WO1992021007A1

WO1992021007A1 - Weighing apparatus

Info

Publication number: WO1992021007A1
Application number: PCT/SE1992/000322
Authority: WO
Inventors: Lauri Holm
Original assignee: Pad Lastceller Ulf Lundman Ab
Priority date: 1991-05-23
Filing date: 1992-05-18
Publication date: 1992-11-26
Also published as: SE469446B; SE9101571L; SE9101571D0

Abstract

An apparatus for dynamic axle-load weighing comprises a base structure, a number of load cells (9) arranged on the base structure, and a weighing platform (7) which is supported by the load cells and over which passes a vehicle to be weighed. The weighing platform (7) has, in the direction of travel (15), an extent which is at least substantially equal to, albeit larger than, the extent of the vehicle pressure surfaces (17) in this direction. The weighing platform (7) has a low weight and is easily mobile and thus of low dynamic inertia. At least one load cell (9) is arranged at each side portion of the weighing platform. The load cells (9) are of a type having a large load-receiving surface and a small height as well as being insensitive to side loads. There are provided means for producing a summation measurement value on the basis of the output signals from the load cells.

Description

WEIGHING APPARATUS

Technical Field

The present invention relates to an apparatus for dynamic weighing comprising a weighing unit with a base structure, a number of load cells arranged on said base structure, and a weighing platform or load board which is supported by the load cells and over which the object to be weighed is to pass, while in motion, the weighing plat- form having a width corresponding at least to the longest distance between the outermost lateral boundaries of the pressure surfaces of the object on the weighing platform when passing thereover, means being arranged to produce a summation measurement value on the basis of the output signals from the load cells.

The apparatus is especially designed for weighing vehicles, in particular axle-load weighing, and will therefore be described primarily in this context. Technical Background The weighing of vehicles for determining the axle load is usually performed under static conditions or at a very low travelling speed of the vehicle, typically one or a few km/h. The weighing apparatus used are of an extreme¬ ly stable design and have a considerable extension in the running direction of the vehicle. The load cells employed have to be acted on by a substantially vertical load owing to the considerable overall height. Therefore, the weigh¬ ing apparatus have high dynamic inertia and are very sen¬ sitive to side loads, which severely restricts the speed that can be allowed in dynamic weighing. Objects of the Invention

One object of the present invention is to provide a new apparatus for dynamic weighing permitting the vehicles to pass over the weighing unit at a much higher speed than has been possible hitherto. Another object of the invention is to provide an apparatus which is light and of small dimensions as well as easy to transport, i.e. is generally easy to handle. A further object of the invention is to provide an apparatus permitting simple signal processing while yield¬ ing accurate measuring results, low sensitivity to distur¬ bances, and the ability to display the measurement values with variable resolution. Summary of the Invention According to the invention, the above objects are achieved by a device exhibiting the distinctive features recited in the appended claims.

Thus, the device according to the invention is essen¬ tially characterised in that the weighing platform has, in the direction of tra¬ vel of the object passing over the weighing platform, an extent which is at least substantially equal to, albeit larger than, the extent of said pressure surfaces in said direction, the weighing platform has a low weight and is easily mobile and thus of low dynamic inertia, at least one load cell is arranged at each side por¬ tion of the weighing platform, one load cell being prefer¬ ably arranged also in the middle of the weighing platform, and the load cells are of a type having a large load- receiving surface and a small height as well as being insensitive to side loads, the load cells preferably being of the type in which a pressure-sensing fluid is provided in the form of a thin layer between two parallel plates.

The special design of the apparatus according to the invention constitutes a new approach aimed at a low weight, a low overall height involving low sensitivity to side loads, a minimum extent in the direction of travel, and a dynamically rapid response. Bearing this in mind, it has been found essential that the weighing unit comprises load cells of the special type stated above. Advantageously, use is made of load cells of the type described in EP Al 0 342 305. The extent of the load cells in the direction of travel is advantageously from about a third to about half of the extent of the weighing platform. Advantageous¬ ly, the load cells have a circular configuration, the rati of the diameter of the load-receiving surface to the heigh being at least about 5. Typically, the weighing platform has an extent in the direction of travel of between about 25 cm and about 30 cm, preferably between about 27 cm and about 28 cm, when vehicles are to be weighed. Preferably, the weighing platform is made of light metal or the like, and has reinforcing profiles in the direction of its breadth. The weighing platform is preferably supported by the load cells in floating and elastically dampening fashion.

The above features result in a mobile weighing plat¬ form having a minimal counteracting mass, i.e. very low dynamic inertia, and adapting itself with great flexibi- lity to the movements of the weighed object. As a result, there are only insignificant signal disturbances caused by pivotal movements, vibrations, impacts, and so forth. According to the invention, separate stabilising platforms on a level with and adjacent to the weighing platform are advantageously provided before and after the weighing unit in order to further reduce the risk of dis¬ turbances. Owing to the provision of such stabilising platforms, vehicle wheels can roll onto and away from the weighing platform in a very even and smooth manner. The means for producing a measurement value are pre¬ ferably adapted to integrate the output signals from the load cells, thereby to achieve a favourable filtering effect, since the impact of signal disturbances in the form of sudden signal leaps, such as signal spikes, and other similar disturbances is substantially eliminated.

The integration is facilitated if, as in a preferred embo¬ diment, use is made of analog-frequency converters for transmitting the analog output signals from the load cells to integrating means in the form of frequency counters.

Different circuits can be used for determining a desired summation measurement value, i.e. essentially the peak value for the summed-up signal from the load cells. Use may, for instance, be made of means for integrating, during a number of brief successive time intervals (typi¬ cally of a duration of a few milliseconds), the summed-up signal at its peak level, and then calculate the average value for the obtained part results. As will be appreciat¬ ed, this average value is proportionate to the desired measurement value.

According to the invention, it is, however, preferred to make use of the fact that the obtained summed-up sig- nal, i.e. the sum of the output signals from the load cells, will normally have a generally triangular course owing to the chosen length of the weighing platform. The means for producing a measurement value are then adapted to integrate the output signals from the load cells and calculate the summation measurement value on the basis of the integration result and the integration time, on the assumption that the course of the signals from the load cells is at least substantially triangular, i.e. that

summation ea- ₌ ⁽integrated summation signal⁾ x 2 surement value ^" ₍integration time₎

According to the invention, the means for producing a measurement value advantageously comprise means for deter- mining the product of the integration time and a peak value for the sum of the output signals from the load cells and for relating this product to the integration result with a view to producing a signal form factor indi¬ cating the quality of the performed weighing, so that the - result thereof may be displayed with a quality-dependent resolution. The form factor is suitably determined as the ratio of the integrated summation signal (i.e. the area under the signal) to the product of the peak value and the inte¬ gration time (corresponds to the associated rectangular area). Ideally, the form factor obviously is 0.5. Natu¬ rally, the inverse ratio may also be used, in which case the ideal form factor is 2.

Deviations of the calculated form factor from the ideal value thus indicate curve course deviations which at worst are so pronounced that the obtained measurement value has to be discarded.

Advantageously, the apparatus according to the inven¬ tion comprises means for producing a separate measurement value for the load cell(s) arranged at each side of the weighing platform, so that the weight distribution between the two sides can be determined. The apparatus may also comprise at least one centrally-arranged load cell, in which case the apparatus advantageously includes means for producing a separate measurement value for this centrally- arranged load cell, such that a comparison of this sepa¬ rate measurement value with the measurement value from the laterally-arranged load cells provides information on whether the weighed object is correctly centred when pass¬ ing. An embodiment of the invention will be described in more detail below with reference to the accompanying draw¬ ings. Brief Description of the Drawings

Fig. 1 is a partly sectional, schematic side view showing a vehicle wheel passing a weighing unit for dyna¬ mic weighing according to the invention,

Fig. 2 is a schematic graph illustrating the course of the weighing signal during the passage of the vehicle wheel over the weighing unit, Fig. 3 is a schematic top view showing a weighing unit whose weighing platform has been removed, and illu- strating the positions of five load cells, some interme¬ diate parts having been left out for reasons of clarity, Fig. 4 is a schematic longitudinal section, through a load cell, of the weighing unit and adjacent stabilising platforms, and

Fig. 5 is a schematic block diagram illustrating an example of a circuit for processing the weighing signal. Description of the Embodiment

Fig. 1 schematically illustrates a vehicle wheel 1 with an axle 3 passing over a weighing unit disposed in a roadway 5 and comprising a weighing platform or load board 7 supported by load cells 9. Stabilising platforms 11 and 13, also flush-mounted in the roadway 5, are arranged "be¬ fore and after, respectively, the weighing unit. The weigh- ing platform 7 is horizontally arranged with its upper sur¬ face on a level with the upper surfaces of the platforms 11, 13 and the roadway 5. Further, the weighing platform is disposed adjacent to the stabilising platforms, to enable the wheel to roll onto the weighing platform as smoothly as possible. The direction of movement of the wheel is indi¬ cated by an arrow 15.

In the direction of travel, the weighing platform 7 has a length of about 27.5 cm, which slightly exceeds the length of the pressure surface 17 of a normal wheel on the ground. The weighing platform is thin and light, and is made of profiled light metal to become torsionally rigid in the direction of its breadth. The weighing platform rests on five load cells distributed in the direction of its breadth (see Figs 3 and 5), two cells being provided at each side end and one cell being provided in the mid¬ dle. The load cells have a small height but a comparative¬ ly large horizontal extent. The load cells employed are of the type disclosed in EP Al 0 342305, hereby incorporated by reference, where more information can be found on the preferred embodiment of the load cells. Further, the load cells 9 rest on a base structure 19 embedded in the road¬ way. Also, the load cells 9 are circular and have a load- receiving diameter which is about half the length of the weighing platform in the direction of travel.

The stabilising platforms 11, 13 are designed to dampen vibrations, for which reason their length in the direction of travel is not a multiple of the length of the weighing platform 7. Typically, the length of the stabi¬ lising platforms is about 1.5 or about 2.5 times the length of the weighing platform.

The weighing unit 7, 9, 19 as well as the stabilis- ing platforms 11, 13 might be arranged on a special base structure embedded in the roadway and having a certain flexibility, e.g. by being made of wood, thereby further reducing the risk of undesired vibrations.

The design of the weighing unit 7, 9, 19 will be described in more detail below with reference to Figs 3 and 4. The load cells 9 are disposed in circular recesses in the profiled bottom of the base structure 19 which is a rigid, rectangular box made of light metal. A circular load-transmitting disc 71 is arranged on the upper side of the load cells. The disc presses against the upper side of the load cell by the intermediary of an element 73 of a certain elasticity. The rectangular weighing platform 7 rests on the discs 71 of the load cells 9, such that it is freely mobile. It will be appreciated that we are dealing with some sort of floating suspension, enabling the weigh¬ ing platform to take up uneven load, if any, in an advan¬ tageous manner. To this contributes the fact that the load cells 9 are also arranged in the base structure 9 via discs and elements corresponding to those on the upper side, albeit this is not necessary.

On its underside, the weighing platform has reinforc¬ ing ribs 75 running in the direction of its breadth and bringing about the application against the discs 71.

The weighing platform 7 and the stabilising platforms 11, 13 (parts of which are shown in Fig. 4) are provided with a wear layer 81, 83 and 85, respectively, on the upper side. Flexible seals 87, 89 are provided in the gap between each stabilising platform 11, 13 and the weighing platform 7.

The weighing platform is 3 m broad and typically has a weight of but about 40-75 kg, especially about 50 kg. The weight of the entire weighing unit may be as low as about 80 kg, and the unit is thus easy to move and handle.

Fig. 2 illustrates a typical course of the summation signal from the load cells 9. Obviously, the course is generally triangular, excepting a strong, spike-type dis¬ turbance immediately before the peak value. Such disturb¬ ances may be caused e.g. by a stone stuck in the tyre sculpture. When the summation signal is integrated, this disturbance will, of course, be substantially filtered out. A corresponding filtering effect is also obtained for other, similar disturbances, which are fairly common, not least in the area of the peak value S_p, but no such dis¬ turbances are illustrated here for reasons of clarity.

It will be appreciated that a conventional determi- nation of the peak value, which is the value of interest when calculating the axle load, is highly unreliable.

It has been found that the peak value can be calcu¬ lated with high accuracy on the basis of the integrated summation signal (i.e. the area under the signal) and the time for the integration (i.e. the time between t.. and t₂), on the assumption that the signal course is trian¬ gular. Then, the signal peak value can be calculated according to the following formula

(integrated summation signal) x 2

S„ = *2 - *1

If the signal course deviates considerably from the generally triangular course, the calculated peak value, i.e. the axle load measurement value, will be less accu¬ rate. This may, according to the invention, be taken into account by determining a signal form factor, in which case the measurement value is displayed with a resolution (i.e. number of figures) depending on the form factor.

Fig. 5 schematically illustrates an example of a cir- cuit for processing the signals from the load cells 9. The Figure also illustrates, schematically and seen from above, the provision of double load cells 9 at each side portion of a weighing unit on both sides of a centrally- arranged load cell, the adjacent stabilising platforms 11, 13 being indicated as well.

The analog output signals from each pair of load cells are first summed up in a circuit 21 and 23, respec¬ tively. Together with the output signal from the central¬ ly-arranged load cell, the output signals from the cir- cuits 21 and 23 are then summed up in a circuit 25. The summation signal from the circuit 25 is converted to a pulse frequency signal in an analog-frequency converter (AFC) 27, whose output signal is transmitted to a counter 31 via a threshold circuit 29. The counter begins to count the supplied pulses (i.e. begins to integrate) when the output signal from the AFC 27 goes above the threshold level and ceases to count the pulses when this signal eventually goes below the threshold level.

A microcomputer 33 is connected to the counter 31, the threshold circuit 29 and a clock circuit 35, and is programmed to perform all necessary calculations, includ¬ ing that of the signal form factor, and to show the resulting axle load measurement value on a display 37 with a suitable resolution. For each pair of load cells, the summation signals from the circuits 21 and 23 are also integrated separately by an analog-frequency converter 41 and 43, respectively, a threshold circuit 45 and 47, respectively, and a counter 49 and 51, respectively. The output signals from the coun- ters 49, 51 are transmitted to the microcomputer 33, which compares the signals to establish the load distribution between the two sides of the weighing unit. The result of this comparison may also be shown on the display 37.

The output signal from the centrally-arranged load cell is also transmitted to an analog-frequency converter 53 which is followed by a threshold circuit 55 and a counter 57 which are controlled by the microcomputer 33 with a view to calculating a separate load measurement value for the centrally-arranged load cell. Normally, this value is zero or very low. A comparison of such a measure- ment value and the measurement values from the ^terally- arranged load cell groups makes it possible to atect whether a vehicle has passed the weighing apparatus incor¬ rectly, e.g. that the vehicle wheel or wheels have passed beside the weighing unit on one side thereof.

Claims

1. An apparatus for dynamic weighing, especially for axle-load weighing, comprising a base structure, a number of load cells arranged on said base structure, and a weighing platform or load board which is supported by the load cells and over which the object to be weighed, espe¬ cially a vehicle, is to pass, while in motion, the weigh- ing platform having a width corresponding at least to the longest distance between the outermost lateral boundaries of the pressure surfaces of the object on the weighing platform when passing thereover, means being arranged "to produce a summation measurement value on the basis of the output signals from the load cells, c h a r a c t e r ¬ i s e d in that the weighing platform has, in the direction of travel of the object passing over the weighing platform, an extent which is at least substantially equal to, albeit larger than, the extent of said pressure surfaces in said direction, the weighing platform has a low weight and is easily mobile and thus of low dynamic inertia, the weighing plat¬ form being preferably made of light metal or the like, at least one load cell is arranged at each side por¬ tion of the weighing platform, one load cell being prefer¬ ably arranged also in the middle of the weighing platform, and the load cells are of a type having a large load- receiving surface and a small height as well as being insensitive to side loads, the load cells preferably being of the type in which a pressure-sensing fluid is provided in the form of a thin layer between two parallel plates.

2. An apparatus as claimed in claim 1, c h a r a c - t e r i s e d in that the load-receiving surface of the load cells has, in the direction of travel, an extent which is between about 1/3 and about 1/2 of the extent of the weighing platform in said direction.

3. An apparatus as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the load cells have a circular configuration, the ratio of the diameter of the load- receiving surface to the height being at least about 5.

4. An apparatus as claimed in any one of the preceding claims, for axle-load weighing of vehicles, c h a r a c ¬ t e r i s e d in that the weighing platform has, in the direction of travel, an extent of between about 25 cm and about 30 cm, preferably between about 27 cm and about 28 cm.

5. An apparatus as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the weighing platform is supported by the load cells in floating and elastically dampening fashion.

6. An apparatus as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the weighing platform comprises a torsionally rigid element that is pro- filed in the direction of its breadth.

7. An apparatus as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the means for producing a measurement value are adapted to integrate the output signals from the load cells and calculate the sum- mation measurement value on the basis of the integration result and the integration time, on the assumption that the signals from the load cells have an at least substantially triangular course.

8. An apparatus as claimed in claim 7, c h a r a c - t e r i s e d in that the means for producing a measure¬ ment value comprise means for determining the product of the integration time and the peak value for the sum of the output signals from the load cells and for relating this product to the integration result with a view to producing a signal form factor.

9. An apparatus as claimed in claim 7 or 8, c h a r¬ a c t e r i s e d in that the means for producing a mea¬ surement value comprise means for converting the analog output signals from the load cells to a signal-magnitude- dependent pulse frequency, as well as counting means for achieving the integration by counting the pulses.

10. An apparatus as claimed in any one of the pre¬ ceding claims, c h a r a c t e r i s e d in that it comprises means for producing a separate measurement value for the load cell(s) arranged at each side of the weighing platform, thereby making it possible to deter¬ mine the weight distribution between the two sides.

11. An apparatus as claimed in any one of the pre¬ ceding claims, comprising a centrally-arranged load cell, c h a r a c t e r i s e d in that it comprises means for producing a separate measurement value for the centrally- arranged load cell, so that a comparison between this value and the measurement value from the laterally-arrang¬ ed load cells provides information on whether the weighed object is correctly centred when passing.