GB2083918A - Force-measuring cells - Google Patents

Abstract

A force or load measuring cell comprises a hermetically sealed chamber. A beam extends across the chamber and is secured at opposed ends to the wall defining the chamber. The beam is in two similar parts 16 symmetrical with respect to a plane normal to the direction of length of the beam and passing through the centre thereof. Two force transmitting members are secured to the centre of the beam and extend in opposite directions away therefrom to extend through the wall of the housing at opposed locations 30. A flexible joint hermetically seals each force transmitting member to the housing at such locations whilst permitting displacement thereof. Stop means in the chamber are operable to limit displacement of the beam centre to a predetermined value. <IMAGE>

Description

SPECIFICATION Improvements in or relating to load and force measuring cells This invention relates to improvements in load and force measuring cells and is concerned with such cells of the kind (hereinafter referred to as being of the "kind referred to") in which the flexure or shear imparted to a beam by the application of the load or force to be measured through a force transmitting member is measured by strain guages attached to the beam.

As is well known, cells of the kind referred to have many applications and are capable of measuring an applied load or force with considerable accuracy and can be constructed to measure magnitudes which vary from quite minute values of the order of less than one gram to several tons. In some cases, the accuracy can be reduced by variations in atmospheric pressure or by the application of the load or force off-centre from the line of application thereof to the beam, for example, where the load is applied to a weighing platform at a location displaced from the axis of the force transmitting member applying the load to the beam so that couples or movements are generated.

Also such cells may be required to operate in a hostile environment. For example, such cells, when employed in the food industry may from time to time be exposed to hot washing solutions used to hose down the equipment or, as in the coffee industry, may be exposed to a dust laden atmosphere which can interfere with the beam displacement which, in some cases may be a maximum of two - one thousandths of an inch.

It is an object of the present invention to provide an improved force or load measuring cell of the kind referred to in which the displacement of the force transmitting member is constrained to a single degree of freedom to substantially obviate the deleterious effect of movements and couples and in which the cell is hermetically sealed to isolate the operative parts from a hostile environment and is barometrically compensated against variations in atmospheric pressure.

The present invention, broadly stated provides a force or load measuring cell comprising a hermetically sealed chamber, a beam extending across the chamber and secured at opposed ends to the wall defining the chamber, the beam being in two similar parts symmetrical with respect to a plane normal to the direction of length of the beam and passing through the centre thereof, two force transmitting members secured to the centre of the beam and extending in opposite directions away therefrom to extend through the wall of the housing at opposed locations, a flexible joint hermetically sealing each force transmitting member to the housing at such locations whilst permitting displacement thereof and stop means in the chamber operable to limit displacement of the beam centre to a predetermined value.

The invention also provides a load or force measuring cell comprising a housing in the form of a rigid frame of closed configuration, a beam extending across the frame within the housing and secured to the frame at its opposed ends, the beam being in two similar parts symmetrical with respect to a plane normal to the direction of length of the beam and passing through its centre, strain guages mounted on said two parts force transmitting means secured to the centre of the beam and extending in opposite directions away therefrom in a direction normal to the direction of length of the beam and extending through the frame at opposed locations, means operable to confine displacement of the force transmitting means to said direction normal to the direction of length of the beam, a flexible joint hermetically sealing the force transmitting means to the frame whilst permitting said displacement thereof, stop means within the housing and rigid with the frame to limit displacement of the centre of the beam to a predetermined value and closure means hermetically sealed to opposed faces of the frame to provide a hermetically sealed chamber therein.

Embodiments of the invention will now be described by way of example, reference being made to the accompanying drawings in which: Fig. 1 is a fragmentary perspective view of a load or force measuring cell according to the invention; Fig. 2 is a section taken along the line Il-Il of Fig. 1; Fig. 3 is a fragmentary perspective view illustrating a detail of the cell shown in Fig. 1; Fig. 4 is a part-sectional, part elevational view of a part shown in Figs. 1 to 3 to a larger scale; Fig. 5 is a view similar to that of Fig. 1 illustrating a second embodiment of a load or force measuring cell according to the invention:: Fig. 6 is a section taken along the line VI--VI of Fig. 5; Fig. 7 is a fragmentary perspective view illustrating a detail of the cell shown in Fig. 5; Fig. 8 is a view similar to that of Fig. 1 illustrating a third embodiment of a load or force measuring cell according to the invention; Fig. 9 is a section taken along the line IX-IX of Fig. 8; Fig. 10 is a fragmentary perspective view illustrating a detail of the cell shown in Fig. 8, and, Fig. 11 illustrates a plurality of cells formed in a single bar.

The first embodiment of the invention illustrated in Figs. 1 to 4 comprises a block generally indicated at 1 of rectangular crosssection cut from a rod of aluminium alloy of similar cross-section. Two windows 2 are formed therein symmetrically with respect to a medial plane of the block which plane contains the axis of the rod, to form a rectangular frame with a centre limb 3, the upper and lower parts of the frame as viewed in the figures being designated 4 and 5 respectively and the left-hand and right-hand sides of the frame being designated 6 and 7 respectively.

Each face of the frame is ground or milled to have upper and lower shoulders 8 and 9 respectively so that the cell may be hermetically sealed by a cover plate 10 applied to each face and electron beam welded thereto around its periphery. Each face of the frame is further machined to provide a recess 11 therein thereby leaving a space under the cover plate 10 to accommodate electrical wiring and/or printed circuits.

In a single operation, a bore is formed through the left and right-hand sides 6 and 7 of the frame and the centre limb 3, for example, of a diameter of 7.5 mm. This bore extends normal to the medial plane referred to and provides a passage 1 2 in the central limb 3 and passages 3 and 14 in the sides 6 and 7 respectively, these passages being enlarged at 1 3a and 1 4a at the outer part of the frame.

A unitary beam of aluminium alloy indicated generally at 1 5 is provided and is in the form of two similar parts symmetrical with respect to a plane normal to the direction of length of the seam and passing through its centre. Each part is in the form of an arm 16 6 of rectangular cross- section shaped at its inner end 1 7 to form a tortuous path or zig-zag configuration whereby it is connected to a central cylindrical load bearing part 18 so as to allow the arms to flex. Each arm 1 6 is formed at its outer end with a cylindrical portion 1 9 coaxial with the central load bearing part and dimensioned to be a close fit within the passages 1 3 and 14.The central load bearing part 1 8 is of slightly smaller diameter than the cylindrical portions 1 9 so that it has clearance within the passage 12. For example, with the passages 12, 13 and 14 of 7.5 mm diameter, the cylindrical portions 19 would be 7.5 mm diameter and the central load bearing part 18 of 7 mm diameter.

The beam 1 5 is inserted through one or other of the passages 1 3 or 14 and passed through the central passage 12 so that the end portions 1 9 are located in the respective passages 13 and 14 and the central load bearing part is within the passage 12 with the arms 1 6 extending across the windows 12. The end portions 19 are electron beam welded into their respective passages 12 and 13 to provide a hermetic seal. Strain gauges 20 are mounted on the arms 1 6 before the beam is mounted in the frame and the wires therefrom are led (in a manner not shown) through a bore 21 which opens to the recess 11 into a communicating bore 22 which leads to the extension of the frame and which is hermetically sealed.

The frame is formed with a through bore 23 the axis of which is in the medial plane referred to and which is normal to the axis of the passages 1 2, 13 and 14 and which passes centrally through the central limb 3 so that it intersects the passage 12.

This bore, on each side of the central passage 12, is formed with an intermediate step 24 and is formed with a further step 25 towards each end thereof to provide an enlarged circular recess 26.

A flexible bearing 27 is mounted in each recess 26. Each bearing comprises an annular plate 28 of stainless steel with an upstanding annular rib 29, a cylindrical end cap 30 of stainless steel and a bellows or spring arrangement 31 also of stainless steel. The cap 30 has a coaxial bore 32 therethrough, an annular flange 33 at one end and an annular rib 34 at the same end as the annular flange. The bellows or spring arrangement 31 comprises a pair of dished stainless steel diaphragms 35 disposed face to face to enclose a space and the diaphragms 35 are electron beam welded together at their periphery. One of the diaphragms is electron beam welded to the annular rib 29 and the other to the rib 34 so that the space between the diaphragms 35 is only open to the bore 32 and to the central opening in the annular plate 28.The annular plate 28 is a close fit in the recess 26 and is welded therein about its periphery by electron beam welding to effect a hermetic seal.

Stainless steel thrust rods 36 are inserted through each bore 32 to extend through the bore 23. The thrust rods 36 are threaded at their inner ends and are threadedly engaged in a correspondingly threaded bore in the central load bearing part 18 of the beam 1 5. The central load bearing part 1 8 is centralised in its passage 1 2 under the rigidity of the beam 1 5 and the thrust rods 36 when engaged therewith assume their own appropriate axiai positions and are then electron beam welded about their periphery in the bore 32 to effect a hermetic seal.

It will be appreciated that the load or force measuring cell is a hermetically sealed unit protected from the environment and that it may be filled with a suitable atmosphere if desired, e.g.

through the bore 22. It will also be appreciated that the thrust rods 36 and the beam arms 1 6 form a symmetrical mechanical bridge which is insensitive to variations in external barometric pressure as such would equally effect both thrust rods 36. The close fit of the annular plates 28 in the recesses 26 and the stiffness of the bellows arrangement 31, ensures that the thrust rods 36 are limited to one degree of freedom, i.e.

displacement in the direction of their axes so that inaccuracies resulting from loads applied offcentre to such axes is avoided.

In use, loads or forces to be measured would be applied to one or both the end caps 30, the arms 1 6 of the beam would be flexed by displacement of the central load bearing part 1 8 in its passage 12 and the strain guages 20 would provide a measure of the load or force being applied. In some applications, the load cell described would stand on and be secured to a base formed with a cavity large enough freely to accommodate one ofi the end caps 30 and the load to be measured would be applied to the other end cap 30.

It will also be appreciated that the clearance of the central load bearing part 1 8 of the beam 1 5 in its passage 12 is sufficient to accommodate the maximum designed flexure of the arms 1 6.

Nevertheless this arrangement provides a positive stop which safeguards the cell against overloads in excess of the designed maximum value of intended applied load or force.

The embodiment illustrated in Figs.5,6 and 7 is similar in many respects to that described with reference to Figs. 1 to 4 and like reference numerals are used to denote like parts.

In this case however, the beam has two arms 1 6a extending across each window 2 and the central load bearing part of the beam is formed by the central part of the central limb 3 and is here referenced as 318. The arrangement illustrated is machined from the solid and in the absence of the previously described zig-zag portion 1 7 of the arms and because the arms 1 6 are unitary with the central portion 318, the arms operate in contra-flexure.

In order to achieve displacement of the central load bearing part 318 the central limb 3 is secured towards its upper and lower ends to provide slits 50 which not only allow the required displacement but also limit such displacement to provide overload protection. In this embodiment, the passages 1 3 and 14 serve no useful purpose other than to facilitate machining of the central passage 12 and do not require to be enlarged at their outer end. They can be used to lead out electrical connections if required but in any event, would be hermetically sealed. As the windows 2 are larger than in the previously described embodiment, the bore 21 is not required and the wiring may be led out directly through the bore 22.

It is important to note that as the central part318 of the central limb 3 is now the load bearing part, it must be connected to the thrust rods without any lost motion. For this purpose, the central passage 12 receives as a tight fit a cylindrical member 11 8 which preferably has two opposed flat faces 11 9 machined thereon and into which the thrush rods are screwed. Thus any load or force applied to the end caps 30 is directly transmitted to the central load bearing portion 318 of the beam and the arms 1 6a operate in contra-flexure.

The third embodiment of the invention illustrated in Figs. 8 to 10 is similar in many respects to that described with reference to Figs. 5 to 7 and like references are used to denote like parts. In this case also, the central load bearing part 318 is part of the central limb 3 and the arms designated 1 6b operate in contra-flexure. In this embodiment however, there are only two arms 1 6h but these have wider faces in the plane of the section shown in Fig. 9 as compared to the corresponding faces of the arms 1 6a of the previous example and the strain guages 20 are attached to these wider faces. In this example, the passages 12,13 and 14 are omitted and the passage 12 of the previous examples is replaced with a passage 1 2a extending at right-angles to that of the previous example.As in the immediately preceding example the load is transmitted through the thrust rods 36 to a cylindrical member 118 which is a close fit in the passage 1 2a so that there is no lost motion.

It will be appreciated from the above described embodiments that the working part of the cell is enclosed in a hermetically sealed casing in a controlled environment and is not sensitive to variations in external barometric pressures. Built in overload stops are provided within the sealed casing so that there is no danger of dust or powder interfering with the operation of the cell or of the cell being destroyed by an applied overload.

Also, as the thrust rods are constrained to move only in their direction of length, the cell is free of errors resulting from movements or couples arising from loads or forces applied off-set from the axis of the thrust rods.

It will be appreciated that metal from which the frame and the beam are made may be selected to suit requirements. In the example described with reference to Figs. 1 to 4 the frame is machined from the solid and a unitary beam is fitted thereto.

In the other examples the beam and the frame are a unitary structure machined from the solid and the cell is easily reproducible, e.g. using a numerically controlled (N/C) machine.

It will be appreciated that as the cells described are machined from the solid, two or more such cells may be incorporated in a single bar as illustrated in Fig. 1 , where two cells are shown identified by the cover plates 10 machined in a bar 100 of aluminium alloy. Two such bars in spaced parallel relationship could be used to support a weighing plate which would be supported towards each of its four corners by an end cap 30 of the four cells provided and could be used for any designed purpose from weighing vehicles to goods. Also, such a bar 100 of appropriate length and with an appropriate number of cells could be used to provide a measure of the load of a body suspended from and moving along an overhead track.In this case, the end caps would be suspended from a fixed overhead member and the lower part of the bar 100 could carry a part of the track from which the body is suspended so that as the body moves over that part of the track a measure of the load would be obtained.

It will be apparent from the above description that the cells described incorporate a pair of flexible bearings 27 which enable motion to transmitted to the interior of the sealed housing without destroying the seal and which constrains the motion transmitted to one degree of freedom.

The invention therefore provides as a further feature a flexible bearing operable to transmit a linear displacement through a sealed chamber, the flexible bearing comprising a plate having an aperture therethrough attached in spaced relationship to a cap member by a pair of dished spring diaphragms disposed in face to face relationship, hermetically sealed to each other at their periphery and hermetically sealed, one to the cap member and one to the plate.

More specifically, the invention further provides a flexible bearing operable to transmit a linear displacement through a sealed chamber comprising a pair of flexible bearings according to the immediately preceding paragraph disposed in spaced face to face relationship in the direction of displacement and hermetically sealed by the plates thereof to opposed walls of a sealed chamber through which the motion is to be transmitted and connected by a rigid member extending between and hermetically sealed to the cap members.

With the parts of the bearing made of stainless steel or other suitable metal, the bearing would have the advantages of being: (a) frictionless.

(b) free from lubrication requirements.

(c) insensitive to dirt.

(d) stable performance.

(e) repeatable manufacture.

(f) suitable for use in a hard vacuum or other selected environment.

(g) controlled characteristics.

(h) simplicity of installation.

(i) negligible hysteresis, and (k) electrical continuity.

Claims (2)

1. A force or load measuring cell comprising a hermetically sealed chamber, a beam extending across the chamber and secured at opposed ends to the wall defining the chamber, the beam being in two similar parts symmetrical with respect to a plane normal to the direction of length of the beam and passing through the centre thereof, two force transmitting members secured to the centre of the beam and extending in opposite directions away therefrom to extend through the wall of the housing at opposed locations, a flexible joint hermetically sealing each face transmitting member to the housing at such locations whilst permitting displacement thereof, and stop means in the chamber operable to limit displacement of the beam centre to a predetermined value.

2. A load or force measuring cell comprising a housing in the form of a rigid frame of closed configuration, a beam extending across the frame within the housing and secured to the frame at its opposed ends, the beam being in two similar parts symmetrical with respect to a plane normal to the direction of length of the beam and passing through its centre, strain guages mounted on said two parts, force transmitting means secured to the centre of the beam and extending in opposite directions away therefrom in a direction normal to the direction of length of the beam and extending through the frame at opposed locations, means operable to confine displacement of the force transmitting means to said direction normal to the direction of length of the beam, a flexible joint hermetically sealing the force transmitting means to the frame whilst permitting said displacement thereof, stop means within the housing and rigid with the frame to limit displacement of the centre of the beam to a predetermined value and closure means hermetically sealed to opposed faces of the frame to provide a hermetically sealed chamber therein.