U.S. patent number RE35,020 [Application Number 08/138,709] was granted by the patent office on 1995-08-15 for fiber optic load sensing device.
This patent grant is currently assigned to Cubic Toll Systems, Inc.. Invention is credited to Thomas J. Quinlan, Jr..
United States Patent |
RE35,020 |
Quinlan, Jr. |
August 15, 1995 |
Fiber optic load sensing device
Abstract
A transverse load sensitive optical treadle switch. The switch
includes a deformable longitudinal housing and a fiber optic
assembly positioned within the housing The fiber optic assembly
includes an optical fiber having a first end and a second end, the
optical fiber being subject to bending upon application of a
transverse load to said housing. Means for mounting the optical
fiber in the housing so as to permit bending of the fiber in
response to application of a transverse load to said housing are
provided, the degree of bending sufficient to cause a significant
decrease in passage of light through said fiber when said fiber is
bent. Means permitting introduction of light into said optical
fiber at the first end of the optical fiber and means for
permitting the exit of light from the second end are also
provided.
Inventors: |
Quinlan, Jr.; Thomas J. (Beach
Haven Terrace, NJ) |
Assignee: |
Cubic Toll Systems, Inc.
(Hauppauge, NY)
|
Family
ID: |
24021560 |
Appl.
No.: |
08/138,709 |
Filed: |
October 15, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
508139 |
Apr 10, 1990 |
05056884 |
Oct 15, 1991 |
|
|
Current U.S.
Class: |
385/13;
250/227.16; 385/32; 385/51 |
Current CPC
Class: |
G01L
1/242 (20130101); G08G 1/02 (20130101); G01D
5/35374 (20130101) |
Current International
Class: |
G01D
5/353 (20060101); G01D 5/26 (20060101); G08G
1/02 (20060101); G02B 006/26 () |
Field of
Search: |
;385/12,13,32,51
;250/227.14,227.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Curtis, Morris & Safford
Claims
What is claimed is:
1. A treadle sensing device for vehicular traffic adapted to be
mounted transversely on a roadway and to receive and sense a
downward load of the type imparted by a vehicle wheel
comprising
a longitudinal elastomeric envelope having spaced apart
longitudinal walls in a lengthwise region of said envelope adapted
to receive said load;
a portion of foam rubber embedded within said envelope; a fiber
optic cable supported by said foam rubber within said region of
said elastomeric envelope and running generally perpendicular to
the direction of the load to be applied substantially the length of
said region, said fiber optic cable having a first end adapted to
be connected to a signal source of light and a second end adapted
to be connected to a receiver;
means permitting deformation of said fiber optic cable within said
envelope when a transverse load of the type applied by a vehicle
wheel is applied to said envelope sufficient to significantly alter
the amount of light transmitted through said fiber optic cable;
and
means for preventing complete compression of said foam rubber where
said fiber optic cable rests against said foam rubber during
application of said load when said device is mounted on a
roadway.
2. A load sensing device adapted to be mounted transversely on a
roadway and to receive downwardly applied loads of the type
imparted by an automotive vehicle wheel comprising
a longitudinal envelope having spaced apart peripheral walls
defining an interior space therebetween in a region of said
envelope adapted to receive said load, said walls remaining
substantially longitudinally parallel to one another substantially
throughout the region of said envelope adapted to receive said
load, at least one of said walls in said region having an
elastically deformable portion having a first interior surface
deflectable into said interior space towards an opposing second
interior surface of one of said peripheral walls upon the
application of said load to said envelope;
a relatively soft compressible elastomeric insert embedded in said
interior space of said envelope;
a fiber optic cable longitudinally positioned within said interior
space of said envelope and resting against said insert, said fiber
optic cable comprising an optical fiber having a first end adapted
to receive light and a second end adapted to permit the exit of
said light, said fiber optic cable being subject to sufficient
deformation into said insert upon deflection of said first interior
surface caused by application of said load so as to permit
localized bending of said optical fiber sufficient to substantially
alter the light transmitted through the optical fiber; and
means for preventing complete compression of said elastomeric
insert where said fiber optic cable rests against said elastomeric
insert during application of said load when said device is mounted
on the surface of the roadway.
3. The load sensing device defined in claim 2, wherein said insert
is a foam elastomeric.
4. The load sensing device defined in claim 2, wherein said fiber
optic cable is bonded to said insert.
5. The load sensing device defined in claim 2, wherein said optical
fiber has an inner core of a first refractive index and an outer
layer of a second refractive index which is lower than said first
refractive index, and wherein said tint end adapted to receive
light and said second end adapted to permit the exit of said light
permits introduction and exit of light from said core of said
optical fiber.
6. The load sensing device defined in claim 5, wherein said fiber
optic cable is a self contained sleeved fiber optic cable assembly
comprising
a sleeve surrounding said optical fiber;
fulcrum means between said sleeve and said optical fiber spaced
apart along the length of said optical fiber a predetermined
average distance sufficient to facilitate said sufficient bending
of said optical fiber between said fulcrum means in response to a
load applied to said deflectable wall portion of said envelope to
substantially alter the light transmitted through the optical
fiber.
7. The load sensing device defined in claim 6, wherein said fulcrum
means comprise a spiral fiber wound around said optical fiber and
disposed within said sleeve.
8. The load sensing device defined in claim 7, wherein said spiral
fiber is wound with a distance between winds in the range of at
least twice the diameter of said optical fiber.
9. The load sensing device defined in claim 6, wherein said
envelope is made of an elastomeric material and said sleeve is made
of a material which is deformable but relatively stiffer than said
elastomeric material of said envelope to facilitate said sufficient
bending of said optical fiber between said fulcrum means when a
load is applied to said envelope.
10. The load sensing device defined in claim 2, wherein said
substantial alteration of said light passing through said optical
fiber is an attenuation which is substantially linear in response
to the magnitude of the applied load.
11. The load sensing device defined in claim 2, wherein said
substantial alteration of said light is substantially complete
attenuation of the amount of light passing through said optical
fiber in response to the applied load.
12. The load sensing device defined in claim 2, wherein said
interior surface of said deflectable wall portion is substantially
fiat when undeformed.
13. The load sensing device defined in claim 2, wherein said
envelope is formed of an elastomeric material.
14. The load sensing device defined in claim 2, wherein said fiber
optic cable is U-shaped within said envelope.
15. The load sensing device defined in claim 2, wherein said region
of said envelope adapted to receive a load includes an upper wall
having an exterior side, said exterior side including a generally
central portion adapted to receive the load to be applied so that
the load to be applied will tend to be concentrated in the
generally central portion of the exterior side of said upper
wall.
16. The load sensing device defined in claim 15, wherein said
generally central portion includes a convex exterior bump.
17. The load sensing device defined in claim 16, wherein said
exterior side of said upper wall includes substantially
horizontally fiat exterior portions to the sides of said convex
exterior bump.
18. The load sensing device defined in claim 2, wherein said
envelope includes exterior retention surfaces adapted to cooperate
with and be retained by a frame assembly.
19. The load sensing device defined in claim 18, wherein at least
one of said exterior retention surfaces includes an inwardly sloped
exterior side surface.
20. The load sensing device defined in claim 2, wherein said
preventing means comprises a frame assembly, said frame assembly
including frame members adjacent said envelope, said frame members
having a vertical height sufficient to prevent said complete
compression of said elastomeric insert where said fiber optic cable
rests against said elastomeric insert during application of said
load.
21. The load sensing device defined in claim 20, wherein said frame
assembly includes a member partially extending over at least a
portion of said envelope, said member leaving a sufficient portion
of envelope exposed to the load to be sensed to permit said
sufficient bending of said optical fiber. .Iadd.
22. A treadle sensing device for vehicular traffic adapted to be
mounted transversely on a roadway and to receive and sense a
downward load of the type imparted by a vehicle wheel
comprising
a longitudinal elastomeric envelope having spaced apart
longitudinal walls in a lengthwise region of said envelope adapted
to receive said load;
a substantially deformable inner member embedded within said
envelope;
a fiber optic cable supported by said substantially deformable
inner member within said region of said elastomeric envelope and
running generally perpendicular to the direction of the load to be
applied substantially the length of said region, said fiber optic
cable having a first end adapted to be connected to a signal source
of light and a second end adapted to be connected to a
receiver;
means permitting deformation of said fiber optic cable within said
envelope when a transverse load of the type applied by a vehicle
wheel is applied to said envelope sufficient to significantly alter
the amount of light transmitted through said fiber optic cable;
and
means for preventing complete compression of said substantially
deformable inner member where said fiber optic cable rests against
said substantially deformable inner member during application of
said load when said device is mounted on a roadway. .Iaddend.
.Iadd.23. The device defined in claim 22, wherein said
substantially deformable inner member includes a relatively
compressible portion. .Iaddend. .Iadd.24. The device defined in
claim 22, wherein said substantially deformable inner member is
adapted to deflect downwardly to a substantial degree where it
supports said fiber optic cable in response to application of said
applied load. .Iaddend. .Iadd.25. The device defined in claim 22,
wherein said substantially deformable inner member is a portion of
air-conditioning elastomeric material. .Iaddend. .Iadd.26. The
device defined in claim 22, wherein said envelope is mounted with
respect to said roadway in such a manner that said deformation of
said envelope during application of said load is limited to a
predetermined amount. .Iaddend. .Iadd.27. The device defined in
claim 26, wherein said predetermined amount is a deformation to a
position substantially flush with an upper surface of said roadway
proximate to said envelope. .Iaddend. .Iadd.28. The device defined
in claim 22, wherein said envelope is mounted with respect to said
roadway such that at least most of said envelope lies below an
upper surface of said roadway and at least a portion of said
envelope is uncovered by roadway material. .Iaddend. .Iadd.29. The
device defined in claim 28, wherein a small portion of said
envelope protrudes above said surface of said roadway. .Iaddend.
.Iadd.30. The device defined in claim 22, wherein said load sensing
device is configured as a vehicle axle counter and further
comprises:
means operatively connected to said optical fiber for introducing
light into said optical fiber;
means operatively connected to said optical fiber for receiving
said light after it has been transmitted through said optical fiber
and for sending said detectable alteration of said light caused by
deformation of said optical fiber;
means operatively connected to said means for receiving and sensing
said light for determining whether a predetermined threshold value
of alteration of said light exists; and
means operatively connected to said threshhold determining means
for causing a count when said threshhold value of alteration of
said light exists due to the passage of said vehicle wheel over
said device.
.Iaddend. .Iadd.31. A sensing device for vehicular traffic adapted
to be mounted transversely on a roadway and to receive and sense a
downward load of the type imparted by a vehicle wheel as the wheel
passes over the device comprising
a longitudinal deformable envelope having a lengthwise region
adapted to receive said load, said envelope being mounted with
respect to said roadway at least partially exposed such that
application of said load will cause deformation of said envelope;
and
a fiber optic cable disposed within said region of said envelope
and running generally perpendicular to the direction of the load to
be applied substantially the length of said region, said fiber
optic cable including an optical fiber adapted to be connected to a
signal source of light and to a receiver, said fiber optic cable
being disposed within said envelope such that deformation of said
envelope in response to said load will cause deformation of said
fiber optic cable and its included optical fiber sufficient to
detectably alter the light transmitted through said optical fiber
without overstressing said optical fiber upon a multiplicity of
repeated applications of said load. .Iaddend. .Iadd.32. A sensing
device for vehicular traffic adapted to be mounted transversely on
a roadway and to receive and sense a downward load of the type
imparted by a vehicle wheel as the wheel passes over the device
comprising
a longitudinal elastomeric envelope having a lengthwise region
adapted to receive said load, said envelope being adapted to be
mounted with respect to said roadway such that application of said
load will cause deformation of said envelope; and
a fiber optic cable disposed within said region of said elastomeric
envelope and running generally perpendicular to the direction of
the load to be applied substantially the length of said region,
said fiber optic cable including an optical fiber adapted to be
connected to a signal source of light and to a receiver, said fiber
optic cable being disposed within said envelope such that
deformation of said envelope in response to said load will cause
deformation of said fiber optic cable and its included optical
fiber sufficient to detectably alter the light transmitted through
said optical fiber, said envelope being mounted with respect to
said roadway in such a manner that said deformation of said
envelope during application of said load is limited to a
predetermined amount to prevent overstressing said optical fiber
upon a multiplicity of repeated applications of said load.
.Iaddend. .Iadd.33. The device defined in claim 32, wherein said
predetermined amount is a deformation to a position substantially
flush with an upper surface of said roadway proximate to said
envelope. .Iaddend. .Iadd.34. The device defined in claim 32,
wherein said envelope is mounted with respect to said roadway such
that at least most of said envelope lies below an upper surface of
said roadway and at least a portion of said envelope is uncovered
by roadway material. .Iaddend. .Iadd.35. The device defined in
claim 34, wherein a small portion of said envelope protrudes above
said surface of said roadway. .Iaddend. .Iadd.36. A sensing device
for vehicular traffic adapted to be mounted transversely on a
roadway and to receive and sense a downward load of the type
imparted by a vehicle wheel as the wheel passes over the device
comprising
a longitudinal elastomeric envelope having at least one deformable
side in a lengthwise region of said envelope adapted to receive
said load and means defining a longitudinal cavity in said
lengthwise region, said envelope being adapted to be mounted with
respect to said roadway such that application of said load to cause
deformation of said deformable side will cause deformation of said
longitudinal cavity;
a compressible cushion within said envelope in a portion of said
envelope disposed below said longitudinal cavity;
a fiber optic cable disposed within said means defining a
longitudinal cavity in said region of said envelope and running
generally perpendicular to the direction of the load to be applied
substantially the length of said region, said fiber optic cable
including an optical fiber adapted to be connected to a signal
source of light and to a receiver,
said fiber optic cable being disposed within said envelope such
that deformation of said cavity in response to said load will cause
deformation of said fiber optic cable and its included optical
fiber sufficient to detectable alter the light transmitted through
said optical fiber,
said envelope being adapted to be mounted with respect to said
roadway as to prevent complete compression of said cushion during
application of said
load. .Iaddend. .Iadd.37. A sensing device for vehicular traffic
mounted transversely on a roadway and adapted to receive and sense
a downward load of the type imparted by a vehicle wheel as the
wheel passes over the device comprising
a longitudinal envelope having at least one deformable wall and
means defining a longitudinal cavity in a lengthwise region of said
envelope adapted to receive said load, said longitudinal cavity
being subject to deformation upon deformation of said deformable
wall;
a compressible cushion disposed in said envelope below said
longitudinal cavity;
a fiber optic cable disposed within said cavity in said region of
said elastomeric envelope and running generally perpendicular to
the direction of the load to be applied substantially the length of
said region, said fiber optic cable including an optical fiber
adapted to be connected to a signal source of light and to a
receiver;
means permitting deformation of said fiber optic cable and said
included optical fiber within said longitudinal cavity of said
envelope when a transverse load of the type applied by a vehicle
wheel is applied to said envelope sufficient to detectably alter
the light transmitted through said optical fiber.
said envelope being mounted with respect to said roadway in such a
manner as to prevent complete compression of said cushion.
.Iaddend. .Iadd.38. The device defined in claim 37, wherein said
envelope is formed of elastomeric material. .Iadd.39. The device
defined in claim 37, wherein said load sensing device is configured
as a vehicle axle counter and further comprises:
means operatively connected to said optical fiber for introducing
light into said optical fiber;
means operatively connected to said optical fiber for receiving
said light after it has been transmitted through said optical fiber
and for sensing said detectable alteration of said light caused by
deformation of said optical fiber;
means operatively connected to said means for receiving and sensing
said light for determining whether a predetermined threshhold value
of alteration of said light exists; and
means operatively connected to said threshhold determining means
for causing a count when said threshhold value of alteration of
said light exists due to the passage of said vehicle wheel over
said device.
Description
FIELD OF THE INVENTION
The present invention relates generally to switches for sensing the
passage of a vehicle over a treadle, and in particular to a novel
treadle switch design utilizing fiber optics as the sensing
means.
BACKGROUND OF THE INVENTION
In toll road and other applications where the passage of a vehicle
is sensed, it is often required to quantify the number of axles
associated with a vehicle. This is generally accomplished by means
of weight sensitive treadles placed across the roadway.
Conventional treadle designs commonly employ a treadle switch unit
consisting of an elastomeric envelope housing an electrical sensing
unit. In one common form, the interior of the envelope is provided
with spaced contact strips defined by upper and lower interior
surfaces which are normally separated by air recesses running
longitudinally. When the weight of a vehicle wheel is upon the
treadle, it deforms the envelope and causes the contacts to engage
each other (electrically) to complete a circuit. The electrical
sensing means for the treadle switch may, to like effect, consist
of a variable resistor which changes its electrical resistance in
response to stress caused by deformation or a piezoelectric sensor
which provides a voltage in response to stress. The important
feature common to these electrical sensing means is that the sensor
produces a readily detectible change in the current and/or voltage
in response to weight applied to the unit.
These treadle switch designs all rely upon electrical current flow
and are therefore particularly subject to failure due to intrusion
of moisture, salts and/or other contaminants. Accordingly, the
lifetime of such electrically sensing treadle designs is unduly
limited. Because these treadles are typically in traffic lanes,
replacement causes undesirable disruption of traffic in addition to
expense.
This invention relates to a completely sealed treadle switch
utilizing a fiber optic sensor of the intrinsic type. For
comparison, with extrinsic fiber optic sensors, light leaves the
fiber and is blocked or reflected before going back into the fiber
optic system. Thus, extrinsic optical sensors have the same
disadvantages as photo-electric controls in that they ar affected
by dirt, contamination and mechanical vibration. By contrast, with
an intrinsic fiber optic sensor, the light is processed (i.e., is
detectably altered due to stress) as it passes along the fiber.
This can be in the form of phase angle change or speckle pattern
detection, or other attenuation.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
treadle switch design which overcomes the foregoing disadvantages
of treadle designs with electrical sensors or extrinsic optical
sensors.
In particular. it is an object of the present invention to provide
a treadle switch having a fiber optic sensor which employs light
rather than electrical current flow for its sensing operation and
is therefore less subject to failure due to presence of moisture,
salts and other contaminants than electrical sensors.
It is a further object of the present invention to provide a
treadle which is simple, robust. long lived, and has a high
inherent reliability.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention,
a transverse load sensitive optical switch is provided which
includes a longitudinal housing having at least one deformable side
and a fiber optic assembly positioned within the housing. The fiber
optic assembly includes an optical fiber having a first end and a
second end, the optical fiber being subject to bending upon
application of a transverse load to said housing.
The fiber optic assembly also includes means for mounting the
optical fiber in the housing so as to permit bending of the fiber
in response to application of a transverse load to said housing,
the degree of bending being sufficient to cause a significant
decrease in passage of light through the fiber when the fiber is
bent. Mean permitting introduction of light into said optical fiber
at the first end of the optical fiber and means for permitting the
exit of light from the second end are also provided.
In a preferred aspect of this embodiment, the means for mounting
the optical fiber include a plurality of fulcrum means positioned
along the length of the optical fiber at intervals sufficient to
permit sufficient bending of the optical fiber between the fulcrums
in response to a transverse load applied to the housing to
substantially alter the amount of light transmitted through the
optical fiber.
In another preferred embodiment of the present invention, the
transverse load sensitive optical switch comprises a longitudinal
housing having at least one deformable side and a fiber optic
assembly positioned within the housing. The fiber optic assembly
includes a graded optical fiber having a first end and a second end
longitudinally positioned in said housing so as to be subject to
bending upon application of a transverse load to the housing, the
fiber having an inner core of a material having a first refractive
index and an outer layer of a material having a second refractive
index which is lower than the first refractive index. Means for
mounting the optical fiber in the housing adapted to permit bending
of the fiber in response to application of a transverse load to the
housing, the degree of bending being sufficient to cause a
significant increase in passage of light from said core to said
sleeve while said fiber is bent, are also provided. Means
permitting the introduction of light into the optical fiber at the
first end of the optical fiber and for permitting the exit of light
from the second end are also provided.
In accordance with a further aspect of this embodiment, the means
for mounting the optical fiber preferably comprises a spiral fiber
wound around the optical fiber, the spiral fiber having adjacent
winds spaced apart from one another a predetermined distance
averaging at least in the range of twice the diameter of the
optical fiber so as to permit sufficient bending of the optical
fiber between the winds of the spiral fiber in response to a
transverse load applied to the housing to substantially alter the
amount of light transmitted through the optical fiber. The turn of
the wound fiber functions essentially as fulcrums to facilitate
bending of the optical fiber.
Advantageously, the switch assembly further comprises a sleeve
surrounding the optical fiber and the spiral fiber to keep said
spiral fiber wound around said optical fiber and to permit the
sufficient bending of the optical fiber. Preferably, the sleeve is
made of a material which is deformable but relatively stiffer than
at least one deformable side of the housing to facilitate bending
of the optical fiber between the winds of the spiral fiber when a
transverse load is applied to the housing.
In a yet further embodiment of the present invention, a treadle
switch unit for vehicular traffic adapted to be mounted
transversely on a roadway is provided which comprises a
longitudinal elastomeric envelope; a portion of foam rubber
embedded within the envelope; and a fiber optic cable supported by
the foam rubber within the elastomeric envelope and running
substantially the length of the treadle switch unit. The fiber
optic cable has a first end adapted to be connected to a signal
source of light and a second end adapted to be connected a
receiving unit, further including means permitting deformation of
said fiber optic cable within the envelope when a transverse load
is applied to the envelope sufficient to significantly alter the
amount of light transmitted through the fiber optic cable.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present
invention will be apparent to those skilled in the art in view of
the following description and drawings, wherein:
FIG. 1 is a plan view of a treadle unit employing the fiber optic
switch of the present invention, with a section broken away;
FIG. 2 is an elevation view of the treadle unit depicted in FIG. 1.
with a section broken away;
FIG. 3 is an end sectional view of an optical fiber having an inner
core of higher refractive index and an outer layer of lower
refractive index employed in a preferred embodiment of the present
invention;
FIG. 4 is a sectional elevation view of the optic cable depicted in
FIG. 3, in its unstressed state, schematically illustrating light
passing through the core of the optic cable relatively
unimpeded;
FIG. 5 is a perspective view, partially in section, of a portion of
a fiber optic cable constructed in accordance with a preferred
embodiment of the present invention;
FIG. 6 is a sectional elevation view of the optic cable depicted in
FIG. 3, in its stressed and bent state, schematically illustrating
light exiting from the core of the fiber optic cable into the outer
layer;
FIG. 7 is a cross-sectional view of the treadle unit depicted in
FIG. 1;
FIG. 8 is an end elevation view depicting the treadle unit depicted
in FIG. 1 and FIG. 2. mounted in a support frame including a
grouping of four treadle switches;
FIG. 9 is a cross-sectional view of a single treadle unit depicted
in FIGS. 7 and 8, shown depressed and distorted under a compressive
force similar to that as shown applied in FIG. 8; and
FIG. 10 is a schematic depiction of a light signal source and light
signal receiving unit adapted for use in conjunction with the
present invention.
DETAILED DESCRIPTION
Turning now to the drawings in detail, and initially to FIGS. 1 and
2 thereof, a treadle switch unit 20 in accordance with a preferred
embodiment of the present invention is depicted. This treadle
switch device is generally designated as designed for vehicular
traffic, and is adapted to be mounted transversely on a roadway. As
depicted in FIG. 8, this treadle unit can be mounted in a fixed
frame and can be used singularity or in quantity up to and
including (4) four treadle switch units 36, 38. 40 and 42, or
more.
The treadle switch unit of the present invention is shown
particularly in the cross-sectional view thereof of FIG. 7 of the
drawings and comprises an envelope preferably of an elastomeric,
such as a rubber or urethane, generally designated as 44. The
fiber-optic cable is bonded to a base 46 of a soft foam rubber
insert supported by the elastomeric envelope of rubber or urethane,
which advantageously serves as a deformable side of the envelope.
This inner foam rubber allows the compression and bending of the
fiber optic cable when compressive force is applied as shown in
FIGS. 8 and 9. The fiber optic cable shown in FIGS. 1 and 2 is an
intrinsic fiber optic cable, 48, bonded to a support base of foam
rubber 46 and connected to the electronic control sending and
receiving unit 50 through optical connectors 52 and 54.
FIGS. 1 and 2 illustrate the treadle 42 in a singular mode and
further illustrate the construction of a rubber-urethane envelope
having a specifically designed contour as shown in FIGS. 7, 8, and
9. This contour allows for rapid mounting on the treadle frame
assembly 34, locked in place by the wedge locking bars, 56, and
secure and protected by the top cover plates 58.
The fiber optic treadle 20 is a pressure sensitive device molded in
a rubber compound. It is installed in a frame assembly 34 which
lays flush with the road surface and counts the number of axles on
each vehicle. The fiber optic portion of the treadle consists of an
optical fiber 22 having a "glass" core 24 with a high refractive
index (bending of wave of light as it passes from one medium into
another) and an outer layer 26 having a lower refractive index. The
fiber also includes a protective hard acrylic coating 31. Any light
which deviates from the center core is bent back into the center by
the lower refractive index of the outer layer. If the fiber is bent
at a point or points along its length, some light is guided out of
the core into the outer layer where it is dispersed. Light passing
down the fiber is virtually switched off by pressure applied the
spiral 30. An infra red light (led) is used to send light into the
sensing fiber.
Turning now to FIG. 3, a preferred embodiment of the transverse
load sensitive fiber optic switch of the present invention
employing a graded optical fiber 22 is depicted. This graded
optical fiber consists of a glass core 24 of high refractive index
and an outer layer 26 having a lower refractive index. Any light
which deviates from the center core is bent back into the center by
the lower refractive index of the outer layer. This is shown in
FIG. 4. When the fiber is bent, some of the light 27 will follow a
path through the core, or follow other possible light paths or
modes of transmission 29 as demonstrated in FIG. 6.
To achieve this micro bending over short or long lengths of fiber,
a plastic spiral 30 is used. Preferably, the spiral is wound around
the optical fiber with the winding spaced apart a distance
averaging at least in the range of twice the diameter of the
optical fiber. This permits bending of the optical fiber to a
degree sufficient to significantly decrease the amount of light
transmitted through the optical fiber core, without damage to the
optical fiber. This is then preferably covered with a sleeve 28 to
hold the assembly together as shown in FIG. 5. The whole assembly
is very flexible and can be taken round corners down to 25 mm
radius.
If the fiber is bent at certain points along its length, some light
is guided out of the core 24 into the outer layer 26 where it is
dispersed, as illustrated in FIG. 6. To the naked eye, light
passing down the fiber appears virtually switched off by pressure
applied to the spiral. In addition, the movement required to give
this attenuation is only 0.04 min. After this movement the fiber
can be squeezed a further 0.1 mm before it is over-stressed. When
operating over the normal attenuation range, the fiber has been
tested to more than 8 million operations without any measurable
change in the level of attenuation. Lastly, there is excellent
linearity in the relationship of attenuation to applied force.
A light signal source and light signal receiving unit 50 with
respective light supply and light return connections 54 and 52 to
the fiber optic cable. Advantageously, an infra-red light emitting
diode (LED) is used to launch light into the sensing fiber. The
light intensity is sensed at the remote end using a photo detector.
By comparing the output level of the photo detector with a
reference, any change in the amount of light passing through the
detector can be measured. The power supply to the LED is pulsed at
2 kHz for maximum light intensity. The pulse train is modified so
that 1 in 11 is missed. The missed pulse is used to verify the
control circuit integrity passing through the fiber.
The light intensity is sensed using a photo-detector and by
comparing the output level of the photo detector with a reference,
any change in the amount of light passing through the detector can
be measured. The contacts are set to open when the signal drops
below a threshold (action similar to on/off switch).
FIG. 8 depicts and assembly of four treadles constructed in
accordance with the invention, three of which (treadles 36, 40 and
42) are in the free, or waiting (unstressed) state shown in FIG. 8.
These treadles carry a pulsed signal light beam from the light
power supply, 4, through the fiber optic cable and back to the
receiving connection of the said light power supply, 4. Treadle 8
in FIG. 8 illustrates compression force bending the fiber optic
cable. By knowing the output level of the infra-red light emitting
diode (LED) any deviation in signal light will be measured by the
detector. The voltage free contacts are set to open when the signal
drops below a threshold. This gives a straight on/off sensor. The
structure and operation of the treadle switch unit and the
improvement thereof over known treadle switch units is believed to
be fully apparent from the above detailed description. It will be
further apparent that changes may be made in the detailed structure
of the improved treadle switch unit of the invention without
departing from the spirit of the invention defined in the appended
claims.
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