Patent Publication Number: US-2017350774-A1

Title: Direct light bend sensor

Description:
FIELD 
     Embodiments of the invention relate generally to flexible bend sensors and more particularly to direct light bend sensors, including fluid-filled direct light bend sensors. 
     BACKGROUND 
     Flexible bend sensors are used to measure the degree of bending of the sensor. Flexible bend sensors are used in a variety of mechanical, industrial, medical applications. Bend sensors may be embedded within an object or attached to objects in order to measure movement of the object or movement of the object relative to another object. For example, bend sensors may be used in automotive applications to measure the flexure of a surface connected to the steering wheel by which a horn is sounded when the surface is flexed sufficiently. A bend sensor may be embedded within a car seat so that when an occupant is seated in the seat the bend sensor flexes and signals that the seat is occupied. Bend sensors may employ electro-optical technologies including fiber optics, as well as electromechanical technologies such as flexible substrates with resistance that varies according to the degree of bending. 
     SUMMARY 
     An apparatus for a flexible direct light bend sensor is disclosed. A system and method also perform the functions of the apparatus. In some embodiments, a direct light bend sensor is disclosed that includes a tube that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. In the embodiment, a light source is disposed in a first end portion of the tube and a photodetector is disposed in a second end portion of the tube. In the embodiment, an inner surface of the tube defines a bend-dependent direct light path from the light source to the photodetector through the tube. 
     In some embodiments, a method for measuring force is disclosed that includes coupling a direct light bend sensor to a first object. In the embodiment, the direct light bend sensor includes a tube that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. The direct light bend sensor of the embodiment further includes a light source disposed in a first end portion of the tube and a photodetector disposed in a second end portion of the tube, where an inner portion of the tube defines a bend-dependent direct light path from the light source to the photodetector. The method further includes positioning the direct light bend sensor to be at least partially bent in response to a first force being applied to at least a portion of the tube and measuring a signal from the photodetector that indicates a magnitude of the first force. 
     In some embodiments, a system is disclosed that includes a direct light bend sensor that is coupled to a first object. In the embodiments, the direct light bend sensor includes a fluid-filled tube that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. The direct light bend sensor further includes a light source disposed in a first end portion of the fluid-filled tube and a photodetector disposed in a second end portion of the fluid-filled tube. In the embodiments, an inner surface of the tube defines a bend-dependent direct light path from the light source to the photodetector through the fluid-filled tube. In the embodiments, the system also includes a controller that measures changes in a signal from the photodetector in response to a bending of a least a portion of the fluid-filled tube, the bending in response to a force being applied to a second object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict merely typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1A  is a schematic block diagram illustrating one embodiment of a resistive bend sensor depicted in a straightened disposition; 
         FIG. 1B  is a schematic block diagram illustrating the resistive bend sensor of  FIG. 1A  depicted in a bent disposition; 
         FIG. 2A  is a schematic block diagram illustrating one embodiment of a light guide bend sensor having optical fibers and depicted in a straightened disposition; 
         FIG. 2B  is a schematic block diagram illustrating the light guide bend sensor of  FIG. 2A  depicted in a bent disposition; 
         FIG. 2C  is a schematic block diagram illustrating an end view of the light guide bend sensor of  FIG. 2A  depicted in a straightened disposition; 
         FIG. 3A  is a schematic block diagram illustrating one embodiment of a fluid-filled light guide bend sensor having a reflective inner surface and depicted in a straightened disposition; 
         FIG. 3B  is a schematic block diagram illustrating the fluid-filled light guide bend sensor of  FIG. 3A  depicted in a bent disposition; 
         FIG. 3C  is a schematic block diagram illustrating an end view of the fluid-filled light guide bend sensor of  FIG. 3A  depicted in a straightened disposition; 
         FIG. 4A  is a schematic block diagram illustrating one embodiment of a fluid-filled direct light bend sensor having a light absorbing inner surface and depicted in a straightened disposition with a direct light path unblocked; 
         FIG. 4B  is a schematic block diagram illustrating an end view of the fluid-filled direct light bend sensor of  FIG. 4A  in a straightened disposition with the direct light path unblocked; 
         FIG. 4C  is a schematic block diagram illustrating a side view of the fluid-filled direct light bend sensor of  FIG. 4A  in a bent disposition with a direct light path partially blocked; 
         FIG. 4D  is a schematic block diagram illustrating an end view of the fluid-filled direct light bend sensor of  FIG. 4A  in a bent disposition with the direct light path partially blocked; 
         FIG. 4E  is a schematic block diagram illustrating a side view of the fluid-filled direct light bend sensor of  FIG. 4A  in a further bent disposition with the direct light path blocked; 
         FIG. 4F  is a schematic block diagram illustrating an end view of the fluid-filled direct light bend sensor of  FIG. 4A  in a further bent disposition with the direct light path blocked; 
         FIG. 5  is a schematic block diagram illustrating one embodiment of system that includes a direct light bend sensor connected to a controller and an output module; 
         FIG. 6  is a graph of an output signal from a fluid-filled direct light bend sensor; 
         FIG. 7A  is a schematic block diagram illustrating one embodiment of a fluid-filled direct light bend sensor with a flared end portion; 
         FIG. 7B  is a schematic block diagram illustrating the fluid-filled direct light bend sensor of  FIG. 7A  coupled to a first object using a cable clamp and configured to measure force applied by a second object; 
         FIG. 8  is a schematic block diagram illustrating one embodiment of a system that includes a fluid-filled direct light bend sensor configured to directly measure flow; 
         FIG. 9A  is a schematic block diagram illustrating one embodiment of a system that includes a fluid-filled direct light bend sensor configured to measure flow using an articulating arm; 
         FIG. 9B  is a schematic block diagram illustrating a top view of the system of  FIG. 9A ; 
         FIG. 9C  is a schematic block diagram illustrating a top view of the fluid-filled direct light bend sensor of the system of  FIG. 9A  configured with a calibration screw; 
         FIG. 10  is a schematic block diagram illustrating an embodiment of a system that includes fluid-filled direct light bend sensor configured to measure pressure applied to a plunger; 
         FIG. 11  is a schematic block diagram illustrating another embodiment of a system that includes fluid-filled direct light bend sensor configured to measure pressure applied to a membrane; 
         FIG. 12  is a schematic flow chart diagram illustrating one embodiment of a method for measuring a force using a fluid-filled direct light bend sensor. 
         FIG. 13  is a schematic flow chart diagram illustrating another embodiment of a method for measuring a force using a fluid-filled direct light bend sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, refer to the same embodiment, but mean “one or more but not all embodiments” unless expressly specified otherwise. The terms “including,” “comprising,” “having,” and variations thereof mean “including but not limited to” unless expressly specified otherwise. An enumerated listing of items does not imply that any or all of the items are mutually exclusive and/or mutually inclusive, unless expressly specified otherwise. The terms “a,” “an,” and “the” also refer to “one or more” unless expressly specified otherwise. The term “substantially” as used herein is intended to mean predominantly or having the overriding characteristic of, such that any opposing or detracting characteristics reach a level of operational insignificance to use of the apparatuses, systems, or methods disclosed herein. Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. §112, ¶6. In particular, the use of “step” or “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. §112, ¶6. Where reference is made herein to a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously (except where the context excludes that possibility), and the method can include one or more other steps which are carried out before any of the defined steps, between two of the defined steps, or after all the defined steps (except where the context excludes that possibility). 
       FIG. 1A  is a schematic block diagram illustrating one embodiment of a resistive bend sensor  100  depicted in a straightened disposition. The resistive bend sensor  100  includes a flexible substrate  102  and a resistive layer  104  that is flexible. The resistive bend sensor has a first end portion  108  and a second end portion  110 . The flexible substrate  102  may be a flexible strip of a polymer film, such as for example, polyimide film Kapton®. The resistive layer  104  may be a thin flexible layer of resistive carbon coupled to or layered upon the flexible substrate  102 . The resistance of the resistive layer  104  changes as the carbon layer is elongated or compressed by the bending of the flexible substrate  102 . A first measurement of the resistance of the resistive layer  104  between the first end portion  108  and the second end portion  110  may be made as a reference for the resistive bend sensor  100  in a straightened disposition. 
       FIG. 1B  is a schematic block diagram illustrating the resistive bend sensor  100  of  FIG. 1A  depicted in a bent disposition. A second measurement of the resistance of the resistive layer  104  between a first end portion  108  and second end portion  110  may be made as a reference for the sensor in a substantially bent position. The radius of curvature  112  is the reciprocal of the curvature of the arc formed by the sensor. By comparing the resistance of the flexible resistive sensor in a first straightened disposition as illustrated in  FIG. 1A  and a bent disposition as illustrated in  FIG. 1B  the range of resistance may be calibrated so that a measurement of a bend curvature may be made at any degree of curvature. 
     Breaks or cracks in the carbon substrate, also referred to as the resistive layer  104 , may be intentionally designed into a resistive bend sensor such that the resistance changes with bending. When this type of sensor is constructed, the resistive layer  104  may be stressed with the intention that no further cracks develop during its intended use. Yet, sometimes unintended cracks  106  may begin to form in the resistive layer  104 . The unintended cracks  106  may alter the resistive properties of resistive layer  104 , for example, by creating an unanticipated increase in resistance during bending This may result in an open circuit that has effectively nearly infinite resistance, which may make the measurements vary over time, thus decreasing the accuracy of the measurements and/or requiring more frequent calibration of the resistive bend sensor  100 . 
     Bending the resistive bend sensor  100  may, in some cases, result in a nearly immediate increase in resistance, but when straightening the resistive bend sensor  100  out, it may take a longer time for the resistance to decrease again. This hysteresis effect may limit the usefulness of the resistive bend sensor  100  to applications requiring dynamic responsiveness. Moreover, the resistance of the resistive bend sensor  100  may drift over time which, for purposes of this application, would also be considered a type of hysteresis. 
     The resistive bend sensor  100  be may a generally planar strip. For example, it may be ribbon-shaped or belt-shaped and thus, may be typically used in applications where a bending force is applied orthogonally to the plane of the sensor as depicted in  FIG. 1B . Bending of a planar strip sideways to the plane of the strip may be difficult and thus planar, belt-like bend sensors maybe less suitable for measuring a degree bending in certain dimensions that are not orthogonal to the plane of the strip. 
       FIG. 2A  is a schematic block diagram illustrating one embodiment of a light guide bend sensor  200  having optical fibers and depicted in a straightened disposition. In some embodiments, the light guide bend sensor  200  includes a tube  202  that is at least partially filled with a transfer medium  204  that is a flexible solid, such as for example, optical fibers, or another flexible solid light transmissive material that extends within the tube  202 , from a light source  206  disposed at a first end portion  208  of the tube  202  to a photodetector  214  disposed at a second end portion  210  of the tube  202 . 
     As used herein, the word tube refers to a conduit that may be hollow and that may, in some embodiments, be filled with an optically transmissive fluid, such as air, inert gas, water, oil. In some embodiments, the tube  202 , may be substantially cylindrical, i.e., may have a substantially circular cross-section. However, in other embodiments the tube  202  may have a non-cylindrical shape, such as for example, a shape having a square, rectangular, triangular, or oval, or arbitrarily shaped cross-section. 
     Moreover, in some embodiments, the exterior of the tube  202  may have a different shape than the interior of the tube  202 . For example, in applications where flow is measured, the exterior of the tube  202  may be shaped like an oar to more readily provide a broader surface upon against which the flow may apply a force. Similarly, a portion of the exterior of the tube  202  may be attached or otherwise coupled to one or more objects such that movement of the one or more objects applies a force to the portion of the tube  202 . 
     Unless otherwise clear from the context, reference to elements of a bend sensor, such as for example the tube  202 , may be understood to similarly refer to other embodiments of a similar tube shown in one or more of the other figures (e.g.,  302 ,  402 ,  502 ,  702 , and  802  which are illustrated and described below with respect to  FIGS. 2B, 3B, 5A, 5B, 6A, 6B, 7A, 8B, and 9-10C . 
     It may be noted by a person of ordinary skill that applying a force to a portion of the tube  202  may cause at least a portion of the tube  202  to bend. In some embodiments, the tube  202  (or similar tubes in other embodiments) may be bent symmetrically, such as for example, as depicted in  FIGS. 3B, 4C-4F, and 5 . In other embodiments, the tube  202  may be bent asymmetrically, such as for example, bent by a portion of the tube being pushed by an object as depicted in  FIGS. 7B, 8, 9A-9C, 10A-10C , and  11 . In other words, in some embodiments, only a portion of the tube  202  (or similar tubes depicted in the embodiments illustrated in other figures) is bent by application of a force to the tube  202 . 
     A guided light  216  is illustrated as representative of light shining through the transfer medium  204  (e.g., optical fibers) from the light source  206  to the photodetector  214 . Because bend sensors are configured to bend, the tube  202  is flexible and may be made be made of polymers, elastomers, and the like. The tubes illustrated in the other figures e.g.,  302 ,  402 ,  502 ,  702 , and  802  may be similarly flexible. 
     In some embodiments in which tube  202  is in a straightened disposition, the guided light  216  may be transmitted through the transfer medium  204  with some refraction and some internal reflection. Transfer media with higher indices of refraction may cause substantial refraction, sometimes visualized as apparent bending of guided light  216 . Transfer media with low indices of refraction such as air or a vacuum or various other light gases may cause substantially less refraction of the guided light  216  as it travels from the light source  206  to the photodetector  214 , such that if an inner surface  212  of the tube  202  is reflective, the bending of the inner surface  212  acts as a light guide by reflecting the guided light  216  towards the photodetector  214 . 
     Although some embodiments depict an inner surface of a tube such as the inner surface  212  of the tube  202 , it may be noted that references to an inner surface of a tube in the various embodiments may refer to either a surface which is composed of the same material as the rest the tube, such as for example latex rubber, or in other embodiments may refer to a surface which is composed of different material from the rest of the tube such as for example a surface coating that is applied to an interior portion of the tube. 
       FIG. 2B  is a schematic block diagram illustrating the light guide bend sensor  200  of  FIG. 2A  depicted in a bent disposition. The transfer medium  204  may include optical fibers or other optically transmissive media with an index of refraction sufficient to cause substantial refraction or reflection of the guided light  216  as it travels from the light source  206  to the photodetector  214 . Much of the guided light  216  is refracted and internally reflected within the transfer medium  204  (e.g. fiber optics or other refractive flexible solid transfer medium) such that the inner surface  212  may be reflective or may alternatively be light absorbing without substantial impact on the amount of light transmitted through the transfer medium  204 . Some degree of light may escape from the transfer medium  204  and be reflected by the inner surface  212  if the inner surface  212  is reflective. Thus, in some embodiments, in which the transfer medium  204  includes for example fiber optics, using a tube that has an inner surface  212  that is light absorbing may be preferable in order to reduce any reflection of the guided light  216  back into the transfer medium  204 , e.g. the optical fibers. 
     The transfer medium  204 , such as for example, the optical fibers may be obtained in very long lengths and with very small bend radii. The bend radius means the minimum radius of curvature at which the optical fibers or the tube may be bent while retaining its intended form, and below which the optical fibers or the tube (e.g., tube  202 ) may become deformed (e.g., kinked or broken). Thus, optical fibers may be quite useful for long bend sensors, or bend sensors used in circumstances where tight bends need to be measured. Optical fibers may add cost to the light guide bend sensor  200 . Use of optical fibers may also lead to an increase in the cost of the photodetector  214  and an associated controller in order to detect with changes in light intensity with sufficient sensitivity. In  FIG. 5 , a controller  526  is depicted and described in the corresponding description. A similar type of controller  526  may be used and/or adapted to electrically connect to and control the various embodiments of the bend sensors depicted in any of the Figures. 
       FIG. 2C  is a schematic block diagram illustrating an end view of the light guide bend sensor of  FIG. 2A  depicted in a straightened disposition. The tube  202  surrounds the transfer medium  204 , e.g., optical fibers, and the guided light  216  is transmitted through the tube  202 . If the inner surface  212  of the tube  202  is reflective, some of guided light  216  may escape the optical fibers and may be reflected back into the fiber. The intensity of the guided light  216  may vary according to the degree of bending or the angle of curvature of the bent tube. The photodetector  214  may need to be more sensitive, and/or a detection algorithm associated with the photodetector  214  may need to be somewhat sophisticated in order to detect subtle changes in the guided light  216  as it passes through the bent or straightened transfer medium  204  that includes fiber optics. 
     In fiber optic type light guide bend sensors, there is typically no aperture or open direct light path between the first end portion  208  and the second end portion  210  of the tube  202  because the inner diameter of tube  202  is filled with optical fiber or another transfer medium  204  that is solid and optically transmissive. As used herein aperture refers to a cross-sectional area of a line of sight direct light path defined by the inner surface of a tube, e.g., tube  202  in any bent, partially bent, or straightened disposition. Since optical fiber is typically inherently refractive and internally reflective, guided light  216  is not considered direct light (e.g. line-of-sight) within the meaning of the term direct light as used within this application. 
       FIG. 3A  is a schematic block diagram illustrating one embodiment of a fluid-filled light guide bend sensor having a reflective inner surface and depicted in a straightened disposition. The tube  302  may be substantially hollow and filled with a fluid, such as for example air, an inert case, water, silicone oil, or any transfer medium  304  that is a fluid, so that a direct light path through the aperture  321  is defined by an inner surface  312  of the tube  302 . For purposes of this application, “direct light” means light traveling along a straight line-of-sight path substantially without reflection, for example, from a light source  306  disposed at a first end portion  308  of the tube  302  to a photodetector  314  disposed at a second end portion  310  of the tube  302 . 
     Although direct light may be transmitted from the light source  306  to the photodetector  314 , a significant amount of reflected light  318  may also be transmitted to the photodetector  314 . Accordingly, the light guide bend sensor  300  may not be regarded as a direct light bend sensor since in addition to the direct light  316  reaching the photodetector  314 , a non-negligible amount of the reflected light  318  may reach photodetector  314 , especially when the tube  302  is in a bent disposition such as will be discussed with respect to  FIGS. 3B-3C . 
     The light source  306  (and similar light sources e.g.,  406 ,  506 ,  706 , and  806 , which are illustrated other Figures and described below) may include any light emitting element or combination of elements such as light emitting diodes, laser diodes, radioluminescent light sources such as tritium tubes, electroluminescent light sources, incandescent bulbs, gas discharge bulbs, and the like. In some embodiments, the light emitting element(s) of the light source  306  emit(s) visible light. In some embodiments, one advantage of visible light is that an assembler or other person working with the light guide bend sensor  300  (or with a direct light bend sensor  400 ,  501 ,  701 , or  801 ) can readily perceive whether the light source is on or off if the light emitting element is not completely enclosed within the tube, e.g., tube  302 . Moreover, some types of photo detectors are more readily available and visible light wavelengths, such as for example, cadmium sulfide photocells. 
     In other embodiments, the light emitting element emits infrared light. Photodetectors such as photodiodes and photo transistors have excellent performance to cost ratios and very good sensitivity as well as excellent linearity in the infrared wavelengths as well as in the visible wavelengths. Accordingly, one of ordinary skill may choose light emitting diodes that emit electromagnetic radiation in the range from about 240 nm to about 940 nm. In some embodiments, the light emitting elements may include emitting diodes (LEDs) that are relatively inexpensive and readily available and may emit light in any selected wavelength from deep ultraviolet LEDs with wavelengths centered at about 240 nm to infrared LEDs with wavelength centered at about 940 nm. 
     The photodetector  314  (and similar photodetectors e.g.,  414 ,  514 ,  714 , and/or  814  which are illustrated other Figures and described below) may include any photo-detecting or light-sensing elements or combination of elements including photoresistors, photodiodes, phototransistors, cadmium sulfide cells, photovoltaic cells, and the like. The wavelength of light detected by the photodetector  314  may generally correspond with, or overlap with, the wavelength of light emitted by the light source  306 . 
     The tube  302  (and similar tubes e.g.,  402 ,  502 ,  702 , and/or  802 , which are illustrated other Figures and described below) may be air-filled or may be filled with another transfer medium  304  that is a fluid, such as for example, inert gases, water, silken oil, or other liquids. Air as a transfer medium has a very low index of refraction. An air-filled tube may also be easier and less costly to manufacture than a tube that includes a transfer medium  304  other than air. In some embodiments, the tube  302  may be substantially sealed such that the transfer medium  304  does not go into or out of the tube  302  during use. 
     In other embodiments, the tube  302  may be unsealed such if the transfer medium  304  is a fluid (liquid or gas), the fluid may go into or out of the tube  302  during use. Whether a tube is sealed or unsealed may also have an effect on the reaction of the tube  302  to a bending force applied to the tube  302 . For example, if the tube  302  is a sealed tube filled with a transfer medium  304  that is a liquid fluid, the tube  302  may be more elastically flexible in response to a force being applied to the tube  302  and then released. 
     In some embodiments, the inner surface  312  of the tube  302  may be reflective as depicted in  FIGS. 3A-3C . In embodiments in which the inner surface  312  is reflective, the reflected light  318  may be transmitted from the light source  306  to the photodetector  314 , in addition to the direct light  316  being transmitted from the light source  306  to the photodetector  314 . The reflectivity of the inner surface  312  may be wavelength dependent. 
     In other embodiments, the inner surface  312  of the tube  302  (and similar inner surfaces e.g.,  412 ,  512 ,  712 , and/or  812 , which are illustrated other Figures and described below) may be light absorbing. As used herein, the term light absorbing means that the inner surface e.g., inner surface  312 , absorbs more light than it reflects for a given wavelength of light emitted from a light source, such as for example, light source  306 . In some embodiments, the inner surface  312  is made of a light absorbing material such as for example a flat black latex, or flat black rubber. In other embodiments, a material such as polyimide e.g., Kapton®, is light absorbing because it absorbs more light than it reflects for a given wavelength of light emitted from the light source  306 . 
       FIG. 3B  is a schematic block diagram illustrating the fluid-filled light guide bend sensor of  FIG. 3A  depicted in a bent disposition. The direct light path (line of sight) may be blocked by an apex  315  of the arc formed at the bend of inner surface  312 . Thus, an aperture  321  may be formed by the inner surface  312  that defines a direct line-of-sight light path, from the light source  306  at first end portion  308  of tube  302  is substantially unblocked by the apex  315  of the inner surface  312 . The aperture  321  or the direct line-of-sight path traveled by the direct light  316  is thus referred to as a bend-dependent direct light path. Similarly, the light path for direct light  416 ,  516 , and/or  716  may also be referred to as bend-dependent direct light paths. 
     Moreover, although the degree of curvature of tube  302  depicted in  FIG. 3B  is readily apparent for purposes of illustration, a much lower degree of curvature may change the aperture  321  by partially blocking direct light in such a way that even a slight degree of bending is detectable. If the radius of curvature is very large (e.g. an ideally fully straightened tube has an infinite radius of curvature), a small change in curvature may result in a small change the size of the aperture  321  and consequently a small change in light intensity, which, nevertheless may be detectable. 
     The reflected light  318  may be reflected off various points of the inner surface  312  to reach the photodetector  314 . Although the reflected light  318  may have somewhat less intensity than direct light  316 , the degree of bending may be detected by measuring the intensity of light received by photodetector  314 . In some embodiments, this measuring may be facilitated by sensitive or sophisticated circuits and algorithms for detecting changes in light intensity associated with bending of tube  302 , as may be the case for detecting light refracted through transfer media such as optical fiber. 
       FIG. 3C  is a schematic block diagram illustrating an end view of the fluid-filled light guide bend sensor of  FIG. 3A  depicted in a straightened disposition, from the perspective of the photodetector  314  with a direct light  316  shining through the center of tube  302  with minimal refraction. The direct light  316  may be distinguished from the reflected light  318  or refracted light. In embodiments in which inner surface  312  is reflective, the reflected light  318  may be reflected off of the inner surface  312  so as to also reach photodetector  314 . 
     The aperture  321  refers to the line-of-sight opening from a first end portion of tube  302  to a second end portion of tube  302 . The paths of direct light  316 ,  416 ,  516 , and  716 , and the apertures  321 ,  421 , are similarly bend-dependent. In some embodiments, the tube  302  may be in a straightened disposition or alternatively in a substantially bent disposition and the difference in the intensity of light that reaches photodetector  314  in the two dispositions will vary less, due to the reflectivity of inner surface  312 , than it would if inner surface  312  had a light absorbing surface. 
       FIG. 4A  is a schematic block diagram illustrating one embodiment of a direct light bend sensor  400  (fluid-filled) that has an inner surface  412  that is light absorbing. The direct light bend sensor  400  is depicted in a straightened disposition with a direct light path the aperture  421  (as shown in  FIG. 4B  below) unblocked such that direct light  416  from light source  406  is substantially unblocked as it passes through tube  402  to photodetector  414 . 
     In one embodiment, the direct light bend sensor  400  includes a tube  402  that is elastically flexible, made of a darkening material, and has an inner surface  412  that is primarily light absorbing or substantially nonreflective. In the embodiment, direct light bend sensor  400  may include a light source  406  disposed in a first end portion of the tube  402  and a photodetector  414  disposed at a second end portion of the tube  402 , such that the tube  402  includes a bend-dependent direct light path from the light source  406  to the photodetector  414  through an interior of the tube  402 . 
     In some embodiments, the tube  402  has a straightened shape in response to the absence of one or more mechanical bending forces being applied to the tube  402 . In other words, in the embodiments, when the tube  402  at least partially bent in response to a force being applied to at least a portion of the tube  402 , the tube  402  returns back to a straightened shape in response to the absence of mechanical bending forces being applied to the tube  402 . 
     In some embodiments, the tube  402  may be hollow and may be fluid-filled. As used herein, the word hollow as applied to a tube refers to the fact that the tube defines a channel that may be filled with something such as for example a gas or liquid. In some embodiments, the tube  402  may be filled with a transfer medium  404  that is an optically transmissive fluid such as for example an inert gas, air, water, and/or another fluid such as for example silicone oil. 
     In some embodiments, the transfer medium  404  may have a relatively low refractive index, such as for example, in the range of about 1.0003 to about 1.001. In other embodiments, the transfer medium  404  may have a moderately low refractive index in the range of about 1.35 to about 1.40. In some embodiments, a particular type of optically transmissive fluid which acts as the transfer medium  404  may have a predetermined refractive index appropriate for a particular type of light source  406  and a particular type of photodetector  414 . In some embodiments, the optically transmissive fluid that comprises transfer medium  404  may further have characteristics that are adapted to a particular application. For example, in an application for measuring flow of a liquid, such as for example water, the transfer medium  404  may also be water so as to minimize possible effects caused by fluid going into or out of the tube  402 . 
     In some embodiments, the tube  402  may be made of a darkening material (as may tubes  302 ,  502 ,  702 , and/or  802 ), meaning not transparent and not primarily translucent to ambient light, so as to minimize the effect that ambient light may have on the amount of light detected by the photodetector  414  for a given degree of bending or unbending of the tube  402 . In some embodiments, being made of a darkening material may refer to being made of a material or a combination of materials that is somewhat translucent but that is sufficiently darkening so as to reduce the amount of ambient light that may potentially interfere with or bias the measurement made of the degree of bending which is determined from the intensity of the direct light  416  emitted from the light source  406  that is detected by the photodetector  414 . In some embodiments, the darkening material effectively increases the signal to noise ratio of the light intensity emitted from the light source  406  that is detected by the photodetector  414  during bending or straightening of the tube  402 . 
     In some embodiments, a darkening material may be a dark or dark-colored material as defined herein. In other embodiments, a darkening material may be a light colored material such as white or amber which at least partially diminishes the intensity of a light exiting a surface of an object made from the material relative to the intensity of the light entering an opposite surface. 
     In some embodiments, some embodiments, as described above with respect to  FIG. 3A-3C , the tube  402  may be substantially hollow and filled with air or with another optically transmissive fluid. In some embodiments, the inner surface  412  may be light absorbing, meaning that the intensity of light which the photodetector  414  may receive or not receive is predominately direct light  416  whether the tube  402  is in a straightened disposition or bent disposition. One example of an inner surface  412  that is light absorbing may be a black surface that is substantially light absorptive. The inner surface  412  may be a layer within the tube  402 , or the tube  402  may be constructed of light absorptive material, for example, black latex rubber, or other dark-colored polymers or elastomers. As indicated, certain embodiments are directed to “dark-colored” materials. As used herein, “dark” or “dark-colored” refers to materials that are black as well as materials having a color approaching black in hue, including, for example, dark grey, dark blue, dark green, dark brown, and the like. As used herein, “black” includes all dark, optically black colors. The term “optically black” refers herein to a material which appears black and at least partially opaque on visual inspection. In certain embodiments, the dark-colored coating compositions of the present invention are optically black. In embodiments in which inner surface  412  is light absorbing, the light  420  is not generally reflected toward photodetector  414 . 
     In some embodiments, the light source  406  may use one or more light emitting technologies. For example, in some embodiments, the light source  406  may include light emitting diodes, laser emitting diodes, incandescent bulbs, gas discharge light sources, electroluminescent light sources, radio luminescent light sources, gas discharge devices, and the like. In some embodiments, the light source  406  may be self-contained and/or self-powered, such as for example, a radioluminescent light source such as tritium ( 3 H) that emits light. Moreover, in some embodiments, the light source  406  may be powered by an external power source such as a battery or another direct current power source or an alternating current power source, for example, to power a gas discharge light source such as a neon bulb. 
     In some embodiments, the photodetector  414  may be a photo resistor, a cadmium sulfide cell, a photodiode, a phototransistor, a solar cell, or any type of photosensitive detector. In the embodiment of  FIG. 4A  and in the other embodiments depicted in other figures, light source  406  (or similar light sources depicted in other figures) is disposed at one end of the tube  402  and the photodetector  414  is disposed at an opposite end of the tube  402 . In other embodiments, the respective positions of the light source  406  and the photodetector  414  may be swapped. 
     Moreover, in some embodiments, both the light source  406  and the photodetector  414  may be disposed at the same end of the tube  402  and a reflective element, such as for example, a mirror (not shown) may be disposed at the opposite end of the tube  402 . In such embodiments, the light  420  that is reflected from the mirror is regarded as a direct light  416  rather than being regarded as a light that is reflected because the path from the light source  406  to the photodetector  414  is a direct path (line of sight) such that bending of the tube  402  may affect the intensity of the direct light  416  significantly more than the fact that it is being reflected at a nearly orthogonal angle off of a reflective element such as a mirror. 
       FIG. 4B  is a schematic block diagram illustrating an end view from the perspective of the photodetector  414  of the direct light bend sensor  400  (fluid-filled) of  FIG. 4A  in a straightened disposition with the direct light path unblocked. The direct light  416  from the light source  406  is transmitted through the tube  402 . In some embodiments, the inner surface  412  may have an inner surface  412  that is light absorbing and does not reflect the light  420  toward the photodetector  414 . In some embodiments, the inner diameter of the tube  402  may be the same at both ends. In  FIG. 4B , the shading of the inner surface  412  illustrates that the direct light  416  shining through aperture  421  is direct light and thus, is not being reflected at an obtuse angle off of the inner surface  412 . 
       FIG. 4C  is a schematic block diagram illustrating a side view of the direct light bend sensor  400  (fluid-filled) of  FIG. 4A  in a bent disposition with a direct light path partially blocked. In the embodiment, the tube  402  is depicted in and at least partially bent disposition such that at least a portion of the direct light  416  is blocked by the apex  15  of the convex arc formed in the inner surface of tube  402  as it is bent. Another portion of the direct light  416  from the light source  406  is transmitted directly to photodetector  414  in this disposition because the bending of the tube  402  merely partially blocks the direct light  416  from the light source  406 . 
     In some embodiments, the inner surface  412  is light absorbing e.g. black, absorptive, or otherwise light absorbing. Accordingly, the intensity of the direct light  416  decreases substantially as the tube is bent from a straightened position to a partially blocking or a substantially blocking bent position. Light other than the direct light  416  such as light  420  strikes the inner surface  412  which is light absorbing and thus light  420  generally does not reach photodetector  414 . 
     The curvature of the tube  402  in a substantially bent position may be much less than depicted in  FIGS. 4C-4F  or as illustrated in any of the other embodiments depicted in  FIGS. 5, 7B, 8, 9A-9C, 10 , and/or  11 . The longer the tube  402  is (for its given diameter), the less bending may be needed to effect a noticeable change in amount of direct light  416  that travels in a straight path through the aperture  421  to the photodetector  414 . 
       FIG. 4D  is a schematic block diagram illustrating an end view of the fluid-filled direct light bend sensor of  FIG. 4A  in a bent disposition with the direct light path partially blocked. In a substantially bent disposition, the direct light  416  is partially blocked by apex  415  as illustrated. Thus, even small changes in the degree of bending may result in significant changes in the intensity of the direct light transmitted from light source  406  to photodetector  414 . 
     In some embodiments, the tube  402  may be comprised of material that is only slightly or merely moderately bendable. For example, the tube  402  (or similar tubes depicted in other Figures) may be made of nylon or polyvinyl chloride (PVC), which are significantly less bendable than latex rubber. Measurements can be made in embodiments in which tube  402  is merely slightly to moderately bendable because the length and the non-reflectivity of inner surface  412  facilitate a measurable change in light intensity. Even though as tube  402  uses stiffer, less bendable materials, the aperture  421  may not become very small compared with the inner diameter of tube  402 . Thus, referring to tube  402  as substantially bent may refer to the fact that the tube  402  appears visibly bent to a human observer and it may also refer to the fact that the tube  402  is at least sufficiently bent such that a measurable difference in amount of the direct light  416  may be detected by the photodetector  414  in response to the tube being in a substantially bent position or alternatively in straightened position. The same maybe similarly noted of other embodiments of tubes depicted in the other Figures. 
     As previously described above, the resistive bend sensor  100  depicted in  FIG. 1  may exhibit signal hysteresis as the sensor is straightened or time drift hysteresis. In other words, there may be a delay for a signal based on the resistance of the resistive bend sensor  100  to decrease as the resistive bend sensor  100  is being straightened or there may be a cumulative drift in signal over time. In some embodiments, the direct light bend sensor  400  exhibits negligible hysteresis. The direct light  416  travels at the speed of light from the light source  406  to the photodetector  414 . Blocking of the direct light  416  by the apex  415  is immediately responsive to an increase in bending and/or straightening of the tube  402 . Similarly, unblocking of the direct light  416  is immediately responsive to a decrease in bending of tube  402 . Thus, direct light bend sensor  400 , or those depicted or described in any of Figures may be especially suitable for applications where quick responsiveness is preferred. 
       FIG. 4E  is a schematic block diagram illustrating a side view of the direct light bend sensor  400  (fluid-filled) of  FIG. 4A  in a further bent disposition with the direct light path blocked. In the embodiment illustrated, the direct light  416  from the light source  406  is substantially blocked by the apex  415  of the convex arc formed in the inner surface  412  of the tube  402  and thus is not direct light transmitted to the photodetector  414  because there is no line-of-sight aperture for a direct light path through the tube  402 . 
     In some embodiments, the light  420  also fails to reach the photodetector  414  because it is not reflected off of inner surface  412  of the tube  402  which is primarily light absorbing rather than primarily light reflecting. Thus, the direct light  416  is substantially blocked by apex  415 , and a significant difference in the intensity of light that may be detected by the photodetector  414  as the tube  402  bends from an angle within a partially blocked range of bending to any angle within a substantially blocked range of bending. 
       FIG. 4F  is a schematic block diagram illustrating an end view of the fluid-filled direct light bend sensor of  FIG. 4A  in a further bent disposition with the direct light path blocked. In the embodiment, the direct light  416  is depicted as substantially blocked by the apex  415  of the inner surface  412  of the tube  402 . Thus, there is no aperture shown in  FIG. 4F  for the direct light  416  to reach the photodetector  414 . 
       FIG. 5  is a schematic block diagram illustrating one embodiment of system  500  that includes a direct light bend sensor  501  connected to a controller  526  and an output module  528 . In some embodiments, the direct light bend sensor  501  includes a light source  506  disposed in a first end portion  508  of the tube  502  and a photodetector  514  disposed in a second end portion  510  that may be opposite the first end portion  508 . In some embodiments, the tube  502  may be a fluid-filled tube that is flexible, made of a darkening material, and light absorbing as described with respect to tube  402 ,  702 , and/or  802 . In some embodiments, the light source  506  may be in electrical communication through a connection  536  with the controller  526 . In some embodiments, the connection  536  may include power and signal connections. In some embodiments, the light source  506  may be self-powered, in which case, the connection  536  may be optional. The connection  536  may be unidirectional or bidirectional depending upon the type of the light source  506  and type of controller  526  used. 
     The connection  536  and another connection  534  may be routed externally to the tube  502  or internally through the tube  502  from either end. In some embodiments, the diameter of thin internally routed wires comprising the connections  534  and  536  may be small compared to an aperture or direct light path through tube  502  and the wires may be small gauge insulated wires for example. For sensitive applications, some embodiments may route the connections  534  and  536  outside of the inner surface  512  so as to avoid potential issues with blockage or interference caused by the connections  534  and/or  536  in the light path from the light source  506  to the photodetector  514 . 
     In some embodiments, the system  500  includes a direct light bend sensor  501  that may function as a flexible photo potentiometer, that is a variable resistor having resistance that varies with the degree of bending. Accordingly, the controller  526  may use any means known to one of skill in the art for measuring variations in resistance. In some embodiments, the photodetector  514  may be a photoresistor (also known as a light dependent resistor). The resistance of a photoresistor decreases with increasing incident light. Conversely, the resistance of a photoresistor increases with decreasing incident light. Thus, the photodetector  514  may be used in a voltage divider circuit with a fixed resistor. The fixed resistor may be connected between a voltage source and an input of the controller and the photodetector  514  may be connected between the input of the controller and a ground. 
     In some embodiments, current flowing to an input of the controller  526  increases as the tube  502  is bent, because the resistance of the photodetector  514  measured by the controller  526  increases as direct light  516  is increasingly blocked by the apex  515  as the tube  502  is bent, while the resistance of the fixed resistor stays the same. Thus, a current measured at an input of the controller  526  increases. An example of this is illustrated in  FIG. 6  (described in more detail below) where the output curve  652  represents the current measured by the controller  526  which increases as the tube  502  is bent. 
     In some embodiments, for example, a simple counter timer circuit with a voltage input may be used to compare a voltage at an output of photodetector  514  with a reference voltage and may provide a pulse frequency that is directly proportional to an increase in resistance or conversely inversely proportional to an increase in resistance depending upon the configuration of the voltage controlled oscillator circuit. 
     Various other means known by a person of skill for measuring output from the photodetector  514  may be used for different types of photodetectors. An output module  528  may be in electrical communication with controller  526  via connection  538 . In some embodiments, the connection  538  may be bidirectional or may be unidirectional and may carry digital or analog signals to and/or from the controller  526 . In some embodiments, the controller  526  converts a measured value have signal from the photodetector to a predetermined unit of measurement and communicates the measured value in the predetermined unit of measurement to the output module  528 . The output module  528  may then display a digital number representing the degree of bend, or may display an analog gauge showing the relative degree of bend pending or may include predetermined limits that enable a user to see whether the degree of bend falls within a specified range or ranges. 
     In some embodiments, the controller  526  and/or the output module  528  may convert signals measured from the photodetector  514  into predetermined units such as Newton meters or foot-pounds to represent a torque measurement. In other embodiments, the controller  526  and/or the output module  528  may convert signals measured from the photodetector  514  into predetermined units such as kilopascals, pounds per square inch, atmospheres, and the like, to represent a pressure measurement. In other embodiments, the controller  526  and/or the output module  528  may convert signals measured from the photodetector  514  into predetermined unit such as cubic feet per second, gallons per minute, or cubic meters per second, and the like for measuring volumetric flow rate of a fluid. The output module  528  may further include a connection  530  that may be connected to other components within a system, such as for example, a computer input. 
       FIG. 6  is a graph  650  of an output signal from a fluid-filled direct light bend sensor. An output curve  652  produced by a controller  526  or alternatively by an output module  528  (or similar elements depicted in the other Figures), in response to bending of a bend sensor e.g.  402 ,  502 ,  702 ,  802 . In one embodiment, as depicted in  FIG. 6 , the output curve  6  represents the value in milliamps of current at various degrees of curvature of a tube, such as the tube  502  of the direct light bend sensor  501 . A reference line  654  (dashed line) illustrates the substantial linearity of the output curve  652  over a wide range of bending of the tube  502 . In some embodiments, the direct light bend sensor  501  operates in a linear portion of the output curve e.g.,  652 . In other embodiments, a non-linear portion of the output curve  652  can also be mapped to degrees of curvature of a direct light bend sensor, e.g.,  502  or similar elements illustrated in the other figures. 
     As depicted in  FIG. 6 , the output curve  652  depicts an input current of about 0.5 mA in response to a curvature of about zero degrees in response to the direct light bend sensor  501  being in a straightened disposition. As the angle of bending of the tube  502  increases from 0 degrees to approximately 50 degrees and the aperture of the tube  702  decreases as direct light  516  is increasingly blocked by the apex  515  of the arc formed in the inner surface  512  of the tube  502 , the output curve  652  increases substantially linearly. 
     As the degree of bending of the tube  502  passes from the partially blocked range to the substantially blocked range, e.g. beyond 60 degrees in this example, the output curve  652 , if continued, would approach a substantially constant value as the direct light  516  from the light source is substantially blocked by the apex  515  (or similar apex as depicted in the other Figures) and continues to be substantially blocked by the apex  515  as the degree of bending of the tube  502  continues to increase. 
       FIG. 7A  is a schematic block diagram illustrating one embodiment of a direct light bend sensor  700  (fluid-filled) with a flared portion  748 . In addition to light source  706  and photodetector  714 , some embodiments may include an indicator  754 , that may be, for example, an LED that is on when direct light bend sensor  700  is powered or in operation. The indicator  754  may also be used to communicate a status, for example, a failure state. 
     A dimension  742  illustrates an inner dimension of tube  702 . Another dimension  740  depicts an outer dimension of a body portion  746  of tube  702 . In one embodiment, the direct light bend sensor  700  includes a flared portion  748  in which the dimensions of tube  702  may be suitably determined for a specific application. For example, in one embodiment where the direct light bend sensor  700  is used to sense torque between two surfaces, the length of the body portion  746  (dimension  752 ) is 0.620 inches. The length of flared portion  748 , as illustrated by dimension  750 , is 0.130 inches. Thus, the overall length of the direct light bend sensor  701  in a depicted embodiment is 0.750 inches. The inner dimension of tube  702  (dimension  742 ) in this embodiment is 0.085 inches and outer dimension of the body portion  746  of tube  702  is 0.156 inches. Dimension  742  illustrates the dimensions of aperture  721  of a bend-dependent direct light path for direct light  716  through tube  702  in a straightened disposition. 
     The outer dimension  744  of flared portion  748  of tube  702  in the embodiment depicted in  FIG. 7A  is 0.188 inches. The flared portion  748  may be conveniently used as a retaining feature. In some embodiments, the flared portion some  48  may simply result from choosing a photodetector  714  with a larger diameter than the diameter of light source  706 . Other dimensions may be used in other embodiments according to the materials used and the bend radius of the tube  702 . 
     The bend radius is typically defined as the minimum radius of curvature at which the tube may be bent while retaining its intended form, and below which the tube becomes deformed (e.g. kinked, or broken). The elasticity of the tube is the ability of the tube to be bent or flexed and to elastically stretch and to return to substantially its original shape in response to releasing a bending force that was applied to result in the bending. Stiffer polymers such as nylon and polyvinyl chloride can bend under a mechanical bending force and return to a straightened shaped when the mechanical bending force is no longer applied. Thus, for purposes of this application, they are elastic. 
     In some embodiments, an inner surface  712  of the tube  702  may be light absorbing. For example, the material used in one torque sensor embodiment was black latex rubber. Alternative materials may also be suitable including black nylon, black polyethylene, polypropylene, black polyvinyl chloride (PVC), and silicone. The elasticity of a tube such as tube  302 ,  402 ,  502 ,  702 , and/or  802  causes it to return to a straightened shaped when no bending forces are applied, which provides for reuse of a direct light bend sensor many times with minimal calibration. It also minimizes drift or memory effect that would be seen if the tube  702  did not return to a straightened disposition after bending forces were applied and then released. 
     An advantage of the direct light bend sensor over other types of bend sensors is that it may be less expensive to manufacture and may be easily customized to meet the needs of particular application. Direct light bend sensors may be designed for general application or customized for a particular use by changing the materials and dimensions of the tubing. A ratio of the length from end to end of the tube to the diameter of the tube of greater than 4 to 1 to less than 10 to 1 may facilitate repeated use of the sensor with minimal deformation or bending, and may provide greater stability. 
       FIG. 7B  is a schematic block diagram illustrating the direct light bend sensor  700  (fluid-filled) of  FIG. 7A  coupled to a first object  760  using a cable clamp  756  and configured to measure force applied by a second object  762 . In some embodiments, the direct light bend sensor  701  may be mechanically coupled to first object  760 , such as by the cable clamp  756  which may be coupled or affixed to the first object  760 . The cable clamp  756  or any coupling device or fastener may be coupled to tube  702  at second end portion  710  or somewhere in the middle such that first end portion  708  or second end portion  710  of the tube may be bent as bending forces are applied to the tube. Light source  706  may be disposed at either end of tube  702  and photodetector  714  may be disposed at the opposite end. 
     Any chosen method of coupling of direct light bend sensor  700  to the first object  760  may be utilized, e.g. chemical, such as glue, or mechanical such as a clamp or fastener. In one embodiment, movement of a second object  762  a distance relative to the first object  760  may cause direct light bend sensor  700  to bend, as force from the movement of the second object  762  is applied to a first end portion  708  of the direct light bend sensor  700 . As depicted, the first end portion  708  may be moved by movement of the second object  762 . This movement and subsequent bending of direct light bend sensor  700  may occur in response to mechanical contact between the second object  762  and the first end portion  708  of the direct light bend sensor  700  causing the tube  702  to bend. 
     Even a small movement between the first object  760  and the second object  762  may cause direct light bend sensor  700  to become substantially bent. Thus, forces that might not easily be visible to a human observer may be measured by observing the response of direct light bend sensor  700  as it is caused to bend by the forces between the first object  760  and the second object  762 . As with light guide bend sensor  300  and direct light bend sensors  400 ,  501 ,  701 , and/or  801 , the light path of direct light  716  is bend-dependent and the size of aperture decreases with increased bending until the direct light path is substantially blocked, in other words, the aperture of the bend-dependent direct light path for direct light  716  is substantially non-existent. In many embodiments, successful measurements can be made in a linear range without ever approaching a substantially blocked range of bending. 
     Light source  706  and photodetector  714  may be connected to a controller and an output module, such as output module  528  discussed with  FIG. 5 . Similarly, an output signal may be sent through a connection, such as connection  530  described with respect to  FIG. 5 , to indicate a degree of bending caused by the movement of first object  760  relative to second object  762 . The output module may execute a calibration routine that calibrates the output signal to predetermined degrees of bending, for example, at a straightened disposition and at the point where the sensor is bent and well within a partially blocked range. The calibration routine may also convert the output into any selected unit of measurement. For example, in a torque sensor application, a unit of measurement may be foot-pounds or newton-meters. 
       FIG. 8  is a schematic block diagram illustrating one embodiment of a system  800  that includes a direct light bend sensor  801  (fluid-filled) configured to directly measure flow. The direct light bend sensor  801  (fluid-filled) is coupled to a portion of a structure  810  that defines a channel for fluid flow  808 . The system  800  further includes a controller  826  that connects to the direct light bend sensor  801  (fluid-filled). In some embodiments, the controller  812  provides power and/or control signals to/from the fluid-filled direct light bend sensor  811 , for example at an end of the fluid-filled direct light bend sensor  811  that includes a light source  806 . In some embodiments, the system  800  further includes an output module  828  that connects to the fluid-filled direct light bend sensor  811 , for example at an end of the fluid-filled direct light bend sensor  811  that includes a photodetector  814 . 
     In some embodiments, the output module  828  displays a value indicative of a measurement of fluid flow  808  in a structure  810  (channel) based on a signal from the photodetector  814 . In response to the fluid flow  808  applying a force to at least a portion of the fluid-filled direct light bend sensor  811 , the at least a portion of the fluid-filled direct light bend sensor  811  is bent so as to form an apex  815  that at least partially blocks direct light from the light source  806  to the photodetector  814 . 
     It may be noted that the portion of the tube  802  of the fluid-filled direct light bend sensor  811  that is facing against the force being applied by the fluid flow  808  may, and some embodiments, bend to a greater degree than another portion of the tube  802  is facing away from the force being applied by the fluid flow  808 . Thus, in some embodiments, the tube  802  of the direct light bend sensor  801  (fluid-filled) may be bent and straightened symmetrically or asymmetrically. In response to a lesser force being applied by the fluid flow  808  to the tube  802 , the tube  802  becomes less bent and the amount of direct light transmitted from the light source  806  to the photodetector  814  increases. Thus, the system  800  may be configured to measure a first force that is directly applied to at least a portion of the tube of the direct light bend sensor in response to movement or flow of a fluid. 
       FIG. 9A  is a schematic block diagram illustrating one embodiment of a system  900  that includes a direct light bend sensor  801  (fluid-filled) configured to measure fluid flow  808  within a channel defined by a structure  810  using an articulating arm  818 .  FIG. 9B  is a schematic block diagram illustrating a top view of the system  900  of  FIG. 9A . The system  900  includes a direct light bend sensor  801  (fluid-filled) that may be coupled to an adapter  816 , for example using a fillet of glue  817 . 
     In some embodiments, the system  900  may include a direct light bend sensor  801  (fluid-filled), a controller  826 , and an output module  828  as described above with respect to  FIG. 8 , except that instead of the fluid flow  808  directly applying a first force to the tube  802  of the direct light bend sensor  801  (fluid-filled), the fluid flow  808  instead applies a first force to an articulating arm  818  that passes through the adapter  816 . 
     In some embodiments, the adapter  816  may include a flexible sealant  823  that surrounds the articulating arm  818  and in response to a first force caused by the fluid flow  808  being applied to a first end of an object, e.g. the articulating arm  818 , causes the articulating arm  818  to apply a second force  827  that causes at least a portion of the tube  802  to be bent, e.g., by a pivotal movement of the articulating arm  818 . Similarly, in response to the fluid flow  808  applying a decreased force to the articulating arm  818 , the second force  827  being applied to the at least a portion of the tube  802  is also decreased, causing at least a portion of the tube  802  to be straightened. In some embodiments, the adapter  816  may include a pin  821  that is coupled to the adapter  816  and about which the articulating arm  818  may pivot. 
       FIG. 9B  is a schematic block diagram illustrating a top view of the system  900  of  FIG. 9A . The system  900  operates as described above with respect to  FIG. 9A  which is a side view. 
       FIG. 9C  is a schematic block diagram illustrating a top view of the fluid-filled direct light bend sensor of the system  900  of  FIG. 9A  configured with a calibration screw. The structure and operation of the system  900  may be substantially the same as described above with respect to  FIG. 12B  except that the embodiment of the system  900  illustrated in  FIG. 9C  further includes a calibration screw  825  that is adjustable. In some embodiments, the calibration screw  825  may be screwed in or out to apply a fixed amount of pressure to the articulating arm  818 . 
     By applying a fixed amount of pressure to the articulating arm  818 , a first reference value may be established at a first position of the second object e.g. the articulating arm  818  relative to the first object e.g. the adapter  816  based on the measurement of the signal from the photodetector  914  in response to the second object e.g. the articulating arm  818  being in the second position e.g. where the second position causes the tube  802  of the direct light bend sensor  801  (fluid-filled) to be bent or straightened. 
       FIG. 10  is a schematic block diagram illustrating an embodiment of a system  1000  that includes direct light bend sensor  801  (fluid-filled) that is configured to measure pressure applied to a plunger. In some embodiments, the direct light bend sensor  801  (fluid-filled) is coupled to a first object such as adapter  816 . In some embodiments, adapter a 16 may be a threaded adapter that includes a plunger  1018  that may move in response to pressure being applied to the plunger  1018 . For example, as increased pressure is applied to an end of the plunger  1018  farthest from the direct light bend sensor  801  (fluid-filled), the plunger  1018  pushes against and bands a portion of the tube  802  so as to form an apex  815  that at least partially blocks direct light being emitted from a light source  806  towards a photodetector  814 . 
     As with at least some of the other embodiments described above, the system  1000  may include a controller  826  that connects power and/or signals to the direct light bend sensor  801  (fluid-filled), for example, to/from the light source  806 . In some embodiments, an output module  828  may be in connection with the direct light bend sensor  801  (fluid-filled), for example at the photodetector  814 . In some embodiments, the controller  826  and/or the output module  828  may convert and/or display the amount of pressure being applied to the plunger  1018  based on at least a portion of the direct light bend sensor  801  (fluid-filled) being bent or straightened. 
       FIG. 11  is a schematic block diagram illustrating another embodiment of a system that includes fluid-filled direct light bend sensor configured to measure pressure applied to a membrane. In some embodiments, the system  1100  have a similar structure to and may operate substantially similarly to the system  1000  described above with respect to  FIG. 10 , except that in the embodiment of system  1100 , the plunger  1118  may be coupled to a membrane  1120  that moves in response to a force being applied to it, e.g., I increase or decrease of pressure being applied to the membrane  1120 . 
       FIG. 12  depicts a method  1200  of measuring force. The method  1200  includes coupling  1202  a direct light bend sensor to a first object, the direct light bend sensor including a tube that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. The direct light bend sensor further includes a light source disposed in a first end portion of the tube and a photodetector disposed in a second end portion of the tube, wherein an inner portion of the tube defines a bend-dependent direct light path from the light source to the photodetector. It may be noted by one of ordinary skill that within the method  1200 , the direct light bend sensor may be any direct light bend sensor script above e.g.,  400 ,  500 ,  700 ,  800 ,  900 ,  1000 , or  1100 . 
     The method  1200  continues and includes positioning  1204  the direct light bend sensor to be at least partially bent in response to a first force being applied to at least a portion of the tube. For example, in the embodiments described with respect to  FIG. 7B , in response to a first force being applied to the tube  702  by the movement of the second object  762 , depicted in the illustration as a downward movement, a first end portion  708  of the tube  702  is positioned to be at least partially bent (e.g., as shown by the dashed lines). 
     In other embodiments, such as for example as depicted in  FIG. 8 , a first force created by fluid flow  808  may be applied to a center portion of the tube  802 . It may be noted in the illustration of  FIG. 8  that a side of the tube  802  facing against the fluid flow  808  may in some embodiments be bent to a greater degree than an opposite side of the tube  802 . 
     The method  1200  continues and includes measuring  1206  a signal from the photodetector that indicates a magnitude of the first force. For example, as illustrated in  FIGS. 8, 9A-9B, 10 , or  11 , an output module  828  may be used to measure a signal from the photodetector  814  that indicates a magnitude of the first force, e.g., the first force being applied directly or indirectly to at least a portion of the tube  802 . In some embodiments, the output module  828  may be part of a controller  826  while in other embodiments, the output module  828  may be separate from controller  826 . In some embodiments, the output module  828  may indicate a calibrated value in predetermined units for the magnitude of the first force. And the method  1200  ends. 
       FIG. 13  is a schematic flow chart diagram illustrating another embodiment of a method  1300  for measuring a force using a fluid-filled direct light bend sensor. In some embodiments, the method  1300  starts and includes coupling  1302  a direct light bend sensor to a first object. The coupling  1302  to an object may be accomplished by any of the means described above. For example,  FIG. 7B  illustrates an embodiment in which coupling the tube  702  of the bend sensor to the first object  760  is accomplished using a cable clamp  756 . In other embodiments such as depicted in  FIG. 9A-9C, 10 , or  11 , the tube  802  of the direct light bend sensor  801  is coupled to the adapter  816  e.g. using glue  817 . In other embodiments, that direct light bend sensor e.g.  801  may be mechanically coupled to an object, such as for example, by cable clamp  756  shown in  FIG. 7B . 
     In the embodiments of the method  1300 , the direct light bend sensor includes a tube that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. The direct light bend sensor further includes a light source disposed in a first end portion of the tube and a photodetector disposed in a second end portion of the tube, wherein an inner portion of the tube defines a bend-dependent direct light path from the light source to the photodetector. The structure of the direct light bend sensor may be substantially as described above with respect to any of  FIGS. 3A- 11B . 
     The method  1300  continues and includes positioning  1304  the direct light bend sensor to be at least partially bent in response to a first force being applied to at least a portion of the tube. In some embodiments, the act of positioning  1304  the direct light bend sensor to be at least partially displaced by an application of force may be done substantially as described above with respect to  FIGS. 7B, 8, 9A-9C, 10 , or  11 . The method  1300  further includes measuring  1308  a signal from the photodetector that indicates a magnitude of the first force. 
     In some embodiments, the method  1300  enables the direct light bend sensor as described herein may be used in a wide variety of applications. For example, in some embodiments such as illustrated in  FIGS. 8, 9A-9C , the direct light bend sensor may be used in a flow sensor. In such embodiments, the first force may be directly applied to at least a portion of the tube of the direct light bend sensor in response to movement/flow of a fluid. 
     In other embodiments, the first force that causes the tube to bend is applied to at least a portion of the tube of the direct light bend sensor in response to a second force being applied to the second object that is movable relative to the first object. For example, the flow sensor application illustrated in  FIG. 8  illustrates a direct application of a first force whereas the flow sensor application of  FIGS. 9A-9C  illustrate methods and systems in which the first force (the pushing of an articulated arm, a plunger, or a membrane, or combinations thereof) is applied to at least a portion of the tube of the direct light bend sensor in response to a second force being applied to a second object that is movable relative to the first object (e.g., first object  760  in  FIG. 7B , or articulating arm  818  in  FIGS. 9A-9C . 
     In some embodiments, such as illustrated in  FIGS. 10 and 11 , the method  1300  enables the direct light bend sensor to be used as a pressure gauge, such as for example, by using a plunger and or a membrane that applies a force against a tube of the direct light bend sensor causing it to be at least partially bent. 
     In some embodiments, the method  1300  further includes converting a measured value of the signal to a predetermined unit of measurement and communicating the measured value in the predetermined unit of measurement. For example, in the embodiment illustrated in  FIG. 8B , the application of the direct light bend sensor may be a torque measuring application which the predetermined unit of measurements are Newton meters or other standard units of torque as described above with respect to  FIG. 7B . 
     In some embodiments, the method  1300  further includes calibrating  1306  an output of the direct light bend sensor prior to measuring  1308  the signal. For example, in some embodiments, the act of calibrating  1306  may include establishing a first reference value at a first position of the second object relative to the first object based on the measuring of the signal from the photodetector in response to the second object being in the first position. 
     The act of calibrating  1306  further includes establishing a second reference value at a second position of the second object relative to the first object based on the measuring of the signal from the photodetector in response to the second object being in the second position and calibrating an output of the direct light bend sensor based upon the first and second reference values. One of skill in the art may apply any known method of calibration to enhance the ability of the direct light bend sensor to provide accurate measurements. 
     In some embodiments, such as illustrated in  FIG. 9C , the act of calibrating  1306  may include adjusting a calibration mechanism such as for example a calibration screw  825 . By adjusting the calibration screw  825  in or out, a repeatability precise reference point of minimal bending may be established. In other words, a zeroing value or another minimum value may be set for the direct light bend sensor. 
     By way of another example, in  FIG. 7B , positioning a first end portion  708  of the tube  702  so that movement of a distance ‘d’ of second surface  762  as shown causes the first end portion  708  to be displaced by corresponding distance, which in turn causes the tube  702  to bend. The bending of the direct light bend sensor  701  decreases the intensity of direct light  716  as the direct light bend sensor  701  goes from the substantially unblocked range to the partially blocked range. The intensity of light may continue to decrease until the bending of the provided directly light bend sensor causes it to go from the partially blocked range to the blocked range at which point the direct light from the light source in the provided direct light bend sensor is blocked. 
     In some embodiments, a second reference may be established at any curvature or degree of bending of the tube, and typically would fall within the partially blocked range of direct light. Establishing a second reference point at a degree of bending well within the substantially blocked range may result in a higher degree of uncertainty regarding the degree of bending since in the substantially blocked range the output of light varies little due to the fact that the intensity of light varies little within the substantially blocked range. 
     When calibration values and algorithms have been established through the act of calibrating  1306  an output of the direct light bend sensor based on the first and second reference values, a measurement of the degree of bending of the direct light bend sensor caused by the relative motion or forces between the first and second objects may be converted to predetermined output units such as by the output module  528  illustrated in  FIG. 5  or other Figures. 
     In some embodiments, the method  1300  of using the direct light bend sensor may be such that the output varies substantially linearly as the direct light bend sensor bands in response to the second object moving from the first position to the second position as described above with respect to  FIG. 7B . Moreover, in some embodiments, the method  1300  of using the direct light bend sensor may be such that the direct light bend sensor response to unbending with negligible hysteresis, whereas the resistive bend sensors described above with respect to  FIG. 1  may exhibit measurable hysteresis. As used herein hysteresis refers to a tendency on the part of bend sensor such as the resistive bend sensor to exhibit over time a tendency or the signal generated by bending and unbending not to return to the same value for a given degree of bending or unbending, or to do so in a less responsive manner or in a manner that exhibits drift. 
     As further illustrated in  FIG. 13 , the method  1300  may be used iteratively to make multiple measurements through the acts of measuring  1308  a signal, converting  1310  measured signal to desired output units, as described above, communicating  1312  the measured value, and determining  1316  whether any additional measurements are to be made. The method  1300  further includes determining  1314  whether additional measurements are to be made, and if so the method  1300  returns to the act of measuring  1308  the signal and continues, or if no additional measurements are to be made the method  1300  finishes. 
     Referring again to  FIGS. 5, 7A-7B, 8, 9A-9C, 10, 11 , in some embodiments, a system (e.g.,  500 ,  700 ,  900 ,  1000 ,  1100 ) is disclosed that includes a direct light bend sensor (e.g., direct light bend sensor  801 ) that is coupled to a first object (e.g., adapter  816 ). In the embodiments, the direct light bend sensor includes a fluid-filled tube (e.g., tube  802 ) that is elastically flexible, made of a darkening material, and has a light absorbing inner surface. The direct light bend sensor further includes a light source (e.g., light source  806 ) disposed in a first end portion of the fluid-filled tube. In the embodiments, the system further includes a photodetector disposed in a second end portion of the fluid-filled tube and a bend-dependent direct light path from the light source to the photodetector through the fluid-filled tube. 
     In some embodiments, the system further includes a controller (e.g.  526 ,  826 ) that measures changes in a signal from the photodetector in response to a bending of a least a portion of the fluid-filled tube, the bending in response to a force being applied to a second object (e.g., articulating arm  818 , plunger  1018 ,  1118 , membrane  1120 , etc.). 
     In some embodiments, the system (e.g.,  500 ,  700 ,  900 ,  1000 ,  1100 ) further includes an output module (e.g.  528 ,  828 ) that displays in predetermined units a magnitude of the force being applied to the second object. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects merely as illustrative and not restrictive. Although very narrow claims are presented herein, it should be recognized the scope of this invention is much broader than presented by the claims. It is intended that broader claims will be submitted in an application that claims the benefit of priority from this application. Insofar as the description above and the accompanying drawings disclose any additional subject matter that is not within the scope of the claims below, the inventions are not dedicated to the public and the right to file one or more applications to claim such additional inventions is reserved.