WEIGHING APPARATUS

Abstract: Weighing apparatus comprises a fixed member (11) and load supporting means such as a weigh pan or weigh platform (31) movable relative to the fixed member. There is a load sensing device (17, 22, 29) directly coupled between the fixed member (11) and the load supporting means (31), the coupling providing a degree of steadying for the load in the horizontal direction. In one embodiment a weigh pan (31) is solely supported, cantilever fashion, on a movable plate (17), the plates (11) and (17) being interconnected by flexible arms (18, 19, 20). A weigh cell (22) extends between plate (17) and an arm (16) rigid with plate (11). The arrangement ensures that there is virtually no interference with accurate weighing from frictional pivots, hinged links, or external supports or steadying means.

Description: Weighing Apparatus The invention relates to weighing apparatus and particularly to weighing apparatus incorporating electrical load cells. When utilising weighing apparatus incorporating load cells, it is desirable to transmit the load to be measured to such a cell in a linear direction such that the line of action is substantially constant and along a fixed path. Preferably the load should be applied to a single cell. Ideally, the load should be measured by a single load cell positioned directly on a vertical line passing through the centre of gravity of the load placed on the weighing platform or weigh pan. When weighing a granular load however, feed and discharge means will normally be positioned on this vertical line and so location of a load cell on the vertical line is not possible. Even en load cells can be placed directly below or above a weighing platform or weigh pan, other supports or steadying means are frequently necessary, particularly with high speed weighing. When a load is displaced to apply load to the load cell, the supports or steadying means accept some load and this may affect the reading or measurement of the load. If a plurality of load cells is used, such that the load is carried directly or indirectly by two or three load cells acting in unison, problems arise from the need to calibrate and match the load cells to provide accurate measuring. It is known to use load cells with a system of beams but pivot friction and the paths travelled by the pivots make it difficult to provide accurate weighing. Load cells incorporated in cantilever beams have also been used, usually in pairs, but such load cells are expensive and the cost may be prohibitive on certain type of apparatus. Furthermore, other supports or steadying means may still be necessary. Load cells are extremely accurate in the measurement of tensile or compressive forces but in many cases do not lend themselves for use in hostile environments such as, for example, the weighing of granular loads such. as fertilizers, where ingress of material to the load cell can cause rapid deterioration and failure of the cell and its electrical connections. Consequently known load cells for this type of weighing application have been protected by encasement in resin or rubber compounds, or by metallic covers, but this tends to affect the temperature or resistance properties of the cells and is generally unacceptable where very accurate weighing is required. The invention seeks to provide weighing apparatus incorporating a singleload cell which does not require steadying means such as anti-sway links or tension wires which are normally required to steady the load, particularly on high speed automatic weighing apparatus. According to the invention weighing apparatus comprises a fixed member, load supporting means such as a weigh pan or weigh platform movable relative to the fixed member, and a load sensing device e.g. comprising a load cell, the load sensing device being directly coupled between the fixed member and the load supportIng means,. the coupling providing a degree of steadying for the load in a horizontal direction. By ensuring that the direct coupling of the load sensing device itself provides a degree of steadying, additional steadying means may be eliminated. The load sensing device may be positioned to one side of the load supporting means, thus making it very suitable for use in weighing granular loads, where feed and discharge means may be positioned respectively above and below the load supporting means. Positioning the load sensing device to one side also makes it possible to isolate and protect the load sensing device from the material being weighed thus preventing contamination from adversely affecting the load sensing device. By directly coupling the load sensing device between the fixed member and the load supporting means it is possible to avoid the need for moving pivots or pivoted links. Controlled relative movement can be permitted between the fixed member and the load supporting means by the use of frictionless hinges or flexible membranes which carry or transmit the load to be measured or at least part thereof. The elimination of moving parts such as levers, pivots or suspension flexures between the load and the load sensing device is also desirous for repeatable, accurate weighing over long periods, such elimination also avoiding movement and wear which has been frequently found in conventional weighing apparatus. Preferably a single load sensing device, for example a single load cell, is used. This allows better access to the load supporting means. It also avoids the need to calibrate and match a plurality of cells and a single cell has a greater working range than a plurality of cells. If, for example, two cells are used, they only carry half the load, and have a correspondingly reduced working range. The coupling may have a first part and a second part arranged for relative movement parallel to the first part, the first and second parts being spaced apart by flexure means controlling the parallel movement, and the load sensing device being arranged to generate a signal in response to the relative movement between the first and second parts. Preferably the load sensing device is positioned such that it is subject to a substantially pure tensile load. Alternatively the load sensing device may be positioned such that it is subjected to a substantially pure compressive load. Arranging for the.load sensing device to be loaded purely in tension or purely in compression makes it possible to avoid the use of a complex and costly shear beam type load cell. - It also means that the weighing measurements are unaffected by the fact that the load may be offset to one side of the load sensing device. The first and second parts, in combination with the flexure means, may form a construction substantially that of a parallelogram. Preferably the flexure means only flexes adjacent to the first and second parts and does not bend along its total length, the load sensing device being attached directly between the first and second parts. Preferably the load sensing device comprises a load cell. Preferably the load supporting means comprises or is attached to a movable one of the two parts, the remaining part being fixed, for example to a frame of the weighing apparatus. Preferably the load supporting means projects, cantilever fashion, from a vertical plate, the plate being mounted for vertical movement on a rigid support frame by two or more flexible members, the plate being supported by a load cell directly coupled between the plate and apart of the rigid support frame. Preferably the load supporting means is releasably connected to the plate. By way of example, specific embodiments of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a side view of an embodiment of weighing apparatus according to the invention, with the weighing pan omitted; Figure 2 is an end view of the apparatus shown in Figure 1; Figure 3 is a view similar to Figure 1 but showing the weigh pan in position; Figure 4 is a diagrammatic side view of an embodiment of batch weighing apparatus according to the invention; Figure 5 is a diagrammatic side view of an embodiment of continuous weighing apparatus according to the invention; Figure 6 is a side view of an embodiment of check weighing apparatus according to the invention; and Figure 7 is a side view of an embodiment of belt weighing apparatus according to the invention. Referring firstly to Figures 1 to 3, the weighing apparatus has a rigid support frame mounted on a chassis 10. The support frame comprises a first vertical plate 11 from which project two spaced apart upper lugs 12, 13 and one central lower lug 14. A central triangular web 15 supports a horizontally extending arm 16. A second movable vertical plate 17 is connected to the plate 11 by three flexure arms 18, 19'and 20. These arms extend respectively from the lugs 12, 13 and 14 to similar lugs on the plate 17. Each flexure arm has portions 21 of reduced thickness enabling the arms to flex adjacent to the plates 11 and 17, the remainder of the length of the arms remaining rigid. Thus the plates and the flexure arms form a parallelogram type construction, guiding the plate 17 for vertical movement parallel to the plate 11 with a minimum of friction and wear. A load cell 22 is connected between the rigid arm 16 and a lug 23 on the plate 17. The load cell 22 is connected by means of upper and lower flexure tapes 24 and each tape is connected to the arm 16 or lug 23 by a threaded rod and nut arrangement 25 which enables the position and/or pretension of the load cell 22 to be finely adjusted; It will thus be seen that downward movement of the plate 17 applies a substantially purely tensile load to the load cell 22. The degree of vertical movement of the plate 17 is limited by upper and lower adjusting screws 26, 27, fitted into further lugs 28 on the plate 17. The downward movement of the plate 17 is limited by engagement of the screw 26 with the upper surface of the arm 16 and the vertical movement of the plate 17 upwardly is limited by engagement of the screw 27 with the underside of the arm 16. Projecting horizontally from the plate 17, away from the load cell 22, in the manner of a cantilever, is a spigot 29 bearing a rectangular flange 30. Figure 3 illustrates the weighing apparatus utilised for automatic weighing. A weigh pan 31 isPeleasably connected to the spigot 29 by means of a claw 32 which engages over the flange 30 thus transmitting all the load of the weigh pan 31 and any material therein to the plate 17. When it is desired to remove the weigh pan 31 it can be uncoupled by lifting it vertically off the flange 30; The weigh pan 31 is in the form of a hopper having a pair of lower doors 33 which can be opened by extending a ram 34 and can be closed again by retracting the ram. Above the hopper 31 is a filling and metering device 35 which is described in more detail in our co-pending U.K. Patent Application of even date. In use the device 35 is used to fill the hopper 31 and when the weighing apparatus indicates that the hopper 31 has received a predetermined desired quantity of particulate material the metering device 35 is closed to prevent the supply of further material and the doors 33 are opened to release the measured quantity of material. The cycle is then repeated and the weighing may take place very rapidly. When carrying out automatic weighing in this way, particularly when it is carried out rapidly, there is a tendency for the hopper 31 to be shaken from side to side as the material falls into it, and this sideways movement can adversely affect measurements of conventional weighing apparatus. For this reason it is customary to attach steadying links to the hopper 31. In the present case, the side attachment of the plate 17 to the hopper itself provides a degree of steadying movement in the horizontal direction. The construction of the weighing apparatus as shown in Figure 3 is analagous to a person holding a saucepan wherein the pan is the hopper 31, the handle of the saucepan is equivalent to the spigot 29, and the person's wrist supporting and sensing the load is equivalent to the load cell 22. In operation the load to be weighed, W (see Figure 1) is applied via the spigot 29 to the plate 17. The plate 17 thus moves under the influence of the load W, travelling parallel to the plate 11 under the control of the flexure arms 18, 19 and 20. Thus the lug 23 of the plate 17 drops with respect to the arm 16, inducing a tensile load in the load cell 22. The electrical signal from the load cell may be transmitted to either a digital or analogue readout device or alternatively may be used for control purposes, for example to control the discharge of material into the hopper. The invention has other applications, some of which are illustrated in Figure 4 to 7. In Figure 4, an apparatus substantially identical to that shown in Figures 1 and 2 has a weigh pan attached thereto in the form of a batch weigher 36. Particulate material is fed continually into the pan 36 and periodically, dependent upon the load in the pan at a given time, a pivotal door 37 is opened to deposit a batch, and is then immediately closed, by means of a ram 38. In the apparatus of Figure 5 an arrangement similar to that shown in Figures 1 and 2 is used to support a platen 39 positioned under a continuous belt 40. Signals from the load cell 22 may for example be fed to the conveyor prime mover to adjust its speed and hence control the flow of material delivered from the conveyor. In Figure 6 apparatus similar to that shown in Figures 1 and 2 is used to support a platen 41 positioned under a continuous belt 42 used to carry individual packages 43. Seach package passes over the platen 41 its weight can be checked. In Figure 7 apparatus similar to that shown in Figures 1 and 2 is is used to support a roller 44 which is positioned between two other support rollers 45 of a continuous belt conveyor 46 carrying material 47. In this arrangement the load cell 22 is continually monitoring the weight of material carried by a given length of the belt. The weighing apparatus described above with referance to the drawings does not require any anti-sway links or tension wires even for high speed automatic weighing. Although the load is applied cantilever fashion to one side of the apparatus, the loading of the load cell substantially in pure tension enables an accurate reading to be obtained. Because the only moving parts involved are the low friction flexure arms 18, 19 and 20, repeatable, accurate weighing can be obtained over long periods, as there is very little movement and substantially no friction or wear. The use of a single load cell mounted to one side of the weigh pan allows good access to the weigh pan. The use of a single cell also means that there is no requirement to match and calibrate a plurality of cells and the capacity of the single cell is increased. The single cell has, for example, a working range which is twice that which would be provided by a pair of identical cells. Since a simple tension cell may be used, there is no need to use the more expensive shear beam type. Not only can the cell be hermetically sealed but it is, in any event, positioned to one side of the weighing area, out of the immediate vicinity of the particulate material being weighed. It can, if desired, be housed in a separate dustproof compartment. The invention is not restricted to the details of the foregoing embodiments. It may be adapted to any type of weighing device, either industrial or domestic, having a single load cell or transducer which carries all the forces from the weighing platform or container. The invention can be used to reduce or eliminate the use of springs, levers and the like, normally associated with known weighing machines. It is possible for devices having the features of the invention to meet all the desirable parameters required with other forms of weighing machines or systems.

Claims: Claims 1. Weighing apparatus comprising a fixed member (11), load supporting means such as a weigh pan or weigh platform (31) movable relative to the fixed member, and a load sensing device (17, 22, 29) e.g. comprising a load cell characterised in that the load sensing device is directly coupled between the fixed member and the load supporting means, the coupling providing a degree of steadying for the load in a horizontal direction. 2. Weighing apparatus as claimed in claim 1, in which the load sensing device is positioned to one side of the load supporting means, thus making it very suitable for use in weighing granular loads, where feed and discharge means may be positioned respectively above and below the load supporting means. 3. Weighing apparatus as claimed in claim 1 or claim 2, in which a single load sensing device is used. 4. Weighing means as claimed in any one of the preceding claims, in which the coupling has a first part and a second part arranged for relative movement parallel to the first part, the first and second parts being spaced apart by fixture means controlling the parallel movement, and the load sensing device being arranged to generate a signal in response to the relative movement between the first and second parts. 5. Weighing apparatus as claimed in claim 4, in which the load sensing device is positioned such that it is subject to a substantially pure tensile load. 6. Weighing apparatus as claimed in claim 4, in which the load sensing device is positioned such that it is subjected to a substantially pure compressive load. 7. Weighing apparatus as claimed in any one of claims 4 to 6, in which the first and second parts, in combination with the flexure means, form a construction substantially that of a parallelogram. 8. Weighing apparatus as claimed in claim 7, in which the flexure means only flexes adjacent to the first and second parts and does not bend along its total length, the load sensing device being attached directly between the first and- second parts. 9. Weighing apparatus as claimed in any one of claims 4 to 8, in which the load supporting means comprises or is attached to a movable one of the two parts, the remaining parts being fixed, for example to a frame of the weighing apparatus. 10. Weighing apparatus as claimed in claim 9, in which the load supporting means projects, cantilever fashion, from a vertical plate, the plate being mounted for vertical movement on a rigid support frame by two or more flexible members, the plate being supported by a load cell directly coupled between the plate and a part of the rigid support frame. 11. Weighing apparatus as claimed in claim 10, in which the load supporting means is releasably connected to the plate.

Inventors: FORD, DEREK WILLIAM
Applicants: DOBSON PARK INDUSTRIES PLC
Application year: 1985
Main classification: G01G3
IPC codes: G01G21, G01G3