GB2534643A

GB2534643A - System and method for detecting a change in shape of at least one predefined region of an optical fibre

Info

Publication number: GB2534643A
Application number: GB1519586.0A
Authority: GB
Inventors: Edward Brock Martin; Villar Gabriel; Jonathan Finchley Cletheroe Daniel; Lewis Emma; Joseph Smallman Ian
Original assignee: Cambridge Consultants Ltd
Current assignee: Cambridge Consultants Ltd
Priority date: 2014-11-05
Filing date: 2015-11-05
Publication date: 2016-08-03
Also published as: GB201519586D0; GB201419732D0; GB2532031A; WO2016071700A1

Abstract

A method for detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during the at least one preconfigured time interval; and identifying that a change in shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval. The method also comprises comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; using a result of the comparison to determine whether to apply a correction to the measured intensity of light during the at one preconfigured time interval; and applying the correction when it is determined that a correction is required.

Description

System and method for detecting a change in shape of at least one predefined region of an optical fibre The present invention relates to detection of a change in shape of at least one predetermined region of an optical fibre. The present invention relates in particular but not exclusively to the measurement of bending, strain and/or displacement at one or more predefined regions along an optical fibre, which is preferably attached to or embedded within a flexible material.

Optical fibres have been used to sense strain and bending, and systems exist that measure the bend-induced attenuation of light transmitted through an optical fibre. However, known techniques monitor only a single point along a fibre, and are therefore difficult to extend to perform measurements at multiple points along the optical fibre. To obtain measurements at multiple points currently requires the use of multiple independent fibre sensors with associated cost and complexity of electrical multiplexing.

Systems are known that make distributed measurements of strain at all points along a single optical fibre attached to a structure. However, known methods use bulky and expensive interrogation systems, either relying on multiple fibre Bragg gratings and wavelength multiplexing, or frequency-domain measurements of Rayleigh-or Brillion-scattering using tuneable lasers or optical modulators. These techniques are not well suited to a low-cost product where, for example, the measurement equipment is to be attached to a flexible material.

In addition, systems are known that rely on the principle of optical time domain reflectometry (OTDR) to detect deformation along an optical fibre. However, in addition to being expensive and bulky, these systems also operate by taking measurements at all points along a fibre. Such systems are therefore limited in the frequency at which measurements can be made, with a known system currently taking around one second to perform a complete sweep along a 30m optical fibre using a 'fast' OTDR.

It can be seen therefore that there exists a need for an improved fibre-optic sensor and -2 -related improved sensing methods. Of particular benefit would be an improved sensor and associated method that avoids or at least partially ameliorates at least one of the above issues.

Such an improved sensor and associated method would, for example, be suitable for use in determining deformation of a flexible material at one or more defined points along a single optical fibre, and at a higher frequency than is currently possible. Indeed, a fibre-optic sensor capable of measuring the bending of a flexible material at one or more defined points within that material using a single optical fibre would offer a significant

advantage of the prior art.

The ability to measure deformation in a flexible material has numerous conceivable applications, such as: 'motion capture' garments for digitising body motion; sports and fitness aids for providing feedback to users; medical garments for monitoring patient health; and condition monitoring of flexible materials used in buildings and civil engineering. In each of these applications, multiple measurements of the bending or flexing of the flexible material at defined points on its surface are typically needed in order to provide useful feedback to the system's users.

According to an aspect of the invention there is provided a method for detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light retumed from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during the at least one (or each) preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval. -3 -

Identifying that a change in shape of the optical fibre has occurred in the at least one predefined region may further comprise comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; using a result of said comparing to determine whether to apply a correction to the measured intensity of light received during the at least one preconfigured time interval; and applying said correction when it is determined that a correction is required.

The intensity of light received during the further preconfigured time interval may be measured more or less frequently than the intensity of light received during the at least one preconfigured time interval.

The further predefined region of the optical fibre may be arranged to inhibit the effect of external mechanical perturbation on the optical fibre such that said external mechanical perturbation has negligible effect on the shape of the optical fibre in said further predefined region. For example, the further predefined region may be fixed in situ (e.g. secured to a garment), or it may be secured to a rigid substrate, or it may be in a region that is not subjected to motion. Said further predefined region may be referred to as a reference region.

The at least one predefined region and the further predefined region may be arranged such that deformation of the optical fibre in the at least one predefined region correlates with a deformation of the optical fibre in the further predefined region. The correlation 25 may be an inverse correlation.

Identifying that a change of shape of the optical fibre has occurred may further comprise comparing changes in the amplitude of light reflected from the predefined region. Identifying that a change of shape of the optical fibre has occurred may further comprise comparing the measured intensity of light received during a time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that time interval, and determining a difference. -4 -

Comparing the measured intensity of light may further comprise determining a degree of stretching, bending or flexing of the optical fibre at the predefined region based on the difference between the measured and the predetermined intensities of light, which may 5 be based on the ratio between the measured intensities from two time different intervals.

Identifying that a change of shape has occurred may further comprise performing a calibration operation to determine an intensity of light expected from a predefined region of the optical fibre before a change in shape occurs. Performing a calibration may further comprise taking regular measurements of an intensity of light returned from the predefined region of the optical fibre to determine the predetermined intensity of light expected. Performing a calibration operation may further comprise predetermining an intensity of light expected from a predefined region of the optical fibre for a range of values for radius of curvature of the optical fibre in that predefined region.

Identifying that a change of shape has occurred may further comprise comparing the measured intensity of light to the predetermined range of values of expected intensities of light and thereby determining the radius of curvature of the optical fibre at the predefined region.

The at least one preconfigured time interval ideally has a linear relationship to the distance to the predefined region of the optical fibre.

Receiving light returned from within the optical fibre may further comprise configuring the 25 at least one preconfigured time interval to be a time period within which any light returned from the selected predefined region of the optical fibre is expected, such that the intensity of light being measured is that of backscatter from the predefined region.

The position of the preconfigured time interval may be determined through an automatic 30 self-calibration process. The position of the preconfigured time interval may be continually optimised through an automatic self-calibration process. -5 -

Receiving light returned from within the optical fibre may further comprise: configuring the at least one preconfigured time interval to be a time period within which any light returned from a region of the optical fibre before the selected predefined region of the optical fibre is expected; configuring a further preconfigured time interval to be a time 5 period within which any light returned from a region of the optical fibre after the selected predefined region is expected; and measuring the intensity of the light received during the further preconfigured time interval. Identifying that a change of shape in the optical fibre has occurred may further comprise comparing the intensity of the light received during the at least one preconfigured time interval with an intensity of light received 10 during the further preconfigured time interval to determine an increase in attenuation through the predefined region of the optical fibre.

Receiving light returned from within the optical fibre may further comprise: receiving light returned from within the optical fibre selectively during a plurality of preconfigured time intervals; measuring the intensity of the light received during each preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in each predefined region based on the measured intensity of said light returned during the corresponding preconfigured time interval.

Transmitting a light pulse may further comprise generating light pulses of a length that corresponds with the length of the predefined region such that there is negligible overlap between light reflected from two adjacent regions along the optical fibre.

Receiving light returned may comprise receiving an optical signal from the light returned, and may further comprise converting the optical signal into an electrical signal. Receiving light returned may further comprise using a time gate to configure the at least one preconfigured time interval. Measuring the intensity of the light received may further comprise integrating a plurality of received optical signals to improve he signal-to-noise ratio of the measurements.

The method may further comprise coiling or folding the optical fibre within the predefined region of optical fibre to increase detection sensitivity. The optical fibre may be attached -6 -to, or embedding within, a flexible material. The method may further comprise securing points of the optical fibre that are adjacent each end of the predefined region to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material. Securing the optical fibre may further 5 comprise securing the optical fibre within the predefined region to the flexible material to be fixed only in a transverse direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material. The method may further comprise modulating the light pulse in amplitude, phase or frequency, with either a smooth or pseudorandom signal, for use in a spread-spectrum 10 system.

According to another aspect of the invention there is also provided a system for detecting a change of shape of at least one predefined region of an optical fibre, comprising: an optical fibre; means for transmitting a light pulse from a first end of the optical fibre towards a second end of the optical fibre; means for receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; means for measuring the intensity of the light received during the at least one (or each) preconfigured time interval; and means for identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval.

The system may further comprise: means for comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; means for using a result of said comparing to determine whether to apply a correction to the measured intensity of light received during the at least one preconfigured time interval; and means for applying said correction when it is determined that a correction is required. -7 -

The further predefined region may be a reference region. The measured intensity of light received from a plurality of said at least one predefined regions may be compared to the measured intensity of light received from said reference region. Alternatively, a plurality of reference regions may be provided on the optical fibre, wherein the measured intensity of light received by each of a plurality of said at least one predefined regions can be compared with the measured intensity of light received from a corresponding reference region (from the plurality of reference regions).

The means for measuring the intensity of the light may be arranged to measure the intensity of the light received from the reference region and the intensity of the light received from the at least one predefined region at different frequencies. The means for measuring the intensity of the light may be arranged to measure the intensity of the light received from the reference region less frequently than it measures the intensity of the light received from the at least one predefined region.

Two other predefined regions may be provided on the on the optical fibre, the two other 15 predefined regions arranged such that deformation of the optical fibre in a first other predefined region correlates with a deformation of the optical fibre in a second other predefined region. The correlation may be an inverse correlation.

Alternatively, the at least one predefined region and the further predefined region may be arranged such that deformation of the optical fibre in the at least one predefined region 20 correlates with a deformation of the optical fibre in the further predefined region. The correlation may be an inverse correlation.

The means for identifying that a change of shape of the optical fibre has occurred may further be arranged to compare the measured intensity of light received during a time interval with a predetermined intensity of light expected to be returned from within the 25 optical fibre during that time interval, and determining a difference.

The optical fibre may be attached to or embedded in a flexible material such that a deformation of the flexible material causes a change in shape of the optical fibre. The optical fibre may be arranged such that certain predefined regions (the sensing regions) -8 -of the optical fibre coincide with one or more regions of the flexible material at which a deformation is expected. The optical fibre may be coiled or folded within the predefined region of optical fibre to increase detection sensitivity. Points of the optical fibre that are adjacent each end of the predefined region may be secured to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material. The optical fibre within the predefined region may be secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.

There may be a plurality of predefined (sensing) regions along the optical fibre, each having a known relationship with a respective preconfigured time interval, such that an intensity of light returned from a plurality of predefined (sensing) regions can be measured and hence a change in shape of the optical fibre can be identified at a plurality of predefined (sensing) regions.

There may be additional predefined regions (the reference regions) of the optical fibre, each also having a known relationship with a respective preconfigured time interval, which are configured to be de-coupled from the material so that a deformation of the flexible material results in no change, or only a minimal change in the intensity of light returned from these additional regions. The fibre may be coiled or folded within the reference regions to result in a larger returned intensity signal. Alternatively, the fibre may be treated to increase backscattering in this region by notches, side-windows, partial or complete cleaving and re-joining of the fibre, mechanical abrasion, exposure to UV light or patterning with a high-power laser, for example. The intensity of the light returned from one or more reference regions may be compared ratiometrically with the intensity of light returned from a sensing region to improve the accuracy and stability of the intensity measurement of the light returned form the sensing region.

Some or all of the predefined sensing regions may be arranged in opposing pairs so that deformation of the material results in an increase in the intensity of light returned from one sensing region in a pair while simultaneously decreasing the intensity of light -9 -returned from the second sensing region in the pair.

The position of the preconfigured time interval may be determined through an automatic self-calibration process. The position of the preconfigured time interval may be 5 continually optimised through an automatic self-calibration process.

The system may further comprise means for splitting the light pulses being transmitted from the first end of the optical fibre and the light returned to the first end of the optical fibre. The system may further comprise a pulse generating means arranged to provide a pulsed signal to control the means for transmitting a light pulse to transmit light pulses. The means for transmitting a light pulse may be synchronised with the at least one preconfigured time-interval such that the amount of light returned can be measured as a function of time since the light pulse was transmitted.

The system may further comprise a signal conditioning means arranged to receive an optical signal output from the receiving means and convert the optical signal into an electrical signal. The system may further comprise an analogue-digital converter arranged to receive an electrical signal output from the signal conditioning means and convert it into a digital signal that is output to the processing means.

The means for identifying that a change in shape has occurred may be configured to determine a change in shape of the optical fibre at a predefined region from the light returned from the predefined region and/or the increase in attenuation in the optical fibre after the predefined region.

The means for transmitting a light pulse, means for receiving returned light, means for measuring and the means for identifying may be combined to form a processing unit that is attachable to the flexible material.

The optical fibre may be a plastic optical fibre. The optical fibre may comprise both a graded refractive index profile and a stepped refractive index profile at regions along its length. The refractive index profile of the optical fibre may vary along its length from a -10 -graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.

The optical fibre may be treated to increase its backscattering properties or bend sensitivity. The optical fibre may be treated at one or more predefined sections of the fibre. The treatment may comprise notches, side-windows, partial or complete cleaving and re-joining of the fibre, mechanical abrasion, exposure to UV light or patterning with a laser.

The system may be arranged to monitor the condition of a building or structure, or deformation in a garment (or other flexible material or textile).

According to another aspect of the invention there is also provided an optical fibre sensor unit for use in the above-described system, comprising: said optical fibre; and said 15 means for receiving light returned from within the optical fibre coupled to said end of the optical fibre.

An optical splitter means may be provided at said end of the optical fibre. The optical fibre may be coiled or folded in the one or more predefined regions. The optical fibre may comprise plastic optical fibre. The optical fibre may comprise both a graded refractive index profile and a stepped refractive index profile at regions along its length. The refractive index profile of the optical fibre may vary along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.

The optical fibre may be treated to increase its backscattering properties or bend sensitivity. The optical fibre may be treated at one or more predefined regions of the fibre. The treatment may comprise notches, side-windows, partial or complete cleaving and re-joining of the fibre, mechanical abrasion, exposure to UV light or patterning with a high-power laser.

The optical fibre sensor may be arranged to be attached to the flexible material at points of the optical fibre that are adjacent each end of the predefined region such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material. The optical fibre within the predefined region may be securable to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.

According to another aspect of the invention there is also provided a processing unit for use with the above-described system, comprising: a light source arranged to transmit a light pulse into an end of an optical fibre; a light detector arranged to receive light returned from said end of the optical fibre; and processing means arranged to measure the intensity of returned light and identify whether the optical fibre has changed shape in a predefined region based on the measured intensity of light compared to a predetermined intensity of light expected from said predefined region.

Identifying that a change shape of the optical fibre has occurred in the at least one predefined region may further comprise comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; using a result of said comparing to determine whether to apply a correction to the measured intensity of light received during the at least one preconfigured time interval; and applying said correction when it is determined that a correction is required.

Identifying that a change of shape of the optical fibre has occurred may further comprise comparing the measured intensity of light received during a time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that time interval, and determining a difference.

Optical splitting means may be arranged to couple with an end of the optical fibre. The optical splitting means may comprise an optical splitter, an optical switch or an optical circulator.

-12 -The light detector may be a photo-diode. A pulse generator may be arranged to control the light source to transmit light pulses.

The processing unit may further comprise signal conditioning means for converting an optical signal received from the light detector into an electrical signal and an analoguedigital-converter for converting the electrical signal into a digital signal to be processed by the processing means. The processing means may be a microprocessor.

The processing unit may be arranged to be secured to a flexible material and to couple with said end of said optical fibre.

According to another aspect of the invention there is also provided a computer programme product for implementing the above-described method.

The invention provides a method of detecting a change in shape of at least one predefined region of an optical fibre and a corresponding system suitable for applications where deformation of the optical fibre must be tracked rapidly and continuously, for example in garments for body motion capture or sports training aids. In such applications, an update rate of 10Hz (and preferably upwards of 100Hz) is required, which is made possible by the present invention.

The present invention may be embedded or otherwise attached to a flexible material. In the simplest embodiment, a single optical fibre traverses the flexible material and 25 crosses each predefined region where flexing is to be measured.

Certain predefined regions of the optical fibre may be designed to have a more sensitive response to local stretching of the material, or to the flexing or bending of the flexible material around an underlying support at defined regions. Multiple folds or coils may be formed in the optical fibre and attached to the material in such a way that stretching the material or flexing it over the underlying support results in a net strain being applied to the material, which in turn changes the shape of the pattern in which the fibre is attached -13 -to the material, for example by changing the average radius of curvature of the optical fibre.

When the material is flexed or stretched the resulting change in shape of the fibre is measured in each defined sensing region. For example, this can be done either by measuring the local increase in backscatter which occurs with reducing bend radius at the location of the associated bend or bends, or by measuring the increase in attenuation of the fibre across the associated bend or bends which occurs with reducing bend radius, or a combination of both.

The invention provides numerous advantages over known systems. For example, with the present invention, no electrical contacts are exposed, thus rendering the sensor waterproof. Furthermore, the sensing mechanism occurs within a small, flexible and tough fibre, rather than at the intersection of two or more conductive fibres or wires resulting in a more robust sensor as a whole than electronic sensors.

Compact, low-cost light sources and photo-detectors are connected to the fibre, illuminate it and measure the light reflections that occur at various points along its length due to backscattering inherent within the fibre itself and additional backscattering from any bends along the fibre. When the fibre is flexed or stretched the system measures either the change in reflection occurring at the associated bend or bends, or the change in backscattering after the bend or bends due to a change in loss, or both.

Therefore the invention comprises a method and system for measuring a change of 25 shape in the optical fibre at one or more predefined regions along an optical fibre, which may be attached to or otherwise embedded in a flexible material.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In, particular, method aspects may be applied 30 to apparatus or system aspects, and vice versa. Furthermore, any, some and/or al: features in one aspect can be applied any., some and/or all features in any other aspect, in any appropriate combination.

-14 -It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

In the figures, similar features of different embodiments are labelled using similar, corresponding reference numerals.

An example of the present invention will now be described, with reference to the 10 accompanying drawings, in which: Figure 1 shows an exemplary embodiment of the present invention; Figure 2 illustrates a signal detected from light reflected from a change in shape occurring at a predefined region of an optical fibre; Figure 3 illustrates a signal detected from light reflected from a change in shape occurring at two distinct predefined regions of an optical fibre; Figure 4 shows an optical fibre incorporated into a garment; Figure 5 is a schematic showing processing optoelectronics; Figures 6A and 6B are further schematics showing partial processing optoelectronics 20 with two different drift compensation techniques; Figures 7 shows another example of an optical fibre incorporated into a garment; and Figures 8A and 8B show the deformation of an optical fibre attached to a flexible material before and after the material undergoes stretching.

Overview Figure 1 shows a particularly beneficial exemplary embodiment of the invention in overview.

In the embodiment of Figure 1, an optical fibre is shown attached to a flexible material at a sensing region Po. A pulse of light provides an input signal that is transmitted from a first end of the optical fibre towards a second end of the optical fibre. Light reflected -15 - (backscatter) from region Po of the optical fibre is received at the first end of the optical fibre, thereby providing a return signal. The return signal is sampled during a sampling window configured to be a time interval (to -ti) within which any light reflected from region Po is expected.

When the flexible material is bent or otherwise deformed it causes a change in shape of the optical fibre. The measured intensity of light returned from sensing region Po is used to identify a change in the shape of the optical fibre. The output illustrated in Figure 2 shows how the intensity of backscattered (reflected) light changes at the location of a bend in an optical fibre, thereby providing a different return signal for a bent optical fibre to the return signal expected for a straight optical fibre.

Additional sensing regions (P1, P2, P3... Pn) may be defined at further positions along the optical fibre, being positioned at predefined regions at which a deformation of the 15 material is expected to provide multiple outputs, as shown in Figure 3.

The sampling window can be configured to coincide with the time period when any backscatter is expected from the sensing region Po. Alternatively, a sampling window may be configured to occur either side of the time period when any backscatter is expected from the sensing region Po to detect attenuation of light within the optical fibre caused by a change in shape in region Po, as will be described in more detail further on.

Beneficially, therefore, by targeting the sampling window at predefined locations in the optical fibre the arrangement shown in Figure 1 allows changes in the shape of the material to be detected rapidly, at potentially several different locations. This rapid detection has the potential to allow real-time or pseudo-real-time movement analysis that has a wide range of beneficial applications.

Background principle

All optical fibre inherently scatters a small amount of light back towards the source. Normally, with increasing length of fibre, the amount of light scattered back decays -16 -smoothly and exponentially in proportion to the length due to attenuation in the fibre. A bend in an optical fibre generally produces two effects when the fibre is illuminated. Firstly, a portion of light is reflected at the bend location. Secondly, the light in the optical fibre becomes attenuated beyond the bend. The attenuation can be measured by comparing the backscattering properties of the fibre before and after the bend location, since the amount of light backscattered will have a discontinuity at the bend location, as illustrated in Figure 2. The present invention takes advantage of this principle.

The present invention In the present invention, a pulse of light is transmitted into an optical fibre and the intensity of the light returned from within the optical fibre is measured at a preconfigured time interval corresponding to a predefined region of the optical fibre. The reflected light (backscattered light caused mainly by Rayleigh scattering) is recorded as a function of time, wherein the time interval between transmitting the light pulse and receiving the reflected signals is linearly related to the distance to the location (i.e. a predefined region) at which the deformation or change in shape of the optical fibre has occurred.

Figure 3 illustrates an output indicating that deformation has occurred at two distinct 20 points along an optical fibre.

If, for example, we consider a 30m long fibre, the round trip delay of this fibre is of the order of 100ns, i.e. it takes a pulse of light 100ns to travel from the light source to the far end of the fibre and back to a photodiode, which measures the backscattered and reflected light. A pulse length of around 1ns could be used to achieve a spatial resolution of about 20cm, which is the typical length of a defined region. A time gate "window" must therefore have an opening time of less than 1ns (otherwise the spatial resolution of the system will be limited by the time-gate window and not the pulse length) and be stepped along each point covering the 100ns delay. This means that a total of at least 100 window positions would therefore need to be sampled. Typically, pulses are be emitted at a repetition frequency of 1MHz, and also typically, 10,000 pulses need to be integrated at each time gate window position to achieve an acceptable signal-to-noise ratio before -17 -the time gate window is swept to the next position. Collecting a complete graph of the form of Figure 3 will therefore take around 1s (100 window positions each requiring the integration of 10,000 pulses at a 1MHz frequency). Hence the update rate is limited to around 1Hz.

With the present invention, however, rather than sampling every point along the fibre, only a few predefined points along the fibre need to be sampled. This can be achieved by knowing the positions of the predefined regions along the optical fibre in advance, or by determining the positions through a self-calibration process based on acquiring a graph similar to that in Figure 3. These predetermined points can be chosen to yield the most information about the deformation of the fibre in each defined region.

For example, if the backscatter from bending is to be measured then the time-gate window positions can be set to coincide with the bend positions, while if the attenuation 15 from bending is to be measured, the window positions can be set to be before and after bend positions so the attenuation through the bent region can be determined.

To illustrate the advantage of the present invention over the known prior art systems, consider the above described example with two predefined regions on 30m of fibre. The present invention requires only two window positions to be measured instead of 100, and hence the achievable update rate increases from 1Hz to 50Hz. Thus, it can clearly be seen that the present invention provides a big advantage when compared to looking along the whole length of an equivalent length of optical fibre.

Figure 4 shows a garment 100 formed of a flexible material 110 into which an optical fibre 120 has been embedded or otherwise attached. The optical fibre 120 traverses the flexible material 110 and crosses each region 130 where flexing is to be measured. An optoelectronic processing unit 140 arranged to illuminate the fibre 120 and to measure the backscattered and reflected light from points along the length of the fibre 120 is also attached to the garment, and connected to the fibre 120.

An additional length of fibre 120 is arranged in certain defined regions 130 of the -18 -garment 100 where increased sensitivity is required. The fibre 120 is formed into multiple folds or coils at the location of the defined regions 130, which in this example correspond to body joints (e.g. shoulder, elbow, etc.) to facilitate the defined of the motion of the body joints. The defined regions 130 are connected by sections of fibre 120 having a larger bend radius so that the backscattering is reduced in these connecting sections 120 where no sensitivity is needed.

The increased sensitivity is achieved by attaching the defined regions 130 to the material 110 in such a way that any stretching of the material 110, or flexing it over the underlying support, results in a net strain being applied to the material 110, which in turn changes the shape of the pattern in which the fibre 120 is attached to the material 110, for example by changing the average radius of curvature of the optical fibre 120. This affects the amount of backscattered and reflected light.

From the measurements of the degree of bending at the various defined regions 130 along the fibre 120, and knowledge of the locations of those defined regions 130, it is then possible to infer and determine information about the shape of the underlying flexible material 110.

Figure 5 is a schematic showing processing optoelectronics 200 used with the present invention. The processing optoelectronics 200 include a pulse generator 210, a light source 220, an optical splitter 230, a photo-diode 240, signal conditioning electronics 250, an analogue-to-digital converter (ADC) 260 and a microprocessor 270. A resonant cavity light emitting diode (RC-LED) may be used, for example, to provide a low-cost, high-bandwidth light source 220.

In use, the pulse generator 210 is controlled by the microprocessor 270 to produce a pulse of electrical current sufficiently short that when converted to light by the light source 220 it results in negligible overlap between the light reflections from two adjacent regions along the fibre 120. The optical splitter 230 allows backscattered and reflected light to return from the fibre 120 into the photo-diode 240 and from there the signal conditioning electronics 250. The optical splitter 230 may consist or any device able to -19 -separate the light transmitted into the fibre 120 from the backscatter and reflected light returned from the fibre 120, for example a passive optical splitter, optical switch or optical circulator.

The optoelectronics 200 convert the optical signal into an electrical signal, use a time-gate to collect selectively only the pulses returning from a variable but defined region of the fibre 120, integrate a number of such pulses and then pass the output to the ADC 260 for digitisation and processing by the microprocessor 270.

The microprocessor 270 synchronises the pulse generator 210 and the time-gate and, by varying the delay before the time-gate opens, is able to measure the amount of light returned as a function of time since the pulse was emitted, resulting in data generally in the form shown in the chart of Figure 2. From such data, the degree of bending at each point of interest along the fibre 120 can be computed using either the direct reflected light from the bend, or the increase in attenuation, or some combination of these. Generally, the signals from many thousands of reflected pulses are integrated to improve the signal-to-noise ratio of the measurements.

The processing optoelectronics 200 may be housed in a compact unit 140, as shown in 20 Figure 4, which is attached to the material 110 when coupled to the fibre 120 attached to or embedded therein.

In any system relying on intensity measurement there is the potential for measurement accuracy to be lost because of drift in the output of the light source, or in the sensitivity of 25 the detector, or because of changes in the transmissions losses of the various optical components.

With reference to Figure 6A, a first example of a drift compensation technique requires one or more reference regions 180 (R1, R2... Rn) defined within the optical fibre 120, which are sampled in a similar manner to the sensing regions 130 (P1, P2... Pn). The reference regions 180 may be treated to increase the backscattered light returned from them, using any of the techniques described herein.

-20 -The optical fibre 120 in the reference regions 180 is arranged to inhibit the effect of external mechanical perturbation of the optical fibre 120 such that external mechanical perturbation has a negligible effect on the shape of the optical fibre 120 in the reference regions 180. The intensity of the light returned from the reference regions 180 therefore depends only on drift in the light source 220, detector 240 and optical components, and not on any deformation of the flexible material 110 being measured.

The measured intensity of light from the reference region 180 can therefore be used to compensate for any errors which this drift may introduce into the measured intensity of light returned from a predefined sensing region 130-1. For example, dividing each measured intensity corresponding to a sensing region 130-1 by a measured intensity corresponding to a reference region 180 will yield a resulting measurement which depends only on the local properties of the optical fibre 120 at each sensing region 130.

The reference region may be located inside the processing unit if it is only desired to compensate for source and detection drift and not connector drift (e.g. if there is no connector or if the connector is unlikely to introduce drift).

Drift is likely to occur relatively slowly in comparison with the measured deformation signals, and therefore the reference region of fibre can be measured less frequently than the sensing regions since drift processes are expected to occur relatively slowly thereby limiting the amount of overall system time spent on these measurements. As a result the system need only spend a small amount of time making (e.g. occasional) reference measurements, and the process of making these reference measurements will therefore only slightly reduce the output measurement update frequency of the system. A single reference region measurement may be used for each sensing region measurement, or a single reference region measurement may be used for multiple sensing region measurements.

With reference to Figure 6B, a second example of a drift compensation technique requires some or all of the sensing regions 130-3, 130-4 to be arranged in pairs so that -21 -deformation of the optical fibre 120 in a first region 130-3 correlates with a deformation in the optical fibre in the second, paired region 130-4. The correlation is preferably an inverse correlation. For example the sensing regions 130-3, 130-4 may be arranged so that an increase of bend radius in the first sensing region 130-3 correlates with a decrease in bend radius in the second sensing region 130-4, and hence that a decrease in the intensity of light returned from the first sensing region 130-3 correlates with an increase in the intensity of light returned from the second sensing region 130-4. A high quality drift-free measurement can thus be made by taking the ratio of the two intensities in each pair of sensing regions 130-3, 130-4. This technique is particularly beneficial when the two sensing regions 130-3, 130-4 can be arranged so that a single underlying physical motion results in the two opposing changes in the two paired sensing regions 130-3, 130-4, such as at an elbow joint, for example.

The two drift compensation techniques may be combined by including one or more reference regions and one or more paired sensing regions, plus possibly some unpaired sensing regions. In this arrangement, the reference regions 180 may be used first, to compensate for intensity drift in all the sensing regions 130 (paired and unpaired). The two compensated intensity measurements from each pair of sensing regions 130-3, 1304 can then be combined in a more sophisticated way than taking a simple ratio. This is possible since there is no longer any drift-dependence in either of the compensated measurements. The more sophisticated combination may include a non-linear function that maps the two interdependent measurements (since the two paired sensors 130-3, 130-4 are arranged in series) into a single estimation of the extent of some underlying deformation. This mapping may be predetermined and may be further updated based on a continuous self-calibration process which operated whenever the state of the underlying deformation is known.

As mentioned above, the reference section of fibre may be measured less frequently than the sensing regions since drift processes are expected to occur relatively slowly and to limit the amount of overall system time spend on these measurements. The intensity measurements from the reference section are used to ratiometrically correct all of the intensity measurements from the sensing regions for any intensity or sensitivity drift in -22 -the opto-electronic unit. The drift-corrected intensity measurements from each group of sensors around each joint can then be processed using a predetermined function which maps a set of intensity measurements into an estimate of joint position. This predetermined function may be automatically updated whenever a joint's position is either known absolutely (for example because the user has been asked to perform a calibration movement) or whenever a joint's position can be inferred with reasonable certainly (e.g. because of the biomechanical limitations at a particular point in time of some underlying physical activity which the users is assumed to be carrying out).

In a garment 300 illustrated in Figure 7, some of the defined sensing regions 130-3, 1304 are arranged in pairs. The pairs of sensing regions 130-3, 130-4 are located on the material 110 forming the garment 300 at the elbow joint position of the garment 300, such that flexing of the elbow joint results in an increase in the stretching of the material 110 on the outside of the elbow joint at the same time as there is a reduction in stretching of the material 110 on the inside of the elbow joint.

The exemplary garment 300 illustrated in Figure 7 has an additional defined reference region 180 of fibre 120 arranged to provide a reference measurement to allow any drift in the processing electronics or its optical components (including the input/output optical connector) to be cancelled out, as described above. In a variant of the system this section of fibre 120 may be placed within the processing optoelectronic unit 140.

Figures 8A and 8B show the fibre 120 attached to a flexible material 110 and arranged so that bending of the material 110 results in a net strain of the material 110 and that this in turn results in a change in the average radius of curvature of the fibre 120, as described earlier. This result can be achieved by ensuring that the fibre 120 is secured to the material 110 at certain predefined points such that when the material 110 is stretched the fibre 120 experiences a change in shape and/or an additional strain within a defined "sensitive" region 130. Both a change in shape and a change in strain cause a change in the loss and backscatter. If the additional strain on the fibre 120 is minimised, then the compliance of the material 110 will not be affected, which is often desirable for a garment. Alternatively, the fibre 120 can be arranged so as to increase the stiffness of -23 -the material 110 if a more supportive and rigid region of the garment is desired.

The material in and around the sensitive region 130 may be elastic and able to compensate for contraction of the fibre pattern along one direction, or it may be inelastic to impose more stretch on the fibre, or inelastic in certain regions where the fibre is attached in order to prevent the fibre from stretching in those regions. The density of restraint points 150,160 may be very high (or continuous in the case of a laminated structure), or relatively low, and the average bend radius of the fibre 120 may increase or decrease in response to stretching of the material 110. For example, the fibre 120 may be arranged as shown in Figures 6A and 6B, where the fibre 120 is secured at each end 150 in the longitudinal direction so that the length of the sensitive region is forced to change in response to a longitudinal stretch of the material 110. Within the sensitive region the attachment 160 of the fibre 120 is looser, so that the fibre can move in the transverse direction, and the average bend radius of the fibre 120 decreases in response to the longitudinal stretching of the material 110.

Although multiple defined regions 130 can be located on a single fibre 120, it may sometimes be advantageous to use multiple fibres (not shown), each with a smaller number of defined regions 130. The signals from these multiple fibres may be combined optically using a passive or electronically controlled splitter (not shown), or processed separately using multiple light sources (not shown) and photo-diodes (not shown). The system may also use multiple processing units, each of which takes measurements using one or more optical fibres.

The fibre may consist of a plastic optical fibre, which may be treated to alter its backscattering properties or bend sensitivity. Such treatment may be applied to selected regions of the fibre or to the entire fibre.

Greater reflections upon bending can be produced by using an optical fibre having a step 30 change in its radial refractive index profile. Alternatively, a graded-index profile can be used to minimise dispersion within the fibre, and hence to maximise spatial resolution, i.e. the system's ability to distinguish bending at each of two closely separated sensing -24 -regions. Depending on the desired spatial resolution and number of sensing regions, the refractive index profile of the optical fibre may (optionally) therefore be stepped, graded, or both. Alternatively, the index profile may be varied along the length of the fibre, for example from a graded profile near the light source (where minimising dispersion is more important to avoid distorting the signals from the multiple sensors located beyond the initial length of fibre) to a stepped profile further from the light source (where maximising the amplitude of the backscattered signal is more important due to the higher losses inherent in traversing a longer length of fibre). In addition, the optical fibre may be treated to increase its backscattering properties or bend sensitivity. For example, it could be notched, partially or totally cleaved and re-joined, side-windowed, mechanically abraded, exposed to UV light or patterned with a high-power laser. The treatment of the fibre may either uniformly increase its scattering, or it may take the form of a fibre Bragg grating (FBG) and be reflective only at a selected wavelength. The treatment may also be applied to either the whole fibre or just those defined regions where increased sensitivity is required.

The processing unit may produce a pulse of light for each measurement. Alternatively, the processing unit may produce an optical signal that is modulated in amplitude, phase or frequency. The light returned from the predefined region of the optical fibre can be characterised by its propagation delay. The optimum propagation delays corresponding to the predefined regions can be determined through a self-calibration process before or during the use of the system.

As an alternative to the pulsed system described above, a spread-spectrum system can be used where the light source is amplitude-, phase-or frequency-modulated, with either a pseudorandom signal, or a smooth signal as in a frequency chirp. The backscattered signals are then detected by cross-correlation using well-known techniques from direct-sequence and pulse-compression radar systems.

Advantageously, an optical fibre sensor is provided for measuring deformation of a flexible material when attached thereto. The optical fibre may traverse multiple regions of the material where flexing is to be measured. The fibre is so arranged as to be -25 -preferentially sensitive to bending or flexing of the material in the desired measurement regions. An optoelectronic processing unit, that uses only compact, low-cost light sources and photo-detectors, connects to the fibre, illuminates it and measures the reflections that occur at each point along its length. The amount of bending in the flexible material is thereby measured at multiple points using reflections of light in a single optical fibre.

It will be understood that the present invention has been described above p.urey by way of examples, and modifications of detail can be made within the scope of the inventttion.

Claims

-26 -CLAIMS1. A method of detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end 5 of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light retumed from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during each preconfigured time interval; identifying that a change in shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the 15 corresponding at least one preconfigured time interval; comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; using a result of said comparing to determine whether to apply a correction to the 20 measured intensity of light received during the at least one preconfigured time interval; and applying said correction when it is determined that a correction is required.
2. The method of claim 1, wherein the further predefined region of the optical fibre is arranged to inhibit the effect of external mechanical perturbation on the optical fibre such 25 that said external mechanical perturbation has negligible effect on the shape of the optical fibre in said further predefined region.
3. The method of claim 1, wherein the at least one predefined region and the further predefine region are arranged such that deformation of the optical fibre in the at least one predefined region correlates with a deformation of the optical fibre in the further 30 predefined region, preferably wherein the correlation is an inverse correlation.

-27 -
4. A method of detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during each preconfigured time interval; and identifying that a change in shape of the optical fibre has occurred in the at least one predefined region by comparing the measured intensity of light received during the at least one time interval with a predetermined intensity of light expected to be returned 15 from within the optical fibre during that time interval, and determining a difference.
5. The method of any preceding claim, wherein comparing the measured intensity of light further comprises determining a degree of stretching, bending or flexing of the optical fibre at the predefined region based on a difference between the measured intensity of light and the predetermined intensity of light.
6. The method of any preceding claim, wherein identifying that a change in shape has occurred further comprises performing a calibration process to determine a predetermined intensity of light expected from a predefined region of the optical fibre before a change in shape occurs.
7. The method of claim 6, where performing a calibration further comprises taking regular measurements of an intensity of light returned from the predefined region of the optical fibre to determine the predetermined intensity of light expected.
8. The method of claim 6 or 7, wherein performing a calibration operation further comprises predetermining an intensity of light expected from a predefined region of the optical fibre for a range of values for radius of curvature of the optical fibre in that -28 -predefined region.
9. The method of claim 8, wherein identifying that a change of shape has occurred further comprises comparing the measured intensity of light to the predetermined range of values of expected intensities of light and thereby determining the radius of curvature 5 of the optical fibre at the predefined region.
10. The method of any preceding claim, wherein receiving light returned from within the optical fibre further comprises configuring the at least one preconfigured time interval to be a time period within which any light returned from the selected predefined region of the optical fibre is expected, such that the intensity of light being measured is that of backscatter from the predefined region.
11. The method of any preceding claim, where the position of the preconfigured time interval is determined through an automatic self-calibration process.
12. The method of claim 11, wherein the position of the preconfigured time interval is continually optimised through an automatic self-calibration process.
13. The method of any preceding claim, wherein receiving light returned from within the optical fibre further comprises: configuring the at least one preconfigured time interval to be a time period within which any light returned from a region of the optical fibre before the selected predefined region of the optical fibre is expected; configuring a further preconfigured time interval to be a time period within which any light returned from a region of the optical fibre after the selected predefined region is expected; and measuring the intensity of the light received during the further preconfigured time interval.
14. The method of claim 13, wherein identifying that a change of shape in the optical fibre has occurred further comprises comparing the intensity of the light received during the at least one preconfigured time interval with an intensity of light received during the further preconfigured time interval to determine an increase in attenuation through the -29 -predefined region of the optical fibre.
15. The method of any preceding claim, wherein receiving light returned from within the optical fibre further comprises: receiving light returned from within the optical fibre selectively during a plurality of 5 preconfigured time intervals; measuring the intensity of the light received during each preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in each predefined region based on the measured intensity of said light returned during the 10 corresponding preconfigured time interval.
16. The method of any preceding claim, wherein transmitting a light pulse further comprises generating light pulses of a length that corresponds with the length of the predefined region such that there is negligible overlap between light reflected from two adjacent regions along the optical fibre.
17. The method of any preceding claim, wherein receiving light returned comprises receiving an optical signal from the light returned, converting the optical signal into an electrical signal, and using a time gate to configure the at least one preconfigured time interval.
18. The method of claim 17, wherein measuring the intensity of the light received 20 further comprises integrating a plurality of received optical signals to improve he signal-to-noise ratio of the measurements.
19. The method of any preceding claim, further comprising coiling or folding the optical fibre within the predefined region of optical fibre to increase detection sensitivity.
20. The method of any preceding claim, wherein the optical fibre is attached to, or 25 embedded within, a flexible material.
21. The method of claim 20, further comprising securing points of the optical fibre that are adjacent each end of the predefined region to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the -30 -flexible material.
22. The method of claim 21, wherein securing the optical fibre further comprises securing the optical fibre within the predefined region to the flexible material to be fixed only in a transverse direction such that the shape of any bends or loops in the optical 5 fibre can change in response to deformation or stretching of the flexible material.
23. The method of any preceding claim, further comprising modulating the light pulse in amplitude, phase or frequency, with either a smooth or pseudorandom signal, for use in a spread-spectrum system.
24. A system for detecting a change of shape of at least one predefined region of an 10 optical fibre, comprising: an optical fibre; means for transmitting a light pulse from a first end of the optical fibre towards a second end of the optical fibre; means for receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; means for measuring the intensity of the light received during each preconfigured 20 time interval; means for identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval; means for comparing the measured intensity of light received during the at least 25 one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; means for using a result of said comparing to determine whether to apply a correction to the measured intensity of light received during the at least one preconfigured time interval; and means for applying said correction when it is determined that a correction is -31 -required.
25. The system of claim 24, wherein the further predefined region of the optical fibre is arranged to inhibit the effect of external mechanical perturbation on the optical fibre such that said external mechanical perturbation has negligible effect on the shape of the 5 optical fibre in said further predefined region.
26. The system of claim 25, wherein the further predefined region is a reference region.
27. The system of claim 26, wherein the measured intensity of light received from a plurality of said at least one predefined regions is compared to the measured intensity of 10 light received from said reference region.
28. The system of claim 26, further comprising a plurality of reference regions, wherein the measured intensity of light received by each of a plurality of said at least one predefined regions is compared with the measured intensity of light received from a corresponding reference region.
29. The system of any of claims 26 to 28, wherein the means for measuring the intensity of the light is arranged to measure the intensity of the light received from the reference region and the intensity of the light received from the at least one predefined region at different frequencies.
30. The system of claim 29, wherein the means for measuring the intensity of the light 20 is arranged to measure the intensity of the light received from the reference region less frequently than it measures the intensity of the light received from the at least one predefined region.
31. The system of any of claims 24 to 30, further comprising at least two other predefined regions arranged such that deformation of the optical fibre in a first other 25 predefined region correlates with a deformation of the optical fibre in a second other predefined region, preferably wherein the correlation is an inverse correlation.

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32 - 32. The system of claim 24, wherein the at least one predefined region and the further predefined region are arranged such that deformation of the optical fibre in the at least one predefined region correlates with a deformation of the optical fibre in the further predefined region, preferably wherein the correlation is an inverse correlation.
33. A system for detecting a change of shape of at least one predefined region of an optical fibre, comprising: an optical fibre; means for transmitting a light pulse from a first end of the optical fibre towards a second end of the optical fibre; means for receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; means for measuring the intensity of the light received during each preconfigured time interval; and means for identifying that a change shape of the optical fibre has occurred in the at least one predefined region by comparing the measured intensity of light received during the at least one preconfigured time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that preconfigured time interval, and determining a difference.
34. The system of any of claims 24 to 33, wherein the optical fibre is attached to or embedded in a flexible material such that a deformation of the flexible material causes a change in shape of the optical fibre.
35. The system of claim 34, wherein the optical fibre is arranged such that the predefined region of the optical fibre coincides with a region of the flexible material at which a deformation is expected.
36. The system of claim 35, wherein the optical fibre is coiled or folded within the predefined region to increase detection sensitivity.

-33 -
37. The system of any of claims 24 to 36, wherein points of the optical fibre that are adjacent each end of the predefined region are secured to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material.
38. The system of claim 37, wherein the optical fibre within the predefined region is secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.
39. The system of any of claims 24 to 38, wherein there are a plurality of predefined regions along the optical fibre, each having a known relationship with a respective preconfigured time interval, such that an intensity of light returned from a plurality of predefined regions can be measured and hence a change in shape of the optical fibre can be identified at a plurality of predefined regions.
40. The system of any of claims 24 to 39, wherein the position of the preconfigured 15 time interval is determined through an automatic self-calibration process.
41. The system of any of claims 24 to 40, further comprising a pulse generating means arranged to provide a pulsed signal to control the means for transmitting a light pulse to transmit light pulses.
42. The system of any of claims 24 to 41, wherein the means for transmitting a light 20 pulse is synchronised with the at least one preconfigured time-interval such that the amount of light returned can be measured as a function of time since the light pulse was transmitted.
43. The system of any of claims 24 to 42, wherein the means for identifying that a change in shape has occurred is further configured to determine a change in shape of 25 the optical fibre at a predefined region from the light returned from the predefined region and/or the increase in attenuation in the optical fibre after the predefined region.
44. The system of any of claims 24 to 43, wherein the means for transmitting a light -34 -pulse, means for receiving returned light, means for measuring and the means for identifying are all comprised in a processing unit that is attachable to the flexible material.
45. The system of any of claims 24 to 44, wherein the optical fibre is a plastic optical fibre.
46. The system of any of claims 24 to 45, wherein the optical fibre comprises both a graded refractive index profile and a stepped refractive index profile at regions along its length.
47. The system of claim 46, wherein the refractive index profile of the optical fibre varies along its length from a graded index profile at a region proximate to the light 10 source to a stepped index profile at a region distal to the light source.
48. The system of any of claims 24 to 47, wherein the optical fibre is treated to increase its backscattering properties or bend sensitivity.
49. The system of claim 48, wherein the optical fibre is treated at one or more predefined sections of the fibre.
50. The system of claim 48 or 49, wherein the treatment comprises at least one of notches, side-windows, partial or complete cleaving and re-joining of the optical fibre, mechanical abrasion, exposure to UV light or patterning with a laser.
51. The system of any of claims 24 to 50 arranged to monitor the condition of a building or structure.
52. The system of any of claims 24 to 50 arranged to monitor deformation in a garment.
53. An optical fibre sensor unit for use in the system of any of claims 24 to 52, comprising: said optical fibre; and said means for receiving light returned from within the optical fibre coupled to said end of the optical fibre.

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54. The optical fibre sensor unit of claim 53, wherein the optical fibre is arranged to be coiled or folded in the one or more predefined regions.
55. The optical fibre sensor unit of claim 54, wherein the optical fibre comprises plastic optical fibre.
56. The optical fibre sensor unit of any of claims 53 to 55, wherein the refractive index profile of the optical fibre varies along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.
57. The optical fibre sensor unit of claim 56, wherein the refractive index profile of the 10 optical fibre varies along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.
58. The optical fibre sensor unit of any of claims 53 to 57, wherein the optical fibre is treated to increase its backscattering properties or bend sensitivity.
59. The optical fibre sensor unit of claim 58, wherein the optical fibre is treated at one 15 or more predefined regions of the fibre.
60. The optical fibre sensor unit of claim 58 or 59, wherein the treatment comprises at least one of: notches, side-windows, partial or complete cleaving and re-joining of the optical fibre, mechanical abrasion, exposure to UV light or patterning with a high-power laser.
61. The optical fibre sensor unit of any of claims 53 to 60 arranged to be attached to the flexible material at points of the optical fibre that are adjacent each end of the predefined region such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material.
62. The optical fibre sensor unit of any of claims 53 to 60 further arranged wherein the optical fibre within the predefined region is arranged to be secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.

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63. A processing unit for use with the system of any of claims 24 to 52, comprising: a light source arranged to transmit a light pulse into an end of an optical fibre; a light detector arranged to receive light returned from said end of the optical fibre; and processing means arranged to measure the intensity of returned light and identify whether the optical fibre has changed shape in a predefined region by comparing the measured intensity of light received during the at least one preconfigured time interval with the measured intensity of light received from a further predefined region during a corresponding further preconfigured time interval; using a result of said comparing to determine whether to apply a correction to the measured intensity of light received during the at least one preconfigured time interval; and applying said correction when it is determined that a correction is required.
64. A processing unit for use with the system of any of claims 24 to 52, comprising: a light source arranged to transmit a light pulse into an end of an optical fibre; a light detector arranged to receive light returned from said end of the optical fibre; and processing means arranged to measure the intensity of returned light and identify whether the optical fibre has changed shape in a predefined region by comparing the measured intensity of light received during the at least one time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that time interval, and determining a difference.
65. The processing unit of claim 63 or 64, further comprising a pulse generator arranged to control the light source to transmit light pulses.
66. The processing unit of any of claims 63 to 65, further comprising signal conditioning means for converting an optical signal received from the light detector into an electrical signal and an analogue-digital-converter for converting the electrical signal into a digital signal to be processed by the processing means.
67. The processing unit of any of claims 63 to 66 arranged to be secured to a flexible -37 -material and to couple with said end of said optical fibre.
68. A method substantially as described herein and shown in the accompanying drawings.
69. A system substantially as described herein and shown in the accompanying drawings.
70. An optical fibre sensor unit substantially as described herein and shown in the accompanying drawings.
71. A processing unit substantially as described herein and shown in the accompanying drawings.