Method and system for determining the distance between a body and the surface of an object by means of low-coherence light interference technology under distortion caused by subsampling

Abstract

The present invention describes a method and a system for determining the spacing distance between an object or material and a processing tool or measuring instrument for the object or material, comprising: generating a measurement beam of low-coherence light radiation, directing the measurement beam towards the object, and directing the measurement beam reflected or diffused from the object towards an optical interference sensor mechanism in a first incident direction; generating a reference beam of low-coherence light radiation and directing the reference beam towards the optical interference sensor mechanism in a second incident direction at a preset incident angle relative to the first incident direction of the measurement beam; superimposing the measurement beam and the reference beam on a common incident area of the sensor mechanism; detecting the position of an interference fringe pattern between the measurement beam and the reference beam on the incident area; and determining the optical path difference between the measurement optical path and the reference optical path based on the position of the interference fringe pattern along the illumination axis of the incident area, which represents the difference between: (a) the current spacing distance between the processing tool or measuring instrument and the object surface, (b) a predetermined nominal spacing distance, wherein the sensor mechanism includes a photodetector arrangement along the illumination axis and controls the incident angle in such a way that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetector.

Description

Technical Field

[0001] The present invention generally relates to industrial processing methods or measurement methods, and more particularly to a method and system for determining the spacing distance between an object or material and the body of the object or material, such as a processing tool or measuring instrument of the object or material.

[0002] According to another aspect, the present invention relates to a processing machine tool for workpieces or materials.

[0003] According to yet another aspect, the present invention relates to a laser processing machine. Background Art

[0004] In this specification and the claims, the term "object" refers to a finished product being measured or a workpiece being processed. When applied to a machine tool, particularly to a laser processing machine, the term "workpiece" and, in the preferred embodiment, "metal part" are used to denote any product, such as a sheet or elongated profile having a closed cross-section (e.g., a hollow circular, rectangular or square cross-section) or an open cross-section (e.g., a flat cross-section or an L-shaped, C-shaped or U-shaped cross-section, etc.). The term "material" or "precursor material" in additive manufacturing denotes the raw material, which is usually a powder, that is subjected to sintering or local melting by a laser beam.

[0005] In industrial processes, it is common to bring a processing tool close to an object or material without contacting it, in order to process it at a certain distance, for example, by emitting radiation or a working fluid. It is also known in the prior art that during the manufacturing process of a product, a measuring instrument approaches a workpiece or material being processed, or a finished product, in order to detect any geometric features or physical properties thereof during an interruption, during the processing or at the end.

[0006] Taking an industrial processing method as an example, during the process of laser processing workpieces (particularly sheets and metal profiles), laser radiation is used as a heat treatment tool for several applications, depending on the parameters related to the interaction between the laser beam and the workpiece being processed, particularly the energy density per incident volume of the laser beam on the workpiece and the time interval of the interaction.

[0007] For example, by guiding a low energy density (on the order of tens of watts per square millimeter of surface) for a long time (on the order of seconds) on a metal part, a hardening process occurs, while by guiding a high energy density (on the order of tens of megawatts per square millimeter of surface) within a time on the order of femtoseconds or picoseconds on the same metal part, a photoetching process occurs. In an intermediate range with increasing energy density and decreasing processing time, welding, cutting, drilling, engraving, and marking processes can be achieved by controlling these parameters. These processes are carried out by emitting a laser beam from a working head, and the operating distance of the working head is far from the workpiece undergoing the process.

[0008] In many processes, including machining processes such as drilling and cutting, it is also necessary to generate an auxiliary gas flow in the processing area where the laser beam interacts with the material, which has a mechanical function of pushing the melt, a chemical function of assisting combustion, or even a technical function of shielding the environment around the processing area. The auxiliary gas flow is also ejected from a specific nozzle whose position is separated from the workpiece being processed.

[0009] In an additive process, due to the auxiliary gas flow, the material can be, for example, in the form of a filament, or in the form of powder ejected from a nozzle, or even alternatively in the form of a powder bed. Thus, the material is melted by laser radiation, and a three-dimensional mold is obtained after the material re-solidifies.

[0010] In the field of laser material processing, laser cutting, drilling, and welding are processes that can be performed by the same machine, which is capable of generating a high-power focused laser beam having a preset transverse power distribution on at least one processing plane of the material, typically a laser beam with a power density of 1 - 10000 kW / mm 2 and controlling the direction and position of the beam incident on the material, and controlling the direction of the auxiliary gas flow as needed. The differences between the various types of processing that can be performed on the material are basically attributed to the power of the laser beam used and the interaction time between the laser beam and the material being processed.

[0011] Figure 1 Shows a machine tool operating on the workpiece being processed.

[0012] Figure 1 Shows a processing and / or measuring head 10 of a machine tool, such as a laser processing machine for a workpiece or material, which is arranged at a distance d from the material WP being processed, and shows a relevant electronic unit ECU for controlling the processing or measurement. The reference numeral 12 generally represents a processing tool or measuring instrument, such as a tool for a mechanical process, a nozzle for ejecting a working fluid, an output end for processing radiation (such as high-power laser radiation for hardening, welding, cutting, drilling, engraving, marking, photoetching, or sintering the material) or a measuring probe.

[0013] The processing tool or measuring instrument 12 can be regarded as the distal part of the working head (with reference to the entire machine) or the proximal end close to the workpiece or material being processed, and these terms will all be used in the specification.

[0014] In a laser processing machine, the output end for processing radiation or the "beam output end" is part of the working head, from which the processing laser beam is emitted into the air, that is, it propagates towards the workpiece or material being processed outside the volume of the working head, and can be the end of an optical focusing system or its protective structure, or the conical end of a nozzle for supplying an auxiliary gas flow for applications that require gas supply during the process.

[0015] The motion actuator mechanism 14 is coupled to the processing and / or measuring head 10 and is controlled by the unit ECU to control the process by means of the servo motor 16 in order to control the mechanical parameters of the process, such as controlling the movement of the working head along the degrees of freedom imparted by a particular embodiment of the machine, so as to follow a programmed working trajectory T on the workpiece or material, in particular controlling the movement towards and away from the workpiece or material along the Z-axis based on its profile or processing profile.

[0016] In an industrial process where a processing or measuring instrument approaches an object or material being processed, the result of the processing or measurement depends on the correct distance between the processing instrument and the object or material. For example, when laser processing a material, specifically when controlling the laser processing of a metallic material to cut, drill or weld the material by laser, or when performing additive manufacturing of a predetermined structure from a precursor powder material, the emphasis is on maintaining a controllable distance between the processing instrument and the material.

[0017] The reference numeral 20 denotes a capacitive sensor associated with the working head 10 which, in the case where the workpiece or material being processed is metallic or conductive, is configured to detect a change in capacitance between the end 12 of the working head approaching the workpiece or material and the surface of the workpiece or material WP at a reference potential. The capacitance signal detected by the sensor 20 is processed by an associated computer module 22 which is programmed to determine the spacing distance between the working head and the workpiece or material based on the acquired capacitance value and to learn the parameters of the machine and the process, and then to forward them to the process control unit ECU to control the movement of the working head using feedback.

[0018] The drawback of this technique is that it is not applicable to processing or measuring non-metallic objects or materials.

[0019] Moreover, it lacks precision because the capacitive effect is not only locally generated between the end of the processing or measuring instrument and the corresponding incidence point on the workpiece or material, but may also be generated due to the relatively large surfaces of the working head and the workpiece or material close to the end of the instrument and the incidence point. If there are significant curves (positive or negative) on the workpiece surface, it is necessary to run an algorithm to calculate the measurement compensation near the edge, or in the case where the working head approaches the workpiece in a direction non-orthogonal to the local surface, the measurement is computationally quite cumbersome and in any case cannot fully compensate for the configuration complexity that may be encountered in reality. SUMMARY OF THE INVENTION

[0020] The object of the present invention is to provide a method for determining the spacing distance between a workpiece or material and a processing or measuring mechanism of said workpiece or material, such as between the working head of a laser processing machine of a workpiece or material and the surface of said workpiece or material, which method is accurate and robust and is not affected by the shape of the workpiece or material on which the machine operates or by the processing conditions (such as the relative translational speed of the working head with respect to the workpiece or material).

[0021] An additional object of the present invention is to provide a method for determining the spacing distance between a workpiece or material and a processing or measuring mechanism for the workpiece or material within a relatively large measurable distance range without compromising the measurement accuracy.

[0022] The present invention also relates to a system for determining the spacing distance between an object or material and a processing or measuring mechanism for the object or material.

[0023] An additional subject matter of the present invention relates to a processing machine tool for a workpiece or material and a laser processing machine for a workpiece or material, which include a system for respectively determining the spacing distance between a processing tool and the surface of the workpiece or material (between the working head and the surface of the workpiece or material) within a predetermined processing area, and the system is configured to perform the above method.

[0024] In summary, the present invention is based on the application of the optical interferometry principle, especially on the low-coherence optical interferometry technology, in which the measurement range is extended by using the distortion effect caused by subsampling.

[0025] The term "optical interferometry" refers to a variety of techniques that utilize the interference phenomenon between a measurement beam and a reference beam, where these beams are superimposed to generate interference fringes. The theory of optical interferometry in coherent light is well-known and is used for relative comparison between distances, but in the case of a temporary interruption of the optical signal, for example, it cannot give absolute and unambiguous measurement information about the said distance.

[0026] The inspiration for the present invention comes from the consideration that absolute distance measurement can be performed in the optical domain by using low-coherence interferometry technology. Low-coherence interferometry is a simple technique for measuring the distance between a probe and a target with high precision. This technique is based on the comparison between the distance that the measurement beam travels from the light source to the detector assembly (the measurement beam is emitted by the probe and back-reflected by the target on this optical path) and the distance that the reference beam travels from the light source to the detector assembly (the reference beam travels on the reference optical path tuned to the measurement optical path) under the condition of the nominal distance between the known probe and the target.

[0027] In low-coherence interferometry, the measurement beam and the reference beam are generated by a low-coherence light source (such as an LED or a superluminescent diode), and the interference fringes between the above beams only appear when the optical paths or the optical path lengths of the corresponding optical paths are corresponding, where the optical path is defined as the sum of the products of the geometric path in each part along the entire optical path and the respective refractive indices, that is, when the optical path length of the measurement optical path corresponds to the optical path length of the reference optical path within the coherence length range. Assuming that the optical path length of the reference optical path is known, the optical path length of the measurement optical path can be obtained by detecting the envelope where the interference fringes exist, and the resolution order of its coherence length is usually in the micron range (from 5μm to 100μm).

[0028] This technique is particularly robust to optical noise because light from other light sources or, for example, from a laser processing process is added incoherently to the interference signal without altering the interference fringe pattern. This measurement is locally applied to a single point where the measurement beam is directed, regardless of the morphology of the surrounding environment. This also allows for precise absolute distance measurement in a direction substantially coaxial with the axis of the instrument carried by the working head of the machine tool.

[0029] Advantageously, compared to detection in the time domain or frequency domain, the low-coherence interferometry technique for detecting an interference fringe pattern in the spatial domain is most promising and effective in terms of the operational flexibility for the purposes of the present invention.

[0030] In fact, in low-coherence interferometry including detection in the time domain, the interference fringe pattern is detected by a photodiode or a photodiode array or by a similar acquisition screen. This is achieved by adjusting the optical path of the reference optical path so as to achieve the condition that the optical paths of the reference optical path and the measurement optical path correspond to each other, except for tolerances of the order of the coherence length. In this case, the available measurement range is limited by the adjustment of the optical path of the reference optical path, which is performed, for example, by translating a retroreflective element arranged along the above optical path. The translation range of the retroreflective element of the reference optical path may be between a few micrometers and a few millimeters, and the size of the translation range may impede the actuation speed or operational complexity.

[0031] Although the implementation of the detection technique in the time domain is quite simple and it is easy to achieve the correspondence between the absolute optical paths of the measurement optical path and the reference optical path, this is not suitable for applications that measure industrial processes in real time. In fact, for dynamic measurements, the optical path of the reference optical path must be continuously modulated to find the condition where it corresponds to the current optical path of the measurement optical path that causes 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 expensive and very precise because the actuation speed at which they operate must be much faster than the sampling rate for distance measurement, typically up to the kilohertz level or above, and this condition is usually not easily achieved, especially in the range of large displacements.

[0032] A different detection technique is based on the Fourier transform relationship between the spectral density function and the cross-correlation of the measurement beam and the reference beam, whereby the differential distance measurement result in real space can be extracted from the spectral profiles of the wavelengths of the two interfering beams. In this way, there is no need for a mechanical actuator to align the optical path of the reference light with that of the measurement light path. A single spectrum acquisition of the superimposed measurement beam and reference beam can be performed using a diffraction grating and a focusing lens downstream thereof to project the spectral distribution of the interfering beams onto a linear sensor device (such as a camera). The spectra of the two interfering beams show periodic modulation, and the periodicity (frequency) of this modulation in the wavelength space changes with the optical path difference between the measurement light path and the reference light path. To extract the measurement result of the signal intensity peak regarding the optical path difference in real space, an algorithm for calculating the Fourier transform, such as the FFT algorithm, is applied.

[0033] This technique also requires high-quality optical components, which must be consistent with sensors of extremely high precision and speed to acquire signals. Additionally, the back-reflected signal may determine the artifacts in the measurement result, and the acquisition sensitivity may decrease due to the presence of the autocorrelation signal, especially in the case of highly reflective surfaces. To calculate the absolute distance, it is necessary to quickly process the signal based on the implementation of the FFT algorithm, which requires dedicated computing tools.

[0034] In the time domain, the scanning of the optical path of the reference light in the low-coherence interferometry technique is distributed in time, while in the frequency domain, the information for comparing the optical paths of the measurement light path and the reference light path is encoded in the wavelength space. Combining the low-coherence interferometry technique for detection in the spatial domain with the above two techniques can directly visualize the measurement result in real space, enabling rapid acquisition with an economical device such as an image sensor (such as a linear sensor).

[0035] In a typical design of a low-coherence interferometry system including detection in the spatial domain, the measurement beam and the reference beam are projected from different directions and superimposed on the surface of the sensor mechanism, and the surface of the sensor mechanism is directly adapted to detect the interference fringe pattern. In this configuration, due to the mutual inclination angle of the two beams, the spatial variation of the measurement light path relative to the reference light path is directly displayed on the sensor mechanism. Therefore, by detecting the position of the interference fringe pattern on the sensor mechanism, and the extension of the interference fringe pattern in the linear dimension of the sensor mechanism is of the order of the coherence length of the light radiation of the beam, the measurement result of the difference between the optical path of the measurement light path and the optical path of the reference light path can be simply extracted.

[0036] In low coherence interferometry techniques that perform detection in the spatial domain, the optical path length of each light beam obliquely incident on the common incident area of the sensor mechanism varies linearly with the position along the illumination axis of the sensor mechanism. As a result, the difference in the optical path between the measurement optical path and the reference optical path also varies linearly. An interference fringe pattern appears within a specific linear range of the image acquired by the sensor mechanism, which corresponds to the condition where the optical path lengths of the measurement optical path and the reference optical path are equal within the coherence length of the light radiation. In other areas of the sensor mechanism, the light beams are superposed incoherently. By detecting the position of the envelope of the interference fringe pattern along the linear extension of the sensor mechanism, the corresponding optical path length of the measurement optical path can be extracted.

[0037] This measure is only limited by the following conditions: within the illumination area of the sensor mechanism, that is, within the sensitive area of the photodetector device forming the sensor mechanism, the envelope of the interference fringe pattern is obtained. The measurement range is determined by the inclination of the light beams on the incident area or, more preferably, by the incident angle between the light beams and the spatial resolution of the photodetection area of the interference image (also known as the pixels of the sensor mechanism), or by the minimum number of areas (pixels) that must be illuminated to demodulate the interference fringe pattern relative to the total number of available areas (pixels) on the sensor mechanism. Under common conditions including a sensor arrangement with thousands of photodetectors, a measurement range of a few tenths of a millimeter corresponding to the subsampled state of the interference fringes can be obtained before the aliasing effect occurs. However, the present inventors have confirmed that the presence of the aliasing effect in the interference fringe pattern does not limit the measurement, but can actually be used to expand the range of measurable distances. In fact, this subsampling system becomes effective in demodulating the fringe pattern at a lower spatial frequency, and this demodulation is directly obtained in an analog manner at the photodetector level of the interference sensor mechanism without inserting additional elements. Due to this demodulation, the number of photodetectors required to detect the envelope of the interference fringes is reduced, which is in turn reflected in a larger distance measurement range.

[0038] Advantageously, an interferometry technique including detection in the spatial domain is adopted, and for each individual acquisition or sampling of the spatial distribution of the light radiation of the superposed measurement light beam and the reference light beam incident on the sensor mechanism, an accurate distance measurement can be performed using a static system of the measurement optical path and the reference optical path. To provide such a system, only standard optical elements are required, and the signals emitted by the sensor mechanism are processed based on simple computational algorithms, thus not causing heavy computations. Using this technique, the defects in detection in the frequency domain are overcome, namely the presence of autocorrelation signal components, artifacts at negative frequencies, and a decrease in sensitivity at higher measurable distance values.

[0039] According to the present invention, the above - mentioned considerations are applied to a machine tool for processing a workpiece or material, such as a machine tool for laser - processing materials, in particular a machine tool for laser cutting, drilling, welding or additive manufacturing, which is achieved by means of an interferometric measurement system arrangement. The interferometric measurement system arrangement includes a measurement optical path that is at least partially integrated in the body of the machine (e.g., in the working head of the machine) and a reference optical path that can also be integrated in the body of the machine (in the working head) or outside thereof and is associated with the measurement optical path. Wherein, the measurement optical path exits from the machine body (from the measurement head) in the region of the processing tool (in the case of a laser - processing machine, it is the output end for measurement) or more generally at the end of the working head close to the surface of the workpiece or material being processed. Description of the Drawings

[0040] The additional features and advantages of the present invention will be presented in more detail in the specific embodiments of the present invention by way of non - limiting examples in conjunction with the accompanying drawings, in which:

[0041] Figure 1 A schematic diagram showing a machine tool working head and a relative control mechanism near a workpiece according to the prior art;

[0042] Figure 2a A schematic diagram of the configuration of a linear low - coherence interferometric measurement system using spatial detection;

[0043] Figure 2b A schematic diagram of the optical path change of the measurement optical path and the reference optical path with respect to the relative incident point on the illumination axis of the sensor arrangement with respect to the interference fringe pattern;

[0044] Figure 2c A schematic diagram of the change in the optical path difference between the measurement optical path and the reference optical path with respect to the relative incident point on the illumination axis of the sensor arrangement with respect to the interference fringe pattern (upper figure) and a schematic diagram of identifying the interference fringe pattern on the illumination axis of the sensor arrangement in a state where the optical paths of the measurement optical path and the reference optical path are equal (lower figure);

[0045] Figure 3 An example diagram showing a system for determining the distance between the working head of a machine tool and the surface of a workpiece or material being processed, which constitutes the subject matter of the present invention;

[0046] Figure 4 An application schematic diagram of a machine for processing a workpiece or material by irradiating a high - power laser beam, in particular the processing laser beam optical path and the low - coherence measurement beam in the working head, according to an exemplary embodiment of the working head of a laser - processing machine;

[0047] Figure 5a A detailed diagram of the relative positions of the processing laser beam and the low - coherence measurement beam at the output end of the working head of a laser - processing machine in the cutting or drilling part of a workpiece or material;

[0048] Figure 5b It is a detailed diagram of the relative positions of a low-coherence measurement beam and a cutting edge at the output end of the working head of a machining tool in the engraving area of a workpiece or material;

[0049] Figure 5c It is a detailed diagram of the relative positions of an outflow nozzle of a working fluid and a low-coherence measurement beam at the output end of the working head of a fluid processing machine in the engraving area of a workpiece or material;

[0050] Figure 6 It shows the trend of the optical radiation intensity simulating the same interference fringe pattern based on signal samples collected with and without aliasing;

[0051] Figure 7 It is a trend line graph showing the contrast or visibility value of the simulated interference fringe pattern as a function of the relationship between the spatial frequency of the interference fringe pattern and the spatial frequency of the photodetectors in the linear arrangement of the photodetectors along the illumination axis of the sensor arrangement of the interference fringe pattern;

[0052] Figure 8a It is a line graph showing the signal indicating the interference fringe pattern identified along the illumination axis of the sensor arrangement as a function of the spacing distance between the working head and the workpiece or material;

[0053] Figure 8b It is an exemplary calibration curve representing the peak trend of the signal indicating the main interference fringe pattern as a function of the spacing distance between the working head and the material; and

[0054] Figure 8c It is a comparison graph of calibration curves representing the peak trends of the signals of the interference fringe patterns indicating the presence or absence of aliasing. Detailed Description of the Invention

[0055] The above has been described in conjunction with the prior art Figure 1 , the content of which is hereby incorporated by reference, provided that such content is commonly used in general-purpose machine tools, especially laser processing machines, for production under control in order to perform the methods taught by the present invention.

[0056] Figure 2aIt is a schematic diagram of the configuration of a low-coherence interferometric measurement system using linear space detection. A collimated measurement beam of optical radiation marked with M and a collimated reference beam of the same optical radiation marked with R are projected and superimposed on a common incident area C of a sensor arrangement S at a predetermined incident angle α. They form an interference fringe pattern F, and the extension of the interference fringe pattern F on the common interference area is of the order of the coherence length of the optical radiation. The widths of the collimated measurement beam of the optical radiation and the collimated reference beam of the optical radiation are preferably designed to substantially illuminate the entire sensor arrangement. To enhance the intensity and contrast of the detected signal, for example, the beams can be focused on the sensor in a direction perpendicular to the illumination axis by means of a cylindrical focusing lens.

[0057] The sensor arrangement S includes, for example, a photodetector arrangement along at least one illumination axis (the x-axis in the figure) of the incident area. The photodetector arrangement is a linear or two-dimensional arrangement of photodetectors, preferably a linear arrangement. The illumination axis of the incident area is determined by the intersection between the plane defined by the incident angles of the measurement beam M and the reference beam R and the sensor surface of the sensor arrangement.

[0058] Figure 2b The line graph in shows the variation of the optical path lengths p of the measurement optical path and the reference optical path in the typical case where the two incident beams are symmetric on the sensor arrangement, with reference to the initial incident wavefronts of the corresponding measurement beam and reference beam on the common incident area of the sensor arrangement S. The x-axis represents the position or x-coordinate along the illumination axis of the photodetector arrangement. The reference numeral p1 represents the additional optical path length of the first optical path (e.g., the measurement optical path of the optical radiation measurement beam M) relative to the initial wavefront incident point of the measurement beam M at the first end x1 of the common incident area C (which is the origin of the measurement axis). The reference numeral p2 represents the additional optical path length of the second optical path (e.g., the reference optical path of the optical radiation reference beam R) relative to the initial wavefront incident point of the reference beam R at the second end x2 of the common incident area (which is opposite to the first end). The reference numeral Δp represents the difference p1 - p2 between the additional optical path lengths of the two optical paths, and this difference is zero at the midpoint coordinate of the sensor arrangement and varies from the value Δp at the end x1 of the common incident area x1 to the value Δp at the end x2 of the common incident area x2 varies.

[0059] In Figure 2c the upper figure shows the curve Δp corresponding to the Figure 2b line graph shown, while the lower figure shows the interference fringe pattern F on the illumination axis (x) of the sensor arrangement S where the optical path lengths of the measurement optical path and the reference optical path are equal. The envelope of the interference fringe pattern F is shaded, and with reference to the upper figure, the corresponding difference Δp in the additional optical path lengths of the measurement beam and the reference beam optical paths p is associated with the coordinate x of the envelope peak p is associated.

[0060] P M and P R represent the measurement optical path and the reference optical path, and their total optical path lengths can be expressed as P M = P1 + p1 and P R = P2 + p2, where P1 is the optical path length 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 length of the reference optical path from the same low-coherence light radiation source to the first wavefront incident on the sensor arrangement, and it is preferably constant. It can be considered that P1 consists of P head + D standoff where P head is the optical path length upstream and inside the working head, including the first part between the low-coherence light radiation source and the end of the workpiece or material WP being processed (processing or measuring instrument 12, such as the laser beam output end) close to the working head and the second part between the above-mentioned proximal end of the working head (processing or measuring instrument 12, such as the laser beam output end) and the sensor arrangement S. These parts have particularly predetermined and invariant geometric paths, and D standoff is the air gap distance between the end of the workpiece or material WP being processed close to the working head and the above-mentioned workpiece or material (such as the surface of the workpiece or material). P2 is the optical path length of the reference optical path, which is equal to the optical path length of the measurement optical path under nominal operating conditions, hereinafter denoted as P1 nom where the distance between the proximal end of the working head (processing or measuring instrument 12, such as the laser beam output end) and the surface of the workpiece or material WP corresponds to the predetermined nominal gap distance D standoff_nom .

[0061] The mathematical expression for the optical path difference between the measurement optical path and the reference optical path is:

[0062] P M - P R

[0063] And interference fringes appear under the condition that it is zero, that is:

[0064] P M - P R = 0

[0065] It can be decomposed into the relationship:

[0066] P1 + p1 - (P2 + p2) = 0

[0067] It can also be written as:

[0068] P head + D standoff + p1 - P2 - p2 = 0

[0069] From which the following conclusions can be drawn:

[0070] P head +D standoff -P2 + Δp = 0

[0071] P head +D standoff -P1 nom +Δp = 0

[0072] P head +D standoff -P head -D standoff_nom +Δp = 0

[0073] Δp = D standoff_nom -D standoff

[0074] That is, the difference between (a) the current spacing distance D between the working head and the surface of the workpiece or material in the processing area standoff and (b) the nominal spacing distance D standoff_nom is equal to the difference in the additional optical path lengths of the measurement optical path and the reference optical path.

[0075] Therefore, by measuring the optical path difference between the measurement optical path and the reference optical path, the current spacing distance between the working head 10 and the surface of the workpiece or material WP is determined. This current spacing distance is different from the nominal spacing distance due to the difference in the additional optical path lengths of the measurement optical path and the reference optical path, that is, due to the movement of the interference fringe pattern along the illumination axis x of the sensor arrangement S relative to the nominal position (such as the midplane of the sensor arrangement S).

[0076] It should be noted that in a laser processing machine for workpieces or materials, the machine operates with a high-power processing laser beam emitted by the working head and guided along a working trajectory including a series of processing areas on the workpiece or material. During a cutting or drilling application under the action of an auxiliary gas flow, the end of the working head close to the workpiece or material being processed is usually the end part of the auxiliary gas nozzle, while during a welding or additive manufacturing application without gas supply, the end of the working head close to the workpiece or material being processed is usually the output end of the processing laser beam.

[0077] In the application solution that forms the subject matter of the present invention, the optical path of the reference optical path is set to correspond to the optical path of the measurement optical path when the working head and the workpiece or material being processed in the processing area are at a preset nominal separation distance, and the difference between (a) the current separation distance between the working head and the workpiece or material in the processing area and (b) the predetermined nominal separation distance results from the optical path difference between the measurement optical path and the reference optical path, which can be identified based on the position of the interference fringe pattern along the illumination axis of the incident area of the sensor arrangement S. Advantageously, the intermediate position of the interference fringe pattern along the illumination axis corresponds to the preset nominal separation distance. Alternatively, the end position of the interference fringe pattern along the illumination axis can correspond to a zero nominal separation distance between the machining tool and the workpiece or material being processed, which is equivalent to contact between the tool forming the proximal end of the working head and the workpiece or material, thus only allowing the separation distance between them to increase, so the interference fringe pattern only moves towards the opposite end of the illumination axis.

[0078] Referring to Figure 2c the following figure in p is the intrinsic position of the light radiation intensity envelope of the interference fringe pattern, and this intrinsic 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 average position of the photodetector weighted by the light intensity of the fringe envelope.

[0079] The detection of the fringe envelope can be performed by means of light intensity profile demodulation techniques, for example by sequentially applying a band - pass spatial filter or high - pass and low - pass filters to present a unique signal component corresponding to the spatial frequency of the interference fringes. For example, in the first step of processing the light intensity data, the light intensity detected by the sensor matrix is integrated in a direction perpendicular to the direction of development of the interference fringes, for example for the columns of the sensor matrix oriented to receive an interference fringe pattern with vertically arranged fringes (if the sensor arrangement is a linear arrangement of photodetectors and the beam is focused on it by means of a cylindrical lens, this operation is not required). Subsequently, the signal generated by the photodetector is normalized with respect to the background signal (for example, the background signal extracted from an image without interference fringes). Thus, a high - pass spatial filter is applied, for example, to 1 / 5 of the spatial frequency of the photodetector in order to remove the baseline and retain the interference fringe pattern. The signal obtained in this way oscillates around zero, so the absolute value of the signal is extracted, and thus a low - pass spatial filter is applied, for example, to 1 / 25 of the spatial frequency of the photodetector in order to extract the envelope of the interference fringe pattern. Finally, the position of the interference fringe pattern is obtained, which is by detecting the position of the fringe pattern envelope, where the maximum of the envelope is found or the envelope is compared with a predetermined model function (such as a Gaussian function) and the peak of this model function is extracted.

[0080] Figure 3Exemplary diagram showing a system for determining the distance between an object or material and a body, the system including a processing or measuring mechanism for the object or material, which forms the subject of the present invention, and is described in particular with reference to an application scenario for determining the distance between the working head 10 of a laser processing machine for a workpiece or material WP and the surface of the workpiece or material.

[0081] In this figure, 100 represents a low-coherence light radiation source, suitably with linear polarization, such as an LED or a superluminescent diode, which operates, for example, in the visible or near-infrared wavelength range. The light radiation emitted by the light source 100 enters an optical waveguide, such as an optical fiber 140, downstream of a suitable optical isolator 120 and is transmitted to a beam splitter 160, which is adapted to generate a measurement beam of light radiation M routed on the measurement optical path P M and a reference beam of light radiation R of the light radiation routed on the reference optical path P R on the reference optical path.

[0082] The measurement optical path P M and the reference optical path P R are guiding optical paths and include optical waveguides (such as optical fibers) adapted to maintain the same beam polarization along the entire optical path.

[0083] As described above, the measurement optical path P M is guided to the working head 10 of a laser processing machine for a workpiece or material and is then emitted therefrom towards the workpiece or material WP being processed and impinges thereon. The region where the measurement beam M is output corresponds to the measurement head section for which the distance from the above-mentioned workpiece or material is to be measured, such as the opening for supplying an auxiliary air flow in the nozzle or the output end of the laser beam. Of course, if applied to different types of machine tools for mechanically processing a workpiece or material or for processing it with a fluid, the measurement head section for which the distance from the workpiece or material is to be measured can be identified as the end of the machining tool or the nozzle for supplying the working fluid.

[0084] Instead, preferably by interposing an optical density filter 200, an optical dispersion compensation element 220, a λ / 4 plate 240 and a focusing lens 260, the reference optical path P R is guided to the retroreflective element 180. The optical reflective element 180 is arranged along the reference optical path such that the optical path from the beam splitter 160 to the optical reflective element 180 corresponds to the optical path of the measurement optical path from the beam splitter 160 to the (reflected) surface of the workpiece or material WP being processed in the operating state, where the surface is at a preset nominal distance D standoff_nom from the working head (i.e., from the end of the working head close to the workpiece or material, such as the opening for auxiliary air in the nozzle or the beam output end).

[0085] The measurement optical path P M and the reference optical path P R, such that the light radiation propagates in two directions along these optical paths and returns to the beam splitter 160 after being reflected on the surface of the workpiece or material WP being processed and on the optical reflection element 180, respectively. Reflection on the surface of the workpiece or material being processed can be understood as reflection on at least a partially reflective surface of the workpiece or material when the workpiece or material is non-metallic but can undergo diffuse or specular reflection at different depths, thus allowing interferometry along the entire depth of the object or material. In the reference optical path P R In this reference optical path, the reference beam R passes twice through the λ / 4 glass plate 240 such that the linear polarization of the beam is rotated by 90°, thus assuming a linear polarization orthogonal to the linear polarization of the measurement beam M. Then, the beam splitter 160 performs recombination of the measurement beam and the reference beam and guides them to superpose along the detection optical path P D (which is shared by a part of the measurement optical path and a part of the reference optical path) towards the sensor arrangement S.

[0086] The measurement beam and the reference beam are guided through the cylindrical focusing lens 280, which can focus the collimated beam only in one direction, in particular in a direction orthogonal to the illumination axis of the sensor arrangement, in order to concentrate the signal along this axis, thus optimizing the illumination of the photodetector and causing the beam to reach the polarization beam splitter 300, which separates the measurement beam M from the reference beam R based on their polarization, directs its first part towards the first reflection element M1 and its second part towards the second reflection element M2; in the latter case, the original polarization can be restored by interposing the λ / 2 glass plate 320. Due to this configuration, the first reflection element M1 and the second reflection element M2 direct the measurement beam and the reference beam towards the sensor arrangement S at the incident angle α, more precisely towards the common incident area of the sensor arrangement.

[0087] The incident angle α can be controlled within a preset value range by means of the reflection elements M1 and M2, which can be translated along the propagation axis of the relative beam and rotated about the normal axis with respect to the incident plane (dashed positions in the figure).

[0088] As described above, the sensor arrangement S includes a plurality of photodetector devices, each of which is adapted to emit a specific signal representing the light intensity incident thereon, and these signals are transmitted as a whole to the processing mechanism 350, which is configured to identify the interference fringe pattern F formed on the common incident area C of the sensor arrangement by acquiring the total incident optical power of the superposed measurement beam and reference beam.

[0089] Preferably, the measurement optical path and the reference optical path include corresponding optical elements. In particular, the reference optical path includes a retroreflective element, whose reflection and light diffusion characteristics correspond to the reflection and light diffusion characteristics of the workpiece or material inserted in the measurement optical path to the maximum extent. Optionally, a light attenuation mechanism and / or a light dispersion mechanism can be provided, which can balance the light intensity and dispersion of the reference light radiation reflected by the retroreflective element with the light intensity of the measurement light radiation reflected by the workpiece or material being processed.

[0090] With the Figure 3 system in or an equivalent system, a method for determining the spacing distance between the working head 10 of a machine tool (such as a laser processing machine for workpieces or materials) and the workpiece or material WP in the processing area defined along a predetermined working trajectory T is performed, and then the processing tool projects from the working head (a processing laser beam is emitted from the working head).

[0091] The method includes generating a measurement beam M of low-coherence light radiation, guiding the measurement beam M through the working head 10 towards the processing area, being reflected or diffused by the workpiece or material WP in the processing area, and guiding it through the working head 10 along a first incident direction towards the sensor arrangement S.

[0092] When processing metal workpieces or materials, it can be assumed that the measurement beam is reflected or diffused on the first surface of the material. In some cases, such as in welding or additive manufacturing processes, it is necessary to measure the distance from the surface of the molten pool (which represents the first surface of the molten metal) rather than the surface of the solid (or substrate) to be welded. In the case of non-metal processing and semi-transparent materials (ceramics, plastics, biological tissues, etc.), or in the case of painted metals, signals are generated in the internal subsurface layer of the material.

[0093] The light radiation measurement beam M particularly travels along a measurement optical path from the light source 100 to the sensor arrangement S, and the sensor arrangement S includes two parts with a special predetermined and invariant geometric path, namely a first part between the light source 100 and the end of the working head 10 close to the workpiece or material WP, and a second part between the end of the working head 10 close to the workpiece or material WP and the sensor arrangement S.

[0094] The reference beam R of the low-coherence light radiation is generated by the same light source 100, and the beam is guided towards the sensor arrangement S along a second incident direction at a predetermined incident angle relative to the first incident direction of the measurement beam M. The reference beam R travels along the reference optical path P R with an optical path equivalent to that of the measurement optical path P M in the nominal operating state, where the distance between the working head 10 and the workpiece or material WP corresponds to a predetermined nominal spacing distance D standoff_nom .

[0095] The measuring beam M and the reference beam R are superposed along a preset illumination axis on a common incidence area C of the sensor arrangement S. By means of the processing means 350, the position of the interference fringe pattern F between the measuring beam M and the reference beam R along the illumination axis on the common incidence area C is detected. As described above, the measuring optical path P can be determined. M and the reference optical path P R The optical path difference therebetween represents the difference between: (a) the current spacing distance between the working head 10 and the surface of the workpiece or material WP in the processing area, and (b) a preset nominal spacing distance.

[0096] This method can be carried out in real time during the working process in order to determine the spacing distance between the working head and the current processing area on the workpiece or material, but it can also be carried out before or after the working process, for example in order to qualify the workpiece to be processed or the working process already carried out.

[0097] Figure 4 and Figure 5a A partial exemplary embodiment of a system for determining the spacing distance between the working head of a laser processing machine and the surface of a workpiece or material in a processing area is schematically shown, in particular an exemplary embodiment of the optical paths of the processing laser beam B and the measuring beam M inside the working head and the relative positions of the processing laser beam B and the measuring beam M in the cutting or drilling part of the workpiece or material WP.

[0098] Figure 4 A reflecting element for deflecting the laser beam is shown, such as a dichroic mirror (labeled DM), which deflects the propagation optical axis of the processing laser beam B from the entry direction of the working head to the incidence direction on the workpiece or material WP being processed. This is a configuration employed in embodiments of working heads including a lateral laser beam input. In the present embodiment, by means of an optical reflection scanning system SM or a folding mirror, the inclination thereof is controlled, for example piezoelectrically, based on the absolute value and direction of the forward rate of the working head along the working trajectory, in order to control the position where the measuring point intersects the surface, so that the light-radiation measuring beam M is directed to the measuring area of the material and passes through the dichroic mirror DM without significant deflection. A focusing lens FL is arranged downstream of the optical reflection scanning system SM, so that the position H where the measuring point intersects the surface of the workpiece or material can be controlled. As can be seen from the figure, the propagation direction of the measuring beam can be controlled by tilting the optical reflection scanning system SM, so as to be superposed on the processing laser beam B non-coaxially, but differently therefrom. Those skilled in the art should also understand that there can be a "dual" or "paired" configuration, in which a dichroic mirror is provided that transmits the processing laser beam but reflects the measuring beam from the lateral input.

[0099] Advantageously, in a machine for laser cutting, drilling or welding a material by means of a laser or for additive manufacturing of a three-dimensional structure, the machine comprising a working head which contains a nozzle for supplying an auxiliary gas flow arranged in the vicinity of the material, guiding a measuring beam of light radiation through the nozzle and guiding it according to a working trajectory to a measuring area of the workpiece or material coaxial with or in the vicinity of, preferably in front of, the current processing area.

[0100] Advantageously, in a machine for laser welding a material by means of a laser or for additive manufacturing of a three-dimensional structure, the machine comprising, downstream of an optical system for focusing the laser beam, a working head which contains an output for a high-power processing laser beam, the system being arranged in the vicinity of the workpiece or material, guiding a measuring beam of light radiation through the said beam output and guiding it according to a working trajectory to a measuring area of the workpiece or material coaxial with or in the vicinity of, preferably in front of, the current processing area.

[0101] Conveniently, the measuring beam M is incident coaxially with the processing laser beam B, for example for evaluating the drilling depth, welding height and height of the structured material during additive manufacturing. The measuring beam M is incident in a retracted position relative to the processing area for verifying the welding quality or additive deposition. The measuring beam M is incident in an advanced position relative to the processing area for early measurement of the distance between the working head and the workpiece or material during cutting and welding, or for identifying the position of the weld seam along the working trajectory. Figure 5a The latter configuration is shown by way of example, where N denotes the nozzle for supplying the auxiliary gas, B denotes the processing laser beam incident on the current processing area of the workpiece or material WP, in which a cutting operation is carried out according to the trajectory indicated by the arrow, which forms a groove G, and M denotes the measuring beam. In the cutting operation, arranging the measuring beam M coaxially with the processing beam B results in highly uncertain measurement, which, in the case of the cutting edge, may make it impossible to control the profile of the groove walls in the workpiece or material, depending on a number of processing parameters.

[0102] Figure 5b A partial exemplary embodiment of a system for determining the distance between the working head of a machine tool for mechanically processing a workpiece or material and the surface of the workpiece or material in the processing area is schematically shown, in particular an exemplary embodiment of a cutting edge CT (e.g. a diamond cutting edge), the end portion of the optical path of the measuring beam M in the working head of the machine tool and the relative position of the cutting edge CT and the measuring beam M in the engraving area of the workpiece or material WP. The direction of the engraving operation is indicated by the arrow, and G denotes the engraving groove produced by the cutting edge CT. In the engraving operation (similar to the cutting process), the measuring beam M is advantageously arranged forward relative to the processing area for early measurement of the distance between the working head and the workpiece or material, from which, for example, the depth of the engraving groove or the entry of the cutting edge CT can be inferred.

[0103] Figure 5c Schematically shows a partial exemplary embodiment of a system for determining the spacing distance between the working head of a machine tool for processing workpieces or materials using a fluid and the surface of the workpiece or material in the processing area, in particular an exemplary embodiment of the outflow nozzle N of the working fluid, the end portion of the optical path of the measuring beam M within the working head of the machine, and the relative positions of the nozzle N and the measuring beam M in the processing area of the workpiece or material WP. The direction of the engraving operation is indicated by an arrow, and G represents the engraving groove produced by the jet J supplied by the nozzle N. In the engraving operation (similar to different types of processes, such as polishing or simple cleaning), the measuring beam M is advantageously arranged forward relative to the processing area in order to measure the spacing distance between the working head and the workpiece or material at an early stage.

[0104] According to the present invention, in the configuration where the measuring beam and the reference beam are incident on a common incident area of the sensor arrangement S, the incident angle α is extended to such an extent that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetector, so as to increase the range of measurable distances. Advantageously, the incident angle α can be controlled within a predetermined value range.

[0105] It is known in the art that assuming that the measuring 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., within the plane of the incident angle) can be approximated as:

[0106]

[0107] where I1 and I2 are the light intensities of the respective beams, and k f is the wave number or spatial frequency of the interference fringe pattern. α1 and α2 represent the incident angles of the measuring beam and the reference beam relative to the normal of the sensor arrangement, and the spacing distance between the interference fringes is given by:

[0108]

[0109] Therefore, the larger the tilt angle, the higher the frequency of the interference fringe pattern, and thus the greater the density of the interference fringes on the sensor arrangement.

[0110] In order to comply with the Nyquist sampling theorem (Nyquist) and avoid the phenomenon of aliasing or sub-sampling, the spatial frequency of the photodetector (which corresponds to the sampling pixel spatial frequency on the sensor arrangement, represented by k p ) must be at least twice the frequency k f of the interference fringe pattern, that is, the ratio k f / k p must be less than 0.5.

[0111] Information on the optical path difference can be directly extracted from the position of the envelope of the interference fringe pattern in the light intensity distribution of the light radiation incident on the sensor arrangement. N p represents the number of photodetectors in the sensor arrangement that are illuminated by the superimposed measurement beam and reference beam; thus, the maximum optical path difference that can be measured is:

[0112]

[0113] Thereby, the measurement range is proportional to the number of photodetectors illuminated by the superimposed beams, which is a result of the resolution of the sensor arrangement and the dimensions of the beams. Therefore, an increase in the range of measurable optical path differences can be obtained by increasing the number of the photodetector devices, which may lead to an increase in the cost of forming the sensor arrangement and an increase in the cost for processing the signals originating from the sensor arrangement. The above-mentioned measurable difference range is also proportional to the k f / k p ratio between the frequency of the interference fringe pattern and the spatial frequency of the photodetector. k f / k p The ratio depends on the incident angle between the measurement beam and the reference beam and the spatial dimensions of the photodetector; thus, a balance needs to be achieved between the inclination of the beam and the number of illuminated photodetectors.

[0114] The present inventors have noticed that, depending on the wavelength and coherence length of the low-coherence light radiation used, dozens of interference fringes are typically visible in the interference fringe pattern formed in the common incident region of the sensor arrangement. While complying with the Nyquist theorem, obtaining a large number of interference fringes over a wide measurement range requires a large number of photodetectors, which leads to over-acquisition of information because only the position of the envelope of the interference fringe pattern is relevant to determining the difference between the measurement optical path and the reference optical path. As a result, the present inventors have disclosed a feasible solution for demodulating the interference fringe pattern at an increasingly smaller spatial frequency in the case where the incident angle between the measurement beam and the reference beam is getting larger, such that the frequency of the interference fringe pattern becomes greater than the spatial frequency of the photodetector - which is the condition for aliasing to occur.

[0115] This is essentially a demodulation technique performed by means of the photodetectors without using a mask. Generally speaking, a mask or lattice that is periodic but has a frequency k p and is interposed along the optical path incident on the sensor arrangement, such as a transmissive or reflective mask, can be used to demodulate at a lower frequency an interference fringe pattern having a frequency k fPeriodic signals in space, such as interference fringe patterns. In this way, the signals detected by the sensor arrangement are modified due to the presence of the periodic mask. As in the prior art, the trend of the new signal detected by the sensor arrangement can be calculated based on the convolution between the original signal and the said mask. The new signal includes periodic components having a spatial frequency different from the original spatial frequency: specifically, the sum value k f +k p and the difference value k f -k p . The component with the lower frequency k f -k p is particularly important because it represents the analog demodulation signal of the original spatial signal. In the specific case of demodulating the fringe pattern at a lower spatial frequency, this method can use a smaller number of photodetectors to detect the pattern for sampling, thereby effectively reducing the number of detected fringes.

[0116] In the present invention, instead of using dedicated demodulation elements, the periodicity of the photodetector matrix for sampling the interference signal is used in a manner similar to the mask, where k p is the frequency of the photodetector or pixel. In the subsampling (aliasing) state where k f >k p , each photodetector (pixel) will obtain an optical signal corresponding to several fringes, and due to the above-mentioned effect, it will result in detecting a distorted fringe pattern at a lower spatial frequency. This lower spatial frequency is reflected in a smaller effective number of photodetectors for detecting the entire fringe pattern; thus, the entire extension of the sensor arrangement can be used for operation over a larger measurement range.

[0117] Considering a constant number of photodetectors, this method can expand the measurement range without losing information, at the cost of only reducing the contrast of the interference fringes, because multiple fringes can be detected by a single photodetector. This phenomenon is referred to Figure 6 , where the line graphs respectively show the reconstructed trends of the optical radiation intensity of the same interference fringe pattern simulated based on signal samples (represented by dots) in the case of no aliasing (upper figure) and with aliasing (lower figure). It can be noted that although the number of detected fringes varies between the two sampling states, the envelope of the fringe pattern remains almost unchanged.

[0118] It can be verified in the prior art that the contrast ν of the interference fringes depends on the aliasing factor k f / k p , specifically according to the following relationship:

[0119]

[0120] which is at the spatial frequency k of the photodetectorp is zero at integer multiples of, such as Figure 7 shown.

[0121] Advantageously, in order to have a locally maximum contrast, the spatial frequency of the interference fringe pattern must be greater than and different from integer multiples of the spatial frequency of the photodetector, preferably approximately a half-integer multiple of the spatial frequency of the photodetector.

[0122] In fact, it can be clearly seen from the line graph in Figure 7 which shows the trend of the contrast ν between the interference fringes in solid line as a function of k f / k p ratio, the local maxima are approximately at half-integer multiples of the spatial frequency of the photodetector, but the maximum contrast drops steeply (qualitatively shown as a dashed line in the figure).

[0123] Advantageously, the incident angle between the measurement beam and the reference beam can be selected to obtain a k f / k p ratio equal to approximately 1.5 (or approximately 2.5, approximately 3.5, etc.).

[0124] Figure 8a is a line graph showing the signals emitted by the photodetector along the illumination axis (vertical axis) of the sensor arrangement S, which signals indicate the intensity of the envelope of the interference fringe pattern formed on the common incident area. In particular, this figure shows the interference signal, and thus the spatial position of the interference fringe pattern along the illumination axis (vertical axis) as a function of the spacing distance (horizontal axis) between the end of the working head and the workpiece or material facing it. For example, such a line graph can be generated in a calibration step by providing a reference optical path with a fixed optical path length and continuously changing the relative position between the working head and the surface of the workpiece or material along the z-axis (i.e., the spacing distance of the working head from the surface of the workpiece or material), and by acquiring interference signal readings as a function of predetermined discrete values of the spacing distance.

[0125] This figure shows the acquisition of strong interference signals in the aliasing state and the translation of the envelope of the interference fringe pattern within a range of approximately 2 mm (corresponding to approximately 1500 pixels), depending on the approximate linear peak trend of the signal indicating the interference fringe pattern, as Figure 8b shown. The sensitivity can be defined as the spacing distance corresponding to the dimension of the photodetector or pixel in the incident area, which is 1.5 μm / pixel in this case. The translation of the envelope of the interference fringe pattern along the entire illumination axis of the sensor arrangement allows the determination of the spacing distance between the working head and the material surface to be approximately 0.25 mm to approximately 2 mm.

[0126] When controlling the incident angle between the measurement beam and the reference beam to obtain a k f / kp When the ratio is such that the sensor arrangement can determine the spacing distance between the working head and the measurement surface to be from about 0.25 mm to about 2 mm, the measurement range is then about 1.8 mm, which is different from the aliasing-free condition, such as k f / k p When the ratio is equal to about 0.3, the measurement range is about 0.3 mm. Figure 8c is a line graph comparing the aliased and aliasing-free calibration curves labeled A and A', respectively. At k f / k p When the ratio is approximately equal to 4.5, a measurement range on the order of 5 mm can be achieved.

[0127] Accurately determining the spacing distance between a body (such as a working head or a machine tool) and the surface of a workpiece or material in a processing area (current processing area or calibration processing area) within an extended range of spacing distances, and appropriately allowing the control unit ECU of the machine to also use feedback to calibrate or control the larger spacing distance between an object or material and the body (which includes the mechanism for processing or measuring the object or material), such as calibrating or controlling the processing / measurement distance or other processing / measurement parameters, such as acting on the motion actuator mechanism 14 to control the movement of the working head along the Z-axis towards or away from the workpiece or material, as a function of a predetermined processing plan and the results of interferometry. This is particularly beneficial for improving the efficiency of the processing or measurement process, as an extension of the control range can be simply obtained by adjusting the optical system.

[0128] It should be noted that the designs proposed for the present invention in the above are substantially only examples and do not limit the present invention. Those skilled in the art can easily implement the present invention in various embodiments, but these embodiments do not deviate from the principles set forth herein and thus fall within the protection scope of this patent.

[0129] This is especially applicable to feasible solutions using a different wavelength of low-coherence light radiation from that in the cited art, or by way of non-limiting example only, using a measurement optical path and a reference optical path with an interposed optical element different from Figure 3 that shown.

[0130] Of course, without departing from the principles of the present invention, the embodiments and implementation details of the present invention may not be exactly the same as those described and illustrated by way of non-limiting examples only, but this does not deviate from the protection scope of the present invention defined by the appended claims.

Claims

1. A method for determining the spacing distance between an object or material and a processing or measuring mechanism for the object or material, characterized in that, The method includes the following steps: - Generating a measurement beam of low-coherence light radiation, guiding the measurement beam through the processing or measuring mechanism towards at least one end of the object close to the object, and guiding the measurement beam reflected or diffused from the object through the processing or measuring mechanism towards the optical interference sensor mechanism along a first incident direction at the end of the object close to the object, wherein the measurement beam travels along a measurement optical path from the corresponding light source to the optical interference sensor mechanism, the measurement optical path including a first section between the light source and the end of the processing or measuring mechanism close to the object and a second section between the end of the processing or measuring mechanism close to the object and the optical interference sensor mechanism, and these sections having respective predetermined and invariant geometric paths; - Generating a reference beam of the low-coherence light radiation and guiding the reference beam towards the optical interference sensor mechanism along a second incident direction at a predetermined incident angle with respect to the first incident direction of the measurement beam, wherein, under the nominal operating condition that the distance between the end of the processing or measuring mechanism close to the object and the object corresponds to a predetermined nominal spacing distance, the optical path of the reference optical path traveled by the reference beam is equal to the optical path of the measurement optical path; - Superposing the measurement beam and the reference beam on a common incident area of the optical interference sensor mechanism along a predetermined illumination axis; - Detecting the position of the interference fringe pattern between the measurement beam and the reference beam along the illumination axis on the common incident area, the extension length of the interference fringe pattern along the illumination axis corresponding to the coherence length of the low-coherence light radiation; and - Determining the optical path difference between the measurement optical path and the reference optical path - which represents the difference between: (a) the current spacing distance between the processing or measuring mechanism and the object surface at the end of the processing or measuring mechanism close to the object during processing or measurement; (b) the predetermined nominal spacing distance - as a function of the position of the interference fringe pattern along the illumination axis of the incident area, wherein the optical interference sensor mechanism includes a photodetector arranged along the illumination axis and controls the incident angle in such a way that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetector, and the spatial frequency of the interference fringe pattern increases as the incident angle increases, resulting in distortion due to factor sampling and spatially demodulating the interference fringe pattern at a lower spatial frequency, whereby the maximum optical path difference that can be determined between the measurement optical path and the reference optical path increases.

2. The method according to claim 1, wherein The spatial frequency of the interference fringe pattern is different from a multiple of the spatial frequency of the photodetector.

3. The method according to claim 1, wherein The photodetector is linearly arranged.

4. The method according to claim 1, wherein, The photodetector is two-dimensionally arranged.

5. The method according to claim 1, wherein The position of the light intensity envelope of the light radiation of the interference fringe pattern along the illumination axis is the inherent position of the light intensity envelope of the light radiation of the interference fringe pattern.

6. The method according to claim 5, wherein The inherent position of the light intensity envelope of the light radiation of the interference fringe pattern is the position of the peak or maximum value of the light intensity envelope of the light radiation.

7. The method according to any one of claims 1-6, wherein, The illumination axis of the incident area is determined by the intersection between the plane defined by the incident angle and the sensing surface of the optical interference sensor mechanism.

8. The method according to any one of claims 1-6, wherein The measurement optical path and the reference optical path include corresponding optical elements. The reference optical path includes a retroreflective element corresponding to an object inserted into the measurement optical path and includes a light attenuation mechanism adapted to balance the light intensity of the reference light radiation reflected by the retroreflective element and the light intensity of the measurement light radiation reflected by the object.

9. The method according to claim 8, wherein The measurement optical path and the reference optical path originate from a common light source, are separated by a beam splitting mechanism, are each separately directed to the object and the retroreflective element, and are focused in a detection optical path where the measurement beam and the reference beam are separated, and the measurement beam and the reference beam are directed to the common incident area of the optical interference sensor mechanism with a controllable orientation that determines the incident angle between the measurement beam and the reference beam.

10. The method according to any one of claims 1-6, wherein The processing or measuring mechanism is carried by the working head of a laser processing machine for a workpiece or material, the laser processing machine operates by means of a high-power processing laser beam conducted along a working trajectory including a series of working areas on the workpiece or material, and the object is the surface of the workpiece or material at the working area.

11. The method according to claim 10, wherein, The end of the processing or measuring mechanism close to the object is a nozzle for distributing an auxiliary air flow in the working head of a machine for laser cutting, drilling or welding a workpiece or for additive manufacturing of a three-dimensional structure from a precursor material by laser, and the measurement beam is conducted through the nozzle.

12. The method according to claim 10, wherein, The end of the processing or measuring mechanism close to the object is the output end of the high-power processing laser beam in the working head of a machine for laser welding a workpiece or for additive manufacturing of a three-dimensional structure from a precursor material by laser.

13. The method according to claim 2, wherein, The spatial frequency of the interference fringe pattern is approximately a half-integer multiple of the spatial frequency of the photodetector.

14. A system for determining the spacing distance between an object or material and a processing or measuring mechanism for the object or material, characterized in that, The system includes: - a mechanism for generating a measurement beam of low-coherence light radiation; - a mechanism for propagating the measurement beam, which is adapted to guide the measurement beam through at least one end of the processing or measuring mechanism close to the object and direct it towards the object, and guide the measurement beam reflected or diffused by the object through the end of the processing or measuring mechanism close to the object and direct it towards the optical interference sensor mechanism along a first incident direction, wherein the measurement beam travels through a measurement optical path from the corresponding light source to the optical interference sensor mechanism, the measurement optical path including a first part between the light source and the end of the processing or measuring mechanism close to the object and a second part between the end of the processing or measuring mechanism close to the object and the optical interference sensor mechanism, and these parts have their respective predetermined and invariant geometric paths; - a mechanism for generating a reference beam of the low-coherence light radiation; - a mechanism for propagating the reference beam, which is adapted to guide the reference beam towards the optical interference sensor mechanism along a second incident direction at a predetermined incident angle with respect to the first incident direction of the measurement beam, wherein, under nominal operating conditions where the distance between the end of the processing or measuring mechanism close to the object and the object corresponds to a predetermined nominal spacing distance, the optical path of the reference optical path traveled by the reference beam is equal to the optical path of the measurement optical path. Wherein, the mechanism for propagating the measurement light beam and the mechanism for propagating the reference light beam are arranged such that the measurement light beam and the reference light beam are superposed on a common incident area of the optical interference sensor mechanism along a predetermined illumination axis; - a mechanism for detecting the position of an interference fringe pattern between the measurement light beam and the reference light beam along the illumination axis on the common incident area, the extension length of the interference fringe pattern along the illumination axis corresponding to the coherence length of the low-coherence light radiation; and - a processing mechanism arranged to determine the optical path difference between the measurement optical path and the reference optical path - which represents the difference between: (a) the current spacing distance between the processing or measuring mechanism and the surface of the object at the end of the object being processed or measured; (b) a predetermined nominal spacing distance - as a function of the position of the interference fringe pattern along the illumination axis of the incident area, Wherein, the optical interference sensor mechanism includes a photodetector arranged along the illumination axis and controls the incident angle in such a way that the spatial frequency of the interference fringe pattern is greater than the spatial frequency of the photodetector, and the spatial frequency of the interference fringe pattern increases as the incident angle increases, resulting in distortion caused by factor sampling and spatial demodulation of the interference fringe pattern at a lower spatial frequency, thereby increasing the maximum optical path difference that can be determined between the measurement optical path and the reference optical path.

15. A processing machine tool for a workpiece or material, comprising a system for determining the spacing distance between a machining tool and the surface of the workpiece or material, the system being arranged to perform the method according to any one of claims 1 to 13.

16. A laser processing machine for a workpiece or material, which operates by means of a high-power processing laser beam emitted by a working head and guided along a working trajectory on the workpiece or material that includes a series of working areas, and includes means for controlling the relative position between the working head and the workpiece or material, wherein, The laser processing machine for the workpiece or material includes a system for determining the spacing distance between the working head and the surface of the workpiece or material at the working area, the system being arranged to perform the method according to any one of claims 1 to 13, and the mechanism for controlling the relative position between the working head and the workpiece or material acts according to a predetermined processing design and the determined spacing distance between the working head and the surface of the workpiece or material.

Images