CA2130429C - Optical sensor displacement monitor - Google Patents

Abstract

An optical sensor comprising an optical fiber having a core covered by a cladding, the cladding having an index of refraction different from that of the core, apparatus for retaining the fiber in a sinuously looped disposition, apparatus for fixing the fiber to an object to be sensed whereby movement to be sensed results in one or both of accordion expansion and contraction of loops of the fiber and microbending of the fiber, apparatus for applying a first optical signal into one end of the fiber, apparatus for detecting a resulting optical signal from the other end of the fiber, and apparatus for comparing the first and resulting optical signals to obtain an indication of the movement.

Description

~' 2~3~29 l This app:Lication is a divisional application of

2 Canadian application S.N. 2~009,033 filed January 31, 1990.
This lnvention relates to a sensor whlch uses an optical fiber Eor monitoring relative movements such as physiological activity, vibrations 6 or movemellt oE struct:llres and apparatus where a strain yauge or displacement monitor would otherwise be used.
8 Strain gauges are usually used to monitor 9 warniny or destructive movements of various lo structures, SUCt1 as pipelines, bridges, buildings, ~11 etc., or to monitor earth movements. Myoelectric 12 potential detecting devices are sometimes used for 13 physiological sensing, e.g. Eor monitoring heart beat ;
14 or other muscle movements. The presence of breathing has been monltored using sound or air pressure sensors 16 attached to the nostrils. Such physiological sensors 17 require electrical connection to the skin, or are ~18 otherwlse uncomeortable or painful to the patient. -~
19 The present invention is a transducer for 20~ monitoring vital signs oE persons, animals, etc. using 21~ a comf~ortable to use structure. The invention can be 22 ~ adapted to detect non-physiological displacements 23 such as the movement of buildings, structures, etc. ~-~24 including machinery, bridges, earthquakes~ blasts, for security and intrusion alarms, etc., e.g. wherever a 26 strain gauge or displacement transducer can be used.
27 The present invention uses an optical fiber 28~ in a particular Eorm as a strain sensor using the 29 phenomenon of microbending, as will be described 30 below. ;
31 U.S. Patent 4,~54,520 which issued 32 March 31, 1987 to Richard W. Griffiths describes the 33 use oE an optical fiber as a strain gauge. The Eiber 34 is fixed to a pipe or another object to be monitored at two or more points. When the pipe or other object 36 bends, the transmission characteristics oE the light 37 signal through the optical fiber change, and are 38 detected. The patent points out that the effect can 39 be enhanced by the use oE the particular phenomenon of ~ .

01 microbending. In microbendin~ ~e~r~e~2active index of 02 an optical fiber cladding is changed to a value close 03 to or equal to that of the core by bending of the ' 04 fiber, and as a result, light that would otherwise be 05 internally reflected at the interface of the core and ~ '' 06 the cladding partly escapes into the cladding~
07 Griffiths states that greatly increased 08 sensitivity can be obtained using an integral 09 continuum of microbend elements. -~
U.S. Patent 4,675,521 issued June 23, 1987 11 to ~iroshi Sugimoto uses what appears to be 12 microbendin~. In this case the cladding of a fiber ~;''' 13 optic cable is pinched or depressed by a contact 14 member, causing the escapé of optical signals from a fiber optic core.
16 ~ - In both of the aforenoted patents, ~' '-':
17 variation of the amplitude of the light signal passing ' -~
18 through the optic fiber and changed by an external 19 force is measured. In both cases it is the bending of the straight segment of fiber which causes the effect 21 to be measured. '' 22 The microbending fiber optic transducers 23 ~ described in the aforenoted two patents are not 24 suitable for use as physiological monitors for several ' '~
reasons. For example, where the chest expansion and 26 contraction of a baby is to be monitored, wrapping the 27 optical fiber around the baby's chest would constrict ~' 28 ~the breathing, causing distress. Secondly, there is 29 no evident way to cause the bending after wrapping the optical fiber around a significant portion of the ' ;~' 31 body since th'e body would be in effect bound up.
32 There is also'no evident way of monitoring delicate 33 movements of part of the'chest wall which is required 34 in order to monitor heart beat.
35' The present invention is an optical sensor 36 which uses exclusively the microbending phenomenon in 37 an optical fiber structure which is ideally suited for '~

-- 213~4~9 ~1 the aforenoted physiological monitoring. The 02 invention is made possible by a unique structure, and 03 also the discovery that one can obtain a high change 04 in the microbending loss in the fiber, and thus high 05 sensitivity,,if the fiber is bent into one or more 06 loops each having a particular maximum smallest radius 07 for a given fiber diameter. This is achieved by 08 creating an optical fiber which is sinuously looped, 09 and which is disposed on a mount which allows the fiber to expand its loops iike the bellows of an 11 accordion with physiological movement of the person or 12 animal or part thereof which is monitored. ThiS
13 allows expansion and contraction of the transducer, 14 while utilizing the microbending phenomenon, which is not possible using the aforenoted prior art fiber 16 structures. As a result the present invention can be 17 used as a physiological monitor, worn on or around the 18 body or over the part of the body to be monitored l9 without invasion of the body, and without pain or discomfort, while being highly effective.
21 With the extremely high sensitivity 22 obtained due to the presence of the multiple loops in 23 the structure, smalL physiological movements such as 24 heart beats can be sensed, while larger movements such as expansion and contraction of the chest resulting 26 from breathing can as~well be sensed. The invention 27 may be used to monitor limb volume during 28 plethysmography. The signal from the device is 29 linearly related to the strain applied to limb~
measured, while blood or fluid is being drained. The 31 invention can also be used to monitor penile 32 tumescence, without the need for bulky or harmful 33 sensors.
34 The transducer can also be used to monitor movement of machinery relative to a fixed point, parts 36 of the machinery relative to other parts, the motion37 of structures relative to a suspended relatively . .......

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1 massive weight, expansion or contraction of devices such as 2 electronic or other substrates, e.g. caused by temperature

3 or pressure variation, movements of parts of buildings, bridges caused by various phenomenon such as blasts, S earthquakes, etc. An example is a large scale vibration 6 detector for intrusion alarm or sound detection. Acoustic 7 pressure waves may be detected using an appropriate ~
8 diaphragm. Indeed, the sensitivity of the sensor can be ~;
g tailored to the application by providing more or fewer loops, even as few as one loop.
11 A preferred embodiment of the invention is an ~ ~ -12 optical sensor comprising at least one loop of optical fiber i3 omprised of a core covered by a cladding having an index of ;
14 refraction different from that of the core, the radius of -~;
15 the loop being sufficiently small so as to obtain ~
16 microbending loss in the fiber with variation in the radius, ~ ~-17 apparatus for retaining the loop on a support whereby the 18 radius can be varied, a diaphragm for receiving sound of i~
1g ultrasonic waves connected to the loop so as to vary the radius with pressure from the waves, apparatus for applying 21 a light signal into one end of the core, and apparatus for '-~
22 detecting a resulting sound or ultrasonic modulated light 23 signal at the other end of the core, forming a microphone.
24 A ~etter understanding of the invention will be obtained by reference to the detailed description below, 26 with reference to the following drawings, in which~
27 Figure l illustrates a segment of a loop of 28 optical fiber, ~' 29 Figure 2 is a schematic illustration of a first 30 embodiment of the present invention, -~
31 Figure 3 is a schematic illustration of a second 32 embodiment of the present invention, 33 Figure 4 is a block diagram of the electronic -~
34 processor portion of Figure 3 which can be 36 ~

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01 used also with the embodiment shown in Figure 2, 02 Figure 5 is another embodiment of the 03 invention, 04 Figure 6 is a schematic view of the present 05 invention in the form of a microphone, and 06 Figure 7 is a plan view of a room 07 illustrating the present invention in the form of an 08 intrusion detector.
09 Figure 1 illustrates a portion of a looped optical fiber which utilizes microbending. The loop 11 is fixed to a resilient or expandable backing, and has 12 ~ a maximum smallest radius R. The radius should be 13 selected to obtain significant microbending optical 14 loss when the loop is strained by bending so that the lS radius increases or decreases. For example, if the 16 optical fiber has a 125 micron diameter, the maXimum I7 smallest radius of the bend should be not more than 18 about 4 mm. Where the optical fiber is 250 microns, ~' 19 ~ the maximum smallest radius of the bend should be not more than about 8 mm.
21 The fiber 1 is formed of a core 2 22~ surrounded by a cladding 3. The cladding should have 23 an index of refraction which is different, preferably 24 ; larger, from that of the core. When the fiber bends, 25 changing the radius R, the cladding is stressed, e.g. ' 26 ~ ~at the cross hatched portion referenced 4. The ~: :
27 portion of the cladding opposite that reference 4 is ~-28 also stressed. The result is ~ariation of the index 29 of re~fraction of the cladding at one or the other sides of the core, so that it closely approaches or 31 becomes equal to the index of refraction to the core 32 2. The result is loss into the cladding of light 33 energy passing through from onè end of the core 2 34 which would otherwise be totally interna}ly -~
reflected. The loss of light from the core or its 36 change can be sensed at the other end of the core, and 37 can be characterized as a variation of the impedance ., . ".. ,j - .

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2 ~ 2 9 ~'1 of the fiber. ',, 02 AS the loop is flexed and the rad,ius R of ,~,', 03 the loop becomes smaller and larger, this change in '~, ,,,''',"
04 impedance can be monitored, and results in a signal 05 which is related to the change in radius R. -:~
06 Figure 2 illustrates an embodiment of the 07 invention. The fiber 1 is retained preferably in a 08 sinuously looped disposition. Each of the loops has a 09 radius of approximately R as described with reference ,' ~,,-;
to Figure 1. The fiber has a light source 5, such as 11 a light emitting diode LED connected at one end 12 thereof in order to apply light energy down the core 13 of the optical~fiber. A light sensor 6 is connected 14 ~ to the opposite'end of the core of the fiber 1.
The fiber is fixed in order to hold its '~
16 general shape when not stressed, to an expandable ' , 17 backing 7. In one successful embodiment of the 18 invention, the flexible backing 7 was formed of ,. :,- . ~
19 elasticized cloth, and the fiber was sewn thereto in -,~ ~, its sinuous disposition.
21 ~ In operation of a successful prototype, the ,,, ~ ,,, 22 backing 7 was disposed over the chest of an infant 23 ~ whose breathing was to be monitored and tied around 24 its body. A light was applied via LED 5 and was sensed at light sensor 6. Breathing of the infant -,''' ~' 2~6 caused expansion of the backing 7, causing the loops 27 of fiber,l to expand and contract like the bellows of '",','~
28 an accordion. This caused increase and decrease of ~ ~k ,' 29 the radii R, and through the phenomenon of microbending, loss and variation of loss of light into 31 the cladding in a rhythmic manner with the breathing 32 of the infant and expansion and contraction of the 33 backing 7.
34 Inspiration by the infant caused expansion of the backing 7 and increase o~ the radii R.
36 Expiration of the infant caused contraction o~ the 37 backing 7 and decrease of the radii R. As a result an ~ , 38 - 6 ~

--~ 213'0429 01 electronic processor receiving the light signal 02 provided a record of the breathing of the infant.
03 The processor could also detect the absence 04 of variation of light, i.e. absence of breathing over 05 a predetermined time period, and should apnea exist, 06 operate an alarm.
07 A structure similar to that shown in Figure 08 2 made smaller and having increased sensitivity by 09 using a large number of loops, taped at its ends to the chest of an infant over the heart, was able to 11 monitor heart beat.
12 With physiological vital sign sensing oF an 13 infant using the structure described above, serious 14 problems such as sudden infant death syndrome would in many cases be able to be averted. The invention has 16 particular application to infants that are predisposed 17 to occurrence of sudden infant death syndrome.
18 Figure 3 illustrates another embodiment of 19 the invention. In this case the fiber 1 is looped in a sinuous manner and is retained on an expandable 21 backing 7, but is looped back upon itself along the 22 same path. This structure provides two additional 23 benefi~ts over that shown in Figure 2. Because there -24 are double the number of loops traversing a given length of backing 7, the amount of attenuation for a 26 certain expansion of the backing 7 can be doubled, 27 resulting in increased sensitivity.
28 Secondly, the LED and sensor 5 and 6 can be 29 brought to a single connection point and connected via a double connector to an electronic processor 8.
31 The embodiment shown in Figure 3 32 illustrates two additional features that can be used.
33 The ends of the backing 7 contain pads 9 of Velcro~. ' 34 ~ith this structure the invention can be removable from a sensor harness that may be attached to a 36 patient, and can be removed during e.g. waking hours 37 and reapplied during sleeping hours. It also allows " ;,.
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2130-~29 ~
Ul the transducer to be fixed to corresponding pads on an ~ -02 article of machinery or the like as described earlier, 03 and to be removed easily.
04 The transducer can have laces, Velcro~ 10 -05 or a similar fastener fixed to the ends thereoE for ~' 06 fixing around the body of a ~person to be monitored.
07 Turniny to Figure 4, a block diagram of an 08 electronic processor 8 is illustrated. LED 5 which 09 applies light energy to the core of optical fiber 1 has its electrical input connected to one input of 11 comparator 11. optical sensor 6 has its electrical i -~
12 output connected to the other input of comparator 11.
13 Comparator 11 provides a signal formed of the 14 difference between the two signals to display 12. If there is no variation in the radius R of the loop, the 16 signals applied to comparator 11 will be virtually 17 identical, assuming little or no loss in the optical 18 fiber. The display, which can be an oscilloscope, 19 printer, alarm, strip chart, or other corresponding device, will provide the appropriate output. ~ -21 With variation in the radius R oE the 22 loops, there will be optical signal loss in the fiber, 23~ and there will be a difference signal output from '~24; comparator 11 to display 12.
~ Should the transducer be monitoring the 26 breathing of an infant, for example, the result on 27 display 12 will be a sinuous line representing 28 expansion and contraction of the chest of the infant.
29 Should breathing stop, the line will be horizontal at a low level. Should this occur, an ancillary alarm 31 can be used to alert medical personnel that breathing 32 has stopped, e.g. after a timeout.
33 Figure 5 illustrates another embodiment o~
34 the invention. Transducer 15 represents the structure comprising the sinuously looped fiber and expandable ~ -36 backing, LED 5 and optical sensor 6. Interface 16 37 represents an analog to digital converter. MC~ 17 is -~

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2130~29 --~ 1 a microcomputer unit. A bus 18 is connected to the 02 microcomputer unit 17, and random access memory RAM 19 03 and erasable programmable read only memory EPROM 20 04 are connected to the bus 18. ~n oscilloscope 22 is 05 connected via a digital to analog converter 23 to the 06 bus 18. The microcomputer unit 17 is also connected 07 to an interEace 24 for connection to a user terminal 08 25, via typically an RS232 port. A power supply 26 is 09 connected to the microcomputer unit.
The microcomputer unit 17 operating by 11 means of programs stored in RAM 19, causes application 12 through interface 16 of an optical signal to :
13 transducer 15, and receives the resulting signal via 14 interEace 16. It performs a comparator function, and causes display of the result on oscilloscope 22 via 16 digital to analog converter 23, the latter converting 17 the digital signals from MCU 17 into analog signals 18 used by oscilloscope 22.
19 The MCU 17 can be controlled from terminal 25 through interface 24.
21 The aforenoted display functions can 22 thereby be generated, and if desired, alarm signals 23 additionally generated. Further, a record oE the ~ -24 results can be stored in memory or on a disk storage ; 25 memory (not shown), and various cycles of ;
26 physiological variation can be compared one with the 27 other, or grouped.
28 Figure 6 illustrates the invention forming -~
29 a microphone. An optical fiber loop of the type ~ '' described above is fixed at positions 27 to a support ;
31 28, so that the loop can flex, changing its radius, -~
32 ~ under external pressure. A diaphragm 29 is fixed from ~ i 33 its center point to the loop of the fiber 1, the . -34 diphragm being located so as to receive sound or ultrasonic waves 26. A light signal such as described -36 above is applied to one end T of the core of the .
37 fiber, while the resulting light signal is received at 38 - 9 ~

, ,,, I , '....,'.''' ,, ',....

2 1 3 ~
~~ the other end, R. ~he resulting light signal is 02 modulated with the sound or ultrasonic waves, which 03 can be detected by demodulation.
04 Figure 7 illustrates in a plan view of a 05 room 32 the invention forming an intrusion detector.
06 Sensor lengths 30 of multiple loop optical fiber, ;~
07 retained to a backing in a manner described earlier, 08 are located around a room to be protected, and are 09 connected in series, with a length or lengths preEerably hidden, running under a floor covering '~
11 where an intruder would step. Preferably the length ~-12 of each sensor is at least 20 times the physical 13 amplitude of the loops. Attachments 31 to windows, '~
14 doors, etc. are made ~rom the sensors so that should a ~ ~
door or window be moved, the fiber will be microbent, ~ ~ -16 resulting in detection using the system described 17 earlier with respect to Figures 4 and 5. Similarly, 18 the fiber will be microbent under pressure from floor 19 coveriny that is stepped on. In this manner, a person intruding into the room by door or window, or stepping 21 on the floor covering over a sensor, will be detected.
22 It should be noted that the sensor can be 23 made in various forms, such as the forms described 24 above, or in the form of a belt,~netting, etc. Rather than being sinuously looped, the fiber can be looped 26 ~ in a circle or oval. It can be made in various sizes ~27 and with different numbers of loops for various 28 applications. A single sensor can have loops having 29 various loop radius to provide variations in sensitivity Eor different bending increments. -31 A person understanding this invention may 32 now conceive of other embodiments or variations in 33 design using the principles disclosed herein. All are 34 considered to be within the sphere and scope of this invention as defined in the claims appended hereto.

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Claims

We claim:

1. An optical sensor comprising at least one loop of optical fiber comprising a core covered by a cladding having an index of refraction different from that of the core, the radius of the loop being sufficiently small so as to obtain microbending loss in the fiber with variation in said radius, means for retaining the loop on a support whereby the radius can be varied, a diaphragm for receiving sound of ultrasonic waves connected to the loop so as to vary the said radius with pressure from said waves, means for applying a light signal into one end of the core, and means for detecting a resulting sound or ultrasonic modulated light signal at the other end of the core, forming a microphone.