GB2087085A - Force transducer with multiple measuring sections - Google Patents

Abstract

A force transducer comprises a force receiver (22) connected to a fixed support (14) by at least two serially connected superimposed measuring sections constituted by for example pairs of flexure strips (10, 12 and 18, 20) with strain gauges (24, 26) provided for producing an electrical measure of the force (F) applied to the force receiver (22). The measuring sections are coupled by an intermediate member (16) which is connectable directly to the force receiver (22) by a lost-motion coupling comprising abutment (26) and an aperture in the force receiver. The flexures (18, 20) may be less stiff than flexures (10, 12) thus enabling the flexures (18, 20) to be used to provide low force measurements and the flexures (10, 12) to be used for larger force measurements, the arrangement being such that the lost motion is taken up when the applied force reaches a certain level thereby bypassing the flexures (18, 20) to avoid overloading them. <IMAGE>

Description

SPECIFICATION Force transducer with multiple measuring sections This invention relates to force measurement and is particularly concerned with improvements in force transducers which enable the applied force to be shared between a number of measuring elements in such a way that improved accuracy can be obtained andlorthe measuring elements can be used selectively for example to provide a multi-range scale.

According to the present invention we provide a force transducer comprising N measuring sections arranged in cascade fashion with the Nth section being connected to a force receiver to which the force to be measured is applied and theMth section (whereM is less than N) being coupled to the force receiver via a force transmission path constituted by the remaining N -M sections andlor becoming coupled to the force receiver by a second force transmission path bypassing the remaining N-M sections when the applied force stresses the preceding section beyond a certain degree.

The invention is particularly applicable to measuring sections which undergo bending stress in response to the application of force and in a typical embodiment of the invention, the measuring sections may each comprise a bending cantilever-type beam provided with one or more strain gauges to provide an electrical measure of the bending strain induced in the beam by the applied force. With such an arrangement, each measuring section may be associated with a respective portion of the overall measuring range of the transducer and circuitry may be provided for selectively processing the outputs of the measuring sections so that, for a given applied force, the output ofthe corresponding section is employed.

In the preferred embodiment, the measuring sections are arranged in series in such a way that when thePth measuring section is responding to the applied force, at least the (P-1)th section also undergoes bending stress so that its output simultaneously increases with the output ofthePth section. In this way, a smooth transition can be made from the Pth section to the (Pth-1) section as the former approaches its limit of bending deflection.

It is envisaged that, in most applications, the stiffness of the measuring sections will increase progressively from the Nth to the first section.

In order to promote further understanding of the invention, reference is now made to the accompanying drawings in which: Figure 1 is a schematic diagram showing one form of transducer in accordance with the invention; Figure 2 is a graph showing the relationship between electrical output and applied load for the transducer of Figure 1; Figure 3 is a schematic view of a modified form of the embodiment in Figure 1; Figure 4 is a graph relating electrical output to applied load forthe embodiment of Figure 3; Figure 5 is a schemactic view of a further form of transducer in accordance with the invention; and Figure 6 is a schematic view of yet another embodiment of the invention.

Referring now to Figure 1, the transducer comprises a first pair of vertically spaced parallel flexure strips 10, 12 each rigidly connected at one end to a fixed support 14 which, in the case of a weighing scale, may be mounted on the base of the machine.

The strips 10, 12 extend in cantilever fashion from the support 14 and their opposite ends are united in parallelogrammic fashion by a vertically extending cross member 16 from which a second pair of vertically spaced parallel flexture strips 18, 20 extend in cantilever fashion. The free ends of the strips 18, 20 are rigidly connected to a force receiver 22 in the form of a vertical cross member which, in the case of a weighing scale, may carry the scale pan or platform.

The force F to be measured is applied to the force receiver 22 and it will be understood that the strips 10, 12, 18, 20 are thereby subjected to bending stresses. The upper flexure strip 10, 18 of each pair is provided with one or more electrical resistance strain gauges 24, 26 on the upper andlor lower face thereof which enable an electrical measure of the bending strains and hence the applied force to be obtained, e.g. by using the gauges in a Wheatstone bridge circuit or such like. Two separate outputs can be obtained, one from each set of strain gauges 24, 26 and Figure 2 illustrates typical responses for the gauges 24, 26 in a situation where the flexure strips 18,20 are less stiff than the strips 10, 12 i.e. the flexure strips can be designed so that one pair gives a greater output than the other.

The outputs from each pair of flexure strips may for example be used in such a way that the output from gauges 26 is used over an initial part of the load range, i.e. up to a preselected level of loading, and the output from gauges 24 is used beyond this level.

Conceivably the outputs may be used in combination for example between certain limits within the load range.

It will be seen that the two pairs of flexure strips 10, 12 and 18, 20 are in effect mounted on the support 14 in series fashion and that this arrangement provides two measuring sections thereby affording the transducer a dual capacity facility andlor enabling greater accuracy to be obtained at for example the lower end of the load range. In this latter context, the greater electrical output from the gauges 26 may be used at the lower end of the range where the output of the gauges 24 may be comparatively small.

Although as described above the flexure strips are designed so that, for a given load, the output from the pair 18,20 is the greater, it is envisaged that in some circumstances it may be desirable for the outputs to be substantially the same or for the output from the strips 10, 12 to be the greater. Thus, for example, the strips 10, 12 may be of the same or lesser stiffness than the strips 18, 20. Also, whilst the The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

strips 18, 20 are shown as lying outside the strips 10, 12 the arrangement may be different, e.g. the strips 18,20 may be located within or may be interdigitated relation with the strips 10, 12, the force receiver 22 being suitably adapted in such circumstances.

Moreover it is to be understood that the invention is not limited to the use offlexure strips -other forms offlexures are possiblenor is it limited to the use of flexure in pairs.

Figure 3 shows a modification of the arrangement of Figure 1 in which means are provided to limit deflection of one pair offlexures so as to prevent overloading thereof. The same reference numerals are used to designate like parts in Figures 1 and 3. In this embodiment such means comprises an abutment 26 which, when the less stiff flexures 18,20 have undergone a certain-degree of deflection, engages the force receiver 22 and provides a second path for force transmission to the flexures 10, 12.

Thus, initially, the force F acts on the flexures, 10, 12 solely through the agency of the flexures 18,20 but, when the lost motion has been taken up and the force receiver engages the abutment 26, the force is transmitted to the flexures 10, 12 primarily through the abutment thereby eliminating or at least minimising funher deflection of the flexures 18,20.

In this embodiment, the abutment 26 is in the form of a stop bar rigidly connected at one end to the cross member 16 and extending in cantilever fashion through openings in the support 14 and the force receiver 22. To minimise the effects of eccentric loads when the stop bar 26 becomes operative, the latter is preferably positioned so that its axis lies in the same vertical plane as the horizontal centre lines oftheflexures 10,12,18,20.

Figure 4 shows the variation of electrical output of the gauges 24,26 with load for the embodiment of Figure 3. It will be seen that when the force receiver 22 engages the stop bar 26, the output from gauges 26 levels out. In practice, because of the nature of the mechanical abutment arrangement, the load at which levelling out occurs may not be accurately repeatable and in Figure 4 the phantom lines indicate such variation as may arise in practice. However, this is not a drawback because the outputs from the gauges 24,26 may be processed in such a way that the output from gauges 24 is brought into use, e.g. at the load point (a), before any levelling out of the other output can take place.

Figure 5 illustrates another form of transducer in accordance with the invention in which two measuring section are again employed. In this embodiment, one measuring section is constituted by parallel vertically spaced flexure strips 30,32 to which the force receiver 34 is connected. The second measuring section comprises a transducer configuration of the type disclosed in our prior pending Patent Application No.7906621 to which reference should be made for further details. The second section is rigidly connected between the fixed support 36 and the cross member 38 with which the flexures 30,32 are rigidly united. As in the embodiments of Figures 1 and 3 the two measuring sections are arranged in series fashion.

Briefly the second measuring section essentially comprises a generally U-shaped portion 40 which includes two flexure strips 42,44 provided with strain gauges. These strips 42, 44 extend vertically and a pair of vertically spaced horizontal flexure strips 46,48 (which will usually be ungauged) are provided to form a parallelogrammic structure with the support 36 and cross member 38 thereby constraining the latter to move substantially vertically when subjected to loading. When the cross member is deflected by the load, the strips 42,44 deflect laterally to one side to enable a measure of the applied force to be obtained from the associated gauges.As a general rule, the arrangement of the strips 42, 44 will be such that they provide a substantially greater stiffness than the flexures 30, 32. Thus, the output from theflexures 30,32 may be used for forces at th'e lower end of the range and the output from the flexures 42,44 may be used at higher loads.

As in Figure 3, the embodiment of Figure 5 is provided with means for preventing overload of the lower stiffness flexures 30, 32. In this case, such means comprises a bracket 50 rigidly connected to the cross member 38 and a stop pin 52 which engages with the bracket 50 when the flexures 30, 32 have undergone a certain degree of deflection.

As shown in Figures 1,3 and 5 each transducer comprises two cascaded measuring sections arranged in series fashion so that the force applied to the force receiver is transmitted to the first measuring section through the agency of the second measuring section and, in the case of Figures 3 and 5, directly to the first measuring section (via the abutmentllost motion arrangement) when the second measuring section has been stressed beyond a certain degree. In a modification there may be more than two measuring sections and in the general case there may beN measuring sections connected in series fashion with the Nth section being connected to the force receiver and the Mth section (where M is less than N) being connected to the force receiver through the agency of the remaining N-M sections.

In such an arrangement, overload/lost motion means may be provided between each successive pair of measuring sections so that, when the Pth section (where P is equal to 2 to N) approaches its overload limit, a new force transmission path from the force receiver to the (P-1)th section is created thereby effectively bypassing thePth section.

Each such measuring section may be gauged and circuitry may be provided for selectively processing the outputs from the measuring sections according.

to the magnitude of the applied load so that for example the output from the Pth section is utilised only when the preceding section has begun to approach or has reached the point at which it is bypassed by the overload means and only until the Pth section itself has begun to approach or has reached its associated bypass point.

In a further modification, which may be understood by reference to the diagrammatic representation in Figure 3, instead of the measuring sections being coupledtin series fashion, they may be mounted independently of each other on a fixed support 60 in a cascade fashion with one section 62 connected directiy to the force receiver 64 and the other section or sections 66,68... being rendered active for force measurment when the preceding section has deflected to a sufficient extent to take up lost motion therebetween, e.g. by means of abutment 70. In such an arrangement, each successive flexure may be stiffer than the preceding flexure with the flexure 62 being the least stiff.

Claims (9)

1. A force transducer comprising N measuring sections arranged in cascade fashion with the Nth section being connected to a force receiver to which the force to be measured is applied andtheMth section (where is less than N) being coupled to the force receiver via a force transmission path constitured by the remaining N -M sections and/or becoming coupled to the force receiver by a second force transmission path bypassing the remaining N - M sections when the applied force stresses the preceding section beyond a certain degree.

2. A force transducer as claimed in Claim 1 in which the measuring sections are arranged in series in such a way that when thePth measuring section is responding to the applied force, at least the (P-1 )th section also undergoes bending stress so that its output simultaneously increases with the output of the Pth section.

3. A force transducer comprising a force receiver connected to a support through the agency of at least two serially connected measuring sections and means operable to couple the force receiver automatically to a location intermediate adjacent measuring sections when the applied force exceeds a predetermined value so as to limit stressing of the measuring section or sections disposed upstream of said location, i.e. between said location and said force receiver.

4. A force transducer as claimed in Claim 3 including means for monitoring the output(s) of the measuring section(s) and utilising the output(s) of the upstream measuring section(s) when the applied force is below said predetermined value or a value close thereto and utilising the output(s) of the downstream measuring section(s) when the applied force is above said predetermined valued or a value close thereto.

5. A force transducer as claimed in any one of Claims 1 to 4 in which the measuring sections are arranged in superimposed relation with each other.

6. A force transducer as claimed in any one of Claims 3 to 5 in which said coupling means comprises a lost motion coupling which only provides coupling between the force receiver and said location when the force reciever is subjected to an applied force greater than said predetermined value.

7. A force transducer as claimed in Claim 6 in which said coupling means comprises an abutment member associated with said location or said force receiver, which abutment member projects into an opening associated with a force receiver or with said location whereby the abutment member engages a boundary surface of the opening when said lost motion has been taken up.

8. A force transducer as claimed in any one of Claims 3 to 7 in which the stiffness of the or each upstream measuring section is less than that of the or each downstream measuring section.

9. A force transducer as claimed in any one of Claims 1 to 8 in which the measuring sections each include at least one elongate flexure member and in which the flexure members of successive measuring sections have adjacent ends rigidly united with an intermediate member, said coupling means being arranged to act between the force receiver and said intermediate member.