WO1994027121A1

WO1994027121A1 - A weighing device

Info

Publication number: WO1994027121A1
Application number: PCT/SE1993/000419
Authority: WO
Inventors: Lars Hedberg; Urban BOLLÖ; Hans Olofsson; Leif Lindholm
Original assignee: Alfa Laval Agri Ab
Priority date: 1993-05-12
Filing date: 1993-05-12
Publication date: 1994-11-24
Also published as: AU4098693A

Abstract

This invention relates to a weighing device (12') having a weighing unit (1) comprising at least one fibre optic sensor (2). Force applied onto a load receiving means (13) is smoothly transmitted by a pair of magnets (14, 16) to the fibre optic sensor, so that damage to the optic fibre (2) is avoided and, hence, a longer durability of the weighing unit is ensured.

Description

A weighing device

FIELD OF THE INVENTION

The present invention relates to a weighing device comprising a receiving means for receiving a force from a load to be weighed, and at least one weighing unit, said weighing unit comprising at least one fibre optic sensor having an optic fibre, which is arranged between at least one pair of first and second pressure plates.

BACKGROUND OF THE INVENTION

Weighing devices in general comprising strain gauges or piezoelectric sensors have proved to be reliable. How¬ ever, they are very expensive, and for this reason efforts have been made to find cheaper weighing devices that are as reliable.

The technique of using fibre optics has developed rapidly in recent years. It has thereby been possible to develop relatively cheap weighing devices using fibre optic sensors instead of e.g. the above mentioned strain gauges and piezoelectric elements.

A weighing device of the kind including such a fibre optic sensor is disclosed in US-A-4 560 016. This fibre optic sensor comprises an optic fibre or cable arranged between two pressure plates, which are provided with protrusions facing the optic fibre. Light emitted from a light source at one end of the fibre and transmitted through the fibre to the other end thereof is measured by a fibre optic receiver or transducer. When the two pressure plates are pressed together by a force, their said protrusions cause microbending of the optic fibre, which in turn causes a light loss in the fibre detected by the receiver or transducer. This light loss corre- sponds to a particular microbending of the fibre, which in turn corresponds to a particular load on the pressure plates. The relationship between various loads and corresponding light losses is practically linear.

As already indicated the pressure plates should be adapted to cause deformation of the optic fibre when the plates are pressed together. In order to achieve this, the pressure plates are suitably provided with a series of alternating grooves and ridges facing said optic fibre. The series of alternating grooves and ridges could have any form, but should preferably have a sinusoidal form, as disclosed in EP-A-0 393 956.

The described fibre optic sensor suffers from a major drawback, though, namely its limited durability because of the repetitive microbending of the optic fibre. This jeopardizes the function of the sensor since the risk is high that the optic fibre gets damaged or even breaks when pressure is applied directly to the pressure plates without any force limiters. Many attempts have been made to overcome this drawback, but none has shown to be successful.

It is therefore an object of the present invention to provide a weighing device having an optic fibre sensor, which weighing device is inexpensive to manufacture and is durable for long periods of use.

Another object of the invention is to provide a weighing device, which can be used for loads in a wide range, only needing adjustment therefore.

According to the invention, these objects can be obtained by a weighing device of the kind initially defined, which is characterized in that at least one pair of first and second interacting, repelling magnets is provided for taking up at least part of said force received by the receiving means; said first magnet is associated with said receiving means and said second magnet is associated with said first pressure plate, the magnets being chosen and arranged such that their repelling magnetic force will keep the receiving means in an initial position of equilibrium when the receiving means is unloaded and will allow said first pressure plate to approach said second pressure plate when the receiving means is being loaded, such that said optic fibre is caused to deform; and guiding means is arranged to keep the repelling magnets positioned relative to each other.

In a weighing device according to the invention, the use of repelling magnets provides a smooth transmission of forces from loads to be weighed to the fibre optic sensor. A long term use of the weighing device is therefore ensured.

Use of magnets for transferring forces is previously known, per se, from DE-A-2 136 003.

Suitably, the weighing device is provided with at least one abutting means arranged to prevent direct or indirect mechanical contact between the magnets, since contact between the magnets caused by a load impact would otherwise be transferred to the optic fibre, which could then be damaged. Said abutting means may comprise a wall arranged between and spaced from the magnets, even though other solutions are possible, as will be seen below. It is also desirable to make possible a calibration of the above mentioned initial position of equilibrium. In order to achieve this, at least either of the magnets may be adjustable in a direction parallel to that of the repelling force of the magnets.

The magnets would need to have a strong magnetic field in order to withstand large forces. It is therefore suggested that spring means be associated with the receiving means in order to provide a predetermined load relief for the weighing unit. The spring means should then be biased to act in the same resulting direction as the magnetic force of the magnets and may be arranged to absorb tensile strain, pressure and/or bending stress. In this connection use could be made of leaf springs, helical springs, coil springs, gas springs or any other means having spring properties, well known to the man skilled in the art.

As mentioned above, the guiding means is arranged to keep the magnets positioned relative to each other. The reason for this is the well known fact that magnets positioned with the same poles opposed to one another create a repelling force such that one magnet can only approach the other with difficulty. Upon use of such repelling force, in practice, guiding means is needed to keep the magnets in their desired relative positions.

According to the invention, the guiding means may be arranged adjacent said receiving means, e.g. beside, underneath and/or above the receiving means, and may comprise at least one guide rail.

However, the guiding means need not comprise a guide rail; also other solutions have been found suitable for achieving the object of the invention. Hence, said spring means may be an integrated part of the guiding means. It is also contemplated that the guiding means may consist solely of spring means.

The skilled man will consider various possibilities of arranging the weighing unit, depending on the desired design of the weighing device. Accordingly, it is possible to arrange the weighing unit such that it measures forces in any suitable direction including rotational directions. Furthermore, it would be possible to arrange the weighing unit beside, underneath, above or even inside the receiving means depending on the physical design of the weighing device, disregarding the direction of the load force. Thus, a weighing unit arranged to measure forces in a horizontal direction need not be mounted beside the receiving means, but could also be mounted e.g. underneath or above the receiving means. This relates mutatis mutandis to forces to be measured in other than the horizontal direction.

According to a first embodiment of the invention, the receiving means comprises a weighing vessel, and said guiding means is arranged such that the weighing vessel is allowed to rotate around a horizontal axis offset from the centre of gravity of the vessel when the vessel is being filled with the load to be weighed, said weighing unit being arranged spaced from said horizontal axis and outside the weighing vessel, such that the weighing vessel when loaded will exert a pressure on the weighing unit.

In a second embodiment of the weighing device according to the invention said guiding means comprises at least one pair of cylinders, each cylinder having a jacket and an end wall; said cylinders are fitted into each other in a sliding relationship, defining an inner and an outer cylinder; said end walls are positioned remotedly from each other such that the end walls and jackets of both cylinders delimit a space, wherein said magnets are arranged inside said cylinders, preferably on the respective end walls of the cylinders; and the axial length of the jacket of said outer cylinder is greater than that of the jacket of said inner cylinder and furthermore greater than the combined extension in the same direction of said magnets and an axial distance counted from the open end of the jacket of said outer cylinder to a support member associable with said fibre optical sensor, the jacket of said outer cylinder there¬ by forming said abutting means.

The weighing device of this embodiment is not restricted to be used in weighing machines of a particular kind, since the design is easily adaptable to mass flow meters as well as to weighing machines for animals, vehicles, bridges, etc. or even to household scales.

Hence, the receiving means may comprise a platform for said animals or vehicles, or a weighing vessel for e.g. liquids, depending on the field of application of the weighing device.

If desired, the platform may be designed like a normal scales for human beings, i.e. two open sheet metal boxes, one partly enclosing the other and containing at least one weighing unit arranged therebetween.

DRAWING SUMMARY

Specific embodiments of the invention will now be described by way of example, with reference to accompanying drawings in which: Figure 1 is a schematic diagram of a fibre optic sensor of a weighing device according to the invention.

Figure 2 is a cross-sectional view of an optic fibre or cable.

Figure 3 is a schematic sectional view of part of a fibre optic sensor.

Figures 4a-4c illustrate a weighing device according to a first embodiment of the invention for measuring forces in a horizontal direction.

Figure 5 illustrates a weighing device according to a second embodiment of the invention.

Figures 6 to 11 show modifications of a weighing device according to said second embodiment of the invention.

Figures 12a and 12b show possible modifications in the arrangement of magnets used in a weighing device according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows a fibre optic sensor 1 comprising an optic fibre or cable 2, which is connected to a light emitting diode (LED) 3 connected to a suitable power source (not shown). The light emitted by the diode 3 is transmitted through the optic fibre 2.

As previously known, see e.g. US-A-4 560 016 or EP-A-0 393 956, deformation of an optic fibre reduces its light transmission efficiency. The amount of light transmitted by the fibre 2 can be measured by a fibre optic transducer 4 arranged to produce an output voltage 5, the magnitude of which depends on the intensity of the light thus measured; the stronger the light, the stronger the output signal. An attenuation of light in the fibre 2, caused by deformation of the fibre, may be used as a basis for calculation of the force having caused said deformation. For the calculation a microprocessor 6 is provided.

Figure 2 shows a cross-section of a typical optic fibre or cable used in fibre optic sensors. Reference numeral 9 designates a core of the fibre, 10 a cladding and 11 an optional protection cover.

In practice, the above mentioned deformation of the optic fibre 2 is caused by a pair of pressure plates 7, 8 between which the fibre 2 is arranged. As seen in figure 3, the plates 7 and 8 have in their opposing surfaces a plurality of parallel alternating grooves and ridges. When a pressure is applied to the pressure plates, the fibre 2 is pinched therebetween. Such pinching is commonly known as "microbending" of the fibre. During microbending there is a risk for cutting marks to be formed in the fibre due to exceedingly sharp ridges on the pressure plates. A sinusoidal cross- sectional form has been chosen for the alternating grooves and ridges in the pressure plates 7, 8 in order to avoid such cutting marks, which is also in accordance with the object of this invention.

Results of research made have shown that the distance A (see figure 3) between two adjacent ridges in a pressure plate should be chosen outgoing from the following equation in order to achieve an optimal pattern for the pressure plates: πrnio— ^" n₉-n₁₀

- where r is the radius of the core 9, n₉ is the refraction index of the core 9 and n₁₀ is the refraction index of the cladding 10.

An arrangement as described above with reference to figures 1-3 may be used as a weighing unit for sensing of loads of various kinds.

Figure 4a-c illustrate a weighing device 12 according to a first embodiment of the invention, using a weighing unit of the kind just described. In figs 4a-c the weighing unit is arranged for sensing loads in a substantially horizontal direction. The weighing device 12 is provided with a load receiving means comprising a holding means 13 which is adapted either to carry a load by itself or to carry for instance a vessel, like vessel 13a in fig 4b, for receiving a load to be weighed.

Fastened to the lower part of the holding means 13 is a first magnet 14. This magnet 14 is situated on the same side of a wall 15 as the holding means 13. On the opposite side of the wall 15 a second magnet 16 is carried by a first pressure plate 7 of the weighing unit. This first pressure plate 7 is horizontally movable relative to a stationary second pressure plate 8. As shown, the optic fibre 2 of the weighing unit extends between the pressure plates 7, 8 and is arranged to be squeezed or pinched therebetween. Means 17 is provided for supporting the stationary pressure plate 8 and for supporting and guiding the movable pressure plate 7. It should be added that the two magnets 14, 16 are turned with the same magnetic poles towards each other and are, thus, subjected to a repelling force when situated close enough together.

Reference numeral 18 designates a guiding means connected directly or indirectly to the holding means 13 and supported by the wall 15. The guiding means 18 consists of a leaf spring, which helps not only to keep the magnets positioned opposite to each other but also provides a predetermined load relief for the magnets. The leaf spring is biased to act in the same resulting direction of torque as the repelling force of the magnets. The use of a leaf spring makes it possible to achieve a compact weighing device 12, even though any kind of spring means may be adapted to the weighing device to interact with the holding means 13.

As already indicated the magnets 14, 16 are separated by the wall 15, which serves as abutting means for the magnet 14 associated with holding means 13. Magnet 14 can thus never touch magnet 16 or, in other words, direct or indirect mechanical contact between the magnets is avoided, which in turn ensures that the optic fibre 2 will not unintentionally be mechanically over¬ loaded. At least either of the magnets 14, 16 may be adjustable in a direction parallel to the repelling force of the magnets, in order to facilitate cali¬ bration. In fig 4a this has been indicated schematically at 19 in connection with the magnet 14.

Holding means 13 is prepared for attachment of different kinds of auxiliary load receiving means. For instance, attachment means may be provided in the form of screws, hook means, dove- ail joint means or any other suitable means for attaching the auxiliary load receiving means to the holding means 13.

Figure 4b shows the weighing device illustrated in figure 4a, having an auxiliary load receiving means or weighing vessel 13a releasably attached to the holding means 13.

In figure 4c a deflection B caused by the weight of a liquid 20 filled into the weighing vessel 13a has been indicated. The deflection B of the vessel causes the magnet 14 to move towards the wall 15. The repelling force of the magnet 14 causes, in turn, the magnet 16 to move away from the wall 15. The magnet 16 is as said before, associated with the pressure plate 7.

The deflection B of the weighing vessel 13a causes movement of the magnet 16 and the pressure plate 7 a certain distance C. As explained above with reference to figures 1-3, the movement of the pressure plate 7 a distance C is immediately recognizable as a certain attenuation of light in the optical fibre 2.

If metering of a mass flow is desired, the weighing device according to figures 4b and 4c may be equipped with an inlet, an outlet and a valve means associated with said outlet and/or inlet. The valve means may open said inlet when the weighing vessel 13a is empty and maintain it open until the weighing unit signals to close it again, the weighed mass being registered and the outlet valve then being opened. The outlet valve is thereafter closed and the sequence is repeated from the beginning. A specific application of such a flow meter is mass flow meters for milking machines.

Figure 5 shows a weighing device 12' according to a second embodiment of the invention. In this embodiment there are provided guiding means in the form of a cylinder 21 closed in one end and open in the other. The cylinder 21 is fitted into a similar but larger cylinder 22. The cylinders have jackets 21a, 22a and end walls 21b, 22b, which latter are situated at the cylinder e:rs turned away from each other, such that the cylinder jackets and the end walls delimit a space. It should be noted that the cylinders 21, 22 need not have a circular cross-section; the man skilled in the art would use any cross-section of the cylinders suitable for his purposes.

Within said closed space said magnets 14, 16 are mounted directly onto the inside of said end walls 21b and 22b, respectively. At least either of the magnets is adjustable in a direction parallel to the repelling force of the magnets, in order to facilitate calibration, as indicated at 23.

In this embodiment of the invention the means for receiving a load to be weighed is constituted by the end wall 22b of the larger cylinder 22. Like in the previously described embodiment of the invention, the receiving means may also in this case be provided with an auxiliary load receiving means to transmit load forces to be measured, such as a vessel or a platform, as will be further explained below.

The inner cylinder 21 is associated with weighing unit of the kind described with reference to figures 1-3. This weighing unit is, in turn, associated with a support member 24. The jacket 22a of the outer cylinder 22 is axially longer than the jacket 21b of the inner cylinder 21. Furthermore, the axial length B of the outer jacket 22a is greater than the combined extension x+y, in the same direction, of the magnets 16, 17 (including any calibration distance), and a distance z counted from the open end of said jacket 22a of the outer cylinder 22 to said support member 24. By such a configuration, the jacket 22a of said outer cylinder 22 may serve as an abutting means, provided that the axial length of the inner jacket 21a is small enough to avoid that it abuts the end wall 22b of the outer cylinder 22 before the outer jacket 22a abuts the support member 24. Spring means (not shown) may be arranged inside or outside said cylinders to relieve the magnets from a predetermined load. It should be noticed that in the figures, the distance betwen the magnets 14 and 16 has been exaggerated for reasons of clarity.

Adjustment of said magnets may be performed by means of the adjustment means indicated at 23. This means may be solely a screw that could be made accessible from outside the cylinders in order to enable adjustment readily.

Figure 6 illustrates a weighing device 12 ' according to figure 5 arranged for sensing loads in a substantially horizontal direction. The weighing device 12 'incorporates not only guiding means comprising cylinders 21, 22 as in fig 5 but also spring means in the form of a leaf spring 18 as in figs 4a-c. Of course, any kind of spring means may be used. A weighing vessel 13a is disconnectably attached to a holding means 13 of the weighing device. As discussed above in connection with figures 4a-c, the intermittent filling and emptying of the weighing vessel 13a creates a limited rotational movement of the vessel around a horizontal axis outside the vessel. Accor- dingly, the connection between the vessel 13a and the cylinder 22 should preferably not be a stiff connection. Accordingly, a ball joint, a sharp edge, a hinge joint or any kind of movable joint may be used for this purpose. In figure 6 the connection is illustrated as including a sharp edge member 25 fastened to the holding member 13 and resting with its sharp edge against the end wall of the cylinder 22.

The cylinder 21 is associated with the fibre optic sensor 1, which is rigidly connected to a wall 26. The leaf spring 18 is also connected to a wall 27, which may be the same wall to which the fibre optic sensor is connected.

Figure 7 shows an installation of a weighing device 12' according to figure 5 for sensing of loads in a substan¬ tially vertical direction, a holding means 13b being suspended by means of a pivot member 28 from the upper end of a substantially vertically extending structure 29. The pivot member 28, which is connected with the holding means 13b, may be arranged to rest directly on the weighing device 12' but, as shown, in this embodiment an exchangeable intermediate member 30 is provided in order to protect the weighing device from being worn.

The holding means 13b is prepared for attachment of different kinds of auxiliary load receiving means. For this purpose, hook means 31 and rest means 32 are provided. Of course, other means obvious to the skilled man would be possible to use, such as dove-tail joints, screws, key lock joints, bayonet couplings etc.

Furthermore, means 33, 34 may be provided for supporting the holding means 13b vertically slidable against the substantially vertical bearing surface of the structure 29. Such means 33, 34 may comprise wheels, sliding surfaces or the like.

Figure 8 shows the weighing device according to figure 7, provided with auxiliary load receiving means 13c. This means 13c comprises a pair of interconnected plates 35, 36, one (35) of which extends substantially vertically and is hooked onto the hook means 31 while, resting with its lower end on the rest means 32, the other plate 36 extends substantially horizontally and has a receiving surface for a load to be weighed. The plates 35, 36 may be fixed relative to each other, but are preferably hinged at reference numeral 37 such that the plate 36 can be folded to a position adjacent to the plate 35 when not in use.

Weighing devices of this kind can be used e.g. as household scales, car weighing machines or animal weighing machines.

Figure 9 shows the weighing device according to figure 7, the holding means 13b being provided with a bowl or a trough 13d that is hooked onto the hook means 31 and rests on the rest means 32. Such a weighing device may be used for weighing liquid, powder, granules or any other flowable mass, e.g. in connection with feeding troughs for animals. The weighing device 12' according to figure 5 may also most suitably be used in weighing machines comprising one or more platforms. Such weighing machines are common for weighing cattle, vehicles, goods or the like. Thus, in such a weighing machine, one single weighing device 12' could be used for one platform but it may be desirable to use at least three weighing devices for each platform for the obtainment of a better accuracy of measurement. It may be possible to improve the measure- ment accuracy even more by use of two or more platforms, each being equipped with one, two or preferably at least three weighing devices.

Figure 10 shows an installation using two weighing devices, each designed broadly as the one shown in fig 7 and both carrying together between themselves a platform 13e. As can be seen, an animal to be weighed stands on the platform. As further illustrated schematically, load signals from both of the weighing devices are to be transmitted to and be treated in a common micro¬ processor 6.

Figure 11 illustrates a weighing installation having two platforms 13f and 13g and weighing devices arranged underneath both platforms. The output signals from the weighing devices are transmitted to a microprocessor 6.

In a weighing device according to the invention the magnetic forces may create deviation of the desired movement of the magnets. In order to avoid such deviation, it is suggested to arrange magnets 14a and 16a not exactly opposite to but somewhat offset from each other, as shown in figures 12a and 12b. This ensures a smooth movement of the magnets also when they are relatively close to each other. Of course, the present invention is not intended to be restricted to any particular form or arrangement, or any specific embodiment disclosed herein, since the same may be modified in various ways without departing from the scope of the appended claims. For instance, the magnets may be exchangeable if stronger magnetic flux is desirable, which could be the case after long term use. Moreover, electromagnets may be used instead of permanent magnets.

It should be emphasized that the same weighing device can be used for a wide range of loads. Fine calibration is made, as explained above, by means of adjustable magnets. Coarse adjustment is made e.g. by means of differently strong springs, which is less cumbersome than exchanging magnets. The strength of a spring is thereby chosen such that it can take up force within a reasonable range of desired loads to be measured. Fine adjustment is thereafter performed with respect to the chosen strength of the spring to set the weighing device at a desired first position of equilibrium. The spring characteristics, thus, have to be well defined and known.

Claims

1. A weighing device (12, 12') comprising:

- receiving means (13, 13a-g) for receiving a force from a load to be weighed, and

at least one weighing unit (1),

said weighing unit (1) comprising at least one fibre optic sensor having an optic fibre (2), which is arranged between at least one pair of first and second pressure plates (7, 8),

c h a r a c t e r i z e d i n that

at least one pair of first and second interacting, repelling magnets (14, 16) is provided for taking up at least part of said force received by the receiving means (13, 13a-g);

said first magnet (14) is associated with said receiving means (13, 13a-g) and said second magnet (16) is associated with said first pressure plate (7), the magnets (14, 16) being chosen and arranged such that their repelling magnetic force will keep the receiving means in an initial position of equilibrium when the receiving means is unloaded and will allow said first pressure plate (7) to approach said second pressure plate (8) when the receiving means is being loaded, such that said optic fibre (2) is caused to deform; and

guiding means (18) is arranged to keep the magnets (14, 16) positioned relative to each other. 2. A weighing device according to claim 1, wherein at least one abutting means (15) is provided for restric¬ ting movement of said first magnet (14) in order to avoid direct or indirect mechanical contact between said magnets (14, 16).

3. A weighing device according to claim 1 or 2, wherein at least either of said magnets (14, 16) is adjustable for calibrating purposes in a direction parallel to that of the magnetic force of the magnets.

4. A weighing device according to claim 1, wherein spring means is associated with said receiving means (13, 13a-g) for providing a predetermined load relief for said weighing unit (1).

5. A weighing device according to any one of the preceding claims, wherein

- said guiding means (14) comprises at least one pair of cylinders (21, 22), each cylinder having a jacket (21a; 22a) and an end wall (21b; 22b),

said cylinders are fitted into each other in a sliding relationship, defining an inner and an outer cylinder,

said end walls (21b; 22b) are positioned remotedly from each other such that the end walls and jackets of both cylinders (21, 22) delimit a space, wherein said magnets (14, 16) are arranged inside said cylinders, preferably on the respective end walls of the cylinders, and the axial length (B) of the jacket (22a) of said outer cylinder (22) is greater than that of the jacket (21a) of said inner cylinder (21) and furthermore greater than the combined extension (x+y), in the same direction, of said magnets and an axial distance (z) counted from the open end of the jacket (22a) of said outer cylinder (22) to a support member (24) associable with said fibre optical sensor, the jacket (22a) of said outer cylinder (22) thereby forming said abutting means.

6. A weighing device according to claim 4, wherein said spring means is an integrated part of said guiding means (18).

7. A weighing device according to claim 6, wherein said guiding means (18) comprises at least one leaf spring.

8. A weighing device according to any one of the preceding claims, wherein said receiving means (13, 13a) has an upper end, a lower end and sides, seen in a vertical plane, and wherein said weighing unit (1) is arranged beside said receiving means to take up load forces substantially in a horizontal direction.

9. A weighing device according to any one of the preceding claims, wherein said at least one weighing unit (1) is arranged underneath said receiving means (13, 13e-g) to take up load forces substantially in a vertical direction.

10. A weighing device according to any one of the preceding claims, wherein said receiving means (13, 13e-g) comprises at least one platform for supporting said load. 11. A weighing device according to any one of claims 2 to 10, wherein said abutting means (15) comprises a wall between said first and second magnets (14, 16), said wall being arranged at a distance from said magnets, respectively.

13. A weighing device according to any one of the preceding claims, wherein said receiving means (13a, 13d) comprises a weighing vessel.

14. A weighing device according to claim 13, wherein said weighing vessel is allowed to rotate around a horizontal axis offset from the centre of gravity of the vessel when the vessel is being filled with the load to be weighed, said weighing unit being arranged spaced from said horizontal axis and outside said weighing vessel such that when loaded the weighing vessel will exert a pressure on said weighing unit (1) .