Pressure sensor utilizing microbending of a fiber optic cable woven through a ladder shaped structure

A pressure detector including a fiber optical cable woven through a ladder-like structure which is then encapsulated and surrounded by a cover. Light transmitted through the fiber optic cable is diminished to a value less than a threshold value upon the occurrence of microbending caused by pressure applied at any location along the length thereof. The rungs of the ladder-like structure are sized and spaced to provide a proper locus about which microbending may be produced. In addition, the rungs may be round in cross-section and/or otherwise shaped to enhance the sensitivity of the system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to pressure sensors incorporating a fiber optic 
cable which provides an indication of pressure upon the onset of 
microbending and, more particularly, a pressure sensor incorporating a 
fiber optic cable which is placed in physical contact by interweaving the 
cable through rungs of a ladder-like structure. 
2. Description of the Prior Art 
Techniques for providing pressure detection are known in the art. One such 
technique includes conducting light through a fiber optic cable and 
sensing a reduction in light transmission therethrough as a result of 
deformation (i.e., microbending) of the cable. In addition, several 
techniques exist for enhancing the sensitivity of a fiber optic cable to 
microbending. Among these include the technique of wrapping a relatively 
thinner diameter member in a serpentine fashion about a fiber optic cable. 
Another technique comprises placing small protrusions on the peripheral 
surface of the fiber optic cable itself. Both of these techniques require 
processing the fiber optic cable in a special manner. Thus, an expensive 
secondary operation must be performed on the fiber optic cable to 
accomplish preparation of such pressure sensors. As such, the fiber optic 
cable is relatively expensive and subject to damage during such secondary 
operations. Another approach known in the art for accomplishing 
microbending of fiber optic cable is to place a fiber optic cable in 
affiliation with a separate structural member having a multitude of 
protrusions disposed thereon. Any pressure between the fiber optic cable 
and the protrusions will result in microbending of the fiber optic cable. 
Such a technique requires placing the fiber optic cable in contact with 
the separate structural member. The effectiveness of the interrelationship 
between these two members is substantially impaired if the fiber optic 
cable and the structural member are not properly positioned with respect 
to one another. As such, an investment is required in structure to achieve 
and maintain this affiliation. Therefore, there is a need for a pressure 
sensor that is inexpensive to construct and uses a standard fiber optic 
cable. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is a pressure detector including a fiber optic cable 
woven through a ladder-like structure which is then encapsulated and 
surrounded by a cover. Light transmitted through the fiber optic cable is 
diminished to a value less than a threshold value upon the occurrence of 
microbending caused by pressure applied at any location along the length 
thereof. The rungs of the ladder-like structure are sized and spaced to 
provide a proper locus about which microbending may be produced. In 
addition, the rungs may be round in cross-section and/or otherwise shaped 
to enhance the sensitivity of the system. 
These and other aspects of the present invention will become more readily 
apparent by reference to the following detailed description of the 
embodiments as shown in the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention incorporates a fiber optic cable 10 which is 
interwoven through rungs of a ladder-like structure 12 encapsulated in a 
foam-like material 14 and then surrounded by a sheath 16. In operation, a 
light source LED 18 transmits a known amount of light through fiber optic 
cable 10 which is received at phototransistor 20. Phototransistor 20 
converts the light amplitude received thereby to a current which is 
supplied to a threshold comparator 22 which provides an indication that a 
light threshold has been interrupted in the event of pressure applied to 
the sensor assembly such as shown by arrow 100. Cable 10 is preferably a 
100 micron multi-mode fiber optic cable with a numerical aperture of 0.1 
and a step index between the core and the cladding. The cable may be 
interwoven as shown in FIG. 1 and 2 through sequential rungs 12 of the 
respective ladder structure. Varying levels of sensitivity may be 
accomplished by spacing the rungs at various intervals and/or skipping 
rungs of ladder 12. 
In the preferred embodiment of the present invention the rungs are circular 
in cross section but may be of any desired cross section to enhance 
sensitivity and/or extend the fatigue life of the cable. Encapsulation 16 
provides a protective barrier around cable 10 and ladder structure 12 
preventing mechanical damage thereof. Cover 16 provides a mechanical 
barrier for the entire assembly. In operation, a signal provided by light 
source 18 is conducted through cable 10 and detected by phototransistor 
20. This signal is characterized by a first transmission loss resulting in 
a first magnitude light quantity incident upon said light detector. The 
sensor assembly is calibrated by adjusting a threshold value at threshold 
comparator 22. A pressure 100 applied to fiber optic cable 10 which 
produces microbending thereof will reduce the light transmitted 
therethrough to an amount less than the threshold amount monitored by 
threshold comparator 22. Pressure 100 deforms cable 10 resulting in a 
second magnitude light quantity conducted therethrough as a result of a 
second transmission loss characteristic. In such an instance, a signal 
will be supplied to indicator 24 providing indication of the detection of 
pressure 100 applied to the pressure sensor assembly. Ladder 12 is free to 
move about within cover 16 in response to any pressure transmitted by 
encapsulation 14. Thus, pressure exerted on one side may result in 
microbending of the cable on the opposite side enhancing sensitivity of 
the entire assembly. In another embodiment of the present invention, as 
shown in FIG. 3, the fiber optic cable of the present invention is woven 
in reverse direction to that of the structure previously described (i.e., 
out of phase thereto) so as to enhance the lengthwise sensitivity of the 
sensor assembly. Such may be necessary to assure sensor sensitivity of 
small diameter objects sought to be sensed thereby. In addition, as shown 
in FIG. 4, the rungs of ladder 12 may be sequentially spaced along 
displaced parallel axes. Such a technique will provide a sensor assembly 
having a heightened sensitivity as the radius which the fiber optic cable 
10 must achieve about rungs of ladder 12 results in a structure which is 
more sensitive to microbending. 
One skilled in the art will readily recognize that certain specific details 
shown in the foregoing specification and drawings are exemplary in nature 
and subject to modification without departing from the teachings of the 
disclosure. Various modifications of the invention discussed in the 
foregoing description will become apparent to those skilled in the art. 
All such variations that basically rely on the teachings through which the 
invention has advanced the art are properly considered within the spirit 
and scope of the invention.