Investigation of an Optical Fibre Sensor for Detecting Lower Limb Location

Size: px

Start display at page:

Download "Investigation of an Optical Fibre Sensor for Detecting Lower Limb Location"

Griffin White
5 years ago
Views:

1 Investigation of an Optical Fibre Sensor for Detecting Lower Limb Location J. Y. C. Chung and B. J. Lithgow Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria 368, Australia. Abstract There is a need for effective sensing of limb position, force and acceleration. This paper investigates the possibility of using optical fibre principles to detect lower limb position in the form of joint angles. The phenomenon of connection loss was chosen as the best candidate for detecting bends. This study shows it is possible to measure reasonable angles using connection loss principles making it a promising basis for sensing joint angles. Introduction One of the main hurdles faced in achieving natural gait for patients with locomotive impairment is the accuracy of control used in their orthopaedic treatment. Walking is a complex combination of sensory feedback and adjustment of force generated in the muscles. This sensory feedback needs to be artificially generated for orthopaedic patients whose sensory system has been damaged due to spinal cord injury, or for control of an active prosthesis. Traditional control systems for lower limb prosthetics and functional neuromuscular stimulation (FNS) have been open loop with only fixed rates of walking. Artificial sensory feedback would mimic the human sensory system by adjusting the response of a closed loop control system to measured information such as limb position, force and acceleration. The use of artificial feedback may make the control of standing and walking a more dynamic and natural process, giving orthopaedic patients confidence to walk without fear of falling. One of the main pieces of feedback information required to control standing or walking is limb position. One step towards providing information on the lower limb s position would be to measure the angle of its joints. Lower limb location can be defined by three joints the hip, knee and ankle. Table gives an indication of the range of angles experienced by these joins for various ambulatory activities. An optical fibre sensor would need to be capable of measurement within these ranges. Measured Variable Range (Degrees) Knee angle Standing Level walking Stair climbing/descent Ankle angle Standing Level walking Stair climbing/descent Hip angle Extension-flexion Lateral Table. Estimation of the range of angles for joints in the lower limb []. In general transducers for orthopaedic treatments like FNS need to be small, light, easy to use and mount. A sensor of limb location also needs to be cosmetically appealing and not hinder natural joint movement. To enhance reliability of a sensor system it should be made from commercially available components. The transducers would need to be stable and not react to environmental variables, such as temperature, humidity, magnetic fields, and electrical noise. Therefore transducers which may be functional in a laboratory setting may not operate properly in the normal environment. The aim of this research was to investigate a way in which we could utilise the characteristics of optical fibres to measure joint angle. An experimental optical system was then designed to test the validity of the proposed method of measuring angles. Three different methods for measuring bends using optical fibres were considered: bending loss, bend induced birefringence, and connection loss.

2 a) Bending loss. The bending loss in single and multimode fibres has been shown to be exponential and only becomes observable at a critical radius []. Bending loss is proportional to the exponential of (R/R C ) where R is the radius of the bend, and R C is the critical radius. Parameter Multimode Single mode n (refractive index of core) a (radius of 5 µm µm core) A (diameter 5 µm 5 µm of whole fibre) %.3.5 % NA.93.3 R C.57 mm.3 mm Table. Typical values and critical radii for single and multimode step-index fibres [3]. It can be seen in Table that the critical radius of single and multimode fibres is extremely small, therefore the intact fibre must be bent at curves close to this size if any power loss is to be measured. However the mechanical support structure of optical fibres limits the minimum radius of curvature. If the mechanical support were removed it would be possible to bend the fibre to very small radii of curvature to cause bending loss. However for the application in mind the fibre will be continuously bent and mechanical stability is required. b) Bend induced birefringence. The other way of measuring bends in an intact fibre is bend induced birefringence in a single mode fibre. The electric field of light launched into a single mode is a superposition of the two propagation modes of the fundamental mode. Applied stresses, twists and bends also cause the LP modes to propagate with different phase velocities and induce birefringence. Induced birefringence is defined as δβ = β y - β x [dimensionless] () Induced birefringence can be measured by launching polarised waves; one more positively polarised than the other. At a bend each is attenuated differently (phase changes are different). The birefringence can be measured by using a special connector that can measure the state of polarisation of the received light wave. However the high cost of this connector makes it prohibitive to use in this application. c) Connection loss. The most promising method investigated was connection loss. The initial connection loss sensor considered was of two ends of identical optical fibre mounted in a flexible join. One fibre end acts as an emitter the other the receiver. Ideally a flexible connection would hold the emitting and receiving surfaces parallel to each other at a fixed distance apart to allow for angular movement. Generally optical fibres are intolerant to large angular misalignment however with careful design small angular movements of the connection could correlate to large bends of the limb (ie for the knee during walking). The join should only allow angular movement in one plane to simplify mapping back to the physical bend of the knee. Connection misalignments lead to power loss as the cones of light emittance and acceptance overlap different amounts depending on the orientation of the two fibre ends. Initial investigation showed that single mode fibres are more sensitive to angular misalignment than multimode as they have smaller cones of emittance and acceptance. Power loss in a single mode connection becomes extremely large at small angles. Since we want the connection loss system to measure a large range of angles the experiment was setup using multimode fibres. Research into connection losses [, 5, 6, 7, 8, 9] has yielded varying results with no definitive theoretical expressions. Three types of misalignments can exist in a connection: end separation (s), lateral (y) and angular (θ). Figure. Exaggerated radial movement of receiver to emitter surface indicating s, y and θ misalignments. The theoretical connection loss of the system was calculated by adding the individual losses caused by the three different types of misalignment. Therefore total connection loss is L = L + L () connection s y + L θ in decibels [db].

3 Connection loss between multimode fibres was calculated using power loss equations found by Tsuchiya et al. [], assuming uniform power distribution. sna L S = log (3) an L y L 6( n n) = log ( + ( n n )) θ = y cos π a ( n n ) ( n n ) y a y a 6 n ( ) θ log + πna () (5) Note: Power losses are given in decibels [db]. All symbols are as stated previously. n is the refractive index of the medium between the emitting and receiving surfaces. Method An experimental system was designed to test if it was feasible to measure bends using the connection loss theory found. Analogue modulation of the optical source was applied since only power loss of the transmission needed to be measured. Instead of using a fibre as the transmitting and receiving surfaces, we used an optical source as the transmitting surface and the fibre as the receiving. The movement of the receiving to transmitting surfaces was approximated as a lateral movement, eliminating the need to include angular misalignment. To approximate the radial movement as shown in Figure as a lateral one the distance between the two fibre ends needs to be at least times larger than the core radius, which for a multimode fibre is 5µm. This made experimentation easier as the receiving fibre could be moved laterally with respect to the transmitting surface to approximate radial movement (or bending of the join ). The transmitting and receiving circuits were a combination of electronic and optoelectronic components. The transmitting circuit produced a khz square wave. A Sharp LT6 laser diode (78nm) with a maximum optical power of 3mW was used The receiving optical fibre was a 5m SMA terminated British standard multimode 5/5µm glass. Fibre specifications were n =.75, = %, NA =.9, optimised at.85µm, attenuation of.36db/km and 63MHz/km. This was then connected to a Honeywell HFD338 5MHz PIN plus preamplifier analogue receiver. This receiver package was chosen as it had maximum spectral responsivity at the operating wavelength (78nm), and a large detection range ( 5µW). The receiver has a responsivity of 7.5mV/µW. A band-pass filter was designed with a centre frequency (f ) of khz, a bandwidth (f BW ) of less than 5Hz and a gain of around. The receiving fibre was positioned maximally by observing the output of the receiving circuit while adjusting the x, y and z positioning separately. First the fibre end was moved as close to the laser diode as possible and this was referenced as zero end separation (s). The z positioning and then the y positioning were adjusted until a maximum signal was observed on the CRO. Finally, this position was considered as maximal transmission where there is zero end separation (s), lateral (y) and angular (θ) misalignment. The first experiment involved measuring the sensitivity of the optical connection to end separation. The average output voltage of the receiver was measured as end separation was increased from the maximal position (s = ) found above. The y optical mount adjuster and dial gauge were used to increase and measure the end separation in 5 µm ± µm increments. The average output voltage was measured and then used to calculate the average optical power received which was in turn used to calculate the connection loss. The second experiment was performed to test whether it is possible to measure bends using connection loss. The optical fibre was positioned at an end separation of 5 µm, and then the fibre was displaced at increasing lateral (y) displacement in increments of approximately 5 µm. This lateral displacement can be equated to angular movement as described previously. The peak to peak signal from the filter was measured and the connection loss calculated in the same way as described for the first experiment. Results and Discussion a) Increasing end separation Connection loss was calculated from the measured amplitude of the received signal

4 Connection loss (db) V L total = log (6) V where V is the amplitude of the signal when angular displacement is zero (ie. maximal transmission), and V is the signal amplitude at different misalignments End separation, s (um) Graph. Theoretical and experimental connection loss due to increasing end separation. The theoretical and measured connection loss for increasing end separation does not increase that dramatically. The loss calculated using the average voltage over the maximum received ( V) obviously shows a lower loss as it is with reference to the maximum signal received, but it gives an indication of the systems sensitivity to increasing end separation. The measured losses for end separation are still high and therefore it would probably be preferable when designing the flexible holder to keep the emitting and receiving ends as close as possible without compromising angular movement. b) Increasing lateral displacement Two sets of measurements were taken to test repeatability of the experiment. Using simple trigonometry if the transmitting and receiving surfaces start at an end separation of 5µm, the equivalent angle measured for a given lateral displacement is: y θ = tan 5µ m (7) It can be seen that the theoretical results predicted that loss measurements would only be possible up to a lateral displacement of 5µm (angles less than ). Experimental results were measurable up to a lateral displacement of 75µm. Using Equation 7 a lateral displacement up to 75µm equates to a bend angle of more than 3. The two trials closely correlated but diverged at higher displacements. This could be attributed to the difficulty in measuring the output as the signal became smaller and it contained a lot of noise. The rate of power loss increased in a similar fashion indicating that the response of the system to lateral displacement is reproducible. The connection loss measured in Graph shows that for small lateral misalignments (less than 5µm) the connection loss is constant. Therefore for lateral displacement within this range most of the receiving fibre core is in line with the laser diode coupling the maximal amount of optical power possible. Only for misalignments more than 5µm does the connection loss increase. For higher lateral misalignments only the edges of the cones of emittance and acceptance are overlapping and connection loss increases more rapidly. Since a lateral misalignment of 5 µm is equivalent to a bend of, this system seems to have little resolution for small changes in angles (< ). This may not necessarily be a problem when using this method to measure limb location during walking, as the range of movement of the joints is relatively large. However the range of angles needing to be measured during standing is only within this small range ( ). Therefore using multimode connection loss to detect small joint angles may be a problem. Connection loss (db) Lateral displacement, y (um) Graph. Theoretical and experimental connection loss due to lateral displacement. Conclusion The principle of connection loss to measure bends was shown to be feasible. Using a multimode mode fibre in the experimental setup allowed lateral displacement, which represented angles of up to 35 to be measured. V V

5 However this was proven in laboratory conditions where the multimode fibre was kept coiled and still. In an ambulatory system the multimode fibre would be subjected to vibrations and environmental effects that would lead to higher attenuation. In theory single mode fibres are less susceptible to transmission loss as their mode is bound more closely to the fibre core but as a result are less tolerant to high misalignments. Further investigation into connection loss using single mode fibres needs to be performed with respect to their sensitivity to misalignment. Environmental effects on the system would also have to be investigated. Obviously a lot more experimental investigation needs to be done before a complete system can be set-up to measure limb location. However the fact that it is possible to measure reasonable angles using connection loss principles makes it a promising basis for sensing joint angles. 8. Nemoto, S., and Makimoto, T., Analysis of splice loss in single-mode fibres using a Gaussian field approximation, Optical Quantum Electron., vol., pp7-57, April Thiel, F. L., and Hawk, R. M., Optical waveguide cable connection, Appl. Opt., vol. 5, pp785-79, Nov Tsuchiya, H., Nakagome, H., Shimizu, N., and Ohara, S., Double eccentric connectors for optical fibres, Appl. Opt., vol. 6, pp33-33, May 977. Acknowledgements I would like to thank Dr Le Binh for his advice and suggestions as well as allowing access to his laboratory facilities. References. Troyk, P. R., Jaeger, R. J., Haklin, M., Poyezdala, J., and Bajzek, T., Design and Implementation of an Implantable Goniometer, IEEE Trans. Biomed. Eng., vol. BME-33, pp5-, Feb Gowar, J., Optical Communication Systems, nd Edition, Prentice Hall International (UK) Ltd, Hertfordshire, Miller, S. E. and Kaminow, I. P., Optical Fiber Telecommunication II, AT&T and Bell Communication Research, Inc., California, Daido, Y., Miyauchi, E., and Iwama, T., Measuring fiber connection loss using steady-state power distribution: a method, Appl. Opt., vol., pp5-56, Feb Di Vita, P., and Rossi, U., of power coupling between multimode optical fibres, Optical Quantum Electron., vol., pp7-7, Gloge, D., Offset and Tilt in Optical Fiber Splices, Bell Syst. Tech. J., vol. 55, pp95-96, Sept Marcuse, D., Analysis of Single- Mode Fiber Splices, Bell Syst. Tech. J., Vol. 56, No. 5, pp , May 977.

Photonic Communications Engineering I

Photonic Communications Engineering I Module 3 - Attenuation in Optical Fibers Alan E. Willner Professor, Dept. of Electrical Engineering - Systems, University of Southern California and Thrust 1 Lead