GB2475081A - A load cell having strain gauges to detect flexure in a beam using parallel resistors. - Google Patents

A load cell having strain gauges to detect flexure in a beam using parallel resistors. Download PDF

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Publication number
GB2475081A
GB2475081A GB0919396A GB0919396A GB2475081A GB 2475081 A GB2475081 A GB 2475081A GB 0919396 A GB0919396 A GB 0919396A GB 0919396 A GB0919396 A GB 0919396A GB 2475081 A GB2475081 A GB 2475081A
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United Kingdom
Prior art keywords
load cell
load
strain gauges
pair
strain
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
GB0919396A
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GB0919396D0 (en
Inventor
Michael William Ford
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Illinois Tool Works Inc
Original Assignee
Illinois Tool Works Inc
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Filing date
Publication date
Application filed by Illinois Tool Works Inc filed Critical Illinois Tool Works Inc
Priority to GB0919396A priority Critical patent/GB2475081A/en
Publication of GB0919396D0 publication Critical patent/GB0919396D0/en
Publication of GB2475081A publication Critical patent/GB2475081A/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G19/00—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups
    • G01G19/14—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for weighing suspended loads
    • G01G19/18—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for weighing suspended loads having electrical weight-sensitive devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G19/00—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups
    • G01G19/14—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for weighing suspended loads
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G21/00—Details of weighing apparatus
    • G01G21/14—Beams
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G3/00—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances
    • G01G3/12—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances wherein the weighing element is in the form of a solid body stressed by pressure or tension during weighing
    • G01G3/14—Weighing apparatus characterised by the use of elastically-deformable members, e.g. spring balances wherein the weighing element is in the form of a solid body stressed by pressure or tension during weighing measuring variations of electrical resistance
    • G01G3/1402—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports
    • G01G3/1404—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports combined with means to connect the strain gauges on electrical bridges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
    • G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
    • G01L1/225—Measuring circuits therefor
    • G01L1/2262—Measuring circuits therefor involving simple electrical bridges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
    • G01L5/0061—Force sensors associated with industrial machines or actuators
    • G01L5/0076—Force sensors associated with manufacturing machines
    • G01L5/009—Force sensors associated with material gripping devices

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Measurement Of Force In General (AREA)

Abstract

Load cell having a first beam with a central axis wherein a torsional force is applied about a mechanical neutral axis which affects both sides of the first beam. At least one pair of strain gauges 1, 2, 3, 4 being positioned at each side of the central axis to detect flexure in the first beam. At least one of the strain gauges is connected in parallel to a resistor 16a, 16b, 16c, 16d of known resistance; The outputs of the strain gauges and electrical resistor forming an at least partial Wheatstone bridge arrangement. The strain gauges may be located on a printed circuit board (PCB). The load sensor may comprise a second beam. The second beam may have a strain gauge on to detect flexure on application of a load to the load cell. The strain gauge on the second beam may be configured to detect either tensile or compression forces on the second beam. The second beam may comprise multiple pairs of strain gauges which may also be connected in parallel with resistors.

Description

LOAD CELL
The present invention relates to a load cell in particular, but not exclusively, to a load cell incorporating strain gauges for weighing loads having a non-vertical force component such as in the case of a suspended
load, for example.
Most strain gauges in accurate applications use a Wheatstone bridge to measure an output voltage proportional to an applied load. Thus, it is known to connect a number of strain gauges together to form a Wheatstone bridge arrangement in load cells to which a load may be applied. The output voltage from the Wheatstone bridge is then measurable and proportional to the applied load. In such applications, the strain gauges may be bonded to a surface of the load cell, using specialised adhesive, so that the strain induced when a load is applied may be transmitted into the strain gauges.
In known configurations using four strain gauges bonded to a load cell, a pair of strain gauges is located such that they will be subjected to compression stress when a load is applied to the load cell, thereby reducing the resistance in the pair of strain gauges, and a further pair of strain gauges is located such that they will be subjected to tensile stress when a load is applied to the load cell, thereby increasing the resistance in the pair of strain gauges.
In a load cell arrangement, the aim is to resolve vertical forces and to measure the output voltage of the Wheatstone bridge formed from the outputs of the four strain gauges to measure the load applied to the load cell. In applications wherein the load is applied eccentrically to the load cell, for example in applications involving a skip attached by chains to one or more load pins wherein the chains do not hang vertically from the load pin(s), forces other than the vertical forces resulting from the skip are transmitted to the strain gauges and, therefore, the resulting measured load does not accurately reflect the actual weight of the skip.
Using a load cell having four strain gauges arranged to form a Wheatstone bridge when the load is eccentrically applied to the load cell has been found to be insufficient to accurately resolve all of the forces such that the actual weight of a load is accurately measured.
Furthermore, during manufacture of the load cell, the strain gauges are usually bonded to the centre line of a beam of the load cell. The load cell may include one or more beams which are the cross members of the load cell in which flexure due to an applied load is measured by the strain gauge(s). By positioning the strain gauges on the centre line of a beam, when torsion is applied to the beam, most of the torsional force is cancelled out by affecting the two halves of a gauge, i.e. the portions of the gauge on either side of the centre line, in equal and opposing ways. In this case, the mechanical neutral axis of the beam, i.e. the axis of the beam where the applied torsional force affects both sides of the beam in equal and opposing ways, is also the measurement axis, i.e. the axis upon which the output voltage of the Wheatstone bridge is measured. Thus, when the measurement axis and the mechanical neutral axis are located on the same axis of the load cell, there is no net effect on the output voltage of the Wheatstone bridge when torsional force is applied to the load cell. At this point, the output voltage from the Wheatstone bridge is directly and accurately attributable to the applied load.
Problems occur when the strain gauges are not perfectly and symmetrically positioned relative to the centre line of the beam. It will be understood that such an error in the position of the strain gauges during manufacture of a load cell will lead to any torsional forces applied to the beam being unequally applied to the strain gauges and, therefore, the compensation of the output of one gauge in a pair of strain gauges to the output of the other in that pair of strain gauges will not be complete. In other words, the mechanical neutral axis is no longer co-aligned with the measurement axis. In production of a load cell utilising strain gauges in this way, such manufacturing errors are unavoidable and so require other means of compensating for the error in output from the Wheatstone bridge.
In current manufacturing processes, the mechanical neutral axis of a mis-aligned load cell is re-aligned and, therefore, moved by removing material from the load cell itself, by filing metal away the material of the load cell, for example. This re-alignment is required to re-position the mechanical neutral axis of the load cell to co-align with the measurement axis.
Without such re-alignment, an error in the electrical output as measured on the Wheatstone bridge would be observed when torsional forces were applied to the load cell. The error would provide inaccurate load measurement in this type of load cell. However, when the mechanical neutral axis of a load cell using strain gauges is co-aligned with the measurement axis, the output voltage from the Wheatstone bridge is directly proportional and attributable to the applied load.
It is an object of the present invention to overcome at least one of the drawbacks of the known systems.
In a first aspect, the present invention provides a load cell comprising a first beam having a central axis (hereinafter "centre line") and further having a mechanical neutral axis wherein a torsional force applied about the mechanical neutral axis affects both sides of the beam in equal and opposing ways, and at least a first pair of strain gauges located and configured to detect flexure in the first beam upon application of a load, the first pair of strain gauges being positioned one at each side of the centre line of the beam, wherein at least one strain gauge is electrically connected in parallel to an electrical resistor of known resistance, the outputs from the pair of strain gauges and the electrical resistor forming an at least partial Wheatstone bridge arrangement.
A pair of strain gauges, when referred to herein, means a pair of compression gauges or a pair of tension gauges. A pair of gauges is located at substantially the same position along the longitudinal axis of the first beam and, therefore, will measure substantially the same bending moments, i.e. tension or compression in the beam.
Thus, in load cells wherein the mechanical neutral axis differs from the measurement axis formed of the pair of strain gauges, the output of one of the arms of the Wheatstone bridge may be altered by the presence of the electrical resistor in parallel with the strain gauge in the same arm. In this way it becomes possible to move the measurement axis towards the mechanical neutral axis.
In preferred embodiments of the invention, a second pair of strain gauges are located and configured to detect flexure in the first beam upon application of a load. More specifically, the second pair of strain gauges are positioned one at each side of the centre line of the beam, the outputs from the first and the second pair of strain gauges forming at least a partial Wheatstone bridge arrangement.
In preferred embodiments of the invention, the first pair of strain gauges are located and configured to detect tensile forces in the first beam upon application of a load to the load cell.
In various embodiments, the second pair of strain gauges are located and configured to detect compression forces in the first beam upon application of a load to the load cell.
In embodiments of the invention wherein the load cell comprises a first and a second pair of strain gauges, at least one strain gauge in each pair is electrically connected in parallel to an electrical resistor of known resistance. More specifically, at least one tension strain gauge and at least one compression strain gauge are electrically connected in parallel to an electrical resistor of known resistance. Even more specifically, at least one tension strain gauge and at least one compression strain gauge, which tension and compression strain gauges are preferably located at a same side of the centre line of the first beam, are electrically connected in parallel to an electrical resistor of known resistance.
In various embodiments, each strain gauge in a pair of strain gauges is electrically connected in parallel to an electrical resistor of known resistance.
In this way, in load cells wherein the mechanical neutral axis differs from the measurement axis formed of the a first pair and a second pair of strain gauges, the output of two of the arms of the Wheatstone bridge may be altered by the presence of at least one electrical resistor in parallel with a strain gauge in the same arm of the bridge.
More specifically, one strain gauge of each pair of strain gauges is connected in parallel with an electrical resistor of known resistance.
Even more specifically, the one strain gauge of each pair of strain gauges connected in parallel with an electrical resistor of known resistance are preferably located on the same side of the centre line of the beam to which they are attached. Thus, by affecting a compression strain gauge and a tension strain gauge on the same side of the centre line of the beam to which the gauges are attached equally, i.e. by connecting them each in parallel with electrical resistors of known and equal resistance, the Wheatstone bridge is kept in balance.
In this way it becomes possible to move the measurement axis towards the mechanical neutral axis.
In load cells wherein the mechanical neutral axis differs from the measurement axis formed of the a first pair and a second pair of strain gauges, the output of two of the arms of the Wheatstone bridge may be altered by the presence of two electrical resistors in parallel with two strain gauges in the same arm of the bridge. In particular, when the load cell comprises a pair of tension strain gauges and a pair of compression strain gauges, and each strain gauge is electrically connected in parallel with an electrical resistor of known resistance, it becomes possible to move the measurement axis, in either direction. Thus, in these embodiments, an error in the alignment of the measurement axis away from the mechanical neutral axis in either lateral direction may be corrected electrically by the electrical resistors. Furthermore, by affecting a compression strain gauge and a tension strain gauge on the same side of the centre line of the beam to which the gauges are attached equally, i.e. by connecting them each in parallel with electrical resistors of known and equal resistance, the Wheatstone bridge is kept in balance.
In embodiments of the invention, at least one strain gauge of the second pair of strain gauges is electrically connected in parallel to an electrical resistor of known resistance. Thus, the outputs from the second pair of strain gauges and the electrical resistor form part of the Wheatstone bridge arrangement. In particularly preferred embodiments, each of the strain gauges of the second pair of strain gauges are electrically connected in parallel to separate electrical resistors of known resistance.
Thus, the outputs from the second pair of strain gauges and the electrical resistors form part of the Wheatstone bridge arrangement.
The electrical resistor(s) provides a way of adjusting the position of the measurement axis relative to the mechanical neutral axis/of the load cell.
In this way, manufacturing errors may be electrically compensated for.
The electrical resistor is preferably of known resistance.
The electrical resistance of the electrical resistor(s) may be any known resistance.
The electrical resistance of the electrical resistor(s) may be selectable.
Thus, depending upon the discrepancy between the mechanical neutral axis and the measurement axis, a user may select the known electrical resistance required to co-align the mechanical neutral axis and the measurement axis of the load cell.
In this way, the electrical resistance of the resistive element may be selected to adjust the measurement axis of the load cell to co-align same with the mechanical neutral axis of the load cell. In various embodiments, the resistance may be 100 to l000kO. More specifically, the resistance may be in the range of 500 to 1 001cC).
It will be understood by the skilled artisan that connecting an electrical resistor in parallel with a strain gauge will alter the output from the arm of the Wheatstone bridge containing the resistor and the strain gauge. By selecting a small resistance, the output from a particular strain gauge can effectively be reduced to close to zero, thereby pushing the measurement axis almost entirely towards the other of the pair of strain gauges. The resistance may be selected to shunt the measurement axis totally or, partially, towards the other strain gauge in a pair as required to co-align the measurement axis and the mechanical neutral axis.
In certain embodiments, the load cell may further comprise a printed circuit board (PCB) to which the pair(s) of strain gauges are electrically connected. More specifically, the pair(s) of strain gauges may be electrically connected in parallel to the circuit board in such a way that a user may connect the required electrical resistor(s) to the circuit board following testing of the load cell to establish the relative positions of the mechanical neutral axis and the measurement axis.
In certain embodiments of the invention, the load cell further comprises a second beam. More specifically, the second beam is arranged in parallel with and spaced apart from the first beam. Even more specifically, the second beam may be located above the first beam with the centre lines of the first and the second beams being parallel and on the same vertical plane.
The load cell may comprise at least one strain gauge located and configured to detect flexure in the second beam upon application of a load to the load cell. In certain embodiments, the load cell comprises at least one strain gauge located and configured to detect compression forces in the second beam upon application of a load to the load cell. Alternatively or, in addition, the load cell may comprise at least one strain gauge located and configured to detect tensile forces in the second beam upon application of a load to the load cell.
In embodiments of the invention, the at least one strain gauge is positioned at a centre line of the second beam. In these embodiments, the output voltage of the strain gauges of the first and the second beams together form at least a partial Wheatstone bridge arrangement.
In preferred embodiments, the load cell further comprises two struts, each strut being attached to an end of the first and the second beam and being at a fixed angle to each of the first and the second beam.
In embodiments of the invention, the first and second beams and the two struts together form a Roberval mechanism load cell. In particularly preferred embodiments, the fixed end of the Roberval mechanism is provided by a load pin coupled to the beams and struts of the load cell and located therebetween.
In certain embodiments, the load pin may be fixedly coupled to the beams and struts of the load cell and located therebetween. More specifically, the load pin may be fixedly coupled to the beams and struts of the load cell such that relative rotation between the load pin and the beams and struts of the load cell is prevented. In such embodiments, the load cell and load pin assembly are pivotable together as a single unit about a longitudinal axis of the load pin.
In alternative embodiments, the beams and struts of the load cell may be rotatably coupled to the load pin. In this way, the load pin may be fixed whilst itself permitting rotation of the beams and struts relative thereto. In such embodiments, the load cell is pivotable about a longitudinal axis of the load pin.
In embodiments of the invention, the load cell may comprise multiple pairs of strain gauges. More specifically, the first beam of the load cell may comprise multiple pairs of strain gauges wherein the strain gauges of each pair are located at each side of the centre line of the first beam. In certain embodiments, one or both of the strain gauges in a pair may be connected in parallel with an electrical resistor of known resistance.
Alternatively, or in addition, the second beam of the load cell may comprise multiple pairs of strain gauges wherein the strain gauges of each pair are located at each side of the centre line of the second beam. In certain embodiments, one or both of the strain gauges in a pair may be connected in parallel with an electrical resistor of known resistance.
By using multiple pairs of strain gauges connected in parallel with an electrical resistor of known resistance, any positioning error in the strain gauges and, therefore, the measurement axis relative to the centre line of the beam and/or the mechanical neutral axis during the manufacture of the load cell may be reduced/compensated for by applying a resistance in parallel with any one of strain gauges of the load cell.
In preferred embodiments, a strain gauge in any pair of strain gauges located and configured to detect compression forces in the beam to which it is attached, has a corresponding strain gauge in a further pair of strain gauges located and configured to detect tensile forces in the beam to which it is attached, and both corresponding strain gauges are located at the same side of the centre line of the beam to which the gauges are attached. In these embodiments, it is preferred that both corresponding strain gauges located at the same side of the centre line of the beam to which the gauges are attached have an electrical resistor of known electrical resistance connected in parallel therewith. In this way, the Wheatstone bridge comprising the gauges is maintained in balance.
In embodiments of the invention, the first beam has at least one strain gauge than the second beam. More specifically, the first beam may have at least one more strain gauge located and configured to detect compression forces in the first beam and/or at least one more strain gauge located and configured to detect tensile forces in the first beam.
In preferred embodiments, when the second beam has one strain gauge located and configured to detect compression forces therein and one strain gauge located and configured to detect tensile forces therein, the first beam has a pair of strain gauges located and configured to detect compression forces therein and a pair of strain gauges located and configured to detect tensile forces therein. In these embodiments, it is much preferred that the strain gauges on the second beam are located on the centre line of that beam and that one of the strain gauges in each the pair of strain gauges of the first beam is located at one side of the centre line of the first beam and the other of the strain gauges in each the pair of strain gauges of the first beam is located at the other side of the centre line of the first beam.
The load cell may comprise multiple pairs of strain gauges wherein the first beam has two or more pairs of strain gauges. Preferably each gauge of a pair of strain gauges is located at a side of the centre line of the first beam and is operable to detect forces applied to the beam. Preferably the pair(s) of strain gauges is located at or close to the centre line of the second beam.
In a second aspect of the invention, the load cell comprises a first beam and a second beam, the beams being parallel and spaced apart from one another, at least two struts, each strut being attached to an end of the first and the second beams and each being at a fixed angle relative to the first and the second beams, at least one strain gauge located and configured to detect flexure in at least the first beam, and a load pin coupled to the first and the second beams and located therebetween, the load pin having a longitudinal axis about which the first and the second beams are pivotable, wherein the at least one of the strain gauge is connected in parallel with an electrical resistor of known electrical resistance which electrical resistor is located and configured to adjust the measurement axis of the strain gauge of the load cell.
In embodiments of the second aspect of the invention, the load cell may comprise at least one strain gauge located and configured to detect compression forces in the first beam and at least one strain gauge located and configured to detect tensile forces in the first beam.
In preferred embodiments, the load cell may comprise at least one pair of strain gauges located and configured to detect compression forces in the first beam and at least one pair of strain gauges located and configured to detect tensile forces in the first beam.
The compression strain gauge and/or the tensile strain gauge(s) may each be connected in parallel with an electrical resistor of known electrical resistance which electrical resistor is located and configured to adjust the measurement axis of the strain gauge(s) of the load cell.
Preferably the at least one electrical resistor of known electrical resistance is located on the first beam.
In embodiments of the invention, each strain gauge in each pair of strain gauges may be connected in parallel with an electrical resistor of known electrical resistance. In this way, an inequality in forces applied to the strain gauges causing a shift in the measurement axis of the load cell away from the mechanical neutral axis of the cell may be compensated for simply by providing a selected electrical resistance in one or more of the electrical resistors of known electrical resistance. Thus, a fining tuning of the measurement axis of the output voltages of the strain gauges and, therefore, the load cell is provided.
The features of one or more of the embodiments of the first aspect of the present invention are also applicable to one or more embodiments of the second aspect of the present invention and vice versa.
According to various embodiments of the second aspect of the present invention, the fixed angle may be equal to 90 degrees. More specifically, each strut may be attached to an end of the first and the second beams and may be substantially perpendicular to both the first and the second beams.
In embodiments of the invention, the longitudinal axis of the load pin is perpendicular to the longitudinal axes of the first and the second beams.
In this way, the beams may rotate about the longitudinal axis of the load pin in a balanced way. In alternative embodiments wherein the load pin is fixedly connected to the beams, the entire load cell is able to rotate about the longitudinal axis of the load pin in a balanced way.
In preferred embodiments, the load pin is fixedly coupled to the first and the second beams and located therebetween such that relative rotation between the load pin and the first and the second beams is prevented.
In preferred embodiments the load pin, struts and beams are rigidly connected to one another.
More specifically, in various embodiments the load pin, struts and beams assembly is pivotable about the longitudinal axis of the load pin.
In certain embodiments, the struts and beams may be integrally formed as a single unit.
In embodiments of the invention, a load pin receiving means is provided between the two beams and the two struts. More specifically, the load pin receiving means is operable to couple the load pin and the beams and struts.
In certain embodiments the coupling is a fixable coupling. More specifically, the coupling provides a fixed rigid connection between the load pin and the beams and struts. Even more specifically the load pin and the beams and struts, once coupled together, are immoveable relative to one another.
In alternative embodiments, the coupling is a rotatable coupling. More specifically, the coupling between the load pin and the beams and struts is operable to allow the beams and struts to rotate relative to the load pin.
Such an arrangement is preferred when the load pin is a fixed load pin which itself does not rotate.
In various embodiments, a load pin receiving means is an aperture formed in the single unit between the two beams and the two struts.
In embodiments of the invention, the load pin is locatable into the aperture for rigid connection therein. More specifically, the load pin and the load pin receiving aperture may be complimentary in shape so as to be releasably coupleable in the initial instance whilst being capable of being rigidly fixed thereafter.
It is much preferred that the complimentary shapes of the load pin and the load pin receiving aperture are such that the load pin is incapable of rotational movement within the aperture. The aperture may, in certain embodiments, be a circular aperture having at least one flattened portion.
The load pin may further comprise a corresponding flattened portion on its circumference.
In preferred embodiments the load pin, struts and beams assembly (hereinafter "transducer-load pin assembly") is pivotable about the longitudinal axis of the load pin. In this way, the load pin is capable of pivoting about its longitudinal axis and the beams and struts, due to their being fixed to the load pin, will pivot together with the load pin.
In preferred embodiments, the longitudinal axis of the load pin is perpendicular to the vertical forces of an applied load. The load pin is preferably rotatable about its longitudinal axis to maintain the longitudinal axis perpendicular to the vertical forces of an applied load in use of the load cell.
The load cell may comprise a split strain gauge in which two strain gauges on a single adhesive backing are applied, as a pair of strain gauges at either side of the centre line of a cross member. In such embodiments, the pair is located to detect either compression or tension in the cross member to which they are attached. Further, the split gauge will from at least part of a single arm in the Wheatstone bridge incorporating the split gauge.
The load cell is preferably a Roberval balance load cell. More specifically, the Roberval balance load cell pivots about a pivot point located between the first and the second beams of the Roberval arrangement.
The Roberval balance load cell is preferably vertically orientated. More specifically, the Roberval balance load cell is configured such that a load may be suspended therefrom.
The load cell may further comprise a load attachment means. Even more specifically, the load cell has a load attachment means below the coupling of the transducer-load pin assembly.
In this way, a load may be suspended to hang below the Roberval load cell and to swing freely until it hangs at its balance point below the load cell.
The load attachment means may be any suitable means for attaching a load to the load cell. In certain embodiments, the load attachment means is a bore through the body of the transducer-load pin assembly proximate a bottom edge thereof. Alternatively, the load attachment means may be a ring, loop, hook, or the like attached to the transducer-load pin assembly.
In preferred embodiments, the load attachment means comprises a pair of holes through the body of the transducer-load pin assembly proximate a bottom edge thereof with each hole being arranged to be coupled to a chain linkage. The chain linkage is preferably the uppermost linkage in a chain having a distal end to which a load may be attached.
The coupling point of the transducer-load pin assembly is located at, or adjacent to, the axis upon which the centre of gravity of the transducer-load pin assembly is located. More specifically, the coupling point is located centrally on, or adjacent to, the longitudinal axis of the transducer-load pin assembly and remote from the edges thereof. The longitudinal axis of the transducer-load pin assembly is perpendicular to the longitudinal axis of the load pin.
By rigidly fixing together the transducer-load pin assembly and allowing the transducer-load pin assembly to pivot as one unit, cabling to and from the strain gauges is simplified. More specifically, the cabling to and from the strain gauges on the load cell may be channelled through the centre of the load pin. In this way, the cabling is protected against wear and damage from collision for example in use of the load cell.
The load cell may further comprise a tilt sensor.
The tilt sensor may be located anywhere where a pitch and/or roll angle is detectable. More specifically, the tilt sensor may be attached to or, alternatively, may be built into the load cell. Alternatively, the tilt sensor may be attached to or, may be built into, the load pin.
Alternatively, or in addition, the load cell may be operably attached to a tilt sensor. More specifically, the tilt sensor may be position on a skip truck dashboard, a skip truck arm or any other suitable position in which it is able to detect the pitch angle of the vehicle/apparatus to which the load cell is attached in use.
The tilt sensor is operable to provide information about the angle of the load cell relative to its datum, which is set with its base being horizontal.
In embodiments of the invention, the load cell will react to only the vertical component of the load.
In exemplary embodiments, when the load cell is attached to a skip truck and the skip truck is tilted, for example by being parked on a slope, the load cell will be tilted through the same angle as an axis passing through the width of the truck. Tilting side to side of the truck is referred to as "roll".
Tilting front to back is referred to as "pitch". The pitch tilt along the length of the truck will be, at least to some extent, self compensating as the load pin and/or the beams will rotate about the longitudinal axis of the load pin to keep the centre of gravity of the load under the centre of the pivot.
It will be understood that friction in the pivot and none symmetric loads will produce errors in the measurement of the applied load.
For small angles of pitch, the magnitude of the error approximates to: Error = weight multiplied by (1-Cosine pitch angle) This will be combined with errors in the other axis to give a combined error for pitch and roll approximates to: Error= weight ((1-Cosine pitch angle)-'-(l-Cosine roll angle)) In embodiments of the invention, the error correction is automatically carried out by a software programme that operates in electronic equipment operably attached to the load cell.
The direct attachment of the load to the load cell removes the necessity for additional couplings for attaching the load to the load cell. This offers the advantage of a reduction in the height of the load cell. This gives a simplified assembly which maintains high accuracy in weight measurement.
It will be understood that the load cell of the present invention has various applications and uses. An exemplary use is in a skip truck wherein the skip is suspended from two load pins, one at each side of a skip truck, by four chains suspended two at each side of the skip. A pair of chains on each side of the skip may be suspended from a single load pin. The load pin(s) is/are then mounted on the truck arm. A load cell according to the present invention may be provided at one or, alternatively at both, sides of the skip truck.
In an alternative application, a load cell according to the invention may be used in a crane weigher. In such applications, the load cell may itself be suspended in use.
Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
There now follows a description of a preferred embodiment(s) of the invention, by way of non-limiting example, with reference being made to the accompanying drawings, in which: Figure 1 shows an arrangement of four stain gauges located on a first beam of a load cell according to the first aspect of the invention; Figure 2 shows a pair of strain gauges located on a second beam of a load cell according to the first aspect of the invention; Figure 3 shows the first beam of the load cell of Figure 1 with electrical resistors in parallel with the strain gauges; Figure 4 shows a Wheatstone bridge formed of the strain gauges and resistors of Figures 2 and 3; Figure 5 shows a load cell according to an embodiment of the second aspect of the invention; and Figure 6 shows an isometric view of the load cell of Figure 5.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Further, although the invention will be described in terms of specific embodiments, it will be understood that various elements of the specific embodiments of the invention will be applicable to all embodiments disclosed herein.
In the drawings, similar features are denoted by the same reference signs throughout.
Figure 1 depicts a first beam 10 of a load cell according to the invention.
The first beam 10 has a centre line 12 about which a torsional force (A) can be applied. A first pair of strain gauges 1, 2 is adhered to the first beam and is positioned on the beam to detect a tensile force applied to the beam when a load is applied to the load cell. A second pair of strain gauges 3, 4 is adhered to the first beam and is positioned on the beam to detect a compression force applied to the beam when a load is applied to the load cell.
The tension gauges 1 and 2 are positioned on each side of the centre line 12 of the first beam 10 and are electrically connected in series with one another to form an arm of a Wheatstone bridge which will measure the output voltages from the strain gauges.
The compression gauges 3 and 4 are also positioned on each side of the centre line 12 of the first beam 10 and are electrically connected in series with one another to form a further arm of the Wheatstone bridge which will measure the output voltages from the strain gauges.
Figure 2 depicts a second beam 20 of a load cell according to the invention. The second beam will be arranged in the load cell to be above and parallel to the first beam such that the centre line 14 of the second beam is parallel to an in the same plane as the centre line 12 of the first beam.
A first strain gauge 5 is adhered to the second beam 20 and is positioned to detect compression in the load cell when load is applied to the load cell.
A second strain gauge 6 is adhered to the second beam 20 and is positioned to detect tension in the load cell when load is applied to the load cell. The first and the second strain gauges 5, 6 are thus a compression gauge and a tension gauge respectively. In the depicted embodiment, the compression gauge Sand the tension gauge 6 of the second beam are located on the centre line 14 of the second beam. In this way, when a torsional force A is applied to the second beam, the torsional force will affect each half of the gauges 5, 6 in equal and opposing ways.
Figure 3 shows the first beam of Figure 1 with a printed circuit board (PCB) 18 adhered to the beam. Each strain gauge (1, 2, 3, 4) has an electrical resistor of 1 5kC) resistance connected in parallel with it. Thus, tension gauge 1 is connected in parallel with resistor 16a, tension gauge 2 is connected in parallel with resistor 16b, compression gauge 3 is connected in parallel with resistor I 6c and compression gauge 4 is connected in parallel with resistor 16d. The PCB 18 may be pads to which an electrical resistor of suitable resistance can be connected, usually by soldering, in use of the load cell. In this way, the resistance of each and all of the electrical resistors 1 6a, 1 6b, 1 6c and 1 6d are selectable and interchangeable by a user.
Figure 4 depicts the Wheatstone bridge formed of the outputs of the strain gauges and resistors of Figure 3. In the load cell, should the mechanical neutral axis differ from the measurement axis formed of the strain gauges 1, 2, 3, 4, 5 and 6, the output of two of the arms of the Wheatstone bridge formed by the strain gauges 1, 2, 3 and 4 may be adjusted by selecting and connecting the electrical resistors 16a, 16b, 16c and 16d in parallel with each of the strain gauges 1 to 4. In the depicted embodiment, it is possible to move the measurement axis in either direction laterally of the centre line by selecting appropriate resistors.
It is preferred that resistors 1 6a and 1 6d and also 1 6b and 1 6c are treated as pairs of resistors. Thus, if resistor 1 6a is 1 5k0 it is preferred that resistor 1 6d is also a 1 5kC) resistor. Likewise, if resistor 1 6b is 1 5kC) it is preferred that resistor 16c is also a l5kC) resistor. In this way, the strain gauges at each side of the centre line of the beam to which they are adhered provides a balanced output to the Wheatstone bridge. This is because a tension gauge and a compression gauge at one side of the centre line will each be in parallel with the same resistance.
In use of a load cell having outputs forming the Wheatstone bridge of Figure 4, applying a small resistance, for example 1 5k0, in the form of resistors 16a and 16d will move the measurement axis towards strain gauges 2 and 3 on the other side of the centre line of the beam.
Conversely, applying a small resistance, for example 1 5k0, in the form of resistors 1 6b and 1 6c will move the measurement axis towards strain gauges 1 and 4 on the other side of the centre line of the beam. It will be understood that the load cell may comprise either resistors 1 6a and 1 6d and/or resistors 1 6b and 1 6c. It will be further understood that if the resistors 16a, 16b, 16c and 16d are all present in the load cell and are all connected to their respective strain gauges, then the measurement axis on the beam will not change.
Referring to Figure 5, a load cell according to an embodiment of the second aspect of the invention is shown. The load cell 30 comprises the first beam 10 (as shown in Figure 1) and the second beam 20 (as shown in Figure 2). The load cell 30 further has a load pin 52 passing through the centre of the Roberval mechanism comprising the first and the second beams 10, 20 and the struts 40, 42. The load pin 52 is rigidly fixed to the single transducer unit 58. The load pin 52 and the single transducer unit 58 in combination are free to rotate about the longitudinal axis of the load pin 52. In this way, the Roberval mechanism is mounted on a rotating load pin 52 located in the centre of the Roberval mechanism which allows the load cell 30 to weigh suspended loads with accuracy. Thus, the load cell 30 is particularly useful for weighing a skip suspended on chains on a skip truck. The strain gauges 1, 2, 3, 4, 5 and 6 are bonded to the single transducer unit 58. As is depicted in Figure 4, the strain gauges 1, 2, 3, 4, and 6 together with the electrical resistors (not shown) are wired up into a Wheatstone bridge in a conventional manner. A skip (not shown) may then be suspended from the apertures 34a and 34b on two chains 60.
The application of weight to the load cell 30 will produce a voltage on the Wheatstone bridge which is proportional to the weight of the skip.
In order to improve the accuracy of weighing the skip, the first beam 10 of the load cell 30 has each of tension gauges 1, 2 and/or compression gauges 3, 4 electrically connected in parallel to an electrical resistor (as shown in Figure 3). By selecting the appropriate resistance of the resistors in parallel with one or more of the strain gauges, the measurement axis of the load cell 30 can be adjusted to co-align with the mechanical neutral axis. In this way, any non-vertical forces applied to the load cell 30 through chains 60 can be a compensated for electrically by the load cell 30 thus removing the necessity for mechanical alteration to the load cell.
Figure 6 shows an isometric view of the load cell 30 of Figure 5. The first links of chains 60 are shown depending from apertures 34a and 34b. The wiring from the strain gauges exits the load cell 30 through a centre hole in the load pin 52 at the point marked 62. In known apparatus requiring relative motion between the load pin and the transducer, the relative motion makes wiring to the strain gauges more difficult. In the present invention, by passing the wires through the centre of the load pin and down the skip arm (not shown), wiring can be protected. This protection of the wires is vital on a skip truck as trees and other objects are often in collision with the vehicle and will damage any exposed wiring.
The Roberval mechanism further allows the load cell to be directly connected to the two chains 60 which support the load to be applied.

Claims (15)

  1. CLAIMS1. A load cell comprising a first beam having a central axis (hereinafter "centre line") and further having a mechanical neutral axis wherein a torsional force applied about the mechanical neutral axis affects both sides of the beam in equal and opposing ways, and at least a first pair of strain gauges located and configured to detect flexure in the first beam upon application of a load, the first pair of strain gauges being positioned one at each side of the centre line of the beam, wherein at least one strain gauge is electrically connected in parallel to an electrical resistor of known resistance, the outputs from the pair of strain gauges and the electrical resistor forming an at least partial Wheatstone bridge arrangement.
  2. 2. A load cell according to claim 1, wherein a second pair of strain gauges are located and configured to detect flexure in the first beam upon application of a load.
  3. 3. A load cell according to claim 2, wherein the second pair of strain gauges are positioned one at each side of the centre line of the beam, the outputs from the first and the second pair of strain gauges forming at least a partial Wheatstone bridge arrangement.
  4. 4. A load cell according to claim 2 or claim 3, wherein at least one strain gauge in each pair is electrically connected in parallel to an electrical resistor of known resistance.
  5. 5. A load cell according to any one of the preceding claims, further comprising a printed circuit board (PCB) to which the at least one strain gauge is electrically connected.
  6. 6. A load cell according to any one of the preceding claims, further comprising a second beam.
  7. 7. A load cell according to claim 6, comprising at least one strain gauge located and configured to detect flexure in the second beam upon application of a load to the load cell.
  8. 8. A load cell according to claim 6 or claim 7, comprising at least one strain gauge located and configured to detect compression forces in the second beam upon application of a load to the load cell.
  9. 9. A load cell according to any one of claims 6 to 8, comprising at least one strain gauge located and configured to detect tensile forces in the second beam upon application of a load to the load cell.
  10. 10. A load cell according to any one of claims 7 to 9, wherein the at least one strain gauge is positioned at a centre line of the second beam.
  11. 11. A load cell according to any one of the preceding claims, wherein the first beam of the load cell comprises multiple pairs of strain gauges wherein the strain gauges of each pair are located at each side of the centre line of the first beam.
  12. 12. A load cell according to claim 11, wherein one or both of the strain gauges in a pair may be connected in parallel with an electrical resistor of known resistance.
  13. 13. A load cell according to any one of claims 6 to 12, wherein the second beam of the load cell comprises multiple pairs of strain gauges wherein the strain gauges of each pair are located at each side of the centre line of the second beam.
  14. 14. A load cell according to claim 13, wherein one or both of the strain gauges in a pair are connected in parallel with an electrical resistor of known resistance.
  15. 15. A load cell substantially as herein described with reference to the accompanying drawings.
GB0919396A 2009-11-05 2009-11-05 A load cell having strain gauges to detect flexure in a beam using parallel resistors. Withdrawn GB2475081A (en)

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CN103787265B (en) * 2012-10-26 2017-04-12 罗伯特·博世有限公司 Mechanical component and manufacturing method for a mechanical component
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