CA1206488A - Electronic scale - Google Patents

Abstract

ABSTRACT
An electronic scale in particular intended to be used as a platform scale. The scale comprises a sensor device having six supporting points which in a plan view form the corners of two triangles. The supporting points situated at the corners of one of the triangles are con-nected to a base whereas the scale pan is supported at the supporting points at the corners of the second triangle. The sensor device comprises a body which is so connected to the different supporting points that is is subject to a bending moment when the scale pan is loaded. The sensor device is also provided with electrical means for indicating the bending moment.

Description

This invention relates to an electronic scale.
Generally, exact measuring of a force in one direction is comparatively simple. Thus, for instance by we;ghing in a suspended scale pan exact weight informa-tion can be obtained by using one measuring sensor. How-ever, in most cases a swinging and unstable scale pan can-not be accepted but a su~stant;ally r-;gid pan or platform for the load is needed.
In order to achieve a stable scale pan, the pan could for instance be suspended on three arms extending from a base and being connected to the pan ~y means of strain gauges. Alternatively, a load cell could be placed under each corner of a platform. Both these solutions require several sensors which have to be trimmed to exactly the same sensitivity in order to indicate the same weight independently of the position of the load on the pan or the platform. Such constructions are technically rela-tively complex and expensive.
If, instead, a central load cell below the middle of the platform is used, large moments are produced when the load is moved to a corner. These moments must be taken up by the load cell without any registration error, which is hardly possible to achieve, even with specially designed load cells, particularly when the platform is large or the demand for accuracy is great.
To eliminate such side forces and undesired moments, known mechanical scale constructions having means for parallel guiding of the platform have been used. The electronic sensor is in these cases so disposed that the force is taken up solely in one direct;on. In some cases, such sensors have been substituted for n wel,~hin~ spring or a system of balanci,ng welghts. ~hen the original mechanical scale constr~ction has been of an expens;ve preci.sion type and the integration of the sensor has been made with great exactness, a goo~ result has been ~chi,eved, altllough at a very high cost.
It is aLso ~revLously known (U.S. Patents 4,274,501 and 4,276,949) to use scales havi.ng i.ntermediate measuring unLts whi.ch are supported at six points and which are provided with sensor n~eans. However, ~he supporting joints are so arran~ed that the levers are very short, and consequently tl~e accuracy is also very low. Another drawback when usi.ng this type of scales i.s that the supporting points are formed as kni.ves which can easily be damaged and subject to wear affecting the exactness of the scale.
The failure of many attempts to modify existing mechanical scales ~or electronic weLghing depends on the fact that sometirnes such short bslanei.ng arms have been used in the mechanical construction that the demand for accuracy with respect to the length of the arm has been of the magnitude of 1/1000 mm. This is difficult both to achieve and to maintain, i.e. due to wear.
The basis of this invention is the desire to provide an electronic scale which is simple and unexpensive from a manufacturing point of view and comprises a sensor which, as far as possible, eliminates undesirable forces and moments caused by the positioning of the load on a platform or the like, and which needs only one sensor. The construction should also be such that short moment arms, which ~enlan(l ~re~t m~nuL~cturin~ l~recision, an~ a~so knives and sockets which can result in friction or wear are a~oided.
The length of the moment arms should be ol~timized to be as lon~ as mechanically possible. Furth~r, no mechanical a~j~stment on assembly should normally be necessary.

~ ccording to one aspect of this invention there is provided an electronic scale including a sensor device, support means having six support points which in a plan view form the corners of two triangles, the support points situated at the corners of one triangle being connected to a base, a weighing receptacle means being supported by the support points at the corners of the other triangle, the apexes of the triangles being oppositely directed, the bases of the triangles being parallel with the triangles partly overlapping each other in a plan view;
a half height point in each triangle is common to both triangles, said sensor device being mounted at said common half height poin~ and comprising a body having sensors, the body being rigidly connected to said six support points via a sys~em of arms so that the action center of ~he sensor body is located at said common hal~ height point.
According to another aspect of the invention there is provided an electronic scale comprising a support means having six support points, said six points defining first and second overlapping triangles with parallel bases and oppositely dixected apexes t the half height points of each triangle being common to the other triangle, means supporting said support means at the points defining one of said triangles and weighing receptacle means supported from said support means at the remaining support points, a support body mounted on said support means in the region of said common half height point, whereby weight on said weighing receptacle means induces strain in said support body, and strain measuring means on said support body at said common half height point.

The above object ;s achieved according to the invention by using a so-called double three-point supporc, meaning that the scale pan or corresponding part is su~ported at three points in a sensor device which i.n turn is supported at three other points in a base. By using two three-point supports ~he constructi.on is statically fully determined.
The i.nvention will now be descri.bed by way of example with reference to the accompanying drawings.
FIG. 1 ~s a partly broken perspective view of an embodiment of a scale accordi.ng to the lnvention.
FIG. 2 is a plan vi.ew, partly in section, of an embodi.ment of a sensor device accordi.ng to the i.nven-tion.
FIG. 3 is a vertical secti.on throu&h a scflle according to the inventi.on.
FIG. 4 shows i.n a side view an alternative embodiment of a central part of the s~nsor devi.ce~of FIC. 1.
FIG. 5 is a plan vi.ew of an nlternative embodi-nlent of a sensor devi.ce accordi.ng to the invention.
FIG. 6 ls a~plan view of another elllboclilnent of a sensor devlce.
FIG. 7 shows Dn embocliment of a llanger.
FIC. 8 i.s a Fasteni.ng means for the hanger of FIC. h.

-3a-FI~. 9 illustrates fastening of a hanger in one of the arms of the sensor device.
The sensor device shown in a plan view in FIG.

2 comp~;ses six supporting points A, B, C, D. E and F, which are connected in pairs by arms 1, 2 and 3. The arms 1 and 3 are also connected to a cross bar ~, which is rigidly connected to one end 5 of a measuring body in the form of a tube 6~ The arm 2 extends freely through the tube and is rigidly connected to the oppos;te end 8 of the tube by means of a sleeve 7. Thus, the complete sensor device forms an integral unit without any linked connec-tion.
Since all supporting points are rigidly connected to one or the other of the ends of the tube 6 by means of arms the latter will, when loading the sensor device, be subject to a bending moment, which in a conventional way is sensed by means of electric strain gauges 9 provided on the upper and lower sides of the tube. The strain gauges 9 are electrically connected to a box, which is provided with an electric source, such as a battery, and means for processing and transforming the s;gnal to a display.
The supporting points A, B and C of the sensor device of ~IG. 2, which are indicated as tips of arrows, are fastened to a base, which can be box-shaped, whereas the scale pan or corresponding part is fastened to the supporting parts D, E and F which are indicated as ends of arrows.
The supporting points Ln the two three-point supports form corners, each defining an isosceles triangle ABC and DEF, respectively. The points A and D form the apexes of the triansles and it ls important that the strain gauges 9 are placed exactly at the half-height points of each triangle. According to FIG. 2 the arm 2 is coaxial with an imaginary height line in the relevant triangle.
Due to the use of the double three-point support the sensor device is acted upon by six main forces when the scale pan is loadecd centrally, all of the forces being directed vertlcally, i.e. perpendicular to the plane of the paper. The effect of the three-point supports is that the force system is statically fully determined and self-adjusting. Thus, lf the base is uneven or inclined thls causes only an error which depends on the shortening of ~he projected length of the moment arm concerned. This error is in most cases negligible so that normally no levellLng of the scale will be necessary.
The sensor device according to FIG. 2 can for instance be suspended in the manner shown in FIG. 3. The arM connecting the supporting points A and D to one end 8 of the measuring tube 6 Ls, as in FIC. 2, denoted by 2. The point A is thus supported in a box 10 serving as a base by means of a leaf spring 11, which is riveted to the wall of the box at 12. In a similar way also the points B and C are supported at the opposite wall of the box.
A scale pan 13 is supported by means of a leaf spring 14 at the supporting point D. The spring 1~ is riveted to the pan at 15. In a s;milar way the pan at its opposite edge is supportecl at the supporting points E and F.

~2~

To avoid turning moments about the length a-~is of the sensor tube 6 the springs 11 and 14, respectively, should be rotatably supported at the ends of the arm 2.
Other fastening points are rigid. P~y us-ing leaf springs some axial displacement between the pan and the base is allowed at the same time as the springs operate to center these parts in relation to each other. In addition to tensile forces the springs should also be able to transmit some compressive forces as will appear below. The leaf springs can however be replaced by other equivalent types of hangers.
In the scale according to FIG. 3, which is pro-vided with a sensor device according to FIG. 2, if the tare weight of the sensor device is not taken into account, the sum of the three downwardly directed forces acting on the sensor device and originating from the scale pan with load will equal the sum of the upwardly directed react;on forces originating from the base. Moving the load between different points of the scale will of course not influence the size of the sum of the forces, which means that the sensor tube 6 will be subject to the same bending moment about its central plane S - S in FIG. 2, where the strain gauges 9 are placed, independently of the position of the load on the scate pan. No turning moment about the length axis is transm;tted because of the above noted fastening of the hangers 11 and 14.
If as an example the forces are considered when the weight P is centrally placed on the scale pan and if the tare weights of the pan and the sensor device are not taken into account, the forces at the points A and D will ~Z~4~8 both be P/2 but directed in opposite directions. The forces at the points B, C and E, F, respectively, will each be P/4 and two and two be directed opposite to one another. It can easily be shown that the moment which is transmitted to the right-hand end of the sensor tube by means of the couple of forces A - D is equal to but directed opposite to the moment which is transmitted to the left-hand end frorn the couples of forces B - E and C - F. If the distance between the points A and D is denoted by L, the size of the momemt M, which is measured at the central plane S - S by the strain gauges 9 will be:
~1 = P x L/2.
If now the load is moved from the center so that it will be situated for instance exactly above the point A, the force at A will increase to the value of P whereas the forces at B and C will be 0. The forces at points E and F will each increase to P/2 and the force at D will be 0. The vertical forces acting on the sensor device of course balance each other and the moment M at the cental plane of the sensor will also in this case be:
M = P x L/2.
If now the load P is assumed to be moved to a corner of the scale pan, for instance exactly above the point E in FIG. 2, the system will tend to turn about an imaginary connecting line between the points A and B and thus be lifted at the point C. However, s;nce the support-ing springs can also take up compressive forces the system will be kept at the point C by an opposite force. There will be no force at the points D and F.

~z~

It i.s easy to show that the force at the pGint amounts to P/2 and that the force at the po;nt A amounts to P and at the point B to P/2. These forces are directed opposite to the force at the point C. This force constel-lation creates a turning moment of the cross bar 4, which however does not influence the sensor 9, which means that the moment at the central. plane S - S still will be:
= P x L/2.
In a slmilar way it is possible to show that if the weight is moved to any other point on the scale pan generally no false moments or forces will be transmit-ted to the sensor tube 6 although in certain cases moments are produced in the rigid cross connection. If the tare weights of the scale pan and sensor devlce are taken into account this will only ~ive a f:i.xed addition to the tare, which can easily be balanced electrically.
The sensor device in FIG. 4 is not tubular but homogenous and has a waist part 16 to wh:ich the strain gauges are applled. The moment is thus transmitted from the poi.nt A via the central arm 2 and a yoke 17 to the right-hand end of the sensor devi,ce, which as in FIG. 2 is in connection with the point D. The moment from the other supporting points is transm:itted as above to the left-hand end of the sensor dev:;.ce by means of the cross bar 4.
This embodiment is in particular suited for very sensitive scales and when it is desired to use a sensor material which cannot easily be manufactured as a tube.
The sensor device can however be modified also in other ways, and may for instance only be an extension of the central arm 2. Also in this case a yoke 17 is ~Ised accord-ing to FIG. ~.

~2~

In a sensor clevice according to FIG. 5 the outer arms and the central bar connecting these arms are generally of ~-shape. Thus, it is possible to save materlal and to reduce the weight by using cast silumin or the like. The function is however the same as that of the sensor device of FIG. 2.
In the modification in FIG. 6 the central arm 2 is displaced with respect to the side arms. As appears frorn the isosceles triangle indicated in the Figure the strain gauges are still placed on a common central plane S - S dividing the height in each triangle into two e~ual parts. This embodiment can in some cases be advantageous ;n order to facilitate and give more space for the hangers.
In the embodiment in FIG. 7 the hanger can be manufactured by photoengraving or punching of a suitable spring metal. The hanger can for instance be fastened to the box 10, as illustrated in FIG. 8, by means of a clamp screw 19 which cooperates with an adjusting screw 20, the length is adjustable by means of a thread 21.
This could be desirable for fine-adjustment of scales of a very high precision.
In scales of very high precis;on it can also be advantageous that the hangers, as shown with respect to a hanger 22 in FIG. 9, are fastened to the arms of the sensor device in such a way that there will be no contact between the hanger 22 and the arm in a position correspond-ing to the central plane 23 of the sensor device. The hanger 22 can -for instance be inserted in a slot and be fixed by means of a screw 24. The hanger 22 can also be so arranged that it will have no contact with the arm in the position which corresponds to the central plane of ~2~

the sensor device when deflected by normal load. This can be of particular irnportance when using highly pre-loaded scales demanding great accuracy.
In order to adjust the length of the arm, i.e.
the supporting point of the hanger, in precision scales of the type described above with respect to the center of the sensor device, it is also possible to arrange a cup-spring washer or the like between the end of the arm and the hanger. This washer can be compressed as much as necessary by means of the mounting screw of the hanger and be locked, for instance by glue, in the desired posi-tion. ~y such an adjustment it is possible to compensate for minor errors when mounting the strain gauges.
The embodiments of a device according to the invention described above should only be regarded as pre-ferred embodiments and a number of further mod;fications are possible within the scope of the principle of the double three-point support as defined in the claims, in particular with respect to the geometrical shape of the supporting arrangement and its hangers. Thus, the position of the supporting triangles with respect to one another could vary and the orientation of the strain gauges be adjusted so that they sense the bending moment.
~ y using this invention it is comparatively simple to manufacture scales which can be converted to different sensing ranges. This can be done by placing a first sensor device, intended to be used for compara-tively small loads, on a second sensor device, intended to be used for greater loads and which ;s tared for the weight of the first device. The scale pan of the second devlce and the base of the first device are suitably built together as one unit.

S;nce all loads are intended to be placed on ~he scale pan of the first sensor device it must be possible to overload this device. For this reason, stop means which operate when the maximum ]oad of the first sensor device has been reached are provided, for instance on the pan.
By means of a switch-over device the signals from the first or the second sensor device can be freely chosen to be connected to the measuring electronics so that the same scale apparatus can be used for weighing small loads with high resolution as well as heavy loads.
Switching-over between different sensing ranges can be effected automatically, for instance in dependence of the magnitude of the measured signal.
In the way described above an optional number of sensor devices can be placecl on each other in order to give the necessary resolution in a desired measuring range.
Although all the examples above relate to plat-form scales, the princ-iple of the solutions can also be applied for instance to load cells, in which the problems are mainly the same, or to other force senslng devices.
It should be observed that ;t is not necessary to use isosceles triangles although such an arrangement is preferred. Thus, it is possible to use triangles having different side lengths as long as the sensor is placed at or near the middle part of the height of the triangles.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electronic scale including a sensor device, support means having six support points which in a plan view form the corners of two triangles, the support points situated at the corners of one triangle being connected to a base, a weighing receptacle means being supported by the support points at the corners of the other triangle, the apexes of the triangles being oppositely directed, the bases of the triangles being parallel with the triangles partly overlapping each other in a plan view;
a half height point in each triangle is common to both triangles, said sensor device being mounted at said common half height point and comprising a body having sensors, the body being rigidly connected to said six support points via a system of arms so that the action center of the sensor body is located at said common half height point.

2. The scale according to claim 1, wherein the apex of each triangle as seen in a plan view is located substantially on the base of the other triangle.

3. The scale according to claim 1 or claim 2, wherein said body is connected to said support points by means of three arms, one of the arms being connected to the support points which form the apexes of the triangles and being rigidly connected to one end of the body and the other of said three arms being rigidly connected to the opposite end of the body.

4. The scale according to claim 1, wherein said body is cylindrical and arranged coaxially with respect to the arm extending between the apexes of the triangles, said last mentioned arm being rigidly connected to the other end of the cylindrical body.

5. The scale according to claim 1, wherein the corners of the two triangles are interconnected by three parallel rods forming said arms, the middle rod extending along the height lines of the triangles and being connected to one end of said body, the two other rods being connected to the opposite end of said body by means of a cross bar.

6. The scale according to claim 1, wherein the sensor device and the receptacle means are so supported at said points that a certain degree of movement is allowed in the direction of the length of the arms.

7. The scale according to claim 1, wherein at least two sensor devices having different characteristics are stacked one above the other in order to create different sensing ranges for the scale.

8. The scale according to claim 7, comprising a stop means which is activated when the maximum load of the relevant sensor device is reached, further increasing weight being registered by a less sensitive sensor device.

9. The electronic scale of claim 1, wherein said weighing receptacle means comprises a scale pan.

10. The scale of claim 6 wherein said support points comprise leaf springs.

11. The scale of claim 4 wherein said other arms are rigidly connected to the other end of the cylindrical body by intermediate spacing means.

12. An electronic scale comprising a support means having six support points, said six points defining first and second overlapping triangles with parallel bases and oppositely directed apexes, the half height points of each triangle being common to the other triangle, means supporting said support means at the points defining one of said triangles and weighing receptacle means supported from said support means at the remaining support points, a support body mounted on said support means in the region of said common half height point, whereby weight on said weighing receptacle means induces strain in said support body, and strain measuring means on said support body at said common half height point.