EP4093584B1 - Method for determining components of a mechanical action torsor at the guiding point of a cutting blade for a cutting machine - Google Patents

Description

Technical area

The present invention relates to the general field of automatic cutting by a vibrating blade of a flexible material placed on a cutting table in the form of a single ply or a stack of plys. It relates more precisely to a method for determining components of a mechanical action twister at the guide point of such a cutting blade.

Prior art

One field of application of the invention is that of the automatic cutting of parts in a flexible textile or non-textile material (such as leather), in particular in the clothing, furniture or automobile upholstery industry. .

A known method for automatically cutting parts from a flexible material consists of bringing the material onto a fixed or movable cutting support of the cutting table, in the form of a single ply or a stack of plys forming a mattress , and cutting the parts by means of a cutting head moving above the cutting support of the table. The cutting head notably carries a vibrating steel blade which is vibrated vertically according to the direction of its cutting edge in order to cut the material.

During this vertical vibration and when cutting the material, the cutting blade is subjected to numerous forces which affect the quality of the cut edges of the parts. In particular, these efforts have a direct impact on the cutting quality and on the geometry of the pieces cut over the entire height of the material, particularly when the latter is formed from a stack of plies.

Also, in order to be able to act on the cutting parameters and on the orientation of the blade, it is necessary to know as best as possible the deformations undergone by the cutting blade.

For this purpose, it is known to position a flexion sensor at the level of the presser foot of the cutting head. In this way, this sensor makes it possible to collect data relating to the lateral flexion of the cutting blade and thus to act on the cutting and orientation parameters of the blade to correct it. We can for example refer to the patent application IT 102017000023745 on behalf of Morgan Tecnica.

However, these data are not sufficient and do not take into account all the forces undergone by the cutting blade. The document US3849712 A discloses an adaptive numerical control machine tool.

Presentation of the invention

The main aim of the present invention is therefore to propose a method making it possible to determine all of the forces undergone by the cutting blade in order to allow finer and more autonomous control of the cutting.

• le positionnement d'un dynamomètre à 6 composantes sur le pied presseur, le dynamomètre comprenant une pluralité de capteurs aptes à déterminer un effort frontal, un effort latéral, un moment de roulis, un moment de tangage et un moment de lacet de la lame de coupe ;
• l'établissement d'une matrice d'étalonnage du dynamomètre ; et
• la détermination des efforts en trois dimensions subis par la lame de coupe à partir des mesures obtenues par les capteurs et de la matrice d'étalonnage.

In accordance with the invention, this goal is achieved thanks to a method for determining components of a mechanical action twister at the guiding point of a cutting blade for a cutting machine, the blade being guided in a presser foot of a cutting head of the machine, the process comprising:

• positioning a 6-component dynamometer on the presser foot, the dynamometer comprising a plurality of sensors capable of determining a frontal force, a lateral force, a rolling moment, a pitching moment and a yaw moment of the blade cut ;
• establishing a dynamometer calibration matrix; And
• determining the three-dimensional forces experienced by the cutting blade from the measurements obtained by the sensors and the calibration matrix.

The method according to the invention is remarkable in that it makes it possible to determine the forces undergone by the blade in the three directions from a dynamometer installed in the presser foot of the cutting head. In particular, five of the six components of the torso of mechanical actions at the blade guide point can be determined, namely: frontal force, lateral force, rolling moment, pitching moment and yaw moment (the force following the (main axis of the blade being excluded).

In this way, from this data, it is possible to ensure management particularly precise and autonomous cutting parameters in order to correct defects.

Preferably, the step of developing the dynamometer calibration matrix includes the development of a theoretical calibration matrix of the dynamometer sensors at different theoretical loads depending on the 6 components of the dynamometer.

Also preferably, the step of developing the dynamometer calibration matrix further comprises, from the theoretical calibration matrix and actual response measurements of the dynamometer sensors, the calculation of a response matrix of the dynamometer sensors. dynamometer sensors at different real loads depending on the 6 components of the dynamometer.

The response matrix of the dynamometer sensors can be calculated by a linear optimization method.

In one embodiment, the dynamometer comprises three triaxial piezoelectric sensors which are mounted in the presser foot being distributed around a longitudinal axis of the blade.

In a second embodiment, the dynamometer comprises at least three - and preferably six - bridges of coupled deformation gauges which are mounted on branches of the presser foot regularly distributed around a longitudinal axis of the blade in order to form at least three - and preferably six - full decks.

In a third embodiment, the dynamometer comprises at least five complete bridges of decoupled strain gauges which are mounted in the presser foot.

Whatever the embodiment, the transmission of measurements from the dynamometer sensors can be carried out without contact or by wire.

Brief description of the drawings

• [ Fig. 1 ] There figure 1 is a schematic view representing a first embodiment of implementing the method according to the invention.
• [ Fig. 2 ] There figure 2 represents a schematic view representing a second embodiment of implementing the method according to the invention.
• [ Fig. 3 ] There Figure 3 represents a schematic view representing a third embodiment of implementing the method according to the invention.

Description of embodiments

The invention applies to the automated cutting of parts in a flexible material in the form of a single ply or a stack of plys.

Such a cutting operation is generally carried out by means of a cutting machine provided with a horizontal cutting support on which the flexible material to be cut is brought.

A cutting head carrying a vibrating blade is mounted on a gantry which is caused to move along the cutting support while the cutting head moves simultaneously along the gantry so as to be able to follow the different cutting trajectories calculated by cutting software.

Typically, a presser foot such as that shown on the figure 1 is mounted on the lower part of the cutting head in order to press the flexible material onto its cutting support using a controlled force during cutting, the position of this presser foot being adaptable depending on the height of the flexible material placed on the support cutting. Thus, the presser foot makes it possible to maintain the guidance of the cutting blade as close as possible to the flexible material.

The invention proposes a method for determining components of a mechanical action twister at the guide point of the vibrating blade of such a cutting head.

Several variants of implementation of the method according to the invention are possible.

According to an embodiment shown schematically on the figure 1 , the method involves positioning a five-component piezoelectric dynamometer on the presser foot P of the cutting head.

More precisely, the piezoelectric dynamometer comprises three triaxial piezoelectric sensors 1 to 3 which are mounted on the presser foot P, preferably being regularly distributed around a longitudinal axis Z of the cutting blade L.

The piezoelectric sensors 1 to 3 are advantageously distributed at 120° being equidistant from the center of the dynamometer. As shown on the figure 1 , their Z axes (respectively Z ₁ , Z ₂ and Z ₃ ) are directed downwards (i.e. towards the cutting support), their Y axes (respectively Y ₁ , Y ₂ and Y ₃ ) are directed towards the outside of the dynamometer to facilitate the passage of the cables, and their X axes (respectively X ₁ , X ₂ and X ₃ ) are parallel to the radii of the dynamometer.

This arrangement allows good integration of the sensors in the environment of the presser foot while guaranteeing good stiffness of the latter.

A top plate (not shown on the figure 1 ) closes the dynamometer integrated into the presser foot. It has holes for the passage of screws allowing the sensors to be pre-loaded by compressing them between the upper plate and the bottom of the presser foot.

The first step of the method according to the invention for determining the 3D forces experienced by the cutting blade is to carry out a calibration of the piezoelectric dynamometer thus mounted on the presser foot.

This calibration consists of establishing a calibration matrix which makes it possible to interpret the different measurement voltages sent by the piezoelectric sensors 1 to 3 in terms of mechanical forces.

Firstly, it is appropriate to create a theoretical or global calibration matrix sensitive to the orientation and geometry of the sensors. Secondly, it is necessary to refine this theoretical calibration matrix towards a response matrix corresponding to the real calibration matrix.

The reflection concerning the theoretical calibration matrix takes place in a context where all geometric shapes are assumed to be perfect without defects, according to an ideal positioning of the axes. It is appropriate to represent in space (X,Y,Z) the positioning of the three triaxial sensors in order to express the torso of the mechanical actions attached to them.

An orthonormal coordinate system (xi, yi, zi) is attached to each sensor i at its center Oi. The twist of actions in Oi can then be written:

By transporting the elementary torsors of each sensor at the origin of the dynamometer reference O, it is possible to determine the contribution of each measurement direction of each of the sensors in the reading of the overall forces.

The theoretical or global calibration matrix is then calculated from these different equations.

The position of the center Oi of each sensor is defined in a cylindrical coordinate system by a radius R corresponding to the distance OOi and an angle βi. Each sensor has its own direct coordinate system (Oi, xi, yi, zi) and their xi axes are collinear to the right (OOi).

The transport of the torsors of each sensor at the origin and in the dynamometer frame is given by the following equation;

The different benchmark changes are as follows:

After simplification, the expression of the torsors of each sensor at the origin and in the reference frame of the dynamometer can then be written:

This calibration matrix is theoretical. It represents the contribution of the different axes of the sensors in measuring the forces of the dynamometer. These measurements depend on the sensitivity K of the piezoelectric sensors used. In reality, no term in the matrix is zero because, despite the care taken in the production and whatever the manufacturing processes of the dynamometer, geometric defects appear. However, the predominant terms must be able to be identified.

Once the theoretical calibration matrix is written, calibration can be performed. It consists of correlating controlled unit loadings applied to the dynamometer with the different electrical signals delivered by the triaxial sensors.

Identified loads should be applied at strategic locations where the theoretical response of the dynamometer is known. By linear optimization, it is possible to correlate the sensor values with the expected values. Through a test campaign, the actual calibration matrix is determined.

The linear optimization results give the following actual calibration matrix:

There figure 2 represents a second embodiment of implementation of the invention in which the method provides for positioning a dynamometer with coupled gauges.

More precisely, the dynamometer comprises at least three and preferably six bridges of coupled strain gauges which are mounted on branches of the presser foot P' distributed around a longitudinal axis Z of the blade L in order to form at least three and preferably six full decks.

In order to guarantee a good reading of the forces, the dynamometer was built around the axis of the blade with branches spaced 120° apart. The three gauges J1 to J3 forming the six gauge bridges are preferably glued equidistant from the axis of the blade and on inclined sides whose extension meets at the point of application of the forces.

Double longitudinal/transverse strain gauges J1 to J3 are used and arranged on each face of each of the branches so that each half-bridge is in opposition. A total of at least three complete bridges are required to instrument this dynamometer.

Calibration consists of matching a known action torque to a deformation value measured by the gauge bridges.

Considering that the gauge bridges are ideally centered on the branches of the test body, the respective centers of the bridges Oi (i=i:6) placed on each branch are combined. They are then distanced from the center of the sensor O by a value r and oriented by an angle α. Finally, the point of application of forces on the blade is moved by -h along the Z axis to point Q.

The following known action torque [T] is applied to point Q:

The movement of this torso [T] at each measuring point of the gauge bridges makes it possible to know the contribution of each of the axes of the bridges in the reading of the forces.

To measure the torque Mz, a force is applied along the Y axis, at point Q with a lever arm of distance l .

For clarity, grouped marks are renamed as follows:

These transports then give:

These values give the components of the theoretical calibration matrix. Now, taking into account the fact that the strain gauges only react along their Z axis, it is possible to simplify the matrix. It is then written:

With K designating the sensitivity of each gauge bridge (here assumed to be common), and F _i the deformation measured by the gauge point i.

The next step in developing the actual calibration matrix consists of applying known forces along well-defined axes and recording the reaction of each half bridge.

This calibration method offers a very large amount of data which requires a certain optimization. Since the signal/load relationships are assumed to be linear, a direct method based on the least squares method is applied.

The approach aims to minimize the least squares of the differences between the imposed values and the measured values according to a linear response model. To this end, we seek to express [A _i,j ], the calibration matrix, using n measurements [m _i ] delivering n different torsors [T _j ]. The equation can be written this way:

The following formatting allows the calculation of the terms aij of the matrix [A] ^t solution by the linear optimization method, identical to the solution of the normal equation of the previous equation.

By way of illustration, the matrix thus obtained for each sensor is given as follows:

As the sensors are all different depending on the variabilities inherent in the machining and bonding of the gauges, it is impossible to obtain an identical matrix. However, the reaction of each sensor to each matrix is good. It is possible to obtain a matrix smoothing the behavior of each sensor, this matrix called a merged matrix takes into consideration all the calibration measurements of the three sensors (see the example below)

After verifications, we see that the response of the three sensors to this matrix is really very close and the measurement difference is very small.

There Figure 3 represents a third embodiment of the invention in which the method provides for positioning a dynamometer with decoupled gauges.

As shown on this Figure 3 , the dynamometer thus comprises five bridges of gauges in complete bridges mounted in the presser foot P". The gauges used are half-bridge rosettes in order to guarantee the reading of the forces in both possible bending directions (for reasons of clarity, only the five gauge bridges P1 to P5 are represented on the Figure 3 ).

The actual calibration matrix is obtained by measuring the strains at the strain gauge positions and doing the calculation for the bridge wiring. As an example, a result is visible in the table below: [Table 1] Fx (%) Fy (%) Mx (%) My (%) Mz (%) Deck 1 - 0.03 0.27 5.61 0.2 Deck 2 1.53 - 0.36 0.85 0.28 Deck 3 0 4.48 - 0.15 0.03 Deck 4 2.49 0.12 0.15 - 2.26 Deck 5 0.08 4.52 0.02 1.5 -

It can be seen that the largest coupling obtained is 5.61% of deformation read by bridge 1 during the application of a moment My.

We also note that this embodiment does not require a prior step of developing a theoretical calibration matrix.

It should be noted that whatever the embodiment, the transmission of measurements from the deformation sensors of the dynamometer is carried out without contact or by wire.

It will also be noted that whatever the embodiment, a set of electronic cards is provided between the piezoelectric sensors or the strain gauge bridges and the computer station using the information received. These electronic cards perform the following functions: power supply and conditioning of signals from the sensors (depending on the typology of these sensors), filtering and amplification of signals in line with the input range of the analog-digital converter, analog-digital conversion, and serialization and transmission of data to the computer station.

Claims (9)

1. Method for determining components of a mechanical action torsor at the guiding point of a cutting blade (L) for a cutting machine, the blade being guided in a presser foot (P; P'; P") of a cutting head of the machine, the method comprising:

   - positioning a six-component dynamometer on the presser foot, the dynamometer comprising a plurality of sensors capable of determining a frontal force, a lateral force, a rolling moment, a pitching moment and a yawing moment of the cutting blade;

   - establishing a calibration matrix of the dynamometer; and

   - determining the forces in three dimensions to which the cutting blade is subjected, on the basis of measurements obtained by the sensors and the calibration matrix.
2. Method according to claim 1, wherein the step of developing the calibration matrix of the dynamometer comprises developing a theoretical calibration matrix of the sensors of the dynamometer at various theoretical stresses as a function of the 6 components of the dynamometer.
3. Method according to claim 2, wherein the step of developing the calibration matrix of the dynamometer further comprises, on the basis of the theoretical calibration matrix and actual response measurements of the sensors of the dynamometer, calculating a response matrix of the sensors of the dynamometer at various actual stresses as a function of the 6 components of the dynamometer.
4. Method according to claim 3, wherein the response matrix of the sensors of the dynamometer is calculated by a linear optimisation method.
5. Method according to any one of claims 1 to 4, wherein the dynamometer comprises three triaxial piezoelectric sensors (1, 2, 3) which are mounted in the presser foot (P) being distributed around a longitudinal axis (Z) of the blade.
6. Method according to any one of claims 1 to 4, wherein the dynamometer comprises at least three coupled strain gauge bridges (J1 to J3) which are mounted on arms of the presser foot (P') regularly distributed around a longitudinal axis (Z) of the blade in order to form at least three full bridges.
7. Method according to claim 6, wherein the dynamometer comprises six strain gauge bridges regularly distributed around the longitudinal axis (Z) of the blade in order to form six full bridges.
8. Method according to any one of claims 1 to 4, wherein the dynamometer comprises at least five decoupled strain gauge bridges (P1 to P5) which are mounted on the presser foot (P").
9. Method according to any one of claims 1 to 8, wherein the transmission of the measurements of the sensors of the dynamometer is performed contact free or by wire.

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