Abstract:
The invention concerns a faseroptic load sensor with a support structure and at least one interference structure shaped like a wire, on which at least one optical fiber is displayed. The optical fiber is cleared through a wireshaped interference structure at intervals from the support structure so that a large number of bent spots occur. An additional pressure load on the optical fiber vertical to the support structure then causes changes in the bending radius of each bend spot.

Description:
FIELD OF THE INVENTION 
     The invention concerns a faseroptic load sensor with a support structure and at least one interference structure on which at least one optical fiber is displayed. 
     BACKGROUND OF THE INVENTION 
     Such a load sensor is, for example, known from DE 33 25 945 A1 and is based on the principle that forces affecting the side of the optical fiber lead to deformations of the corresponding fiber sections. In the case of the known structure, the optical fiber is displayed on a periodically  i.e., cyclically) shaped support structure so that, in the case of a load vertical to the fiber axis, a periodic deformation occurs. When the latter stands in correct proportion to the beat length of two of the modes contained in the optical fibers, the changes in pressure cause corresponding changes in the mode coupling. The known structure is, in other words, strongly dependent on the geometry of the support structure and, ultimately, on its coefficient of expansion in cases of temperature change. Constant interference is also caused by tensile strain, as for example, when such a sensor is laid in a road surface. 
     SUMMARY OF THE INVENTION 
     One of the objectives of the present invention is to create a faseroptic load sensor that is laid, in particular, in the road surface for the detection of vehicles, and which withstands well the mechanical and thermal loads that occur and is able detect the wheel of a vehicle driving over it with the lowest possible error rate. This objective is achieved according to the present invention by an optical load sensor in which an optical fiber is wound or spirally rolled about an elongate, substantially radially incompressible support structure so that the sensor is extensible. The fiber will undergo periodic bending moments along the longitudinal axis of the support structure, whereby light transmitted through the fiber is measurably attenuated by the deformations in the optical fiber attributable to the bending moments when the sensor is in its normal unloaded state, i.e., the optical fiber is already deformed, or pre-stressed, before any load to be measured is applied. Consequently, the sensor is much more sensitive to transverse loads produced by the vehicles traversing the road surface in which the sensor is embedded. Also, the sensor is less sensitive to longitudinal stress whether attributable to changes in ambient temperature of the roadbed or other factors. 
     While known load sensors generally detect the deformation of optical fibers in an essentially undeformed state, in the case of the load sensor according to the invention, a large number of distortions per unit of length are present in a no-load condition, whereby an additional pressure load causes changes in the respective bending radii, which in turn cause changes in the attenuation of the optical fibers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described more closely in the following by using both a schematic representation based on the function principle and two working examples. 
     FIG. 1 shows the schematic construction of a faseroptic load sensor; 
     FIGS. 2 and 3 show two forms of load sensors shaped like a rope; and 
     FIG. 4 shows a section through a road surface with the load sensor laid into it. 
    
    
     DETAILED DESCRIPTION 
     The basic construction of a load sensor according to the invention in accordance with FIG. 1 displays a support structure (1) with a smooth surface, an interference structure (2) shaped like a wire set up on top of it, and an optical fiber (3), which is spaced by the interference structure (2) at intervals from the support structure in such a way that two areas I and II, in which the fiber optic light guide is not supported, arise between two such elevations. The interference structure can also be designed like a bump of a support structure that is the same in all other ways. In the example represented in the diagram, the diameter of the structure shaped like a wire (2) is about twice the size of the diameter of the optical fiber (3): the interval between two neighboring elevations of the optical fibers (3) and the support structure (1) corresponds to about eight times the diameter of the optical fiber (3). As a result of this, the optical fiber is deformed almost up to the physically given limits of its bending radii even in the load sensor&#39;s normal condition. An additional load that affects the surface of the support structure (1) vertically, for example in the form of a pressure load (D), which is symbolized by the broken arrows, will further bend the optical fibers in the areas I and II, whereby these changes in the bending radius cause changes in the intensity of the light contained in the optical fibers. 
     Changes of length caused by temperature or a tensile strain of the support structure (1) will only be transferred to the optical fibers (3) to a small extent so that such interferences influence the sensitivity of the load sensor or the measuring signal only marginally. 
     In the case of the embodiment of a load sensor shaped like a rope represented in FIG. 2, the support structure consists of a plastic core (21), on which a metal wire or a quartz fiber (22) is rolled up in twists with a steep slope as an interference structure. On this support and interference structure shaped as a rope, a first optical fiber (23.1) is wound in a wrapping, also with a steep slope, that is made into an interference structure (22) that works in the opposite direction. Around this rope designed as a support structure (21), interference structure (22), and first fiber optic light guide (23.1), a second optical fiber (23.2), in which the light is fed in the opposite direction to the optical fiber (23.1), is wound in the same direction as the first optical fiber (23.1). The optical fibers (23.1) and (23.2) are connected optically on one end of the rope or formed as a loop of an uninterrupted optical fiber so that the light supply and the evaluation of the signal can occur on the same end of the rope. 
     By winding the interference structure and fiber optic light guides in the opposite direction, they are cleared at periodic intervals, i.e., at each point of intersection of the wrappings of the plastic core (21), which represents the wrapping core. The slope of the wrappings determines the intervals of the elevations. The intersection points are either distributed linearly or helically on or around the plastic core (21), depending on the relationship of the slope of individual wrappings with respect to each other so that the entire load sensor displays an essentially radially symmetric sensitivity to outside sources of pressure. 
     The shrinkdown plastic tubing (24) leads to an initial tension of the optical fiber in the direction of the surface of the core, whose consequence is a decrease in the light transmitted from the fiber. Each additional radial load from outside leads immediately to another further elastic deformation of the optical fiber and thus to a reversible further drop in the light transmitted, which is detected as a measuring signal. 
     The wrapping intervals of the interference structure (2) are at least twice the diameter of the optical fiber (3) or the interference structure (2) depending on which of them is larger. The upper limit for the twisting intervals is relatively uncritical, but should not be exceeded by ten times the above-mentioned diameter. Also uncritical is the twisting interval of the optical fibers (23.1 or 23.2); the radial pressure sensitivity nonetheless improves as the wrapping interval becomes smaller. The diameter of the plastic core (21) is uncritical as well; favorable values lie at around two to ten times that of the diameter of the fiber optic light guide. Furthermore, it has proven beneficial when the interference structure (2) and the optical fiber (3) have about the same diameter. 
     In the working example of a load sensor represented in FIG. 3, two optical fibers (33.1 and 33.2) made of quartz fiber are twisted with one another and wound together around a plastic core (31). In this embodiment, the optical fibers mutually take on the function of the interference structure in accordance with FIG. 2 so that one can do without an interference structure. The two optical fibers (33.1 and 33.2) are optically connected to each other on one end of the load sensor, which, for instance, is also caused by a loop formation and feedback of a single fiber optic light guide. The plastic core (31) consists of a flexible, but hard, material and is enclosed, together with the fiber optic light guides (33.1 and 33.2), by the shrinkdown plastic tubing (34). 
     For the detection of vehicles and airplanes, a load sensor according to the invention, for example in accordance with the embodiments of FIGS. 2 or 3, is inserted in the desired position in the road surface. For a guaranteed, trouble-free detection of a wheel rolling by, embedding a faseroptic load sensor (40), represented in FIG. 4, in a slit (41.1) of the road surface (41) has proven itself with an elastic sealing compound (42) surrounding it. The depth (t) of the slit (41.1) stands in a 3-to-1 ratio to its width (b).