Process for conditioning and for placing a traffic sensor

Process for packaging a sensor (1) to be placed in a road (2), characterized in that it consists in covering said sensor (1) with threads (8) of organic or inorganic material extending over the entire length of sensor (1) and in burying the whole in a binder that hardens by polymerization, so as to obtain a bar (7) containing said sensor (1).

BACKGROUND OF THE INVENTION 
This invention relates to a process for conditioning a traffic sensor to be 
placed across a road transversely to to the direction of circulation of 
cars and trucks. 
It applies more particularly to sensors that by their positioning in the 
road must be protected from the static or dynamic load stresses that the 
road undergoes. Among these sensors, the invention finds a very particular 
advantage for sensors requiring an installation over a considerable 
length, in particular the width of the road or the width of a traffic 
band, such as for example sensors for passage of axle assemblies. 
Conventionally, sensors sensitive to the passage of axle assembles are of 
the piezoelectric type, so that they supply data as voltage pulses in an 
electric signal in correspondance with the pressure variations caused by 
the weight of axle assemblies at the passage of vehicles. They consist of 
a conductive core covered with piezoelectric ceramic in a coaxial 
conductive tubular shield and they are connected at one or both ends to 
external circuits for electric supply and treatment of the electric 
signals delivered. 
The most frequent sensors are of the piezoelectric, capacitive, resistive 
or inductive type and are connected by a coaxial electric cable to a 
system for ensuring electric current supply and treating the outlet 
electric signals that reflect the measurements. This electric signal is 
generally delivered to one end of the sensor by means of a coaxial cable 
connected by its core to the core of the sensor, its shielding being at 
the mass or ground voltage. 
Conventionally, the sensors are conditioned for their placement in the road 
in an aluminum U shape profile filled with a mixture of sand and of resin 
wherein they are embedded. The structural self-supporting bar thus 
obtained is then placed in a trench made in the road and also filled with 
a mixture of sand and resin; the sensor, its encapsulating mass and the 
trench extending over the width of the road. 
The drawback of this technique resides in the fact that the bar containing 
the sensor undergoes considerable stresses, particularly shearing stresses 
during passage of vehicles on the road, and in the fact that it breaks 
under the effect of these stresses particularly because of the different 
moduli of elasticity that, on the one hand, the sand and resin mixture 
possesses while sand and resin are in approximately equal proportions and, 
on the other hand, the surfacing or pavement and the foundation layers of 
the road. 
It has been proposed to attempt to absorb these stresses by providing the 
bar laterally with foam cover layers, but this solution is not 
satisfactory, since there exists the need to have a sensor encapsulated or 
embedded so that shows sufficient hardness in relation to the purpose of 
the sensor such as for example dynamic weighing, such hardness being 
necessary so as not to disturb the electric signal issuing from the 
sensor. 
SUMMARY OF THE INVENTION 
The governing idea of this invention is to impart to the sensor some 
flexibility in order to reduce the dangers of breaking the sensor, while 
maintaining a good hardness and mechanical strength for the reliability of 
the measurements, through encapsulating it in an appropriate embedding 
mass either prior to or in situ during its placement into a trench across 
the road. 
To do this and according to its main characteristic, this invention relates 
to a process for conditioning a traffic sensor that comprises at least one 
step of encapsulating said sensor in a strengthening organic mass 
comprising threads of organic or inorganic material extending continuously 
and unidirectionally over the entire length of the sensor and enveloping 
said sensor all around. 
According to a first mode of implementation of the invention, such 
encapsulation of a sensitive element to be placed in a road transversely 
to the traffic path is performed in a separable mold, the process 
comprising enveloping said element all around with threads of organic or 
inorganic material extending continuously and unidirectionally over the 
entire length of the element and in embedding the whole in a binder 
composition that hardens by polymerization, so as to obtain a self 
supporting three-dimensional structural bar detector incorporating said 
sensitive element inside the cured embedding mass. 
The bar thus obtained possesses a certain flexibility thanks to the 
cohesion provided by the threads that extend over the entire length of the 
bar. It is nonetheless hard and resistant due to the effect of the 
polymerization of the binder, which can be selected from this 
characteristic according to criteria known to those skilled in the art. 
The binder can be preferably an epoxy resin. 
Among the organic or inorganic materials that can be used for the threads, 
such as carbon, Kevlar (trade name), glass or a combination of various 
fibers, it will be preferred to use glass threads because, in addition to 
their low cost, they impart on the one hand better flexibility in 
comparison with carbon threads and, on the other hand, they are easier to 
handle in the process compared to Kevlar threads. 
Glass threads thus used for covering the sensor all around inside the 
encapsulating composition impart excellent quality of flexibility and 
mechanical strength to the bar, especially when being disposed without 
weaving or entwining unidirectionally parallel to the sensor, but grouped 
into bundles that are softly assembled together to form individual 
unidirectional sheets of threads. 
In practice, a mold is used that is filled with class threads and in which 
the sensor is positioned, then the binder composition consisting of a 
mixture of epoxy resin and a hardening agent is injected so as to fill all 
voids between the threads or the threads and the sensor, and after curing 
through polymerization, a self supporting detector bar is obtained. 
Care will preferably be taken during that encapsulating operation to make 
sure that the sensor is kept in a position in which it is at constant 
distance over its entire length from a face of the bar which, preferably, 
has a parallelepiped cross section. This keeping in position can, for 
example, be assured by applying axial tension efforts to tighten the 
sensor while it is in the mold, or by placing centering elements at 
intervals along the sensor. Such centering elements can advantageously be 
made of an organic material compatible with the polymerizable binder and 
having a form such that it does refrain the encapsulating composition to 
penetrate and circulate so as to fill in all voids and avoid the 
apparition of air bubbles in the encapsulating mass. 
This positioning is of importance considering the application for which the 
sensor is intended so that once placed in the road, it is approximately 
equidistant from the surface of the road over its entire effective length, 
from over the length where it must be sensitive to supply information. 
Since the sensor should be connected by one of its ends to an electrical 
connection cable, it is advantageously provided, with again an eye to 
avoiding the dangers of breaking, to surround this end with a flexible 
protective sleeve so as not to render the cable or the connection with the 
sensor at the limit of the bar brittle or fragile during placement in the 
road. In case the encapsulated sensor requires electrical connection at 
both its ends, a sleeve surrounding the connection will be provided at 
each end. 
The above process leads to a traffic detector according to the invention 
which is characterized in that it consists of a self-supporting bar 
comprising a rectilinear piezoelectric sensor element centered axially in 
a stratified encapsulating mass which comprises continuous threads 
extending longitudinally over all the length of the sensitive element, 
said threads being tightly embedded with the sensitive element in a cured 
composition comprising a polymerizable binder. 
In comparison with the bars of the prior art of same dimensions, a detector 
bar according to the invention shows a breaking resistance that is 
considerably higher because of the flexibility that is imparted to it. In 
addition, there is no longer any need to resort to an aluminum jacket as 
was necessary in the prior art because of slight cohesiveness of the bar 
over its length, since cohesiveness is now inherent to the bar itself. 
According to a further implementation of the invention, the encapsulating 
step is performed in situ by molding it in a trench accross a road. 
Conditioning of the sensor is thus performed as a process for placing a 
traffic detector in a road. Said detector can either a simple sensitive 
element not yet encapsulated, or a sensor encapsulated into a bar 
according to the prior art, or preferably a bar detector of the invention 
as described above. 
Thus the invention can also be implemented as a process for placing a 
traffic detector as a longitudinally extending sensitive element or bar 
including said element transversely accross a road which due to similar 
use of longitudinally extending not woven threads meets substantially the 
same aims as explained above, namely imparting to the bar once placed a 
certain flexibility to limit the dangers of breaking the sensor while 
maintaining good hardness and mechanical strength for reliability of the 
measurements. 
Such process preferably comprises the steps of: 
making, in the road, a trench with a cross section considerably greater 
than the cross section of said detector; 
preparing in the bottom of the trench, a lower base layer consisting of at 
least one unidirectional sheet of non woven long threads of organic or 
inorganic material in a composition hardenable by polymerization of an 
organic binder and advantageously charged with short fibers of organic or 
inorganic material; 
positioning said detector on said lower base layer by keeping it by means 
of brackets resting on the edges of the trench to prevent it from being 
driven into said base layer during curing of said composition; 
preparing on said bar an upper covering layer similar to said lower base 
layer and ended on top by a finishing layer consisting of said composition 
that is flush with the road; 
letting said composition harden by curing it through polymerization of the 
binder. 
For the placement in the road of the composite bar according to the 
invention, use is therefore also made of threads of organic or inorganic 
material but that are here preferably brought together in sheet to 
facilitate the use of the process which must be performed on the 
installation site of the sensor. Further, this makes it possible to obtain 
a layer of threads extending beyond the sensitive element over the entire 
length of the trench that thus impart to the whole a good mechanical 
strength and a cohesiveness over the entire length of the trench. 
The fact of using a nonwoven unidirectional sheet in which bundles of 
threads are connected by weft insertion, aims at an improved natural bed 
by not creating an obstacle for the pouring of the binder thus making 
possible a better cohesiveness. 
Advantageously, the organic binder used is in a composition charged with 
fibers of organic or inorganic material. As is well known in the field of 
glass products, such fibers are non uniform and short, with length 
generally less than 1 cm, whereas threads are produced as continuous 
material over length of 1 m or more. Furthermore, a fiber charge will have 
rough short fibers irregularly in all directions and crossed to each 
other, whereas the strengthening threads as used in the invention have a 
length of 1 m at least and are extending continuously substantially 
parallel to each other. 
For reasons similar to those explained for the manufacture of the bar, the 
threads of the sheets as well as the fibers in the binder are made out of 
glass prerably to other inorganic material or organic materials. 
According to a particularly advantageously characteristic of the placement 
process according to the invention, said composition consists of a charge 
of glass fiber and sand and a polymerizable binder having an epoxy resin 
base. 
The use of sand in the charge makes it possible to absorb partially the 
heat released by the polymerization of the resin and thus to limit the 
effects of it. Actually, it is desired to control the exothermy during the 
polymerization, on the one hand because of the use of the process in a 
road that can contain foundation layers or a pavement sensitive to 
temperature and, on the other hand to prevent an expansion of the 
structure installed in the trench while would induce a raised area of the 
surface of the road at this very place. 
In this same context, a forced ventilation of the surface of the road is 
preferably set up at the location of the trench to help to remove the heat 
produced by the polymerization. 
To strengthen the performance of the structure installed in the road and 
therefore facilitate the anchoring of this structure in the trench, this 
trench is preferably made in undercut shape of approximately trapezoidal 
form directed toward the top of the road. 
In this same contact, care will advantageously be taken during the making 
of the trench that its bottom does not correspond with an edge between two 
constituent layers of this road, or two foundation layers or the pavement 
and the final foundation layer. 
The structure obtained by the use of the placement process according to the 
invention is particularly suitable for the applications for which it is 
intended while considerably improving the service life of the sensors thus 
installed. 
Preferably, the base and covering layers each comprise at least three 
layers of thread sheet, the maximum number of layers of sheets depending 
on the intensity of the dynamic stresses that the structure is to undergo 
and on the depth of the trench.

DETAILED DESCRIPTION OF THE FIGURES 
The example of embodiment represented in the figures applies to the 
conditioning and to the placement of a long-shaped sensor 1 that must be 
placed transversely in a road 2 such as for example a sensor intended to 
measure the dynamic load supported by the road. 
As represented in FIG. 1, sensor 1 extends therefore over the entire width 
of road 2 or at least over the entire width of a strip of this road on 
which the measurements must be made. Current dimensions are within the 
range from 1.5 to 6 meters, for instance 1.7 or 3.2 meters. 
Sensor 1 is a sensor of a type known in the art consisting for example of a 
metal core 3 covered with ceramic layer 4 (FIG. 2) and is connected at one 
of its ends 5 (FIG. 3) to a cable (6), for example of coaxial type, for 
connection to a computer system using the electric signals to calculate 
measurements. The outer shields are at ground voltage. 
The sensor 1 is conditioned according to the invention in the form of a 
composite bar 7 made by covering the sensor with glass threads 8 extending 
over the entire length and by embedding the whole in a binder that hardens 
by polymerization of an epoxy resin. 
This bar, made for reasons of convenience in a parallelepiped shape, is 
obtain from a mold (not shown) that is filled with glass threads 8, 
generally presented in bundles each containing from 5 to 100, preferably 
from 10 to 50 threads, considering their slight diameter (which is lower 
than 0.5 mm, for instance within the range from 0.1 to 0.5 millimeters). 
Thence it is preferably made use of layers of glass threads known 
commercially as unidirectional sheets wherein a number of such bundles are 
assembled together parallel to each other using some fiber material 
loosely interlaces around the bundles from place to place along them. 
The sensor 1 is positioned in the mold while taking care that over its 
entire length it is equidistant from one of the faces of the mold, more 
particularly from the face which will correspond for the finished bar 7 to 
its top face 9 when placed in the road 2. 
This equidistance can be obtained by maintaining the sensor axially in the 
center of the mold which advantageously makes it possible not to have to 
locate a determined face on the finished bar for it to be placed in the 
road and to make, without risk to the reliability of the measurements, a 
bar operating as a cylindrically sensitive detector. 
In practice in the example described, the sensor has a diameter of 3 mm 
(which could be from 1 to 10 mm in other cases) and the mold is fully 
filled with glass threads around the sensitive element. This is obtained 
by placing a first series of unidirectional sheets, then the sensor, then 
a second series of sheets. The number of sheets in each series is from 10 
to 30, and their bundles place by themselves in alternate position, so 
that it can be observed finally that the sensor is completely surrounded 
with threads. Seen in cross-section, it can be admitted that a least a 
dozen of threads will face each point or the sensor on a thickness 
substantially equal to its diameter. 
In practice also, two or three pieces of an organic material compatible 
with the binder and clasped onto the sensor at different places along it 
will be sufficient to maintain the sensor in place laterally, while, 
vertically, it is supported in the correct place by the a accumulated in 
the mold. 
The end of sensor 1 which is connected to connection cable 6 is sheathed 
with a flexible sleeve 10 for protection of the connection. This sleeve 10 
extends toward cable 6 to protect this cable 6 also over the portion of 
its length that could be contained in bar 7 and in the structure that will 
be made during the placement of bar 7 in road 2. 
Once glass threads 8 and sensor 1 are placed in the mold, the latter is 
closed and injection of the binder with an epoxy resin base is performed. 
Injection can, for example, occur from one of the ends of the mold by 
suction with a vacuum pump at the other and, so as to obtain a uniform 
dense filling penetrating all around threads and sensor and not leaving 
any void or air bubbles. 
After polymerization of the resin and removal of the mold, composite bar 7 
is obtained which, while exhibiting a sufficient hardness by the resin 
used, offers a good resistance to breaking thanks to the flexibility and 
the cohesiveness over its entire length provided by glass threads 8. 
By way of example, for a bar of square cross section 15 mm.times.15 mm with 
a length of 3 m, the breaking threshold obtained is 650 Kg and the flexion 
is not higher than 30 mm while a bar of the previous art exhibits with no 
flexion a breaking threshold at 200 Kg. 
For the placement of bar 7 thus obtained in road 2, a trench 11 over the 
entire width of road 2 is made according to the invention. This trench 11 
can be made by any conventionally used means. However, care will be taken 
to make sure that bottom 12 of trench 11 does not correspond with edge 13 
between pavement 14 and final foundation layer 15 of road 2 or, if the bar 
is placed more deeply, between two foundation layers of the road. 
In this same context, to assure a better performance in the structure 
produced, trench 11 is preferably made in undercut shape, i.e., it 
exhibits a trapezoidal-shaped cross section directed toward the top of 
road 2, as is seen in FIG. 2. The inclination of the walls of the trench 
does not need to be great, it has only the aim of preventing any danger of 
heaving up of the structure produced. This inclination can be replaced 
with irregularities left in the lateral walls of the trench making posible 
an anchoring of the structure when it is hardening. 
A first layer 16 of a previously prepared composition having a base of an 
organic binder that hardens by polymerization of an epoxy resin, and 
charged with sand and glass fibers, is then poured in trench 11. This 
first layer 16 is distributed on bottom 12 with a sufficient thickness to 
make up for the differences of level and of irregularities that come from 
the making of trench 11. 
By glass fibers or small glass fibers are meant sections of slight length 
that can be incorporated into the composition without hampering the 
fluidity that is desired for it to be poured into the trench and to 
closely envelop the glass threads. Their direction is not controled. 
Then by successive layers, nonwoven sheets 17 of glass threads are 
deposited, on each of which a new layer of the composition is poured so as 
to obtain a lower base layer 18. This base layer 18 preferably comprises a 
number of sheets 17 between 3 and 10 depending on the depth of trench 11 
and on the level at which it is desired to place bar 7. 
Sheets 17 of glass threads (FIG. 4) consist of bundles 17a of non-entwined, 
non-twisted and unidirectional threads so that the sheets are buried in 
the composition more easily; the bundles being connected to one another by 
a weft insertion 17b to facilitate their placement. The number of bundles 
17a of each sheet 17 depends on the width of trench 11 and on the cross 
section of the bundles 17a, it is for example between 5 and 15. 
The bar 7 is then positioned in the trench with U-shaped brackets 19 that 
are flush with base layer 18 and that are suspended by resting by edges 
19a that they have on the top of road 2. These brackets have as their 
object to prevent the driving of bar 7 into base layer 18 during the 
hardening of the composition. 
An intermediate layer 20 of the composition is poured to cover bar 7 and an 
upper covering layer 21 is made in a manner similar to lower base layer 
18, i.e., by depositing of successive layers of sheets 17 of glass threads 
on each of which a layer of the composition is poured. In practice, while 
it is not clear from the figures, both layers meet on the sides of the 
bar. The covering layer 21 is ended by a finishing layer 22 of the 
composition that is flush with the top of road 2. 
Once the structure thus made is hardened, i.e., at the end of the 
polymerization of the resin contained in the composition, brackets 19 are 
cut off flush with road 2 to eliminate their edges 19a and thus obtain a 
continuity of the surface of road 2. 
By way of example, for a bar 7 of square cross section 15 mm.times.15 mm, a 
trench 11 with a depth of 70 mm and a width of 50 mm will be made. 
The proportions used in the hardening composition are, for example, those 
of a charge consisting in relation to the volume of the composition from 
10 to 40% glass fibers, from 10 to 40% sand, and from 20 to 80% binder 
with an epoxy resin base. 
To prevent an expansion of the structure under the effect of the heat 
release by the polymerization and although this release of heat is already 
limited by the addition of sand in the charge of the composition, a forced 
ventilation of the surface of road 2 is set up at the level of trench 11. 
For example, for this purpose a tunnel (not shown) of sheet metal or of 
plastic placed on the road over the entire length of trench 11 is used and 
a fan is placed at one of its ends to cause a circulation of forced air 
and thus remove the released heat. This tunnel also has the role of 
protecting the surface of the structure when it is hardening, from rain 
and from foreign bodies that could deposit on its surface. 
The complete detection system finally obtained can operate much longer than 
sensors conditioned as in the prior art when used in similar qualities of 
road and similar traffic. Further, it can be used safely in roads of 
comparatively bad quality and they will follow progressive deflexion of 
the road without breaking. In an example wherein the encapsulation is 
performed in situ using glass threads as described above, with a 
conventional detector as the bar, it can be observed that weights up to 19 
tons are supported before encountering a risk or rupture, which could then 
occur through destruction or the threads continuity, whereas a similar bar 
embedded in a composition comprising resin, sand, and fibers, but no 
threads would break by weight charges if 9 tons. 
Of course, the invention is in no way limited to the examples of embodiment 
described above. 
In particular the conditioning process with a first encapsulating step as a 
bar and/or a second encapsulating step in situ during the placement 
process in the road, can be used with other types of sensors, for example 
localized sensors buried in the placement structure. Also, the glass 
fibers and the glass threads can be replaced by other organic or inorganic 
materials provided that these materials meet the same objects, and the 
sand could be replaced by another charge, such as for example silica gel 
provided that this charge fulfills the desired functions. 
Also, although the placement process has been described for an embodiment 
flush with the pavement of the road, this process can be applied to the 
placing of sensor in a foundation layer during the making of the road, the 
structure being then covered by the pavement of the road.