CA1069945A - Device for measuring loads, especially transient loads - Google Patents
Device for measuring loads, especially transient loadsInfo
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- CA1069945A CA1069945A CA249,974A CA249974A CA1069945A CA 1069945 A CA1069945 A CA 1069945A CA 249974 A CA249974 A CA 249974A CA 1069945 A CA1069945 A CA 1069945A
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- threads
- capacitance
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Abstract
ABSTRACT OF THE DISCLOSURE
A device for sensing or measuring loads, especially transient loads, such as a load imposed by a wheel of a vehicle driving over the device.
The device comprising two electrically conductive plates and an intermediate dielectric and elastomeric or elastic material. A series of steel wires is embedded in a central plane of the dielectric material. Said threads are connected together electrically at one end. The wires form one capacitor electrode while another electrode is formed by the plates. The variation in the capacitance is linearly proportional to the magnitude of the applied load.
The two plates are pressed towards each other in order to create a mechanical preload in the intermediate material, in order to give said linear propor-tionality.
A device for sensing or measuring loads, especially transient loads, such as a load imposed by a wheel of a vehicle driving over the device.
The device comprising two electrically conductive plates and an intermediate dielectric and elastomeric or elastic material. A series of steel wires is embedded in a central plane of the dielectric material. Said threads are connected together electrically at one end. The wires form one capacitor electrode while another electrode is formed by the plates. The variation in the capacitance is linearly proportional to the magnitude of the applied load.
The two plates are pressed towards each other in order to create a mechanical preload in the intermediate material, in order to give said linear propor-tionality.
Description
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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
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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.
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
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. ~ , . . . . .... . ., .. . ~. . ~ . . . .
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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.
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',' ' " '. ' ' ' ' " " ' ' " ' ' ' ~',` ' ~' ' ', '',. ,',, ",' ,, " ' ' , ~ ;, 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. , '~"
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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.
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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.
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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 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 : . :- :, . .
:: :
.
(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 _ ,, ,- , ",,-, ,., ,-, .,. , . -.
',' ' " '. ' ' ' ' " " ' ' " ' ' ' ~',` ' ~' ' ', '',. ,',, ",' ,, " ' ' , ~ ;, 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. , '~"
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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.
.. .. . : ~ :,:,: : : : :,.: : , ,,.:, :. .~: .,,. : .. .: . . :, , :
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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.
~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 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 : . :- :, . .
:: :
.
(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 _ ,, ,- , ",,-, ,., ,-, .,. , . -.
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A variable capacitance device for sensing and measuring loads, especially transient 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 connected to said threads, and (f) a second electrode of the device connected to one of said plates.
2. A device according to claim 1, wherein a stiff bond is applied between the plates and the dielectric layer.
3. A device according to claim 1, wherein the threads are oriented in a central plane of the dielectric layer.
4. A device according to claim 1, wherein the threads are further oriented longitudinally in the layer and are equally spaced.
5. A device according to claim 1, wherein compressing means consists of a stretched elastic strip.
6. A device according to claim 1, wherein the breadth of the plates exceeds that of the dielectric layer.
7. A device according to claim 1, wherein the dielectric layer consists of two parts.
8. A device according to claim 7, wherein the two parts are bonded to each other by a flexible bond.
9. A weighing station including a plurality of the devices of claim 1, wherein each device rests upon a road surface and a drive is arranged on both sides of the devices.
10. A weighing station according to claim 9, wherein a plate covers the devices.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA249,974A CA1069945A (en) | 1976-04-09 | 1976-04-09 | Device for measuring loads, especially transient loads |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA249,974A CA1069945A (en) | 1976-04-09 | 1976-04-09 | Device for measuring loads, especially transient loads |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1069945A true CA1069945A (en) | 1980-01-15 |
Family
ID=4105680
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA249,974A Expired CA1069945A (en) | 1976-04-09 | 1976-04-09 | Device for measuring loads, especially transient loads |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1069945A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6006386A (en) * | 1995-11-16 | 1999-12-28 | International Road Dynamics Inc. | Capacitive transducer |
-
1976
- 1976-04-09 CA CA249,974A patent/CA1069945A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6006386A (en) * | 1995-11-16 | 1999-12-28 | International Road Dynamics Inc. | Capacitive transducer |
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| MKEX | Expiry |