WO2003006942A1

WO2003006942A1 - Ring shaped load cell

Info

Publication number: WO2003006942A1
Application number: PCT/NO2002/000259
Authority: WO
Inventors: Audun Andersen; Karstein Bransvik; Bjørn BURMANN; Andre Larsen
Original assignee: Boltsafe As
Priority date: 2001-07-13
Filing date: 2002-07-12
Publication date: 2003-01-23
Also published as: NO314369B1; EP1430282A1; NO20013483L; KR20040032861A; NO20013483D0

Abstract

A ring-shaped load cell for measuring compression loads, comprising a number of strain gauges (6) mounted on a ring body (2) and connected in an electric circuit for delivery of an output signal representing occuring loads on the cells. The ring body (2) of the load cell is bipartite and comprises two half-ring bodies (3, 4), each half-ring body being provided with respective strain gauges (6) which are connected in an associated electric circuit for delivery of said output signal.

Description

Ring shaped load cell

The invention relates to ring-shaped load cell for measuring compression loads, comprising a number of strain gauges mounted on a ring body and connected in an electric circuit for delivery of an output signal representing occurring loads on the cell.

Ring-shaped load cells of this type are known in different embodiments. As examples of the prior art reference can be made to GB 1 162 926, GB 2 126 357, US 3 422 671 and DE 2 357 484 C2. In these previously known constructions, the load cells consist of a solid ring body which is manufactured in one piece.

In a number of constructions, especially in connection with bridges, dams and other constructions which contain pre-stressed anchorings, the construction is characterised in that wire ends, or bundles of wires, and tension rods are anchored and pre-stressed. It is here essential to control the pre-stressing in the stay or wire, either only when setting up, in a periodic control, or as a permanent monitoring during the lifetime of the construction.

When mounting the known ring-shaped load cells in anchorings of the above- mentioned type, the pre-stressing must be removed, the coupling must be disassembled, a load cell must be treaded onto the structure, and the coupling must be assembled once more in order to apply a new pre-stressing. Such an operation is often quite unacceptable because of the risk for breake down of the whole construction when removing the pre- stressing.

The object of the invention therefore is to provide a load cell structure which eliminates the above-mentioned drawbacks and problems, and which is shaped in such a manner that it can be installed by only jacking back wires or tension rods in order to mount the load cell.

The above-mentioned object is achieved with a load cell of the introductorily stated type which, according to the invention, is characterised in that the ring body of the load cell is bipartite and comprises two half-bodies, each half-body being provided with respective strain gauges which are connected in an associated electric circuit for delivery of said output signal.

The mechanical boundary conditions in constructions of the above-mentioned type, an uneven support and an oblique tension in wires or tension rods, result in that the accuracy may become very poor if the sensor elements (the strain gauges) are mounted in normal manner, i.e. in the centre of theoretical lines of force. In order to overcome this problem, an advantageous embodiment of the load cell according to the invention is characterised in that the strain gauges are mounted peripherally spaced in each half-ring body on respective ones of two concentric circular surfaces, so that the strain gauges sum up forces over an area. In this manner one achieves a reduced sensitivity for boundary conditions, and an increased measuring accuracy with different mechanical boundary conditions.

The load cell according to the invention is particularly intended for use on end stays, earth anchors, wires and similar pre-stressed structures. The design with a bipartite ring entails that the units can be installed and replaced without removing the pre-stressing in the anchoring in question.

The invention will be further described below in connection with exemplary embodiments with reference to the drawings, wherein

Fig. 1 shows a schematic plan view of an embodiment of a load cell according to the invention, with partly shown connection cables and an electric signal adaptation unit;

Fig. 2 shows a side view of the load cell, as viewed in the direction of the arrow A in Fig. 1 ;

Fig. 3 shows a segment of Fig. 2, with a cavity in which there are mounted two strain gauges as shown in the enlarged detail in the figure; Fig. 4 shows the load cell in Fig. 1, and the different excitation signal conductors forming part of the connection cables;

Fig. 5 shows diagrams of coupling arrangements for the strain gauges which are mounted in the two half- ring bodies; and

Fig. 6 is a circuit diagram showing a bridge connection of the pairs of strain gauges, in the load cell according to Fig. 1, and an amplifier on the output of the bridge connection.

As appears from Fig. 1, the ring-shaped load cell or dynamometer 1 according to the invention comprises a ring body 2 which is bipartite and consists of two half-ring bodies 3 and 4 having a constructive design which is preferably symmetrical about an axis X-X.

Each half-ring body 3, 4 has an essentially rectangular cross-section in a radial plane, and is provided with a number of recesses or cavities 5 which are arranged along the outer peripherial surface and receive respective sensor elements in the form of strain gauges 6, as shown in Fig. 3. For sealing purposes a lid (not shown) will be welded over each cavity.

As further described below, the strain gauges 6 in each ring half are connected in an electric circuit comprising electric leads which, via connection cables 7, are brought out to a signal adaptation unit 8. The connection cables 7 are partly shown in Fig. 1, and in practice generally consist of hydraulic cables which are covered by strong jackets 9, to stand the often rough environment in which such load cells are applied.

In order to obtain an acceptable measuring result when using strain gauges, it is a condition according to the conventional technique that the mechanical strain occurring in the area wherein the strain gauges are mounted, is homogeneous and representative of the applied force. In order to ensure that these conditions are fulfilled, load cells normally are made with a relatively great height. However, this is often very impractical, and frequently not possible since there is not a sufficient space for assembly.

In the present load cell the strain gauges are positioned so that the demand for a homogeneous load field is no longer important, so that the height of the load cell thereby can be reduced. This is achieved in that the strain gauges are positioned in such a manner that they sum up the forces over an area. For this purpose the strain gauges 6 in each half- ring body 3 and 4 are mounted on respective ones of two concentric circular surfaces which are shown stippled in Fig. 4 and designated 10 and 11, more specifically in that the strain gauges are mounted at the bottom of the respective cavities 5 whose bottom surfaces are located on said circular surfaces.

The shaded field A_L in Fig. 4 shows the circular load area. Any applied force lying inside of this area will be summed up, and one achieves thereby an increased measuring accuracy with different mechanical boundary conditions (uneven load on the area). One has found that an optimum measuring accuracy is obtained when the circular surfaces 10 and 11 are located at a radial distance outside of the inner diameter of the ring body 2 which is equal to 1/3 and 2/3, respectively, of the radial width of the ring body.

If one imagines that the strain gauges where placed on a centre line between the outer and inner diameters of the ring body, one would get a correct measuring result only if points along this line were loaded by a homogeneous load. However, this does not occur in practice, as an applied load over an area will always be somewhat uneven because of unevenness in the support, non-homogeneity in materials, etc. If one further imagines that the strain gauges were placed on the circular surface at the outer or inner diameter of the ring body, forces applied on the opposite side would not be registered, or registered to a small extent, and the measuring accuracy thereby would be poor. A placing of the strain gauges which covers an area between the extreme points, therefore would be optimal. Said location of the circular surfaces which is defined by said distances of 1/3 and 2/3, respectively, of the radial width of the ring body, appears as an optimal compromise in that one sums up the forces over a part of the area having such a great rigidity that measurements are not influenced to any appreciable extent by effects occurring at the outer and inner edges. As mentioned, by means of this strain gauge placing, the height of the load cells can be reduced, as the demand for a homogeneous load field is no longer important, since a summing up of the forces over an area is carried out.

As shown in Fig. 3, two strain gauges 6 are mounted at right angles to each other at the bottom of each cavity 5. Thereby stretch in the material is registered both in the height direction and in the radial plane of the load cell, since the height decreases and the diameter increases when applying a load. In the illustrated embodiment there are arranged four cavities 5 in each half-ring body, but there may also be arranged for instance six or eight gravities, dependent on the diameter of the load cell. In the illustrated embodiment the load cell has inner and outer diameters of 84 mm and 135 mm, respectively.

The electrical interconnection of the strain gauges 6 is shown more in detail in Fig. 5 and 6, where Fig. 5 shows the connection arrangement for each of the half-ring bodies, whereas Fig. 6 shows a circuit diagram for one half-ring body.

As appears, the strain gauges 6 are connected in a measuring bridge 12, each pair of strain gauges constituting a branch of the bridge. The measuring bridge is excited with a direct voltage V of normally 10 volts. The output signal is amplified in an amplifier 13 forming part of the signal adaptation unit 8 in Fig. 4. All strain gauges are active, which means that, when applying a load, all the strain gauges will deliver a signal which is proportional to the load or stretch that each strain gauge "sees". Thereby a summing up of the signal from all the strain gauges is achieved, so that the output signal represents the total applied load.

In an embodiment of the load cell wherein each half-ring body is provided with eight cavities and thereby eight pair of strain gauges, the strain gauges advantageously may be connected in two measuring bridges connected in parallel, with four strain gauge pairs in each bridge.

The wiring in the connection cables 7 is schematically shown in Fig. 4. As appears, each cable contains two excitation conductors 14 extending between the topical direct turret source and the inputs of the measuring bridge 12, and two signal conductors 15 between the outputs of the measuring bridge 12 and the amplifier 13 in question. As shown in Fig. 1, each half-ring body 3 and 4 in one of its side surfaces is provided with a groove 16 for receiving the necessary leads for interconnection of the strain gauges.

As shown in Fig. 4, each of the amplifiers 13 is connected to an amplifier 17 which adds the output signals from the two halves of the load cell, so that there is obtained a signal for the totally applied load on the load cell. If desired, the summed up signal can be converted to a current signal or a serial digital signal.

Claims

P a t e n t c l a i m s

1. A ring-shaped load cell for measuring compression loads, comprising a number of strain gauges (6) mounted on a ring body (2) and connected in an electric circuit for delivery of an output signal representing occurring loads on the cell (1), characterised in that the ring body (2) of the load cell is bipartite and comprises two half-ring bodies (3, 4), each half-ring body being provided with respective strain gauges (6) which are connected in an associated electric circuit for delivery of said output signal.

2. A load cell, according to claim 1, characterised in that the strain gauges (6) are mounted peripherally spaced in each half-ring body (3 resp. 4) on respective ones of two concentric circular surfaces (10, 11), so that the strain gauges (6) sum up forces over an area.

3. A load cell according to claim 2, characterised in that the strain gauges (6) are mounted at the bottom of respective cavities (5) of which the bottom surfaces are located on said circular surfaces (10, 11), the circular surfaces being located at a radial distance outside of the inner surface of the ring body (2) which is equal to 1/3 and 2/3, respectively, of the radial width of the ring body.

4. A load cell according to claim 3, characterised in that two strain gauges (6) are mounted at right angles to each other at the bottom of the cavity (5) in question.

5. A load cell according to claim 4, characterised in that each half-ring body (3 resp. 4) comprises four pairs of strain gauges (6) wherein each pair constitutes a branch of a bridge connection (12) forming part of said electric circuit.

6. A load cell according to one of the preceding claims, characterised in that the output of the electric circuit for each half-ring body (3 resp. 4) is connected to an amplifier (17) summing up the signals from the two circuits and delivering a signal representing the total load on the cell (1).

7. Use of a load cell according to one of the preceding claims, for measuring mechanical pre-stress when controlling tie rods, wires and the like in mechanically pre- stressed anchorings.