WO2009027951A1 - A load measuring device - Google Patents

A load measuring device Download PDF

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Publication number
WO2009027951A1
WO2009027951A1 PCT/IB2008/053518 IB2008053518W WO2009027951A1 WO 2009027951 A1 WO2009027951 A1 WO 2009027951A1 IB 2008053518 W IB2008053518 W IB 2008053518W WO 2009027951 A1 WO2009027951 A1 WO 2009027951A1
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WO
WIPO (PCT)
Prior art keywords
measuring device
load measuring
sensor
load
sensing points
Prior art date
Application number
PCT/IB2008/053518
Other languages
French (fr)
Inventor
Fredrik Francois Pieterse
Peter Mark Bidgood
Original Assignee
University Of Johannesburg
Csir
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by University Of Johannesburg, Csir filed Critical University Of Johannesburg
Publication of WO2009027951A1 publication Critical patent/WO2009027951A1/en

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Classifications

    • 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
    • G01L1/2206—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports
    • 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
    • G01G3/125—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 wherein the weighing element is an optical member
    • 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
    • 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
    • G01L1/2287—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 constructional details of the strain gauges
    • 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
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • 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
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
    • G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings

Definitions

  • This invention relates to a load measuring device and more particularly, to a load measuring device that measures relative displacement of two points on a load measuring element thereof.
  • Load cell or load balance design requires the expansion of the load envelope or alternatively the sensitivity of load measurement devices.
  • These devices include particularly, but are not limited to, internal six component strain gauge balances which are used in determining the loads acting on a structure under wind loading as are typically found in wind tunnel testing environments. These types of devices require a high level of stiffness, strength, accuracy as well as high sensitivity.
  • a wind tunnel balance is a complex apparatus that measures the aerodynamic forces and moments which act on a scale model during a wind tunnel test.
  • Most of the conventional internal wind tunnel balances also called sting balances
  • electrical resistance strain gauge sensors for measuring normal, side and axial forces as well as pitching, yawing and rolling moments.
  • a wind tunnel balance is a multiple axis force and moment transducer that usually, but not always, makes use of a bending beam.
  • the advantage of using a bending beam is that similar compressive and tensile stresses occur simultaneously at opposite sides of the beam, enabling the maximizing of sensitivity of the sensors when connected in a Wheatstone bridge. Compensation for temperature effects is however not always possible as temperature gradients may exist between sensors. The same is true for shear type elements.
  • electromagnetic interference can have a significant negative impact on the quality of output of the sensors.
  • Sensors such as strain gauges are normally glued to the measuring element or beam for measuring compression or expansion of the beam.
  • a length of such a sensor, or the whole sensor (in the case of a strain gauge) is glued to the element so that the gauge deforms with the element. Mechanical amplification becomes difficult if one is restricted by the size or stiffness of the measuring element.
  • a load measuring device comprising a load measuring structure within which are sensing points, the sensing points being movable relative to each other upon application of a force to the structure, and a displacement sensor for measuring relative displacement of the two sensing points.
  • the displacement sensor is distinguishable from a load sensor by virtue of typical displacement sensor characteristics.
  • the most relevant of these characteristics in this application is the ability to measure a displacement with minimum effect on the displacement itself.
  • the displacement sensor is distinguishable from a load sensor in that it has a lower modulus of elasticity underlying device structure.
  • the sensing points prefferably be two opposing points on opposing sides of a cut-out in the measuring element.
  • a further feature of the invention provides for the sensing points to be two points on protrusions from the measuring element.
  • sensing points are on opposite edges of the cut-out in the measuring element.
  • the cut-out may be a slot which is substantially u-shaped in cross-section.
  • the cut-out may be substantially c-shaped in cross-section so that the two sensing points are at the ends of the "c" closer together than the length of a central diameter of the "c".
  • the displacement sensor may be a physically deformable sensor such as a strain gauge or an optical fiber.
  • a physically deformable sensor two sensor attachment points of the sensor are attached to the two sensing points so that a sensor measuring section of the sensor is defined between the two sensor attachment points which section is not attached to the measuring element.
  • the fiber is pre-stressed along its length prior to attachment to the two sensing points, so that the fiber is under elongate tension when the measuring element is in a rest position.
  • a still further feature provides for the measuring element to be a beam or shear plate.
  • the measuring device may form part of a wind tunnel balance.
  • Figure 1 shows a perspective view of a measuring device having a measuring element in the form of a cantilever beam moveable between a rest position as shown by the broken lines, to a loaded position, as shown by the solid lines;
  • Figure 2 shows a perspective side view of a displacement sensor in the form of an optical fiber attached between two sensing points on opposite edges of a cut-out in the measuring element of figure 1 ;
  • Figure 9 shows a perspective view of a second embodiment of the measuring element of figures 1 and 2.
  • a load measuring device is generally indicated by reference numeral 1.
  • the device 1 has a measuring element 2 in the form of a cantilever beam 2.
  • One end of the beam is anchored between two clamps 7 with a free end 14 suspended in mid-air.
  • the beam 2 has two cut-outs 8, one in a top side of the beam and one in a bottom side of the beam.
  • the cut-outs are open-ended elongate slots, u-shaped in cross-section, extending transverse to the beam.
  • the cantilever beam measures 50 x 30 mm in cross section.
  • the cut-outs or slots are 10 mm deep as shown by reference numerals 3, 10 mm wide, as shown by reference numerals 4 and 30 mm long across a top side and a bottom side of the beam.
  • Each slot has an operatively vertical front wall 10, an operatively vertical rear wall 11 and an inner operatively horizontal wall 9.
  • the slots are located closer to a secured end of the beam i.e. the opposite end of the free end 14 of the beam.
  • strain gauges 5 were bonded in conventional manner to the inner walls 9 of the slots and wired to a full Wheatstone bridge. The strain gauges were used during the tests and do not form part of the invention insofar as their attachment and position of attachment is concerned.
  • the slots provide mechanical magnification. If the dimension of the sides of the slots 10 an 11 increase, so will the mechanical amplification and thus the relative displacement between two opposing sensing points on the top edges of the slots.
  • Optical fibers 6, under tension, are glued across the slots so that they are coaxial with the beam.
  • the fibers are glued to a longitudinal axis of top and bottom surfaces of the beam. These surfaces are indicated as 12 and 13 on top and underneath the beam respectively.
  • Surfaces 12 are operatively top surfaces of the beam between the secured end of the beam and each slot.
  • Surfaces 13 extend on operatively top and bottom surfaces of the beam between the free end and the two slots.
  • the fibers 6 are under elongate tension when the beam is at rest so as to enable them to compress, shorten when the sensing points move closer together.
  • the optical fibers extends centrally and longitudinally along these surfaces (12 and 13) and are glued to these surfaces so that a measuring section of each fiber, the section that spans across each slot and between sensor attachment points as described below, is under tension.
  • the optical fibers are thus suspended over the slots, under tension.
  • the sensor attachment points 14 on each fiber are attached to two opposing sensing points 15 on the top edges of the slots.
  • the measuring section of each fiber which is not attached to the measuring element (beam) as described above, includes a Bragg grating.
  • the sensor will show relative displacements of the sensing points on the beam when under load.
  • the embodiment described herein is implemented and tested on a cantilever beam and compared with electrical resistance strain gauges 5.
  • the strain gauges being bonded to the balance material hence measuring the strain of the balance material while sensors spun across a measurement zone will measure displacement which results in strain of the sensor being independent of the strain in the balance material and which can be enhanced by reducing the depth of the slots without reducing the displacement.
  • the cantilever beam will subjected to primary normal loads, secondary side loads, and variable temperature conditions.
  • An optic fiber Bragg grating is one type of sensor used for displacement sensing, because of its sensitivity, stability and EMI immunity.
  • Bragg gratings are made by using laser light to place a grating with a particular wavelength on to the doped core of an optical fiber. This changes the refractive index of the core which serves as a reflector of light at a specific wavelength down the fiber.
  • the wavelength at which light is reflected by the Bragg grating changes.
  • the maximum stretch in the fiber is 5% of the stretched length.
  • a maximum of 50000 micro strain deformations can be obtained without permanent damaging the fiber.
  • the tension decreases and thus a decreasing in wavelength of the reflected light is experienced.
  • the beam was loaded to check the wavelength shift. As seen from the graph of figure 4 the top fiber stretches as the load increases thus an increase in wavelength and vice versa for the bottom fiber that compresses, thus a decrease in wavelength.
  • the beam was loaded (pos) up and down to check for hysteresis and linear lines fitted, as is shown in the graph of figure 6.
  • the beam was rotated by 180 degrees and loaded (neg) up and down to check for hysteresis and linear lines fitted, as is shown in the graph of figure 7.
  • the wavelength shift (a to a') was 0.196 nm.
  • the wavelength shift b to b' was 0.199 nm.
  • the difference in wavelength ⁇ (1.383 nm) represents the load of 10.7 kg under stable temp conditions of 21.5 0 C.
  • ⁇ ' (1.386 nm) represents the same load of 10.7 kg under changing temp conditions of 20.88 to 28.72 0 C. Thus an error of 0.003 nm.
  • the slots 8 have opposing protrusions or lips 13 extending from and integral to, upper edges of each slot (from the outer ends of surfaces 10 and 11), towards each other, parallel to an elongate axis of the beam.
  • the lips terminate short of each other to leave an opening of which the optical fibers are spanned under tension as described above.
  • This embodiment will lead to an amplified ⁇ L/L ratio as compared with the slots of the beam of figures 1 and 2, where L equals the distance between opposing lip ends (distance between sensing points) and ⁇ L the change of the distance L when the free end of beam is bent down or up.
  • L equals the distance between opposing lip ends (distance between sensing points)
  • ⁇ L the change of the distance L when the free end of beam is bent down or up.
  • a strain gauge can be used instead of using the optical fiber.
  • a physically deformable sensor such as the optical fiber or strain gauge, a light or other electromagnetic wave distance measuring device can be used instead of using the optical fiber.
  • the sensor in the case of a physically deformable sensor, the sensor is spanned over a gap formed by a slot (for example also the substantially c-shaped slot of figure 9) or could ever be spanned over protrusions such as rod pillars extending away from the beam or measuring element.
  • the sensor would then be spanned between the free ends of the pillars.
  • the sensor should have a lower modulus of elasticity than the measuring element.
  • a light or other electromagnetic wave distance measuring device could be used with the result that it has no influence on the relative movement of the sensing points.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

This invention relates to a load measuring device and more particularly, to a load measuring device that measures relative displacement of two points on a load measuring element thereof. In accordance with this invention there is provided a load measuring device comprising a load measuring structure within which are sensing points, the sensing points being movable relative to each other upon application of a force to the structure, and a displacement sensor for measuring relative displacement of the two sensing points.

Description

A LOAD MEASURING DEVICE
FIELD OF INVENTION
This invention relates to a load measuring device and more particularly, to a load measuring device that measures relative displacement of two points on a load measuring element thereof.
BACKGROUND TO THE INVENTION
Load cell or load balance design requires the expansion of the load envelope or alternatively the sensitivity of load measurement devices. These devices include particularly, but are not limited to, internal six component strain gauge balances which are used in determining the loads acting on a structure under wind loading as are typically found in wind tunnel testing environments. These types of devices require a high level of stiffness, strength, accuracy as well as high sensitivity.
A wind tunnel balance is a complex apparatus that measures the aerodynamic forces and moments which act on a scale model during a wind tunnel test. Most of the conventional internal wind tunnel balances (also called sting balances) use electrical resistance strain gauge sensors for measuring normal, side and axial forces as well as pitching, yawing and rolling moments. A wind tunnel balance is a multiple axis force and moment transducer that usually, but not always, makes use of a bending beam. The advantage of using a bending beam is that similar compressive and tensile stresses occur simultaneously at opposite sides of the beam, enabling the maximizing of sensitivity of the sensors when connected in a Wheatstone bridge. Compensation for temperature effects is however not always possible as temperature gradients may exist between sensors. The same is true for shear type elements.
In the case of electrically based sensors electromagnetic interference (EMI) can have a significant negative impact on the quality of output of the sensors.
Sensors such as strain gauges are normally glued to the measuring element or beam for measuring compression or expansion of the beam. A length of such a sensor, or the whole sensor (in the case of a strain gauge) is glued to the element so that the gauge deforms with the element. Mechanical amplification becomes difficult if one is restricted by the size or stiffness of the measuring element. OBJECT OF THE INVENTION
It is an object of this invention to provide a load measuring device that will, at least partially, alleviate some of the abovementioned difficulties.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a load measuring device comprising a load measuring structure within which are sensing points, the sensing points being movable relative to each other upon application of a force to the structure, and a displacement sensor for measuring relative displacement of the two sensing points.
The displacement sensor is distinguishable from a load sensor by virtue of typical displacement sensor characteristics. The most relevant of these characteristics in this application is the ability to measure a displacement with minimum effect on the displacement itself.
This can be achieved with sensitive elastic displacement sensors, or electromagnetic wave distance measuring device, such as an acoustic wave distance measuring device or a laser light distance measuring device. The displacement sensor is distinguishable from a load sensor in that it has a lower modulus of elasticity underlying device structure.
There is provided for the sensing points to be two opposing points on opposing sides of a cut-out in the measuring element.
A further feature of the invention provides for the sensing points to be two points on protrusions from the measuring element.
There is provided for the sensing points to be on opposite edges of the cut-out in the measuring element.
The cut-out may be a slot which is substantially u-shaped in cross-section.
Alternatively, the cut-out may be substantially c-shaped in cross-section so that the two sensing points are at the ends of the "c" closer together than the length of a central diameter of the "c".
Alternatively, the displacement sensor may be a physically deformable sensor such as a strain gauge or an optical fiber. In the case of a physically deformable sensor two sensor attachment points of the sensor are attached to the two sensing points so that a sensor measuring section of the sensor is defined between the two sensor attachment points which section is not attached to the measuring element. In the case of an optical fiber, the fiber is pre-stressed along its length prior to attachment to the two sensing points, so that the fiber is under elongate tension when the measuring element is in a rest position.
A still further feature provides for the measuring element to be a beam or shear plate.
The measuring device may form part of a wind tunnel balance.
These and other features of the invention are described in more detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention is described below, by way of example only and with reference to the drawings in which:
Figure 1 shows a perspective view of a measuring device having a measuring element in the form of a cantilever beam moveable between a rest position as shown by the broken lines, to a loaded position, as shown by the solid lines;
Figure 2 shows a perspective side view of a displacement sensor in the form of an optical fiber attached between two sensing points on opposite edges of a cut-out in the measuring element of figure 1 ;
Figures 3 to 8 show graphs of experimental results; and
Figure 9 shows a perspective view of a second embodiment of the measuring element of figures 1 and 2.
DETAILED DESCRIPTION OF THE DRAWINGS
With reference to the drawings, a load measuring device is generally indicated by reference numeral 1.
The device 1 has a measuring element 2 in the form of a cantilever beam 2. One end of the beam is anchored between two clamps 7 with a free end 14 suspended in mid-air. The beam 2 has two cut-outs 8, one in a top side of the beam and one in a bottom side of the beam. The cut-outs are open-ended elongate slots, u-shaped in cross-section, extending transverse to the beam. The cantilever beam measures 50 x 30 mm in cross section. The cut-outs or slots are 10 mm deep as shown by reference numerals 3, 10 mm wide, as shown by reference numerals 4 and 30 mm long across a top side and a bottom side of the beam. Each slot has an operatively vertical front wall 10, an operatively vertical rear wall 11 and an inner operatively horizontal wall 9.
The slots are located closer to a secured end of the beam i.e. the opposite end of the free end 14 of the beam.
Four foil strain gauges 5 were bonded in conventional manner to the inner walls 9 of the slots and wired to a full Wheatstone bridge. The strain gauges were used during the tests and do not form part of the invention insofar as their attachment and position of attachment is concerned.
The slots provide mechanical magnification. If the dimension of the sides of the slots 10 an 11 increase, so will the mechanical amplification and thus the relative displacement between two opposing sensing points on the top edges of the slots.
Optical fibers 6, under tension, are glued across the slots so that they are coaxial with the beam. The fibers are glued to a longitudinal axis of top and bottom surfaces of the beam. These surfaces are indicated as 12 and 13 on top and underneath the beam respectively. Surfaces 12 are operatively top surfaces of the beam between the secured end of the beam and each slot. Surfaces 13 extend on operatively top and bottom surfaces of the beam between the free end and the two slots.
The fibers 6 are under elongate tension when the beam is at rest so as to enable them to compress, shorten when the sensing points move closer together.
The optical fibers extends centrally and longitudinally along these surfaces (12 and 13) and are glued to these surfaces so that a measuring section of each fiber, the section that spans across each slot and between sensor attachment points as described below, is under tension. The optical fibers are thus suspended over the slots, under tension. The sensor attachment points 14 on each fiber are attached to two opposing sensing points 15 on the top edges of the slots. The measuring section of each fiber, which is not attached to the measuring element (beam) as described above, includes a Bragg grating.
In this way the sensor will show relative displacements of the sensing points on the beam when under load.
If the sensing points are selected correctly, minimum cross coupling between forces and moments will occur. It is postulated that this method will allow for mechanical and strain magnification which will result in a more sensitive balance with greater stiffness with reduced effects from temperature and EMI.
The embodiment described herein is implemented and tested on a cantilever beam and compared with electrical resistance strain gauges 5. The strain gauges being bonded to the balance material hence measuring the strain of the balance material while sensors spun across a measurement zone will measure displacement which results in strain of the sensor being independent of the strain in the balance material and which can be enhanced by reducing the depth of the slots without reducing the displacement.
The cantilever beam will subjected to primary normal loads, secondary side loads, and variable temperature conditions.
The stresses and strains in the beam was analyzed with finite element software.
An optic fiber Bragg grating is one type of sensor used for displacement sensing, because of its sensitivity, stability and EMI immunity.
Bragg gratings are made by using laser light to place a grating with a particular wavelength on to the doped core of an optical fiber. This changes the refractive index of the core which serves as a reflector of light at a specific wavelength down the fiber.
When the optic fiber is stretched, the wavelength at which light is reflected by the Bragg grating changes.
The maximum stretch in the fiber is 5% of the stretched length. Thus, over a 10 mm groove, a maximum of 50000 micro strain deformations can be obtained without permanent damaging the fiber.
As a normal force is applied to the free end 14 of the beam, the tension on the top optical fiber increases and result in an increase in wavelength of the reflected light.
On the other side, the bottom optical fiber the tension decreases and thus a decreasing in wavelength of the reflected light is experienced.
Thus a differential wave length shift (being double that experienced by each sensor i.e. increased sensitivity) is experienced in the reflected light which is proportional to the applied normal force
Side force and temperature change: As seen from the bottom part of figure 3, both optical fibers experience the same wavelength shift in the same direction, thus the differential shift in the wave length of the reflected light remains unchanged. This indicates the insensitivity of the sensors to side loads or strain changes due to temperature effects (if both fibers are at the same temperature).
Primary data (Cantilever beam)
The beam was loaded to check the wavelength shift. As seen from the graph of figure 4 the top fiber stretches as the load increases thus an increase in wavelength and vice versa for the bottom fiber that compresses, thus a decrease in wavelength.
Primary data (Cantilever beam)
In the graph of figure 5 the foil strain gauge (material strain) is plotted versus the the sensor's strain. As indicated, the material experience a strain of 197.10 micro-strain versus the sensor strain of 1737.14 micro-strain. Thus an increase of 676.57%. resulting from the achieved mechanical amplification Normal force (Pos)
The beam was loaded (pos) up and down to check for hysteresis and linear lines fitted, as is shown in the graph of figure 6.
Normal force (Neq)
The beam was rotated by 180 degrees and loaded (neg) up and down to check for hysteresis and linear lines fitted, as is shown in the graph of figure 7.
Temperature compensation test
As seen from figure 8, the wavelength shift (a to a') was 0.196 nm. And the wavelength shift b to b' was 0.199 nm. Thus an error of 0.003 nm.
The difference in wavelength λ (1.383 nm) represents the load of 10.7 kg under stable temp conditions of 21.5 0C.
Λ' (1.386 nm) represents the same load of 10.7 kg under changing temp conditions of 20.88 to 28.72 0C. Thus an error of 0.003 nm. Strain Amplification
In the embodiment of figure 9, the slots 8 have opposing protrusions or lips 13 extending from and integral to, upper edges of each slot (from the outer ends of surfaces 10 and 11), towards each other, parallel to an elongate axis of the beam. The lips terminate short of each other to leave an opening of which the optical fibers are spanned under tension as described above. This embodiment will lead to an amplified ΔL/L ratio as compared with the slots of the beam of figures 1 and 2, where L equals the distance between opposing lip ends (distance between sensing points) and ΔL the change of the distance L when the free end of beam is bent down or up. Thus, by introducing the protrusions the ratio of the change in length of the measuring section of the sensor to the distance between sensing points (width of the slot) is increased, hence the amplification.
The invention described above will thus provide mechanical as well as strain amplification.
The invention is not limited to the precise details as described herein and those skilled in the art will appreciate that other embodiments are possible without departing from the scope of the invention. For example, instead of using the optical fiber, a strain gauge can be used. Further, instead of using a physically deformable sensor such as the optical fiber or strain gauge, a light or other electromagnetic wave distance measuring device can be used.
The idea is that, in the case of a physically deformable sensor, the sensor is spanned over a gap formed by a slot (for example also the substantially c-shaped slot of figure 9) or could ever be spanned over protrusions such as rod pillars extending away from the beam or measuring element. The sensor would then be spanned between the free ends of the pillars. The sensor should have a lower modulus of elasticity than the measuring element. Ideally, a light or other electromagnetic wave distance measuring device could be used with the result that it has no influence on the relative movement of the sensing points.

Claims

CLAIMS:
1. A load measuring device comprising a load measuring element having two sensing points, the sensing points being movable relative to each other upon application of a force to the sensing element, and a displacement sensor for measuring relative displacement of the two sensing points.
2. A load measuring device as claimed in claim 1 in which the sensing points are two opposing points on opposing sides of a cut-out in the measuring element.
3. A load measuring device as claimed in claim 1 in which the sensing points are two points on protrusions from the measuring element.
4. A load measuring device as claimed in claim 2 in which the sensing points are on opposite edges of the cut-out in the measuring element.
5. A load measuring device as claimed in claim 2 or 4 in which the cut-out is a slot which is substantially u-shaped in cross-section.
6. A load measuring device as claimed in claim 2 or 4 in which the cut-out is substantially c-shaped in cross-section so that the two sensing points are at the ends of the "c" closer together than the length of a central diameter of the "c".
7. A load measuring device as claimed in any one of the preceding claims in which the displacement sensor has a lower modulus of elasticity than the measuring element.
8. A load measuring device as claimed in any one of the preceding claims in which the displacement sensor is an electromagnetic wave distance measuring device.
9. A load measuring device as claimed in claim 8 in which the displacement sensor is acoustic wave distance measuring device.
10. A load measuring device as claimed in claim 8 in which the displacement sensor is a laser light distance measuring device.
11. A load measuring device as claimed in any one of claims 1 to 7 in which the displacement sensor is a physically deformable sensor.
12. A load measuring device as claimed in claim 11 in which the displacement sensor is a strain gauge.
13. A load measuring device as claimed in claim 11 in which the displacement sensor is an optical fiber.
14. A load measuring device as claimed in any one of claims 11 to 13 in which two sensor attachment points of the sensor are attached to the two sensing points so that a sensor measuring section or sensor is defined between the two sensor attachment points which section is not attached to the measuring element.
15. A load measuring device as claimed in claim 13 in which the fiber is pre- stressed along its length prior to attachment to the two sensing points, so that the fiber is under elongate tension when the measuring element is in a rest position.
16. A load measuring device as claimed in any one of the preceding claims in which the measuring element is a beam.
17. A load measuring device as claimed in any one of the preceding claims in which the measuring device forms part of a wind tunnel balance.
PCT/IB2008/053518 2007-08-30 2008-08-29 A load measuring device WO2009027951A1 (en)

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ZA200707382 2007-08-30
ZA2007/07382 2007-08-30

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Cited By (3)

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GB2540430A (en) * 2015-07-17 2017-01-18 Airbus Operations Ltd Calibration of transducers
EP2549254A3 (en) * 2011-06-30 2017-03-15 General Electric Company Method and system for a fiber optic sensor
CN112834163A (en) * 2020-12-31 2021-05-25 中国航天空气动力技术研究院 Multi-component force measuring balance based on photoelectric sensor principle

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US3992933A (en) * 1975-08-21 1976-11-23 Randolph Jr George J Electromechanical moment-independent deflection sensor
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Publication number Priority date Publication date Assignee Title
EP2549254A3 (en) * 2011-06-30 2017-03-15 General Electric Company Method and system for a fiber optic sensor
GB2540430A (en) * 2015-07-17 2017-01-18 Airbus Operations Ltd Calibration of transducers
CN112834163A (en) * 2020-12-31 2021-05-25 中国航天空气动力技术研究院 Multi-component force measuring balance based on photoelectric sensor principle

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