Characteristics of fabrics in automated handling systems

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2002 24: 15Transactions of the Institute of Measurement and ControlP. M. Taylor, R. Bicker, P. J. Abbott and D. M. Pollet

automated handling systemsParameters affecting the low-force frictional characteristics of fabrics in

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Transactions of the Institute of Measurement and Control 24,1 (2002) pp. 15–31

Parameters affecting the low-forcefrictional characteristics of fabrics inautomated handling systemsP.M. Taylor1, R. Bicker1, P.J. Abbott1 andD.M. Pollet2

1Department of Mechanical, Materials and Manufacturing Engineering, Universityof Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK2European Patent Of�ce, Munich, Germany

Much research has been carried out on the automation of garment assembly but, in practice,many of the techniques suffer from unreliability. In order to investigate why this is so, it isnecessary to determine the critical mechanical properties on the fabrics under the conditionsthat will be found in the handling equipment. This paper concentrates on the frictional proper-ties of fabrics against engineering materials with particular emphasis on these properties underzero applied normal loads, i.e., self-weight. Several tests are described and the main resultsgiven. These highlight the importance of humidity and the strong in�uence of the fabric struc-ture and the supporting surface on the frictional characteristics. It is concluded that either theassembly must be carried out using well de�ned fabrics and a degree of climate control orthat these effects must be carefully considered in the design of handling systems.

Key words: automation; environmental effects; fabric properties; fabric testing; test equipment.

1. Introduction

During the 1980s and early 1990s there was an upsurge in research on the auto-mation of garment and shoe manufacture. In Japan the TRAAS programme

Address for correspondence: Prof. P.M. Taylor, Department of Mechanical, Materials and Manufac-turing Engineering, Stephenson Building, University of Newcastle upon Tyne, Newcastle upon TyneNE1 7RU, UK. E-mail: p.m.taylorKncl.ac.uk

Ó 2002 The Institute of Measurement and Control 10.1191/0142331202tm044oa by guest on September 8, 2012tim.sagepub.comDownloaded from


16 Frictional characteristics of fabrics

(Iguchi, 1990) focused on jacket production, in the USA part of the AMTEX project(Black, 1993) was targeted at the garment industry and in Europe (European Com-mission, 1991) there were a number of initiatives sponsored by the EuropeanUnion under its BRITE and ESPRIT programmes, complemented by variousnational and in-company schemes. Although many ingenious mechanisms andmachines were invented, relatively little has been developed for use in production.Not only is advanced machinery often dif�cult to justify on cost grounds but itmust also be very reliable. In the experience of the authors, this reliability is oftendif�cult to achieve in practice. In particular, slight changes in fabrics and environ-mental conditions can cause an operation to fail catastrophically. Figure 1 showsthe result of a faulty pick-up-and-place of a rectangular fabric panel by a four-element pinch gripper. This is an extreme case but even small errors in positioningor alignment of fabric panels can lead to mis-sized garments with faulty seamssuch as that shown in Figure 2.

It has been observed (Taylor et al., 1998) that changes in humidity can havesigni�cant effects on fabric properties and hence on the performance of automatedgripping. Large, hourly changes in humidity are not untoward, with changeswithin a day of 20–30% relative humidity being common throughout the worldand through many seasons. For example, consider Asheville, North Carolina. At07.30 h in January the average (over 1880 to 1949) relative humidity (r.h.) is 82%,at noon it is 61% (Air Ministry Met. Of�ce, 1958). In Mexico City the change iseven more abrupt. From 07.00 to 13.30 h the corresponding average drops from79% to 34%. There are similar variations when moving from one location to

Figure 1 Failed pick and place of a rectangular fabric panel



Taylor et al. 17

Figure 2 The results on a completed garment of an alignmenterror at an earlier stage of the production

another in the world. Note that most fabrics are affected; indeed their ability toabsorb water is one of the reasons for their comfort.

Humidity is just one possible cause of unreliability; in order to determine whyautomated handling systems are not as reliable as they need to be, a programmeof research was initiated at the University of Hull and transferred in 1996 to theUniversity of Newcastle upon Tyne. This activity comprises two parts, �rstly thedetermination of key fabric properties as required in automated systems and theeffect on these of environmental changes. The second aspect is an analysis of theautomated handling processes, using the above properties to predict the behaviourof equipment and to use the new knowledge of the processes and materials todesign more reliable second-generation machines.

Key fabric properties are the tensile and compressive characteristics, the bend-ing behaviour and the frictional characteristics against engineering surfaces for arange of applied normal forces going down to zero. Most studies on fabric frictionare concerned with subjective fabric handle (the ‘feel’ of a fabric) or have studiedfriction between pairs of fabrics (Thorndike and Varley, 1961; Wilson, 1963; Carret al., 1988). Little work has been reported on friction between fabrics and non-�brous materials, and most of this has been carried out under medium(Nishimatsu and Sawaki, 1984a, b; Yoon et al., 1984; Ajayi, 1992) to high normalforces (Virto and Naik, 1997). None of this can be applied with con�dence for thecase of very low or zero applied normal load, such as occurs when fabric is slidover a surface. Such sliding occurs in aligning devices and vibratory feeders. It




is, however, applicable much more widely than this; fabrics are often only grippedlocally, with the bulk of the material left to trail across the working area. Thislatter material will usually slide or be distorted by the local gripping and manipu-lation task. Consequently, the zero-load frictional data is relevant to many manualand automatic operations such as gripping, moving and sewing. Recent work bythe authors (Pollet, 1998; Taylor and Pollet, 2000a, b) describes tests which wereused to determine the low load frictional characteristics of a range of fabrics overthree surfaces. The emphasis of these papers is on the textile technology and indetermining models for the behaviours and relating them to the earlier workdescribed above. The aims of this paper are to describe the test procedures indetail and to summarize the key results with a view to establishing those para-meters of the fabrics, the handling surfaces and the environment which will affectthe frictional forces from an engineering viewpoint. Conclusions are then drawnregarding the implications of these results for designers of automated equipment.

2. The fabric samples and surfaces

Six types of fabric were chosen to represent the wide range of fabrics used in theapparel industry. They cover cotton, wool, acrylic and polyester �bres and twomain types of construction: woven and knitted. Their surface �nishes areunknown, being as supplied by the producers. The main technical details of thefabrics are given in Table 1. The setts were counted under a conventional micro-scope and the results con�rmed from the laser scans described below.

A laser sensor (LD1605–4 by m e Ò ) was used to characterize the surface pro�leof the fabrics. The 300-m m diameter beam of light of wavelength 675 nm waspointed downwards onto the fabric measurement surface. The sensor uses triangu-lation to convert the re�ected beam into a distance. A 12-bit analogue-to-digital(A/D) converter (PC30AT-card) was used for these measurements giving a 5-mVresolution corresponding to 1 m m displacement. The fabric sample was movedunder the sensor using an x–y table in order to get the surface pro�le made upfrom six scan lines 1 cm in length and 1 mm apart. For each of the six chosen

Table 1 Fabric descriptions (Courtesy of the Textile Institute)

Fabric Fibre Fabric Sett (threads/cm) Mass Thicknesscode content structure (g/m2) at

0.5 cN/cm2Warp/wale Weft/course(mm)(P) (T)

C1 Cotton Twill weave 19 17 517 1.034P1 Polyester Plain weave 46 39 105 0.190W1 Wool Twill weave 12 13 352 3.095KC1 Cotton Fleece knit 16 11 257 1.428KC11 Cotton Double knit 14 15 183 1.030KA11 Acrylic Double knit 15 10 264 1.230



Taylor et al. 19

fabric types, six sample pieces were used for the warp/wale tests and six separatesample pieces for the weft/course tests. Six line pro�les, each 1 cm in length and0.1 mm apart, are scanned at 1 mm/s on every sample. The surface roughness,SMD, of the fabrics is, in accordance with the KES-FB4 test (Kawabata, 1980),calculated from their height pro�le, hi, obtained by the laser sensor at s differentpoints and is de�ned as follows:

SMD =Os

1

(hi 2 h̄)

s(1)

h̄ represents the mean value of the heights. Large-scale, low-frequency undulationsof the fabric can occur, so, taking the approach of Ramgulam et al. (1993), twomean heights and the local roughness based on them are calculated for 4-mmsubdivisions, SMD1 being calculated between 2 and 6 mm and SMD2 between 6and 10 mm. The �lled area in Figure 3 between the mean value and the actualsurface height of the second subdivision represents the numerator of Equation(1). The results are used later in the section dealing with the dependency of frictionon the surface properties of the fabric.

Three representative engineering materials were chosen for the test surface: alu-minium as a metallic, lightweight construction material, rubber as a surface usedin grippers and conveyors, and Formica as a typical tabletop material. Detailsregarding the roughness of the surfaces can be found in Table 2. Each surface isin the form of a plate which can be clamped to the moving table described below.

Figure 3 First pro�le scan of KC11 along the wale direction(courtesy of the Textile Institute)




Table 2 Speci�cations of the engineering surfaces (Courtesy of the Textile Institute)

Aluminium Formica Rubber

Material 99% pure Natural grade BThickness 0.15 mm 0.9 mmCharacteristics Half hard AntistaticRoughness,a Ra ( m m) 0.243 0.959 0.600

aThe roughness test is performed on a Form Talysurf Series with a diamond-tipped stylus (tipradius 1.5–2.5 m m) spring-loaded onto the material with a force of 70–100 mgf.

3. Equipment

The apparatus is based on Dreby’s principle (Dreby, 1943) and is shown inFigure 4. A 10-cm square sample of fabric is placed on a test surface. The sampleis constrained by two light threads connecting it to a static ring whilst the surfaceis slid in a controlled manner beneath it. The spring steel ring is of diameter150 mm and has four single-axis semiconductor strain gauges bonded to it at themid-length of the ring. These are connected in a bridge network to a high-gain

Figure 4 Friction-testing equipment



Taylor et al. 21

ampli�er mounted immediately below the ring such that a full-scale de�ection of5 g will give an output voltage of 10 V. This signal is connected to a PC via a12 bit A/D converter, thereby giving a resolution of 1.22 mgf. The computer logsthe data and provides the control signals to the motors moving the test surface.In these particular experiments the test surface is clamped onto an x–y table withonly the x-axis energized.

The equipment is housed in a room where the temperature can be controlledto 6 1°C and the humidity to 6 4% r.h. Many of the tests are carried out under‘standard conditions’. In this paper this means 20 6 1°C and 65 6 4% r.h. Notethat true ‘standard conditions’ as de�ned by ASTM, 20 6 2°C and 65 6 2% r.h.(ASTM D1776–85), require the relative humidity to be more closely controlled. Itis not possible to achieve this tight speci�cation in our laboratory but it is found,in practice, that repeatabilities are very good and experimental curves are smoothso that this imprecision is deemed acceptable for the tests carried out here. Duringthe friction tests, the air is ionized with a portable ionizer close to the table toprevent the fabric samples from becoming electrically charged.

4. Test procedures

In general, every test is carried out six times, each time with a different piece offabric cut from adjacent sections of a roll. All fabric samples are preconditionedby leaving them, technical face upwards, in the controlled environment for at least1 day. This conditioning period was determined experimentally as describedbelow in the �rst section of results. After conditioning, the mass of the samplewas measured using an electronic balance (Mettler Toledo PB302) having a resol-ution of 0.01 g. This mass is entered into the computer program along with thetest parameters: table velocity, distance of travel and sampling frequency (75 Hz).

The fabric sample was then connected to the measurement ring with a �nepolyester thread and temporarily rested, measurement face upwards, on the coverof the ring and its ampli�er. Following a command to return the table to its homeposition, the fabric sample was placed measurement face down onto the test sur-face. A short measurement was taken to calibrate the zero level and offset theweight of the �bre attachment, followed by a dwell time of 30 s in accordancewith ASTM D3334–80.

Figure 5 shows a typical friction trace where the ‘friction coef�cient’ m i at a giventime is taken as the instantaneous force measured by the ring, divided by the normalforce exerted by the self-weight of the sample. The static friction coef�cient, m s, istaken as the highest peak at the start of the motion and the mean of the peaks andtroughs is taken as an estimation of the dynamic (kinetic) friction coef�cient m d.

The discrimination, however, between the static and dynamic friction for low-pressure tests is not always very clear, especially at higher velocities. The dynamicfriction coef�cient is indicated in Figure 5 by the horizontal line through the pulsesof the friction trace and calculated as follows:

m d =Os

1

m i

s(2)

where s is the number of samples m i, starting from the static friction reading.




Figure 5 Friction trace for KC11 fabric (wale direction) againstthe aluminium surface (courtesy of the Textile Institute). Standardconditions, velocity = 0.5 mm/s, mass sample = 1.78 g, area of thesample = 1 dm2, static friction coef�cient m s = 0.341, dynamicfriction coef�cient m d = 0.339

5. Experimental results

An extensive set of experiments was then carried out to determine the effects onfriction of a number of key parameters: roughness and direction of the fabric, themorphology of the table on which the fabric rests, the speed of the fabric relativeto the table, triboelectric charge, small applied loads (multiple thicknesses offabric) and humidity.

5.1 Effect of fabric surface roughness and direction

The results of Figure 6 show that the smoother the fabric’s surface the higher thefriction coef�cient will be. In addition, note that all knitted samples have a higherfriction coef�cient than the woven samples.

The linear regression �ts (least squares) in Figure 6 indicate the trend betweenthe friction coef�cient and the roughness for both woven and knitted structures.The results indicate models of the form m d = aSMD + b with a = 2 1.34, b = 0.44for knitted and a = 2 0.34, b = 0.28 for woven (SMD = mean roughness). Figure 6



Taylor et al. 23

Figure 6 Scatter plot of dynamic friction for unloaded samplesagainst aluminium as a function of roughness. Sliding velocity =1 mm/s (courtesy of the Textile Institute)

also shows that for some fabrics, such as P1 and to a lesser extent KC1, the direc-tion of movement with respect to the principal direction of the fabric is also sig-ni�cant, whereas for others, such as KC11, C1 and W1, the directionality effectsare negligible.

5.2 Effect of the table surface

Generally, the variation in friction coef�cient between fabrics for a given surfaceis less pronounced than between fabric–fabric, which can give a variation betweenindividual materials of up to 160% (Ajayi, 1992). He ascribes this to the smoothersolid surface, which cannot obstruct the movement compared to fabrics where theyarn crown and ribs can �t together when slid across each other. Here, as indicatedin Figure 7, three different sliding surfaces (aluminium, Formica and rubber) havebeen used. It can be seen from Figure 7 that the friction for all fabrics in the weftor course direction is lowest when slid across aluminium, highest against rubberand intermediate for Formica. Similar results are found in the other fabric direction(Pollet, 1998) and correspond with recent �ndings by other researchers (Yoonet al., 1984; Ajayi, 1992; Virto and Naik, 1997), who found the same classi�cationsfor loaded samples according to the sliding material. However, the morphology




Figure 7 Friction coef�cients for set 1 fabrics in the weft (course)direction against different surfaces. Standard conditions, velocity= 1 mm/s, sample area = 1 dm2

of the sliding surface (see Table 2) does not seem to be directly responsible forthis friction increase.

5.3 Effect of table velocity

Experiments were undertaken to investigate the velocity dependency of dynamicfriction of the fabrics against the test surfaces. A group of six samples was usedfor each principal direction. For each group, a velocity was chosen at random,the tests carried out and a recovery period of 1 day allowed before testing at adifferent velocity.

The friction, as displayed in Figure 8, initially decreases with increasing speed,reaches a minimum, and eventually increases again with increasing speed towardsa constant value, following the distinctive form of the Stribeck curve. Similarcurves are obtained for �brous materials (lubricated or clean yarn) and journalbearings (Hansen and Tabor, 1957; Kalyanaraman, 1988a). Four regimes of lubri-cation can be distinguished: static friction (velocity independent), boundary lubri-cation, semi-boundary lubrication at the minimum friction and hydrodynamiclubrication towards higher speeds.

5.4 Effect of triboelectric charge

Another factor which can in�uence the handling behaviour of fabrics is thepresence of triboelectric charge. All the tests now described were carried out under



Taylor et al. 25

Figure 8 Velocity effect on the dynamic friction of KC11 (wale)sliding against a rubber surface (courtesy of the Textile Institute)

fairly dry atmospheric conditions: 33% r.h. and 21°C. For test 1, all of the unloadedfabric samples were left on a charged perspex plate for 30 min and then testedon the aluminium plate as above. Table 3 shows the effect on the static anddynamic friction coef�cients. For Test 2, the samples are left charged for 1 daybefore the friction coef�cients were measured. Following this, for test 3, the fabricswere each rubbed 10 times on the aluminium surface. The friction coef�cients ofP1 (polyester) are 0.28 at 33% r.h. when no charge is present so, from Table 3, itcan be seen that they are almost 10 times larger when an electrostatic charge ispresent. Note that the electric charge on the fabric will be carried away by watermolecules when they evaporate (Onogi et al., 1996) and so an increase in humiditywill reduce the triboelectric effect.

Table 3 Triboelectric effect on the friction coef�cients (33% r.h., 21°C)

KC11 W1 P1

Friction coef�cient

Static Dynamic Static Dynamic Static Dynamic

Test 1 0.39 0.34 0.35 0.31 3.60 3.10Test 2 0.48 0.46 0.41 0.38 4.39 3.90Test 3 0.52 0.47 0.37 0.34 2.47 2.00




5.5 Effect of fabric pressure

The normal pressure on the fabric samples was slightly increased by adding simi-lar fabric samples at random on top of the samples under test. The results areshown in Figure 9, which yields relationships of the form:

Fp = CNn (3)

where Fp is the frictional force, A the area, N the normal force (including self-weight), and C and n coef�cients. This is the same form as proposed by Wilson(Wilson, 1963). The coef�cient n is given by the slope of the characteristic inFigure 9. The above experiment was repeated for all three sliding surfaces andboth directions. Three clusters can be seen in Figure 10 clearly showing the sur-face dependency.

5.6 Effect of humidity

Before carrying out an extensive set of experiments for different conditions oftemperature and humidity, it is necessary to determine the length of time thesamples must be left undisturbed in the changed environment so that one can besure that they have reached close to steady-state conditions. Experiments werecarried out to determine this and simultaneously �nd the time constants of change

Figure 9 Logarithmic relationship between the frictional forceper area (log F/A) and the normal pressure (log N/A) for fabricssliding across an aluminium surface. Standard conditions, velocity= 1 mm/s, aluminium surface, weft or wale direction, sample area= 1 dm2 (courtesy of the Textile Institute)



Taylor et al. 27

Figure 10 The in�uence of the surface type on the Wilson modelcoef�cients n and C (all fabrics in both principal directions; cour-tesy of the Textile Institute)

in frictional properties to a step change in humidity. This has practical implicationssince it determines the importance of daily or faster changes in the environment.The speci�c test procedure was as follows; an oven-dried specimen at a near zerohumidity was brought into the standard conditioned laboratory, weighed and itsfriction tested at time zero. The weighing and testing were then repeated six timesover a 90-min period.

A typical trace is shown in Figure 11 for fabric KC11 which exhibits anexponential increase in the dynamic friction coef�cient following a step increasein humidity. All other samples show this characteristic, except for the polyestersample, which is almost hydrophobic, and therefore neither the moisture contentnor the friction changes signi�cantly over the 0–65% r.h. range used in this test.

In order to see the effects of humidity on friction, the humidity level in thelaboratory is changed and six samples are allowed to condition for at least 1 daybefore the friction tests are carried out. Figure 12 shows the absorption curve forKC11, �tted to the points as the humidity is increased. The desorption points arealso indicated and these are quite close to the absorption points, showing thatthere is little hysteresis in this case. The range bars show the maximum variationacross each set of six samples.




Figure 11 Variation of dynamic friction (1 mm/s) for KC11caused by a sudden humidity change from 0% to 65% r.h.(courtesy of the Textile Institute)

Figure 12 Dynamic friction of KC11 against an aluminiumsurface, as a function of relative humidity



Taylor et al. 29

6. Evaluation

Every factor investigated above has been shown to have a signi�cant effect onthe low-force frictional characteristics of fabric samples. Bearing in mind the rela-tively few samples tested, preliminary evaluations are as follows.

The �bre from which the fabric is made seems to have less in�uence on thefriction than the type of construction. An example here is the friction coef�cientagainst an aluminium surface where the lowest coef�cient is obtained from thetwill cotton samples, the highest from the double-knit cotton samples. However,the type of �bre becomes crucially important when tribolectric effects are con-sidered. Here, a 10-fold increase in friction was observed for the polyester samplein a dry atmosphere, the effect on other samples being much lower. Again, thetype of construction affects the surface pro�le and in general the smoother thesurface of the fabric the higher will be the friction coef�cient. Anisotropy wasgenerally not signi�cant in the samples tested, apart from P1 and KC1.

Just as the surface pro�le of the fabric is crucial, so is the nature of the surfaceon which the sample is supported. For all samples, the aluminium surface gavethe lowest friction coef�cients, Formica being the next highest and the rubbersurface giving the maximum values in the range of 50–100% higher than theresults for aluminium.

If we now consider handling variables, all fabrics exhibited Stribeck character-istics of friction against velocity. Four regimes of lubrication can be distinguished:static friction (velocity independent), boundary lubrication, semi-boundary lubri-cation at the minimum friction and hydrodynamic lubrication towards higherspeeds. With reference to the velocity effect in yarn–metal friction, the followinghypothesis has been proposed (Kalyanaraman, 1988b). At low speed (boundarylubrication), the �brous materials are supported on their hairs. As the speed isincreased a ‘bristle effect’ lifts the �brous material up and reduces the contact areathat in turn decreases the friction. With the speed increasing further, the projectingsurface hairs �atten and the contacting surface area increases, contributing to arise in friction. In the case of man-made �bres, the explanation has to be moredirected towards various lubricating effects. In addition to this hypothesis, airentrapped between and underneath the �bres probably plays an important role.A typical characteristic showed a range of friction coef�cients of 0.49–0.57 (KC11against rubber). The above results are all for single-thickness samples with zeronormal force applied. When small normal forces are applied by multiplying thethicknesses of the samples, as would occur in a partially constructed garment, itis found that the friction force F against weight N characteristics are of the formfound by Wilson (1963) for larger loads, namely Fp = CNn. The coef�cients C andn are, unsurprisingly, also a function of the supporting surface.

Humidity is shown to have a large in�uence. Firstly, the friction coef�cientsrespond exponentially to a step change in humidity, with time constants of 10 minor so being typical. Not only does this mean that samples should be conditionedbefore testing, but also that environmental variations will have a rapid effect onthe handling properties of the fabric. It is seen that a change in relative humidityfrom 40% to 80% can double the friction coef�cients. The characteristics are non-linear so a 5% change in relative humidity has a bigger effect at 80% r.h. than




about 50% r.h. When fabrics acquire charge through rubbing in dry conditions,the triboelectric effect can cause a dramatic 10-fold increase in the friction coef-�cients of the polyester material whereas other fabrics are almost unaffected. Sub-sequent increases in humidity cause charge dissipation and hence a decrease infriction for the polyester material. Consequently, the effect of humidity is inextri-cably linked with the nature of the �bres used in the fabric and itself affects theeffect of the fabric becoming electrically charged. Other work (Taylor et al., 1998)has shown that humidity affects the bending stiffness of fabrics as well as theirfrictional characteristics, so humidity is clearly a major factor in variations in hand-ling properties.

7. Conclusions

One of the main advantages of human operators in a garment factory is theirinherent adaptability to changes in material properties. Humans can pick up andplace a wide variety of fabrics in a large range of environmental conditions, onlyhaving real problems when the temperature is low and we lose the ‘feel’ in our�ngers.

Automated equipment is not so adaptable. From the above studies it is clearthat the ideal situation for the application of automated equipment is to have asingle, well de�ned fabric made so that the variation in properties from roll toroll and along and across the roll are all minimized. Furthermore, the rolls shouldbe handled carefully from fabric producer to manufacturer and during storage.Garment production should be in reasonably constant conditions of humidity,achieved either by judicious location of the factory or by use of air conditioningequipment. In this way variations in fabric properties and environmental con-ditions are minimized.

Such care is very much the exception rather than the rule, has associated costsand is only suitable for those manufacturers with large-scale production of stan-dard items. The vast majority of manufacturers require equipment to work in verymuch less constrained conditions. This is only possible if the equipment isdesigned, from the start, to tolerate the changes in process variables. The approachtaken here is to gain knowledge of the variations in the key parameters andeventually combine these with analytical understanding of the handling. Thisforms part of ongoing research. At the moment we are beginning to understandwhy equipment fails. The next step will be to apply this knowledge to the designof more reliable equipment.

Acknowledgement

The authors wish to acknowledge the support of the EPSRC for part of thiswork.



Taylor et al. 31

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