next sect ions deal with the experiments

per formed to invest igate the performance of the systems for monitoring

axle presence and their weighing potentials. Consideration is given to a

means of improving system performance and extending its capability.

Conclus ions are drawn from the results and an indication of the future

work which is needed is given.

page -2-

2. AVAILABLE SYSTEMS

The opera t ion of the two sys t ems i n v e s t i g a t e d re l ies u p o n two

fundamentally different optical effects. These effects are described before

discussing the operation of the individual systems.

2.1 The Coneents o f M i e r o h e n d i n g and Sneck le P a t t e r n s

An optical fibre consists o f a core th rough which the main opt ical

transmission occurs and around which there is a protect ive cladding. The

cladding performs two main functions. Firstly it protects the core f rom

being damaged and gives the fibre greater mechanical strength. Secondly

because it has a refractive index greater than that of the core, it promotes

total internal reflection of the core transmitted light. As a result the light

may be propagated through the fibre via a series of total in ternal

reflections (figure 2.1).

~ cladding

Figure 2.1 Light propagation through an optical fibre. At angles of incident greater than the critical angle, 0a total internal reflection occurs. However for angles less than the critical angle 0b, refraction into the cladding occurs.

Irregularities at the core cladding interface and bending of the fibre cause

changes in the propagation characteristics of the fibre. (f igure 2.2). The

light rays no longer maintain the same angle with the fibre axis. Thus if

the angle of incidence is greater than the critical angle, then the ray

passes through the cladding and is lost. These perturbations can be caused

internally in the construction of the fibre or external ly by bending the

fibre. If the fibre is subjected to bending at a particular periodici ty on a

microscopic scale transmission losses f rom the fibre core are enhanced

(microbending) [Ref 1]. It can be shown that microbend losses are related

to wavelength and its effect on the higher wavelengths is greater than on

the shorter ones [Ref 2]. A more detailed descr ipt ion of such phenomena

requires the use of wave optics and propagation modes [Ref 3].

page -3-

4

c l a d d i ~ c o r e O.

b ctackllnl~com

Figure 2.2 Diagram illustrating fibre losses a) due to micro bending, where if the angle of incidence on core cladding interface is smaller than the critical angle then internal reflection does not occur, and b) due to bending the fibre.

P ropaga t ion of monoch roma t i c l ight f rom a laser through a fibre occurs

via a number o f d i f ferent modes having different propagation rates [Ref 4].

Such e f fec t s combined with the highly coherent nature of the laser light

leads to the ex is tence of des t ruc t ive and construct ive interference patterns

at the ex i t o f the f ibre . This g ives the emerg ing l ight a speckled

appearance . Dis tu rbances of the light p ropaga t ion (for instance by small

changes in core c l add ing in te r face due to minute s t resses) causes the

spat ia l d i s t r ibu t ion o f the br ight and dark regions to vary so that the

speck le pat tern f l ickers . Figure 2.3 shows three pictures of the end of a

f ib re wh ich is t ransmi t t ing laser light. The f igure shows the ef fec t of

bending the fibre and then returning it to its original position. The speckle

pat tern is s teady for any posi t ion and any movement of the fibre causes an

u n r e p e a t a b l e change in the pat tern.

Figure 2.3 Diagram illustrating speckle pattern seen at the end of an optical fibre transmitting laser light a) The speckle pattern is steady until the fibre is bent and b), the pattern is seen to flicker c) Returning the fibre to its original form does not revert the speckle pattern.

2 .2 O o e r a t i o n o f H e r ~ a s e n s o r a n d e l e c t r o n i c s

The Herga sys tem cons is t s o f a f ibre sensor and instrumentat ion which

inc lude opt ical emit ter and de tec tor sys tems. The optical sensor consists of

an optical fibre which has a core diameter of 85gm and a cladding diameter

p a g e -4-

5

of 125 v.m. Around the fibre is wrapped a coil in a helical fashion. The

system is encased in a plastic tape, (Figure 2.4.) and the pitch of the helical

winding is arranged to optimise microbending effects. [Ref 5]. T ~

. /

iii i : i!i !i ii iiiiiii!iiii iiiiiiiii!i !ii!i i i!iii i!i !iiii!:iiiii iiiiiiii ii i:iiiii !iiiiiiii ii!iiiiiiiiii iiiiii i i iiiiiiii iiii !!iiiiiii!ii iii iii!i i iiiii!iiiiiiii ii iiiii i iiii!iiiii! Figure 2.4 Diagram showing arrangement of Herga Fibre.

Light is injected into the fibre from a L.E.D operat ing in the infrared

region. When pressure is applied to the tape, the helical wire induces

microbends in the optical fibre. These microbends cause a s igni f icant

change in the attenuation of the light within the optical fibre according to

the discussion of section 2.1 above. The changes in attenuation are detected

by a photodiode and this signal is processed by appropr ia te e lectronic

c i r c u i t r y .

The Herga electronic sys tem operates in a pulse ampl i tude modula t ion

mode. i.e. the emitting L.E.D is pulsed at a given rate to produce a square

waveform. The amplitude of the transmitted pulse is modula ted intrinsically

with stress applied to the opt ical-f ibre sensor. The ampl i tude of the

received signal is monitored and if the signal goes , below an adjustable

threshold then a relay is activated to implement a safety inter lock.

The Herga system therefore operates in a fail safe manner in the sense that

regular pulses of the L.E.D establishes system integrity whilst in a dormant

mode. For operation in the detect mode a suff ic ient modula t ion in the

amplitude of the pulse with respect to noise and drift is necessary. This

would therefore set the limit of resolution of the Herga system.

As delivered, the Herga system utilised low quali ty fibre connectors which

could unnecessari ly affect per formance .

2.3 . O n e r a t i o n o f P i l k i n u t o n s e n s o r a n d e l e c t r o n i c s

The Pilkington sensor is constructed out of 100~. fibre around which is a

tightly buffered jacket with kevlar strain relief, The tight buffering of the

page -S-

6

fibre makes the fibre more sensitive to external stresses. Figure 2.5 shows a

schemat ic of the fibre. Apart f rom the tight buffering, the fibre is

essent ial ly the same as normal communicat ion fibre. The principle of

operation of the sensor is to transmit laser light down the fibre and to

detect the speckle pattern at the output. The detection electronics are

arranged to be sensitive to small changes in the speckle pattern which are

caused by small disturbances to the stress applied to the fibre.

ncht o ~ Pvc ~

Kevl~ a r m

Figure 2.5 Diagram showing construction of Pilkington sensor

The optical emit ter used in the Pilkington system is a laser diode

transmitting at 780 nm. An elaborate circuit is used to maintain stability of

the laser using feedback. The transmitter operates in a steady mode only

and has an output power of lmW. The detector operates in a.c. mode only i.e.

all the d.c. bias is removed from the received signal and only the transient

changes in the optical signal are monitored. This enables the detection to

be highly optimised to respond to small transient signal changes and so

provide an extremely sensitive system.

The optical detector is a photo diode and the signal is passed through

various stages of filtering and rectification which convert the a.c changes

i n t o a d.c. value. The d.c. signal is then checked using a threshold detection

circuit and the output produces pulses when the signal is above a given

threshold. A block diagram of the detection circuit is shown in figure 2.6.

Current to Volage 8uffw' with small Prec~lon ful l~er.~ion a~ converter and flltm" gain. wave mdlfler to etc converter wah h~h ~qeency cu td f

Photo diode CamoNk~ ec fllt~

Out;~at .,* TP3 on board

Figure 2.6 Block diagram showing the principle of operation of the Pilkington sensor

page -6-

The Pilkington system is therefore h ighly sensi t ive to t rans ient signal

changes. Al though there does not appear to be an au tomat ic sys tem

integrity check there would appear to be no apparent reason why the d.c

optical signal which is t ransmit ted cont inuous ly during dormant per iods

could not be monitored to provide such system checking.

2.4 S u m m a r v o f A v a i l a b l e S y s t e m s

It is clear that the two systems provided for invest igat ion are based upon

qui te d i f fe ren t ope ra t i ng p r i n c i p l e s and u t i l i s e q u i t e d i f f e r e n t

instrumentation. Thus whereas the Herga system relies upon microbending

induced changes, with pulse amplitude modulat ion activated with an L.E.D.,

the Pilkington system relies on highly sensitive transient detect ion of a.c.

signals, with a laser diode source. The optical fibre of the Herga system

appears more fragile in its normal state than the Pilkington fibre and the

Herga optical connectors are of poorer quality than the Pi lk ington ones.

The Herga system would however appear to have cost advantages over the

Pi lk ington system.

The relevant properties of the two systems are summarised in table 2.1.

S y s t e m H e r g a P i l k i n g t o n

Principle of operation M i c r o b e n d i n g

of sensor

Speck le pa t te rn

c h a n g e s

Principle of operation Pulse ampli tude

of electronics m o d u l a t i o n

Robustness of the

s e n s o r

Unprotected fibre and

is fragile

a.c. changes on the

rece ived s ignal

Fibre has pvc covering

and kevlar strain

r e l i e f .

Connectors used Poor quality RS chap

c o n n e c t o r s

Standard SMA

connec to r s used

Emitter used Infrared L.E.D Laser diode Table 2.1 Summary of properties of Herga and Pilkington system

!

page -7-

3, IAXLE DETECTION EXPERIMENTS

3.1 P e r f o r m a n c e C r i t e r i a

The in i t ia l s tage o f the expe r imen ta l p rogramme was to assess the

pos s ib i l i t y of us ing each sy s t em as an axle de tec t ion system. The

requirements are that the sensor should be able to detect the presence of

both static and dynamic load applied to the sensor by an axle. The sensor

needs to be encapsula ted in a tough resin so that it can be embedded for

road ins t a lmen t . The tests requi red are that of sens i t iv i ty to load,

temperature dependence of the circuit and sensor, the drifts in the circuit

and the e f fec t o f encapsu la t ion of the f ibre on these parameters. The

resul ts are p resen ted with a view to assessing the systems' merits and

l i m i t a t i o n s .

3 ,2 E n c a o s u l a t i o n T e c h n i a u e s

For tests on an encapsulated sensor, it was necessary to construct a number

o f devices of d i f ferent lengths. The sensors were cast in a modified "Tee"

sect ion mould. The mould was split horizontal ly at two levels so that thin

threads could he woven across to form suspensions for the outward and

return fibres. A cross-sect ion of a complete Pilkington sensor is shown in

in figure 3.1. At the far end of the sensor, the cable was formed into a loop

with a min imum bending radius of 25ram and held under light tension with

fur ther threads. The bare cable was sheathed with heat-shrink sleeving at

its entry and exit points on the mould and coated with primer. The mould

was filled with Devcon Flexane 80 liquid and allowed to cure for one week.

Ree~ E m ~ l o ¢ ,

Fig 3.1 Diagram showing encapsulated Pilkington sensor

p a g e -8-

9

3.3 The P i lk ing ton Svstem

As already indicated the Pilkington system relies upon responding to

transient changes in the speckle pattern. The system is highly sensitive

even to movement of the fibre and also low level disturbances remote from

the fibre.

Tests were attempted with various loads in a dynamic mode i.e. transient

loading and unloading but no correlation of load with signal output could

be discriminated.

Because of the transient nature of the Pi lkington system it was not

meaningful to undertake controlled steady load tests for uniformity of

response along the fibre l eng th . Neither were drift and temperature

dependence tests meaningful to make, since changes associated with such

effects occur on time scales which are too long for the Pilkington system to

r e s p o n d .

When encapsulated, the Piikington system remained sensit ive to remote

movement so that for practical purposes encapsulat ion may be taken to t

have insignificant effect upon the system response.

The highly sensitive nature of the Pilkington system meant that it not only

responded to influences upon the encapsulated section of the optical fibre

but that it was equally sensitive to perturbations to the unencapsulated

fibre section (which connected the sensing element proper to the detection

electronics). Tests were performed with an intermediate length of ordinary

fibre of various types (figure 3.2) in an attempt to r educe this undesirable

over-sensitivity. No simple means has been discovered of eliminating such

ef fec ts .

\ o~ean~ f ~

I " | [

Figure 3.2 Arrangement for test to reduce over-sensitivity of interconnecting fibre.

,page -9.

10

a) Lon~ Term Road Tests With Encansulated Pilkington Sensor

A Pi lk ington f ibre, encapsu l a t ed as a 3m Tee sect ion axle sensor was

p r o d u c e d t o assess count a ccu racy and l i fe t ime under real is t ic operat ing

condit ions. It was decided that the most stringent testing ground would be a

mo to rway near Theale in Berkshire at a T R R L test site.

A 40ram deep slot was cut with a 12ram wide blade and deepened to 75mm at

the offs ide end (to locate the f ibre loop). Feeder cable slots were also cut.

The slots we re t h o r o u g h l y d r i ed with a gas torch and c leaned with

c o m p r e s s e d air.

The slot was half f i l led with Stag Traff ic Coil Adhes ive and the sensor

gradual ly lowered in f rom the of fs ide end towards the kerb so that a bow-

w a v e fo rmed p reven t ing air f rom be ing t rapped under the sensor. The

adhes ive squeezed up the side of the sensor and under the horizontal arms

o f the Tee sect ion. Feeder cables were bur ied in adhesive and topped with

ho t b i tumen .

f o r compar i son , four other sensors were similarly installed one metre apart

next to the f ibre-optic sensor. The other sensors were :

• T r i b o e l e c t r i c made and suppl ied by Traff ic Technology Limited. This

has a Tee section as descr ibed in TCC specification MCE 1325C. A unity

gain buf fe r amplif ier was used and fed into a TRRL processing board

which has a va r i ab le high gain ampl i f ie r fo l lowed by a window

c o m p a r a t o r .

• P i e z o e l e c t r i c made by Pioneer Wes ton and supplied by Golden River

Traffic Limited. This was a replaceable type made to TCC specification

MCC 2075B, and was connec ted to one channel of a Golden River

"Oakley" p rocessor board .

• P i e z o e l e c t r i c - H i g h S e n s i t i v i t v from the Gates Rubber Company. This

was a rep laceable type and similar construct ion and was connected to

a second channel o f the Oak ley board.

p a g e - lO-

11

• Piezores is t ive From the Gates Rubber Company was also a replaceable

type. The circuit for processing the signal was based on the Gates

application notes.

The processed output pulse of each sensor was connected to a counter to

establish that they were working correctly and also to estimate the point at

which they failed. There were about 34000 axles over the sensor each day

which is about 1 million axles each month.

Figure 3.3 shows a typical response of the fibre-optic sensor to a vehicle

travelling at approximately 90km/h (55-60 miles/h). Channel 1 shows the

output signal obtained from the end of the amplification and filtering stage

of the electronics and channel 2 shows the output after the signal is

digitised. Table 3.1 shows the lifetime and failure mode of the various

sensors .

Sensor

Piezores is t ive

Piezoelectric '(pioneer Weston)

Piezoelectric (Gates)

Trboe lec t r i c

Fibre-Optic

l i f e

6 weeks

31/2 weeks

61/2 weeks

>12 months

>12 months

Failure mode

Feeder cable joint failure

n o i s y

n o i s y

Table 3.1 Results showing life time and failure mode for different sensors

embedded on the motorway.

The results, based on a small number of sensors, are only illustrative but do

show the importance of construction technique and installation practice to

sensor life. Table 3.2 shows the results of a parallel counting trial over a

period of about one week.

Sensor

T r i b o e l e c t r i c

Piezoelectric (A)

Piezoelectric (B)

Fibre-Optic

Count

264687

263982

265898

263089

Table 3.2 Comparison of axle counts from four axle sensors.

page -11-

f J.

i I

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f

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T

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C~ ,--4

O

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C~

t" 0

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13

The results show a count accuracy better than 1% although since the fibre-

optic was reading the lowest, there is the possibility of an occasional missed

axle caused by slow recovery or inappropriate signal process ing during

the passage of two closely spaced axles.

3.4 The Her~a System

Tests have been performed on two types of Herga fibres which differ in the

manner in which the f ibre bend ing is p r o d u c e d . T h e s e sys tems are

designated HS1 (structure shown on figue 2.4) and HS2 (structure described

below in section 3.4.2 part a).

3.4.1 HSI Tvne Svstem

The Herga system with the unencapsula ted Herga Fibre and the Herga

electronics was tested for uniformity of response along the fibre length,

the sensit ivi ty of the sys tem and the t empera tu re dependence of the

system. The influence of these various factors upon the changes in the

threshold of response once set by an operator could then be established.

a) Uniformity of Sensitivitv

Tests on the uniformity of the fibre's sensitivity to load as a funct ion of

position along the fibre length were performed using the system shown in

figure 3.4. This consisted of a brass cantilever which could be loaded at its

free end with a movable weight and whose other end was arranged to stress

a portion of the optical fibre by pressing the latter against a fixed stop. The

fibre stress was varied by moving the weight on the free end of the

cantilever and the response of the fibre at var ious posi t ions along its

length was measured by changing the part of the fibre c lamped between

the fixed stop and the cantilever.

The length of fibre s t ressed be tween the can t i l eve r and s top was

approximately 2.5 cm.

p a g e -13-

14

b)

ntovable load Brass oantllever fitye

Figure 3.4. Diagram showing a)fibre loading apparatus and b) the set up of the experiment

The results of these tests are shown in graph 3.1 in the form of pulse

height voltage as a function of position along the fibre. The zero load

output was 1.8V and the full load output (loads which produce no further

change in output voltage) was 0.3V. For a load intermediate between zero

and full, a variation between 1.0 and 1.3V is apparent along the fibre

length. This means that the uniformity along the length of the fibre is 20%

of the full scale. It is recognised that this exper iment identifies

manufactur ing variations, rather than expected performance of the fibre

as an axle sensor. Encapsulation, and s road tyre of constant width 100mm

will tend to mask any non-uniformity along the fibre length.

A > • ,.,- 1.5 ¸ Q O) ¢0

O

> 1.0 - !

0

.~ 0.5 co Q

0 . 0

0

Graph 3.1

T - '~ : _./~--~." I ~--o-.~L,,,._

m Peak output voltage (v) m

20 40 60 Position along fibre (cm)

Graph showing uniformity of sensitivity along fibre length for the bare Herga fibre using the Herga board.

8 0

page .14 -

15

b) Variation of Outnut with Load

Tests were performed on the variation of the peak output voltage from the

amplifier with respect to the load applied to the fibre at one position along

the fibre length using the apparatus shown above in figure 3.4. The results

are shown in graph 3.2. For small loads (i.e. loads less than 100g) the system

is not linear, being insensitive to loading. However in the load range 100-

300 g the system responds linearly. The resolution (VoutYunit load) is

5.6mV/g for the linear region and the noise level is .5mV peak to peak (i.e.

the signal to noise ratio is 2300:1). A major difficulty encountered during

these tests was the poor repeatability which was ascribed to the low quality

optical connectors attached to the Herga fibre.

1.5,

A >" 1.0. v

Q

Ca

0

0.5

a .

:3 0

0.0 0

Graph 3.2

\

B Output voltage (V)

100 200 Load (g)

Variation of output voltage of Herga electronics with load on bare Herga fibre

300

c) Stability of Svstem

The stability of the Herga system was assessed during a 72 hour test with

the sensor loaded at a constant load. The ambient temperature was also

monitored during the test. The results of this test are shown in graph 3.3.

Two major features are noteworthy. The first is that the system exhibited a

long term drift which correlates with the var ia t ion i n the ambient

temperature. The drift corresponds to 4mV for a temperature change of 6°C.

This represents a temperature coefficient of 0.7 mV/C. (The graph of long

term tests show a rectified d.c. signal rather than the square wave output

p a g e -15 -

16

fnr convenience). The second feature of graph 3.3 relates to the presence

of sudden step changes in the output of the system, the reason for which is

not known.

d~ Effect of Temoerature Variation on System

Because the sensor and possibly the electronics will need to operate over a

large temperature range i.e from -5 to 70 degrees centigrade, and because

the implications of the long term drift results (Section c above) better

control led temperature tests were needed to identify the reasons for the

temperature dependence and to establish operational specifications.

I HJrga Peek Voltage Output I 2 Houm ~----=- Temperature

~ M

Tln~ (Houm) • Graph 3.3 Long term drift test using the unencapsulated Herga Sensor and Herga Electronics. The graph shows the peak output of the Herga system and the temperature of the system.

A specially configured oven which could accommodate reasonable lengths

of fibre was constructed for these tests. The system layout is shown in

figure 3.5b. The system consists of a brass tube (diameter 10cm, length

120cm) around which a heating tape was wound. The temperature inside

the tube, measured using a thermocouple probe, could be controlled by the

current through the tape. Heat losses from the tube were minimised with

an outer insulating jacket. The fibre under test was laid within the tubular

oven as shown in figure 3.5a. The fibre was held inside the oven along two

metal rods to keep it away from the the oven wall.

It was recognised that temperature stability tests were needed not only on

the optical fibre sensing element itself but also on the transmitting and

detect ing electronics. For this reason the Herga electronic system was

p a g e -16-

17

placed inside a second temperature controlled oven so that the temperature

of the electronics could be controlled separately from the fibre.

In the first test the temperature of the fibre oven was varied and that of

the electronics was kept constant. The output was monitored for both

heating and cooling of the fibre. The next stage of the experiment involved

keeping the temperature of the f ib re cons tan t and va ry ing the

temperature of the electronics, again for both heating and cooling. In each

case the temperature was varied from 25 to 80°C.

Temperature ~obe~ F: Oven b)

Own I,

I I i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i )

T~um

6moeellng circuit

Rloontlr

Figure 3.5 Diagram to illustrate heating apparatus, a) the sensor heating oven b) The experimental set up for the thermal tests.

The results of these tests are shown in graph 3.4. which gives the system

response (output voltage) as a function of temperature during the heating

and cooling individually of the fibre and the electronics.

Changing the temperature of the fibre sensing element itself leads to a

change in the system output voltage of 150mV. On re turning the

temperature to its original value of 25°C there remains a permanent

change in output of lOOmV. If the zero load output is 2.5V, this corresponds

to a 4% zero point change after cycling and a 6% temperature effect on the

system for the range of temperatures used.

p a g e -17-

18

0.2

0.1 & .= "~ 0.0

& -0.1

3 -~ -0.2

-0.3

• Cooling electronics

I • Cooling sensor r - " - - -=~ ' '~

r ' - - .-- .... [ ..... • H e a t i n g s e n s o r : . . . . . . . . . . j j

l ~ I k ' ~ L ~ , = • Heating Electronics

. . . . . . . . 1 . . . . . . . L . . . . . .

W

30 40 50 60 70 80 Temperature(C)

Graph 3.4 Variation of output voltage of the bare fibre sensor and Herga electronics to thermal variations.

The effect of varying the temperature of the electronics alone is also

shown on graph 3.4. The system output voltage reduces by 0.2 V for a

temperature change up to 80°C. During cooling of the electronics the

system voltage increases back to its original value but along a different

route so showing a hysteresis effect. The implication is that there is a 10%

output variation on heating to 80°C but no memory effect on the original

zero point.

e~ Effect of Encaosulation of Sensint, Fibre

Since the sensor is to be embedded into the road, it is necessary for it to be

encapsulated by proven techniques (see section 3.2). The encapsulation is a

tough resin with a "T" shaped cross section as shown in figure 3.1. It is

anticipated that such encapsulat ion will affect the sensitivity, uniformity

and the temperature dependence of the system. The extent to which these

parameters are affected by encapsulat ion have been investigated and are

descr ibed below.

f) Tempera ture

To test the effect o f temperature on the Herga encapsulated sensor, the

same apparatus as shown on figure 3.5 was used. A temperature monitoring

p a g e -18-

19

probe was placed inside the encapsulation so that an accurate reading of

the temperature of the sensor could be obtained. Graph 3.5 shows the

variation of output voltage of the Herga electronics when the encapsulated

fibre was cycled through separate temperature changes.

The results show that there is a permanent change after one thermal cycle

of 50mV and the maximum change in output for one cycle is 210 mV. These

correspond to approximately a zero point change of about 2% and an 9%

effect due to temperature cycling. Thus encapsulation reduced the zero

point change from 4% to 2% but there is an increase in the effect due to

temperature cycling from 6% to 9%. However there is also an indication

that temperature cycling may reduce the sensi t ivi ty to tempera ture

changes since the second temperature cycle produced a smaller voltage

change and no zero point drift.

1 .30 . . . . . . . . . . : - --

Q o~ 1.20 m

o >

3 1.1o,

0

G r a p h 3 . 5

• Heating 2 ,

.Z. % L . . . .

r ~

~' m Heating 1

% • Cooling

i

• Cooling 2 I

I

1.00, 20 30 40 50 60 70

" romp (C)

Variation of Voltage output of encapsulated Herga fibre sensor with embedded temperature probe.

• ~ Sensitivitv and Uniformity

In order to determine the sensitivity and the uniformity of the sensing

fibre, a commercial loading apparatus needed to be used because greater

forces were needed to produce an output. A schematic of the apparatus used

is shown in figure 3.6.

p a g e -19-

20

m L o a d e o n t r o l

Heq~ seato En(=p=ulaled \ . L~.~ ]r~ unit

ovu H=r~ ~ ~ [ ] a

Figure 3.6 Diagram showing setup for experiments to determine sensitivity and uniformity of the encapsulated fibre.

Using the above system the fibre sensor was loaded to a given value at

various points along its length and the uniformity of the response

determined. Graph 3.6 shows the variation of output voltage as a function of

position along the fibre length. The length of encapsulated fibre exposed to

each stress test was 15 cm.

The test results indicate a change of only 40 mV along the entire fibre

length which corresponds to uniformity to within 2%. This is an order of

magnitude improvement over the unencapsulated fibre (Graph 3.1). The

improvement is probably due to the encapsulation distributing the load

over a longer length of the fibre sensor.

1.0

= 0 .5 .

3 O

[1 rl ,.+ ! ,,J . i ~

= Output (V)

0.0 0 10 20 30 40 50 60

Displa~mont(em) Graph 3.6 Uniformity along sensor length for the encapsulated Herga sensor using the Herga electronics

p a g e -20 -

21

The sensitivity of the sensor to load was also tested using the same

apparatus shown in figure 3.6 The results for these tests are given in graph

3.7.

1.2

A

v

Q

m . . ,

o

g a . g o

1.0

0.8

0.6

0.4

0.2

0.0 - 0 2O

. / f

/ / .

r Output Voltage (v)

4 0 6 0 8 0 1 0 0 Load(Kg)

Graph 3.7 Variation of output voltage of Herga electronics with encapsulated sensor

The results show a change from zero output voltage to 1.3V for full load. The

region of greatest change is between 25 and 65 kg and over this region the

resolution is 20 mV/Kg. For an unencapsulated fibre the sensitivity was 5.6

mV/g (graph 3.2) so that the sensitivity on encapsulation is reduced by

2 . 8 x 1 0 2. This result thus provides a calibration factor for giving an

approximate indication of the effect of encapsulation. The noise on the

signal is 0.5 mV peak to peak giving a signal to noise ratio of 2300:1

h) Tests usin~ Imoroved Road Axle Simulation

The tests performed on the Herga system have shown that encapsulation in

a form suitable for road embedment reduces the system sensi t ivi ty

significantly (X280). It was therefore essential to simulate the loading

which would be experienced by the fibre under real axle detect ion

conditions. Such tests would need to take account of the fact that the

vehicle weight is distributed over the road surface, through the area of

tyre in contact with the road. Thus the embedded fibre sensor is likely to be

loaded directly by the full weight on the wheel . Fur thermore the

encapsulated sensor is restricted by fixing into a groove in the road

surface. In order to take better account of these factors , improved

simulation tests were performed.

page -21-

2 2

The road/groove effect was s imulated by setting the encapsulated fibre into

a groove in a wooden block (figure 3.6a). To simulate the true vehicle axle

e f fec t a tyre was used which could be connected to the loading apparatus

( f igure 3.6b).

Tes t s wi th this more real is t ic s imula t ion sys tem showed that the Herga

sys tem was too insensit ive to detect real load condit ions and even though an

opt ica l s ignal was pass ing th rough the f ibre , no modula t ion could be

observed above the noise level. The system was loaded up to 250 Kg which is

about the load applied by one axle on the sensor.

smoothl~

od

Tym

om~m~

unit

r .1 r - J

Illlilllli

Figure 3.6 a) wooden block used to simulate road/sensor arrangement and b) Diagram showing set up for experiments on load test on the encapsulated Herga sensor using a better road/axle simulation.

3 .4 .2 H S 2 T v n e S y s t e m .

A new design of optical sensor was presented by Herga and encapsulated by

T R R L so that its per formance could be assessed before embedding into the

road. Figure 3.8 shows the structure of the new sensor. The sensor consists

o f a f ibre-opt ic loop which is sandwiched be tween layers of r ibbed plastic

and cork. The length of sensor encapsulated was 3m and 1.5m of each end of

the fibre loop was outs ide the encapsulat ion but was protected with a loose

page -22-

23

fitting PVC jacket. The -"fibres were terminated using standard SMA

connectors. The total length of the fibre was about 9m. Tests using the real

road-axle was performed to determine the best combination of sensor and

detection electronics.

a) Load Tests Using Herea Electronics

Tests using the new Herga sensor connected to the Herga board were

performed using the real road tyre simulation explained in section h above.

The attenuation of the optical signal was much lower than that observed

with the old Herga fibre, and sufficient optical power was transmitted

through the sensor that the effect of modulating the sensor using a load

could be observed.

I PI ImUc

Figure 3.8 Diagram showing the structure of the modified Herga sensor

Graph 3.8 shows the variation of output voltage of the Herga electronics

with load applied to the sensor. The load was cycled from 0 to 250 Kg and

applied on the sensor using a tyre. The load cycling was performed three

times to monitor any drift due to cycling the load. 1.3 I

• Loading 1 • Unloading 1

12 ~ " • Loading2 • Unloading 2

~ • Loading3 11 i o Unloading3 - -

1.0 , , IIL

0 0.9

0 50 100 150 200 250 Load (Kg)

Graph 3.8. Variation of output of Herga Electronics to load using the modified Herga sensor

p a g e -23.

24

The graph shows a near ly l inear curve for loading and unloading. The

sensit ivi ty of the sensor is lmV/kg . The noise on the system is 5inV. The

reso lu t ion then at bes t is 5kg. A zero point drif t o f 40mV/cyc le was

observed after the first loading cycle.

b) Uni formi ty of Sensor

Tests on the uni formi ty o f the fibre 's sensit ivi ty to load as a function of

posi t ion along the f ibre length was per formed using the sys tem shown in

figure 3.7 o f section g above. The results are shown in graph 3.9 below.

1.2

1 . 0

• " 0.8 :1 Q.

0 0.6

ca

Q ..r

0 . 4

20 cm " ' - - ~ b .

. . . . . . . . . . = ...... 3 0 c m ' - ' " , , ° , , I , - - - 5 0 c m

. . . . . • * - ' - 60 cm '

- - " - - 7 0 c m

- , - - - - . P - - 80 cm i !

~e~e iD Q ~ Q L

0.2 ' 0 1 0 0 L o a d ( K g ) 2 0 0 3 0 0

Graph 3.9 Graph showing variation of sensitivity to load with position.

The graph shows that there is a significant change in sensit ivity to load as

we move along the length of the sensor. At 50cm from the loop end of the

senso r the sens i t iv i ty was a max imum, (2 .4mV/kg) and at 80cm the

sens i t iv i ty became a lmost zero. It was observed that non-uniformi ty was

caused b y the in f luence o f the in ter face b e t w e e n the road and sensor.

B e c a u s e there are s l igh t pe r tu rba t i ons in the encapsu la t ion , in some

locat ions there was a t ight buffer ing be tween the sensor and the block. In

these posi t ions the sys tem exhibi ted reduced sensitivity. However when the

buffer ing was not so tight then the sensi t ivi ty was greatly enhanced.

f ,

This test illustrates that i f uniformity to load is required then care must be

taken to achieve not only a uni form buffer ing be tween road and sensor but

also to maintain a f lexible adhesion be tween the two.

p a g e . 2 4 -

2 5

To check that the non-uniformity was caused by the road sensor interface,

load tests were performed with a secondary load of 100kg applied to the

sensor at a different location. The results are shown in graph 3.10 below.

The result shows that the output response does correspond to a horizontal

shift in the graph shape.

1.2 ~ ~ Is Loading • Unloading

1.1 ...... • Loading • Unloading

1.0, k~

o 0.9,

m Q

0.8, 0 1 0 0 Load (Kg) 2 0 0

Graph 3.10 Graph showing the variation of output of Herga electronics for load tests using the modified Herga Sensor I) System without secondary load applied and 2) with a secondary load of 100kg.

c~ Temnerature Effect on Sensor

The effect of temperature was investigated using the apparatus shown in

figure 3.5 of section d. An embedded probe was used to monitor the

temperature of the section of sensor inside the oven. To obtain a more

repeatable experiment, the temperature was cycled three times to monitor

the drift due to temperature cycling. The results are shown in graph 3.11

The results show that the form of the intensity variation with temperature

is similar for either repeated heating or cooling but that the magnitude

decreases with cycling. The effect of increasing temperature on the

intensity is a maximum at 50°C giving a change of 40mV for the first cycle

and 20mV for the last cycle. For the cooling curve the maximum change

due to temperature was 30mV for the first cycle and 20mV for the last cycle.

The results indicate that repeated heating and cooling of the sensor would

reduce the temperature dependence of the sys tem. The m a x i m u m

sensitivity to load is 2.4mV/Kg. The maximum effect of temperature is 30mV

over 30°C or lmV/C. So Temperature has a 50% effect on the signal

page -25 -

26

received. The temperature effect can be reduced with further cycling of

temperature. A zero point drift of 10mV was observed due to temperature

cycling. This corresponds to a load difference of 5kg applied to the sensor.

1 . 8 0 I

• Heating1

• Cooling i 1.76 t Jj ~ Heating2 ~ ~ L

o • Heating3

- - 1 . 7 2

1 . 6 8 i , !

25 35 45 55 65 Temp(C)

Graph 3.11 Intensity variation with temperature cycling for the modified Herga Sensor.

3.5 S u m m a r y of Axle De tec t ion E x n e r i m e n t s

The experiments performed on the Pilkington and Herga systems lead to

the following results:

i) The first Herga sensor (HSI) in its present form does not appear to be

sufficiently sensitive to detect axle loading when embedded in a real road

situation. However if this lack of insensitivity could be overcome, the

experimental evidence is that problems due to drift, temperature changes

and load non-uniformity are not prohibitive. The system can cope with

both stationary and transient loads (the pulse rates used are sufficiently

rapid to monitor the most rapid road speed expected.)

ii) The New Herga sensor (HS2) is sensitive enough to measure axles when

embedded in the road but attention must be given to the road sensor

interface to maintain sensitivity and uniformity along sensor length. The

effect of temperature has to be considered. Repeated temperature cycling

could be used to reduce the temperature dependence.

iii) The Pilkington system is extremely sensitive and has no difficulty in

detect ing vehicle axle type loads transiently even when encapsulated. It

page .26-

2 7

can withstand extended operat ion under heavy traffic condi t ions However

it has two inherent disadvantages which are:

over-sensit ivi ty to per turbat ions of. the fibre connec t ing the road

sensing element to the instrumentat ion.

its inability to respond to a time invariant perturbation which would

be caused by a stationary vehicle.

The former disadvantage can lead to spurious signals not caused by road

vehicles and could make the system unsuitable for moni to r ing a single

motorway lane in isolation from the others. This over-sensi t ivi ty can also

cause problems with regard to quas i - s ta t ionary vehic les because the

slightest movement of the vehicle tyre in the vicinity of the fibre sensor

will also produce spurious signals.

%

p a g e -27 .

28

4 V E H I C L E WEIGHING CONSIDERATIONS.

4.1 P e r f o r m a n c e C r i t e r i a .

The experiments described in the previous section assessed the potential of

the Herga and Pi lkington sensors for use as axle detection systems. The

P i lk ing ton sys tem responded to dynamic loads only and because of its

re l iance on speckle pat tern changes which are random, is incapable of

d i s c r imina t i ng be tween d i f fe ren t loads ( sec t ion 3.3). Both the Herga

systems could in principle cope with both static and dynamic loads. The load

exper iments on the Herga systems showed that there was a proportional

change in output with an increase in applied load. Consequently the Herga

sys tems has the potential for being used as a vehicle weighing system.

However the l imitat ions of the first Herga system are its inability to

r e s p o n d to typ ica l loads w h e n a p p r o p r i a t e l y encapsu la t ed and a

susceptibi l i ty to drift and to temperature induced variations. The modified

Herga sensor on the o ther hand is able to respond to load when

encapsu la ted and embedded in the road. It however suffers from other

p rob lems l ike tempera ture dependence .

It is the re fo re appropr ia te to examine whether these l imitat ions are

fundamenta l in nature or mere ly associated with the particular mode of

s y s t e m a d d r e s s i n g u t i l i s e d in bo th the Herga and P i l k i n g t o n

ins t rumen ta t ion . For this purpose tests have been per formed using a

d i f f e ren t m o n i t o r i n g t echn ique in con junc t i on with the fibre sensing

e l e m e n t s .

The requi rements for a vehicle weighing system include the ability to

d i s c r imina t e b e t w e e n d i f f e ren t loads under both static and dynamic

condi t ions . The sys tem needs to be able to respond with the sensor

embedded in the road and give enough resolution to be able to distinguish

different vehicle classes. The system should be able to respond to both a

bicycle and a heavy goods vehicle.

/ ,

p a g e -28-

29

4.2 Chromat ic Modulat ion Technioues

Chromatic modulation i s a technique which has been developed at the

University of Liverpool [REF 6,7] for sensing with optical fibres using

polychromatic light. In the context of the present investigation it provides

a versatile means of addressing the optical signals transmitted through the

stress sensitive optical fibres. As such the relative capabilities of both fibre

sensors may be compared with a common monitoring technique and

assessment made of ultimate performances.

The principle of operation of chromatic modulation is presented followed

by a description of the tests performed with the method.

4.2.1 Princinles of Chromat ic Modulat ion

The chromaticity of an optical signal is determined by the form and not the

magnitude of its spectral intensity distribution. The chromaticity therefore

only varies with the non-uniform changes of spectral intensities.

Consequently it is independent of signal intensity variations caused by

spurious results.

In its simplest form chromatic modulation utilises two photo-detectors to

address two different parts of the optical spectrum. (It differs from single

wavelength ratio techniques since substantially large and overlapping

regions of the spectrum are addressed and this provides added advantages).

The ratio of the two signals from the two detectors gives an indication of

the chromaticity of the signal. If there is the same reduction in the

intensities of the signal at all wavelengths, then there is no chromatic

change since the ratio of the two signals remains unchanged. In practice

optical sensors which modulate the intensity of a polychromatic optical

signal usually produce simultaneously an associated chromatic change.

This is because the modulation action affects each of the wavelength

regions differently.

The instrumentation for detecting such chromatic changes is in its basic

form relatively unsophisticated.

page -29-

30

Spectral s ignature changes are anticipated with the Herga microbend

technique since the optical attenuation produced by microbending is

wavelength dependant. (Section 2.2 above). The speckle pattern of the

Pilkington system are due to monochromatic light interference and should

therefore not produce any chromatic changes; the use of broad band,

polychromatic sources are unlikely to provide an attenuation since the

speckle pattern itself will become obscured.

4 . 2 . 2 S n e c t r a l R e s n o n s e s o f t h e C a n d i d a t e s e n s o r s

The spectral responses of both the Herga and Pilkington fibre sensors

when subjected to stress were investigated in order to establish their

potential for use in conjunction with the chromatic modulation technique.

For this purpose each type of fibre element was addressed with a white

light source and the spectrum of the transmitted light measured under

various stress conditions using an Optical Spectrum Analyser.

Graph 4.1 shows the spectral distribution for the Pilkington fibre when

subjected to various degrees of bending. (specified in terms of bend radii of

10cm, 5cm, etc.) The attenuation suffered is limited and the chromatic

changes even smaller. This result is in accordance with the anticipated

speckle pattern changes. (section 2.1) being indistinct with polychromatic

l i gh t .

Graph 4.2 shows the spectral changes for the first Herga fibre (HS1) when

subjected to various loads up to 700g (unencapsulated). The significant

a t tenuat ion shows that there are also chromat ic i ty changes which

accompany the attenuation. This behaviour is in accordance with the

anticipated effects of wavelength dependent microbending. (section 2.1).

Graph 4.3 shows the the spectral characteristics of the modified Herga fibre

(HS2) when subjected to various loads up to 250 kg. The sensor was loaded

using the apparatus shown in f igure 3.6. The character is t ic are

significantly different to that of the first Herga fibre.

From the two spectral characteristics a number of important features can

be observed. The modified Herga fibre exhibits no peak in output at around

page -30-

31

%

500nm as in the first Herga sensor. This could be caused by the fibre being

pre tens ioned inside the sensor e l ement and this causes an ef fec t o f

preloading the sensor to around 7008 on graph 4.2. This would not

contradict the results obtained from the load experiments performed on the

two sensors using intensity modulat ion (see graph 3.2 and 3.9). Graph 3.2

shows the effect on intensity of the first Herga sensor with load and 3.9

shows the results of the load tests using the modif ied Herga sensor. Graph

3.2 shows a region of reduced sensit ivity to loading at the start o f the

graph. This region is not present in the exper iments per formed on the

modified sensor (graph 3.9). This could be explained if the modif ied sensor

was pretensioned inside the encapsulat ion.

Another feature of two spectral characteristics is the posi t ion of the peak

amplitude of response. For the first Herga sensor (graph 4.2), there is a

very small shift in the wavelength o f peak response ( .04nm/g). However

there is a much greater shift in the modif ied sensor ( .gnm/kg).

The two graphs 4.2 and 4.3 also seem to have different spectral shapes.

There are a number of possible causes for these differences. These are

listed below.

i) The modi f i ed Herga sensor is p r e t e n s i o n e d when encapsu la t ed

compared with the old Herga sensor.

i i ) The spectral differences are caused by the d i f fe rent m e t h o d of

inducing microbending in the two sensors.

iii) The type of fibre used is different in the two sensors.

On balance the possibi l i ty o f p re tens ion ing the sensor seems to be

consistent with the results obtained.

The significance of these results is that chromatici ty changes are l ikely to

be p r o d u c e d wi th the two H e r g a f i b r e s w h e r e a s w i t h the

page -31-

Graph 4.1

Graph 4.2

I Spectral Characteristics of Pilkington Sensor

I Wilvelonilth (tim) 641 nm 1008nm

Graph 4.3

I

Spectral Characteristics of Herga Sensor (HS1)

I Wavelength (nm) 641nm

Spectral Characteristics of Modified Herga Sensor (HS2)

32

Wllvelmr~th (rim) 641nm lOOGnm

p a g e -32-

33

Pilkington fibre such changes are subdued. Thus the two Herga fibres are

more likely to provide chromatic changes compat ible with the chromatic

monitoring instrumentation. It is expected that the two types of Herga

sensors will produce different chromatic characterist ics when subjected to

load and temperature.

4.2.3 Tests us in~ the P i l k i n ~ t o n s y s t e m

Tests on the Piikington system using the chromatic technique showed that

there was insufficient change in output for applied load and no conclusive

experiments could be performed using the system. This is perhaps not

surprising in view of the system reliance upon speckle pat tern changes

which are of course random. It is also consistent with the spectral changes

shown in graph 4.1.

4.2.4 E x n e r i m e n t a t i o n us in~ the H e r ~ a S e n s o r s

Tests pe r fo rmed on the two Herga sys tems are d e n o t e d as HSI

(corresponding to the or ig inal ' fibre sensor which p r o d u c e d spect ra l

changes given on graph 4.2) and HS2 (corresponding to the second Herga

fibre which produces the spectral change shown on graph 4.3.)

t)__I, Ltu

A number o f exper iments have been p e r f o r m e d to d e t e r m i n e the

performance of the Herga fibre sensor in terms of chromatic change. One

immediate advantage was that the chromat ic de tec t ion sys tem ut i l ises

s tandard SMA fibre connec tors . This made the e x p e r i m e n t s more

repeatable. Simple load tests were made with the loading apparatus already

shown in figure 3.5 whilst proper tyre load tests were performed using the

apparatus shown in figure 3.6.

The results of these tests are given respectively on graphs 4.4 and 4.5

p a g e -33-

34

. - . 3

v

¢= 4 -

o 2

,e,,,=

¢1

¢1 0 1

0

I I .e~ ira,= - 1 m

I ,

• ~ V o l t a ( i t ( V )

0 1 O0 Load(g) 2 0 0 3 0 0

Graph 4.4 Load response of chromatic instrumentation using bare Herga sensor and loaded to 0.3 kg

r /

2.50 ......... I

Un;oading 3 "

& -- -

E m i ~ a d i n o

~ 1.00 ~ ~ ~ ~ ~ • 2

0.50 ~ w Unloading

0.00

~e Unloading 1

! !

_ Loading 1

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 Load (Kg)

Graph 4.5 Variation of output chromaticity with load of the encapsulated Herga fibre with a tight jacket and loaded using a tyre.

Graph 4.4 shows the chromatic variation for the uncapsulated fibre. The

output is similar to that of the Herga instrumentation system (graph 3.2).

There is a total change in output form zero to 300g load of 2.5V. The graph is

non-linear but for larger loads exhibits some linearity. The sensitivity of

the system is 10mV/g and the noise on the system was 5mV. this

corresponds to a signal to noise ratio of 500:1

p a g e .34 -

35

Graph 4.5 shows the chromatic variation for the embedded fibre using the

real tyre simulation (figure 3.6). The variation of the output signal with

load is again non-linear. In this case there is a change in output voltage of

2V from an unloaded state to a load of 250kg. The noise on the signal was

about 1OmV peak to peak giving a signal to noise ratio of 200:1 . Graph 4.5

shows three successive stress tests to examine test to test variation. The

results show quite good reproducibility but with a zero point change of

10mV. The change is uniform over the range of loads investigated.

The above results suggest that even though there i s monotonic drift in the

system, the total change in output for a fixed load may be constant since the

actual wave shape is preserved in successive loading and unloading tests.

The results of graph 4.5 are significant in that they show that chromatic

modulation is capable of responding under condit ions that the Herga

instrumentation becomes insuff icient ly sensit ive.

b) Drift Tests

The results of graph 4.5 suggested that more extensive tests for long term

drift susceptibility were needed. Graph 4.6 shows a chart recording of an

experiment where the load was repeatedly cycled between 50 to 250kg. The

experiment was performed over-night and the system was still operational

at the end of the experiment. The result showed that even though there is a

long term drift the amplitude of the signal remains constant to better than

5%. The cause of the long term drift is not known and at this stage should

not necessarily be ascribed to the fibre sensor itself.

Two samples of unencapsulated Herga fibre were connected one to the

Herga e lec t ron ics and the o ther to the c h r o m a t i c m o n i t o r i n g

instrumentation. The output of each section was monitored over a period of

10 days. The results are shown in graph 4.7. The ambient temperature was

also monitored. The drift observed on the Herga electronic system is

approximately 3% of full load whilst with the chromatic instrumentation it

is 10% of full load. (amplifier gain is increased on the chromat ic

i n s t r u m e n t a t i o n ) .

page -35.

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38

c~ Temperature Effects

The influence of ambient temperature changes upon the chromatic signal

obtained from the Herga fibre was investigated using the apparatus of

figure 3.3. The effect of temperature on the encapsulated fibre is illustrated

in graph 4.8 below.

1 . 6 5 r

E :~ 1.40 r 0

1 .15 _ i

i .......

V" ,,71 [ • Cooling 2

I I I T ~i r-~r T I e, Heating 1 ~ • Heating 2

0 . 9 0 ' I ' : ; 20 30 4 0 5 0 60 70

TemD (C) Graph 4.8 Variation of chromaticity with temperature of encapsulated Herga sensor with probe embedded into encapsulant

The results of Graph 4.8 suggests that the heating and cooling cycles have

unrelated characterist ics. After the first temperature cycle a permanent

change in zero point voltage is apparent. The change after one cycle is

about 27 mV and the maximum change in output for one cycle is 450 inV.

However the experimental evidence suggests that repeated heating and

cool ing of the sensor may make the response of the sensor more

r e p r o d u c i b l e .

To put the variations due to heating in perspective it should be recalled that

the chromatic change for unloaded to fully loaded is about 2.5V. This means

that the effect of temperature is 18% and that of cycling the temperature is

only of the order of 1% at worst.

p a g e . 3 8 .

39

Tests on the improved Herga sensor described above, (chapter 3.4 section i),

were performed to observe the effect of chromaticity with load. Tests on the

sensitivity and uniformity along sensor length in terms of load applied are

described in the section below.

a] Sensitivitv to Load

Tests on the change in chromaticity with load were performed on the new

sensor using the apparatus shown in figure 3.7. The results are illustrated

in graph 4.9 below. 0.2

v

~, 0.1

i 0

O.O

J

f

)

Loading I Unloading I

0 1 0 0 Load (kg) 2 0 0

Graph 4.9 Graph showing the variation of chromaticity with load for the modified Herga sensor.

The results show that the variation is nonlinear and that the chromatic

signal increases to 1.6V and reach a peak at around 100kg before

decreasing as the load is further increased to 200kg. Furthermore during

load cycling tests a change in output from I50mV for 100kg loading to 75mV

for the unloading was observed. The unloading cycle is seen to be of the

same shape as the loading cycle but with a much reduced sensitivity. The

load was cycled six times to observe the effect on sensitivity to load. The

results are shown in graph 4.10. This indicates that there is a gradual

reduction in sensitivity with the cycling of load. The zero point drift was

25mV. However the point of greatest modulation (i.e. at 100kg) the change

was 65mV. The noise on the system was 5mV. This gives a signal to noise

ratio of 30:1 and a resolution of 10kg at best.

p a g e -39-

40

The shape o f the cha rac te r i s t i c s can be exp la ined by cons ider ing the

spectral charac ter i s t ics o f the Herga sensor (graph 4.3). For small loads

there is a grea ter shif t at the higher wave leng ths and as the load is

increased there is a spectral shift in the peak of response towards the lower

wavelengths . The non-monotonic nature of graph 4.9 is not restrictive as a

su i table choice of chromat ic de tec tors can be used to adjust the response

(see note 1).

0.20

~ 0.15

"~ 0.10 •

0.05 - ~ , ,

• Loading 1 • Unloading 1 • Loading 2 • Unloading 2 • Loading 3 • Unloading 3 • Loading 5 • Unloading 5 • Loading 6 • Unloading 6

0.00 0 100 Load (kg) 2 0 0

Graph 4.10 Graph showing the effect of cycling load on the chromaticity using the modified Herga sensor.

b~ Uni fo rmi tv Along Sensor Length

Tests on the uniformity along the length of the sensor were performed. The

resu l t s are g iven in graph 4 .11. The resul ts indicates that the output

cha rac te r i s t i c s with load changes with pos i t ion along the sensor. Here

three outputs for loading cycles are shown.

The effect of a secondary load on the sensor was tested by applying a 100kg

load 30cm away f rom the loading section. The results are given in graph

4.12. These conf i rm the non-l inear i ty of the sensor. The sensit ivity of the

sensor is reduced f rom 1.25mV/kg to 0 .25mV/kg.

Note 1. Prelimenary theoretical calculations indicate that the non-monotonic nature of the response (graph 4.10) is governed by tuning the chromatic detection system and can indeed be conveniently made monotonic.

p a g e -40-

41

== Unloading 20cm I • Loading 20cm

~ ~ ~ ~ UnloadingSOcm "~ o 0.0

~ ' 1 Loading 80ore

o u Unloading 80cm -0.1 ! I

0 100 Load(kg ) 2 0 0

Graph 4.11 Variation of sensitivity to load of modified Herga sensor in terms of chromatici ty

• Loading 1 7 ~ , ~ = ,, . . . . . . . .

~, ~ " Unloading 1

O 0.1

, , . . . . . . . . . . . . . . . . . . -

a • Loading 2 E .o ~ , .W ~- ~"'- I --'----~ • ~= : t "~Ik .,--~, _- .- ......... ~ ~" - _

0 ~ - - " - ~ • Unloading 2 I 0.0 . .

0 100 Load (Kg) 2 0 0

Graph 4.12 The effect of a secondary load on the system, l) is the output of the chromatic modulation system for load cycling the modified Herga sensor and 2) the response of the same system with a secondary load.

These results indicate that the new Herga sensor has comple t e ly d i f ferent

chromatic characterist ics to the previous type.

c) Temnerature Effects

Tests on the effect of temperature on the mod i f i ed Herga sensor were

performed using the system descr ibed in f igure 3.5 above . The results are

shown in graph 4.13 below.

The results indicate that even though temperature a f fec ts the intensi ty o f

the sensor, the e f fec t on the ch romat i c i ty is s i gn i f i can t ly lower . The

chromat ic change is less than 50mV for the r ange 45°C. Thus the

temperature effect is less than lmV/C.

page -41 .

42

~>' 9.6- m

0 0

E 0 i m

,4= o

9.4 25

I I !

[] Heating 1 • Cooling I • Heating 2 • Cooling 2

1 ! J.

3 5 4 5 5 5 6 5 Temperature (C)

Graph 4.13 Variation of chromaticity with temperature for the modified Herga sensor.

4 .2 .5 D i s c u s s i o n

Tests have shown that the Herga fibre sensors HS1 and HS2 both produce

s igni f icant chromat ic modu la t i on but that the Pi lk ington fibre system

produces insuf f ic ien t change .

The HSI sys tem sens i t iv i ty is cons ide rab ly improved by using the

chromatic technique giving a modulat ion of 8mV/kg and a signal to noise

ratio of 200:1. Insuff ic ient signal was available for the Herga intensity

system itself to be used when the fibre was encapsulated for road use. The

chromatic approach for reducing drift and temperature problems which is

no t c o n v e n i e n t l y pos s ib l e wi th the in tens i ty m o d u l a t i o n technique

presently employed with the Herga system.

The HS2 system produces non-monotonic chromatic variations probably on

account of the method of pre tens ioning used which may be el iminated

using proper chromatic tuning of the system. The resolution is typically

10kg with a signal to noise ratio of 30:1. The corresponding values for the

intensity system were 5kg and 20:1. The chromatic system is insensitive to

t e m p e r a t u r e e f f ec t s ( I m V / ° C compared to 10mV/°C for the intensity

system). The Chromatic moni tor ing also has the capability in combination

wi th the intensi ty moni to r ing of d iscr iminat ing load location along the

f i b r e .

p a g e - 4 2 -

43

5 CONCLUSIONS

t~

W

4'

C',

The results of the present invest igat ion lead to a number o f important

c o n c l u s i o n s :

• The Pi lk ington fibre sensor sys tem based upon speck le pat tern

changes is a highly sensit ive system capable of vehic le detect ion

when road embedded. It has successful ly wi ths tood ex tended road

embedment trials.

However the system's high sensitivity can lead to spurious signals due

to non -veh i c l e sources and p o s s i b l y i n t e r f e r e n c e f rom non

moni tored traffic lanes. Also its inherent mode of opera t ion to

respond to transient changes makes it unsu i t ab le for de tec t ing

stat ionary vehicles .

The first Herga system shows sufficient uni formi ty along the fibre

length, sufficient insensit ivi ty to drift and tempera ture effects and

sufficient insensi t ivi ty to non-veh ic le in te r fe rence for use as a

vehicle de t ec to r . It is also capable of moni tor ing both moving and

stat ionary traffic.

However the first Herga fibre system when encapsula ted for road

embedment is insuff ic ient ly sensi t ive for veh ic le de tec t ion when

used in c o n j u n c t i o n wi th the Herga a m p l i t u d e m o d u l a t i o n

i n s t r u m e n t a t i o n .

Nonetheless, when used with chromat ic modu la t i on ins t rumenta t ion

the Herga fibre system becomes su f f i c ien t ly sens i t ive f o r axle

detection when properly encapsulated.

The modified Herga sensor is suitable for use in conjunct ion with a

properly tuned chromatic modula t ion sys tem and is operat ional with

intensity modulation. However care must be taken in the development

of the road sensor interface to optimise uniformity and sensit ivity if

its load measuring properties are to be used. Also the effect of

temperature on the system must be reduced.

page -43-

44

The Pi lk ington sys tem does not have the inherent capabil i ty for

vehicle weighing since it only responds to optical signal changes in

the speckle pattern as opposed to the magnitude of the signal.

Both the Herga systems do possess the capability for vehicle weighing

because the basic pr inc ip le o f operat ion (microbending) gives an

output which is a function of stress or loading.

However the first Herga f ibre sensor needs to be ut i l ised in

conjunct ion with chromatic modulat ion in order to take advantage of

its inheren t po ten t ia l for we igh ing . This not 0nly yields the

necessary degree of sensitivity but also provides a potential route for

compensa t ion for drift and temperature effects to provide improved

rel iabi l i ty and resolut ion.

The modif ied Herga sensor ,used with chromatic modulat ion, has the

potent ial o f d iscr iminat ing the posi t ion, of load along the sensor,

which could for instance be useful for traffic lane description.

The performance capabilities of the three systems Pilkington, HS1 and HS2

are compared on table 5.1.

r ~

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46

6 RECOMMENDATIONS

The investigations have proved that optical fibre based methods have the

potential for development for not only axle detection but also vehicle

weighing. Existing fibre sensors in conjunction with chromatic modulation

techniques offer the best possibility for future development.

Further development will clearly be necessary with regard to road tests,

establishing weighing limitations and improving drift and temperature

response characteristics. It is believed that such objectives would be

achievable fol lowing a l imited amount of further research and

d e v e l o p m e n t .

Such development need to be conducted simultaneous to further road

testing and the joint participation of the manufacturers of the fibre

elements and researchers

p a g e . 4 6 .

4 7

7 REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

[7]

J.M.Senior,"Optical Fibre Communications",Prentice/HaU

International 1985, pp 73-76.

J.M.Senior,"Optical Fibre Communications',Prentice/Hall

International 1985, p74.

J.M.Senior ,"Optical Fibre Communications", Prentice/Hall

International 1985,pp 24-42

J.M.Senior ,"Optical Fibre Communications", Prentice/Hall

International 1985,pp29-34

J.M.Senior ,"Optical Fibre Communications",Prentice/Hall

International 1985,p75

G.R.Jones, S Kwan C.M Beaven,"Displacement Measurement using a

Focusing Chromatic Modulator ",Meas. Sci. Technol. 1 (1990) 207- 215.

G.R.Jones,S Kwan, C.M Beaven,P.Henderson and E.Lewis,"Optical

Fibre based Sensing using Chromatic Modulation", Opt. Laser

Technol. 9(1987) 297-303.

p a g e -47-