DESCRI PTION

1 . Technical Field

This invention relates generally to the art of strain measuring devices (extensometers) used in weighing systems particularly, but not exclusively, weighing systems which are located in use on a vehicle.

2. Background

There is a need to weigh product before, or as it is loaded onto a vehicle for the purpose of transporting it to a different location.

In the waste removal and recycling industries there are several known types of vehicle. Two types of vehicle that are commonplace are the front end loader and the side loader. Both of these types are typified in having one or more cantilever structures which are part of a mechanism which clamps onto a container containing the product to be transported away and then lifts the container into such a position that its contents are emptied into the vehicle itself. The empty container is then lowered and left whilst the vehicle moves on to a new location where the process is repeated.

It is desirable to weigh the product at the point where it is picked up for many differing reasons, among them the apportioning of recycling credits and the planning of the most cost effective routes for the operators of these vehicles.

There are already ways in which the weighing function can be accomplished. For instance, traditional on-board weighing systems can be used whereby load cells are interposed between the chassis

and the body of the vehicle, or sensors are applied to the axles of the vehicles, sensing either strain in the axle itself or by monitoring the physical displacement, under load, of the vehicle's suspension system. However, all these systems have the disadvantage that they are necessarily designed to withstand the complete weight of the vehicle when it is filled to capacity (or beyond) and, as such, are not sensitive enough to the small weight increments represented by changes in product load. Secondly they all require significant modifications to the vehicle itself in order to install the weight sensors.

Parts of the lifting mechanism of such vehicles are subjected to mechanical stress when weights are lifted and these stresses will be proportional to the weight being lifted. We have looked at ways of accurately monitoring the stress in the lifting mechanism in order to realise a weighing system.

Bolt-on strain sensors (extensometers) can be used. Also conventional bonded strain gauges or other bondable strain sensors could be employed to do much the same task although not without extreme practical difficulties.

However, when such bolt on sensors or bondable strain sensors are applied to cantilever structures, all suffer from the fundamental deficiency that the strain in the cantilever is not only proportional to the weight applied at a point along the cantilever, but also to the applied moment of the force. This means that weight sensed in this way w ll be centre-of-gravity dependent.

SUMMARY OF THE INVENTION

We have appreciated that it is possible to measure the load supported at an indeterminate position at one end of a cantilever beam by looking at the difference between the bending strains in the beam at two positions along the beam, because it can be shown that the said difference is substantially independent of the position of

the centre of gravity of the load.

Thus, when the load is a rubbish skip, for example, supported by a cantilevered pick-up arm, it is possible in accordance with the present invention to measure the weight of the loaded skip utilising a pair of longitudinally spaced-apart strain gauge assemblies on the pick-up arm, and the resulting measurement of the load is independent of the distribution of the skip contents which can affect the position of the centre of gravity of the loaded skip.

According to one aspect of the invention a weighing apparatus for weighing a load supported by a cantilever beam comprises first and second deflection or extension sensor assemblies arranged to monitor the deflection or extension in the beam at first and second longitudinally spaced-apart positions along the beam, to produce first and second deflection or extension sensor outputs respectively, and difference means responsive to the first and second outputs to produce a difference signal corresponding to the difference between said outputs, said difference signal being representative of the weight of said load.

Preferably the first deflection or extension sensor assembly is in the form of a strain gauge.

Also preferably the second deflection or extension sensor assembly is in the form of a strain gauge.

The strain gauge assemblies are preferably arranged to monitor the longitudinal extension of an upper surface of the beam, or longitudinal contraction of a lower surface of the beam.

According to a second aspect of the invention a method of weighing a load supported by one end of a cantilever beam comprises using the difference between strains in the beam at first and second longitudinally spaced-apart positions along the beam as a measure of the load.

According to a third aspect of the invention a differential strain gauge unit for providing measurements of the difference in strain in two adjacent portions of a structure comprises a bar for securing alongside the structure, first, second and third securing means spaced-apart along the bar and adapted to secure the bar locally to the structure at first, second and third spaced-apart securing locations respectively, the bar being relieved at a first strain gauge position intermediate the first and second securing means, and at a second strain gauge position intermediate the second and third securing means, the relieved portions of the bar being elastically deformable to permit the bar to be longitudinally extended and contracted in use in response to changes in the longitudinal spacing of the first, second and third locations of the structure due to strain of the structure, and first and second strain gauges associated respectively with the relieved portions to provide strain gauge outputs responsive to the elastic deformations of the respective relieved portions.

The relieved portions of the bar are preferably created by machining staggered, oppositely directed transverse slots in the bar so as to define a transverse web which becomes joggled on extension of the bar.

It will be appreciated that in use the output of the first strain gauge will be a measure of the surface strain in that portion of the structure intermediate the first and second securing locations, and that the output of the second strain gauge will be a measure of the surface strain in that portion of the structure intermediate the second and third securing locations.

In order to avoid drilling and tapping of the structure itself, preferably the differential strain gauge unit comprises mounting pads which are at least locally attached in use to the structure by welding or by adhesive bonding adjacent to the first, second and third securing loc ons, the securing means comprising respective screws adapted to rge the bar towards the pads, and locating

members received between opposed formations provided in or on the confronting surfaces of the bar and pads, the formations with respective locating members being provided in the vicinity of each of the first, second and third locations.

The formations are preferably pockets in the bar and pads, and the locating members are preferably ball-bearings, but other possible locating members are knife edges, indented needles, or rollers.

Preferably the three pads are in the form of an integral mounting bar which is reduced in thickness at intermediate positions between the first, second and third locations to permit flexing of the mounting bar.

BRIEF DESCRIPTION OF THE DRAWINGS

A differential strain gauge unit in accordance with the invention, and a system employing that gauge unit on a front end loader, will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a cantilever beam subject to a load W.

Figure 2 is a side elevation of a front end loader vehicle showing a typical mounting position of the strain gauge unit in accordance with the invention,

Figure 3 is a side elevation of the gauge unit employed on the vehicle of Figure 2,

Figure 4 is a plan view of the gauge unit, and

Figure 5 shows the electrical connection in a Wheatstone Bridge for the outputs of the gauge unit to provide a differential response.

DETAILED DESCRIPTION

The theory behind the invention will now be set out with reference To Figure 1.

A measure of the bending moment within an elastic bending beam is proportional to the strain, and stress within the beam as shown by equations (i), (ii) and (iii) below.

(i)

E cr

(ii) e where:

M - applied bending moment

I - first moment of area

E - Young's Modules p - radius of curvature - stress e - strain

combining equation (i) and equation (ii)

(f

I p p£ therefore: = -ISL iϋ

_P e

Figure 1 shows a cantilever beam which is fixed at one end and has a load W acting at the beam's opposite free end. The bending moment at point A on the beam is given by equation (iv) below and the bending moment at point B is given by the equation (v) below.

Bending moment at Point A (BMA) = a.W = (c+b)W (iv)

Bending moment at Point B (BMB) = b.w (v)

The difference between the bending moment at point A and the bending moment at point B is given by the equation (vi) below.

BMA - BMB = (c+b)W - bw = cW + bW - bW BMA - BMB = cW (vi)

It has been shown above that the bending moment is proportional to the strain and stress as shown in equation (iii).

Substituting equation (iii) above for BMA and BMB in equation (vi) above gives equation (vii) below.

Iff IC cW (v )

P ^e A ^e B

It can be seen from equation (vii) that the applied force W is proportional to the difference between the bending moment strain measured at two locations, so that by measuring the strain in the beam associated with bending of the beam by load W, at points A and B and taking the difference it becomes possible to measure the load W, and this measurement of load W is independent of the precise positioning of the load W along the beam.

Figure 2 shows a typical front end loader 1 provided with a pair of transversely spaced-apart cantilever arms 2 pivoted at one end 3 and carrying prongs 4 at their free ends 5 for engaging with a container which is to be emptied in known manner into the body 6 by pivoting of the cantilever arms 2.

The inventive strain gauge unit 7 to be described with reference to

Figures 3 and 4 is mounted on the upper surface of one of the cantilever arms at a convenient location intermediate the ends 3 and 5.

With reference to Figures 3 and 4, the strain gauge unit 7 comprises an elongate main bar 8 and a mounting bar 9 of similar length and width but of reduced depth, the mounting bar 9 being deeply slotted at 13 and 14 effectively to define three separate mounting pads 10, 11 and 12 each of which is welded to the underlying structure, the cantilever arm 2 in this case, the surface strain of which is to be monitored.

The elongate main bar is secured locally to the pads 10, 11 and 12 by respective securing screws 15, 16 and 17 which are received 1n counter-bored holes 18, 19 and 20 respectively in main bar 8 and threadedly engage with holes 18', 19' and 20' in pads 10, 11 and 12, the holes being arranged on the longitudinal centre-lines of the bars. In order to define the precise regions of connection between the bars 8 and 9, pairs of ball-bearings 21 are associated with each mounting screw, the ball bearings of each pair being located on a transverse line of the bars, and on either side of the respective screw 15, 16, 17; suitable part-spherical recesses being provided in the confronting faces of the bars 8, 9 to locate the balls.

The screw 15 and the two balls 21 adjacent thereto constitute a first securing means which locally secures one end of the bar 8 to the structure 2. The screw 16 and the two balls 21 adjacent thereto constitute a second securing means, and the screw 17 with the two balls 21 adjacent thereto constitute a third securing means, the first, second and third securing means being spaced apart along the longitudinal direction of the structure 2.

The main bar 8 is relieved by opposed transverse slots 22, 23 at a first strain gauge position 24, and by similar opposed transverse slots 25, 26 at a second strain gauge position 27, the first strain gauge position 24 being intermediate the screws 15 and 16, and the second strain gauge position 27 being intermediate the screws 16 and 17.

The slots 22, 23 define a transverse web 28 on which is attached a pair of strain gauges 29, 30 of known type, the strain gauges 29, 30 being connected in series in a Wheatstone bridge (Figure 5) and being responsive to the joggling of the web which is produced by extension of the left-hand portion of the bar 8 when the separation of screws 15 and 16 is increased due to strain in the structure 2, in that portion of the structure 2 between holes 18' and 19'. Thus the output of gauges 29, 30 is a measure of the surface strain in structure 2 in the region between holes 18' and 19'.

Similarly slots 25, 26 define a web 31 on which identical strain gauges 32, 33 to gauges 29, 30 are mounted and which are connected in a similar manner. The combined output of gauges 29, 30 provides a measure of the strain in structure 2 in the region between screws 16 and 17.

With reference to Figure 5 the differential output of the strain gauge assemblies at the first and second strain gauge positions 24, 27 is taken from bridge B and D, bridge corners A and C being provided with a suitable input signal. The output from BD represents the difference in the surface strains in structure 2 in the two regions between holes 18' and 19', and 19' and 20 ^' respectively.

It will be appreciated that it is possible to provide deflection or extension sensors in the form of variable capacitance sensors, variable reluctance sensors, variable inductance sensors, optical sensors and laser sensors in order to produce the respective outputs.