The invention relates to machine tools for laser cutting and in particular it relates to a machine tool with a laser cutting head, provided with a system for controlling the collisions of the cutting head. The invention further relates to a method for controlling collisions of a laser cutting head of a machine tool during its operation.

In the field of machine tools for processing metal sheets, plates, slabs the use of laser cutting heads is known and widespread for performing cutting, slicing and welding processes on workpieces.

As known, laser is a device able to produce a beam of monochromatic, spatially coherent light, i.e. focused in a rectilinear ray and having very high brightness (brilliance), by means of a process of stimulated emission. The possibility to concentrate a big amount of energy in a very restricted area enables laser devices to cut, slice, weld metals. Typically, cutting metal materials occurs through steaming, and above all, through fusion. In this last case, the laser fuses a small piece of metal and the fused metal (slag) is removed by a blow or jet of assist gas. More precisely, the laser beam or ray focused by a specific optical unit inside the cutting head exits therefrom through a cutting nozzle concentrating the assist gas jet that moves away slags generated by the metal fusion and reduces the likelihood that they reach the inner cutting head.

Different types of laser sources can be used to generate a beam of light suitable for cutting metals. Typically gas lasers (carbon dioxide C0 ₂ ) and solid-state lasers (doped glass laser diodes and fibre lasers) are used.

In the machine tools, due to high energies required for cutting sheets of even large thickness, the size and weight of laser emitting apparatuses are such that it is not possible to position them on the machine. The laser beam is thus focused on workpieces by a laser cutting head or focusing head that is connected to the emitting apparatus by means of an optical chain (C0 ₂ laser) or a transmitting fibre (optic fibre, for example in YAG laser diodes). Thanks to the reduced size and low weight, the laser cutting head can in fact be moved by the machine tool precisely and quickly to cut the piece. More precisely, the laser cutting head is mounted on a supporting structure of the machine tool and is moved by suitable actuators so as to be movable along three orthogonal axes X, Y, Z, the first two axes X, Y thereof are parallel to a horizontal work plane on which the workpiece is positioned and orthogonal between them, while the third axis Z is vertical and orthogonal to the aforesaid work plane.

During the operation of the machine tool, the laser cutting head is moved along the first two axes X, Y to perform the required processing and along the third axis Z to maintain a distance from the surface of the workpiece constant in order to compensate for planarity irregularities of the latter. Such distance is minimum in order to reduce power leakages of the laser beam in the air and to optimize the assist gas flow which removes the slag generated by the metal fusion.

In the laser cutting process collisions can occur between the cutting head (in particular between the cutting nozzle that represents the part of cutting head closer to the workpiece) and the workpiece, which can be divided in two types according to the mass of the bodies against which the cutting head collides: low inertia and high inertia collisions.

The first type of collisions is represented by impacts against "free" scraps and cutting residues which, by virtue of their small mass, are immediately pushed away from the cutting head with no damage to the latter and to the supporting structure of the machine tool and without requiring to stop the machine thereof as an emergency.

The second type of collisions is represented by impacts against scraps and residues that got welded to the workpiece during the cutting process or that got trapped among grids of the work plane and the workpiece, by impacts against bubbles of a protection film applied on the workpiece surface formed by the assist gas partially detaching the film itself (the latter being sufficiently strong for transferring motion to the piece), by impacts against stepped discontinuities of the workpiece (sheet metal) caused by scraps of material present on the grids of the work plane that make the latter non-planar.

In the second type of collisions, they are involved masses comparable to that of the entire piece or, if the friction between the workpiece and grids of the work plane is particularly high, comparable to the supporting structure of the machine tool.

As impacts are substantially inelastic, the kinetic energy of the laser cutting head is very quickly absorbed by the supporting structure which can consequently get deformed and/or damaged permanently. In particular, the impact can cause damage to the kinematic chain that moves the cutting head with consequent loss of calibration of mechanisms of the aforesaid kinematic chain, damages to fixing elements of the cutting head to the supporting structure of the machine, breakage of sensors present on the cutting head to detect the distance from the surface of the workpiece (capacitive sensors with ceramic components). In order to at least partially avoid this possibility it is necessary to dramatically reduce the interaction time between the cutting head and the obstacle by detecting and identifying as quickly as possible the collision event and reacting consequently, in particular moving quickly the cutting head away from the workpiece.

In the known machine tools provided with laser cutting heads, detecting the collision is implemented by monitoring a distance from the workpiece by a capacitive sensor mounted on the cutting head in a closed loop control system. In particular, a collision event is detected and signalled when the distance measured by the sensor is null for a predefined time range, typically in the order of tens of ms. This predefined time range is necessary to avoid the so- called "false-positive" due to low inertia collisions, as described above, or to the steaming process of the material composing the piece, generally of metal, which, can create an interference near the nozzle in measuring the distance. Such measure is also submitted to interferences (the so-called "background noise") and a delay due to the transduction processing chain of the analog signal emitted by the sensor.

Considering the intrinsic delay for detecting the collision, its intrinsic noise (background noise) and its extrinsic noise (false-positives), using the capacitive sensor is not a suitable solution to solve the problem of high-inertia collisions with the aim of dramatically reducing the interaction time between the cutting head and the obstacle.

Furthermore, the capacitive sensor provides an "isotropic" measure i.e. not related to the advancing direction of the cutting head on the work plane. The capacitive sensor is in fact able to detect abnormal quota variations, i.e. variation of the distance from the workpiece at 360° in the immediate proximity of the nozzle of the cutting head, and hence to identify the presence of obstacles on the work plane or on the surface of the workpiece. However, based on its operation physical principle it is not able to associate a precise direction on the plane to the quota variation and/or to the obstacle. Therefore, in case the capacitive sensor is used as anti-collision sensor, collision false alarms are generated anytime the cutting head is close to variations of quota and/or obstacles that do not lie along the movement direction of the cutting head, for example scraps that do not intersect the cutting direction and thus do not represent real collision obstacles for the cutting head.

Known machine tools, so-called CNC (Computer Numerical Control) machines comprise a control unit capable to control, in a precise and replicable way, position, speed and acceleration of the cutting head along three axes X, Y, Z by operating on respective actuators and motors. The latter ones are provided with sensors (encoders) capable of detecting the actual or real values of motion parameters, among which accelerations. However, it is not possible to use the values of actual decelerations measured by sensors of the actuators to detect a possible collision of the cutting head against an obstacle.

In fact the numeric control of the machine tool driving the cutting head considers possible decelerations of the latter as an abnormal load condition (for example an increase in the resistance of the kinematic chain due to wear and/or residues on components of the aforesaid kinematic chain) and at least within a short time after such decelerations, it attempts to compensate them increasing the pushing value of actuators (for example the torque in case of rotative electric motors). These variations of acceleration/deceleration occur frequently (and unforeseeably) in the operation of the machine tool and cannot obviously generate stop signals of the machine thereof.

However, in case decelerations are actually caused by a collision of the cutting head against an obstacle, the normal feedback control procedure of the path of the cutting head by the control unit must be absolutely avoided as the pushing increase on actuators (e.g., of the torque) induced by said control unit, in an attempt to restore the right nominal acceleration values, would submit the cutting head and the supporting structure of the machine tool to a further mechanical strain, which is very dangerous for the integrity of the latter ones.

An object of the present invention is to improve the known machine tools with laser cutting head, in particular machine tools provided with systems for detecting impacts and collisions of the cutting head.

Another object is to provide a machine tool with laser cutting head enabling to detect safely, precisely and efficiently during the cutting process possible impacts or collisions, particularly of the high inertia type, by the laser cutting head against obstacles, such as cutting scraps and residues, placed on a workpiece and/or on a working plane.

An additional object is to implement a machine tool capable of rapidly and promptly moving the laser cutting head away from the obstacle in case it detects an impact and collision with the latter such as to maintain the integrity of the cutting head and of the machine tool.

Still another object is to provide a method for detecting safely, precisely and efficiently in a machine tool provided with a laser cutting head, possible impacts and collisions which the latter can be submitted to.

Another additional object is to provide a method for controlling the handling of the cutting head such as to move it rapidly and promptly away from an obstacle in case it detects an impact and collision with the latter so as to maintain the integrity of the cutting head and of the machine tool.

In a first aspect of the invention a machine tool is provided with a laser cutting head according to claim 1.

In a second aspect of the invention a method for detecting and controlling collisions in a machine tool with laser cutting head according to claim 13 is provided.

The invention will be better understood and implemented referring to the enclosed drawings that illustrate an exemplary and non-limiting embodiment, wherein: Figure 1 is a perspective schematic view of the machine tool of the invention provided with a laser cutting head;

Figure 2 is a partial and enlarged view of the machine of Figure 1 that illustrates in particular a cutting nozzle of the laser cutting head in a collision condition;

Figure 3 is a block diagram relative to the method of the invention for detecting and controlling collisions of the laser cutting head.

Referring to Figure 1, a machine tool 1 is illustrated having a laser cutting head 2, for example feedable by a laser emitting apparatus, of the known type and not illustrated in the figures, by means of optical transmission means and able to perform cutting, slicing and welding processes on a workpiece 50, typically a sheet metal or metal plate. The laser cutting head 2, of the known type too and thus not illustrated in detail in the figures, is provided with a cutting nozzle 3 that enables to emit a laser beam operating on the workpiece 50 and a jet of assist gas that moves away slags generated by the metal fusion carried out by the laser beam and reduces the likelihood that they can reach the inside of the cutting head.

The machine tool 1 comprises actuating means 10 arranged to move the cutting head 2 along three orthogonal axes X, Y, Z that include one first axis X and one second axis Y that are parallel to a work plane 4 of the machine tool 1 on which the workpiece 50 can be placed and orthogonal between them, and one third axis Z orthogonal to the work plane 4.

The cutting head 2 is further supported by a supporting structure 6 of the machine tool 1 so that it is movable along the aforesaid three orthogonal axes X, Y, Z. In the embodiment illustrated for exemplary purposes, the supporting structure 6 comprises a portal 7 connected to the basement 8 of the machine tool 1 that supports also the work plane. The portal 7 slidably supports a support beam 9 movable along the first axis X which in turn slidably supports a first carriage 11. The latter is movable along the second axis Y and slidably supports a second carriage 12 to which the cutting head is fixed. The second carriage 12 is movable along the third axis Z.

The actuating means 10 of the known type and not illustrated in the figures operate in particular on the support beam 9, the first carriage 11 and the second carriage 12 to move the cutting head 2. In particular, the actuating means 10 comprise a first actuator 13, a second actuator 14 and a third actuator 15 which respectively move the supporting beam 9, the first carriage 11 and the second carriage 12 by means of respective transmission means, not shown. The machine tool 1 also includes control means 30, 35 connected to the actuating means 10 to control at least the latter ones such as to move the cutting head 2 along a defined working path T, for example for cutting the workpiece 50.

An accelerometer 5 is mounted on the cutting head 2 and it is arranged to detect and measure at least a measured acceleration a _zm of the cutting head 2 along the third axis Z and send a relative signal to the control means 30, 35 to which said accelerometer 5 is connected.

In the embodiment shown, the control means 30, 35 comprise a main control unit 30 adapted to control the whole operation of the machine tool 1 and in particular connected to the actuating means 10 to control the latter ones such as to move the cutting head 2 along the working path T.

As better explained in the hereinafter description, the control means 30, 35 are arranged to detect an existing collision condition CP, in which the cutting nozzle 3 collides against an obstacle 40, for example positioned on the workpiece 50 and/or on the work plane 4 when, after the aforesaid collision, the measured acceleration a _zm is different from a nominal acceleration a _zn along the third axis Z that is imposed by the actuating means 10 to the cutting head 2 moved along the working path T. In other words, the accelerometer 5 detects a sudden acceleration along the third axis Z, which, not being imposed or planned by the control means 30 (controlling the actuating means 10) in the movement of the cutting head 2 or being different from a nominal acceleration a _zn planned by the main control unit 30, identifies a high inertia impact or a collision against an obstacle 40. The latter is for example composed of a scrap which was welded to the workpiece 50 during the cutting process or interlocked between the work plane 4 and the workpiece 50, of a bubble of protection film formed by the assist gas partially detaching said film applied on the workpiece 50 surface, of a stepped discontinuity of the workpiece 50 caused by scraps of material present on the work plane and making the latter non-planar.

In the collision condition CP, i.e. as soon as the collision condition CP is detected by the control means 30, the latter ones are arranged to control the actuating means 10 so as to move the cutting head 2 along the third axis Z away from the work piece 50.

The accelerometer 5 is preferably a three-axial accelerometer capable of detecting and measuring linear accelerations of the cutting head 2 along the three orthogonal axes X, Y, Z, in particular measured accelerations a _xm , a _ym , a _zm .

The accelerometer 5 of the known type is mounted and fixed to the cutting head 2 such that it forms a single stiff body with the latter that is submitted to the same dynamic stresses, in particular the same accelerations. In order to compare at least the measured acceleration a _zm with the nominal acceleration a _zn along the third axis Z and identify a collision condition CP or no-collision condition CA (wherein the cutting head 2 does not collide against an obstacle 40), the control means 30, 35 of the machine tool 1 of the invention comprise a collision detector or estimator 20 adapted to implement an estimation algorithm, in particular of the linear type. The estimator 20 receives as input at least nominal accelerations a _xn , a _yn , a _zn along said three orthogonal axes X, Y, Z imposed by actuating means 10 to the cutting head 2 moved along the working path T and at least the measured acceleration a _zm along the third axis Z. The estimator 20 is thus capable of comparing at least such measured acceleration a _zm with the nominal acceleration a _zn along the third axis Z, hence identifying and providing as output the collision condition CP or no-collision condition CA.

Preferably, the estimator 20 is arranged to receive as input also positions x, y, z of the cutting head 2 along the three orthogonal axes X, Y, Z detected by the actuating means 10, operating parameters p _x , p _y , p _z of the actuating means 10 along the three axes X, Y, Z (for example power supply currents for actuating means 13, 14, 15) and measured accelerations a _xm , a _ym , a _zm along the three orthogonal axis X, Y, Z detected by the three-axial accelerometer 5 (Figure 3).

The estimator 20 can comprise a Kalman filter, of the known type and not described in further detail, which receives as input positions x, y, z, nominal accelerations a _xn , a _yn , a _zn , measured accelerations a _xm , a _ym , a _zm of the cutting head 2 along the three orthogonal axes X, Y, Z and operating parameters p _x , p _y , p _z of the actuating means 10 along the three orthogonal axes X, Y, Z and provides as output an estimate of the collision or no-collision condition, i.e. of the state of the dynamic system consisting in the cutting head 2 from the magnitudes input and/or measured, after filtering noise and interferences of the latter ones.

The estimator 20 is a mathematic model included i.e. inserted or implemented preferably in a secondary control unit 35 (microcontroller) of the control means 30, 35 of the machine tool which makes it possible to operate with sampling intervals having amplitude reduced with respect to those used by the main control unit 30 which controls the actuating means 10, such as to minimize a delay in detecting a possible collision. The secondary control unit 35 (microcontroller) is linked to the main control unit 30.

Alternatively, the estimator 20 can be included or implemented or inserted in the same main control unit duly structured and configured to operate with sampling intervals having amplitude reduced with respect to sampling intervals normally used to control the path, in order to minimize the delay in detecting a possible collision. As a consequence of the above, the estimator 20 is adapted to operate with sampling intervals having amplitude reduced with respect to sampling intervals used by control means 30, 35 for controlling actuating means 10 along the working path T.

The machine tool 1 of the invention also comprises one first controller 21, or operation controller, adapted to control the actuating means 10 so as to move the cutting head 2 with the best possible precision and accuracy, along the working path T without collision i.e. in a no-collision condition CA. The first controller 21 is incorporated in the main control unit 30 and it is capable of controlling the actuating means 10 according to the working path T set and calculated by an interpolator or path generator 23 based on the processing to be carried out on the workpiece 50. In particular, in the normal operation of the machine tool 1, the first controller 21 makes a compromise between the maximum dynamics of the motion and the precision of the motion thereof i.e. it performs the maximum correspondence of the real path of the cutting head 2 with the ideal path planned by the interpolator of the main control unit 30. For example, the first controller 21 in the cutting step and during movements along the first axis X and the second axis Y keeps the cutting head at an almost constant height (i.e. it keeps its position on the third axis Z constant) or it performs micro-movements along the third axis Z to follow the non-perfectly planar surface of the workpiece 50. In said cutting conditions, the planned acceleration along the axis z is substantially null or limited with respect to accelerations along the axis X and Y.

The main control unit 30, and more precisely the first controller 21, is able to feedback control the actuating means 10 i.e. to adjust the operation thereof based on the operative parameters p _x , p _y , p _z of the actuating means 10 along the three axes X, Y, Z (for example power supply currents of the actuators 13, 14, 15) and based on the values of the motion parameters (displacement, speed and acceleration) measured by suitable sensors mounted on the aforesaid actuating means 10 (for example encoders).

The main control unit 30 comprises, for example, an industrial computer of the known type and the first controller 21 and the interpolator 23 are parts of the management and control programme of the machine tool 1 object of the invention.

The machine tool 1 also comprises a second controller 22, or emergency controller, adapted to control the actuating means 10 to move the cutting head 2 at maximum speed and acceleration values along the third axis Z away from the work plane 4, in particular at a safety quota or height, in a collision condition CP, as in this case precision is not required in the working path T, but rather high speed in disengaging the cutting head from the collided obstacle 40. The second controller 22 is itself incorporated in the main control unit 30 and comprises a specific and distinct part of the management and control programme of the machine tool 1. Alternatively, the first controller 21 and the second controller 22 can match and constitute a single controller of the main control unit 30 which the calibration parameter set for controlling the actuating means 10 is rapidly switched to in case a collision condition is detected. More precisely, a first calibration parameter set can be provided to configure the single controller as the first controller 21, in normal operation of the machine tool 1, and a second calibration parameter set can be provided to configure the single controller as the second controller 22 in case a collision is detected.

The cutting nozzle 3 which is, as known, the part of the cutting head 2 closest to the workpiece 50 and to the work plane 4 has truncated cone shape so that its collision against an obstacle 40, even when the working path T is substantially parallel to the work plane 4 i.e. when the working head is moved along the first axis X and/or the second axis Y, generates in any case a reaction stress FR _Z on the cutting head 2 also along the third axis Z, thus determining an acceleration along such third axis Z measurable by the accelerometer 5 (measured acceleration a _zm ). More precisely, the impact of the cutting nozzle 3 against the obstacle 40 causes on the cutting head 2 and on the support structure 6 that supports the latter an impulsive-type reaction stress FR having components both along the first axis X and/or the second axis Y (F _RXy ) and along the third axis Z (FR _z ) (Figure2).

In the normal machine operation the main control unit 30 controls the actuating means 10 by the interpolator or path generator 23 and the first controller 21 so as to move the cutting head 2 along a defined working path T, which is characterized in cutting conditions by nominal acceleration a _zn values along the third axis Z that are almost inexistent, or very limited, and by a very reduced height - so-called "altitude" - above the workpiece 50 surface, defining a very hazardous operating condition from the viewpoint of collisions.

The main control unit 30 feedback controls the actuating means 10 based on actual values of motion parameters (displacement, speed and acceleration) measured by sensors mounted on the aforementioned actuating means 10 (for example encoders). Thereby, for example, possible decelerations of the cutting head along the first axis X and the second axis Y measured by the aforesaid sensors are considered as abnormal load conditions by the main control unit 30 (for example due to an increase in the resistance of the kinematic chain due to wear and/or residues on components of the aforesaid kinematic chain) which determine a procedure of reaction/rollback of the main control unit 30 in which the latter attempts to compensate the aforesaid decelerations increasing the pushing values of the actuators (for example the torque in case of rotary electric engines) to restore the correct nominal acceleration values.

These acceleration/deceleration variations can occur frequently in the machine tool normal operation.

The main control unit 30 is further programmed such as not to consider and assess in the feedback control of the actuating means 10 the measured acceleration/decelerations by sensors (encoders) along the third axis Z which exceed a predefined threshold value i.e. exceeding of a predefined tolerance value, values imposed or planned by the main control unit 30, i.e. the values of the nominal acceleration a _zn along said third axis Z imposed to the cutting head 2 by the actuating means 10. These accelerations may be ascribed in fact to a collision of the cutting head against an obstacle and if they are managed by the main control unit 30 as normal conditions of abnormal load (thus increasing the pushing force of actuating means in an attempt to restore the correct acceleration values) in the normal reaction/restoring procedure, the cutting head 2 and the support structure 6 of the machine tool would be submitted to an additional mechanical strain, in addition to that related to the collision, which is very dangerous for the safety of the whole machine tool. In particular a collision against an obstacle 40, even when the working path T is substantially parallel to the work plane 4, i.e. when the cutting head 2 is moved along the first axis X and/or the second axis Y, generates in any case a reaction stress FR _Z component directed upwards along the cutting head 2 also along the third axis Z. If the compensation of said component was possibly managed by the main control unit 30 it would cause the actuating means to act downwards with dramatic consequences for the integrity of the cutting head 2. Such compensation action would also remove (or reduce) the difference between said nominal acceleration a _zn and measured acceleration a _zm values jeopardizing the process of identifying the high inertia collision against an obstacle 40, by the estimator 20, which occurs, as mentioned above, exactly when said abnormal difference occurs between said acceleration values along the third axis Z. In other words, in said conditions, the compensation action by the main control unit 30 would interfere with the process of identifying the collision by the estimator 20. Consequently, in said conditions, said compensation action by the main control unit 30 is inhibited.

Consequently, in said conditions, accelerations/decelerations along the third axis Z measured by the accelerometer 5 mounted on board the cutting head 2 can be managed to control the machine tool 1 object of the invention by the estimator 20 of the secondary control unit 35 of the control means 30, 35. In fact, the aforesaid estimator 20 with sampling intervals having reduced amplitude compares the measured acceleration a _zm along the third axis Z detected by the accelerometer 5 with the nominal acceleration a _zn along the third axis Z calculated by the interpolator 23 and imposed by actuating means 10 to the cutting head 2 moved along the working path T. The estimator 20 itself is arranged to identify a possible collision condition CP, in which the cutting head 2, or more precisely the cutting nozzle 3, collides against an obstacle 40, when the measured acceleration a _zm is different from the nominal acceleration a _zn or a no-collision condition CA, in which the cutting nozzle 3 does not collide against an obstacle 40, when the measured acceleration a _zm is almost equal to the nominal acceleration a _zn .

In case the estimator 20 verifies a collision condition CP, the control of the actuating means 10 is switched to the second controller 22 which controls the actuating means 10 so as to move the cutting head 2 with the highest acceleration and speed along the third axis Z rapidly away from the workpiece 50, in particular to a safety quota or height from the obstacle 40, the main control unit 30 subsequently stopping the machine tool 1 in an emergency condition.

By contrast, in case the estimator 20 verifies a no-collision condition CA, the actuating means 10 keep on being controlled by the interpolator 23 and by the first controller 21.

The method according to the invention for controlling possible collisions of a laser cutting head 2 of a machine tool 1, the aforesaid cutting head 2 being provided with a cutting nozzle 3 for the exit of a laser beam operating on a workpiece 50 to be processed, comprises the steps of:

moving the cutting head 2 along a defined working path T by actuating means 10 of the machine tool 1 controlled by control means 30, 35 and operating along three orthogonal axes X, Y, Z, including a first axis X and a second axis Y that are parallel to a work plane 4 of the machine tool 1 on which said workpiece 50 can be positioned and orthogonal between them, and a third axis Z orthogonal to the work plane 4;

detecting and measuring by means of an accelerometer 5 mounted on the cutting head 2 at least one measured acceleration a _zm of the cutting head 2 along the third axis Z; comparing, by means of an estimator 20 of the control means 30, 35 with sampling intervals having amplitude reduced with respect to sampling intervals used by the control means 30, 35 to control the working path T, the measured acceleration a _zm with a nominal acceleration a _zn along the third axis Z imposed by the actuating means 10 to the cutting head 2 moved along the working path T ; identifying, by means of the estimator 20 with sampling intervals having amplitude reduced with respect to sampling intervals used by control means 30, 35 to control the working path T, a collision condition CP, in which said cutting nozzle 3 collides against an obstacle 40, when the measured acceleration a _zm is different from said nominal acceleration a _zn or a no-collision condition CA, in which the cutting nozzle 3 does not collide against an obstacle 40, when said measured acceleration a _zm is almost equal to said nominal acceleration a _zn .

The method further provides to control the actuating means 10 so as to move the cutting head 2 along the third axes Z rapidly away from the workpiece 50 in case of collision condition CP.

It is provided to detect measured by means of the accelerometer 5 accelerations a _xm , a _ym , a _zm along the three orthogonal axes X, Y, Z along which the cutting head 2 is moved.

The method provides to repeat the steps of comparing the measured acceleration a _zm with the nominal acceleration a _zn and identifying a collision condition CP or a no-collision condition CA with a definite sampling interval. Comparing accelerations and identifying collision conditions occurs by means of a collision detector or estimator 20 which implements an estimation algorithm, in particular of the linear type. The estimator 20 receives as input at least nominal accelerations a _xn , a _yn , a _zn along the three orthogonal axes X, Y, Z imposed by the actuating means 10 to the cutting head 2 moved along the working path T and at least the measured acceleration a _zm along the third axis Z. The estimator 20 provides as output information relative to the collision condition CP or no-collision condition CA.

Preferably, the estimator 20 is arranged to receive as input also positions x, y, z of the cutting head 2 along the three orthogonal axes X, Y, Z detected by the actuating means 10, operating parameters p _x , p _y , p _z of the actuating means 10 along the three axes X, Y, Z (for example power supply currents for the actuating means 13, 14, 15) and measured accelerations a _xm , a _ym , a _zm detected by the three-axial accelerometer 5 (Figure 3) along the three orthogonal axes X, Y, Z.

The estimator 20 includes a Kalman filter, i.e. it compares the measured acceleration a _zm and the nominal acceleration a _zn along the third axis Z and identifies a collision condition CP or a no-collision condition CA by means of a Kalman filter of the known type and not described in further detail. The Kalman filter receives as input the positions x, y, z, the nominal accelerations a _xn , a _yn , a _zn , the measured accelerations a _xm , a _ym , a _zm of the cutting head 2 along the three orthogonal axes X, Y, Z and operating parameters p _x , p _y , p _z of the actuating means 10 along the three orthogonal axes X, Y, Z and provides as output an estimate of the collision or no-collision condition, i.e. of the state of the dynamic system consisting in the cutting head 2 from the magnitudes input and/or measured, after filtering noise and interferences of the latter ones.

The method according to the present invention can thus provide that the step of comparing by means of the estimator 20 the measured acceleration a _zm with the nominal acceleration a _zn along the third axis Z, includes implementing an estimation algorithm, in particular comprising a Kalman filter.

The estimator 20 is a mathematic model introduced or implemented into a secondary control unit 35 (microcontroller) of the control means 30, 35 of the machine tool 1 which allows to operate with sampling intervals having amplitude highly reduced with respect to those normally used in the main control unit 30, which controls the entire operation of the machine tool 1, so as to reduce the delay in detecting the collision.

The method of the invention provides to control, in a no-collision condition CA (in which the cutting head does not collide against an obstacle and moves freely along the cutting direction T), the actuating means 10 by means of a first controller 21, or operation controller, so as to move the cutting head 2 with precision and accuracy along the working path T, and to control, in a collision condition CP in which the cutting head collides against an obstacle 40, the actuating means 10 by means of a second controller 22 so as to move the cutting head 2 away from the work plane 4, with maximum speed and acceleration values along the third axis Z in particular at a safety quota or height.

The method provides to feedback control the actuating means 10 through a main control unit 30 that adjusts the functioning of the aforesaid actuating means 10 based on operating parameters p _x , p _y , p _z of the latter ones along the three axes X, Y, Z (for example power supply currents of actuators 13, 14, 15) and based on actual or real values of motion parameters (displacement, speed and acceleration) measured by suitable sensors (for example encoders) mounted on the actuating means 10.

The main control unit 30 is further programmed so as not to consider and assess in the feedback control of the actuating means 10 the measured acceleration/decelerations by the sensors (encoders) along the third axis Z which exceed a predefined threshold value i.e. exceed of a predefined tolerance value, values imposed or planned by the main control unit 30, i.e. the values of the nominal acceleration a _zn along said third axis Z imposed to the cutting head 2 by means of the actuating means 10. Thanks to the machine tool 1 and the control method of the invention it is thus possible to safely, precisely and efficiently detect possible impacts and collisions, in particular high inertia ones, of the machine against obstacles represented by scraps and cutting residues placed on the workpiece and/or on the machine work plane, during the cutting process performed by a laser cutting head of the machine.

In particular, by virtue of the accelerometer 5 mounted on and in a single-piece with the cutting head 2, it is possible to detect accelerations along the third axis Z which are not imposed or planned by the main control unit 30 controlling the actuating means 10 in moving the cutting nozzle 2 or that are in any case different from nominal accelerations planned by the main control unit 30 and thus identifying an impact or collision against an obstacle 40. Detecting the collision (collision condition CA) is particularly rapid enabling to minimise the response delay by the machine since a specific secondary control unit 35 (microcontroller) is provided, which allows to operate with sampling intervals having amplitude highly reduced with respect to those normally used in the main control unit 30, which controls the entire operation of the machine tool 1.

Furthermore, using a second controller 22 which replaces the first controller 21 in case of collision in controlling the actuating means 10 allows to move the cutting head 2 along the third axis Z away from the work plane 4 with maximum speed and acceleration values, in particular at a safety quota or height, as in this case precision is not required in the direction for moving away (as in the normal machine functioning), but only high speed in disengaging the cutting head from the obstacle 40.

In case an impact or collision is detected it is thus possible to rapidly and promptly move the laser cutting head 2 away from the obstacle 40 so as to protect the integrity not only of the cutting head but also and foremost of the support structure 6 and of the actuating means 10 of the machine tool.