Method and system for determining and controlling the separation distance between a laser processing machine work head and the surface of a processed object by low coherence optical interferometry

Abstract

A method and system for determining and controlling a separation distance between a machine tool work head and a material surface is described, comprising: - generating a measuring beam of low-coherence optical radiation, directing the measuring beam towards the material and directing the measuring beam reflected or diffused from the material surface towards an optical interferometric sensor arrangement in a first direction of incidence, - generating a reference beam of low-coherence optical radiation and directing the reference beam towards the optical interferometric sensor arrangement in a second direction of incidence at a pre-set angle of incidence with respect to the first direction of incidence of the measuring beam; - superimposing the measuring beam and the reference beam on a common area of incidence of the sensor arrangement; - detecting a position of an interference fringe pattern between the measuring beam and the reference beam on the area of incidence; and - determining, based on the position of the interference fringe pattern along an axis of illumination of the area of incidence, an optical path difference between the measuring optical path and the reference optical path, which optical path difference represents a difference between (a) a current separation distance between the work head and the material surface and (b) a predetermined nominal separation distance.

Description

TECHNICAL FIELD

[0001] The present invention relates to the laser processing of materials, preferably metallic materials, and in particular to improving the control of the laser processing of materials, for example for laser cutting, drilling or welding of said materials, or additive manufacturing of a predetermined structure of said materials.

[0002] More specifically, the present invention relates to a method and a system for determining a separation distance between a work head in a machine for laser processing of materials and a material surface.

[0003] According to another aspect, the present invention relates to a machine for laser processing of materials, comprising a system for determining a separation distance between said work head and a material surface, provided for carrying out the above-mentioned method.

[0004] In the description and in the appended claims, the terms "material" and "metallic material" in the preferred embodiments are used to identify any product, for example a sheet or an elongated profile having a closed cross-section, for example hollow circular, rectangular or square, or an open cross-section, for example a planar cross-section or an L-shaped, C-shaped or U-shaped section, etc. In additive manufacturing, the term "material" indicates a raw material, typically a powder that is locally sintered or melted by a laser beam. BACKGROUND

[0005] In particular in the industrial processing of materials, sheets and metal profiles, lasers are used as thermal tools for various applications, depending on the parameters related to the interaction between the laser beam and the material being processed, in particular the energy density per unit volume of the laser beam incident on the material, and the interaction time.

[0006] For example, by directing a low density of energy (tens of W per mm 2 of surface) on a metal piece over a long time (several seconds), a hardening process occurs, while by directing a high density of energy (tens of MW per mm 2 of surface) on the same metal piece over a time of a few femtoseconds or picoseconds, a photoablation process occurs. In an intermediate range of increasing energy density and decreasing processing time, controlling these parameters makes it possible to achieve welding, cutting, drilling, engraving, marking processes.

[0007] In several processes, including drilling and cutting processes, it is necessary to generate a flow of auxiliary gas in the processing area where the laser beam interacts with the material, which has a mechanical function of driving the melting or a chemical function of assisting combustion, or even a technical function of shielding the processing area from the surrounding environment.

[0008] In additive processing, the material can be, for example, in the form of a filament, or in the form of a powder shot from a nozzle due to the flow of an auxiliary gas, or even, alternatively, in the form of a powder layer. This material is thus melted by laser radiation, so as to obtain, after re-solidification of said material, a three-dimensional mould.

[0009] In the field of laser processing of materials, laser cutting, drilling and welding are processes that can be carried out on the same machine, which is able to generate a focused high-power laser beam (typically a laser beam with a power density of between 1 kW / mm 2 and 10000 kW / mm 2 ) with a preset transverse power profile on at least one work plane of the material, and to control the direction of incidence and position of the beam along the material. The differences between the various types of processes that can be performed on the material are mainly due to the power of the laser beam used and to the interaction time between the laser beam and the material subjected to the process.

[0010] The laser processing machines according to the prior art are shown, for example, in Figure 1 and Figure 2 .

[0011] Figure 1 A machine for industrial processing using a CO2 laser with a light path of the laser beam in air is schematically shown, which comprises an emission source 10, for example a CO2 laser generator suitable for emitting a single-mode or multimode laser beam B, and a plurality of mirrors 12a, 12b and 12c suitable for directing the laser beam emitted by the emission source along a light path for the transmission of the beam towards a work head, indicated as a whole by 14, arranged in the vicinity of a material WP. The work head 14 comprises an optical system 16 for focusing the laser beam, which is typically composed of a focusing lens suitable for focusing the laser beam along a propagation optical axis which impinges on the metal material. A nozzle 18 is arranged downstream of the focusing lens, through which the laser beam directed towards the material work plane region passes. The nozzle is suitable for directing towards the material on the processing region an auxiliary gas beam emitted by a corresponding device (not shown). The auxiliary gas serves to control the execution of the process (drilling or cutting) and the quality of the process that can be obtained. For example, the auxiliary gas can comprise: oxygen, which promotes exothermic reactions with the metal, for example oxidation of the iron, which generates various types of iron oxides through endothermic reactions, thus releasing energy into the material contributing to the process, in addition to the energy released from the laser beam, so as to maintain a dynamic balance of the process, thus making it possible to increase the cutting speed; or inert gases such as nitrogen, which do not cause the material to melt, but act as a propellant for the material melting, protecting the (metal) material from unwanted oxidation at the edges of the processing profile, protecting the work head from possible splashes of the melt, and can also be used to cool the sides of the groove generated in the material, thus closing the extension of the heat-affected zone.

[0012] Figure 2 An industrial processing machine is schematically shown comprising a laser beam emitted in an optical fiber. The machine comprises an emission source 10, for example a laser generator which can emit a laser beam in a transmission fiber, for example a Ytterbium-doped fiber laser or a direct diode laser, suitable for emitting a single-mode or multimode laser beam, and a fiber guide 12d suitable for guiding the laser beam emitted by the emission source towards a working head 14 arranged in the vicinity of a material WP. At the working head, the laser beam exiting the fiber with controlled divergence is collimated by a dioptric collimation system 20 and reflected by a reflecting system 22, then focused along the optical propagation axis of the impacting material WP by an optical focusing system 16 generally consisting of a focusing lens.

[0013] As a first approximation, an ideal laser beam, i.e. a laser beam ideally collimated with parallel rays, is concentrated in a focused spot having finite dimensions at its waist downstream of the optical focusing system. Typically, in industrial processing uses, the best processing conditions are reached, even to the order of a tenth of a millimeter, in terms of material wall impacted by the beam and material wall from which the beam exits, when the transverse position of the plane corresponds to a precisely defined beam waist.

[0014] The power density distribution of a normally collimated laser beam is generally of Gaussian shape with rotational symmetry in the case of a single-mode beam, i.e. the power is concentrated around the longitudinal axis (Z axis) of the beam and decreases gradually along the peripheral coverage, or can be described as a Gaussian profile envelope with rotational symmetry in the case of a multimode beam.

[0015] In the field of high-power applications of lasers, the use of single-mode or multimode laser radiation beams, described in first approximation as Gaussian, responds to technical control needs. Indeed, the Gaussian beam is easily described with small parameters and can be easily controlled by propagation along the light transmission path of the emission source at the processing machine head, since it benefits from the property of propagating without changing the power distribution, so it can be described by the ray value and the divergence value in the far-field propagation conditions (in this case, the geometrical optics approximation can be used). In the near-field propagation conditions of the focused beam, along the working trajectory which is irrelevant to the geometrical optics approximation, the beam maintains the Gaussian shape of the power distribution in each of its cross-sections.

[0016] On the contrary, laser beams containing high-order transverse modes have a non-Gaussian power distribution. These conditions are generally obtained by using a dioptric system (transmission optical system, i.e. lens) or a reflecting system (reflection optical system, i.e. mirror), in which the dioptric system or the reflecting system modifies the shape of the beam coming from the Gaussian distribution.

[0017] The control of the propagation direction or of the transversal power distribution shape of the laser beam, for example in relation to the controlled distribution of the auxiliary gas or due to the separation distance between the work head and the material, to the work trajectory to be travelled and to the type of machining to be performed, is advantageous for the machining process, where the transversal power distribution shape is different from the Gaussian shape and can have a symmetry different from the rotational symmetry in the material machining area. For example, it is possible to control the power distribution of the laser beam by breaking the rotational symmetry of the beam, so that the power distribution can be identified or expanded, if necessary, in relation to the separation distance between the work head and the material and to the work trajectory.

[0018] It is clear that the control of the propagation direction of the laser beam or of the transversal power distribution shape of the laser beam must be as precise and repeatable as possible, so that the indicated advantages can be achieved. For this reason, it is necessary to move the work head and the proximal position of the work head with respect to the material, i.e. the position of the laser beam output, as well as the position of the nozzle for the outflow of the auxiliary gas with respect to the material, in particular with respect to the point at which the laser beam impacts the material, are controlled in real time with extreme precision based on the current machining conditions and the current position along the work trajectory. On the contrary, there is the risk of focusing the laser power in a work plane in which the material thickness is not ideal and of using an auxiliary gas with a pressure that is too high or insufficient at the material surface.

[0019] For these reasons, in the field of laser machining it is desirable to be able to accurately determine the separation distance between the work head, i.e. the proximal end of the work head with respect to the material, and the surface of said material.

[0020] When machining a metal material, it is known to use a capacitive sensor provided to detect the change in capacitance between the metal end of the work head, for example a hole in the nozzle, and the material surface.

[0021] Such a solution is given as an example in Figure 3 . This figure shows: a work head 14 according to one embodiment of the prior art, arranged at a separation distance d from the material WP being machined; and the relevant electronic unit (ECU) for controlling the machining. The laser beam generated by an emission source, which is transmitted to the work head 14 through an optical path in air with multiple reflections or in an optical fibre, is collimated towards an optical focusing system along an optical propagation axis in a direction of incidence on the material being machined, and emerges from the beam output 32, preferably at or downstream of a protective glass (not shown) suitable for protecting the focusing system from possible splashes of melt.

[0022] The expression "beam output" in the following description indicates a portion of the working head from which the machining laser beam emerges into open air, i.e. outside the head volume, propagating towards the material being processed, and can be the end of the optical focusing system or a protective structure thereof, or the conical end of a nozzle for supplying a flow of auxiliary gas for applications that require the supply of gas to the process. This portion can be considered as the distal portion of the working head (with reference to the entire machine), or as the end close to the material being processed, and these terms will be used equally in the description.

[0023] The mobile actuator device 40 is coupled to the working head 14 and is controlled by the unit ECU for controlling the machining by means of a servo motor 42, in order to control the mechanical parameters of the machining, for example controlling the movement of the machining head along the degrees of freedom conferred to it by the specific embodiment of the machine in order to follow the programmed work trajectory T on the material being processed, in particular for movements along the Z axis towards and away from the material based on the profile of the material or the machining profile. Means are also provided (not shown) for mechanically adjusting the optical focusing system, in order to calibrate the position of the system transversely to the beam propagation direction (X-Y axes) and in the beam propagation direction (Z axis).

[0024] The capacitive sensor associated with the working head 14 is indicated by 44 and is configured to detect the variation in capacitance between the end 32 of the working head close to the material, here the hole in the nozzle coinciding with the laser beam output, and the surface of the material WP placed at a reference potential. The capacitive signal detected by the sensor 44 is processed by the relative computer module 46 programmed to determine the separation distance between the working head and the material based on the acquired capacitance values, knowing the parameters of the machine and the process, and is forwarded from it to the unit ECU for controlling the process, in order to use the feedback to control the movement of the working head.

[0025] This technique is disadvantageous and not applicable when machining non-metallic materials.

[0026] This also lacks accuracy, since the capacitive effect is generated not only locally between the laser beam output and the corresponding point of incidence on the material, but also due to the large surfaces of the working head and of the material close to the laser beam output and the point of incidence. In the case of a material surface with a significant curve (positive or negative) close to the edge, or in the case of a working head approaching the material in a direction non-orthogonal to the local surface, it is necessary to run calculation algorithms for compensating the measured values, which are quite burdensome in terms of calculation, and in any case cannot completely compensate the complexity of the configurations that can be encountered in reality. SUMMARY

[0027] The present invention aims at providing a method of determining the separation distance between a work head of a machine for laser processing a material and the surface of said material, which is accurate and robust and not affected by the shape of the material operated on by the machine or by the processing conditions such as the relative speed and direction of translation of the work head with respect to the material.

[0028] Another object of the present invention is to provide a method of determining the separation distance between a work head of a machine for laser processing a material and the surface of said material over a wide range of measurable distances without compromising the measurement accuracy.

[0029] According to the present invention, these objects are achieved by a method of determining the separation distance between a work head of a machine for laser processing a material and the surface of said material. The machine operates with a high-power processing laser beam emitted by the work head and is guided along a work trajectory on the material comprising a succession of work zones. The method comprises the steps of: generating a measurement beam of low-coherent optical radiation, directing the measurement beam through the work head towards a work zone and directing the measurement beam reflected or diffused from the material surface in the work zone through the work head and along a first direction of incidence towards an optical interferometric measurement sensor device, wherein the measurement beam travels through a measurement optical path from the respective source to the optical interferometric measurement sensor device comprising a first portion comprised between the source and the work head and a second portion comprised between the work head and the interferometric measurement sensor device, the first portion and the second portion having respective predetermined and constant geometrical lengths; generating a reference beam of the low-coherent optical radiation and directing the reference beam along a second direction of incidence at a predetermined angle of incidence with respect to the first direction of incidence of the measurement beam towards the optical interferometric measurement sensor device, wherein, under nominal operating conditions in which the distance between the work head and the material surface corresponds to a predetermined nominal separation distance, the reference beam travels through a reference optical path having an optical path equal to that of the measurement optical path; superimposing the measurement beam and the reference beam along a predetermined irradiation axis on a common incidence area of the optical interferometric measurement sensor device; detecting the position of an interference fringe pattern along the irradiation axis on the incidence area between the measurement beam and the reference beam, wherein the extension of the interference fringe pattern along the irradiation axis corresponds to the coherence length of the low-coherent optical radiation; and determining the optical path difference between the measurement optical path and the reference optical path from the position of the interference fringe pattern along the irradiation axis of the incidence area, said optical path difference being representative of the difference between (a) the current separation distance between the work head and the material surface at the work zone and (b) the predetermined nominal separation distance.

[0030] The particular embodiments constitute the subject matter of dependent claims and their content is to be understood as integral part of the present description.

[0031] The present application also relates to a system for determining the separation distance between a work head of a machine for laser processing a material and a surface of said material. The machine operates with a high-power processing laser beam emitted by said work head and is guided along a work trajectory on the material comprising a succession of work areas. The system comprises: means for generating a measuring beam of low-coherent optical radiation; means for propagating said measuring beam, said means being adapted to direct said measuring beam, by means of said work head, towards a work area and for directing the measuring beam reflected or diffused by the surface of the material in said work area through said work head and towards an optical interferometric sensor device along a first direction of incidence, wherein said measuring beam travels through a measuring optical path from each source to said optical interferometric sensor device, said measuring optical path comprising a first portion between said source and said work head and a second portion between said work head and said interferometric sensor device, said first portion and said second portion having respective predetermined and constant geometrical lengths; means for generating a reference beam of said low-coherent optical radiation; means for propagating said reference beam, said means being adapted to direct said reference beam towards said optical interferometric sensor device along a second direction of incidence at a predetermined angle of incidence with respect to the first direction of incidence of said measuring beam, wherein, under nominal operating conditions in which the distance between said work head and said surface of the material corresponds to a predetermined nominal separation distance, said reference beam travels through a reference optical path having an optical path equal to that of said measuring optical path; wherein the means for propagating the measuring beam and the means for propagating the reference beam are arranged to superimpose said measuring beam and said reference beam on a common area of incidence of said optical interferometric sensor device along a predetermined axis of illumination; means for detecting the position of an interference fringe pattern between said measuring beam and said reference beam along said axis of illumination on said area of incidence, wherein the extension of said interference fringe pattern along said axis of illumination corresponds to the coherence length of said low-coherent optical radiation; and processing means arranged to determine, from the position of said interference fringe pattern along said axis of illumination of said area of incidence, an optical path difference between said measuring optical path and said reference optical path, said optical path difference being representative of the difference between (a) the current separation distance between said work head and the surface of the material at said work area and (b) the predetermined nominal separation distance.

[0032] The present application also relates to a machine for laser processing a material, comprising a system for determining the separation distance between a work head and a surface of the material, said system being configured to perform the method described above.

[0033] In summary, the present application is based on the application of the principle of optical interferometry.

[0034] The term "optical interferometry" denotes a variety of techniques that exploit the phenomenon of interference between a measurement beam and a reference beam that superimpose and produce interference fringes. The theory of optical interferometry in coherent light is well known and is used for relative comparisons between distances, but cannot give unique absolute measurement information of the distances, for example in case of a temporary interruption of the optical signal.

[0035] The present invention is inspired by the consideration that absolute distance measurements can be performed in the optical domain by using low-coherence interferometry techniques. Low-coherence interferometry is a simple technique for measuring distances between a probe and a target with high precision, and is based on a comparison between the distance travelled by a measurement beam that propagates from a source to a detector assembly, and that is emitted by the probe and back-reflected by the target, and the distance travelled by a reference beam that propagates from the source to the detector assembly under the condition of a known nominal distance between the probe and the target, and that is tuned to a reference path of the measurement path.

[0036] In low-coherence interferometry, the measurement beam and the reference beam are generated by a low-coherence source, such as a light-emitting diode (LED) or a superluminescent diode, and interference fringes between the above beams occur only when the respective optical paths or optical path lengths correspond, i.e. when the length of the measurement path corresponds to the length of the reference path within the coherence length range, the optical path being defined as the sum of the products between the geometrical lengths and the respective refractive indexes within each portion of the travelled optical path. Assuming that the length of the reference path is known, the length of the measurement path can be derived by detecting the presence of the interference fringes envelope, with a resolution of the order of the coherence length, typically in the micrometric range (from 5 um to 100 um).

[0037] This technique is particularly robust against optical noise, since light coming from other sources or from the laser machining process is non-coherently added to the interference signal without changing the interference fringes pattern. The measurement is applied locally at the point pointed by the measurement beam, and is independent from the morphology of the surrounding environment. This also allows to make precise absolute measurements of distances in a distribution that is substantially coaxial to the laser machining.

[0038] Advantageously, the low-coherence interferometry technique that detects the interference fringes pattern in the spatial domain is the most promising and more efficient in terms of operational flexibility for the purposes of the present invention, compared to detection in the temporal or frequency domain.

[0039] In fact, in low-coherence interferometry detected in the time domain, the interference fringe pattern is detected by a photodiode or by a photodiode array or similar acquisition screen, in such a way as to adjust the length of the reference path so as to reach the condition of correspondence of the lengths of the reference path and of the measurement path, with a tolerance of the order of the coherence length. In this case, the limitation of the available measurement range is associated with the adjustment of the length of the reference path, which is performed, for example, by means of the translation of a back-reflection element arranged along the path, the spatial range of translation of the back-reflection element of the reference path can be between a few microns and a few millimeters, the size of the range of translation can compromise the actuation speed or the complexity of the operation.

[0040] Although the time-domain detection technique is relatively simple to implement and allows an easy correspondence between the absolute optical paths of the measurement path and of the reference path to be achieved, this is not suitable for applications for the real-time measurement of industrial processing processes. In fact, for dynamic measurements, the length of the reference path must be constantly modulated to find the condition of correspondence of its length with that of the current measurement path, which leads to the appearance of the interference fringe pattern. This can be obtained by means of various types of control devices, including refractive index modulators or fast-acting mechanical actuators, such as piezoelectric actuators; however, these types of devices are rather expensive and very delicate, since they must operate at actuation speeds much faster than the sampling rate used for measuring the distance, typically higher than kHz, which is often not easily obtainable, especially in the range of large displacements.

[0041] A different detection technique is based on the Fourier transform relationship between the spectral density functions of the measurement beam and of the reference beam, by means of which it is possible to extract the differential distance measurement in real space from the spectral profiles of the two interference beams. In this way, the mechanical actuators are not required to align the length of the reference path with that of the measurement path. The single spectral acquisition of the superimposed measurement beam and of the reference beam can use a diffraction grating and a focusing lens downstream thereof, for projecting the spectral distribution of the interference beams onto a linear sensor device, for example a video camera. The spectra of the two interference beams show a periodic modulation, and the periodicity (frequency) of this modulation in the wavelength space varies as a function of the difference between the optical paths of the measurement path and of the reference path. An algorithm for calculating the Fourier transform is applied, for example the fast Fourier transform (FFT) algorithm, in order to extract the measurement of the peak of signal intensity related to the difference between the optical paths in real space.

[0042] This technique also requires high-quality optical elements that must be aligned with extreme precision and fast sensors to acquire the signal. Moreover, back-reflected signals can determine artifacts in the measurement, and the sensitivity of the acquisition can be reduced by the presence of autocorrelation signals, especially in the case of highly reflective surfaces. In order to calculate the absolute distance, it is necessary to quickly process the signal on the basis of the FFT algorithm, which requires specific computing tools.

[0043] Unlike low-coherence interferometry techniques in which the scan of the reference path length is in the time domain and the comparison information between the lengths of the measurement path and the reference path is encoded in the frequency domain in the wavelength space, low-coherence interferometry techniques detected in the spatial domain combine the previous two techniques and can directly visualize the measurement results in real space, so that they can be quickly acquired by economic devices such as image sensors (for example, linear sensors).

[0044] In a typical design of a low-coherence interferometry system detected in the spatial domain, the measurement beam and the reference beam impinge in superposition on the surface of a sensor device from different directions, and the surface of the sensor device is directly adapted to detect the interference fringe pattern resulting from this superposition. In this configuration, the spatial variation of the measurement optical path with respect to the reference optical path is directly displayed on the sensor device as a result of the mutual inclination angle of the two beams. Therefore, the measurement of the difference between the measurement optical path length and the reference optical path length can be simply extracted by detecting the position of the interference fringe pattern on the sensor device, the extension of which in the linear dimension of the sensor device is of the order of the coherence length of the beam optical radiation.

[0045] In low-coherence interferometry techniques detected in the spatial domain, the optical path length of each beam obliquely impinging on the common entrance area of the sensor device varies linearly with the position along the irradiation axis of the sensor device, therefore the difference between the measurement optical path and the reference optical path also varies linearly. The interference fringe pattern appears in a certain linear range of the image acquired by the sensor device, which corresponds to the condition that the optical path of the measurement path and the reference path are equal within the coherence length of the optical radiation, while in other areas of the sensor device the beams are incoherently superimposed. By detecting the envelope position of the interference fringe pattern along the linear extension of the sensor device, it is possible to extract a certain length of the measurement path.

[0046] This measure is limited only by the condition that the envelope of the interference fringe pattern is formed within the irradiation area of the sensor device, i.e. within the sensitive area of the photodetector apparatus forming the sensor device. The measurement range is determined by the beam inclination on the area of incidence, or better by the angle of incidence between them, and, in the case of identical beam inclinations, by the minimum of the number of photodetectors or photodetector areas (also called pixels of the sensor device) irradiated by the superimposed beams and the total number of photodetectors of the sensor arrangement, or by the minimum number of areas (pixels) that must be irradiated to demodulate the interference fringe pattern for the total number of areas (pixels) available on the sensor device. In common conditions, including sensor arrangements with thousands of photodetectors, a measurement range of a few tenths of a millimeter can be obtained before the interference fringe aliasing effect occurs. However, the inventors have demonstrated that the presence of the interference fringe pattern aliasing effect does not limit the measurement, but can in fact be used to increase the range of measurable distances, as will be described in more detail below. In fact, this subsampling system reflects on the effective demodulation of the fringe pattern at lower spatial frequencies, obtained in a similar way directly at the level of the photodetectors of the interferometric sensor arrangement, without the need to insert additional elements.

[0047] Advantageously, the use of an interferometric measurement technique that detects in the spatial domain makes it possible to use a static system of the measurement and reference optical paths for precise distance measurements, and to perform each individual acquisition or sampling of the spatial distribution of the optical radiation of the superimposed measurement and reference beams that impinge on the sensor device. To provide this type of system, standard optical elements are the only ones required, and the signals emitted by the sensor device are processed on the basis of simple calculation algorithms, so that the calculation effort is not great. Using this technique, the drawbacks of the frequency domain detection technique, i.e. the presence of the autocorrelation signal component, the artifact at negative frequencies and the reduction in sensitivity at high values of the measurable distance, are also overcome.

[0048] According to the present application, the application of the above considerations to a machine for laser processing of materials, in particular for laser cutting, drilling or welding or additive manufacturing, is achieved by means of an arrangement of an interferometric measurement system comprising a measurement optical path integrated at least in part in the working head and a reference optical path associated with the measurement optical path, which can also be integrated within the working head or outside it, wherein the measurement optical path comes out of the measurement head in the beam output area, or more generally at the end of the measurement head close to the surface of the material being processed. BRIEF DESCRIPTION OF DRAWINGS

[0049] The additional features and advantages of the present application will be more fully apparent in the following detailed description of one embodiment, given by way of non-limiting example, with reference to the attached drawings, in which:

[0050] Figure 1 and Figure 2is an example of a laser machining machine according to the prior art;

[0051] Figure 3 shows a schematic example of a laser machine working head and related control means close to the material being machined according to the prior art;

[0052] Figure 4a is a configuration scheme of a linear low-coherence interferometry system with spatial detection;

[0053] Figure 4b is a scheme of the length variation of the relative points of incidence of the measurement and reference optical paths on the irradiation axis of the sensor arrangement with respect to the interference fringe pattern;

[0054] Figure 4c is a scheme of the length difference variation of the relative points of incidence of the measurement and reference optical paths on the irradiation axis of the sensor arrangement with respect to the interference fringe pattern (upper graph), and the identification of the interference fringe pattern on the irradiation axis of the sensor arrangement in the condition of equal optical path of the measurement and reference paths (lower graph);

[0055] Figure 5 is a simplified diagram of a system for determining the separation distance between a machine working head for machining a material and the surface of the material, which is the subject of the present invention;

[0056] Figure 6 shows schematically the paths of the machining laser beam and of the low-coherence measurement beam in the working head according to a simplified embodiment of the working head;

[0057] Figure 7 is a detail of the relative position of the machining laser beam and of the measurement low-coherence beam at the output of the working head in the cutting or drilling area of the material;

[0058] Figure 8 is a graph showing the trend of the contrast or visibility value of a simulated interference fringe pattern as a function of the ratio between the spatial frequency of said interference fringe pattern and the spatial frequency of the photodetectors in the linear arrangement of photodetectors along the irradiation axis of the sensor arrangement of the interference fringe pattern;

[0059] Figure 9a is a graph showing the dependence between the results of the interferometric readings (expressed as optical path difference between the measurement and reference optical paths) and the pressure of an auxiliary gas (for example used in the process of laser cutting) formed along a portion of the measurement optical path for achieving a predetermined separation distance between the working head and the surface of the material;

[0060] Figure 9bis a graph showing the dependence between the results of interferometric readings (indicative of local changes in the position of the surface of the optical element protecting or delimiting the auxiliary gas chamber along the measurement beam) and the trend (increase, decrease) of the pressure of the auxiliary gas;

[0061] Figure 10a is a graph showing the main signal indicative of the recognition of the interference fringe pattern along the irradiation axis of the sensor arrangement as a function of the separation distance between the work head and the material, and the respective additional multiplexed signals indicative of the recognition of the respective additional interference fringe patterns along the irradiation axis of the sensor arrangement, the additional interference fringe patterns resulting from the travel along an additional measurement light path or reference light path having a different geometrical length than the geometrical length of the main measurement light path or reference light path, the surface of the optical element inserted along the light path of the machining laser beam comprising at least one partially back reflecting;

[0062] Figure 10b is an exemplary calibration curve indicative of the trend of the signal peak indicative of the main interference fringe pattern as a function of the separation distance between the work head and the material;

[0063] Figure 10c shows a series of graphs indicative of the measurements made during the cutting process. DETAILED DESCRIPTION

[0064] Figures 1 to 3 Reference has been made to the prior art and their content is hereby referred to insofar as they are generic to the controlled production of machining machines in order to carry out the method according to the teachings of the present invention.

[0065] Figure 4a is a schematic representation of the configuration of a low coherence interferometric measurement system with linear space detection. A measurement collimated beam of optical radiation denoted by M and a reference collimated beam of the same optical radiation denoted by R impinge at a predetermined angle of incidence a onto a common incidence area C of a sensor arrangement S, wherein they form a pattern of interference fringes F, the extension of which over the common incidence area is of the order of the coherence length of the optical radiation. The width of the optical radiation measurement collimated beam and the width of the optical radiation reference collimated beam are preferably of such a dimension as to illuminate substantially the entire sensor arrangement. In order to enhance the strength and the differentiation of the detected signal, these beams can be concentrated on the sensor in a direction perpendicular to the irradiation axis, for example by means of a cylindrical focusing lens.

[0066] The sensor arrangement S comprises a photodetector arrangement, for example along at least one irradiation axis (x-axis in the figure) of the incidence area. The photodetector arrangement is a linear or two-dimensional arrangement of photodetectors, preferably a linear arrangement. The irradiation axis of the incidence area is determined by the intersection between the plane defined by the angle of incidence of the measurement beam M and the reference beam R and the sensor surface of the sensor arrangement.

[0067] In Figure 4b the figure schematically shows the variation of the length p of the measurement and reference optical paths of the initial incident wavefronts of the respective measurement and reference beams on the common incidence area of the reference sensor arrangement S in a typical configuration in which the two incident beams are symmetrical on the sensor arrangement. The x-axis represents the position or x-coordinate along the illumination axis of the photodetector arrangement. Reference numeral pi represents the additional length of the first optical path (e.g. the measurement optical path of the measurement light radiation beam M) with respect to the initial incidence point of the wavefront of the measurement beam M at the first end xi of the common incidence area C, xi being the origin of the measurement axis. Reference numeral p2 represents the additional length of the second optical path (e.g. the reference optical path of the reference light radiation beam R) with respect to the initial incidence point of the wavefront of the reference beam R at the second end x2 of the common incidence area, x2 being opposite to the first end. Reference numeral Δp represents the difference pi - p2 between the additional lengths of the two paths, which is zero at the middle coordinate of the sensor arrangement and varies from the value Δp x1 at the end xi of the common incidence area to the value Δp x2 .

[0068] In Figure 4c the figure, the upper graph shows the curve Δp corresponding to the figure in Figure 4b , while the lower graph shows the identification of the interference fringe F pattern on the illumination axis (x) of the sensor arrangement S that occurs when the optical paths of the measurement and reference paths are equal. The envelope of the interference fringe F pattern is represented in shading and the specific difference Δp p between the path additional lengths of the measurement and reference beams is associated with the coordinate x p of the peak of the envelope of the upper graph.

[0069] P M and P R represent the measurement and reference paths, the total length of which can be represented as P M = pi + P1 and P R = p2 + P2, where P1 is the optical path of the measurement optical path from the low coherence light radiation source to the first wavefront incident on the sensor arrangement, P2 is the optical path of the reference optical path from the same low coherence light radiation source to the first wavefront incident on the sensor arrangement and is preferably constant. It can be considered that P1 is composed of P head + D standoff , where P headis the optical path length upstream and inside the working head, including a first portion between the low-coherence light radiation source and the end of the working head close to the material WP under processing (e.g. the laser beam output 32) and a second portion between the above-mentioned proximal end of the working head (e.g. the laser beam output 32) and the sensor arrangement S, having a certain predetermined and unaltered geometrical length, and D standoff is the separation distance in open air between the end of the working head close to the material WP under processing and the surface of said material. P2 is the optical path of the reference optical path, which is equivalent to the optical path of the measurement optical path under nominal operating conditions, hereinafter denoted as Pl nom where the distance between the proximal end of the working head (e.g. the laser beam output 32) and the surface of the material WP corresponds to the predetermined nominal separation distance D standoff_nom .

[0070] The optical path difference between the measurement optical path and the reference optical path is mathematically expressed as:

[0071] P M -P R

[0072] The interference fringes appear under the condition that this optical path is zero, i.e.:

[0073] P M -P R = 0

[0074] The relationship can be decomposed as:

[0075] P1+p1-(P2+p2) = 0

[0076] which can be written again as:

[0077] P head +D standoff +p1-P2-p2 = 0

[0078] From this it is derived that:

[0079] P head +D standoff -P2+Ap = 0

[0080] P head +D standoff -Pl nom +Ap = 0

[0081] P head +D standoff -P head -D standoff_nom +Ap = 0

[0082] Ap = D standoff_nom -D standoff

[0083] That is, (a) the current separation distance D between the working head and the material surface in the machining area. standoff (b) Nominal separation distance D standoff_nom The difference is equal to the difference in additional length between the measuring optical path and the reference optical path.

[0084] Therefore, the current separation distance between the working head 14 and the surface of the material WP, which is different from the nominal separation distance determined by the optical path difference between the measuring optical path and the reference optical path, can be attributed to the difference between the additional lengths of the measuring optical path and the reference optical path, and thus to the movement of the interference fringe pattern along the illumination axis x of the sensor arrangement S relative to the nominal position (e.g., the mid-plane of the sensor arrangement S).

[0085] Note that in laser cutting or drilling applications involving an auxiliary gas flow, the end of the working head closest to the workpiece is typically the end of the auxiliary gas nozzle, while in welding or additive manufacturing applications where no gas is supplied, the end of the working head closest to the workpiece is typically the output of the processing laser beam.

[0086] In the application forming the subject matter of this invention, the length of the reference optical path is set to correspond to the length of the measuring optical path at a predetermined nominal separation distance between the working head and the material surface within the processing area, and the difference between (a) the current separation distance between the working head and the material surface within the processing area and (b) the predetermined nominal separation distance is derived from the length difference between the measuring optical path and the reference optical path, which can be identified based on a portion of the interference fringe pattern along the incident region irradiation axis of the sensor arrangement S. Advantageously, the middle portion of the interference fringe pattern along the irradiation axis corresponds to the predetermined nominal separation distance. Alternatively, the end position of the interference fringe pattern along the irradiation axis can correspond to a zero nominal separation distance between the nozzle and the workpiece, which is equivalent to the contact between the nozzle and the material constituting the proximal end of the head, thereby allowing the separation distance between them to increase only, and thus the interference fringe pattern to move only toward the opposite end of the irradiation axis.

[0087] refer to Figure 4c In the image below, the interference fringe pattern is located along the illumination axis at position x. p It is the inherent position of the light radiation intensity envelope of the interference fringe pattern, and this inherent position of the light radiation intensity envelope of the interference fringe pattern is, for example, the position of the peak or maximum intensity of the light radiation envelope, or the middle position of a photodetector weighted by the light intensity of the fringe envelope.

[0088] The detection of the fringe envelope can be performed by an intensity profile demodulation technique, for example by applying a band-pass spatial filter or, sequentially, a high-pass filter and a low-pass filter, in order to reveal the only signal component corresponding to the spatial frequency of the interference fringes. For example, in a first step of processing the light intensity data, for example for the columns of the sensor matrix oriented to receive the interference fringe pattern aligned vertically, the light intensity detected by the sensor matrix is concentrated in a direction perpendicular to the development direction of the interference fringes (if the sensor arrangement is a linear arrangement of photodetectors focused by a beam-through cylindrical lens, this operation is not required). Subsequently, the signal generated by the photodetectors is normalized with respect to a background signal, for example extracted from an image without interference fringes. Therefore, a high-pass spatial filter is applied, for example to 1 / 5 of the photodetector spatial frequency, to remove the baseline and keep the interference fringe pattern. Since in this way a signal oscillating around zero is obtained, the absolute value of the signal is extracted, and therefore a low-pass spatial filter is applied, for example to 1 / 25 of the photodetector spatial frequency, to extract the interference fringe pattern envelope. The position of the interference fringe pattern is finally obtained by detecting the position of the fringe pattern envelope, either by finding its maximum or by comparing the envelope with a predetermined model function, for example a Gaussian function, and extracting the peak of the model function.

[0089] Figure 5 An example diagram of a system for determining the separation distance between a machine working head 14 for laser processing a material WP and the surface of said material, forming the subject of the present application, is shown, according to the currently preferred embodiment.

[0090] In the diagram, 100 indicates a source of low-coherence light radiation, for example an LED or a superluminescent diode, suitably with linear polarization, for example the low-coherence light radiation source works in the visible or near-infrared wavelength range. The light radiation emitted by the source 100 is injected into an optical waveguide, for example an optical fiber 140, downstream of a suitable optical isolator 120, and is transmitted to a beam splitter 160 suitable for generating, in a measurement optical path P M the measurement optical radiation beam M routed upstream, and in a reference optical path P R the reference optical radiation beam R routed upstream.

[0091] the measurement optical path P M and the reference optical path P R are guided paths and comprise optical guides, for example optical fibers, suitable for maintaining the same polarization of the beams along the entire path.

[0092] As mentioned above, the measurement optical path P Mis directed to the machine working head 14 for laser processing of a material and from the working head 14 towards the impacted processed material WP. The area from which the output measurement beam M is emitted corresponds to the area of the measurement head intended for measuring the distance of said measurement head from the above mentioned material, for example the opening in the nozzle for supplying the flow of auxiliary gas or the output of the laser beam.

[0093] Conversely, the reference light path P R is directed to the reflective return element 180, preferably through the interposition of the optical density filter 200, the optical dispersion compensation element 220, the λ / 4 plate 240 and the focusing lens 260. The optical reflection element 180 is arranged along the reference light path so that the optical path of this path from the beam splitter 160 to the optical reflection element 180 corresponds to the optical path of the measurement light path from the beam splitter 160 to the (reflective) surface of the processed material WP in operating condition, wherein said surface is located at a preset nominal separation distance D standoff_nom from the working head, i.e. from the end of the working head close to the material, for example the opening in the nozzle for the auxiliary gas or the output of the beam.

[0094] The measurement light path P M and the reference light path P R are such that the optical radiation travels in both directions along these paths, returning to the beam splitter 160 after reflection at the surface of the processed material WP and at the reflective optical element 180, respectively. In the reference light path P R , the passage of the reference beam R twice through the λ / 4 plate 240 causes a rotation of the linear polarization of the beam by 90°, so as to assume a linear polarization orthogonal to that of the measurement beam M. The beam splitter 160 therefore performs the recombination of the measurement beam and the reference beam and directs them along the detection light path P D (shared with a portion of the measurement light path and a portion of the reference light path) towards the sensor arrangement S in superimposed manner.

[0095] Both the measurement beam and the reference beam are directed through a cylindrical focusing lens 280, which is suitable for focusing the collimated beam in only one direction, in particular orthogonal to the irradiation axis of the sensor arrangement, with the aim of concentrating the signal along this axis, so as to optimize the irradiation of the photodetectors, and reach a polarizing beam splitter 300, which separates the measurement beam M from the reference beam R on the basis of the polarization of the measurement beam M and of the reference beam R, directing the first of these towards a first reflecting element Ml and the second of these towards a second reflecting element M2; in the last case, a λ / 2 plate 320 is inserted, suitable for restoring the original polarization. Thanks to this configuration, the first reflecting element Ml and the second reflecting element M2 direct the measurement beam and the reference beam respectively towards the sensor arrangement S, and more precisely towards the common incidence region of the sensor arrangement, at an incidence angle a. When the system is formed so that the reflecting elements Ml and M2 are able to move in translation along the propagation axis of the respective beam and in rotation about an axis perpendicular to the incidence plane (dashed position in the figure), the incidence angle a can advantageously be controlled within a preset range of values.

[0096] As mentioned above, the sensor arrangement S comprises a plurality of photodetector devices, each suitable for emitting a specific signal representative of the intensity of the light incident thereon, and these signals, as a whole, are transmitted to a processing device 350, which is configured for identifying the pattern of the interference fringes F formed on the common incidence region C of the sensor arrangement by acquiring the total incident light power of the superimposed measurement beam and reference beam.

[0097] Preferably, the measurement optical path and the reference optical path comprise corresponding optical elements, in particular the reference optical path comprises a reflecting return element whose reflecting and light diffusing properties correspond as much as possible to those of the surface of the material inserted in the measurement optical path. If necessary, light attenuator means and / or light disperser means can be provided, which are suitable for balancing the intensity and dispersion of the reference light radiation reflected by said reflecting return element with respect to the intensity of the measurement light radiation reflected by the material being processed.

[0098] By means of a system as in Figure 5 or an equivalent system, a method is performed for determining the separation distance between a machine working head 14 for laser processing a material and the surface of the material WP within a processing region defined along a predetermined working trajectory T and then by a processing laser beam emitted by the head.

[0099] The method comprises generating a measurement beam M of low coherence light radiation, which is directed by the working head 14 towards the processing region and is reflected or diffused by the surface of the material WP in the processing region, in a first incidence direction, towards a sensor arrangement S through the working head 14.

[0100] When processing metallic materials, it can be assumed that the measuring beam is reflected or diffused at the first surface of the material. In some cases (such as welding or additive manufacturing), instead of the surface of the solid (or substrate) to be welded, it is necessary to measure the distance from the surface of the molten pool, which represents the first surface of the molten metal. The internal subsurface layer of the material generates signals in non-metallic and translucent materials (ceramics, plastics, biological tissues, etc.) or painted metals.

[0101] In particular, the measuring optical radiation beam M travels along the measuring optical path from the source 100 to the sensor arrangement S, which includes two parts with specific predetermined and constant geometric lengths: a first part between the source 100 and the end of the working head 14 near the material WP, and a second part between the end of the working head 14 near the material WP and the sensor arrangement S.

[0102] The low-coherence optical radiation reference beam R is generated by the same source 100, and this beam is guided toward the sensor arrangement S at a second incident direction with a predetermined incident angle relative to the first incident direction of the measurement beam M. The reference beam R is along the reference optical path P. R As it travels, the reference optical path P R It has a measurement optical path P equivalent to that under nominal operating conditions. M The optical path length, wherein the distance between the working head 14 and the material WP corresponds to the predetermined nominal separation distance D. standoff_nom .

[0103] The measurement beam M and the reference beam R are superimposed on the common incident area C of the sensor arrangement S along a predetermined illumination axis. The position of the interference fringe F pattern between the measurement beam M and the reference beam R along the illumination axis on the common incident area C is detected by the processing device 350, and as described above, the measurement optical path P can be determined. M and reference optical path P R The optical path difference between (a) the current separation distance between the working head 14 and the surface of the material WP within the processing area and (b) the preset nominal separation distance.

[0104] This method can be executed in real time during processing to determine the separation distance between the working head and the currently processed area on the material, or it can be executed before or after processing, for example, to make the workpiece to be processed or the process that has already been executed qualified.

[0105] refer to Figure 6 and Figure 7 This schematically illustrates an exemplary embodiment of the paths of the machining laser beam B and the measuring beam M inside the working head and their relative positions when cutting or drilling a portion of the material WP.

[0106] Figure 6A reflective element deflecting the laser beam is shown, for example a dichroic mirror denoted DM, which deflects the propagation optical axis of the machining laser beam B from the head entry direction to the incidence direction on the material WP being machined. This is the configuration employed in one embodiment of a work head comprising a transverse laser beam input. In this embodiment, the measurement optical radiation beam M is guided to the measurement region of the material through a reflective optical scanning system SM or a fold mirror, through the dichroic mirror DM without significant deflection, the tilt of the reflective optical scanning system SM or fold mirror being piezoelectrically controlled for example on the basis of the absolute value and direction of the advancement rate of the work head along the work trajectory, to control the position of the measurement point of intersection with the surface. Downstream of the reflective optical scanning system SM a focusing lens FL is arranged, so that the position H of the measurement point of intersection with the material surface can be controlled. As shown, the propagation direction of the measurement beam can be controlled by the tilt of the reflective optical scanning system SM so as to be superimposed not coaxially with the machining laser beam B, but differently therefrom. The skilled person will understand that a "dual" or "opposite" configuration is also possible, in which a dichroic mirror is provided which is transparent to the machining laser beam but reflects the measurement beam coming from the transverse input.

[0107] Advantageously, in a machine for laser cutting, drilling or welding of materials or for additive manufacturing of three-dimensional structures by means of a laser, in which the machine comprises a work head comprising a nozzle for supplying a flow of auxiliary gas arranged in the vicinity of the material, the measurement optical radiation beam is guided through the nozzle and is directed towards a measurement region of the material coaxial with or in the vicinity of the current machining region, preferably in front of the current machining region according to the work trajectory.

[0108] Advantageously, in a machine for laser welding of materials or for additive manufacturing of three-dimensional structures by means of a laser, in which the machine comprises a work head comprising an output for a high-power machining laser beam, downstream of the optical system for focusing the laser beam, arranged in the vicinity of the material, the measurement optical radiation beam is guided through the above-mentioned beam output and is directed towards a measurement region of the material coaxial with or in the vicinity of the current machining region, preferably behind the current machining region according to the work trajectory.

[0109] The measurement beam M is incident coaxially to the machining laser beam B, for example for evaluating the drilling depth, the welding height and / or the depth of structured materials during additive manufacturing. The incidence of the measurement beam M in a set-back position relative to the machining region serves for verifying the welding quality or the additive deposition. The incidence of the measurement beam M in a front position relative to the machining region serves for early measurement of the separation distance of the head from the material during cutting and welding processes or for identifying the topography of the material surface after a spatial scan, for example to track the welding joint along the work trajectory. The latter configuration is shown in Figure 7 by way of example in Fig. 1, wherein N denotes a nozzle for supplying auxiliary gas, B denotes a machining laser beam incident within a current machining region of the material WP in which a cutting operation is performed according to the trajectory indicated by the arrow, thereby forming a groove G, and M denotes a measurement beam. In the cutting operation, the coaxial arrangement of the measurement beam M with the machining beam B makes the measurement very uncertain, as it will be at the cutting edge, where the wall of the groove in the material has a profile that can not be controlled depending on many machining parameters.

[0110] The improvements of the present invention will be described in the following description of the specification.

[0111] Advantageously, in a configuration in which the measurement beam and the reference beam impinge on a common incidence region of the sensor arrangement S, the incidence angle a is controlled and extended to the extent that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetector, to increase the range of measurable distances.

[0112] It is known in the art that, assuming that the measurement beam and the reference beam propagate as plane waves, the total light intensity as a function of the x-coordinate along the illumination axis of the sensor arrangement S, i.e. in the plane of the incidence angle, can be approximated as

[0113]

[0114] where I1and I2are the intensities of the respective beams, k f is the wave number or spatial frequency of the interference fringe pattern. The incidence angles of the measurement beam and the reference beam relative to the normal of the sensor arrangement are denoted a1and a2, and the spacing between the interference fringes is represented by the following equation:

[0115]

[0116] and therefore a greater tilt angle results in a higher frequency of the interference fringe pattern, and therefore a greater density of interference fringes on the sensor arrangement.

[0117] According to the prior art, in order to respect the Nyquist sampling criterion and avoid aliasing or sub-sampling phenomena, the spatial frequency of the photodetectors (indicated by k p ) must be at least twice greater than the frequency k f of the interference fringe pattern, i.e. k f / k p must be less than 0.5.

[0118] The information relating to the optical path difference can be extracted directly from the position of the interference fringe pattern envelope in the intensity distribution of the optical radiation incident on the sensor arrangement. N p represents the number of photodetector devices of the sensor arrangement illuminated by the superimposed measurement and reference beams; therefore, the maximum path difference that can be measured is

[0119]

[0120] Therefore, the measurement range is directly proportional to the number of illuminated photodetectors of the superimposed beams, which is a result of the resolution of the sensor arrangement and of the beam size. Therefore, an increase in the range of measurable differences between the paths can be obtained by increasing the number of photodetector devices, which can introduce greater costs in forming the sensor arrangement and greater expenses in processing the signals deriving therefrom. The range of measurable differences is also directly proportional to the k f / k p ratio between the frequency of the interference fringe pattern and the spatial frequency of the photodetectors. The k f / k p ratio depends on the angle of incidence between the measurement and reference beams and on the spatial size of the photodetectors; therefore, it is necessary to find a balance between the inclination of the beams and the number of illuminated photodetectors.

[0121] The inventors have noticed that, depending on the wavelength and coherence length of the low-coherence optical radiation used, several tens of interference fringes are usually visible in the interference fringe pattern formed in the common incidence region of the sensor arrangement. The acquisition of a large number of interference fringes in a wide measurement range while respecting the Nyquist criterion requires a large number of photodetectors, which leads to an excessive amount of information acquisition, since only the position of the envelope of the interference fringe pattern is relevant for determining the difference between the measurement and reference paths. As a result, the inventors have explored the possibility of demodulating the interference fringe pattern at increasingly small spatial frequencies, which can be obtained by increasingly large angles of incidence between the measurement and reference beams, so that the frequency of the interference fringe pattern becomes greater than the spatial frequency of the photodetectors, which is a condition for the occurrence of aliasing phenomena.

[0122] This method can increase the measurement range without losing information, with a constant number of photodetectors, simply by reducing the contrast of the interference fringes, since a single photodetector detects a plurality of fringes.

[0123] From the prior art it is demonstrated that the contrast v of the interference fringes depends on the aliasing factor k according to the following relation f / k p :

[0124]

[0125] and is zero at integer multiples of the spatial frequency k p of the photodetector, as shown in Figure 8 .

[0126] Advantageously, in order to have a local maximum contrast, the spatial frequency of the interference fringe pattern must be greater than the spatial frequency of the photodetector and different from a multiple of the spatial frequency of the photodetector, preferably close to a half-integer multiple of said spatial frequency of the photodetector.

[0127] In fact, it is clear from the graph in Figure 8 (illustrating the trend of the contrast v between the interference fringes, calculated as a function of the k f / k p ratio, in continuous line) that the local maximum is close to a half-integer multiple of the spatial frequency of the photodetector, but the maximum contrast rapidly decreases (qualitatively represented in the graph in discontinuous line).

[0128] Advantageously, the angle of incidence between the measurement beam and the reference beam can be chosen so as to obtain a k f / k p ratio equal to about 1.5 (or about 2.5, about 3.5, etc.).

[0129] The above invention can also be improved as described below.

[0130] In general, the propagation characteristics of a measurement beam of optical radiation are affected by the physical parameters (temperature, pressure, mechanical deformations) of the transmission device in which said beam propagates, mainly because the refractive index of the transmission device can vary as a function of these parameters.

[0131] In a machine for laser cutting, drilling or welding materials or for the additive manufacturing of three-dimensional structures by means of a laser (including a working head with a nozzle for supplying a flow of auxiliary gas, which wants to determine the distance between the nozzle and the material being processed), the measurement beam of optical radiation is made to propagate through the nozzle. The propagation characteristics of the measurement beam are therefore affected by the pressure of the auxiliary gas.

[0132] Figure 9aThe results of the interferometric readings (expressed as optical path difference between the measurement optical path P M and the reference optical path P R are shown as a function of the pressure of the auxiliary gas, which is established along a portion of the measurement optical path (typically in the chamber of the nozzle for supplying the auxiliary gas) for achieving a predetermined separation distance of 1 mm between the nozzle (of the working head) and the material surface. From the experimental values and the partially discontinuous interpolation curve carried out on two measurements, it can be seen that the dependence is practically linear.

[0133] In order to improve the accuracy of the method of the present application, the determination of the optical path difference between the measurement optical path and the reference optical path can thus preferably be based on a normalized optical path of the measurement optical path, which length is calculated from the geometrical length and the normalized refractive index of the portion of the measurement optical path passing through the auxiliary gas chamber or nozzle. The normalized refractive index calculation is calculated from the predetermined nominal dependence between the refractive index of the auxiliary gas and the pressure of the gas, according to the pressure of the auxiliary gas in the chamber.

[0134] For these reasons, the pressure of the auxiliary gas in the auxiliary gas chamber of the nozzle can be detected directly by a pressure sensor facing said chamber, or this can be derived indirectly from the measurement of the local position variation of the surface of the optical element for protecting or delimiting the auxiliary gas chamber along the axis of the measurement beam, according to a predetermined nominal relationship between the position of the surface of the optical element with respect to a certain predetermined nominal position and the pressure of the auxiliary gas.

[0135] The measurement of the local position variation of the surface of the optical element is determined, for example, from the difference in length between (a) a supplementary measurement optical path of said optical element, which comprises at least one of the following: (i) a first portion between the source of the measurement beam and the optical element whose first surface is impacted by said measurement beam in a partially back-reflected manner, and (ii) a second portion between the surface of the material being processed and the optical element whose second surface is impacted by said measurement beam in a partially back-reflected manner; and (b) a certain supplementary reference optical path, the optical path of which in the nominal operating state is equal to that of the supplementary measurement optical path of said optical element, which comprises the partially back-reflection of the measurement beam at said first or second surface of the optical element for obtaining a predetermined reference pressure value of the auxiliary gas inside the auxiliary gas chamber of the nozzle, when said optical element is located in a predetermined nominal position along the axis of the machining laser beam.

[0136] Figure 9bThe dependence between the interferometric reading results (expressed in terms of local position variation of the surface of the optical element protecting or delimiting the auxiliary gas chamber along the measurement beam axis) and the trend (increase, decrease) of the pressure of the auxiliary gas in the chamber described above is shown. Curve A represents the variation of the local position of the surface of the optical element protecting or delimiting the auxiliary gas chamber as a function of the increase in pressure inside the chamber. Curve B represents the variation of the local position of the surface of the optical element protecting or delimiting the auxiliary gas chamber as a function of the decrease in pressure inside the chamber.

[0137] More generally, the determination of the optical path difference between the measurement optical path and the reference optical path can be based on a normalized optical path of the measurement optical path, calculated from the geometric length and the normalized refractive index of a transmission device of said measurement optical path or of a portion of said measurement optical path, wherein the refractive index is calculated from a predetermined nominal relationship based on the temperature of said optical path portion.

[0138] Alternatively or in combination with the above, the determination of the optical path difference between the measurement optical path and the reference optical path can be based on a normalized optical path of the measurement optical path, calculated from the normalized geometric length and the refractive index of a material transmission device of a portion of said measurement optical path, wherein the normalized geometric length is calculated from a predetermined nominal relationship based on the mechanical deformation of said material transmission device.

[0139] Even more advantageously, the technique forming the subject of the present application makes it possible to determine a perturbation of the current optical path of at least one portion of the measurement optical path with respect to the current optical path of the corresponding portion of the reference optical path, and to correct the value determined for the separation distance between the work head and the material surface on the basis of said perturbation, for example by subtracting the perturbation measurement value (possibly after application of a correction factor) from the measurement value of the machining distance. The perturbation is formed, for example, as a result of a variation in at least one physical parameter of the transmission device in which the measurement optical path extends.

[0140] For these reasons, the measurement beam impinging on the sensor arrangement S comprises at least one measurement calibration beam generated by the travel along a measurement calibration optical path, wherein the measurement beam is reflected or diffused by at least one back-reflection surface of a static optical element inserted along the measurement optical path, and wherein the reference beam impinging on the sensor arrangement S comprises a specific reference calibration beam generated by the travel along a reference calibration optical path, the optical path of which is equivalent to the optical path of the measurement calibration optical path in a nominal calibration operating state, wherein the geometric length and the refractive index of the transmission devices of the measurement calibration optical path and of the reference calibration optical path are equal within a predetermined tolerance range. The static optical element can for example be the optical focusing system 16 of the laser beam.

[0141] The determination of the perturbation in the current optical path of at least one portion of the measurement optical path comprises the following steps:

[0142] - superimposing the measurement calibration beam and the reference calibration beam along the irradiation axis on a common incidence area of the sensor arrangement S;

[0143] - detecting a position of an interference fringe pattern between the measurement calibration beam and the reference calibration beam along the irradiation axis in the common incidence area; and

[0144] - determining, based on the position of the interference fringe pattern along the irradiation axis of the incidence area, a difference in optical path length between the measurement calibration optical path and the reference calibration optical path, which difference in optical path length represents (a) a difference between a geometrical length of the measurement calibration optical path and a geometrical length of the reference calibration optical path and / or (b)

[0145] a difference between a refractive index of the measurement calibration optical path and a refractive index of the reference calibration optical path. The difference in optical path length between the measurement calibration optical path and the reference calibration optical path represents the above-mentioned perturbation in the current optical path of at least a portion of the measurement optical path.

[0146] In case the position of the interference fringe pattern is detected, a predetermined reference position of the interference fringe pattern - corresponding to a condition of equal optical path length of the measurement calibration optical path and the reference calibration optical path - can be, by way of non-limiting example only, an intermediate position or an end position along the irradiation axis of the photodetector.

[0147] Another solution to increase the range of measurable differences between the lengths of the measurement path and the reference path by forming the subject of the present invention technique is to exploit partial back reflections at surfaces of at least one optical element inserted along the optical paths of the machining laser beam and the measurement optical radiation beam, or to exploit a reference optical path having a predetermined length different from the length of the main reference optical path.

[0148] In one embodiment, the measurement beam impinging on the sensor arrangement S comprises: a main measurement beam resulting from a travel along a main measurement optical path, which is reflected from the surface of the material in the machining area and is transmitted through each optical element inserted along the optical path of the high-power machining laser beam; and at least one additional multiplexed measurement beam resulting from a travel along an additional measurement optical path, which is reflected from the surface of the material being machined and the additional measurement optical path has a geometrical length greater than the geometrical length of the main measurement optical path, for example because it comprises at least one partial back reflection at a surface of an optical element inserted along the optical paths of the high-power machining laser beam and the measurement optical radiation beam.

[0149] In this embodiment, the inventive method is based on the detection of the position of an additional interference fringe pattern on the common incidence area C of the sensor arrangement S, which is determined by the interference between the additional measurement beam and the reference beam. The additional interference fringe pattern comprises, for example, (i) a peak or maximum intensity of the light radiation envelope, which is different (e.g. lower) than the peak or maximum intensity of the light radiation envelope of the main interference fringe pattern, or (ii) an intrinsic position of the light radiation intensity envelope, which is different than the intrinsic position of the light radiation intensity envelope of the main interference fringe pattern, in the case of a simultaneous occurrence with the main interference fringe pattern between the main measurement beam and the reference beam.

[0150] Figure 10a is a diagram showing the signal emitted by the photodetector along the illumination axis (vertical axis) of the sensor arrangement S, which represents the envelope intensity of the interference fringe pattern formed on the common incidence area. In particular, the diagram shows the interference measurement signal and thus the spatial position of the interference fringe pattern along the illumination axis (vertical axis) as a function of the separation distance between the working head and the material facing the working head (horizontal axis). Such a plot can be generated in a calibration step, for example, by providing a fixed length of the reference optical path and continuously varying the relative position between the working head and the material surface (i.e. the separation distance of the working head from the material surface) along the z-axis, and by taking readings of the interference measurement signal as a function of a predetermined discrete values of the separation distance.

[0151] The diagram shows the signal according to Figure 10b The shown approximate linear trend of the signal peak representing the interference fringe pattern, the acquisition of the strong interference measurement signal in the aliasing state and the translation of the interference fringe pattern envelope in the range of about 2 mm (corresponding to about 1500 pixels). The sensitivity can be defined as the separation distance corresponding to the size of the photodetector or pixel of the incidence area, in this case 1.5 um / pixel. The translation of the interference fringe pattern envelope over the entire illumination axis of the sensor arrangement makes it possible to determine the separation distance between the working head and the material surface from about 0.25 mm to about 2 mm.

[0152] In the circle area, the respective additional multiplexed signals are marked, which indicate the identification of the individual additional interference fringe patterns along the illumination axis of the sensor arrangement, which are generated by the travel along the additional measurement optical path or the reference optical path having a geometrical length greater than the geometrical length of the main measurement optical path or the reference optical path, which comprises at least one partial back reflection at the surface of an optical element inserted along the optical path of the machining laser beam.

[0153] In the case where the several separate operating ranges are not faced or superimposed on the sensor arrangement but are separated to a sufficient extent so as to show the respective interference fringes in alternation, the selection of the interference fringes is automatic due to the attainment of a separation distance between the working head and the material, in which only one of the main measurement beam and the additional measurement beam interferes with the reference beam, thereby generating an interference fringe pattern falling on the surface of the sensor arrangement S.

[0154] In the previous condition, therefore, the position of the additional interference fringe pattern along the axis of illumination of the incident region determines an optical path difference between the measurement optical path and the reference optical path, which represents the difference between (i) the current separation distance between the working head and the surface of the material in the machining region and (ii) the predetermined nominal separation distance.

[0155] In different embodiments, the reference beam impinging on the sensor arrangement S comprises a main reference beam generated by travel along a main reference optical path and at least one additional multiplexed reference beam generated by travel along an additional reference optical path having a geometric length different from that of the main reference optical path.

[0156] In this embodiment, the method of the present application is based on detecting a position on the common incident region of the sensor arrangement S, which is determined by the interference between the measurement beam and the additional reference beam.

[0157] In this case, the additional interference fringe pattern also comprises, for example: (i) a peak or maximum intensity of the light radiation envelope, which is different (for example lower) than the peak or maximum intensity of the light radiation envelope of the main interference fringe pattern; or (ii) an intrinsic position of the light radiation intensity envelope, which is different from the intrinsic position of the light radiation intensity envelope of the main interference fringe pattern, in the case where it appears simultaneously with the main interference fringe pattern between the main measurement beam and the reference beam.

[0158] In the case where the several separate operating ranges are not faced or superimposed on the sensor arrangement but are separated to a sufficient extent so as to show the respective interference fringes in alternation, the selection of the interference fringes is automatic due to the attainment of a separation distance between the working head and the material, in which only one of the main measurement beam and the additional measurement beam interferes with the reference beam, thereby generating an interference fringe pattern falling on the surface of the sensor arrangement S.

[0159] In the previous condition, therefore, the position of the additional interference fringe pattern along the axis of illumination of the incident region determines an optical path difference between the measurement optical path and the reference optical path, which represents the difference between (i) the current separation distance between the working head and the surface of the material in the machining region and (ii) the predetermined nominal separation distance.

[0160] From Figure 10aIt is understood that the illumination axis of the sensor arrangement S has a small extension, for example half the extension employed for the measurement in question, however, this will allow the separation distance between the working head and the material surface to be detected within the same value range (0.25-2 mm) from the signal readings representative of the main interference fringe pattern in the first range and from the signal readings representative of the interference fringe additional pattern in the second range.

[0161] Figure 10c A series of graphs representing the measurements made in a cutting process (in this example a process of cutting a square cutout having a width of 40 mm on a flat made of soft steel having a thickness of 3 mm) in which the nominal distance of the working head from the plate can vary between 1.3 and 1.2 mm is shown.

[0162] The upper graph shows the trend of a series of process parameters as a function of time. In particular, the curve represented by V x and V y represent the translation speed of the cutting head along the orthogonal directions x and y on the cutting plane, the curve represented by P L represents the light intensity of the machining laser beam, and the curve Pr represents the pressure of the auxiliary gas. The central graph shows the trend measured for the actual separation distance between the working head and the workpiece. The lower graph shows the trend of the interference fringes as a function of time obtained by the spatial domain detection technique.

[0163] The accurate determination of the separation distance between the working head and the material surface in the machining area (whether it is the current machining area or a calibration machining area) properly allows the unit ECU for controlling the laser machining machine to also use feedback to correct or control the machining distance or other machining parameters, for example acting on the movement actuator means 40 to control the movement of the working head, for example along the Z axis, towards or away from the material according to the interference measurements, for example to keep the distance between the working head and the material near a predetermined value according to a predetermined machining project. This is particularly useful, for example, to increase the efficiency of the cutting process. Alternatively or in addition, the determination of the separation distance between the working head and the material surface allows the unit ECU for controlling the process to use feedback to control the movement of the working head along an alternative axis to the Z axis, for example after a spatial scan, by acting on the movement actuator means 40, for example in order to keep the trajectory of the working head along a predetermined path suitable for the surface morphology being machined, thus also having a translation or tilting movement with respect to the surface. This is particularly useful, for example, to optimize a welding process due to the tracking of a welding joint.

[0164] It is noted that the design of the application proposed in the above discussion is purely exemplary and does not limit the application. Experts in the field will be able to easily implement the application in various embodiments which do not depart from the principles set out here and therefore fall within the scope of the present patent.

[0165] This is particularly true for the use of low-coherence light radiation wavelengths different from the cited wavelengths, or having a coherence length different from the cited coherence length. Figure 5 the possibility of measuring or referencing the optical path of the inserted optical element, different from the optical element shown purely by way of non-limiting example.

[0166] Of course, embodiments and implementation details can be subject to great modifications with respect to what is described and illustrated purely by way of non-limiting example, without departing from the scope of protection of the present application, as defined by the appended claims, without prejudice to the principles of the present application.

Claims

1. A method of determining a separation distance between a workhead in a machine for laser processing material and a surface of the material at a work area, the machine operating with a high power processing laser beam emitted by the workhead and directed along a work trajectory on the material comprising a series of work areas, characterised in that, The method comprises the steps of: - generating a measurement beam of low-coherence optical radiation, directing the measurement beam through the working head towards a working area, and directing a measurement beam reflected or diffused from a material surface in the working area through the working head and along a first direction of incidence towards an optical interferometric measurement sensor arrangement, wherein the measurement beam travels through a measurement optical path from the respective source to the optical interferometric measurement sensor arrangement, the measurement optical path comprising a first portion comprised between the source and the working head and a second portion comprised between the working head and the interferometric measurement sensor arrangement, the first portion and the second portion having respective predetermined and unvaried geometrical lengths; - generating a reference beam of the low-coherence optical radiation, and directing the reference beam along a second direction of incidence at a predetermined angle of incidence with respect to the first direction of incidence of the measurement beam towards the optical interferometric measurement sensor arrangement, wherein, under nominal operating conditions in which the distance between the working head and the material surface corresponds to a predetermined nominal separation distance, the reference beam travels through a reference optical path having an optical path equal to that of the measurement optical path; - superimposing the measurement beam and the reference beam along a predetermined irradiation axis on a common area of incidence of the optical interferometric measurement sensor arrangement; - detecting the position of an interference fringe pattern along the irradiation axis on the common area of incidence between the measurement beam and the reference beam, wherein the extension of the interference fringe pattern along the irradiation axis corresponds to the coherence length of the low-coherence optical radiation; and - determining from the position of the interference fringe pattern along the irradiation axis of the area of incidence an optical path difference between the measurement optical path and the reference optical path, the optical path difference being representative of the difference between (a) the current separation distance between the working head and the material surface at the working area and (b) the predetermined nominal separation distance.

2. The method of claim 1, wherein, The position of the interference fringe pattern along the irradiation axis is an intrinsic position of an optical radiation intensity envelope of the interference fringe pattern.

3. The method of claim 2, wherein, The intrinsic position of the optical radiation intensity envelope of the interference fringe pattern is the position of a peak or maximum of the optical radiation intensity envelope.

4. The method of claim 1 or 2, wherein, The optical interferometric measurement sensor arrangement comprises an arrangement of photodetectors along the irradiation axis, and the angle of incidence is controlled so that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetectors.

5. The method of claim 4, wherein, The spatial frequency of the interference fringe pattern is different from a multiple of the spatial frequency of the photodetectors.

6. The method of claim 1, wherein, The optical interferometric measurement sensor arrangement comprises an arrangement of photodetectors along the irradiation axis, and the arrangement of photodetectors is a linear arrangement of photodetectors.

7. The method of claim 1, wherein, The optical interferometric measurement sensor arrangement comprises an arrangement of photodetectors along the irradiation axis, and the arrangement of photodetectors is a two-dimensional arrangement of photodetectors.

8. The method according to any one of claims 1-3 and 5-7 in a machine for laser cutting, drilling or welding material or for additive manufacturing of three-dimensional structures by laser, wherein, The machine comprises a working head with a nozzle for dispensing a flow of auxiliary gas arranged in the proximity of the material and the measurement beam is directed through the nozzle and towards a material measurement region coaxial with or adjacent to the current working region.

9. The method according to any one of claims 1-3 and 5-7 in a machine for laser welding material or for additive manufacturing of three-dimensional structures by means of a laser, wherein, The machine comprises a working head with an output of a machining laser beam arranged in the proximity of the material and the measurement beam is directed through the output of the machining laser beam and towards a material measurement region coaxial with or adjacent to the current working region.

10. The method of claim 8, wherein, The measurement beam is directed towards the material measurement region through an optical scanning system whose inclination is controlled as a function of the absolute value and direction of the advancement rate of the working head along a working path.

11. The method of any one of claims 1-3 and 5-7, wherein, The intersection between the plane defined by the incidence angle of the illumination axis of the incidence region and the sensing surface of the optical interferometric sensor device determines the point of incidence of the measurement beam on the sensing surface.

12. The method of any one of claims 1-3 and 5-7, wherein, The measurement optical path and the reference optical path comprise corresponding optical elements, the reference optical path comprising a reflecting return element corresponding to the surface of the material inserted in the measurement optical path, and a light attenuator device adapted to balance the light radiation intensity of the reference beam reflected by the reflecting return element with respect to the light radiation intensity of the measurement beam reflected by the machined material.

13. The method of claim 12, wherein, The measurement optical path and the reference optical path originate from a common source, are separated by a beam splitting device, are directed individually to the surface of the machined material and to the reflecting return element, respectively, and are reunited in a detection optical path; In the detection optical path, the measurement beam and the reference beam are separated, the measurement beam and the reference beam being directed towards a common region of the optical interferometric sensor device with a controllable orientation which determines the incidence angle between the measurement beam and the reference beam.

14. The method of claim 1, wherein, In a machine for laser cutting, drilling or welding a material or for additive manufacturing of three-dimensional structures by means of a laser, the machine comprises a working head with a nozzle for dispensing a flow of auxiliary gas, wherein the measurement beam is directed through the nozzle, the determination of the optical path difference between the measurement optical path and the reference optical path being based on a normalized optical path of the measurement optical path calculated from the geometrical length and a normalized refractive index of a portion of the measurement optical path through an auxiliary gas chamber of the nozzle, the normalized refractive index being calculated as a function of the pressure of the auxiliary gas in the chamber based on a predetermined nominal relationship of the refractive index of the auxiliary gas as a function of the pressure of the gas.

15. The method of claim 1, wherein, The determination of the optical path difference between the measurement optical path and the reference optical path is based on a normalized optical path of the measurement optical path calculated from the geometrical length and a normalized refractive index of a portion of the measurement optical path, the normalized refractive index being calculated as a function of the temperature, pressure or other physical parameter of the transmission medium of at least a portion of the measurement optical path based on a predetermined nominal relationship of the refractive index as a function of the temperature, pressure or other physical parameter of the transmission medium of the measurement beam.

16. The method of claim 1, wherein, The determination of the optical path difference between the measurement optical path and the reference optical path is based on a normalized optical path of the measurement optical path, the normalized optical path being calculated from a normalized geometrical length and a refractive index of a material transmission medium of a portion of the measurement optical path, wherein the normalized geometrical length is based on the geometrical length depending on a predetermined nominal relationship of a mechanical deformation of the material transmission medium of the measurement beam, calculated from the mechanical deformation of the material transmission medium.

17. The method of any of claims 1-3, 5-7, and 13-16, comprising: determining a perturbation of a current optical path of at least a portion of the measurement optical path with respect to a current optical path of a corresponding portion of the reference optical path, and correcting the determined value of the separation distance between the working head and the material surface based on the perturbation, wherein the measurement beam incident on the optical interferometric sensor device comprises at least one calibration measurement beam resulting from a travel of a calibration measurement optical path, wherein the measurement beam is reflected or diffused by at least one retroreflective surface of a static optical element interposed along the measurement optical path, and wherein the reference beam incident on the optical interferometric sensor device comprises a corresponding calibration reference beam resulting from a travel of a calibration reference optical path having an optical path equal to that of the calibration measurement optical path, under nominal calibration operating conditions in which the geometrical length and the refractive index of the transmission medium of the calibration measurement optical path are equal to the geometrical length and the refractive index of the transmission medium of the calibration reference optical path within predetermined tolerance ranges, and wherein the determination of the perturbation of the current optical path of at least a portion of the measurement optical path comprises: - superimposing the calibration measurement beam and the calibration reference beam along a predetermined irradiation axis on a common incidence area of the optical interferometric sensor device; - detecting a position of an interference fringe pattern between the calibration measurement beam and the calibration reference beam along the irradiation axis on the common incidence area; and - determining the optical path difference between the calibration measurement optical path and the calibration reference optical path, respectively, from the position of the interference fringe pattern along the irradiation axis of the incidence area, the optical path difference being indicative of (a) a difference between the geometrical length of the calibration measurement optical path and the geometrical length of the calibration reference optical path, and / or (b) a difference between the refractive index of the calibration measurement optical path and the refractive index of the calibration reference optical path, the optical path difference between the calibration measurement optical path and the calibration reference optical path being indicative of the above-mentioned perturbation in the current optical path of at least a portion of the measurement optical path.

18. The method of claim 14, wherein, The pressure of the auxiliary gas within the auxiliary gas chamber of the nozzle is directly detected by a pressure sensor facing the chamber.

19. The method of claim 14, wherein, based on a predetermined nominal relationship between the position of the surface of the protected or delimited optical element with respect to the corresponding predetermined nominal position and the pressure of the auxiliary gas in the auxiliary gas chamber of the nozzle, the pressure of the auxiliary gas in the auxiliary gas chamber of the nozzle is indirectly derived from a measurement of the local position variation of the surface of the protected or delimited optical element of the auxiliary gas chamber along the axis of the measurement beam, the measurement of the local position variation of the surface of the protected or delimited optical element of the auxiliary gas chamber being determined from a difference in length between an additional measurement optical path for measuring the protected or delimited optical element and a corresponding additional reference optical path, wherein the additional measurement optical path comprises at least one of (i) a first portion between the source of the measurement beam and the protected or delimited optical element whose first surface is impacted in a partially back-reflected manner by the measurement beam and (ii) a second portion between the surface of the material being processed and the protected or delimited optical element whose second surface is impacted in a partially back-reflected manner by the measurement beam; and the optical path of the corresponding additional reference optical path under nominal operating conditions is equal to the optical path of the additional measurement optical path of the protected or delimited optical element, the additional reference optical path comprising a partially back-reflection of the measurement beam onto the first or second surface of the protected or delimited optical element to obtain a predetermined reference pressure value of the auxiliary gas in the auxiliary gas chamber of the nozzle, when the protected or delimited optical element is located at the predetermined nominal position along the axis of the processing laser beam.

20. The method of any one of claims 1-3, 5-7, 13-16, and 18-19, wherein, The measurement beam incident on the optical interferometric sensor device comprises: a main measurement beam resulting from the travel of a main measurement optical path, which is reflected from the surface of the material within the work area and transmitted through each optical element inserted along the optical path of the high-power processing laser beam; and at least one additional multiplexed measurement beam resulting from the travel of an additional measurement optical path, which is reflected from the surface of the material being processed, and the additional measurement optical path has a greater geometrical length than the geometrical length of the main measurement optical path, which comprises at least a partially back-reflection at the surface of the optical element inserted along the optical path of the high-power processing laser beam, The method comprises the following steps: - detecting the position of an additional interference fringe pattern on the common incidence area, the additional interference fringe pattern having: (i) a peak or maximum of light radiation intensity different from the peak or maximum of light radiation intensity of the main interference fringe pattern between the main measurement beam and the reference beam; or (ii) an intrinsic position of the light radiation intensity envelope that deviates from the intrinsic position of the light radiation intensity envelope of the main interference fringe pattern; and - determining the optical path difference between the additional measurement optical path and the reference optical path from the position of the additional interference fringe pattern along the irradiation axis of the incidence area, the optical path difference representing the difference between (i) the current separation distance between the work head and the surface of the material at the work area and (ii) the predetermined nominal separation distance.

21. The method of any one of claims 1-3, 5-7, 13-16, and 18-19, wherein, The reference beam incident on the optical interferometric sensor arrangement comprises a main reference beam resulting from the travel of a main reference optical path and at least one additional multiplexed reference beam resulting from the travel of an additional reference optical path having a geometric length different from the geometric length of the main reference optical path, The method comprises the steps of: - detecting the position of an additional interference fringe pattern on the common incidence area, the additional interference fringe pattern having (i) a peak or maximum of light radiation intensity different from the peak or maximum of light radiation intensity of the main interference fringe pattern between the measurement beam and the main reference beam or (ii) an intrinsic position of the light radiation intensity envelope deviating from the intrinsic position of the light radiation intensity envelope of the main interference fringe pattern; and - determining the optical path difference between the measurement optical path and the additional reference optical path from the position of the additional interference fringe pattern along the illumination axis of the incidence area, the optical path difference representing the difference between (i) the current separation distance between the work head and the material surface at the work area and (ii) the predetermined nominal separation distance.

22. The method of claim 5, wherein, The spatial frequency of the interference fringe pattern is close to a half-integer multiple of the spatial frequency of the photodetector.

23. The method of claim 8, wherein, The material measurement area precedes the current work area along the work trajectory.

24. The method of claim 9, wherein, The material measurement area follows the current work area along the work trajectory.

25. The method of claim 9, wherein, The measurement beam is directed to the material measurement area by an optical scanning system, the tilt of which is controlled according to the absolute value and the direction of the advancement rate of the work head along the work path.

26. A method of controlling the relative position between a work head of a machine for laser processing material and the material at a work area, the machine operating with a high power processing laser beam emitted by the work head and directed along a work path on the material comprising a series of work areas, characterised in that, The method comprises performing the method according to any one of claims 1 to 25 and moving the work head towards or away from the material or translating or tilting relative to the surface according to a predetermined work design and the determined separation distance between the work head and the material surface.

27. A system for determining a separation distance between a workhead in a machine for laser processing material and a surface of the material at a work area, the machine operating with a high power processing laser beam emitted by the workhead and directed along a work trajectory on the material comprising a series of work areas, characterised in that, The system comprises: - means for generating a measurement beam of low-coherence light radiation; - means for propagating the measurement beam, the means being adapted to direct the measurement beam through the work head towards a work area and to direct the measurement beam reflected or diffused by a material surface in the work area through the work head and along a first incidence direction towards an optical interferometric sensor arrangement, wherein the measurement beam travels through a measurement optical path from the respective source to the optical interferometric sensor arrangement, the measurement optical path comprising a first part between the source and the work head and a second part between the work head and the interferometric sensor arrangement, the first part and the second part having respective predetermined and constant geometric lengths; - means for generating a reference beam of the low-coherence light radiation; - means for generating a reference beam of the low-coherence light radiation; - means for propagating the reference beam, said means being adapted to direct the reference beam along a second direction of incidence at a predetermined angle of incidence with respect to a first direction of incidence of the measurement beam towards the optical interferometric sensor means, wherein, under nominal operating conditions in which the distance between the working head and the material surface corresponds to a predetermined nominal separation distance, the reference beam travels through a reference optical path whose optical length is equal to the optical length of the measurement optical path; wherein the means for propagating the measurement beam and the means for propagating the reference beam are arranged to superimpose the measurement beam and the reference beam along a predetermined axis of illumination on a common area of incidence of the optical interferometric sensor means; - means for detecting the position of an interference fringe pattern between the measurement beam and the reference beam along the axis of illumination on the common area of incidence, wherein the extension of the interference fringe pattern along the axis of illumination corresponds to the coherence length of the low-coherence light radiation; and - processing means arranged to determine, from the position of the interference fringe pattern along the axis of illumination of the area of incidence, an optical path difference between the measurement optical path and the reference optical path, said optical path difference being representative of the difference between (a) the current separation distance between the working head and the material surface at the working area and (b) a predetermined nominal separation distance.

28. A machine for laser processing material, operating by means of a high-power processing laser beam emitted by a working head and guided along a working trajectory on the material comprising a succession of working areas, and comprising means for controlling the relative position between the working head and the material, characterized in that, The machine comprises a system for determining the separation distance between the working head and the material surface at the working area, said system being arranged to perform the method according to any one of claims 1 to 25, the means for controlling the relative position between the working head and the material operating in accordance with a predetermined working design and with the determined separation distance between the working head and the material surface.

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