A method for determining the three-dimensional forces acting on a cutting blade at its guide points for a cutting machine.

Abstract

The present invention relates to a method for determining the torsional components of mechanical action at a guiding point of a cutting blade (L) of a cutting machine, the cutting blade being guided by a presser foot (P) of a cutting head of the cutting machine, the method comprising: placing a five-component dynamometer on the presser foot, the dynamometer including a plurality of sensors for determining the frontal force, lateral force, rolling moment, pitching moment and yawing moment of the cutting blade; creating a calibration matrix for the dynamometer; and determining the three-dimensional forces experienced by the cutting blade based on the measurements obtained by the sensors and the calibration matrix.

Description

[Technical Field]

[0001] The present invention relates to the general field of automatic cutting by means of a vibrating blade of flexible materials placed on a cutting table in the form of a single layer or multiple layers. More precisely, the invention relates to a method for determining the component of the torsor of the mechanical action at the guide point of such a cutting blade. [Background technology]

[0002] The field of application of the invention is that of automated cutting of parts made of flexible woven or non-woven materials (such as leather), in particular in the clothing, furniture or automotive interiors industry.

[0003] A known method for automatically cutting parts of flexible material consists of providing the material in the form of a single layer or multiple layers forming a mattress on a fixed or mobile cutting support of a cutting table, and cutting the parts by means of a cutting head which moves over the cutting support of the table, the cutting head supporting in particular an oscillating steel blade which oscillates perpendicularly to the direction of the cutting edge in order to cut the material.

[0004] During this vertical vibration and cutting of the material, the cutting blade is subjected to many forces that affect the quality of the cutting edge of the part. In particular, these forces directly affect the cut quality throughout the height of the material (especially if it is made up of multiple layers) and the shape of the cut part.

[0005] It is also necessary to know as much as possible the strains to which the cutting blade is subjected in order to be able to influence the cutting parameters and the blade direction.

[0006] To this effect, it is known to arrange a bending sensor on the presser foot of the cutting head, which in this way collects data on the lateral bending of the cutting blade and can act on the cutting parameters and blade direction to correct it. Reference can be made, for example, to Italian patent application no. 2017000023745 in the name of Morgan Tecnica.

[0007] However, these data are incomplete and do not take into account all the forces to which the cutting blade is subjected. Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore a primary object of the present invention to provide a method for determining all forces experienced by a cutting blade in order to allow for finer and more autonomous control of the cutting. [Means for solving the problem]

[0009] According to the invention, the object is a method for determining the torsion component of the mechanical action at the guide point of a cutting blade of a cutting machine, the cutting blade being guided by a presser foot of the cutting head of the cutting machine, the method comprising: - placing a six-component dynamometer on the presser foot, the dynamometer including a plurality of sensors capable of determining the frontal force, the lateral force, the rolling moment, the pitching moment, and the yawing moment of the cutting blade; - creating a calibration matrix for the dynamometer; - determining the three-dimensional forces experienced by the cutting blade based on measurements obtained by the sensor and a calibration matrix.

[0010] The method according to the present invention is characterized by the fact that it is possible to determine the forces acting on the cutting blade in three directions using a dynamometer attached to the presser foot of the cutting head. In particular, it is possible to determine five of the six components of the torsion of the mechanical action at the cutting blade's guide point: the frontal force, the lateral force, the rolling moment, the pitching moment, and the yawing moment (excluding the force along the blade's main axis). This ensures particularly precise and autonomous control of the cutting parameters to correct defects based on this data.

[0011] Preferably, the step of creating a calibration matrix for the dynamometer includes creating theoretical calibration matrices for the dynamometer's sensors at various theoretical stresses as a function of the six components of the dynamometer.

[0012] Preferably, the step of creating a calibration matrix for the dynamometer also further includes the step of calculating response matrices for the dynamometer sensors at various actual stresses as a function of the six components of the dynamometer based on the theoretical calibration matrix and actual response measurements of the dynamometer sensors.

[0013] The response matrix of the dynamometer sensor is calculated by a linear optimization method.

[0014] In one embodiment, the dynamometer includes three triaxial piezoelectric sensors attached to the presser foot distributed around the longitudinal axis of the blade.

[0015] In a second embodiment, the dynamometer comprises at least three (preferably six) linked strain gauge bridges mounted on the arms of the presser foot, regularly distributed around the longitudinal axis of the blade to form at least three (preferably six) full bridges.

[0016] In a third embodiment, the dynamometer includes at least five full bridges of isolated strain gauges attached to a presser foot.

[0017] In any embodiment, transmission of measurements from the dynamometer sensors can be contactless or wired. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram illustrating a first embodiment of the implementation of the method according to the invention; [Figure 2] 2 is a schematic diagram illustrating a second embodiment of the implementation of the method according to the invention; [Figure 3] 3 is a schematic diagram illustrating a third embodiment of the implementation of the method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The invention applies to the automated cutting of parts of flexible material in single or multi-layer form.

[0020] Such cutting operations are generally carried out by means of a cutting machine having a horizontal cutting support on which the flexible material to be cut is provided.

[0021] The cutting head, which supports the oscillating blade, is mounted on a gantry that moves along the cutting support while the cutting head moves along the gantry so that it can follow various cutting paths calculated by the cutting software.

[0022] Typically, a presser foot as shown in Figure 1 is attached to the bottom of the cutting head to apply a controlled force to the flexible material on the cutting support during cutting, and the position of this presser foot is adjustable depending on the height of the flexible material placed on the cutting support. Thus, the presser foot allows for guiding the cutting blade to be held as close as possible to the flexible material.

[0023] The present invention proposes a method for determining the torsional component of the mechanical action at the guide points of the vibrating blade of such a cutting head.

[0024] Several implementation alternatives of the method according to the invention are possible.

[0025] According to one embodiment, which is shown diagrammatically in FIG. 1, the method comprises: 6 It is envisaged that a component piezoelectric dynamometer will be placed.

[0026] More precisely, the piezoelectric dynamometer comprises three triaxial piezoelectric sensors 1 to 3 attached to the presser foot P, preferably regularly distributed around the longitudinal axis Z of the cutting blade L.

[0027] The piezoelectric sensors 1 to 3 are advantageously distributed at 120° equidistant from the center of the dynamometer. As shown in Figure 1, their Z axes (Z1, Z2, and Z3, respectively) are directed downward (i.e., toward the cutting support), and their Y axes (Y1, Y2, and Y3, respectively) are directed toward the outside of the dynamometer to facilitate the passage of cables. Their X axes (X1, X2, and X3, respectively) are parallel to the radius of the dynamometer.

[0028] This arrangement allows for good integration of the sensor around the presser foot whilst ensuring good stiffness of the presser foot.

[0029] A top plate (not shown in Figure 1) closes the dynamometer, which is mounted on the presser foot. The top plate has holes for screws, which can be pressed between the top plate and the bottom of the presser foot to energize the sensor.

[0030] The first step of the method according to the invention for determining the forces experienced by the cutting blade in 3D is thus to calibrate the piezoelectric dynamometer attached to the presser foot.

[0031] This calibration consists in creating a calibration matrix that makes it possible to interpret the various measured voltages transmitted by the piezoelectric sensors 1-3 as mechanical forces.

[0032] First, a theoretical or global calibration matrix must be generated that is sensitive to the orientation and geometry of the sensor. Second, this theoretical calibration matrix must be refined to produce a response matrix that corresponds to the actual calibration matrix.

[0033] A theoretical calibration matrix is ​​considered in a situation where all geometric shapes are assumed to be perfect and defect-free, according to the ideal arrangement of the axes. Representing the arrangement of three triaxial sensors in space (X, Y, Z) helps to represent the torsion of the mechanical actions attached to the sensors.

[0034] A Cartesian reference frame (xi,yi,zi) is attached to the center Oi of each sensor i. Therefore, the torsion of the action at Oi can be written as:

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[0035] By moving the base torsion of each sensor to the origin of the reference coordinate system of the dynamometer O, the contribution of each measurement direction of each sensor in the total force reading can be determined.

[0036] A theoretical or global calibration matrix is ​​then calculated based on these various equations.

[0037] The position of each sensor center Oi is defined in a cylindrical coordinate system by the radius R and angle βi corresponding to the distance OOi. Each sensor has its own direct reference coordinate system (Oi, xi, yi, zi), whose x-axis is collinear with the line (OOi).

[0038] The translation of each sensor to the origin of the torsion in the dynamometer reference frame is given by:

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[0039] The various modifications of the reference frame are as follows:

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[0040] After simplification, the equations for the origin of the dynamometer and the torsion of each sensor in the reference coordinate system can be written as follows:

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[0041] This calibration matrix is ​​theoretical. It represents the contribution of the various axes of the sensor to the force measurement of the dynamometer. These measurements depend on the sensitivity K of the piezoelectric sensor used. In reality, none of the terms in the matrix are zero, since, despite great care in production, geometric imperfections appear in any manufacturing process. However, the dominant term must be identifiable.

[0042] Once the theoretical calibration matrix is ​​written, the calibration can be performed by correlating a controlled unit load applied to the dynamometer with the various electrical signals delivered by the triaxial sensor.

[0043] It is convenient to apply specified loads at key points where the theoretical response of the dynamometer is known. Linear optimization allows the sensor values ​​to be correlated with expected values. The calibration matrix is ​​determined by a test campaign.

[0044] The linear optimization results in the actual calibration matrix:

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[0045] FIG. 2 shows a second embodiment of the implementation of the invention, which envisages deploying a dynamometer with coupled gauges.

[0046] More precisely, the dynamometer comprises at least three, preferably six, connected strain gauge bridges attached to the arms of the presser foot P', distributed around the longitudinal axis Z of the blade L, to form at least three, preferably six, full bridges.

[0047] To ensure a good force reading, the dynamometer is constructed with arms spaced 120° apart around the axis of the blade. Three gauges J1-J3, forming a six-gauge bridge, are glued to an inclined surface, preferably equidistant from the axis of the blade, with their extensions meeting at the point of force application.

[0048] Dual longitudinal and transverse strain gauges J1 to J3 are used, positioned on each side of each arm so that each half bridge faces the other. A total of at least three full bridges are required for this dynamometer instrumentation.

[0049] Calibration consists in matching a torsion of known action to the value of strain measured by the gauge bridge.

[0050] Considering that the gauge bridge is ideally located in the center of the arm of the test specimen, the centers of the bridges Oi (i = i:6) placed on each arm are coincident. They are spaced apart by a value r from the sensor center O and are oriented at an angle α. Finally, the point of force application on the blade is shifted by -h along the axis Z to point Q.

[0051] The following known action tosser [T] is applied to point Q:

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[0052] The movement of this torso [T] at each measuring point on the gauge bridge allows the contribution of each axis of the bridge to the force reading to be determined.

[0053] To measure the torsional moment Mz, a force is applied along axis Y at point Q using a lever arm of distance l.

[0054] For clarity, the grouped reference frames are renamed as follows:

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[0055] These transports provide:

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[0056] These values ​​give the elements of the theoretical calibration matrix. Now, by taking into account the fact that the strain gauges only respond along the Z axis, we can simplify the matrix, which can be written as follows:

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[0057] K represents the sensitivity of each gauge bridge (assumed to be common here), and F i is the strain measured by gauge bridge i.

[0058] The next step in creating the actual calibration matrix is ​​to apply known forces along well-defined axes and record the response of each half-bridge.

[0059] This calibration method provides a large amount of data on which to impose a specific optimization. The relationship between signal and load is assumed to be linear, and a direct method based on the least squares method is applied.

[0060] This approach aims to minimize the least squares of the difference between the imposed and measured values ​​according to a linear response model. To this effect, n different tors [T j ], which gives n measurements [m i ] to obtain the calibration matrix [A i,j ]. The formula can be written as follows:

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[0061] The solution matrix [A] can be calculated using linear optimization methods using the following formulation: tWe can calculate the terms aij, which are the same as the solution of the normal equations in the previous equations.

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[0062] As an example, the matrix thus obtained for each sensor is given as follows:

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[0063] Because the sensors are all different due to inherent variability in machining and gage bonding, it is impossible to obtain identical matrices. However, the response of each sensor to each matrix is ​​good. A matrix can be obtained that smooths the behavior of each sensor; this matrix is ​​called the integration matrix, and it takes into account all three sensor calibration measurements (see example below).

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[0064] After checking, it is observed that the responses of the three sensors to this matrix are generally very close, with very low measurement deviations.

[0065] FIG. 3 shows a third embodiment of the implementation of the invention, which envisages deploying a dynamometer with separated gauges.

[0066] As shown in this diagram, Figure 3, the dynamometer comprises five gauge bridges as a full bridge attached to a presser foot P''. The gauges used are half-bridge rosettes to ensure force readings in the two possible bending directions (for clarity, only five gauge bridges P1 to P5 are shown in Figure 3).

[0067] The actual calibration matrix is ​​obtained by measuring the strain at the strain gauge locations and performing calculations on the bridge wiring. For example, the results are shown in the table below. [Table 1]

[0068] The maximum coupling obtained is observed to be a strain of 5.61% read by bridge 1 during application of moment My.

[0069] It is also observed that this embodiment does not require the prior step of creating a theoretical calibration matrix.

[0070] It should be noted that in any embodiment, the transmission of measurements from the strain sensors of the dynamometer may be contactless or wired.

[0071] It should also be noted that in any embodiment, a set of electronic cards is provided between the piezoelectric sensors or strain gauge bridges and the computer station that utilizes the received information. These electronic cards perform the following functions: supplying and conditioning the signals from the sensors (as a function of the type of these sensors), filtering and amplifying the signals to suit the input range of the analog-to-digital converter, converting from analog to digital, serializing, and transmitting the data to the computer station.

Claims

1. A method for determining the three-dimensional forces exerted on a cutting blade (L) of a cutting machine at the guide points of said cutting blade, said cutting blade being guided by a presser foot (P; P'; P'') of a cutting head of said cutting machine, said method comprising: - placing a six-component dynamometer on the presser foot, the dynamometer including a plurality of sensors capable of determining the frontal force, the lateral force, the force along the major axis of the blade, the rolling moment, the pitching moment and the yawing moment of the cutting blade; - creating a calibration matrix for said dynamometer; determining the three-dimensional forces experienced by said cutting blade based on the measurements obtained by said sensor and said calibration matrix.

2. 2. The method of claim 1, wherein creating the calibration matrix for the dynamometer comprises creating theoretical calibration matrices for the sensors of the dynamometer at various theoretical stresses as a function of the six components of the dynamometer.

3. 3. The method of claim 2, wherein the step of creating the calibration matrix for the dynamometer further comprises the step of calculating a response matrix of the sensor of the dynamometer at various actual stresses as a function of the six components of the dynamometer based on the theoretical calibration matrix and actual response measurements of the sensor of the dynamometer.

4. The method of claim 3 , wherein the response matrix of the sensors of the dynamometer is calculated by a linear optimization method.

5. 5. The method according to any one of claims 1 to 4, wherein the dynamometer comprises three triaxial piezoelectric sensors (1, 2, 3) attached to the presser foot (P) distributed around the longitudinal axis (Z) of the blade.

6. 5. The method according to any one of claims 1 to 4, wherein the dynamometer comprises at least three linked strain gauge bridges (J1-J3) mounted on arms of the presser foot (P') regularly distributed around the longitudinal axis (Z) of the blade to form at least three full bridges.

7. 7. The method of claim 6, wherein the dynamometer comprises six strain gauge bridges regularly distributed around the longitudinal axis (Z) of the blade to form six full bridges.

8. The method according to any one of claims 1 to 4, wherein the dynamometer comprises six separated strain gauge bridges attached to the presser foot (P'').

9. The method according to any one of claims 1 to 8, wherein the transmission of the measurements of the sensors of the dynamometer is done contactlessly or by wire.

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