Light waveguide sensor for small pulling or pressing forces

A light waveguide sensor for small pulling or pressing forces comprises a primarily coated light waveguide, a coil wound on the light waveguide and composed of an element having a diameter which is smaller than a diameter of the primarily coated light waveguide, a casing surrounding the primarily coated light waveguide and the coil and composed of a fiber reinforced synthetic plastic material with longitudinally extending, pulling resistant fibers embedded in a synthetic plastic matrix, the coil being composed of a plurality of coils arranged in cross-lay relative to one another on the light waveguide and each having a pitch length greater than 2.2 times the diameter of the primarily coated light waveguide.

BACKGROUND OF THE INVENTION 
The present invention relates to a light waveguide sensor for small pulling 
or pressing forces. More particularly, it relates to such a light 
waveguide sensor which has a primarily coated light waveguide, a coil 
wound on the light waveguide, and a casing surrounding the light waveguide 
and the coil. 
Light waveguide sensor of the above mentioned general type are known in the 
art. One of such light waveguide sensors for pulling forces is disclosed 
for example in the German document DE-OS 3,526,966. In this reference, a 
coil wound on the light waveguide is composed of a metal wire, for example 
a steel wire with a thickness of 0.08 mm or a glass fiber, and then a 
pulling-resistant casing is applied around them. The casing is composed of 
a glass fiber reinforced thermoplastic material and is wire-shaped with an 
outer diameter of approximately 2 mm. 
For forming the coils numerous approaches have been taken to the diameter 
and the pitch length and several coils can be wound around the primarily 
coated light waveguide in a parallel or cross-lay arrangement. Various 
materials have been proposed for the casing, for example fiber reinforced 
synthetic plastic resin (Duroplast), such as polyester resin with 
unidirectionally oriented glass fibers, and also thermoplasts. 
This light waveguide sensor for pulling forces is more sensitive than the 
older sensor described in the German reference DE-OS 3,305,234 in which 
the coil of resin-impregnated glass fibers is wound on the light 
waveguide, or in homogenous synthetic plastic layer with addition of 
grainy glass or Corundum powder is applied. The utilization of such a 
sensor is described for monitoring concrete structural works such as a 
prestressed concrete bridge. The light waveguide sensor is located in a 
meander-like bedded prestressing wire of the bridge and the measuring ends 
of the light waveguide are connected with a light-passage testing device 
(damping measuring device) to allow a continuous mechanical monitoring of 
the bridge. 
Such light waveguide sensors can be easily converted from pulling sensors 
to pressing sensors. 
The German document DE-OS 3,628,083 described a ground plate from a beam 
with embedded light waveguide pressure sensors, in which the inhomogenous 
layer between the light waveguide and the casing is formed as a metal wire 
coil. This ground plate is embedded in buildings or environment for object 
protection in the ground. 
The German Pat. Application P 3,809,957.8 discloses a light waveguide 
pressure sensor in which the inhomogenous layer between the light 
waveguide and the casing (protective casing) is formed as the above 
mentioned grainy synthetic plastic layer. The sensor acts as a signal 
generator or for release of protective devices at the forces of 
approximately 1 N. It is used predominantly as contact sensors in the 
safety technique for clamping, contacting or overriding protection. 
In the light waveguide pulling sensor described in the German document 
DE-OS 3,526,966 only the embodiment with one coil is practically 
operative. For a sensor without casing (sensor core) composed of a 
primarily coated multimode lightwave guide with an outer diameter of 
approximately 175 um and gradiant fiver 50/125 and with a wound steel wire 
with a diameter of approximately 90 um the following is true: 
By applying an axially acting force, this sensor is expanded and the steel 
wire is constricted into the light waveguide and therefore imparts light 
damping properties to the light waveguide due to the microbanding effect. 
The damping increase is linear with the expansion as long as the 
microbanding effect takes place. In all circumstances this is the case 
with a expansion of the sensor up to 0.3%. With greater expansions the 
sensor is rearranged so that finally the steel wire is directly clamped 
and now the light wave-guide is wound around it. Later on, in this 
expansion condition the damping of the sensor no longer changes. 
A linearization of the damping is achieved by encasing of the sensor with 
fiber reinforced synthetic plastic material. With suitable selection of 
the parameters for light waveguide, wire and casing, an expansion sensor 
with linear damping-expansion ratio up to over 1.5% expansion is produced. 
Thereby values from 0.2 to 1.5 dB damping per one centimeter expansion are 
achieved. 
The coordination of the parameters of the light waveguide-wire-casing is 
difficult and complicated to reproduce. This makes the manufacture of such 
an expansion sensor expensive, time consuming and complicated. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a light 
waveguide sensor of the above mentioned general type, which avoids the 
disadvantages of the prior art. 
More particularly, it is an object of the present invention to provide such 
a light waveguide sensor which is simple to produce and also has an 
increased measuring sensitivity. 
In keeping with these objects and with others which will become apparent 
hereinafter, one feature of the present invention resides, briefly stated, 
in a light waveguide sensor in which several, preferably two, coils are 
wound on a primarily coated light waveguide in cross-lay and the pitch 
length of each coil is greater than 2.2 times the diameter of the 
primarily coated light waveguide. 
When the light waveguide sensor is designed in accordance with the present 
invention, it eliminates the disadvantages of the prior art and 
significantly increases the sensor sensitivity. The function of the sensor 
is performed solely by the sensor core including the light waveguide and 
two cross-laid coils, and the later encasing with fiber composite material 
no longer changes the optical properties of the sensor and serves 
exclusively for increasing the mechanical stability. 
In accordance with another feature of the present invention, the primarily 
coated multimode light waveguide has an outer diameter between 0.1 and 0.3 
mm preferably 0.15 mm, the two steel wires have a diameter between 0.6 and 
0.12 mm preferably 0.09 mm, and the pitch length of the cross-lay is 
between 8 and 12 mm, preferably 10 mm. 
Still a further feature of the present invention is that the coil pitch 
length corresponds to a pitch length of a gradient fiber, or in other 
words, the double lens focal length of the calimating lens sequence 
simulated by the gradient fiber, with ratio n=3, 4, 5 ...&gt;10. 
A further feature of the present invention is that the pitch lengths of the 
coils can be different from one another. 
For utilization of the light waveguide sensor as expansion sensor in 
concrete construction works, its casing can be composed of a high-strength 
fiber composite material in which unidirectionally oriented glass fibers 
are bonded with polyester resin or it is composed of a thermoplast. 
For utilization of the inventive sensor as contact sensor, its casing can 
be composed of a thermoplast in which a low number or no reinforcing 
fibers are embedded. 
Finally, the outer surface of the casing can be provided with an implant 
structure for improved embedding into a structural element to be 
monitored. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A light waveguide sensor in accordance with the present invention is shown 
as an example in form of a light waveguide sensor provided with a casing 
of fiber composite material, on which two steel wire coils are wound in an 
insert cross-lay manner. 
A light waveguide is identified with reference numeral 1. It can be formed 
as a 50/125 primarily coated light waveguide with an outer diameter for 
example 175 .mu.m. One steel wire is identified with reference numeral 2 
and can have a diameter of 90 .mu.m. The other steel wire 2' is similar to 
the first steel wire and extends so as to intersect the latter. Reference 
numeral 3 identifies the casing of a glass fiber reinforced polyester 
resin. The casing is provided with a structured outer surface 3'. A pitch 
of the coils of the steel wires is identified as S. 
The second wire 2' which is wound on the light waveguide 1 so as to 
intersect the first wire 2 prevents rearrangement of the light waveguide 
in the event of great expansion and fixes it in its original position 
along the longitudinal axis of the sensor in the event of great expansion. 
Both wires can be stranded in one working step with cross-lay our 
counter-lay with identical parameters such as for example pitch lengths, 
pulling forces, etc., around the light waveguide rotating in the center. 
It is also possible however to arrange the second wire after arranging the 
first wire. The casing in accordance with a preferable embodiment is 
composed of a high strength fiber composite material in which 
unidirectionally oriented glass fibers are bonded with polyester resin. 
In such sensor not only the damping-expansion diagram is linearized to 
greater expansion, but also the sensor's sensitivity is considerably 
increased. When the damping-expansion diagram is within the region of 
1.5-2 dB damping per one centimeter expansion, the value up to 5 dB/cm is 
achieved without further mechanical amplifying mechanism. This pertains 
especially for the forces which act from the steel wire-intersection 
points onto the light waveguide. 
If the periodicity of these intersection points is adjusted by respective 
selection of the coil pitch lengths to the so-called pitch length of the 
gradient fiber or in other words the double lens focal length of the 
collimating lens sequence simulated by the gradient fiber, the sensor 
sensitivity increases to the value over 10 dB/cm. In this case, however, 
the pulling force of the steel wires must be reduced so that no excessive 
base damping of the sensor is produced. 
The high transverse pressure sensitivity of this expansion sensor can be 
used for utilizing the sensor as a sensitive contact sensor. When it is 
supplied in the regions where transverse pressures for example by the 
weight of a person occur, then it is easy to exceed an opto-electronically 
predetermined threshold. In this case the sensor produces an adjustable 
contact indicator which reacts in an exceptionally sensitive manner. 
Thresholds arranged in series can be provided for contact selection. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
light waveguide conductor, it is not intended to be limited to the details 
shown, since various modifications and structural changes may be made 
without departing in any way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.