GB2178179A - Vehicle weighing apparatus - Google Patents

Abstract

Apparatus for measuring weight transmitted by a vehicle wheel includes a pair of laterally spaced beams (12) with vertical webs (30) and upper and lower horizontal flanges (31,32) to provide lateral and torsional stiffening, tubular cross members (19) connecting the beams, and supports (13) for the ends of the beams. A load platform (10), with ramps (11) for the wheel, is carried on the cross members. During weighing, strains in the vertical webs (30) of the beams between the supports (13) and the cross members (19) are measured by semiconductor straing gauges (34,35) bonded to a flat surface (36) in the web. A stiffening box section (14) around the strain gauges reduces horizontal and torsional strain. Cut-outs (41,42) in the web above and below the strain gauges concentrate the vertical shear forces. Two mutually perpendicular strain gauges at 45 DEG to the vertical compensate for torsion. A Wheatstone bridge circuit measures resistance changes, enabling the load on the apparatus to be determined after preliminary calibration. <IMAGE>

Description

SPECIFICATION Weighing apparatus This invention relates to weighing apparatus, and in particular to apparatus for measuring the weight transmitted by the wheel of a vehicle.

Total vehicle weights can be determined on conventional weighbridges but there has long been a need for accurate portable vehicle weighing equipment. In order to be portable, such equipment is required to measure separately the weight transmitted by each wheel or set of closely spaced wheels, and the total weight is found by aggregating the separate weights. The same apparatus can be used similarly to determine the load carried by any one axle of the vehicle.

Individual wheel weighing equipment of this kind has been known for some years, in which a load platform is supported on a number of load cells which are in turn carried by a base structure. Typically four separate load cells might be provided, each supporting one corner of a rectangular load platform. Each load cell includes elements machined from high tensile steel, for protection against overloads, and the whole apparatus is relatively costly and complex.

An object of the present invention is to provide vehicle wheel weighing apparatus that is of a relatively simple construction and that can be economically fabricated from readily available materials without the need to incorporate separate costly load cells into the structure.

According to the present invention there is provided weighing apparatus for measuring the weight transmitted by a wheel of a vehicle, comprising a load platform adapted to carry a vehicle wheel; a stiff beam extending between spaced apart beam supports and supporting the load platform on an intermediate portion of the beam lying between and spaced from the beam supports; and a semiconductor strain gauge bonded to the beam at a location between said intermediate load platform supporting portion and each beam support.

According to a further aspect of the invention there is provided weighing apparatus for measuring the weight transmitted by a wheel of a vehicle, comprising a substantially rigid support structure including a pair of generally parallel stiff beams spaced laterally apart, each extending between a pair of spaced apart beam supports, and at least one connecting member rigidly connecting together intermediate portions of each beam lying between and spaced from their respective beam supports; a load platform adapted to carry a vehicle wheel and supported by the at least one connecting member; and semiconductor strain gauges bonded to each beam at locations between said intermediate portion and each beam support.

In apparatus in accordance with the invention separate load cells are not required, since the weight transmitted by the wheel through the load platform is carried by an intermediate portion of the or each beam between the beam supports, which may rest directly on firm ground. The strain gauges are situated between each beam support and the intermediate load-carrying portion of.the beam and respond directly to the deflection of the beam at that point due to the load. The beam or beams may be of low tensile structural steel and the whole support structure and the load platform may be fabricated by welding from readily available steel sections and/or steel sheet or plate.

The semiconductor strain gauges that are bonded to the beam or beams are at the present time resistive elements of long single or U-shaped doped silicon crystals provided with electrical conductor leads connected to each end of the crystal. The gauges are bonded to the beams by means of a suitable adhesive, such as an epoxy resin, and respond with a change in their electrical resistance to strain in the beam occasioned by local compressive or tensile stresses. The changes in electrical resistance are monitored in a known manner by a suitable form of Wheatstone bridge circuit.

After preliminary calibration to relate beam deflection to load, the separate loads associated with each beam support can be aggregated to determine the weight carried by the load platform.

The strain gauges are suitably bonded to a surface of the or each beam that extends in a substantially vertical plane parallel to the longitudinal axis of the beam. The beam may comprise a substantially vertical web or flange, extending the length of the beam between the beam supports, to which the strain gauges are bonded. The beam may also comprise at least one lateral web or flange to stiffen the beam against lateral loads and torsion. The vertical web or flange may be cut away above and/or below each strain gauge to concentrate shear stresses in the beam due to the load into the vicinity of the strain gauges. The beam may also be provided with stiffening members around the strain gauges adapted to stiffen the beam against lateral and torsional deflections in the vicinity of the strain gauges.

Since the strain gauges are long thin crystals that are effectiveiy responsive only to axial strain, their alignment on the beam to which they are bonded affects the response of the apparatus to load. Preferably the strain gauges are mounted at an angle of about 45" to the vertical, in a substantially vertical plane parallel to the longitudinal axis of the beam to which they are bonded. Whether the strain gauge is stretched or compressed when the apparatus is under load, will be governed by which of the two possible 45" aiignments is chosen for the crystal.In order to be able to compensate more readily for torsional strains in the apparatus, two strain gauges may be bonded to the beam or beams between each said intermediate portion and each beam support, each strain gauge being mounted at about 45" to the vertical but the two strain gauges being mutually perpendicular.

One embodiment of the invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is an isometric general view of vehicle wheel weighing apparatus; Figure 2 is a plan view of the load platform support structure of the apparatus; Figure 3 is a cross section taken on the line 3-3 in Fig. 2; Figure 4 shows the load platform added to the view in Fig. 3; Figure 5 is a detailed elevation of one end of one beam in the support structure of the apparatus; Figure 6 is a cross section taken on the line 6-6 in Fig. 5; and Figure 7 diagrammatically represents four strain gauges in an electrical Wheatstone bridge circuit.

As shown in Fig. 1, a load platform 10 for supporting a vehicle wheel is provided at.each end with integral downwardly sloping ramps 11 whereby one wheel of a vehicle can be driven on and off the load platform. A substantially rigid support structure for the load platform comprises two parallel spaced apart stiff channel section beams 12, the ends of which are supported on two narrow flat plates 13 each extending between the beams under the ramps 11. Stiffening boxes 14 surround and protect semiconductor strain gauges bonded to the beams.

The support structure is shown in more detail in Figs. 2 and 3. It is of welded mild steel construction. In addition to the beam support plates 13, two rectangular section cross tubes 19 rigidly connect the beams 12 at locations between and spaced from the support plates.

Further rectangular section tubes 20 welded to the beams and to the tubes 19 reinforce the inward sides of the beams between the tubes 19. The tubes 19 and 20 support the load platform as will be described below, and accordingly the weight of the load platform and of any load thereon is taken by intermediate portions of the beams 12 extending between the two cross tubes 19.

Fig. 4 shows the load platform 10 positioned on the support structure. The platform is reinforced by four transverse flanges 24 welded to its underside, and the ramps 11 are reinforced by longitudinal triangular flanges 25 welded to their undersides. The reinforcing flanges 24 and 25 also serve to locate the load platform in its correct position on the tubes 19 and 20.

Figs. 5 and 6 illustrate the regions of the beams 12 at which the strain gauges are mounted. Each beam 12 comprises a vertical web portion 30, an upper outwardly extending horizontal flange 31 and a lower outwardly extending horizontal flange 32. Semiconductor strain gauges 34 and 35 are bonded to the web 30 on the outwardly facing side thereof on a flat mounting surface 36 for the strain gauges. The vertical location of the flat surface 36 is centrally between the beam flanges 31 and 32. The longitudinal location of the flat surface 36 is between the inner edge 38 of the beam support plate 13 and the outer edge 39 of the closer cross tube 19 (shown in dashed outline), which represents the outer end of the intermediate portion of the beam supporting the load platform 10 (shown in phantom outline).Thus the flat surface 36 on which the strain gauges are mounted is located at a region of the beam that is subject to shear stresses between the load acting downwardly on a line corresponding to the cross tube edge 39 and the upward reaction from the support plate 13 acting on a line corresponding to the support edge 38.

These stresses are concentrated in the region of the strain gauges by the provision of cut-out portions 41 and 42 in the web 30 above and below the flat surface 36.

The flat surface 36 carrying the strain gauges is surrounded by the stiffening box 14, which is formed from a short length of rectangular section tube welded end-on to the web 30. This protects the strain gauges from excessive strains that might be caused by side loads on the support structure. The box 14 overlaps with the cross tube 19 on the other side of the web 30, and in the overlap region an aperture 44 is provided in the web to permit electrical connecting leads (not shown) to pass between all the strain gauges on the support structure through the tubes 19 and 20.Each box 14 has a removable end cap (not shown) to protect the strain gauges and electrical connections against dust, dirt, moisture and the like, and one box 14 has an electrical junction box and connector socket 46 mounted in an aperture in a side wall thereof by means of which all the strain gauges of the apparatus can be connected via a suitable plug on a flexible connector lead into a suitable resistance-measuring circuit for determining the weight carried on the load platform.

As shown in Fig. 5, the two strain gauges 34 and 35 are bonded to the web 30 perpendicularly to one another and each at 45" to the vertical in the plane of the web. A downward load on the intermediate portion of the beam 12 induces tension in the gauge 34 and compression in the gauge 35. By using a pair of strain gauges so oriented between each beam support and the load-carrying intermediate portion, it is possible to introduce compensation for torsion in the apparatus, which can easily arise if the beam supports are not lying on an absolutely flat bed.

Fig. 7 shows the basic resistance measuring circuit, in which resistances R, to R4 representing four strain gauges are connected in a bridge circuit with an excitation voltage V and an output voltage e which will be zero when the bridge is exactly balanced. In practice the circuit will be more complex, as is known in the art, to correct for factors such as thermal effects. Compensation for torsion in the case of four strain gauges can be achieved by suitably locating and orienting the strain gauges so that, for example, R, and R2 are subjected to compressive strain and R3 and R4 are subjected to tensile strain.

While one embodiment only of the invention is illustrated in the drawings, it will be appreciated that the invention may be put into effect in other ways. For example, the load platform may be carried on a single stiff beam, and the beam conformation may be other than a channel section. The apparatus may be located in a prepared site below ground level so that no ramps are required to drive a vehicle wheel on to the load platform. The load platform itself need not be a flat sheet, but could for example be constituted by a plurality of spaced parallel bars extending between two end pieces.

Claims (13)

1. Weighing apparatus for measuring the weight transmitted by a wheel of a vehicle, comprising a load platform adapted to carry a vehicle wheel; a stiff beam extending between spaced apart beam supports and supporting the load platform on an intermediate portion of the beam lying between and spaced from the beam supports; and a semiconductor strain gauge bonded to the beam at a location between said intermediate load platform supporting portion and each beam support.

2. Weighing apparatus for measuring the weight transmitted by a wheel of a vehicle, comprising a substantially rigid support structure including a pair of generally parallel stiff beams spaced laterally apart, each extending between a pair of spaced apart beam supports, and at least one connecting member rigidly connecting together intermediate portions of each beam lying between and spaced from their respective beam supports; and load platform adapted to carry a vehicle wheel and supported by the at least one connecting member; and semiconductor strain gauges bonded to each beam at locations between said intermediate portion and each beam support.

3. Weighing apparatus according to claim 1 or claim 2 wherein the or each said strain gauge is bonded to its respective beam by means of an adhesive.

4. Weighing apparatus according to any one of the preceding claims wherein the or each said strain gauge is bonded to a surface of its respective beam that extends in a substantially vertical plane parallel to the longitudinal axis of the beam.

5. Weighing apparatus according to any one of the preceding claims wherein the or each said beam comprises a substantially vertical web or flange, extending the length of the beam between the beam supports, to which the strain gauge or strain gauges are bonded.

6. Weighing apparatus according to claim 5 wherein the or each said beam further comprises at least one lateral web or flange adapted to stiffen the beam against lateral loads and torsion.

7. Weighing apparatus according to claim 6 wherein the vertical web or flange is cut away above and/or below the or each said strain gauge whereby to concentrate shear stresses in the beam due to the load into the vicinity of each strain gauge.

8. Weighing apparatus according to any one of the preceding claims wherein the or each beam is provided with stiffening members around the or each strain gauge site adapted to stiffen the beam against lateral and torsional deflections in the vicinity of the or each strain gauge.

9. Weighing apparatus according to claim 8 wherein the stiffening members comprise a box section welded to the beam around the or each strain gauge site.

10. Weighing apparatus according to any one of the preceding claims wherein the or each strain gauge comprises an elongate crystal which is mounted at an angle of about 45" to the vertical in a substantially vertical plane parallel to the longitudinal axis of the beam to which it is bonded.

11. Weighting apparatus according to claim 10 wherein two strain gauges are bonded to the or each beam between each said intermediate portion and each beam support, each of the two strain gauges being mounted at about 450 to the vertical but the two strain gauges being mutually perpendicular (in the same vertical plane).

12. Weighing apparatus according to any one of the preceding claims wherein the load platform is provided with ramps whereby a wheel of a vehicle can be driven on and off the platform.

13. Weighing apparatus substantially as herein described with reference to and as illustrated in the accompanying drawings.