Patent Description:
High power lasers have important applications in the laser processing of industrial materials. Pulsed lasers, with peak powers exceeding 10kW, are used in marking, engraving, cutting, welding, and drilling applications. Continuous wave lasers with powers exceeding 500W are used in cutting and welding applications. These high power lasers advantageously have optical fibre beam delivery systems for delivering the laser radiation from the laser to a work piece, which work piece can be located tens or hundreds of metres from the laser.

Laser cutting of materials typically commences with piercing through the material in order to form at least one hole. There can be many holes required and so the piercing can contribute a significant amount of time to the cutting process. The ability to know accurately when the material is pierced allows the apparatus to start the cut or move on to another pierce, hence improving reliability and productivity as dwell times for piercing are kept to a minimum.

There are many industrial lasers that use a beam delivery cable between the laser and the work piece. These lasers include fibre lasers, disk lasers, and rod lasers. Fibre lasers are especially attractive owing to their excellent reliability, cost, and wall plug efficiency. Also, the fibre lasers provide power levels with good beam quality to multi kilowatts. The beam delivery cable contains optical fibre, which can be tens or even hundreds of metres long, allowing the laser to be located in a location remote from the work piece. There is thus a continuous optical path through the optical fibre from the laser to the work piece through which optical radiation emitted from the work piece can propagate back to the laser itself. Backward travelling optical radiation is generally considered a nuisance and needs to be removed in a controlled manner to prevent damage to the laser.

It is an aim of this invention to analyze the optical radiation emitted from the work piece in order to control the laser processing of a material.

<CIT> disclose apparatus for controlling laser processing of a material.

According to a non-limiting embodiment of the present invention there is provided apparatus for controlling laser processing of a material, as claimed in claim <NUM>.

The apparatus of the invention is advantageous in that significant time is often devoted to piercing material. Detecting and analyzing the optical radiation that is emitted by the material enables the apparatus to commence another piercing or a cutting operation more quickly after piercing the material. The optical radiation emitted from the material may be laser radiation that is reflected from the material together with optical radiation that is emitted by the material as a consequence of it being heated by the laser radiation.

The apparatus may be one wherein the means for directing the laser radiation onto the material includes an optical fibre and a laser processing head, and wherein the optical fibre is configured to propagate the laser radiation from the laser to the laser processing head. Use of the optical fibre enables the laser to be located tens or hundreds of metres from the material being processed. This can reduce constraints on factory layouts, for example in the provisioning of services such as cooling and electrical power for the laser.

The apparatus may include a coupler configured to couple the optical radiation from the optical fibre to the detector. Guiding the optical radiation from the material along the optical fibre towards the laser, and coupling the optical radiation from the optical fibre to the detector helps to prevent damage to the laser caused by the backward travelling optical radiation. In addition, it enables the detector, the electronic filter, and the discriminator to be located within or near to the laser. This has the advantage for the end user that no equipment for detecting laser piercing is needed around the machining area, saving on cost and complexity. This is particularly advantageous for machining installations where it is difficult or impractical to include sensors at the material for detecting laser piercing.

The detector may be located in or on the laser processing head. This can be advantageous if other sensors are used for process control as signals from the detector and these other sensors can be combined together.

The electronic filter may comprise a peak detector. Peak detectors can be designed simply and reliably using analogue or digital electronics. The peak detector can, for example, output the maximum value, or maximum minus the minimum value of the electronic signal over pre-determined time intervals. Such maximum or maximum minus minimum values can then be low pass filtered and level detected by the discriminator.

The electronic filter may comprise a root mean square filter. Root mean square filters can provide a better signal to noise ratio than peak detection if there is interference in the signal, caused for example by electrical interference from machinery.

The electronic filter may comprise a bandpass filter. A bandpass filter can be used to analyze particular frequency components present in the electronic signal that are indicative of certain dynamics of the laser processing of the material. Such analysis can provide better process control and improve quality.

The electronic filter may comprise a high pass filter. High pass filters are useful for removing the direct current component of an electronic signal.

The electronic filter may comprise an integrator. The electronic signal is typically very noisy while piercing the material, and the noise reduces after the material has been pierced. Integrating the electronic signal, preferably after high pass or band pass filtering, can provide a signature that enables the laser processing to be better understood.

The electronic filter may comprise a band stop filter configured to reject interference. This can be advantageous in factory environments where signal interference and pick up can interfere with signal processing.

The discriminator may include a level detector for detecting a predetermined amplitude. The predetermined amplitude may be selected to correspond to the material being pierced.

The discriminator may include a slope detector for detecting a predetermined slope. The slope detector is useful for analyzing an electronic signal that has been integrated. Once the material has been pierced, the slope of the integrated electronic signal typically reduces.

The electronic filter and the discriminator may be configured in a digital electronic circuit. Digital electronic implementation enables a wide variety of filter and discriminator settings to be implemented, each optimized for different materials and material thicknesses.

The electronic filter may utilize spectral frequency analysis. Spectral frequency analysis can be provided using fast Fourier transforms. It can be used to select particular frequency components present in the electrical signal that correspond to dynamics of laser piercing, cutting, or welding processes.

The electronic filter and the discriminator may be implemented with fuzzy logic. Fuzzy logic is useful for analyzing complex waveforms and processes.

At least one of the electronic filter and the discriminator may comprise a neural network. Neural networks can provide self learning functions that can be used to optimize the laser piercing process. Use of neural networks can avoid having to consult laser experts when materials are changed, or if the thickness or surface finish of a material is altered. The neural network can train the apparatus to adjust the filter and discriminator parameters.

The electronic filter may comprise an analogue electronic filter. Although digital electronic implementation offers many advantages, analogue electronics are simple to implement and may be a lower cost option if the apparatus is only used to machine a single component.

The apparatus may comprise an optical filter. This can be advantageous in order to monitor and control other parameters associated with laser cutting, such as the temperature of the material while piercing and while cutting the material. The higher the temperature of the material, the more wavelengths in the optical spectrum that are emitted by the material. More than one optical filter can be provided each connected to a different detector. The filter detector combinations can be configured to analyse the optical spectral content at different wavelengths.

Melting or vaporization of the material during the piercing process is believed to cause fluctuations in the power and direction of the optical radiation emitted by the material, which can be observed as noise in the electronic signal.

The characteristic feature may provide an indication that the material has been pierced. Alternatively or additionally, the characteristic feature may provide an indication as to when the optics requires cleaning or replacing in part. The characteristic feature that provides an indication as to when the optics requires cleaning or replacing in whole or in part may be different from the characteristic feature that provides an indication that the material has been pierced. Surprisingly the inventor has observed an increase in the amplitude of the electronic signal after the material has been pierced. This increase is associated with optics that require cleaning or replacing in whole or in part. The increase can be removed by replacing contaminated or defect cover slides that protect the lens from spatter from the work piece.

The material may comprise a test piece. This aspect of the invention is particularly useful in cutting machines. A test piece such as a <NUM> to <NUM> thick sheet of copper can be provided to one side of a cutting bed and the test piece can be pierced periodically for quality control purposes. Thus if the optics becomes dirty through spatter ejected from the material in a work piece such as steel or other materials, the state of the optics can be detected and the optics cleaned or cover slides replaced. Other features of the invention, such as pierce detection, can be utilized to control the piercing of the work piece.

As an example, copper has a thermal conductivity of approximately <NUM> W/m/K which causes heat absorbed by the material during piercing to be conducted away from the hole more rapidly than with material such as iron that has a thermal conductivity of approximately 83W/m/K. Consequently the hole formed in the piercing process has a similar diameter to the spot size of the laser radiation on the surface of the material. Contaminated or dirty optics can cause scatter or thermal lensing, which causes interaction of the laser radiation with the copper after piercing, and which can cause additional reflection or emission of the laser radiation from the surface. Thus monitoring the amplitude of the electronic signal or the rate of change in amplitude of the electronic signal after the material or a test piece has been pierced provides a useful and valuable indication that the optics needs cleaning or replacing in whole or in part.

The material may have a thermal conductivity greater than <NUM> W/m/K, preferably greater than <NUM> W/m/K and more preferably greater than <NUM> W/m/K. The material may comprise copper.

The apparatus may be for controlling the laser piercing of the material. Alternative or additional control purposes include controlling the cutting speed of materials in order to optimize the quality of the cut. If the cutting speed is too fast, then this can lead to poor edge quality of the cut. If the cutting speed is too slow, then this can lead to excessive dross on the underside of the cut. The optimal cutting speed can also vary across a material, particularly at the edges of a sheet of metal owing to a change in the local heat sinking provided by the remainder of the sheet. Being able to control the cutting speed by monitoring and analyzing the optical radiation emitted from the material is therefore advantageous. In particular, an increase in the noise of the electronic signal indicates that the cut quality is diminishing.

The apparatus may be for controlling the quality of a laser weld process. The laser is focussed on to the material and forms a melt. The nature of the melt can vary and is classified as either being in a conduction mode or keyhole mode. For both welding processes instabilities can lead to porosity and varying penetration depth and ultimately weak weld joints. Being able to detect and adjust the welding parameters by monitoring of the optical radiation emitted from the material is therefore desirable. Other features of the invention such as optical integrity monitoring described above can also be provided, either with a test piece or without.

The apparatus may be for actively monitoring the quality of a melt produced in an additive manufacturing process (also known as 3D printing). For a laser based additive process, layers of powder are successively laid down and selectively melted using a laser to form complex 3D structures. This is known as powder bed fusion. It is key to the structural integrity of the process that the quality of the fusion is maintained throughout the structure. Owing to the complex structures, the thermal properties can alter during the build leading to variations in the fusion process. Monitoring and analysing of the optical emission from the melt is therefore desirable. Other features of the invention such as optical integrity monitoring described above can also be provided, either with a test piece or without.

In a further aspect the invention may comprise a method as claimed in claim <NUM>.

The method may comprise providing an optical fibre and a laser processing head, and propagating the laser radiation from the laser through the optical fibre to the laser processing head.

The method may comprise providing a coupler, and coupling the optical radiation from the optical fibre to the detector.

In the method, the detector may be located in or on the laser processing head.

In the method, the electronic filter may comprise a peak detector.

In the method, the electronic filter may comprise a root mean square filter.

In the method, the electronic filter may comprise a bandpass filter.

In the method, the electronic filter may comprise a high pass filter.

In the method, the electronic filter may comprise an integrator.

In the method, the electronic filter may comprise a band stop filter configured to reject interference.

In the method, the discriminator may include a level detector for detecting a predetermined amplitude.

In the method, the discriminator may include a slope detector for detecting a predetermined slope.

In the method, the electronic filter and the discriminator may be configured in a digital electronic circuit.

In the method, the electronic filter may utilize spectral frequency analysis.

In the method, the electronic filter and the discriminator may be implemented with fuzzy logic.

In the method, at least one of the electronic filter and the discriminator may comprise a neural network.

In the method, the electronic filter may be an analogue electronic filter.

The method may comprise providing an optical filter. This can be advantageous in order to monitor and control other parameters associated with laser cutting, such as the temperature of the material while piercing and while cutting the material. The higher the temperature of the material, the more wavelengths in the optical spectrum that are emitted by the material. More than one optical filter can be provided each connected to a different detector. The filter detector combinations can be configured to analyse the optical spectral content at different wavelengths.

In the method, the characteristic feature may provide an indication that the material has been pierced. Alternatively or additionally, the characteristic feature may provide an indication as to when the optics requires cleaning or replacing in part. The characteristic feature that provides an indication as to when the optics requires cleaning or replacing in whole or in part may be different from the characteristic feature that provides an indication that the material has been pierced. Surprisingly the inventor has observed an increase in the amplitude of the electronic signal after the material has been pierced. This increase is associated with optics that require cleaning or replacing in whole or in part. The increase can be removed by replacing contaminated or defect cover slides that protect the lens from spatter from the work piece.

In the method, the material may comprise a test piece. This aspect of the invention is particularly useful in cutting machines. A test piece such as a <NUM> to <NUM> thick sheet of copper can be provided to one side of a cutting bed and the test piece can be pierced periodically for quality control purposes. Thus if the optics becomes dirty through spatter ejected from the material in a work piece such as steel or other materials, the state of the optics can be detected and the optics cleaned or cover slides replaced. Other features of the invention, such as pierce detection, can be utilized to control the piercing of the work piece.

In the method, the material may have a thermal conductivity greater than <NUM> W/m/K, preferably greater than <NUM> W/m/K and more preferably greater than <NUM> W/m/K. The material may comprise copper.

The method may be for controlling the laser piercing of the material. Alternative or additional control purposes include controlling the cutting speed of materials in order to optimize the quality of the cut. If the cutting speed is too fast, then this can lead to poor edge quality of the cut. If the cutting speed is too slow, then this can lead to excessive dross on the underside of the cut. The optimal cutting speed can also vary across a material, particularly at the edges of a sheet of metal owing to a change in the local heat sinking provided by the remainder of the sheet. Being able to control the cutting speed by monitoring and analyzing the optical radiation emitted from the material is therefore advantageous. In particular, an increase in the noise of the electronic signal indicates that the cut quality is diminishing.

The method may be for controlling the quality of a laser weld process. The laser is focussed on to the material and forms a melt. The nature of the melt can vary and is classified as either being in a conduction mode or keyhole mode. For both welding processes instabilities can lead to porosity and varying penetration depth and ultimately weak weld joints. Being able to detect and adjust the welding parameters by monitoring of the optical radiation emitted from the material is therefore desirable. The method may also include monitoring the optical integrity of the optics as described previously, either with a test piece, or without.

The method may be for actively monitoring the quality of a melt produced in an additive manufacturing process (also known as 3D printing). For a laser based additive process, layers of powder are successively laid down and selectively melted using a laser to form complex 3D structures. This is known as powder bed fusion. It is key to the structural integrity of the process that the quality of the fusion is maintained throughout the structure. Owing to the complex structures, the thermal properties can alter during the build leading to variations in the fusion process. Monitoring and analysing of the optical emission from the melt is therefore desirable. The method may also include monitoring the optical integrity of the optics as described previously, either with a test piece or without.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:.

<FIG> shows an apparatus for controlling laser processing of a material <NUM>, which apparatus comprises a laser <NUM> for emitting laser radiation <NUM>; means <NUM> for directing the laser radiation <NUM> onto the material <NUM>; at least one detector <NUM> for detecting optical radiation <NUM> that is emitted by the material <NUM>; an electronic filter <NUM> for filtering an electronic signal <NUM> emitted by the detector <NUM> in response to the detector <NUM> detecting the optical radiation <NUM>; and a discriminator <NUM> for analysing the output <NUM> from the electronic filter <NUM>. The apparatus is characterised in that the electronic filter <NUM> and the discriminator <NUM> are configured to determine at least one characteristic feature <NUM> of the electronic signal <NUM> that is indicative of the processing of the material <NUM> by the laser radiation <NUM>.

The apparatus of the invention is advantageous in that significant time is often devoted to piercing material. Detecting and analyzing the optical radiation <NUM> that is emitted by the material <NUM> enables the apparatus to commence another piercing or a cutting operation more quickly after piercing the material <NUM>. The optical radiation <NUM> emitted from the material <NUM> may be laser radiation that is reflected from the material <NUM> together with optical radiation that is emitted by the material <NUM> as a consequence of it being heated by the laser radiation <NUM>. For example, metals often emit white light during laser processing, and analysing the optical spectral content of such light can be useful for ensuring that the correct laser processing temperature has been achieved. Such analysis can be at one or more of visible, infra-red and ultraviolet wavelengths.

The characteristic feature <NUM> can be a reduction in the noise of the electronic signal <NUM>. Such reductions are observed to occur when the laser radiation <NUM> pierces the material <NUM>. The characteristic feature <NUM> can be a rise in the noise of the electronic signal <NUM> that occurs with some materials when the laser radiation <NUM> starts to pierce the material <NUM>, followed by a reduction in the noise amplitude or power of the electronic signal <NUM> that occurs when the laser radiation <NUM> has pierced the material <NUM>. The characteristic feature <NUM> can be a reduction in the amplitude or power of at least one frequency component of the electronic signal <NUM> when the laser radiation pierces the material <NUM>. Such frequency components are thought to be present because of the interaction of the laser radiation <NUM> with the material <NUM> during the piercing process. It is believed that molten metal may oscillate, reflecting the laser radiation <NUM> at angles that vary with a characteristic frequency.

The laser <NUM> can be a fibre laser, a disk laser, or a rod laser. Other types of laser may also be used.

The means <NUM> for directing the laser radiation <NUM> onto the material <NUM> includes an optical fibre <NUM> and a laser processing head <NUM>. The optical fibre <NUM> is configured to propagate the laser radiation <NUM> from the laser <NUM> to the laser processing head <NUM>. Optics <NUM> focuses the laser radiation <NUM> onto the material <NUM>. The optics <NUM> can be or can comprise a single or a multi-element lens. The optics <NUM> can include a plurality of lenses, including collimation and beam expanding optics. The optics <NUM> would preferably include a cover slide (not shown) to protect the lens from spatter from the work piece. The laser radiation <NUM> may be used to cut a hole <NUM> in the material <NUM>. Use of the optical fibre <NUM> enables the laser <NUM> to be located tens or hundreds of metres from the material <NUM> being processed. This can reduce constraints on factory layouts, for example in the provisioning of services such as cooling and electrical power for the laser <NUM>.

A controller <NUM> receives a signal <NUM> from the discriminator <NUM>. The controller <NUM> is used to control the laser <NUM> and the processing head <NUM>.

The detector <NUM> can be on or in the processing head <NUM>, as shown with reference to <FIG>. Alternatively, the apparatus can include a coupler <NUM> as shown in <FIG>. The optical radiation <NUM> is coupled into the optical fibre <NUM>, and routed back towards the coupler <NUM>, which directs the optical radiation <NUM> to the detector <NUM>. An optical fibre <NUM> can transmit the optical radiation <NUM> from the coupler <NUM> to the detector <NUM>. The coupler <NUM> separates the laser radiation <NUM> from the optical radiation <NUM>. Guiding the optical radiation <NUM> from the material <NUM> along the optical fibre <NUM> towards the laser <NUM>, and coupling the optical radiation <NUM> from the optical fibre <NUM> to the detector <NUM> helps to prevent damage to the laser <NUM> caused by the backward travelling optical radiation <NUM>. In addition, it enables the detector <NUM>, the electronic filter <NUM>, and the discriminator <NUM> to be located within or near to the laser <NUM>. This has the advantage for the end user that no equipment for controlling laser processing such as laser piercing is needed around the machining area, saving on cost and complexity. This is particularly advantageous for machining installations where it is difficult or impractical to include sensors at the material <NUM> for detecting laser piercing.

<FIG> shows a variety of alternative couplers <NUM>, each directing the optical radiation <NUM> to a different one of the detectors <NUM>.

The laser <NUM> can comprise a plurality of lasers <NUM> which are combined together with a laser signal combiner <NUM> and coupled into the optical fibre <NUM>. The optical fibre <NUM> can be terminated with an optical connector <NUM>. The laser signal combiner <NUM> is typically a fused optical fibre component that combines a plurality of input fibres <NUM> into a single laser output fibre <NUM>. The coupler <NUM> can be the laser signal combiner <NUM>. The laser signal combiner <NUM> can be configured to act as a cladding mode stripper, directing the optical radiation <NUM> to a detector <NUM> which may be placed adjacent to the laser signal combiner <NUM>. Alternatively or additionally, an input fibre <NUM> can direct the optical radiation <NUM> from the laser signal combiner <NUM> to a detector <NUM>.

As shown in the insert of <FIG>, the optical fibre <NUM> comprises a core <NUM>, a cladding <NUM>, and a coating <NUM>. The laser radiation <NUM> propagates along the core <NUM> of the fibre. The optical radiation <NUM> typically propagates along both the core <NUM> and the cladding <NUM> of the optical fibre <NUM>. The cladding <NUM> is typically significantly larger than the core <NUM> in fibres that are used for cutting applications, and consequently most of the optical radiation <NUM> typically propagates in the cladding <NUM>, and can be removed by an optional cladding mode stripper <NUM> as shown in <FIG>.

The coupler <NUM> can be the cladding mode stripper <NUM>, which can be configured to direct the backward travelling optical radiation <NUM> to a detector <NUM>. The detector <NUM> can be placed adjacent to the cladding mode stripper <NUM>. Preferably, the cladding mode stripper <NUM> and its configuration within the laser <NUM> is such that the cladding mode stripper <NUM> directs the backward travelling optical radiation <NUM> to the detector <NUM> more effectively than guiding any forward going laser radiation <NUM> that is removed by the cladding mode stripper <NUM> to the detector <NUM>. By "backward travelling" it is meant in a direction that is opposite to the forward going laser radiation <NUM>, which laser radiation <NUM> is directed from the laser <NUM> to the material <NUM> as shown with reference to <FIG>.

The coupler <NUM> can be the laser <NUM> or in the case of a single laser, the laser <NUM> of <FIG>. The optical radiation <NUM> can comprise light at different wavelengths from the wavelength of the laser radiation <NUM> emitted by the laser <NUM>. At least a portion of the optical radiation <NUM> can pass through the laser <NUM> to a detector <NUM>.

The coupler <NUM> can comprise a beam splitter, a dichroic mirror, an optical fibre coupler, a pump and signal beam combiner, or a laser signal combiner. Other forms of coupler <NUM> are also possible.

The coupler <NUM> shown in <FIG> and <FIG> can be in the laser <NUM>, or external to the laser <NUM>.

The coupler <NUM> can be formed from a composite optical fibre <NUM> comprising a first fibre <NUM> and a second fibre <NUM> as shown with reference to <FIG>. The first fibre <NUM> and the second fibre <NUM> are in optical contact with each other and are surrounded by a common coating material <NUM>. The first fibre <NUM> preferably comprises a core <NUM> for guiding the laser radiation <NUM>. The refractive index of the coating <NUM> is preferably lower than a refractive index of both the first and the second fibres <NUM>, <NUM> in order that the optical radiation <NUM> can be guided along the second fibre <NUM>. The second fibre <NUM> can be separated from the first fibre <NUM> by removing a portion of the coating <NUM>, and the second fibre <NUM> can be directed to the detector <NUM>. The first fibre <NUM> can be the optical fibre <NUM> in <FIG>.

The apparatus shown in <FIG> can comprise at least one optical filter <NUM>. The optical filter <NUM> is useful for transmitting certain optical wavelengths in the optical radiation <NUM> that are indicative of certain processing conditions, such as temperature. The optical filter <NUM> is shown located between the material <NUM> and the coupler <NUM>. Alternatively or additionally the optical filter <NUM> can be located between the coupler <NUM> and the detector <NUM>. The apparatus may comprise more than one optical filter <NUM> and detector <NUM>, each filter detector combination configured to analyse the optical spectral content at different wavelengths. The apparatus of <FIG> may also comprise at least one optical filter <NUM>.

The electronic filter <NUM> shown in <FIG> comprises a band stop filter <NUM> and a peak level detector <NUM>. The band stop filter <NUM> is useful for removing interference signals that are sometimes present in large factory settings.

The peak level detector <NUM> removes high frequency content from the signal <NUM> and emits a signal <NUM> that is indicative of the peak amplitude <NUM> of the signal <NUM> over a time constant (not shown) selected to be short enough to reduce latency, and long enough to reduce false triggering of the discriminator <NUM>. The peak level detector <NUM> can comprise a rectifier. The reduction in the amplitude (peak value) <NUM> of the signal <NUM> is indicative of the laser beam <NUM> piercing through the material <NUM>. The apparatus of <FIG> can also include one or more of the band stop filter <NUM> and the peak level detector <NUM>.

The electronic filter <NUM> shown in <FIG> can comprise a pre-filter <NUM> shown with reference to <FIG>. The pre-filter <NUM> is configured to improve the signal to noise ratio of the discriminator process. The pre-filter <NUM> can comprise a bandpass filter. A bandpass filter is advantageous for improving signal to noise ratio if the characteristic feature <NUM> in the electronic signal <NUM> occurs at a characteristic frequency. The centre frequency of the bandpass filter would be set at the characteristic frequency. A bandpass filter is also advantageous if the characteristic feature <NUM> is noise. The pre-filter <NUM> can comprise a high pass filter. A high pass filter is useful for removing the direct current value from the detector signal <NUM>. A high pass filter can be used if the characteristic feature <NUM> is noise amplitude or noise power. If a particular frequency band is subject to electrical noise from interference from machinery, then the pre-filter <NUM> can be configured to reject such interference by, for example, selecting the centre frequency of a bandpass filter such that the filter rejects the interference. If the laser <NUM> is operated as a pulsed laser, then the centre frequency of the bandpass filter can be selected to be either higher or lower than the pulse repetition frequency of the pulsed laser in order to reject "noise" at the pulse repetition frequency arising from, for example, unwanted reflections within the laser apparatus. Other pre-filters, for example band stop filters, are also possible.

The electronic filter <NUM> of <FIG> can comprise a non-linear function <NUM> shown with reference to <FIG>. The non-linear function <NUM> can comprise a demodulator, a peak level detector, a half wave rectifier, a full wave rectifier, or a root mean square filter. The non-linear function <NUM> can include a low pass filtering function such that the characteristic feature <NUM> is output as a baseband signal such as the signal <NUM> shown with reference to <FIG>. Other non-linear functions are also possible.

The discriminator <NUM> of <FIG> can comprise a level detector or a comparator for detecting a predetermined amplitude of the signal output by the filter <NUM>. The level detector or comparator preferably comprises hysteresis to improve the signal to noise ratio or reliability of the pierce detection process.

The electronic filter <NUM> of <FIG> can comprise an integrator <NUM> as shown with reference to <FIG>. The integrator <NUM> is for integrating the output of the non-linear function <NUM>. The discriminator of <FIG> may include a slope detector (not shown) for detecting a predetermined slope of the signal output from the integrator <NUM>. Alternatively or additionally, the output <NUM> of the integrator <NUM> can be delayed by a time delay <NUM> to provide a time delayed integrator output <NUM>, which output <NUM> is subtracted from the output <NUM> of the integrator in subtracting means <NUM>. The output <NUM> of the subtracting means <NUM> is compared to a reference value <NUM> in the discriminator <NUM> to produce the discriminator signal <NUM>.

The electronic filter <NUM> and the discriminator <NUM> of <FIG> may be configured in a digital electronic circuit. The digital electronic circuit may comprise a computer such as a microprocessor. The microprocessor may utilize spectral frequency analysis such as fast Fourier transforms to perform at least a portion of the filtering. The microprocessor may use fuzzy logic or a neural network in order to analyze the electronic signal <NUM> in order to provide the signal <NUM> that is indicative of the laser beam <NUM> piercing the material <NUM>. Alternatively, or additionally, the electronic filter <NUM> may comprise an analogue electronic filter.

<FIG> shows the electronic signal <NUM> emitted by the detector <NUM> when the optical radiation <NUM> is emitted from the material <NUM> in a typical pierce process. The process commences when the laser radiation <NUM> first interacts with the material <NUM>, resulting in the laser radiation <NUM> being reflected. Depending on the material <NUM> and the wavelength of the laser radiation <NUM>, the reflection can be <NUM>% to <NUM>% of the incident power, and can be higher for copper or gold. The amplitude of the signal <NUM> typically falls at a time <NUM>, indicating that the laser radiation <NUM> has initiated a phase change in the material <NUM>. The phase change can correspond to the material <NUM> melting, or to gases or particulates being emitted by the material <NUM>. Once the phase change occurs, the material <NUM> absorbs a greater proportion of the incident laser radiation <NUM>, allowing the hole <NUM> to be drilled. Typically, the optical radiation <NUM> that is coupled into the optical fibre <NUM> represents only about <NUM>% of the incident laser radiation <NUM>. The signal <NUM> can be very small and hard to detect. The hole <NUM> pierces through the material <NUM> at a time <NUM> and the optical radiation <NUM> reduces to near zero. As a consequence, the signal <NUM> reduces to near zero, with much of the signal being caused by other optical reflections or imperfections with the laser system. A level detector that monitors the signal <NUM> can be set with a threshold <NUM> corresponding to the time <NUM> when it is safe to conclude that the material <NUM> has been pierced. The laser <NUM> can then be turned off at a time <NUM>, whereupon the signal <NUM> falls to zero.

As shown in <FIG>, it can be difficult to detect reliably the time <NUM> at which the material <NUM> has been pierced. This problem is exacerbated if there is interference caused by electrical machines, electrical power line interference, or other forms of electromagnetic interference. Other problems include the power stability of the laser radiation <NUM> that is emitted by the laser, variations in the power of the laser radiation <NUM> used in different processes and materials, and drifts in the electronic threshold <NUM> for example caused by temperature. Additionally some materials such as copper can be pierced but still have a residual reflectivity to some of the incident laser radiation <NUM> being incident on copper surrounding the hole <NUM>, making pierce detection based on the amplitude of the detected optical radiation <NUM> complicated.

The apparatus and method of the present invention improve the reliability of the pierce detection by analysing at least one characteristic feature <NUM> of the electronic signal <NUM>. The characteristic feature <NUM> can be the amplitude or power of the noise content of the electronic signal <NUM>. The characteristic feature <NUM> can be the amplitude or power of a particular frequency component of the electronic signal <NUM>. The method is preferably independent of the power level of the incident laser radiation <NUM>. As will be described with reference to <FIG>, this can be achieved by deriving reference signals such as the reference <NUM> shown with reference to <FIG> from the signals that are related to the power level of the incident laser radiation <NUM>.

The apparatus shown in <FIG> was used in the following Examples. The laser <NUM> was a 2kW ytterbium-doped fibre laser, model number JK2000FL manufactured by SPI Lasers UK Limited of Southampton, England, that was modified by placing the detector <NUM> in close proximity to an internal cladding mode stripper that removes the optical radiation <NUM> that propagates in the cladding <NUM> of the optical fibre <NUM> back towards the laser <NUM>. The detector <NUM> was an Indium Gallium Arsenide photodiode which is sensitive to the infrared one micron laser radiation emitted by the fibre laser. The output of the detector <NUM> was amplified by an electronic amplifier.

The material <NUM> was <NUM> thick mild steel, and the laser radiation <NUM> was focussed to a diameter of <NUM> onto the surface of the material <NUM>. <FIG> shows the signal <NUM> that was output by the detector <NUM> while piercing the material <NUM>. The laser <NUM> took approximately <NUM> to pierce the material <NUM>. The signal <NUM> was noisy between times <NUM> and <NUM>, indicating that the pierce process is inherently unstable. The effective reflectivity of the material <NUM> is oscillating, resulting in the signal <NUM> being heavily modulated over a wide frequency range (<NUM>-<NUM> for a typical detector system). In addition, there is very little change in amplitude of the signal before and after the time <NUM> at which the material <NUM> was pierced by the laser radiation <NUM>. It is believed that this is because the back reflected signal was much smaller than the signal caused by spurious reflections within the apparatus shown in <FIG>. As can be seen in <FIG>, it would have been difficult to set a threshold value <NUM> to provide a reliable indication of piercing.

The signal <NUM> was amplified, digitized with an analogue to digital converter sampling at <NUM>, and filtered using the filter <NUM> shown in <FIG>. The pre-filter <NUM> was a second order bandpass filter having a centre frequency of <NUM> and a quality factor Q of <NUM>. The non-linear function <NUM> was a digital rectifier characterized by an output equal to the modulus of the input to the digital rectifier. The integrator <NUM> was digital integration by which the output at a particular sample time was equal to the output at the preceding sample time plus the input to the integrator <NUM>. The delay <NUM> corresponded to <NUM> samples. The reference <NUM> was set to provide a reliable indication of pierce detection of the material <NUM> by the laser radiation <NUM>.

The output <NUM> of the digital rectifier is shown in <FIG>. The signal <NUM> reduces in amplitude to near zero after the time <NUM>. The output <NUM> of the integrator <NUM> is shown in <FIG>. The slope of the output <NUM> reduces to near zero after the time <NUM>. <FIG> also shows the output <NUM> of the subtracting means <NUM> and the discriminator signal <NUM>. The output <NUM> has been level shifted by <NUM> units for clarity. The discriminator signal <NUM> was obtained by comparing the output <NUM> of the subtracting means <NUM> to a reference value <NUM>. When the output <NUM> fell below the value of the reference value <NUM>, the discriminator signal <NUM> rises to provide a positive indication of the piercing of the material <NUM>.

Alternatively, the discriminator <NUM> in <FIG> can analyze the slope of the output <NUM> to determine the time <NUM>, or can analyze the signal <NUM>, for example by performing a rolling variance of the signal <NUM> shown with reference to <FIG>, and looking for the variance to reduce significantly in amplitude. Other non-linear functions and discrimination methods can also be used in isolation or in addition.

The signals <NUM> and <NUM> shown with reference to <FIG> and <FIG> illustrate how the method of the invention can be used to detect piercing of the material <NUM> when the slope of the output <NUM> of the integrator <NUM> of the selected high frequency band falls to near zero. For this example <NUM> has been chosen as the centre frequency of the band pass filter, but the algorithm works equally well with centre frequencies between <NUM> and <NUM> and a quality factor Q of <NUM>. Other quality factors also work well. It should be noted that latency is reduced as the ratio of centre frequency to quality factor Q is increased. Reducing latency allows the discriminator decision within the discriminator to be made more rapidly.

It is preferable that the laser radiation <NUM> emitted by the laser <NUM> is stable. In this Example, the "noise" within the bandpass of the <NUM> filter without the presence of the optical radiation <NUM> being emitted by the material <NUM> was <NUM>% peak to peak, whereas the "noise" in the presence of the optical radiation <NUM> was approximately <NUM>% peak to peak. The centre frequency of the bandpass filter is preferably selected to increase the signal to noise ratio of the piercing detection process, and may vary from laser type to laser type. The optimum centre frequency can be found by experimentation. In this Example, the characteristic feature <NUM> was the amplitude or power of a particular frequency component of the electronic signal <NUM>.

Experiments were performed to investigate laser piercing of more reflective materials, including brass and copper.

The material <NUM> was <NUM> thick copper, and the laser radiation <NUM> was focussed to a diameter of <NUM> onto the surface of the material <NUM>. <FIG> shows the signal <NUM> that was output by the detector <NUM> while piercing the material <NUM>. The laser <NUM> took approximately <NUM> to pierce the material <NUM>. The signal <NUM> is very noisy between times <NUM> and <NUM>, indicating that the pierce process is inherently unstable. Interestingly, the signal <NUM> increases in amplitude after the time <NUM> which would make a purely amplitude discrimination process, such as described with reference to <FIG>, difficult.

The signal <NUM> was passed through the same filter <NUM> as used in Example <NUM>. The output <NUM> of the integrator <NUM> is shown in <FIG>, together with the output <NUM> of the subtracting means <NUM> and the discriminator signal <NUM>. As before, the output <NUM> has been level shifted by <NUM> units for clarity. The discriminator signal <NUM> provides a positive indication of the piercing of the material <NUM>. In this Example, there were two characteristic features <NUM>:.

The increase of the amplitude of the signal <NUM> was indicative that the optics <NUM> needs cleaning or replacing in whole or in part. In the experiment, replacing the cover slide that formed part of the optics <NUM> with a new and clean cover slide eliminated the increase in the amplitude of the signal <NUM> after the time <NUM>.

Without wishing to limit the scope of the invention, it is believed that the increase occurred because (i) the optics <NUM> was contaminated, and (ii) the material <NUM> was copper. Copper has a higher thermal conductivity than the steel used in Example <NUM>. Consequently heat is conducted away from the melt zone caused by the interaction of the laser radiation <NUM> with the copper more rapidly than it is with steel. The resulting hole <NUM> in copper therefore has a similar diameter to the spot size of the laser radiation <NUM> on the material <NUM>. Contaminants or damage to the optics <NUM> can cause scattering of the laser radiation <NUM>, or can result in thermal lensing within the optics <NUM>. Either may result in drift in the position of the laser radiation <NUM> on the surface of the material <NUM>, resulting in an increasing amount of interaction of the laser radiation <NUM> with the material <NUM>. This interaction can cause additional melt zones on the material and the reflection from these can increase with time with a rate of change <NUM> that increases with the magnitude of the imperfections in the optics <NUM>.

Steel has a lower thermal conductivity than copper, and the hole <NUM> can have a diameter that is twice or more then the diameter of the laser radiation <NUM> on surface of the material <NUM>. Once the material <NUM> is pierced, there will therefore be less interaction of the laser radiation <NUM> with steel than there is with copper.

The ability to detect damaged or contaminated optics, that is optical integrity monitoring, provides important advantages in the laser processing of materials. The material <NUM> can have a thermal conductivity greater than <NUM> W/m/K, preferably greater than <NUM> W/m/K and more preferably greater than <NUM> W/m/K. The material <NUM> can be a test piece that can be pierced periodically by the laser radiation <NUM> in order to validate the integrity of the optics <NUM>. Including a test piece such as copper is particularly useful when laser processing material such as steel that has a lower thermal conductivity. The electrical signal <NUM> can be stored and analyzed each time it is pierced for quality control purposes including monitoring the cleanliness and integrity of the optics <NUM>. If the rate of change <NUM> of the electrical signal <NUM> is greater than a predetermined value, then an alarm can be generated to alert the user that the optics <NUM> needs cleaning or replacing in whole or in part. Although this optical integrity monitoring feature has been described with reference to Example <NUM>, it can be used in any of the other embodiments and examples of the invention described herein.

The material <NUM> was <NUM> thick aluminium, and the laser radiation <NUM> was focussed to a diameter of <NUM> onto the surface of the material <NUM>. <FIG> shows the signal <NUM> that was output by the detector <NUM> while piercing the material <NUM>. The laser <NUM> took approximately <NUM> to pierce the material <NUM>. The signal <NUM> becomes increasingly noisy as the hole <NUM> is pierced through the material <NUM>, and is especially noisy between a time <NUM> and the time <NUM> that the piercing was achieved.

<FIG> shows the frequency spectrum <NUM> of the signal <NUM> shown in <FIG> between times of <NUM> to <NUM>. The frequency spectrum <NUM> was obtained using fast Fourier transforms. Also shown is the frequency spectrum <NUM> obtained without the optical radiation <NUM> being emitted by the work piece. The frequency spectrum <NUM> is larger than the frequency spectrum <NUM>, illustrating the amount of noise or information that is contained in the signal <NUM> during the piercing process.

Although the frequency spectrum <NUM> is relatively broad, it contains certain frequency components that are stronger than others. One of the most prominent frequency components <NUM> is at <NUM>. The digital filter used in Examples <NUM> and <NUM> was therefore modified such that the central frequency of the digital band pass filter was equal to <NUM>. The quality factor Q was reduced from <NUM> to <NUM>.

The output <NUM> of the integrator <NUM> is shown in <FIG>. The slope of the output <NUM> reduces at the time <NUM> at which the piercing of the material <NUM> was complete. There is then another increase in slope at time <NUM> when the laser <NUM> was turned off. This increase in slope occurs because the step change in the signal <NUM> at time <NUM> shown with reference to <FIG> has a large frequency content, including at the centre frequency of the digital band pass filter. Also shown is the output <NUM> of the subtracting means <NUM> and the discriminator signal <NUM>. As before, the output <NUM> has been level shifted for clarity. The discriminator signal <NUM> provides a positive indication of the piercing of the material <NUM>.

Referring again to <FIG>, there is also shown an increase in the slope of the output <NUM> at time <NUM>, corresponding to the increase in noise of the signal <NUM> in <FIG> and output <NUM> in <FIG>. This increase in noise was evidenced in other experiments with aluminium, and using other sensors, was found to correspond to the time that an incomplete pierce of the aluminium was achieved. It is believed that the increase in noise corresponds to a change in dynamics of the melt pool within the hole <NUM>. In this Example, the characteristic feature <NUM> was an increase of the amplitude or power of a particular frequency component of the electronic signal <NUM>, followed by a decrease of the amplitude or power at that particular frequency.

This Example illustrates an optional feature of the invention, namely that additional information can be gathered about the dynamics of the melt within the hole <NUM> as the material <NUM> is being pierced by analysing specific frequency bands of the signal <NUM>.

<FIG> shows an implementation of the filter <NUM> shown in <FIG>. The filter <NUM> comprises a high pass filter <NUM>, and rectifier <NUM>, a first low pass filter <NUM>, and a second low pass filter <NUM>. The high pass filter <NUM> was configured to have a bandwidth of <NUM>. The rectifier <NUM> was configured to invert negative going signals and pass positive going signals. The first low pass filter <NUM> was configured to have a bandwidth of <NUM> and a gain of unity. The second low pass filter <NUM> was configured to have a bandwidth of <NUM> and a gain of <NUM>/<NUM>. The discriminator <NUM> was configured to output a positive going signal when the signal <NUM> was larger than the signal <NUM>.

The signals <NUM> shown with reference to <FIG>, <FIG> and <FIG> were analysed using the filter <NUM> and discriminator <NUM> shown in <FIG>. The results are shown in <FIG>, <FIG> and <FIG> respectively. Referring to <FIG>, the signal <NUM> was at the input <NUM> to the high pass filter <NUM>. The waveform <NUM> was at the output <NUM> of the high pass filter <NUM>. The waveform <NUM> was at the output <NUM> of the rectifier <NUM>. The waveform <NUM> was at the output <NUM> of the first low pass filter <NUM>. The waveform <NUM> was at the output <NUM> of the second low pass filter <NUM>. The waveform <NUM> was at the output <NUM> of the discriminator <NUM>. The same numbering convention was used for <FIG> and <FIG>. In each of <FIG>, the waveform <NUM> provides a robust measure of laser piercing, which piercing is indicated by the first positive going signal <NUM>. The characteristic feature <NUM> in the analysis shown in <FIG> is the noise amplitude or power of the electronic signal <NUM>.

The bandwidths, gains, and configuration of the filters shown in <FIG> can be adapted and modified, and different types of analogue or digital filters can be used. The second low pass filter <NUM> can be replaced with a simple threshold value (voltage for analogue electronics, number for digital filtering), which can be selected by the user through experimentation or via a look up table. However using the second low pass filter <NUM> as described has advantages in that the waveform <NUM> scales with the waveform <NUM>, and thus the pierce detection process is independent of the amplitude of the signal <NUM>.

The invention described with reference to the Figures and the Examples can be used in a variety of ways, including:.

Claim 1:
Apparatus for controlling laser processing of a material, which apparatus comprises:
• a laser for emitting laser radiation;
• means for directing the laser radiation onto the material;
• at least one detector for detecting optical radiation that is emitted by the material;
• an electronic filter for filtering an electronic signal emitted by the detector in response to the detector detecting the optical radiation; and
• a discriminator for analysing the output from the electronic filter;
and the apparatus being characterised in that
• the electronic filter and the discriminator are configured to determine at least one characteristic feature of the electronic signal that is indicative of the processing of the material by the laser radiation; and
• the characteristic feature comprises at least one of noise of the electronic signal thereby to provide an indication of when the material has been pierced, and an increasing amplitude of the electronic signal after the material has been pierced thereby to provide an indication of when optics in the apparatus needs cleaning or replacing in whole or in part.