Note: Descriptions are shown in the official language in which they were submitted.
~C~6~99~S
This invention relates to a device for sensing or measuring loads,
especially transient loads. The invention is especially adopted for
measuring axle weights of vehicles passing over or driving over the device
situated on a road surface.
A previously known device for sensing and measuring a transient
load imposed by at least one wheel on an axle of a passing vehicle, comprises
a resiliently flexible mat formed of a dielectric and elastomeric material
adapted to be engaged by the wheel. At least two vertically spaced capacitor
electrodes are embedded in and extend over substantially the entire operative
area of the mat. The electrodes are connected to an apparatus for detecting
variations in the electrode spacing due to transient loads. Void areas are
located in the body of the mat to provide for mat deformation that is linearly
proportional to the magnitude of an applied load. These void areas are
uniformly distributed in the mat and extend at least substantially from one
of said electrodes to the other. *
Also previously known are other pressure transducers for measuring -;
axle weights but these mostly require structural modification of the road -~
surface.
The development of portable dynamic weight bridges, which include
& portable or at least mobile laboratory for the instrumentation, has
necessitated development of a new pressure transducer which is easy to
conatruct and can be applied onto the road without any structural modifica-
tion of the road surface. I-t has also been found that the pressure trans-
ducer must have a linearal proportionality between the applied load and the
output electrical information and especially there must be linearal pro-
portionality between the transient load and the variation in capacity.
According to this invention there is provided a variable
capacitance device for sensing and measuring loads, especially transient
~ J
:. . . . .................. ~: : . ....... . . .
. ~ , . . . . .... . ., .. . ~. . ~ . . . .
~L~69~
loads, wherein the capacitance varies as a linear -function of the loads to
be measured, said device comprising: (a) two conductive plates, (b) an
elastic dielectric layer between said plates, (c) means encompassing said
device for compressing said two conductive plates toward each other and for
maintaining pressure on said conductive plates to preload said dielectric
layer to cause the capacitance of the device to vary as a linear function
of the loads to be measured, (d) a plurality of conductive threads embedded
in said dielectric layer, (e) a first electrode of the device connec-ted to
said threads, and (f) a second electrode o-f the device connected to one o-f
said plates.
One object of the present invention is to provide a device for
measuring loads, especially transient loads, in which the output is linealy
proportional to the transient load. ~-
Another object is to provide a device which can be applied to the -
road surface easily and without any structural modlfication of the road
surface. Further, the device shall be so constructed that it is of a small
height, in order to reduce the influence of dynamic forces caused by the
vehicles when passing over the device.
Another object is to provide a device which is rugged and un-
complicated and relatively free -from electrical interference and which may
be manufac-tured cheaply.
Still another object is to provide a device having two electrically
conductive plates and an intermediate dielectric and elastomeric or elastic
material, in which the two plates are pressed towards each other in order
to cause a mechanical preload in the intermediate material, to provide the
linear proportionality. -
Still another object is to provide a preload by winding a rubber
ribbon under stress around said plates.
- 2 -
~ ~.
',' ' " '. ' ' ' ' " " ' ' " ' ' ' ~',` ' ~' ' ', '',. ,',, ",' ,, " ' ' , ~ ;,
1~69945
A further object is to provide a series of strands of steel wire
in a central plane of the dielectric material which are used, as a first
capacitor electrode. Each plate is used as a second capacitor electrode
and may be connected to ~ero potential or earth potential.
In order to fully describe and illustrate the invention, an
example is given hereunder with reference to the accompanying dra,wing in
which
Figure 1 is a perspective view of an embodiment of the inven-tion ~
in a non-assembled state. '
Figure 2 is a sectional view of a part of the apparatus in
Figure 1.
Figure 3 is a perspective view of the device as the device being
subjected to a preload b~ winding a rubber ribbon under stress around the -',
-two plates, forming a part of the device. ~
Figure 4 is a sectional view of the device shown in Figure 3. ~-,
Figure 5 is a graph showing output signal as applled force in a ~ '
device according to Figure 7 withou-t preload.
Figure 6 is a graph showing output signal as applied force in a ~'
device according to Figure 8 having a preload of approximately 3kp/cm2.
Figure 7 is a front elevation view o~ a device without preload
in an assemblied state and having six strands of steel wire material.
Figure 8 is a sectional view of the device shown in Figure 7
and in which the device is subject to a preload.
Figure,9 is a plan elevation view of the complete device in
smaller scale. ,
Figure 10 is a side elevation view of a drive used by vehicles
to move up to a weighting station equipped with a device according to the
present invention. ,
'~"
~C36~4~
Figure 11 is a side elevation view of a drive used by vehicle
to move i~rom the weighing station.
Figure 12 is a side elevation view of a weighing station including
the drives shown in Figures 10 and 11. `~
Figure 13 is a plan elevation view of the weighing station accord- ,
ing to Figure 12 and in which seven devices are used and
Figure 1~ is a perspective view of the weighing station equipped
with electronic devices to evaluate the output signals emitted by the
devices.
The device according to the present invention is shown in Figure 1
in a perspective view and in a non-assembled condition. The device comprises
two parts, subassemblies identified by reference numerals 1 and 2. Sub- -`
assembly 1 consists of an electrically conductive plate 10 and a dielectric
and elastomeric or elastic sheet material 12. Subassembly 2 consists of
an electrically conductive plate 11 and a dielectric and elastomeric or
elastic sheet material 12a. A number of strands 18 are applied to the
material 12. In Figure 1 five strands are shown and in Figure 7 six threads
are shown. ~he number chosen is a matter of application ~ield and material
in other parts ~f the device. -
In Figure 2 is subassembIy 1 shown in front elevation view.
In accordance with this invention the twQ conductive plates 10
and 11 are pressed towards each other in order to cause a preload in the
intermediate dielectric and elastomeric or elastic material. This preloading
is accomplished, as sho~m in Figure 3, by the use of a rubber ribbon 3. This
ribbon is wound in a spiral form around the plates 10 and 11 under stress
in the direction indicated by "~" in Figure 3. It is suggested that the ;
stress in the ribbon shall be so adjusted that the preload shall have the
magnitude of approximately 3kp/cm2.
.. .. . : ~ :,:,: : : : :,.: : , ,,.:, :. .~: .,,. : .. .: . . :, , :
- , . . . , :. , :::, :.: :.:: :: ::- :: ;: :, ::,: ,: . :: -.. .: , . . :
6~S
Figure 4 is a front elevation sectional view of the completed device
shown in accordance to Figure 3.
Owing to the molecular makeup of rubber the relation between
received output signal and applied force is well known. In Figure 5 is shown
the characteristic diagram. This diagram starts with a non-linear portion "A"
followed by a linear portion "B". If the material used as intermediate
material has the dimension and the characteristics mentioned hereinafter it
has been found that there is a transition of the non-linear portion "A" *
into a linear portion "B" at about 3kp/cm2.
If the plates 10 and 11 are pressed towards each otherJ as indicated
in ~igures 3 and 4 and having a preload of 3kp/cm2 the output signal is
linear with the applied force or load. This is illustrated in Figure 6 by
the line "Bl". If the device is subject to a number of loads, especially
high transient loads, it has been found that the line may get a smaller
inclination as illustrated by line "B2".
Referring to Figure 7 there is shown a device in a non-preloaded
state. The upper conductive plate 10 and the lower conductive plate 11 are
arranged on each side of an intermediate dielectric homogenous material 12
and 12a. The material 12 or 12a consists~ in this application, of a sheet of
crude rubber having a thickness of 2mm. The conductive plate has a thickness
of 1~5 mm. The crude rubber has a shore hardness of 40 and a modulus of
rigidity G=3.5 kp/cm2. (Shear modulus).
The upper sheet material 12 is at its upper side rigidly fixed to
the undersurface of the plate 10 with the bond being designated by the
reference numeral 13. The lower sheet material 12a is at its underside
rigidl~ fixed onto the upper surface of the plate 11 and the bond has been
given the reference n~eral 14. Any conventional flue such as cyanic
acrylic glue may be used in order to obtain a secure hond.
5 -
~969!~
The sheets 12 and 12a have essentially identical cross-sections
and are arranged to cooperate with each other. Before the surfaces 12'
and 12" are glued together, a number of strands of steel wire are applied
to the surface 12'. These steel wires may be music wires having a diameter
of 0;25 mm. One end of each wire is welded or soldered to a connecting
wire 18a which is connected to the central conductor in a coaY~ial cable 19.
The glue used to bond the surfaces 12' and 12" together may be any conven-
tional glue providing a soft bond.
The plates 10 and 11 extend beyond the side edges 15 and 16 of `
the material 12, 12a in order to render the rubber sheets 12 and 12a as
uniform and deformation free as possible during the pressure provided by
the rubber ribbon 3 (Figure 3) and the load. The plates 10 and 11 have
the same length as the sheets 12 and 12a.
The bonded surfaces 12' and 12" are located in a central plane
of the device and the wires 18 are each oriented in side-by-side relation
with the intermediate distance equal.
The central conductor of the coaxial cable 19 is connected to
the wires 18 and the shield conductor which is at earth potential, is
connectea to the plates 10 and 11. An output signal is received which -
senses the variation in the capacitance depenaing upon the load (transient
load).
Figure 8 shows a sectional view of the device according to Figure 7 -
which has been subject to preloading.
One device of the construction illustrated above is shown in
plan elevation view in Figure 9 and has been given the reference numeral 20.
It is to be noted that Figure 9 is shown in a larger scale. ~`
Before describing the device or a number of devices when ;
incorporated into a weighing station, there shall be a description in
- 6 -
I., . . ,- : . ,: .:: ::- : ; : ,: :. , ., : :
~ .
6g~45
somewhat more detail as to the manner in which the deformation changes the
capacitance of the device. This description is based upon the embodiment
shown in Pigures7, 8 and 9.
The strands 18 between the rubber sheets 12 and 12a do not resist
the deformation. The formula used for deformation is thc same as the formula
used for rubber bearings. If it is assumed that the active surface of the
load has a length exceeding its breadth (strip form), then the deformation
will follow the formula
~ = (p X t3)/(G X b2)
in which ~ = deformation
p = force/unit of surface
t = the total thickness of the strip
G = module of rigidity
b = the breadth of the strip.
As mentioned the strands 1~ do not cause any resistance towards
horizontal deformation in a direction perpendicular to the length direction
of the strip. As a result of the use of the wire strandsa greater deforma-
tion takesplace than would be the case if a metal sheet had been used. In
this application the rela~ive deformation exceeds the deformation caussd by
a metal sheet by a factor four. It is obvious from the formula that said
deformation varies with a second power of the thickness of the sheet. As a
soft glue is used between the rubber strips 12 and 12a no shearing strain
occurs in this bond.
From the given formula it appears that the deformation is linear
with reference to the applied force or load.
It may be assumed that b=35 mm, t=4 mm, 1=2000 mm (the length of
the strip) and G=3.5 kp/cm2.
A relative changing of volume between the plates 10 and 11 will be
-- 7 --
: . :- :, . .
:: :
.
(using cm and kp) Mp
~/t = ~p X t2 X 1000)/CG X b2 X p X 1 X b) = 0.53%/Mp.
It is obvious that the electrical construction of the device can
be seen as two capacitors having a common electrode (the strands 18). Due
to the fact that the plates 10 and 11 surround the strands and the strips the
sensitivity of the device to e~ternal currents is practically eliminated.
The capitance for each capacitor may be calculated by '~
C = ~dielec~ric constant for rubber) ~area)/~ X 4.
distance between the plates).
If the dielectric constant for the used rubber is 2.9 and the
threads have the breadth 6 X 0.55 cm = 3.3 cm then the average breadth of
the plate is ~3.3 + 4.0)/2 - 3.65 cm and the capacitance is 1680pF.
It has been necessary to reduce the capacitance if strands are
used instead of a plate (edge effect). Increasing the dis~ance between the
threads causes a further decrease in capacitance. Measurements have shown
that the device has a capacitance of about 800 pF.
For smaller defoI~ations the changing in capacitance is a linear -~
function to the deformation ~compression). The total changing in the
capacitance is linear with respect to change in volume. Thus gor each
2~ device
800 X 0.53/100 = 4pF/Mp.
Tests have shown that it is possible to detect changing in ~he
force down to 20kp, which means a changing in the capacitance of 0.08pF.
In Figures 10-14 is shown a complete portable weighing bridge
having a portable weighing station. The weighing station comprises of a
first drive, the weighing station and a second drive.
The first drive will be described with reference to Figure 10.
Figure 10 is a side elevation view of a drive used by vehicle to move up to
'~jJ
.,',,.~.. ',~ . .
~6~45
the weighing station. The weighing station is designated by the reference ;
numeral 39 the first drive by 38 and the second drive by 40. (Figure 12).
The weighing station comprises of a number of elements or devices `~r
20 each of which is covered by a plate 21. A rubber sheet 22 is applied by
glue to the ~mdersurface of the plate 21.
The plate 21 is glued to a part 23 which is made o~ rubber. This
part 23 is posi-tioned to cooperate with the upper end surface 21a of the
plate.
The drive 38 comprises a plate 27 which is glued to a rubber base
portion 25 and an overlying rubber sheet 26. A screw 24, which passes
through the plate 27 secures the end surface 21a to the plate 27. These
components are secured by a nut (not shown) in recess 41 in the part 23.
The plate 27 and the rubber sheet 26 extend from the part 23
and cooperate with a drive section 45, made of rubber. Section 45 is glued
onto the plate 27 in the region 42 and onto the sheet 26 in the region 42a.
When the device 20 and the part 23 are secured to the drive 38,
said drive 38 is secured to the support 29 by means of screws or nails
oriented into recesses 28 which are intended to withstand hori~ontal forces.
Further recesses or holes 48 extend in a plane parallel to the devices 20
and incline towards the support 29 and are intended to receive nails or
screws which withstand vertical forces. This drive is of the same length
as the elements or devices 20. The dimensions of the parts forming the ~ -
drive may be as follows.
Rubber sheet 26 (1 mm), plate 27 (1 mm), rubber sheets 45, 23 and
25 (4 mm). This gives a total height of 11 mm. The plate 21 and the rubber
sheet 22 may have a thickness of 1 mm.
Figure 11 shows a side elevation view of a drive 40 used by
vehicle in order to move from the weighing station. This drive 40 is
6~45
identical to the drive 38 and the same parts have been given the same
reference numerals adding a prim mark. This drive has a rubber sheet 25',
a metal plate 27' and a rubber sheet 26'. The parts to the right and the '
part to the left in Figure 11 are identical to the drive already described.
In the intermediate portion 40a of the drive 40 there is situated
a side indication device. This device comprises of rubber strips 43 oriented
perpendicular to the drive direction. These strips are placed between the
plate 27' and the rubber sheet 45' and in the drive direction oriented ;,
metalic strips 44. Although only one strip 44 is shown there is a plurality
of strips 44 adjacent each other.
Each metal strip 44 is connected to electrical output line 37.
The function of the side indication device is as follows. One
metal strip 44 a~ong all strips will upon leading of rubber sheet 45' be
pressed by the rubber strip 43' into electrical contact with the metal plate
27' closing an electrical circuit. BY sensing this signal it is possible
to determine where the load was acting.
Due to the fact that the weighing s-tation has seven elements or
devices 20 (Figure 12) and each is sensing the transient load it will be
possible to determine the velocity of the vehicle. The aistance between
a sensed device 20 and the indication device 43', 44 is known and the time ;
between activation may be evaluated electronically. -
- In figure 12 the arrow indicates the direction of movement for
the vehicle.
Figure 13 is a plan elevation view of the weighing station and
associated drives and shows the cables 19-19f. Each cable is connected to
its respective device 20. Also shown is cable 37, which includes a wire
for each strip 44. The length of the devices may be half the breadth
of the road or less, but may also be larger.
- 10 - ~-
::.
: ", . ~,; ! . , . ", ", ; . , j
~ ~ ~ g g ~tj
Figure 14 shows in a perspective view the application of the
weighing station with associated drives using electronic devices to evaluate
the output signals emitted by the devices and other means.
The electronic devices which are used for treating the signals
from the devices do not form any part of the present invention but their
function will be described.
Each device is fed by square formed pulses having a frequency
of 20kH~. The capacitance in the device is changed by the load and thus
the time of discharging said capacitor over a resistance is changed. By
evaluating the time of discharge it is possible to provide an analog output
signal. This signal is amplified and detected applied to an analog/digital `
converter.
The registration may be continuous or only based upon the maximum
value. By this electronic device it is possible to set a threshold value
under which no registration occurs. This is a way of separating passenger
vehicle from lorries. By using the side indication device it is possible,
as previously is mentioned, to evaluate the velocity of the vehicle. It
is also possible to correct and amend any deviations due to dynamic forces,
which may occur.
In Figure 14 is shown a DTC-device (discharge to time converter~,
a SCU-device (signal conditioning and control unit) and DCU-device in
which the signals are collecting unit).
The lines 19-19f are connected to the DTC-device in which the
signals are treated. The result is fed to the SCU-device via line 141,
line 142 is a power supply. Cable 37 is connected directly to the SCU-device.
A cable 143 is arranged to feed information from the SCU-device to the DCU-
device.
While prime importance is given to the device 20 and its use in
a weighing station 39 for measuring axle weight of vehicles driving over
- l] -
~L~6~
the applicant envisages the invention as being used generally for the
measuring of various types of forces, transient of sta-tic.
It must be noted that the strands or wires may be in the form
of strips or in the form of a sheet material. In addition other means may
be used to provide preloading in the device 20
- 12 _
,, ,- , ",,-, ,., ,-, .,. , . -.
