WO2012164538A1 - Apparatus for measuring the stiffness of a sample of material in a plurality of directions laying in a same plane - Google Patents

Apparatus for measuring the stiffness of a sample of material in a plurality of directions laying in a same plane Download PDF

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Publication number
WO2012164538A1
WO2012164538A1 PCT/IB2012/052776 IB2012052776W WO2012164538A1 WO 2012164538 A1 WO2012164538 A1 WO 2012164538A1 IB 2012052776 W IB2012052776 W IB 2012052776W WO 2012164538 A1 WO2012164538 A1 WO 2012164538A1
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Prior art keywords
rotation
deformation
cam
axis
sample
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PCT/IB2012/052776
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French (fr)
Inventor
Lorenzo Masia
Giulio Sandini
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Fondazione Istituto Italiano di Tecnologia
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Fondazione Istituto Italiano di Tecnologia
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/02—Details
    • G01N3/04—Chucks
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/40—Investigating hardness or rebound hardness
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/003—Generation of the force
    • G01N2203/0032—Generation of the force using mechanical means
    • G01N2203/0037—Generation of the force using mechanical means involving a rotating movement, e.g. gearing, cam, eccentric, or centrifuge effects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/0076—Hardness, compressibility or resistance to crushing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/025—Geometry of the test
    • G01N2203/0254—Biaxial, the forces being applied along two normal axes of the specimen

Definitions

  • the present invention refers to an apparatus for measuring the stiffness of a sample of material in a plurality of directions, in particular in a plurality of directions laying in a same plane.
  • the expression "stiffness of a sample of material in a given direction" is to be intended as referring to the scalar ratio of the force generated in static conditions by the material as a result of a deformation produced on the sample of material in that direction to the amount of the same deformation.
  • the stiffness of the sample of material in that direction is the scalar ratio of the force produced by the material as a result of the imposed deformation to the same deformation.
  • reaction force the force which is produced by the material as a result of the imposed deformation
  • the invention is based on the idea of providing an apparatus for measuring the stiffness of a sample of material in a plurality of directions, the apparatus comprising: deformation means arranged to impose cyclically a predetermined deformation on the sample of material along a plurality of directions of deformation,
  • moving means arranged to vary each time the direction of deformation along which the deformation means impose the predetermined deformation on the sample of material, in such a manner that for each deformation cycle the direction of deformation is different from the direction of deformation of the immediately preceding deformation cycle, and force sensor means arranged to detect, during each deformation cycle carried out on the sample of material, a signal indicative of the reaction force produced by the sample of material along the direction of deformation.
  • the apparatus according to the invention By imposing on the sample of material, through the deformation means, a predetermined deformation in a known direction and by measuring, through the force sensor means, the reaction force produced by the sample of material along that direction, the apparatus according to the invention allows to measure the stiffness of the sample of material in that direction. By varying for each measure cycle, through the moving means, the direction of deformation, the apparatus according to the invention allows to change each time the direction of measure of the stiffness of the sample of material.
  • Figure 1 is a perspective view of an apparatus for measuring the stiffness of a sample of material in a plurality of directions laying in a plane, according to a preferred em- bodiment of the present invention
  • Figure 2 is a side elevation view of the apparatus of Figure 1 ;
  • Figure 3 is an axial section view of the apparatus of Figure 1 , taken through the plane indicated III-III in Figure 2;
  • Figure 4 is an exploded view of the apparatus of Figure 1 , from which some components have been removed for the sake of clarity in order to make the moving means forming part of the apparatus more visible;
  • Figure 5 is a perspective view of the apparatus of Figure 1 , which further shows a sample of material on which the stiffness measure is to be performed and also indicates the different directions along which the apparatus is able to perform the stiffness measure;
  • Figure 6 is an exploded view showing the sample of material, the force sensor forming part of the apparatus of Figure 1, as well as the components used to restrain the sample of material;
  • Figure 7 is a section view showing the connection of the sample of material with the force sensor of the apparatus of Figure 1 ;
  • Figure 8 shows an example of profile of the cam used as moving means in the apparatus of Figure 1 ;
  • Figure 9 shows the law of motion imposed by the cam profile of Figure 8.
  • an apparatus for measuring the stiffness of a sample of material S in a plurality of directions laying in a same plane is generally indicated 10.
  • the apparatus 10 is able in particular to measure the stiffness of the sample of material S in a given number N of radial directions (shown in Figure 5) which are perpendicular to a given axis z (in the illustrated embodiment a vertical axis) and are arranged angularly evenly spaced in a plane perpendicular to the axis z, the angular distance between a measure direction and the adjacent measure direction being equal to 360 N (degrees).
  • the apparatus is able to measure the stiffness of the sample of material S along eight directions angularly spaced from each other by an angle equal to 45 degrees.
  • the apparatus according to the invention can be configured so as to measure the stiffness in any other number of directions.
  • the apparatus 10 is provided with moving means comprising a drive shaft 12 arranged with its own axis of rotation coinciding with the axis z and an electric motor (not shown), or a similar actuation device, arranged to control, either directly or through a motion transmission and/or a motion reduction mechanism, the rotation of the drive shaft 12 about the axis z.
  • the drive shaft 12 is supported for rotation (in a manner which is not shown but is however of per-se-known type) at its bottom end by a support structure (also not shown) forming part of the device 10.
  • the electric motor is also preferably supported by the support structure.
  • the apparatus 10 further comprises a cam 14 keyed on the drive shaft 12, preferably at the top end thereof, so as to be drivingly connected for rotation with this latter about the axis z.
  • the cam 14 is made as a disc-shaped element, which extends in a plane perpendicular to the axis z and has its own centre of rotation on the axis z, and is arranged to generate a linear motion in that plane.
  • the moving means of the apparatus 10 further comprise a reduction mechanism, generally indicated 16, which is arranged to transmit, with a transmission ratio less than one, the rotary motion of the drive shaft 12 about the axis z into a rotary motion of an output member 18 also about the axis z.
  • the reduction mechanism 16 is a planetary gear.
  • the planetary gear comprises a solar pinion 20, mounted on the drive shaft 12 so as to be drivingly connected for rotation with this latter about the axis z, an internally-toothed ring gear 22, which is fixed to the support structure and is arranged with its own axis coinciding with the axis z, a plurality of planet gears 24 (three planet gears, in the proposed embodiment) which are radially interposed between the solar pinion 20 and the ring gear 22 and permanently mesh each both with the solar pinion 20 and with the ring gear 22, and a planet carrier formed by the output member 18, the planet carrier being rotatably mounted about the axis z and rotatably carrying each of the planet gears 24 about a respective axis of rotation z' parallel to the axis z.
  • the planetary gear 16 thus transmits, in per-se-known manner, the rotary motion to the output member 18 through the drive shaft 12 with a given transmission ratio R less than one (in the illustrated example, R - 1/8), which depends on the number of teeth of the ring gear 22, of the solar pinion 20 and of the planet gears 24. Accordingly, a 360-degree rotation of the drive shaft 12, and hence of the cam 14, about the axis z corresponds to a rotation of the output member 18 about the axis z by an angle equal to 360-R (degrees).
  • the apparatus 10 further comprises a support plate 28 mounted on the output member 18 by means of linear guide means, which in the proposed embodiment are formed by a pair of linear guides 30 which are placed on opposite sides of the axis z and are oriented parallel to an axis perpendicular to the axis z, i.e. oriented radially.
  • the support plate 28 is therefore drivingly connected for rotation with the output member 18 about the axis z, but is slidable relative to the output member 18 along a sliding direction x which coincides with the radial direction along which the linear guides 30 extend and rests in a plane perpendicular to the axis z and hence parallel to the plane in which the cam 14 extends.
  • the sliding direction x is stationary relative to the output member 18 and is therefore drivingly connected for rotation about the axis z with the output member when this latter is driven to rotate about the axis z by the drive shaft 12 via the planetary gear 16.
  • the support plate 28, which is for instance made as disc-shaped member, is interposed axially (i.e. in the direction of the axis z) between the output member 18 and the cam 14.
  • the apparatus 10 further comprises a sensor-carrying structure 32, which is mounted on the support plate 28 and is rigidly connected to this latter, and a multi-axis force sensor 34, which is, fixed to the sensor-carrying structure 32 and is arranged with its own axis parallel to the axis z.
  • the sensor-carrying structure 32 comprises an upper plate 36 which is arranged above the cam 14 and a connecting portion 38 which connects the upper plate 36 to the support plate 28 enclosing laterally part of the cam 14.
  • the force sensor 34 is of per-se- known type and therefore will not be described in further detail.
  • the force sensor 34 is mounted so as to be idly rotatable on the upper plate 36 by means of a rotary joint having a bearing 26, whereas at its top end it is rigidly connected to the sample of material S by means of a threaded pin 40 so as to be drivingly connected for rotation with the sample of material S about the axis z.
  • the apparatus 10 further comprises a follower 44 which cooperates with a profile 42 of the cam 14 (hereinafter referred to as cam profile 42) and is restrained to the sensor-carrying structure 32.
  • the follower 44 comprises a cylindrical lower portion 46 slidable along the cam profile 42 and an upper pin 48 by means of which the follower 44 is connected to the upper plate 36 of the sensor-carrying structure 32, in such a manner that the upper plate 36 of the sensor-carrying structure 32 is drivingly connected with the follower 44 in the translational movement of this latter in the radial direction (direction x).
  • the cam profile 42 is suitably shaped so as to bring about, as a result of the rotation of the cam 14 about the axis z, a rectilinear translational motion of the follower 44 in the direction x by means of the linear guides 30, and hence a rectilinear translational motion of the whole sensor-carrying structure 32 in the radial direction with a given law of motion, such as in particular the one shown in Figure 9. More specifically, the cam profile 42 is designed to bring about, for each 360-degree rotation of the cam 14 about the axis z, first (first 60 degrees of rotation of the cam) a radial rectilinear motion of the follower 44 having a given length ⁇ 1 equal to the throw of the cam 14.
  • the radial rectilinear motion of the follower 44 allowed by the linear guides 30 is transmitted to the whole sensor-carrying structure 32 causing the deformation of the sample of material S by virtue of the connection with the force sensor 34.
  • the cam profile 42 is shaped as an arc of circumference and therefore does not cause any radial movement of the follower 44.
  • the sample of material S remains in the deformation state generated in the preceding phase, thus allowing the force sensor 34 to measure the reaction force produced by the sample of material.
  • the cam profile 42 might be different from the one shown in Figure 8, provided it brings about, at each rotation of the cam 14, a to and fro rectilinear motion of the follower 44 with suitable ascending, steady and descending phases, thereby causing a corresponding radial displacement of the sensor-carrying structure 32 and a corresponding deformation of the sample of material S rigidly connected to the force sensor 34. Therefore, at each complete turn of the cam 14 about the axis of rotation z a radial displacement, the amount ⁇ 1 of which is equal to the throw of the cam profile 42, is imposed on the sample of material S and is kept constant for a given angle of rotation of the cam 14.
  • the force sensor 34 Since the force sensor 34 is drivingly connected with the sample of material S and is free to rotate about its own axis relative to the sensor-carrying structure 32, the rotation of the cam 14 and its action on the follower 44 produce a to and fro radial movement of the force sensor 34 which is repeated in different radial directions for each complete turn of the cam 14, as the directions of the linear movement of the follower 44 change.
  • the cam 14 is configured to carry out a deformation cycle during each complete turn about the axis z, during a complete turn of the output member 18 the cam 14 will make N complete turns and hence will carry out N deformation cycles, thus allowing to detect N values of stiffness of the sample of material S in N different radial directions laying in a plane perpendicular to the axis z and angularly spaced from each other by an angle equal to 360/N (degrees).
  • the cam 14 is caused to rotate about the axis z with a given angular speed CD.
  • the reduction mechanism 16 the output member 18 rotates at an angular speed equal to ra/8 in the same direction as the cam 14. Accordingly, the sensor-carrying structure 32 and the force sensor 34 move with a roto translation motion given by the overlapping of a rotary motion about the axis z with an angular speed equal to ro/8, imposed by the reduction mechanism 16, and of a translation motion in a direction perpendicular to the axis z, imposed by the cam profile 42 on the follower 44.
  • the angle of rotation of the cam 14 comprised between 0 and 60 degrees corresponds to the ascending phase in which a deformation, the amount ⁇ of which is equal to the throw ⁇ 1, is imposed on the sample of material S in a first radial direction xi laying in a plane perpendicular to the axis z.
  • the force sensor 34 detects the reaction force Fj applied by the sample of material S in the radial direction xi along which the deformation ⁇ has been imposed.
  • the sample of material S is brought back to the initial undeformed position, and remains in this position until a complete turn has been made by the cam 14 (last 180 degrees of rotation of the cam 14).
  • the cam profile 42 will bring about the ascending phase, and hence the deformation of the sample of material S, in a second radial direction x 2 angularly spaced from the first radial direction xi by the angle a with an imposed displacement the amount ⁇ 2 of which is equal to ⁇ 1.
  • the force sensor 34 will detect the reaction force F 2 applied by the sample of material S along the second radial direction x 2 , thereby allowing to calculate the stiffness K 2 of the sample of material S in that direction as the ratio of the intensity of the reaction force F 2 to the amount ⁇ 1 of the imposed deformation.
  • the apparatus allows to measure the stiffness of a sample of material in a plurality of directions laying in a plane, in particular in a plurality of radial directions angularly evenly spaced from each other, in a time interval depending on the angular speed ⁇ with which the drive shaft 12 is set into rotation.
  • the reduction mechanism allowing to transmit the motion from the drive shaft 12 to the output member 18 with a transmission ratio less than 1 is a planetary gear
  • any other motion reduction mechanism such as for instance a harmonic drive, a bevel gear etc. can be provided for.

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Abstract

The apparatus (10) comprises: deformation means (14, 28, 32, 44) arranged to bring about cyclically on the sample of material (S) a deformation cycle comprising a predetermined displacement (Δ1) along a direction of deformation (x1, x2, Xi, x8), moving means (12, 16, 18) arranged to vary each time the direction of deformation (x1, x2, Xi, x8), and force sensor means (34) arranged to be connected to the sample of material (S) to detect, during each deformation cycle, the reaction force (F1, F2, Fi, F8) applied on the sample of material (S) along the direction of deformation (x1, x2, Xi, x8). The deformation means (14, 28, 32, 44) are configured in such a manner that the direction of deformation (x1, x2, Xi, x8) is a radial direction perpendicular to an axis of rotation (z), and the moving means (12, 16, 18) are arranged to vary the direction of deformation (x1, x2, Xi, x8) by rotation about that axis of rotation (z).

Description

Apparatus for measuring the stiffness of a sample of material in a plurality of directions laving in a same plane
The present invention refers to an apparatus for measuring the stiffness of a sample of material in a plurality of directions, in particular in a plurality of directions laying in a same plane.
For the purposes of the present invention, the expression "stiffness of a sample of material in a given direction" is to be intended as referring to the scalar ratio of the force generated in static conditions by the material as a result of a deformation produced on the sample of material in that direction to the amount of the same deformation. To say it in other words, assuming that a known deformation is imposed in a given direction on the sample of material, the stiffness of the sample of material in that direction is the scalar ratio of the force produced by the material as a result of the imposed deformation to the same deformation. For the sake of simplicity, the force which is produced by the material as a result of the imposed deformation will be referred to hereinafter as reaction force.
In an anisotropic material the stiffness varies depending on the direction and therefore the same displacement imposed on a sample of such a material in different directions will produce different values of the reaction force in the different directions considered.
To date there are no mechanisms enabling to measure the stiffness of a sample of material in a plurality of directions, but the stiffness of a sample of material in a plurality of directions is obtained by complex computational techniques which require a huge amount of work for post-processing the data acquired by experiments.
It is therefore an object of the present invention to provide an apparatus which enables to measure in a simple, quick and automatic way the stiffness of a sample of material in a plurality of directions, in particular in a plurality of directions laying in a same plane, and hence to characterize the planar stiffness of the sample of material.
This and other objects are fully achieved according to the present invention by virtue of a stiffness measuring apparatus having the features set forth in the enclosed independent claim 1.
Preferred embodiments of the present invention are the subject-matter of the dependent claims, the content of which is to be regarded as being an integral and integrating part of the description which follows.
In short, the invention is based on the idea of providing an apparatus for measuring the stiffness of a sample of material in a plurality of directions, the apparatus comprising: deformation means arranged to impose cyclically a predetermined deformation on the sample of material along a plurality of directions of deformation,
moving means arranged to vary each time the direction of deformation along which the deformation means impose the predetermined deformation on the sample of material, in such a manner that for each deformation cycle the direction of deformation is different from the direction of deformation of the immediately preceding deformation cycle, and force sensor means arranged to detect, during each deformation cycle carried out on the sample of material, a signal indicative of the reaction force produced by the sample of material along the direction of deformation.
By imposing on the sample of material, through the deformation means, a predetermined deformation in a known direction and by measuring, through the force sensor means, the reaction force produced by the sample of material along that direction, the apparatus according to the invention allows to measure the stiffness of the sample of material in that direction. By varying for each measure cycle, through the moving means, the direction of deformation, the apparatus according to the invention allows to change each time the direction of measure of the stiffness of the sample of material.
Further features and advantages of the present invention will appear from the following detailed description, given purely by way of non-limiting example with reference to the appended drawings, in which:
Figure 1 is a perspective view of an apparatus for measuring the stiffness of a sample of material in a plurality of directions laying in a plane, according to a preferred em- bodiment of the present invention;
Figure 2 is a side elevation view of the apparatus of Figure 1 ;
Figure 3 is an axial section view of the apparatus of Figure 1 , taken through the plane indicated III-III in Figure 2;
Figure 4 is an exploded view of the apparatus of Figure 1 , from which some components have been removed for the sake of clarity in order to make the moving means forming part of the apparatus more visible;
Figure 5 is a perspective view of the apparatus of Figure 1 , which further shows a sample of material on which the stiffness measure is to be performed and also indicates the different directions along which the apparatus is able to perform the stiffness measure;
Figure 6 is an exploded view showing the sample of material, the force sensor forming part of the apparatus of Figure 1, as well as the components used to restrain the sample of material;
Figure 7 is a section view showing the connection of the sample of material with the force sensor of the apparatus of Figure 1 ;
Figure 8 shows an example of profile of the cam used as moving means in the apparatus of Figure 1 ; and
Figure 9 shows the law of motion imposed by the cam profile of Figure 8.
With reference first to Figures 1 to 5, an apparatus for measuring the stiffness of a sample of material S in a plurality of directions laying in a same plane is generally indicated 10. The apparatus 10 is able in particular to measure the stiffness of the sample of material S in a given number N of radial directions (shown in Figure 5) which are perpendicular to a given axis z (in the illustrated embodiment a vertical axis) and are arranged angularly evenly spaced in a plane perpendicular to the axis z, the angular distance between a measure direction and the adjacent measure direction being equal to 360 N (degrees). In the embodiment illustrated in Figure 5, the apparatus is able to measure the stiffness of the sample of material S along eight directions angularly spaced from each other by an angle equal to 45 degrees. As will be apparent from the following description, the apparatus according to the invention can be configured so as to measure the stiffness in any other number of directions. The apparatus 10 is provided with moving means comprising a drive shaft 12 arranged with its own axis of rotation coinciding with the axis z and an electric motor (not shown), or a similar actuation device, arranged to control, either directly or through a motion transmission and/or a motion reduction mechanism, the rotation of the drive shaft 12 about the axis z. The drive shaft 12 is supported for rotation (in a manner which is not shown but is however of per-se-known type) at its bottom end by a support structure (also not shown) forming part of the device 10. The electric motor is also preferably supported by the support structure.
The apparatus 10 further comprises a cam 14 keyed on the drive shaft 12, preferably at the top end thereof, so as to be drivingly connected for rotation with this latter about the axis z. The cam 14 is made as a disc-shaped element, which extends in a plane perpendicular to the axis z and has its own centre of rotation on the axis z, and is arranged to generate a linear motion in that plane.
The moving means of the apparatus 10 further comprise a reduction mechanism, generally indicated 16, which is arranged to transmit, with a transmission ratio less than one, the rotary motion of the drive shaft 12 about the axis z into a rotary motion of an output member 18 also about the axis z. In the proposed embodiment, the reduction mechanism 16 is a planetary gear. More specifically, the planetary gear comprises a solar pinion 20, mounted on the drive shaft 12 so as to be drivingly connected for rotation with this latter about the axis z, an internally-toothed ring gear 22, which is fixed to the support structure and is arranged with its own axis coinciding with the axis z, a plurality of planet gears 24 (three planet gears, in the proposed embodiment) which are radially interposed between the solar pinion 20 and the ring gear 22 and permanently mesh each both with the solar pinion 20 and with the ring gear 22, and a planet carrier formed by the output member 18, the planet carrier being rotatably mounted about the axis z and rotatably carrying each of the planet gears 24 about a respective axis of rotation z' parallel to the axis z. The planetary gear 16 thus transmits, in per-se-known manner, the rotary motion to the output member 18 through the drive shaft 12 with a given transmission ratio R less than one (in the illustrated example, R - 1/8), which depends on the number of teeth of the ring gear 22, of the solar pinion 20 and of the planet gears 24. Accordingly, a 360-degree rotation of the drive shaft 12, and hence of the cam 14, about the axis z corresponds to a rotation of the output member 18 about the axis z by an angle equal to 360-R (degrees).
The apparatus 10 further comprises a support plate 28 mounted on the output member 18 by means of linear guide means, which in the proposed embodiment are formed by a pair of linear guides 30 which are placed on opposite sides of the axis z and are oriented parallel to an axis perpendicular to the axis z, i.e. oriented radially. The support plate 28 is therefore drivingly connected for rotation with the output member 18 about the axis z, but is slidable relative to the output member 18 along a sliding direction x which coincides with the radial direction along which the linear guides 30 extend and rests in a plane perpendicular to the axis z and hence parallel to the plane in which the cam 14 extends. The sliding direction x is stationary relative to the output member 18 and is therefore drivingly connected for rotation about the axis z with the output member when this latter is driven to rotate about the axis z by the drive shaft 12 via the planetary gear 16. The support plate 28, which is for instance made as disc-shaped member, is interposed axially (i.e. in the direction of the axis z) between the output member 18 and the cam 14.
The apparatus 10 further comprises a sensor-carrying structure 32, which is mounted on the support plate 28 and is rigidly connected to this latter, and a multi-axis force sensor 34, which is, fixed to the sensor-carrying structure 32 and is arranged with its own axis parallel to the axis z. The sensor-carrying structure 32 comprises an upper plate 36 which is arranged above the cam 14 and a connecting portion 38 which connects the upper plate 36 to the support plate 28 enclosing laterally part of the cam 14. The force sensor 34 is of per-se- known type and therefore will not be described in further detail. As shown in Figures 6 and 7, in this embodiment the force sensor 34 is mounted so as to be idly rotatable on the upper plate 36 by means of a rotary joint having a bearing 26, whereas at its top end it is rigidly connected to the sample of material S by means of a threaded pin 40 so as to be drivingly connected for rotation with the sample of material S about the axis z.
As shown in particular in Figure 3, the apparatus 10 further comprises a follower 44 which cooperates with a profile 42 of the cam 14 (hereinafter referred to as cam profile 42) and is restrained to the sensor-carrying structure 32. In this connection, the follower 44 comprises a cylindrical lower portion 46 slidable along the cam profile 42 and an upper pin 48 by means of which the follower 44 is connected to the upper plate 36 of the sensor-carrying structure 32, in such a manner that the upper plate 36 of the sensor-carrying structure 32 is drivingly connected with the follower 44 in the translational movement of this latter in the radial direction (direction x). With reference also to Figures 8 and 9, the cam profile 42 is suitably shaped so as to bring about, as a result of the rotation of the cam 14 about the axis z, a rectilinear translational motion of the follower 44 in the direction x by means of the linear guides 30, and hence a rectilinear translational motion of the whole sensor-carrying structure 32 in the radial direction with a given law of motion, such as in particular the one shown in Figure 9. More specifically, the cam profile 42 is designed to bring about, for each 360-degree rotation of the cam 14 about the axis z, first (first 60 degrees of rotation of the cam) a radial rectilinear motion of the follower 44 having a given length Δ1 equal to the throw of the cam 14. The radial rectilinear motion of the follower 44 allowed by the linear guides 30 is transmitted to the whole sensor-carrying structure 32 causing the deformation of the sample of material S by virtue of the connection with the force sensor 34. In a subsequent phase of the rotation of the cam 14 (for instance in the subsequent 60 degrees of rotation) the cam profile 42 is shaped as an arc of circumference and therefore does not cause any radial movement of the follower 44. During this phase the sample of material S remains in the deformation state generated in the preceding phase, thus allowing the force sensor 34 to measure the reaction force produced by the sample of material. In the subsequent 60 degrees of rotation of the cam 14, the cam profile 42 is descending and therefore the follower 44 is subject to a rectilinear motion having the same amplitude but opposite direction with respect to the first phase (ascending phase), thus getting back to the initial position before the start of the deformation cycle and remaining in this position until the cam 14 has completed a turn (interval of rotation from 180 degrees to 360 degrees), as shown in Figure 9.
Anyway, the cam profile 42 might be different from the one shown in Figure 8, provided it brings about, at each rotation of the cam 14, a to and fro rectilinear motion of the follower 44 with suitable ascending, steady and descending phases, thereby causing a corresponding radial displacement of the sensor-carrying structure 32 and a corresponding deformation of the sample of material S rigidly connected to the force sensor 34. Therefore, at each complete turn of the cam 14 about the axis of rotation z a radial displacement, the amount Δ1 of which is equal to the throw of the cam profile 42, is imposed on the sample of material S and is kept constant for a given angle of rotation of the cam 14. By measuring, through the force sensor 34, the reaction force produced by the sample of material S as a result of its deformation throughout the angle of rotation of the cam 14 in which the deformation produced on the sample of material is kept constant, it is therefore possible, the amount Δ1 of the deformation imposed being known, to obtain directly the value of the stiffness of the sample of material S in the radial direction in question by calculating the ratio of the force measured by the sensor 34 to the amount Δ1 of the deformation imposed by the cam profile 42 through the linear movement of the follower 44.
Since the force sensor 34 is drivingly connected with the sample of material S and is free to rotate about its own axis relative to the sensor-carrying structure 32, the rotation of the cam 14 and its action on the follower 44 produce a to and fro radial movement of the force sensor 34 which is repeated in different radial directions for each complete turn of the cam 14, as the directions of the linear movement of the follower 44 change. Moreover, since the cam 14 is drivingly connected with the drive shaft 12 in the rotary motion about the axis z and since the reduction mechanism 16 imposes a given transmission ratio R (in the proposed example, as already stated, a transmission ratio R equal to 1/8) between the drive shaft 12 and the output member 18, a number N = 1/R of complete turns (in the present case, 8 complete turns) of the cam 14 is obtained for each complete turn of the output member 18. Since the sample of material S is not free to rotate, but is restrained to a frame T (schematically shown in Figure 6), as well as to the force sensor 34, it is deformed as a result of the to and fro radial movement of the force sensor 34 in radial directions perpendicular to the axis z.
More in general, if the cam 14 is configured to carry out a deformation cycle during each complete turn about the axis z, during a complete turn of the output member 18 the cam 14 will make N complete turns and hence will carry out N deformation cycles, thus allowing to detect N values of stiffness of the sample of material S in N different radial directions laying in a plane perpendicular to the axis z and angularly spaced from each other by an angle equal to 360/N (degrees). Even more generally, if the number of deformation cycles carried out by the cam 14 during each complete turn about the axis z is indicated n, during a complete turn of the output member 18 the cam 14 will make n-N complete turns and hence will carry out n-N deformation cycles, thus allowing to detect n'N values of stiffness of the sample of material S in n-N different radial directions laying in a plane perpendicular to the axis z and angularly spaced from each other by an angle equal to 360/(n N) (degrees).
Even though the operation of the stiffness measuring apparatus is to be regarded as already being clear enough in the light of the description given above, the main steps thereof will be summarized here below with reference in particular to Figure 5, where n = 1 , R = 1/8 and hence N = 8.
Once the apparatus is set going, i.e. once the electric motor connected to the drive shaft 12 is operated, the cam 14 is caused to rotate about the axis z with a given angular speed CD. By virtue of the reduction mechanism 16, the output member 18 rotates at an angular speed equal to ra/8 in the same direction as the cam 14. Accordingly, the sensor-carrying structure 32 and the force sensor 34 move with a roto translation motion given by the overlapping of a rotary motion about the axis z with an angular speed equal to ro/8, imposed by the reduction mechanism 16, and of a translation motion in a direction perpendicular to the axis z, imposed by the cam profile 42 on the follower 44.
Using the cam profile 42 shown in Figure 8, the angle of rotation of the cam 14 comprised between 0 and 60 degrees corresponds to the ascending phase in which a deformation, the amount Δχι of which is equal to the throw Δ1, is imposed on the sample of material S in a first radial direction xi laying in a plane perpendicular to the axis z. During the subsequent phase in which the deformation imposed on the sample of material S is kept constant (angular interval from 60 to 120 degrees), the force sensor 34 detects the reaction force Fj applied by the sample of material S in the radial direction xi along which the deformation Δχι has been imposed. The apparatus allows therefore to calculate the stiffness Ki of the sample of material S in the direction xi as the ratio of the intensity of the reaction force Fi to the amount Δχι = Δ1 of the deformation imposed. In the subsequent descending phase (angular interval from 120 to 180 degrees), the sample of material S is brought back to the initial undeformed position, and remains in this position until a complete turn has been made by the cam 14 (last 180 degrees of rotation of the cam 14). Meanwhile, the output member 18 rotates by an angle a equal to 360/8 degrees = 45 degrees, which leads to a change in the orientation of the axis x of the linear guides 30, and hence of the sensor- carrying structure 32, by the same angle a with respect to the force sensor 34 and to the sample of material S. During the subsequent turn, therefore, the cam profile 42 will bring about the ascending phase, and hence the deformation of the sample of material S, in a second radial direction x2 angularly spaced from the first radial direction xi by the angle a with an imposed displacement the amount Δχ2 of which is equal to Δ1. In the angular interval of rotation of the cam 14 corresponding to the phase during which the imposed deformation is maintained, the force sensor 34 will detect the reaction force F2 applied by the sample of material S along the second radial direction x2, thereby allowing to calculate the stiffness K2 of the sample of material S in that direction as the ratio of the intensity of the reaction force F2 to the amount Δ1 of the imposed deformation.
During a complete turn of the output member 18 there will be therefore 8 complete turns of the cam 14 and hence 8 different measures in sequence of the stiffness of the sample of material S along 8 radial directions which are perpendicular to the axis z, lay in a same plane and are angularly evenly spaced from each other by an angle a equal to 45 degrees.
As is clear from the description given above, the apparatus according to the invention allows to measure the stiffness of a sample of material in a plurality of directions laying in a plane, in particular in a plurality of radial directions angularly evenly spaced from each other, in a time interval depending on the angular speed ω with which the drive shaft 12 is set into rotation.
Naturally, the principle of the invention remaining unchanged, the embodiments and the constructional details may vary widely from those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the appended claims.
For instance, even though in the embodiment described and illustrated here the reduction mechanism allowing to transmit the motion from the drive shaft 12 to the output member 18 with a transmission ratio less than 1 is a planetary gear, it is clear that instead of a planetary gear any other motion reduction mechanism, such as for instance a harmonic drive, a bevel gear etc. can be provided for.

Claims

1. Stiffness measuring apparatus (10) for measuring the stiffness of a sample of material (S) in a plurality of directions (xi, x2, xi; x8), the apparatus (10) comprising: deformation means (14, 28, 32, 44) arranged to impose bring about cyclically on the sample of material (S) a deformation cycle comprising a predetermined displacement (Δ1) along a direction of deformation (x\, x2, Xj, x8),
moving means (12, 16, 18) arranged to vary each time the direction of deformation (xi, x2, Xj, x8), in such a manner that in each deformation cycle the direction of deformation is different from the direction of deformation of the immediately preceding deformation cycle, and
force sensor means (34) arranged to be connected to the sample of material (S) to detect, during each deformation cycle, a signal indicative of the reaction force (Fi, F2, Fj, F8) generated by the sample of material (S) along the direction of deformation (x\, X2, Xj, x8).
2. Apparatus according to claim 1 , wherein said deformation means (14, 28, 32, 44) are configured in such a manner that the direction of deformation ( i , x2, Xj, xg) along which the predetermined displacement (Δ1) is applied each time is a radial direction passing through an axis of rotation (z) of the apparatus, wherein said moving means (12, 16, 18) are arranged to vary the direction of deformation (xi , x2, Xj, x8) from a deformation cycle to the subsequent one by rotation about said axis of rotation (z).
3. Apparatus according to claim 2, wherein said moving means (12, 16, 18) are arranged to vary the direction of deformation (xj , x2, Xj, x8) from a deformation cycle to the subsequent one by rotation of a constant predetermined angle (a). »
4. Apparatus according to claim 2 or claim 3, wherein said deformation means (14, 28, 32, 44) comprise a cam (14) which has a driving profile (42) suitably shaped to bring about said predetermined displacement (Δ1) and which is rotatably mounted about said axis of rotation (z), a cam follower (44) which is arranged in contact with the driving profile (42) of the cam (14) to slide along the same when the cam (14) rotates about said axis of rotation (z), and a sensor-carrying structure (28, 32) on which said sensor means (34) are mounted,
wherein the sensor-carrying structure (28, 32) is rotatably mounted about said axis of rotation (z) and is slidable by means of linear guide means (30) in a radial direction (x), and wherein the sensor-carrying structure (28, 32) is drivingly connected for translation with the cam follower (44), whereby due to the rotation of the cam (14) about said axis of rotation (z), the cam follower (44) and the sensor-carrying structure (28, 32) move in the radial direction (x) with a law of motion defined by the driving profile (42) of the cam (14).
5. Apparatus according to claim 4, wherein said moving means (12, 16, 18) are arranged to set into rotation at the same time the cam (14) arid the sensor-carrying structure (28, 32) about said axis of rotation (z) with a given predetermined ratio (R) of the angular velocity of the sensor-carrying structure (28, 32) to the angular velocity of the cam (14).
6. Apparatus according to claim 5, wherein said predetermined ratio (R) is less than 1.
7. Apparatus according to claim 5 or claim 6, wherein said moving means (12, 16, 18) comprise a driving shaft (12) rotatable about said axis of rotation (z) and a motion transmission mechanism (16) interposed between the driving shaft (12) and the sensor-carrying structure (28, 32), wherein the cam (14) is drivingly connected for rotation with the driving shaft (12), wherein the motion transmission mechanism (16) comprises an input member (20) drivingly connected for rotation with the driving shaft (12) and an output member (18) drivingly connected for rotation with the sensor-carrying structure (28, 32), and wherein the motion transmission mechanism (16) is arranged to transmit the rotary motion from the input member (20) to the output member (18) with a transmission ratio equal to said predetermined ratio (R).
8. Apparatus according to claim 7, wherein the motion transmission mechanism (16) is a planetary gear and comprises a solar pinion (20) forming said input member, a fixed ring gear (22) having inner teeth, a plurality of planet gears (24) radially interposed between the solar pinion (20) and the ring gear (22) and permanently meshing each with the solar pinion (20) and with the ring gear (22), and a planet gear which is rotatably mounted about said axis of rotation (z) and supports each planet gear (24) for rotation about a respective axis of rotation (ζ') parallel to said axis of rotation (z) of the apparatus (10), the planet gear being formed by the output member (18) of the motion transmission mechanism (16).
9. Apparatus according to claim 7 or claim 8, wherein the sensor-carrying structure (28, 32) is mounted (30) to slide in a radial direction (x) by means of said linear guide means (30) on the output member (18) of the motion transmission mechanism (18).
10. Apparatus according to any of claims 4 to 9, wherein the driving profile (42) of the cam (14) is shaped in such a manner that the cam (14) brings about, during each 360- degree rotation about said axis of rotation (z), at least one deformation cycle comprising a first step in which the cam (14) produces said predetermined displacement (Δ1), a second step in which the cam (14) holds said predetermined displacement (Δ1), and a third step in which the cam (14) produces a radial movement which has the same length as that of the first step, but is directed in the opposite direction, so as to bring the cam follower (44) back to the starting position, and hence to bring the sample of material (S) to an initial condition in which it is not deformed.
PCT/IB2012/052776 2011-06-03 2012-06-01 Apparatus for measuring the stiffness of a sample of material in a plurality of directions laying in a same plane Ceased WO2012164538A1 (en)

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IT000482A ITTO20110482A1 (en) 2011-06-03 2011-06-03 EQUIPMENT FOR MEASURING THE STIFFNESS OF A SAMPLE OF MATERIALS IN MORE DIRECTIONS IN A PLAN
ITTO2011A000482 2011-06-03

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