CA1259342A - Force sensing device for measurement apparatus - Google Patents

Abstract

ABSTRACT
A force sensing device comprises a parallelepipedic block the height of which is larger than its width, and which is provided with an opening in the direction of its width so as to form a parallelogram made deformable by the presence of thinner zones in the direction of its height; flexural deformation sensitive elements are fixed to a substantially horizontal section of the block opposite two of the thinner zones which are of a higher flexibility than the other thinner zones. This allows significantly easier manufacturing of low costs while combining high accuracy of measurement and low sensitivity to accentered loads.

Description

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This invention relates to a Forre ~ensing ~:le.vi,~
for measurement apparatus such as a we:igh].rlg instrument.
Force sensing devices are alread\J known, consisting o-f a parallelepipeclic blu-(,l< formecl wi:th receC.ses 5 therein defining thinner sections nf lesser strength of which the c!eforma-tion is measuled under the action of a mass to be weighed wi:th elements adapted to c~eter,-t deformations such as strain gauges.
The force sensing devices of the mentioneci type 10 generally comprise an application area cantilevered, located close to a corner af the block on its lower face and a receptlon area 4Or the force to be measured located .close to a corner diagonally opposite:to that mentioned above in the upper face of the block.
: 15 Pratically, to the reception area of the force to be measured there is a~ded a plate on which a rnass to bè
weighed is placed in any point. It is to be understood that ~; for an accurate measurement of such mass it is appropriate to eliminate (or reduce to minimum) the spuriou~s influence 20 of the, position af the mass on the plate; there-fore, it appears to be suitable for, such devices to be made as ,; little~ sensitive as possible to f:lexure moments created by ; eccente~red foroes.
To~ this end, various block geometrie:s~,have been 5:proposed, one of which, proposed in the French patent FR~-2,386,024 ~or ~the patent ~EP-0 060 766~ 15 entirely satisfactory.~
: ::: k These patents propose a parallelepipedic~block the maximum and minimum con.stant dimensions of: which - 3~0~(respectively~ called "length" and "wldth") are oriented in a ~hori:zontal p:lane while the third dimension intermediary:
between the~ width and the length ca:lled the "height" is vertically~ orie~nted. Various recesses are formed:in ,the bl~ock in the dlrection of its .width'so as to define a :: 35 de;formable parallelogram~ comprising two vertical uprights ~: h connected. by thinner :zones to ~horizontal transverse ': : ' : :
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elements, with both vertlcal uprlghts being connected to one another between the hor.i-zont-~il transverse elements, by two horizontal heams connectec~ by a cen-tral vertiral flexural element which may be flexecl perpellciicu.lar to the 5 direction of the width. This central el.ement permlts elimlnation of -fle~ural moment gerlerated b\l the ecce~ erecl forces. Measurement of the app~.ied load is made~by gauges disposed on either sic3e of a thinner zorle through which one of the beams is connected to the corresponding upright on 0 the side o~ the application area.
Another geometry is proposed by USA patent N
4~,432,247 which proposes such a block having its smallest dimension (height) oriented vertically; such block is formed with recesses therein in the direction of the width 5 this latter dimension, although being lower than the length talso oriented horizontally) is much higher than the height. Such recesses define a very flattened parallelogram comprising two massive horizontal uprights connected through thinner zones of large dimension lin the~direction 20 of the Width) to upper and lower plates. Measurement of the ; ~ mass to be weighed is given by strain gauges secured above the thinner zones borderi-ng the upper plate of such block ~ ~, . , :
which therefore behaves as a whole as a fle~xure b~eam. Some insensitivity to flexural moments generated by the Z5 eccentered forces is obtained~due to the high width which confers good rigidity in the horizontal plane to~the block.
; It is to be noted that sensitivity o~ a force sensor formed of a recesses parallelepipedic block of any one of the above-mentioned types diminlshes where its width 30 increases, thereb~y resulting that the low sensitivity to the eccent~ered forces of the flexural beam of USA patent N
4~,432,247 corrèsponds to low sensitivity and therefor~e~low accuracy in measurements.
This invention relates to a force sensor of the 3~5 type comprlsing a parallelepipedic block recessed in the ::
~ ~ ~ direction of its width which both presents large : :

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sensitivity ancl theref(lre h1gh arlurac\/ (c~ue -to a low wiclth much lc)wer than the height of said blc)rk) ancl a ver~) :I.ow sensitivity to eccentr-rl3cl forl-es, wi~h an overall geometry which is however simplf!r and easier to ohtain (at l.esser 5 costs) than those proposecl in the ahove mentionecl docurnents FR-2,386,024 and even EP-0 oGn ~
To this encl, the inventLon proposes a force sensor as a weighing instrument of the type comprising a generally paral7elepipedic hlock the length and wiclth of 10 which are measured accordlng to perpendicular harlzontal axes and the height of which is measured according to a vertical axis, such block being recessed therein in the direction of its width so as to comprise two vertical uprights connected by two pairs of thinner zones to two 15 upper and lower horizontal beams, one of -these uprights being intended for fixation of the block to a base and the : other of such uprights being intended for securement to a ~:: reception plate for a load to be weighed, such sensor also comprising elements responsive to a flexural deformation Z associated with at least one zone of the block, thinner in~ :
the direction of the height, such block being cha:racterized in that, in combination :
the block is reduced to a deformable : parallelogram comprising a~ single central recess therein ~ c~
and its height lower than its length is substantially higher than its width:
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the elements sensitive to flexural deformatibn are disposed on a substantially planar horizontal~face of :the block opposite two thinner zones through which one: of:
3~0 the horizontal beams:is connected to the:vertical uprights;
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. the ~fl;exibility of the thinner zones o:f the~
; beam~ carrying saicl sensitive elements is higher than the : flexibility of :the thinner zones of the other of the ~:; 35:horizontal beams.
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1 It will be noted that this invention proposes a group of characteristics the combination of which is neither described nor suggested in the above mentioned documents.
It will also be noted that it was already known to use a force sensor for weighing such a block having a central recess therein but that these previous sensors keep high symmetry in the block geometry with strain gauges associated with each thinner zone, the various thinner zones presenting one and the same flexibility. The invention proposes however to renounce symmetry taught actually by almost all of the known devices as regards both the disposition of the deformation detection members and the geometry and mechanical characteristics of the different thinner zones.
In the case where upper and lower beams are of a same material, such difference ir. flexibility may be obtained through a difference (in opposite sense) between the thicknesses of the thinner zones of both beams. Such flexibility difference may also be obtained thanks to appropriate differences between widths or mechanical properties of said thinner zones (or of beams in which the latter are provided).
The sensitive elements or members are preferably carried by the upper beam.
In a preferred mode of embodiment of the invention the members sensitive to flexural deformation are flexible and preferably belong to a supple circuit adapted to follow up deformations if the upper beam of the block consisting for example of a printed circuit board. The above mentioned sensitive members are advantageously added directly to the upper face of the block, by glueing for example, or they are connected thereto through a ~upple supporting layer; they may be obtained on this block thanks to any other known method.

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For simplici.t~J of mac:hining th~ central Lecess o-f a block is advantayeolJs].y -formed b~J a central cyllndric bore conne,-ting four srnaller cyli.nclrir bor(?s bolclering the thinner zones clnd di5ptJsed a5 a rectanglP. As a matter c)f 5 fact it is known that perforatL~-In of hclles is a particularly cheap rnachining operation. In fact, only the geometry of the thinner zones is important especially those of the upper beam so that the rest o-f the central recess can be obtained with entirely medium tolerances.
According to an advantageous -form of embodiment of the invention the parallelepipedic block is made of two parts advantageously from two materials i.e. a flexure blade portion carrying the sensitive elements and a U-shaped fra~e. This form of embodiment in two portions 5 facilitates manufacturing of the block (through cuttiny for example of two U-shaped frames disposed head to foot in a plate with a lesser loss of metal than in case of a single piece block), on the one hand and on the other hand, ~ facilitates fixation about the sensitive elements of a ! ~ : ' 20 sealed insulation sleeve such as a metallic beIlows. Such ,, , arrangement is also advantageous in that it permits a plurality of fle><ural blades of different performances to ; be mounted on one and the same frame whereas such a blacle ~can be used uncler certain applications all alone without 25 being added to ~1 U-shaped frame or on the contrary be mounted selectively on any of a plurality of U-shaped fram2s with differing mechanical properties and compositions.
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~ Such a structure in two-parts of the :: ~
~ ~ 30 parallelepipedic block allows thanks to combination of :: :
~ différent materials for the two-parts or through selection : :
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~ aimeci at according to the invention even when using ; ~ thinner zones of a same thickness.
~ Objects, char~acteristics and aclvantages of this invention will appear from the followlng description given ~: :
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by way of non lirnitative examples in the ligh-t o~' the ~attached clrawings .in which : ~
- figule 1 is a :Latera.1. elevationaii. view of a force sensor according to the i.nvention;
S - figure 2 is a top V:Lf'W thereof on a larger scale;
~ figure 3 is a lateral elevational vlew of a force sensor according to a modified form of embodiment of the sensor of fiyure 1;
- figure 4 is a perspective view of a force sensor according to the invention in accordance with another form of embodiment, and - figure 5 is an elevational view of an example of application of the force sensor of figure 4.
Figure 1 represents by way of e~ample a force sensor 1 comprising a substantially parallelepipedic homogeneous block 2 presenting an application bearing 3 for securement thereof to a base, shown as at ~ and a reception bearing 5 diagonally opposite thereto adapted to be 20 submitted to a force to be measured.
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In practice, such reception bearing is a~apted to receive for fixation a plate shown in dashed lines by reference~ character 6 and whioh is intended for supporting a~ mass to be weighed. The block 2 is formed with threads 7 5~and~ 8 therein ~for cooperation with -fixation members not shown such as screws for securement thereof to the base 4 and the plate 6. ~ ~
Said block has its larger dimens.ion called "length" L according to a horizontal axis X-X and its 30 sma~ller dimension called "width" according~;to an horizontal axis~ Y-Y perpendicular to the plane of figure 1. This~block presents according to a vertical aXis Z-Z parall0l to the direction of the force to be measured F a dimension called height'! which is lower than the length and substantiallY
35 higher than t!le wldth.
The block 2 is recessed in the direction o-F its : : ~ : ::

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width perpendicul.arl\J ~o axes X X and Z-Z such that it comprises a first vert:Lcal upright 2A perpenclicu1.ar to the application area 3, a sel-oncl vertical upright 2e benPath the reception area 5 and two horizon-ta] beams 2C ancl 2n S respectively an upper and a lower one, connected to such uprights -through thinner zone.s sA and 9~, ~C and 9[). The thickness e1 of the thinner zones sA ancl 9~ bordering the upper beam 2C as measured according to axis Z-Z is lower than the thickness e2 o-F the thinner zones 9C and 3D
10 bordering the lower beam 2U: flexibility of zones 9A and 9 is thus hiyher than flexibility of thinner zones 9C and 9D.
Advantageously, the central recess in the block 2 consists of a central cylindric bore 11 of radius R and of center O connecting four cylindric bores 12A through 12D of : 15 smaller radius r, each defining one of the thinner zones and centered according to the corners in a rectangle 01,02,03,04 with sides parallel to axes X-X and Z-Z.
; By way of example, in a block of 110 x 40 x 20 in mm) R is equal. to 17.5 mm, r to 7.5 mm and the

2~0~thicknesses of the thinner zones are respectively e1 = 2.5 -~ ~ mm and e2 = 3.5 mm, with the thinner zones spaced 3a mm :apart parallel to axis X-X ~distance 01,02, or 03,04).
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It has been experienced that for obtaining good accuracy it was desirable for th~ sum e1 + e2 to be higher :25 than a nominal value ~of for example 1.8 mm in the above-mentioned case) and that it was advantageous for obta~ining low sensitivity to eccentered loads that the u~difference e2 - e1 should be higher than a minimum value different from O (o-f 0.6 for example in the case :: 30 considered) ~On the upper face of block 2 and opposite the :
thinner zones ~A and ge of maximal flexibility, sensitive elements 13A and 133 are disposed, responsive to flexural ::deformation o~f such zones. These sensitive elements are : 35 connected within a supple circuit 13 comprising a residual : portion 13C above the vertical upright 2A, i.e. in a :

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neutral z~ne as regal-cis straln or c1eformation.
~ etails of such suF)p:Le ci~cuit )3 appear in figure 2 which shows the upper section o-f block 2.
~ It is t.o be notecl that accorc~ g tu an 5 advantageous embocliment o-f the i.nvention, the sensi.tive elements and the rest o-f the c:ircui-t 13 are brougt)t to a very narrow surFace.
The circuit 13 o-F figl.1re 2 comprises sensitive elements 13A and 13B consistiny of strain ga-lyes which are 10 connected through tin-plated wires 14A,1~B,14C and 14~ to the partial circuit 13C comprising a grid 15 to compensate for temperature drift in sensitivity, and the inactive gauges 1 6A and 1 6e ~ Such circuit comprises output terminals 17.
In a modified form of embodiment not shown, the inactive gauges are secured to a vertical face of the block.
The supple circuit 13 is obtained directly by . glueing active gauges 13A and 13B to the thinner zones 9A
20 and 9B ~glueing zones appear at 18 in figure 2~ by glueing a circuit carrier 19 (printed circuit board) comprlsing conductive lines, then by glueing the grid 15 and the ~ .
ina~tive gauges 16A and~16B to such carrier. These glueings~
are:prac~tically followed wit~ disposal thereof into oven~to 25~thermally c3nsolidate under pressure fixation of these~
e~.ement~s.
In a~ modified form of embodiment not shown the residual portion 13C of the circuit 13 is located between the sensitive elements 13A and 13B (this is for example a 30~polyamide layer or an epoxy resin layer of a thicknes~s of about 10 microns). Again according to another modified form of embodiment, the portions 13A, 13B and 13C belong to one and~the~same contlnuous circuit.
Figure 3 shows a modified form of embodiment of ` 35 figure 1 in which the vertical upright 2'A and 2'B:of the~
block 2 have an increased height 2~H relative to the : :
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remainder of the block (tll(3reb\J to permit easi.er ~eCIJrement to a hase or a plate) and the th:Lrlrler zorles s~A~s~s~c anrl 9'D are determinell by l1ttle bores corln(cte(.t toyethel b~ a rr-~ctangular central bclre. The bl.ock 2' also preserlts bores 5 20 in the direr-tion of the wldth i.n the vertica:L uprights and by means of which such uprlghts can be securecl to said base or sald plate. The supple circuit 13' is continuous and extencJs up to the lef~t-hand upper corner of the block.
According to an advantageous form of embodiment oF the invention, the parallelepipectic block o-f a fnrce sensor according to the invention is made of two sections ; possibly obtained .in parallel manufacturing steps which are however distinct from materials that may be clifferent (metal and/or plastics, resin...).
Figure 4 illustrates a block 21 comprising a U~shaped frame 22 comprising vertical uprights 22A and 22B
connected at their bases through thinner zones Z3A and 23B
to a horizontal beam 24 as well as a flexural blade 25 adapted to be added to the upper portion of the frame 22 20~ for example through screws by means of bores and threads 26 therein formed at the ends of said blade and at the upper portion of the uprights 22A and 22B. To this flexure blade Z5~ there is added a continuous circuit 28 extending cantinuously above and between the thinner zones 27A and 25 ~27~B~. With such circuit 28 there are associated output wires 29, `~ ~
Figure 5 shows an arrangement which illustrates the~ advantage of the conformation of the block 21 from two sections. In such figure the fl~xura1 blade 25 ~ is 30~effectively secured to the U-shapecl frame 22, the latter being added to ~a~ base 4 and a diagonally opposite end of~
the~flexural blade~constitutes a reception area for a force F to be measured.
Aclvantageously, securement means (not shown) 35 provide for fixation of both a reception plate for a load to be weighed and~such flexure blade to the upright 22B of ; the frame.
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i~S9342 1~3 The clrcult 2~ aciclel to -the upper face uf the flexural blade 25 is cllsposecl as a who:le wlthln a sealecl enclosure consisting of a sealed lnsul.ation .sl.eeve, constituted here by meta.~ c beLlows 3n adapted to fol.low 5 up flexions oF such flexur.ll. bar fiXatiOIl O~ said beLlows is facilitated by the preserlce of shoulders 31 projec-ting vertically. It is~ posslble to mount the be]lows 30 witholJt longitudinal welding according to axis X-X in ~/iew of the removable mounting of the blade 25 in respect to the 10 remainder of the blocl< 21.
The flexibility of thinner zones 27A and 27B is higher to the flexibility of thinner zones 2~A and 23~.
This difference may be ohtained as previously stated through selecting e"1 less than e"2; ta~ing profit of the 15 2-part structure of the block, one may also use a flexural blade of width less than the width of the U-shaped frame, : or a blade made in a more flexible material. There may also : be contemplatec, for the thinner zones, differences in : their height, in their width (or even in their length, with 20 different curvature radii) as well as in their constitu:tive material.
It will be understood that the preceding description was only given lndicatively ; and not : limitatively and that many varied forms of embodiment can 25 be~proposed by the man of the art without however departing ~ : : from the :scope of the invention, in particular, one can :~ ; combine the above mentioned advantageous forms of : :embodiment so that the blocks of figures 1 and :3 can be : realized in two sections and so that the form of the 30 central recess can be modified in many ways.
The thinner zones of maximal flexibility may be located at lower portion of the block; flexural blades may :
constitute a lower beam o~ a bloc~ according to the : :invention.
:~ 35~It may be noted that, according to different preferred features of the invention, which are common to :~

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, ` i;~59342 'I 1 the illustratecl ernbodirnrlnts - the c:entral open:i.ng is symmr!-trical w:ith re~spect to an hori7On-tal plane para.~lr-!l tr, axes X-X anci Y-Y;
- ~he central opening is symm(~trica] ~ith respec1 5 to a transverse verti.cal plane containiny axes Y-Y ancl Z-Z
or parallel thereto;
- the thinner zones respectively loca~ed on upper and low~r beams facing each othe.r are centered wi-th respect : to two parallel planes;
1Q - these thinner zones are preferably delimited by transverse boles, the axes of which are lying by pairs in two parallel planes; and - the strain gauges are locatecd in a:symmetrical manner with respect tc corresponding thinner zones, : 15 parallel to axis X-X.

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Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A force sensing device for use in a weighing instrument, comprising a substantially parallelepipedic block with maximum and minimum dimensions, respectively called length and width, measured according to perpendicular horizontal axes and an intermediate dimension called height measured according to a vertical axis, such block being provided with a central opening in the direction of its width so as to consist in a deformable parallelogram with two vertical uprights, connected through two pairs of thinner zones to two upper and lower horizontal beams, with one of these uprights being intended for fixation of the block to a base and the other of such uprights being intended for securement to a plate for reception of a load to be weighed, said sensing device further comprising means sensitive to flexural deformation and disposed on a substantially planar horizontal face of the block opposite two thinner zones through which one of the horizontal beams is connected to the vertical uprights the parallelepipedic block being made of a flexural blade comprising the beam carrying said sensitive means and an upper portion of each of the vertical uprights, and of a U-shaped frame to which said flexural blade is removably attached.
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2. A force sensing device according to claim 1, wherein said flexural blade and said U-shaped frame are made of different materials.

3. A force sensing device according to claim 1, wherein said flexural blade extends through a deformable sleeve for sealingly isolating both thinner zones of such blades with respect to the outside.

4. A force sensing device according to claim 1, wherein said flexural blade has a width which is smaller than said width of the U-shaped frame.

5. A force sensing device according to claim 1, wherein said sensitive means are carried by said upper horizontal beam.

6. A force sensing device according to claim 5, wherein said flexural deformation sensitive means are a part of a supple electronic circuit added to the upper face of the block.

7. A force sensing device according to claim 6, wherein said supple circuit comprises two active strain

Claim 7 continued...

gauges opposite said thinner zones and two inactive strain gauges connected to the active strain gauges so as to form a wheatstone bridge and disposed opposite one zone of the upper surface of the block where deformation is zero, as well as a grid to compensate for the temperature drift in sensitivity on a power supply section.

8. A force sensing device according to claim 1, wherein said opening defining both pairs of thinner zones is formed from a main cylindric bore connecting four cylindric bores together disposed as a rectangle bordering said thinner zones.

9. A force sensing device according to claim 1, wherein said thinner zones of higher flexibility are of a thickness which is smaller than thickness of said other thinner zones.

10. A force sensing device according to claim 1, wherein the central opening is symmetrical with respect to a horizontal plane.

11. A force sensing device according to claim 1, wherein the central opening is symmetrical with respect to a transverse vertical plane.

12. A force sensing device according to claim 1, wherein said thinner zones respectively located on upper and lower beams facing each other are centered with respect to two parallel planes.

13. A force sensing device according to claim 1, wherein said thinner zones are preferably delimited by transverse bores, the axes of which are lying by pairs in two parallel planes.

14. A force sensing device according to claim 1, wherein said sensitive means are located in a symmetrical manner with respect to corresponding thinner zones, parallel to length axis.