Temperature measurement assembly and method

Abstract

Pyrometer assembly 150 comprises spectrometer 156, perhaps a charge-coupled device spectrometer, to determine the thermal emission spectrum of light from a surface, and controller 130 to determine sur

Description

Technical Field The present disclosure relates to a pyrometer assembly and a temperature measurement assembly, such as a Wire Arc Additive Manufacturing (WAAM) assembly, and is particularly, although not exclusively, concerned with a pyrometer assembly which is adapted for use in a WAAM assembly. Background Wire Arc Additive Manufacturing (WAAM) is a 3D manufacturing process in which multiple layers of a weld material, e.g. metal, such as steel, nickel, aluminium, titanium or alloys thereof, are sequentially deposited on top of each other, for example, using a welding process, to form a desired 3D shape. The development of WAAM has improved the ease of production of complex parts due to the freedom of motion available to the welding torch. This has assisted in the transfer of complex 3D computer-aided designs to physical components. Additionally, component manufacture through WAAM can be more cost-effective than traditional manufacturing techniques, as material waste can be reduced. In a WAAM process, a plasma welding process may be used, in which a jet of a plasma forming gas, such as argon gas, is ejected through a nozzle in a weld torch and an electric arc is formed between the weld torch and a weld substrate via a plasma formed by ionising the plasma forming gas. The electric arc causes a melt pool to form in the weld substrate. A wire formed from the weld material, is fed into the melt pool to deposit additional weld material onto the bead which is formed as the torch is in motion. The weld torch is translated over the underlying substrate to deposit layers of the weld material over the substrate, in order to build up the 3D shape. Additionally or alternatively, the manufacturing process may involve a laser welding process, in which a high power laser is used to provide a concentrated heat source to produce the melt pool into which weld material is fed. During laser welding, a metal vapour may be produced from the weld substrate, which may become ionised to form a plasma. In some cases, the WAAM process may include both a laser and a weld torch for forming the plasma. Although WAAM is a promising production method, there are multiple operating parameters that could be measured and monitored during the manufacturing process to provide improved process control. Measuring and monitoring such parameters can enable the production of components with improved quality and / or reduce the requirement for post manufacture quality inspections such as non-destructive testing. In particular, it may be desirable to measure the temperature of the melt pool, the deposited metal bead and / or the area of the substrate adjacent the melt pool during the WAAM process. Previously proposed temperature measurement systems, which have been applied to measure the temperature of the melt pool in a WAAM process, have experienced inaccuracies due to the high temperature environment in which the system is operating, and the presence of the plasma and other background radiation sources. For example, existing one or two channel pyrometers typically operate by measuring the intensity of light within one or two approximately 18 nm wide bands of the object’s thermal emission spectrum. Any interfering radiation within the bands of the pyrometers can therefore result in significant measurement errors. Furthermore, in order to determine the temperature of the weld material using a one or two channel conventional pyrometer, it is often necessary to know the emissivity of the weld material, which may not be available or may change unpredictably during the WAAM process. Statements of Invention According to an aspect of the present disclosure, there is provided a temperature measurement assembly for measuring temperature of a surface in the presence of light interference, wherein the assembly comprises: a source of light interference; and a pyrometer assembly, the pyrometer assembly comprising: a spectrometer configured to determine a thermal emission spectrum of light from the surface; and a controller configured to: determine a temperature of the surface based on a plurality of intensities of the determined emission spectrum. The source of light interference may generate light, e.g. visible and / or non-visible light, for example, in a wavelength range of 425nm to 2000nm. The spectrometer may be configured to operate within a wavelength range of 425nm to 2000nm. By using the pyrometer assembly to measure the temperature of the surface, there may be no requirement for knowing surface emissivity. The temperature measurement assembly may comprise an assembly for performing a laser-based additive manufacturing processes, a fusion and / or gas welding processes, or may comprise an assembly including, or operating at least partially within, a plasma environment, an internal combustion engine and / or a gas-turbine engine. The assembly may be a Wire Arc Additive Manufacturing (WAAM) assembly. The source of light interference may comprise a weld torch and / or laser. The weld torch and / or laser may be configured to generate a plasma at a weld substrate and create a melt pool on a surface of the weld substrate. The surface may comprise the surface of the weld substrate. The spectrometer may be a Charge-Coupled Device, CCD, spectrometer. The temperature may be determined based on intensities measured by a plurality of light sensing elements of the CCD spectrometer, e.g. which correspond to a plurality of different wavelengths. The controller may be configured to determine the temperature of the surface based on a function of the intensities, e.g. a plurality of intensities, and wavelengths of the determined thermal emission spectrum. The controller may be configured to determine a gradient of a line, e.g. a straight line, fitted to a relationship between functions of the intensities and wavelengths of the determined thermal emission spectrum. The temperature may be determined based on the gradient of the line. The line may be fitted to a plurality of points in the relationship between functions of the intensities and wavelengths of the determined emission spectrum at a number of predetermined wavelengths. The line may be fitted to the relationship between the functions of the intensities and wavelengths of the determined emission spectrum so as to disregard the effect of lines / peaks in the emission spectrum resulting from the light interference. The line, e.g. the straight line, may be fitted to a relationship between C2 / A and Ln(A51), wherein C2 is a constant, A is wavelength and I is intensity. C2 may be 14388 pm K. The controller may be configured to transform the thermal emission spectrum to Wien co-ordinates. The temperature of the surface may be determined based on the transformed emission spectrum. For example, the temperature may be determined based on a gradient, e.g. linear gradient, of the transformed thermal emission spectrum. The controller may be configured to subtract a line emission spectrum of a constituent of the plasma from the determined thermal emission spectrum prior to determining the temperature of the surface. The controller may be configured to correct raw data from the spectrometer for the baseline error by subtracting a background error value from the raw data from the spectrometer. The background error value may be determined by determining a background thermal emission spectrum using the spectrometer for an object at a predetermined temperature, and averaging the values of the background thermal emission spectrum to determine the background error value. The controller may be configured to determine a black body thermal emission spectrum for a black body radiation source and correct the thermal emission spectrum for the spectral deviation from the ideal black body distribution based on the determined black body thermal emission spectrum. The controller may be configured to control the operation of the weld torch and / or laser based on the determined temperature. The controller may be configured to control the operation of the weld torch and / or laser to maintain a temperature of the melt pool or an area surrounding, e.g. within a predetermined threshold distance of, or adjacent the melt pool within a predetermined temperature range. The assembly may further comprise an optical sighting tube for receiving light from the surface and an optical fibre extending from the optical sighting tube to the spectrometer. The assembly may further comprise an optical splitter coupled to the optical fibre, and a guide light source configured to transmit a light on a return path through the optical fibre towards the surface. According to another aspect of the present disclosure, there is provided a pyrometer assembly, e.g. for the temperature measurement assembly of any of the preceding claims, wherein the pyrometer assembly comprises: a spectrometer configured to determine a thermal emission spectrum of light from a surface; and a controller configured to: determine a temperature of the surface based on a plurality of intensities of the determined emission spectrum. The pyrometer may comprise one, more than one or each of the features of the above-mentioned pyrometer of the temperature measurement assembly. According to another aspect of the present disclosure, there is provided a temperature measurement method, the method for measuring a temperature of a surface in the presence of light interference from a light interference source, wherein the method comprises: determining a thermal emission spectrum of light from the surface; and determining a temperature of the surface based on a plurality of intensities of the determined thermal emission spectrum. The method may comprise generating the light interference. For example, the method may be performed by the above-mentioned temperature measurement assembly. According to another aspect of the present disclosure, there is provided a method for a Wire Arc Additive Manufacturing (WAAM) assembly, the assembly comprising a weld torch and / or laser, and a pyrometer assembly including a spectrometer, wherein the method comprises: generating a plasma at a weld substrate and creating a melt pool on a surface of the weld substrate; determining an emission spectrum of light from the melt pool, or an area surrounding or adjacent the melt pool; and determining a temperature of the melt pool, or area surrounding or adjacent the melt pool, based on a plurality of intensities of the determined emission spectrum. To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention. In particular, features described in relation to the first mentioned aspect may be combined with the features of the subsequently mentioned aspects. Brief Description of the Drawings For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic view of a welding assembly according to arrangements of the present disclosure; Figure 2 is a graph showing a typical short-wave infra-red spectrum measured during a titanium alloy plasma arc deposition process; Figure 3 is a graph showing the typical spectrum of Figure 2 transformed to Wein coordinates; Figure 4 is a flow chart illustrating a temperature measurement method according to arrangements of the present disclosure; and Figure 5 is a flow chart illustrating a method for a welding assembly according to arrangements of the present disclosure; and Figure 6 is a flow chart illustrating another method for a welding assembly according to arrangements of the present disclosure. Detailed Description With reference to Figure 1, temperature measurement assembly, e.g. a welding assembly, such as a Wire Arc Additive Manufacturing (WAAM) assembly 100 according to arrangements of the present disclosure will now be described. In the arrangement depicted in Figure 1, the welding assembly 100 comprises a weld torch 112 supported on a holder 114, e.g. a weld torch holder, and may comprise a bed 116, for supporting a weld substrate 118. The welding assembly 100 further comprises a power supply 120, a gas source 122, configured to supply plasma forming gas and / or shield gas to the weld torch, and a source of weld wire 124, e.g. a metal weld wire, such as weld wire formed of steel, nickel, aluminium, titanium or alloys thereof. When the shield gas comprises a different gas or mixture of gases from the plasma forming gas, the welding assembly 100 may comprise a further gas source 123 for providing the shield gas or additional component of the shield gas. In use of the welding assembly 100, a jet of the plasma forming gas is emitted from the weld torch 112 towards the weld substrate 118 supported on the bed 116. A potential difference between an electrode of the weld torch 112 and the bed 116, e.g. generated by the power supply 120, causes the plasma forming gas to become ionised as a plasma and an electrical current to pass between the weld torch and the substrate via the plasma. The electric current causes a melt pool to form on the substrate. In alternative welding assemblies, the current may pass into the torch instead. Weld wire is fed, through or adjacent to the weld torch, to the melt pool (or fed directly at an angle) and material from the weld wire is deposited onto the substrate at the melt pool. The welding assembly 100 may further comprises a shield gas emitter 113 configured to emit a shield gas around the melt pool produced by the welding assembly 100 and / or around the plasma. In the arrangement shown in Figure 1, the shield gas emitter 113 comprises a portion of the weld torch. For example, the weld torch may comprise a nozzle for emitting the plasma forming gas and an opening arranged about the nozzle for emitting the shield gas, so that the shield gas surrounds the plasma. In other arrangements, the weld substrate 118 may be arranged within an enclosure 140. The shield gas may be provided within the enclosure, such that the shield gas is around the plasma and / or melt pool. In other arrangements, the welding assembly 100 may comprise a laser configured to act as a heat source for producing a melt pool on a surface of the weld substrate 118. The laser may be mounted on the weld torch holder 114. Heating of the substrate by the laser may lead to the metal of the substrate being vaporised and ionised to form a plasma. The laser may be provided in addition, or as an alternative, to the weld torch 112. When the weld torch 112 is not provided, the shield gas may be emitted by a separate shield gas emitter, e.g. comprising a nozzle or opening for emitting the shield gas around the plasma and / or melt pool. The welding assembly 100 may further comprise a holder actuator 126 for moving and / or rotating the weld torch holder 114 in one or more axes relative to the bed 116. For example, the holder actuator 126 may comprise an end effector of a robot arm configured to move and or rotate the weld torch holder in 3 or more, such as 4, 5, 6, 7 or 8, axes. In some arrangements, the welding assembly 100 may further com prise a bed actuator 128 for moving and / or rotating the bed 116 in one or more axis relative to the weld torch holder 114. The holder actuator and bed actuator may be together configured such that the weld torch can be moved and / or rotated relative to a weld substrate supported on the bed in more than 3 movement axes, such as 4, 5, 6, 7 or 8 axes. Movement of the weld torch holder 114 and bed 116 may be controlled, e.g. based on Computer Numerical Control (CNC), in order to produce a component through the WAAM process. For example, the welding assembly 100 may comprise a CNC controller for controlling the holder actuator 126 to move the weld torch holder 114 and the bed actuator 128 to move the bed 116 in order to produce a component. During manufacture of a component using a WAAM process, the weld torch 112 is moved and / or rotated relative to the substrate in order to deposit layers of weld material over the substrate to build up the three-dimensional shape of the component. During manufacture of the component, a number of parameters of the welding system 110 may be adjusted in order to affect, e.g. improve, a quality of the weld. In particular, the feed rate of the weld wire, flow rates of the plasma forming gas and / or shield gas, the welding current passed through the torch, and / or movement and / or rotation speed of the weld torch and / or bed may be adjusted in order to affect weld quality. One operating parameter that affects the quality of the manufactured part is the temperature of the melt pool and a surrounding area, e.g. an area within a predetermined threshold distance of a centre of the melt pool, and / or an area adjacent the melt pool, e.g. within a predetermined distance behind the melt pool relative to the weld torch. The melt pool and area surrounding and / or adjacent the melt pool may be referred to as a melt pool region. In particular, it may be desirable for the temperature of the melt pool to be maintained within a predetermined threshold range, such as between about 2500°C and 500°C, e.g. when the weld material comprises a titanium alloy. The temperature of the melt pool region during the WAAM process, e.g. during material deposition and / or between passes (the inter-pass temperature) may affect the material properties of the layers of weld material deposited. Controlling the temperature of the melt pool and / or the region behind the melt pool, is therefore desirable in order to achieve the desired material properties of the deposited material. Further, if the temperature varies during deposition of a layer of weld material and / or between layer, the material properties of the weld material may vary between different parts of the manufactured components, which may be undesirable. The temperature of the melt pool region may be affected by the weld current. Additionally or alternatively, the temperature of the melt pool may be affected by the number of layers previously deposited, inter-pass temperature, feed rate of the weld wire, flow rates of the plasma forming gas and / or shield gas, and / or movement and / or rotation speed of the weld torch and / or work piece travel speed. In order to provide an improved determination of the temperature at and / or around the melt pool, the welding assembly 100 according to the present disclosure comprises a pyrometer assembly 150, which is configured to determine the temperature at and / or in an area around the melt pool, e.g. within a predetermined distance of, or offset by a predetermined distance from the melt pool. In the arrangement shown in Figure 1, the pyrometer assembly 150 comprises an optical fibre or optical fibre cable 152, such as a multi-mode fibre optic cable. A first end 152a of the optical fibre may be mounted on the weld torch holder 114, and may be oriented to receive light from the melt pool, and / or an area around the melt pool and / or offset from the melt pool, when the welding assembly 100 is operating. The pyrometer assembly 150 may further comprise a lens 154, such as a collimating lens, mounted on the weld torch holder. The lens 154 may be configured to focus light from a surface of the weld substrate and / or weld material to the first end 152a of the optical fibre. The pyrometer assembly 150, e.g. the lens 154 and optionally the optical fibre / fibre optic cable may be configured to receive light from a collection area, e.g. around or offset from the melt pool. In some arrangements, the pyrometer assembly 150 may further comprise a sighting tube provided on an opposite side of the lens 154 from the optical fibre cable 152. The sighting tube may reduce optical radiation originating from elsewhere other than the collection area being transmitted to the optical fibre cable 152. In the arrangement depicted, the pyrometer assembly 150 further comprises a spectrometer 156. A second end 152b of the optical fibre is coupled to the spectrometer 156. The spectrometer 156 comprises optical and electrical components configured to receive the light from the optical fibre 152 and generate an emission spectrum of the light. The spectrometer 156 may comprise a plurality of light sensing elements, and the spectrometer 156 is configured such that light within different wavelength bands is sensed by different ones of the light sensing elements. The light sensing elements may comprise, e.g. be manufactured from, an Indium Gallium Arsenide (InGaAs) semiconductor material. Alternatively, the light sensing elements may comprise a silicon semiconductor material or any other suitable material, e.g. sensitive in the spectral region 425nm to 2000nm . The spectrometer 156, e.g. optical components and the light sensing elements of the spectrometer, may be configured such that light within wavelength bands of approximately 3 nm are sensed by respective light sensing elements. In some arrangements, the spectrometer may comprise a Charge-Coupled Device (CCD). The plurality of light sensing elements may be provided on the CCD. The spectrometer 156 may comprise a multi-spectral and / or hyper-spectral camera, as an alternative or in addition to the spectrometer. A controller of the spectrometer, or the pyrometer controller 130 mentioned below, may be configured to capture intensity measurements from the plurality of light sensing elements, e.g. corresponding to the different wavelength bands. Due to the high intensity of light that may be received at one or more wavelengths, e.g. from the plasma, the spectrometer controller may be configured to capture the intensity measurement from the one or more light sensing elements at a high sensing frequency, in order to reduce the likelihood of the one or more light sensing elements becoming saturated. The spectrometer controller may be configured to capture the intensity measurement from the one or more light sensing elements at wavelengths of between 425nm and 2000nm. In other arrangements, as an alternative to the spectrometer, the pyrometer assembly 150 may comprise one or more band pass optical filters arranged to receive the light from the melt pool, e.g. via the optical fibre cable 152, and pass light of different respective predetermined wavelengths or bands of wavelengths. The pyrometer assembly 150 may further comprise one or more optical detectors configured to detect the light passed by the respective optical filters. When the pyrometer assembly 150 comprises one or more optical filters and associated optical detectors rather than the spectrometer, the pyrometer assembly 150 may comprise a controller, such as the pyrometer controller 130 mentioned below, configured to determine intensity measurements of the light passed by the one or more optical filters from the one or more optical detectors respectively. In some arrangements, the pyrometer assembly 150 may further comprise an optical splitter 153 coupled to the optical fibre 152, and a guide light source 154 configured to transmit a light on a return path through the optical fibre 152 via the splitter to the first end 152a, so as to illuminate the weld material and guide placement of the optical fibre to capture radiation from a desired location on the weld material, e.g. ata desired location relative to the melt pool. The pyrometer assembly 150 further comprises a pyrometer controller 130. The pyrometer controller, or another controller (such as the WAAM controller described below), is configured to determine a temperature of, e.g. at and / or around and / or in an area offset from, the melt pool based on information received from the spectrometer 154 (or optical detectors if present). In particular, as described in greater detailed below, the pyrometer controller 130 is configured to determine the temperature of the melt pool based on the thermal emission spectrum, e.g. a plurality of intensities of the emission spectrum, determined by the spectrometer 156 (or optical detectors). Figure 2 illustrates an example thermal emission spectrum 200 that may be determined by the spectrometer 156 of the pyrometer assembly 150 from light sensed from an area at and / or around and / or offset from the melt pool of a WAAM assembly, such as the WAAM assembly 100. As shown, the emission spectrum 200 may comprise one or more plasma spectral lines 202 corresponding to emissions from components of the plasma, such as argon, e.g. in addition to the desired thermal emission spectrum. Additionally or alternatively, the emission spectrum 400 may include further spectral lines corresponding to other background sources of radiation. When determining the temperature of, e.g. at and / or around and / or in an area offset from, the melt pool, the high intensity lines within the emission spectrum may lead to inaccuracies in the determined temperature. Accordingly, it is desirable to account for the presence of the high intensity spectral lines when determining the temperature. Conventional pyrometers apply Planck’s radiation law to intensity measurement(s) of radiation at one or more particular wavelengths, in order to determine the temperature of the body emitting the radiation. The determined temperature may be corrected using the emissivity of the particular body in order to correct for black-body assumptions of the application of Planck’s radiation law. According to the present disclosure, and as described above, the spectrometer 156 is able to directly observe the intensity distribution of the emission spectrum. The pyrometer controller 130 may be configured to determine the temperature of the melt pool by comparing the determined emission spectrum to a Planck distribution. For example, the pyrometer controller 130 may be configured to fit a Planck distribution corresponding to the emission of a black body of a particular temperature to the determined emission spectrum. The temperature of the melt pool may correspond to the particular temperature associated with the fitted Planck distribution or Planck distribution. The spectral intensity I(A,T) of the thermal emission can be expressed by: l(A,T) = £ Ci A-5 / [exp(C2 / AT) -1] (1) Where £ is emissivity, T is temperature, Ci=37418 Wpm4 / cm2, 02=14388 pm K, the wavelength (A) is expressed in microns and the intensity is expressed in W / cm2 pm. In the Wein region of the emission spectrum (C2 / AT » 1), transformations lead to the following form of expression (1): Ln(A5l) - Ln(£ Ci)= - C2 / (AT) (2) If £=constant, a spectrum portion in this region is linearized in the (x, y) coordinate plane where y= Ln(A5l) and x = C2 / A and the slope of the fitted line is determined by the radiation temperature. Figure 3 illustrates the emission spectrum shown in Figure 2 transformed into Wein coordinates, e.g. by plotting C2 / A on the x-axis and Ln(A5l) on the y-axis. The gradient of the transformation of the emission spectrum, plotted in Figure 3, e.g. a straight line fitted to the transformed emission spectrum, may be equal to T. The pyrometer controller 130, or other controller, may be configured to determine the temperature of the melt pool region based on a relationship between functions of the intensities and wavelengths of radiation received from the melt pool, e.g. an area at and / or around and / or offset from the melt pool. For example, the pyrometer controller 130 may be configured to determine a relationship between C2 / A and Ln(A5l). The controller 130 may determine the temperature of the melt pool based on the relationship. For example, the pyrometer controller may be configured to fit a straight line to the relationship between the functions of intensities and wavelengths of radiation and determine the temperature based on a gradient of the fitted line. In some arrangements, the controller may be configured to fit the straight line to a plurality of points in the relationship between C2 / A and Ln(A5l) at a number of predetermined wavelengths, such as at wavelengths of 1.4393, 1.2206, 1.1241 and 0.9905 microns. Alternatively or otherwise, the straight line may be fitted to the relationship so as to disregard the effect of peaks in the emission spectrum resulting from light interference, e.g. from the plasma. In other words, the pyrometer controller 130, or other controller, may be configured to transform the emission spectrum determined by the spectrometer 156 into Wein coordinates and may be configured to determine the gradient of a straight line fitted to the transformation of the emission spectrum. The pyrometer controller 130, or other controller, may be configured to determine the temperature of the melt pool based on the gradient of the transformed emissions spectrum. In some arrangements, the pyrometer controller may be configured to correct raw data from the spectrometer for the baseline error, e.g. prior to determining the temperature. In particular, when no welding process is being performed, the pyrometer controller may be configured to capture a baseline emission spectrum of an object at a predetermined temperature, such as 20 degrees Celsius, over a predetermined time corresponding to a time over which measurements are made during a welding process. The baseline emission spectrum may be averaged. The baseline spectrum, or average, may be subtracted from the measured emissions captured during a welding process. As mentioned above, the emission spectrum determined from radiation received from the melt pool may include unwanted, e.g. interfering, radiation emitted by a component of the plasma, such as argon. In some arrangements, the controller 130 may be configured to subtract a known emission spectrum, such as a known emission spectrum of argon, from the determined emission spectrum prior to determining the temperature. Returning to Figure 1, the WAAM assembly 100 further comprises a WAAM controller 160. The WAAM controller may be configured to control the operation of the WAAM assembly. In particular, the WAAM controller 160 may be configured to control the operation of the weld torch 112 and / or laser and the pyrometer assembly 150. Although the WAAM controller 160 is described as being separate from the pyrometer controller 130 and the spectrometer controller, it will be appreciated that in some arrangements, the functions of one, more than one or each of the WAAM controller, pyrometer controller and spectrometer controller may be performed by a single controller, e.g. by one or more modules of a single controller, or a combination of two or more other controllers. The WAAM controller 160 may be configured to control the operation of the WAAM assembly 100 based on the determined temperature of the melt pool. For example, the WAAM controller 160 may be configured to control an operating parameter of the WAAM assembly 100, such as the feed rate of the weld wire, flow rates of the plasma forming gas and / or shield gas, the welding current passed through the torch, and / or movement and / or rotation speed of the weld torch and / or bed, based on the determined temperature. The WAAM controller 160 may control the operating parameter in order to maintain the temperature of the melt pool or melt pool region within a predetermined threshold range of temperatures for example 2500 - 500 deg. C. In the arrangements described above, the pyrometer assembly is provided as part of a WAAM assembly for measuring a temperature of a surface of a weld substrate in the melt pool region in the presence of light interference from the weld torch and / or laser, which generates a plasma during operation of the WAAM assembly. However, in other arrangements, the pyrometer assembly may be provided as part of any other temperature measurement assembly within which it is desirable to determine the temperature of a surface in the presence of light interference, e.g. originating from a light interference source included in the temperature measurement assembly and, optionally, where the surface emissivity is unknown. For example, the temperature measurement assembly may comprise a laser-based additive manufacturing processes, a fusion and / or gas welding processes, an assembly operating in a plasma environment, an internal combustion engine and / or a gas-turbine engine. Features described above in relation to the WAAM assembly may equally apply to such a temperature measurement assembly. With reference to Figure 4, a temperature measurement method 400 according to the present disclosure, will now be described. The method may be for measuring the temperature of a surface, e.g. of a weld substrate, in the presence of light interference from a light interference source, such as light from plasma generated by a weld torch or laser. The method 400 may be performed by a temperature measurement assembly comprising the pyrometer assembly described above. The pyrometer assembly may be arranged to measure the temperature of the surface. The method 400 may comprise a first step 402, in which light interference is generated. The method 400 comprises a second step 404, in which a thermal emission spectrum of radiation from a surface is determined. For example, the emission spectrum may be determined using a spectrometer. The method 400 comprises a third step 406, in which a temperature of the surface is determined based on the determined emission spectrum. The method 400 may further comprise a control step 408, in which an operating parameter of the temperature measurement assembly is controlled based on the temperature of the melt pool. For example, in the control step, a parameter of the temperature measurement assembly may be adjusted in order to maintain the temperature of the surface within a predetermined threshold temperature range. With reference to Figure 5, a method 500 for a welding assembly, e.g. a WAAM assembly, such as the WAAM assembly 100 shown in Figure 1, will now be described. The WAAM controller 160, and / or another controller, may be configured to control the operation of the WAAM assembly to carry out the method. The method 500 comprises a first step 502, in which a plasma is generated at a weld substrate, e.g. using a weld torch and / or laser, and a melt pool is created in the weld substrate. The method further comprises a second step 504, in which a thermal emission spectrum of radiation from the melt pool is determined. For example, the emission spectrum may be determined using a spectrometer. The method 500 comprises a third step 506, in which a temperature of the melt pool is determined based on the determined emission spectrum. The method 500 may further comprise a control step 508, in which an operating parameter of the WAAM assembly is controlled based on the temperature of the melt pool. For example, in the control step, a weld current may be adjusted in order to maintain the temperature of the melt pool within a predetermined threshold temperature range. With reference to Figure 6, another method 600 for a welding assembly, e.g. a WAAM assembly, such as the WAAM assembly 100 shown in Figure 1, will now be described. The method 600 may be similar to the method 500 and may comprise one, more than one or each of the first 502, second 504, third 506 and control 508 steps described above. The method 600 may further comprise a fourth step 608, in which a relationship is determined between two or more functions of the intensities and wavelengths of radiation in the emission spectrum. For example, in the fourth step 608, a relationship between C2 / A and Ln(A5l) may be determined. Alternatively, in the fourth step 608, the determined emission spectrum may be transformed, e.g. into Wein coordinates. The temperature of the melt pool may be determined, e.g. in the third step 606, based on the relationship or transformed emission spectrum. For example, the temperature may be determined based on a gradient of a straight line fitted to the relationship or transformed emission spectrum. The method 600 may further comprise a fifth step 610, in which a known emission spectrum originating from a known source of interference, such as from the plasma, e.g. a component of the plasma, is subtracted from the determined emission spectrum. For example, in the fifth step 610 a known emission spectrum of argon within the plasma may be subtracted from the emission spectrum. Features of the method 600 described in relation to measuring the temperature of the surface of the weld substrate, e.g. in the melt pool region, may be equally applicable to measuring the surface by the temperature measurement assembly, e.g. as part of the method 400. In other words, the method 400 may comprise the features of the method 600. In particular, the method 400 may comprise the fourth and fifth steps 608, 610. It will be appreciated by those skilled in the art that although the invention has been described by way of example, with reference to one or more exemplary examples, it is not limited to the disclosed examples and that alternative examples could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. A temperature measurement assembly for measuring temperature of a surface in the presence of light interference, wherein the assembly comprises:a source of light interference; anda pyrometer assembly, the pyrometer assembly comprising:a spectrometer configured to determine a thermal emission spectrum of light from the surface; anda controller configured to:determine a temperature of the surface based on a plurality of intensities of the determined emission spectrum.

2. The temperature measurement assembly of claim 1, wherein the assembly is a Wire Arc Additive Manufacturing (WAAM) assembly, andwherein the source of light interference comprises:a weld torch and / or laser, the weld torch and / or laser configured to generate a plasma at a weld substrate and create a melt pool on a surface of the weld substrate, wherein the surface comprises the surface of the weld substrate.

3. The temperature measurement assembly of claim 1 or 2, wherein the spectrometer is a Charge-Coupled Device, CCD, spectrometer, wherein the temperature is determined based on intensities measured by a plurality of light sensing elements of the CCD spectrometer.

4. The temperature measurement assembly of any of the preceding claims, wherein the controller is configured to determine the temperature of the surface based on a function of the intensities and wavelengths of the determined thermal emission spectrum.

5. The temperature measurement assembly of any of the preceding claims, wherein the controller is configured to determine a gradient of a line, e.g. a straight line, fitted to a relationship between functions of the intensities and wavelengths of the determined thermal emission spectrum, wherein the temperature is determined based on the gradient of the line.

6. The temperature measurement assembly of claim 5, wherein the line, e.g. straight line, is fitted to a relationship between C2 / A and Ln(A51), wherein C2 is a constant, A is wavelength and I is intensity.

7. The temperature measurement assembly of any of the preceding claims, wherein the controller is configured to transform the thermal emission spectrum to Wien coordinates, wherein the temperature of the surface, is determined based on the transformed emission spectrum.

8. The temperature measurement assembly of any of the preceding claims, wherein the temperature is determined based on a gradient, e.g. linear gradient, of the transformed thermal emission spectrum.

9. The WAAM assembly of claim 2, or any of the preceding claims when depending on claim 2, wherein the controller is configured to subtract a line emission spectrum of a constituent of the plasma from the determined thermal emission spectrum prior to determining the temperature of the surface.

10. The temperature measurement assembly of any of the preceding claim, wherein the controller is configured to correct raw data from the spectrometer for the baseline error by subtracting a background error value from the raw data from the spectrometer.

11. The temperature measurement assembly of any of the preceding claims, wherein the controller is configured to determine a black body thermal emission spectrum for a black body radiation source and correct the thermal emission spectrum for the spectral deviation from the ideal black body distribution based on the determined black body thermal emission spectrum.

12. The WAAM assembly of claim 2 or any of the preceding claims when depending on claim 2, wherein the controller is configured to control the operation of the weld torch and / or laser based on the determined temperature.

13. The WAAM assembly of claim 2 or any of the preceding claims when depending on claim 2, wherein the controller is configured to control the operation of the weld torch and / or laser to maintain a temperature of the melt pool or an area surrounding or adjacent the melt pool within a predetermined temperature range.

14. The temperature measurement assembly of any of the preceding claims, wherein the assembly further comprises an optical sighting tube for receiving light from the surface and an optical fibre extending from the optical sighting tube to the spectrometer.

15. The temperature measurement assembly of any of the preceding claims, wherein the assembly further comprises an optical splitter coupled to the optical fibre, and a guide light source configured to transmit a light on a return path through the optical fibre towards the surface.

16. A pyrometer assembly for the temperature measurement assembly of any of the preceding claims, wherein the pyrometer assembly comprises:a spectrometer configured to determine a thermal emission spectrum of light from a surface; anda controller configured to:determine a temperature of the surface based on a plurality of intensities of the determined emission spectrum.

17. A temperature measurement method, the method for measuring a temperature of a surface in the presence of light interference from a light interference source, wherein the method comprises:determining a thermal emission spectrum of light from the surface; and determining a temperature of the surface based on a plurality of intensities of the determined thermal emission spectrum.

18. A method for a Wire Arc Additive Manufacturing (WAAM) assembly, the assembly comprising a weld torch and / or laser, and a pyrometer assembly including a spectrometer, wherein the method comprises:generating a plasma at a weld substrate and creating a melt pool on a surface of the weld substrate;determining an emission spectrum of light from the melt pool, or an area surrounding or adjacent the melt pool; anddetermining a temperature of the melt pool, or area surrounding or adjacent the melt pool, based on a plurality of intensities of the determined emission spectrum.

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