WO2010010253A1

WO2010010253A1 - Platen and method for measuring a cutting force

Info

Publication number: WO2010010253A1
Application number: PCT/FR2009/000907
Authority: WO
Inventors: Arnaud Larue; François LAPUJOULADE
Original assignee: Arts.
Priority date: 2008-07-22
Filing date: 2009-07-22
Publication date: 2010-01-28
Also published as: FR2934368A1; FR2934368B1

Abstract

The invention relates to a platen for measuring force with inertial compensation and comprising a base plate (4), an upper plate (5) and piezoelectric force sensors bridged together by each of the plates (4, 5), characterised in that the lower plate (5), intended for rigid interaction with a part to be machined of a machining tool, is triangular and in that the number of sensors is at least three, and in that it further comprises a set of six single-axis accelerometers (10-15), three of which (10, 11, 12) are oriented in the direction of the platen body while the three others (13, 14, 15) are located in the plane of the platen, two of the latter accelerometers (13, 14) being perpendicular to one of the sides thereof while the last one (15) is parallel thereto. The measuring method that uses the platen comprises identifying the sensor by a least square method in order to define the mass (m) of the assembly and the position of the centre of gravity thereof, and processing by calculation the measuring signal value of the platen in order to derive the value of the cutting force by subtracting, from the measure value, the calculated value of the mass-type inertia forces multiplied by the acceleration.

Description

Platinum and method for measuring a cutting force.

The present invention relates to a platen for measuring cutting forces for a machine tool and the method it allows to implement.

BACKGROUND OF THE INVENTION

The measurement of cutting forces is important in the field of machining. It is necessary to have a technology that allows this measurement. Indeed, in a machining machine, forces exist during the operation between the part being machined and the tool. There is of course a correlation between the deformation and the force that generates it. In addition, the measurement of forces during the process is of great importance for pre-programmed machining operations. A divergence of the measured values with respect to the setpoints makes it possible to make the necessary corrections.

In addition, the quantitative observation of the cutting phenomenon makes it possible to understand and model it to provide assistance in the choices concerning the optimization of the cutting process.

Several measurement sensors are known, differing on the one hand by the number of components of the effort they are able to measure (at most six components: resultant and moment of effort projected in the three directions of space), and secondly, by their architecture (stationary or rotary dynamometers). The measuring sensors of the rotary dynamometers are directly connected to the rotating tool, while the stationary dynamometers are connected to the cutting forces by the fact that they belong to a fixed element forming the support of the tool or the workpiece.

Three-component dynamometers comprise, in known manner, between two parallelepipedic plates, four three-component sensors, each sensor comprising two shear quartz and one compression quartz. The two plates are clamped one on the other with prestressing. These known sensors are analyzed as elastic systems with a rigid plate mounted on springs that are piezoelectric sensors: they have a very pronounced resonance frequency and a high sensitivity to the movements of their support which make them poorly suited to measurement of cutting forces during transient operating conditions of the machine.

Indeed, a dynamometer subjected to variable forces is the seat of movements that induce inertial forces that are superimposed on the forces to be measured. This results in a deterioration of the measured signal which can go as far as complete denaturation if the inertial forces become of the same order of magnitude or are greater than the forces to be measured. This effect is particularly important when the excitation frequency is close to the own frequencies of the dynamometer.

The resonant frequencies of the known sensors extend over a range generally between 0.5 kHz and 5 kHs. These frequencies are different from one measurement axis to another (the different directions of the components of the measured effort).

These sensors are suitable, for example, for a grooving operation in which the cutting force signals are mainly constituted by the harmonic corresponding to the frequency of passage of the teeth of the tool because in this case, the cutting excitation usually does not exceed the self-frequency values of the sensor. For example, a four-tooth tool that grooves a workpiece at 15,000 rpm gives an excitation frequency of 1 kHz, which is less than the value of the natural frequency of some dynamometers. Measurement cutting forces are then possible. However, the error increases when the rotational speed rises.

On the other hand, in a high-speed machining operation or a machining operation of the type milling profile on a small thickness, the cutting forces are much more discontinuous. Their frequency spectrum then comprises very high harmonics likely to cause resonance of the top plate of the dynamometers. Under these conditions, the measurements collected are very disturbed.

There is therefore an unmet need for a measuring plate that is insensitive to the vibrations of the sensors, which are important in particular in the transient regimes of cutting operations which characterizes high-speed, highly discontinuous cutting operations at low levels of effort. or the machining of thin walls or polynomial profiles.

In addition, the demand for a reliable cutting force measurement, regardless of the cutting conditions, is great for many reasons. Indeed, it is more and more useful as regards the preparation of cutting processes (machine reception phase, implementation of tool / material pair, etc.), when doing research and development (development, analysis, optimization of operation, determination of laws governing dynamic cutting operations, simulation ... and when real-time monitoring of cutting processes.

OBJECT OF THE INVENTION The present invention firstly relates to an inertial compensation force measuring plate comprising a base plate, an upper plate and force sensors clamped between each of the plates, in which the upper plate intended to cooperate rigidly with a workpiece or a machining tool is indeformable so that its natural frequencies are located at a level greater than 10 kHz, in that the number of force sensors is at least three and in that it comprises a set of at least six unidirectional accelerometers .

By means of the accelerometers, it is possible to determine the six components of the acceleration of the upper plate which are exploited by the calculation to subtract the results of the measurement carried out by the force sensors, the inertial component which comes from to superimpose.

Preferably, the upper plate is of triangular shape, the number of sensors being equal to three and three accelerometers are oriented in the direction of the thickness of the plate while the other three are housed in the plane of the plate, with two of them perpendicular to one of its sides and the last parallel to it.

A very simple calibration procedure makes it possible to determine the mass and the position of the center of inertia of the assembly formed by the upper plate and the object attached thereto.

The second object of the invention is a method for measuring cutting forces, especially in the transient period or for high cutting speeds by using the plate above, characterized in that it comprises a stage of identification of the plate. before it is put into operation, in order to know the mass (m) of the assembly and the position of its center of inertia and to process by calculation the value (Fmes) of the measurement signal of the plate to extract the value cutting force (Fapp).

This identification step is remarkable in that it comprises the application of a shock on the top plate of the plate by means of a non-instrumented hammer. Other features and advantages of the invention will emerge from the description given below of an exemplary embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the accompanying drawings in which:

FIG. 1 is a diagram illustrating the theoretical foundation of the stage according to the invention,

FIGS. 2 and 3 show, by two orthogonal schematic views, a measuring plate according to FIG.

The invention.

The basic principle of compensation is simple. In the case of a plate, theoretical, as illustrated in Figure 1, a single sensor, the plate 1 comprises two parallel plates 2 and 3 between which a piezoelectric sensor 4 is clamped under prestressing. The sensor is assimilated to a spring to put in equations the behaviors of the plate in dynamic operation. The upper plate 2 of the plate is subjected to the action of the external force Fapp that one seeks to measure and the action of the sensor (spring) 1. The lower plate 3 is secured to a support 5. By making the assumption that the measured value Fmes corresponds to the force exerted by the upper plate 2 on the sensor 1, it is easy to write the equation of equilibrium of the mass m corresponding to the upper plate 2:

mx = Fmes + Fapp, ^ _*

The simultaneous knowledge of the measured force Fmes and the acceleration ("x") makes it possible to determine the force applied if the mass m: Fapp = mx - Fmes is known.It is noted that this relation does not involve the baseplate and its eventual movement The inertial compensation gives the force exerted true even in the presence of a movement of the support. external force is applied to the dynamometer, the movements of the base generates a force of inertia mx which results in a measured force: Fmes = mx The dynamometer then functions as an accelerometer. The mass m is difficult to know a priori. But it is determined by identification in the case of a known applied force. A remarkable particular case is that in which this force is null. If the measured force and the acceleration are known at times ti (i = 1,.,.,., N), then the best value of m is the one that minimizes the quantity: n ε = ^ (mx, - FtHeS ₁ ) ²

' ^{= 1} (2)

The same approach is applicable to the case of a dynamometer comprising, for example, four multidirectional or multiaxial sensors. By construction, one must be close to the condition of indeformability of the upper part of the dynamometer. It is then possible to write its dynamic equilibrium equation which is given in its complete form by Chung and Spiewak. We will limit ourselves to the equations which involve the three orthogonal components of the applied force Fapp = [Fapp _x , Fapp _yf Fapp _z ] without considering the use of moment equations:

_G x = _Xj + ΣFmes Fαpp my _G = _x + _yj ΣFmes Fαpp _y - / ^{= 1} J ^≈1

mz _G = ΣForms _Zj + Fαpp _z

J = I

(3)

In these equations, x _G , y _G , z _G are the accelerations of the center of gravity of the upper part. The fourth Tri-directional sensors provide the twelve measurement signals:

FMES _Xi FmeSy FMES _j and _Zi.

The acceleration values x _G , y _G , z _G can be obtained from acceleration measurements at six points of the solid provided that the position of the center of gravity is known. In the same manner as above, it is possible to obtain the value of the mass and the three position parameters of the center of gravity by identification in the case of a zero applied force.

The plate according to the invention constitutes the particular apparatus which makes it possible to obtain in a simple manner, with widely acceptable approximations, the variables necessary for solving the above equations. The apparatus 6 proposed and shown in FIGS.

3, has three triaxial or three-directional piezoelectric force sensors, not shown and conventionally clamped under prestressing between two parallel plates as in FIG. 1. This arrangement ensures the isostatism of the system, which is an advantage because this isostatism guarantees a certain independence vis-à-vis the deformations due to thermal expansions which is not true for a system with 4 sensors.

Thus, for this apparatus, the system of equations (3) becomes:

m - x _G = + Fapp _y

3m - z _G = -Female _Zj + Fapp _{z (4)} J = ι

In a classic way, the results on each direction are obtained by summation of the electrical charges (paralleling) from the three sensors. This technique reduces the number of charge amplifiers from nine to three, making the system more economical.

The architecture of the proposed apparatus is such that at least the upper plate 7 of the plate (see Figure 3), that is to say that which will be coupled to the part or the tool, sits cutting force to measure, is triangular.

The triangular shape of the upper plate has been determined because it is the most compatible with the presence of three force sensors and allows to get closer to the non - deformability condition which is the basis of the system. equations (4). In addition, with this device, a maximum deformability of the upper plate of the plate is obtained. The modes of bending, torsion of the upper plate are then reported beyond the target bandwidth of 1OkHz, which is not possible with a rectangular plate.

In addition, the plate has three accelerometers 10, 11, 12, oriented in the direction Z perpendicular to the platen plates, two accelerometers 13 and 14 oriented in the direction X parallel to the plane of the plates and perpendicular to one of the sides of the plate. the upper plate and an accelerometer 15 oriented in the direction Y which belongs to the plane of the direction X and which is perpendicular to this direction. Az1, Az2, Az3, Ax4, Ax5, Ay6 denote the accelerations measured by the accelerometers 10, 11, 12, 13, 14 and 15, respectively.

The position x _G , y _G , z _G of the center of gravity will be assumed to be known in a reference frame linked to the upper plate. The six accelerometers are placed at the points [P ₁ , P ₂ , ..., P ₆ ], in the directions ψ _ι , d ₂ , ..., d ₆ j. With the approximation of the small displacements, one obtains the values of the accelerations in according to that of the center of gravity G:

with:,

= {x _dι , y _dj , z _dι }.

By developing the vector product and the scalar product, equation (5) becomes:

α, = (x _G + θ _y .z _{GP /} -θ _z .y _GPι ) x _d + (y _G -θ _x .z _{G /} + + θ _t _{.x GPι} ) y _d + (z _G + θ _x .y _GPι -θ _y .x _GPj ) z _d (6) Equation (6) is a linear system of 6 equations with 6 unknowns. It makes it possible to obtain the 6 components of the acceleration at the center of inertia if the system is not singular, that is to say if the positions and orientations of the accelerometers are well chosen. With the configuration described in FIG. 3, the system becomes simpler and becomes the system (7):

The previous system of equations has the distinction of being split into 3 subsystems. The first, consisting of the first 3 lines, makes it possible to obtain z _G , Θ _X , θ _y . Then the next two lines give x _G , θ _z and the last line gives y _G. This separation is very useful during the calibration of the dynamometer because it allows to operate direction by direction. This is achieved thanks to the particular arrangement of the accelerometers.

The theoretical exposition above demonstrates that knowing the mass m of the upper plate and the acceleration of its center of gravity, one can achieve, by calculation, the desired result, namely, the value of the force Fapp. In machining applications, a part (or tool) is attached to the top plate. The accuracy of the results implies that the plate and workpiece assembly is an indeformable assembly, which imposes strict limits both to the shape of the workpiece fixed on the plate and on the fastening means.

The coordinates x _G , y _G , z _G of the center of gravity are not known. They must be determined for each piece mounted. It is the same with the mass m.

The knowledge of these variables therefore passes through a procedure called the identification procedure of the characteristics of the sensor. This procedure is repeated whenever, due to the removal of material resulting from machining for example, the mass and the center of gravity of the plate / workpiece assembly has varied significantly. This identification procedure is of the type corresponding to equation (2) above. A disturbance is exerted on the system and the measurements are carried out after its cessation when the force applied is zero. This will advantageously be a simple shock on the platen, given for example by a hammer un-instrumented.

The identification is normally made between two uses of the dynamometer. But in the case of intermittent machining, it is possible to proceed on periods where the cutting force is zero, to follow the changes in the mass and the position of the center of gravity.

Among the many advantages of the measuring plate according to the invention and its method of use include: the measurement of cutting forces for the dynamic identification of cutting laws in milling to great speed,

- the possibility of studying machining cases of machined walls at high speed,

- help with the study and the memorization of models of cutting forces, the apprehension of the problems of cutting under transient regime (phenomena of tailgating, ...),

- finally, the apprehension of recurring machining problems (vibrations of the tool / door / tool / spindle system mounted in the machine, etc.)

The invention is not limited to the embodiment described above. In particular, the equilateral triangle shape of the top plate can suffer transformations to the isosceles form. It is also possible to increase the number of force sensors and change the shape of the upper plate while respecting the requirement of rigidity: it is it in fact that simplifies calculations.

The application of the invention illustrated to the measurement of cutting forces is not limiting. Platinum can indeed be used to measure the forces and their variations for any part fixed on its upper plate regardless of the origin of these efforts. Furthermore, the part concerned by the claims may be a part in contact with a tool or a tool in contact with a workpiece.

Claims

1. A force measuring plate subjected to a three-axis dimensionally stable piece with inertial compensation comprising a base plate (4), an upper plate (5) and clamped force sensors between each of the plates (4,5). ), characterized in that the upper plate (5) is intended to cooperate rigidly with the workpiece and is dimensionally stable so that its natural frequencies are located at a level greater than 10 kHz, in that the number of force sensors is at least three and in that it comprises a set of at least six uni-axial accelerometers (10 to 15).

2. Platinum according to claim 1, characterized in that the upper plate is of triangular shape, the number of sensors being equal to three and in that three accelerometers (10,11,12) are oriented in the direction of the thickness. of the plate while the other three (13,14,15) are housed in the plane of the plate, with two of them (13,14) perpendicular to one of its sides and the last (15) parallel to the latter.

3. A method of measuring cutting forces especially in transient period or for high cutting speeds using the plate according to claim 1 or claim 2, characterized in that it comprises a stage of identification of the front plate. its operation, in order to know the mass (m) of the assembly and the position of its center of inertia and to process by calculation the value (Fmes) of the measurement signal of the plate to extract the value of the cutting force (Fapp).

4. Method according to claim 3, characterized in that the stage of identification of the plate comprises the application of a shock on the upper plate of the plate by means of a non-instrumented hammer.