Patent Description:
Over the past years, there has been an increasing interest in the use of lightweight materials in for instance the automotive and aerospace industry, where the main goal has been to reduce carbon emissions during transportation. For instance, it has been increasingly common that the vehicle or aircraft components are made of fiber composites. An emerging swap from thermoset resins to thermoplastic matrices allow the parts to be welded together, for example through induction welding.

There are several problems associated with induction welding of such materials, particularly when it comes to temperature measurement and control. For example, it is difficult to control the direction of the heat towards the weld zone without overheating or underheating parts of the workpiece, as well as knowing when to terminate the welding process. Moreover, it is difficult to measure the temperature of a weld zone without integrating sensors such as thermocouples, resistance thermometers or fiberoptical sensors, thereby interfering with and affecting the thermal and/or mechanical performance of the material. Thermocouples in particular might also be heated by the induction or disturbed by the electromagnetic fields, which can complicate accurate temperature readings.

Several attempts to avoid this type of interfering measures have been made, e.g. indirect measurement from for example pyrometers or other external temperature sensors, all of them having drawbacks such as being costly, inaccurate and requiring complex geometrical arrangements, without guaranteeing reproducibility in manufacturing. Examples of prior art can be found for instance in <CIT>, <CIT>, <CIT> and <CIT>. <CIT> (see the preamble of claims <NUM>, <NUM>) discloses a method and a device for joining moulded parts by electromagnetic welding, whereby a sensing inductor is first moved along a contact plane of the parts to be moulded, followed by a joining inductor which performs the joining of the parts.

Thus, it is clear that there is a need for developing a new induction welding technique which can overcome the problems stated above.

An object of the present invention is to solve or at least mitigate the problems related to prior art. This object is achieved by means of the technique set forth in the appended independent claims; preferred embodiments being defined in the related dependent claims.

According to an aspect of the invention, a system for controlled induction welding of at least one weld seam area of at least two surfaces of at least one workpiece is provided. The system comprises an inductor configured to be arranged in conjunction with the at least one workpiece, and a processing means configured to generate an electromagnetic field by applying an alternating voltage to the inductor so as to inductively heat the at least one of the surfaces so that the weld seam area is welded together. The processing means is also configured to simultaneously measure at least one parameter of the at least one workpiece, at least based on the generated electromagnetic field. The processing means is further configured to detect a change of the at least one parameter, and determine a temperature estimation of the at least one workpiece, based on said detected change.

Hence, the inductor acts both as a welding device and a sensing device. The inductor is thus configured to both heat the workpieces to cause welding and to act as a sensor simultaneously.

In one embodiment, the processing means is further configured to control, based on the determine temperature estimation, the operation of the system.

In one embodiment, the control of the operation of the system at least comprises altering the applied voltage and/or frequency, and/or causing a movement of the inductor.

In a one embodiment, the system further comprises a movement means configured to cause a movement of the inductor, and wherein the processing means is further configured to control the movement of the inductor.

In one embodiment, the system comprises a pressure means configured to apply a pressure to the system, and wherein the processing means is further configured to control the applied pressure.

In one embodiment, the pressure is applied in a direction substantially perpendicular to the weld seam area.

In one embodiment, the inductor and the processing means are in operable communication with each other.

In one embodiment, the processing means comprises a frequency converter.

In one embodiment, the processing means is further configured to provide a voltage/current with a certain frequency to the inductor.

In one embodiment, the at least one parameter measured by the processing means is at least one of frequency, a phase angle, a duty cycle, a resistance, an inductance, a peak, mean or root mean square (RMS) value of a current, a power, and/or energy.

In one embodiment, the estimation of temperature is performed by using at least one neural network being fed by the at least one parameter and/or using at least one transfer model being fed by the at least one parameter and/or by at least one autoregressive model being fed by at least two parameters.

The at least one workpiece is made of a composite material or carbon fiber reinforced plastics (CFRP).

Preferably, the matrix of the composite is a thermoplastic material or semi crystalline. The matrix material could also be a metal. Hence, there is a possibility for it to be remelted. Thus, the workpieces can be repaired when damaged, increasing recyclability and lifetime of the workpiece.

In one embodiment, at least one of the workpieces are made of carbon fiber reinforced plastics (CFRP). An advantage of using carbon fiber composites is that they are classified as semiconducting materials. Hence, they can be heated directly throughout the whole body of the workpiece. The at least one workpiece may also be fiberglass composites, or fiberglass reinforced plastics, with a thermoplastic matrix. Fiberglass is generally cheaper than carbon fiber. The fiber reinforcement can be any type of technical fiber such as flax fibers, aramid, ultra high molecular weight polyethylene UHMWPE. The composite can also be a hybrid fiber reinforced material, containing more than one fiber type, e.g. glass and carbon fiber. The fibers can be continuous or chopped, plain or woven layups or randomly oriented fibers. Different types of fibers and layups have their particular advantages, such as stiffness, density, cost, appearance, environmental impact, dielectric properties etc., obvious to a person skilled in the art.

In one embodiment, the weld seam area is defined by a first portion of a first workpiece arranged on or adjacent to and facing a second portion of a second workpiece. The workpieces might also belong to the same part, for example in the case of welding a tube together.

Advantages of this system is that it has a relatively low cost and a less complicated setup than prior art. No external temperature sensors need to be integrated inside the material to get the temperature of the weld zone, not seen in prior art. The system has a <NUM>-in-<NUM>-principle where the inductor acts as both a temperature sensor and a heater. The processing means is configured to both generate an electromagnetic field and at the same time measure parameters of the at least one of the workpieces to determine the temperature. Even without external temperature sensors, the system can still run in a closed loop manner to ensure the proper temperature is reached, unlike open loop control, where the system relies on pre-determined settings, which works great in certain cases, but with a huge risk for over/under heating of the weld.

Another advantage of the claimed system is that a temperature deviation at the weld zone can be estimated during induction welding using only the inductor itself and a processing means, such as a frequency converter. There is no need to integrate for example thermocouples or fiber Bragg gratings into the structure to monitor the heating/welding process. In other words, the system provides for measurement of a hidden layer without mechanical interference with the material. This leads to for instance weight reduction and lower production cost during welding. The system is configured to measure differences in electromagnetic signals or properties in an inductor while at the same time heating the components to be welded, i.e. the workpieces, by the same inductor. Hence, the measurement of electromagnetic properties can be performed when the workpieces are both in a cooled state and a heated state. Welded joints provide large savings in component weight and assembly costs, but can only be applied if a reliable process can be guaranteed. Fiber composites loses a lot of its mechanical strength when holes are made, thus welded parts can be made significantly thinner, with maintained mechanical properties and reduced risk for fatigue.

Furthermore, the system provides for a controlled process with shorter cycle-times and ensures proper adhesion between the two workpieces due to improved temperature control and for instance the pressure means and appropriate welding time. The result is a high quality weld with reduced risk of locally overheating the material at the weld zone since the welding process is stopped by the processing means before the workpieces are overheated. The system also reduces the risk of insufficient temperature.

According to a second aspect, a method is provided for induction welding at least two surfaces of at least one workpiece using the system as described previously. The method comprises the steps of providing an inductor configured to be arranged in conjunction with the at least one workpiece, providing a processing means, generating an electromagnetic field by applying an alternating voltage to the inductor so as to inductively heat at least one of the surfaces so that the weld seam area is welded together, and simultaneously measuring at least one parameter of the at least one workpiece at least based on the generated electromagnetic field. The method further comprises detecting a change of the at least one parameter, and determining a temperature estimation of the at least one workpiece based on said detected change. In this context, in conjunction with is defined as electromagnetically coupled to.

In one embodiment, the method further comprises controlling the operation of the system based on said temperature estimation.

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, in which:.

Embodiments of the invention will now be described with reference to the accompanying drawings. The invention may, however, be embodied in many different forms within the scope defined in the appended claims, and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the particular embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention.

<FIG> illustrates a part of a system <NUM> according to the invention, where an object is to monitor changes in electrical signals to achieve a process temperature sufficient to weld at least two workpieces <NUM>, <NUM> without overheating or underheating the materials, or workpieces, to be welded. The changes in electrical signals are characterized through inductive measurement. The inductive power <NUM> is caused by an inductor <NUM> that induces a current into the at least one workpiece <NUM>, <NUM>, as will soon be described more in detail.

In one embodiment, at least two surfaces of at least two workpieces <NUM>, <NUM> are to be welded to each other. The at least two workpieces <NUM>, <NUM> are configured to be inductively welded in a weld seam area A. The weld seam area A is defined by a first portion <NUM>' of the first workpiece <NUM> arranged on or adjacent to, and facing, a second portion <NUM>' of the second workpiece <NUM>. The inductor <NUM> is arranged in conjunction with, or close to, at least one of the two workpieces <NUM>, <NUM>. In one embodiment, the two workpieces <NUM>, <NUM> are arranged in conjunction with the inductor by being in direct contact with each other. In yet one embodiment, the two workpieces <NUM>, <NUM> are arranged in conjunction with the inductor <NUM> without being in direct contact with each other. Hence, the at least one workpiece <NUM>, <NUM> may be in direct contact or in indirect contact with the inductor <NUM>. In one embodiment the inductor <NUM> is at a predetermined distance from the at least one workpiece <NUM>, <NUM>.

The inductor <NUM> induces a current in the workpieces <NUM>, <NUM> which are susceptive to electromagnetic heating. They are inductively heated at the weld seam area A and are melted or fused together when they reach a predetermined melting or processing temperature corresponding to the material properties of the workpieces <NUM>, <NUM>. The time required for melting the workpieces <NUM>, <NUM> at the weld seam area A is determined by the material properties and geometry of the respective workpieces.

In the embodiment shown in <FIG>, the workpieces <NUM>, <NUM> are to be welded when arranged on top of one another. Here, the second workpiece <NUM> is placed on top of the first workpiece <NUM> and the weld seam is generated in the weld seam area A on two respective end portions so that the two workpieces <NUM>, <NUM> are welded in a side-to-side configuration. There are several variations to this arrangement. For instance, the second workpiece <NUM> may be arranged sideways in parallel with the first workpiece <NUM> so that they are welded in an edge-to-edge configuration. Other possibilities are for instance a side-to-edge configuration (not shown).

In the embodiment shown in <FIG>, the workpieces <NUM>, <NUM> are rectangularly shaped. However, as should be understood by a person skilled in the art, they may have any shape. For instance, they may be a beam and skin of an aircraft component. The thickness may vary from sub millimeter to tenths of millimeters and the workpieces to be welded may have complex geometries.

The workpieces <NUM>, <NUM> are susceptors meaning that they have the ability to absorb electromagnetic energy and convert it to heat. Typically, they are carbon fiber composite materials such as carbon fiber reinforced plastics (CFRP). The fiber reinforcement can be any type of technical fiber such as flax fibers, aramid, ultra high molecular weight polyethylene UHMWPE. The composite can also be a hybrid fiber reinforced material, containing more than one fiber type, e.g. glass and carbon fiber. The fibers can be continuous or chopped, plain or woven layups or randomly oriented fibers. Different types of fibers and layups have their particular advantages, such as stiffness, density, cost, appearance, environmental impact, dielectric properties etc., obvious to a person skilled in the art.

The workpieces <NUM>, <NUM> may for instance be made from unidirectional laminates where each layer is arranged with a different angle with respect to adjacent layers. As a non-limiting example, each workpiece may contain e.g. <NUM> layers of carbon fibers embedded in a thermoplastic matrix. Alternatively, the workpieces <NUM>, <NUM> are made of a woven web of carbon fibers, for example chopped carbon fibers, organized or randomly oriented. Optionally, the web is nonwoven. The matrix may be for example polypropylene (PP), polyamide (PA), polycarbonate (PC) or it can be a semicrystalline thermoplastic material such as polyphenylenesulfide (PPS) or polyetereterketone (PEEK) etc., but it can also be a metal such as aluminum or titanium. Carbon fiber composites are usually classified as semiconductors and can be directly heated efficient at distributing heat throughout the material thickness. Due to the thermoplastic matrix, it is possible to melt the workpieces <NUM>, <NUM> and thereby create a weld seam in a weld seam area A shared by the two workpieces <NUM>, <NUM>.

The workpieces <NUM>, <NUM> may also be fiberglass composite materials. The fibers can also be of any other technical textile, such as flax fibers, aramid, ultra-high molecular weight polyethylene, etc. As a non-limiting example, glass fibers may be used as a reinforcement agent in a polypropylene based matrix. If the workpieces <NUM>, <NUM> to be welded are fiberglass composites, one may need to introduce an additional layer (not shown) in an interface between the two workpieces. This is due to the insulating properties of fiberglass. The additional layer placed in the interface may also be called a susceptor. This susceptor may be for instance a woven web of metal or carbon fiber. The webs constituting the additional layer may also be nonwoven. It may also be that the additional layer is something else than a web. For instance, the additional layer may be randomly oriented carbon fibers applied to the surface of one or both of the workpieces <NUM>, <NUM> to be welded.

It should be noted that the two workpieces <NUM>, <NUM> may be of different material. Hence, the first workpiece <NUM>,<NUM> may be of a first material and the second workpiece may be of a second material. In the embodiment described above, one workpiece may be made of a fiberglass composite material and one workpiece is a susceptor. The composite materials can also be built by hybrid fiber reinforcement, for example glass fiber and carbon fiber, typically with the carbon fiber at least at the surface or close to the surface of the material. This is shown in <FIG>.

In <FIG>, a pressure is applied to the workpieces <NUM>, <NUM> to ensure good contact between the two workpieces to be joined. The pressure may be applied through pressure means <NUM>, as shown and discussed more in relation to <FIG> and <FIG>.

<FIG> illustrate three different situations where the inductor <NUM> is moved in relation to the workpieces <NUM>, <NUM>. The arrows indicate the directional movement. In <FIG>, the second workpiece <NUM> is arranged on top of the first workpiece <NUM> and the weld seam is generated in the weld seam area A defined by the area between the two workieces in which they overlap, i. e where a first portion <NUM>' of the first workpiece <NUM> is facing a second portion <NUM>' of the second workpiece <NUM>. Here, the workpieces are welded in a side-to-side configuration.

The first workpiece <NUM> comprises a first surface 20a and a second surface 20b. The second workpiece <NUM> comprises a first surface 21a and a second surface 21b. In the embodiment shown in <FIG>, the second workpiece <NUM> is arranged on top of the first workpiece <NUM>. The second surface 21b of the second workpiece <NUM> is arranged facing the first surface 20a of the first workpiece <NUM>. The inductor <NUM> is arranged below the second surface 20b of the first workpiece <NUM> so as to indirectly heat the first surface 20a of the first workpiece <NUM> and the second surface 21b of the second workpiece <NUM>.

In <FIG>, the workpieces <NUM>, <NUM> are welded in a side-to-side configuration but with the inductor arranged and moving on top of the second workpiece <NUM>.

In <FIG>, the second workpiece <NUM> is be arranged sideways in parallel with the first workpiece <NUM> so that they are welded in an edge-to-edge configuration. Here, the inductor <NUM> is arranged and configured to be moved along both the first workpiece <NUM> and the second workpiece <NUM>. Alternatively, the inductor <NUM> may be placed on the opposite side (underneath) the two workpieces <NUM>, <NUM>.

In all cases, the workpieces <NUM>, <NUM> may be supported by a support element (not shown). The support element may be arranged opposite the pressure means <NUM> such that the workpieces <NUM>, <NUM> are be sandwiched between the pressure means <NUM> and the support element. Alternatively, the workpieces <NUM>, <NUM> may be supported mainly by the supporting force generated by the pressure coming from the pressure means <NUM> when acting in a direction towards the inductor <NUM> in its housing <NUM>. In that case, the support means may be the inductor housing <NUM>.

As shown in <FIG>, the movement of at least a part of the inductor <NUM> may be in the lengthwise direction illustrated by the arrow M. The direction may for instance be along the x-axis. Alternatively, the movement is in a direction along the length of the two workpieces <NUM>, <NUM> while lying on a surface or tool. However, as should be understood, the movement of the inductor <NUM> could be along the width of the two workpieces <NUM>, <NUM> (only a cross-sectional side view is shown in <FIG>) while lying on a surface or tool. Although not shown, it should be realized that the movement could be in multiple directions in a single embodiment as well, as long as the inductor <NUM> is close to the at least one workpiece <NUM>, <NUM>. Preferably, the translational movement of the inductor <NUM> is actuated by a movement means <NUM>. This will be further discussed in relation to <FIG> and <FIG>.

In the embodiments described above, welding has been described as welding of two surfaces of two workpieces <NUM>, <NUM>. However, as should be understood by a person skilled in the art, two surfaces of one single workpiece <NUM>, <NUM> could also be welded together. This is shown in <FIG> where the two surfaces <NUM>', <NUM>" of the workpiece <NUM> have been welded together in a weld seam area A. In such a situation, the workpiece may have been bent so as to create a shape where two end portions <NUM>', <NUM>" meet. The workpiece <NUM>, <NUM> may be bent and welded in the shape of a metal profile with four corners and having a quadratic cross-section. Several other shapes may also be provided, such as a pipe-like shape with a circular cross-section. In these cases, the workpiece <NUM>, <NUM> has two end portions, forming surfaces <NUM>', <NUM>", where the weld is created in a weld seam area A between two end portions of the same blank. Put differently, a single blank made of carbon fiber and/or fiberglass composite may be welded together.

<FIG> shows an embodiment where surfaces of three workpieces <NUM>, <NUM>, <NUM> are welded together. This may be the case when the two of the workpieces <NUM>, <NUM> are fiberglass composite materials. In order to weld these workpieces <NUM>, <NUM> together, an additional workpiece <NUM> is introduced. Such additional workpiece <NUM> have been discussed above, where it was referred to as a susceptor. The additional workpiece <NUM> may be of the same size as the workpieces <NUM>,<NUM> or may be smaller.

In this configuration, two weld seam areas A1, A2 are created. One weld seam area A1 is created between the first workpiece <NUM> and the additional workpiece <NUM>, and one weld seam area A2 is created between the second workpiece <NUM> and the additional workpiece <NUM>.

<FIG> is an illustrative top view of a conductor. The inductor <NUM> comprises at least one coil 12a, 12b. In one embodiment the inductor <NUM> comprises at least two coils 12a, 13b. In this configuration one coil may be designated to measure, whereas the one or more additional coils are configured to heat. If a plurality of coils are present in the inductor <NUM>, the coils are electromagnetic mutually coupled to each other.

In one alternative embodiment the inductor <NUM> comprises one single coil. The single coil is thus configured to both cause the heating of the workpieces as well as measuring the parameters of the system.

The system <NUM> for controlled induction welding of at least one weld seam area A of the at least two workpieces <NUM>, <NUM> is further illustrated in <FIG>. Here, the weld seam created in the weld seam area A is shown as a black line between two workpieces <NUM>, <NUM>. The system <NUM> is seen from a side view. A pressure means <NUM> is provided to apply a pressure to the system <NUM>, and more specifically to the at least two workpieces <NUM>, <NUM> to ensure good contact between the workpieces during welding.

The inductor <NUM> may be arranged in a housing <NUM>. A purpose of the housing <NUM> is to protect the inductor <NUM> from coming into contact with the workpieces <NUM>, <NUM> to be welded. Another purpose is to provide a stable ground for the inductor to be supported for instance by a table in a workspace. Yet another purpose of the inductor housing <NUM> is to support the workpieces <NUM>, <NUM> during welding so that they experience proper contact between the pressure means <NUM> and the inductor <NUM>. This ensures a stable and controlled welding process. The inductor housing <NUM> also facilitates controlled and stabilized movement of the inductor <NUM> when it is to be moved. The housing may also prevent the coil from heating undesired objects in close proximity. Other than coil unit, another word for the inductor <NUM> may be heating element.

The system <NUM> also comprises a processing means <NUM> which is configured to generate an electromagnetic field by applying an alternating voltage to the inductor <NUM> so as to inductively heat the two workpieces <NUM>, <NUM> so that the weld seam area A is welded together. Simultaneously, the processing means <NUM> is configured to measure at least one parameter P of at least one of the two workpieces <NUM>, <NUM> at least based on the generated electromagnetic field. The processing means <NUM> is configured to detect a change of the at least one parameter P. Based on said change, the processing means is further configured to determine a temperature estimation of the at least one temperature of the workpiece <NUM>,<NUM>. The temperature estimation will be discussed more in reference with <FIG>.

Preferably, the processing means <NUM> is or comprises a frequency converter <NUM>. It can also be a control unit. The processing means <NUM> may further comprise a display unit <NUM> to provide an operator or user with process information, and coupling means <NUM> so that the processing means <NUM> is in operative communication with the inductor <NUM>. Movement means <NUM> is also provided and operatively coupled to both the processing means <NUM> and the inductor <NUM>. The movement means <NUM> is configured to cause a movement of the inductor <NUM> based on process information received and/or determined by the processing means <NUM>. In other words, the processing means <NUM> is configured to control the movement of the inductor <NUM>. Alternatively, the inductor <NUM> is stationary and the movement means <NUM> is configured to move the workpieces <NUM>, <NUM> in relation to the inductor <NUM>.

Preferably, the processing means further comprises an interface for transmitting the data obtained by inductor. The interface may be of any suitable type, including simple wiring, a serial interface such as Ethernet, RS485, USB, a wireless interface such as Bluetooth or WiFi, etc..

The processing means may comprise a programmable device, such as a microcontroller, central processing unit (CPU), digital signal processor (DSP) or field-programmable gate array (FPGA), discrete digital synthesizer (DDS) with appropriate software and/or firmware, and/or dedicated hardware such as an application-specific integrated circuit (ASIC). The processing means <NUM> can be connected to or comprises a computer readable storage medium such as a disk or memory. The memory may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR, SDRAM or some other memory technology.

<FIG> discloses an embodiment of the system <NUM>. Here, the system <NUM> comprises an inductor <NUM> and a processing means <NUM> as has previously been described. The system <NUM> may further comprise one or more of a movement means <NUM>, pressure means <NUM> and a cooling means <NUM>.

The movement means <NUM> is configured to move the inductor <NUM> relative to the workpieces <NUM>, <NUM> to be welded. The movement means <NUM> may be in operative communication with the processing unit <NUM> and in operative communication with a drive unit <NUM> that causes the movement of the inductor <NUM>. The drive unit <NUM> may be part of the movement means <NUM>, or be arranged externally of the movement means. The drive unit <NUM> may be a motor such as an electrical motor or pneumatic actuator or similar. Alternatively, the drive unit <NUM> can be a brushless DC electric motor. The brushless DC electric motor may be a stepper motor. A stepper motor divides a full rotation into a number of equal steps. A benefit with a stepper motor is that it is possible to move and hold at one of these steps without having a position sensor for feedback. The drive unit could also be any type of servo motor.

The processing means <NUM> may instruct the drive unit <NUM> to move the inductor <NUM> along a predetermined path in small steps. The steps are preferably in the sub-millimeter scale. The drive unit <NUM> can be controlled wirelessly by the processing means <NUM> or by wire or fiber optic. The drive unit <NUM> may be configured to follow a predefined protocol stored in an associated memory to the processing means <NUM>, and/or the drive unit <NUM> is configured to follow instructions caused by a user of the processing means <NUM>. The drive unit <NUM> may be arranged as a part of the system <NUM> or as a separate external part, being in operative communication with the system <NUM>.

Optionally, the movement means <NUM> may be a part of the processing means <NUM>, or arranged externally of the processing unit <NUM>.

The movement means <NUM> may for example comprise a track, frame, a rod or similar arrangement that allows the movement to be controlled in a precise manner. The movement means <NUM> may further be a robotic arm, parallel kinematic robot or gantry. Preferably, the movement means <NUM> can move in incremental steps to better control the welding process. The movement means <NUM> can also move continuously. The movement means <NUM> may also be manually driven. For instance, the movement means <NUM> may be arranged in a housing, for example a longitudinal arrangement along which it can move, not shown) for allowing the movement of the inductor <NUM>.

The movement means <NUM> may in one embodiment comprise a telescopic arm that is able to be lengthened or shortened during welding so as to allow for different positions of the inductor <NUM>.

As previously stated, the system <NUM> may comprise cooling means <NUM> configured to cool down the system <NUM>. The cooling means <NUM> is configured to cool the workpieces <NUM>, <NUM> during and/or after having reached the processing temperature and the weld has been formed. The cooling can be controlled by the processing means <NUM>. If for instance the welding process is continuous, the system <NUM> may comprise a roller or cooling cylinder configured to cool the welded area. This roller or cooling cylinder may then be arranged behind the inductor <NUM> as it moves across the workpieces <NUM>, <NUM>. Alternatively, the inductor <NUM> may chill the newly welded area through a process called thermal conduction. In that case, there may be for instance a suction element arranged in conjunction with the system <NUM> to retrieve the energy loss associated with the cooling process.

The system <NUM> further comprises a pressure means <NUM> configured to apply a pressure to the workpieces <NUM>, <NUM>. In <FIG>, the inductor <NUM> is arranged underneath the two workpieces <NUM>, <NUM> and the workpieces are sandwhiched between the pressure means <NUM> and the inductor <NUM>. In general, the pressure means <NUM> and the inductor <NUM> are arranged on respective sides of the two workpieces <NUM>, <NUM> to be welded and follow each other across the areas to be welded. The inductor <NUM> and the pressure means <NUM> may as well be arranged on the opposite sides. This is shown in <FIG> where that the inductor <NUM> is arranged on top of the second workpiece <NUM> and the pressure means <NUM> is arranged to contact the first workpiece <NUM>.

The processing means <NUM> is configured to control the applied pressure. Typically, the pressure is applied in a direction substantially perpendicular to the weld seam area A of the at least two workpieces <NUM>, <NUM>. This is illustrated in <FIG> and <FIG>.

Now turning to <FIG>, illustrating further parts of the system <NUM>. In <FIG>, a part of the process of measuring at least one parameter P is shown. Here, a frequency F, a current I and a voltage V serves as input to the inductor which in turn delivers a parameter P of the at least two workpieces <NUM>, <NUM> as output to the processing means <NUM>. The frequency F, current I and voltage V are all examples of signals initially provided as input by the processing means <NUM>.

In operation, the workpieces <NUM>, <NUM> are inductively heated by the inductor <NUM> which receives certain electromagnetic signals from the processing means <NUM> as input in terms of a voltage V/current I with a certain frequency F. The frequency of at least one of these signals (power output) can be either constant or ramped up. Concurrently, as the inductor <NUM> heats the workpieces <NUM>, <NUM> to weld them in a weld seam area A, electromagnetic signals, or parameters P, of the workpieces <NUM>, <NUM> are sensed by the inductor <NUM>. The parameters P are transmitted back to the processing means <NUM> via the inductor <NUM>. The processing means <NUM> is configured to measure the at least one parameter P and detect/assess a change in the at least one parameter P. Put differently, the processing means <NUM> is configured to evaluate the at least one parameter P by comparing it with predetermined reference values.

The predetermined reference values may be obtained through system modelling/identification or from an AI model. The values are typically calculated from previous input data. The predetermined reference values may for instance be material characteristics of the workpieces <NUM>, <NUM> to be welded.

The processing means <NUM> is also configured to control the operation of the system <NUM> based on this evaluation of changes in at least one parameter P. When the processing means <NUM> determines, based on the measured parameters P, that the appropriate welding temperature has been reached at the weld seam area A, the welding process is changed. The welding process may be changed by stopping or altering the welding process, for instance by maintaining the temperature or moving the inductor <NUM> through the movement means <NUM> and/or by cooling the weld seam area A through the cooling means <NUM>. Other examples are for instance by altering the applied voltage and or frequency or the applied pressure given by the pressure means <NUM>. All of these actions/events are preferably determined by the processing means <NUM>.

In other words, the system <NUM> functions as follows. The movement means <NUM> moving either the workpieces <NUM>, <NUM> or the inductor <NUM> is controlled, via the processing means <NUM>, based on output from the inductor <NUM>. This output comes in the shape of electromagnetic signals or parameters P resulting from changes in material characteristics of the workpieces <NUM>, <NUM> to be welded. Moreover, the pressure means <NUM> is controlled, via the processing means <NUM>, based on the output from the inductor <NUM>. Further, the cooling means <NUM> is controlled, again via the processing means <NUM>, based on output from the inductor <NUM>. Lastly, the output, or directives, coming from the processing means <NUM> is in turn determined based on output from the inductor <NUM>. One can see this interaction as a type of feedback system.

As previously mentioned, the processing means <NUM> is configured to generate an electromagnetic field by applying input signals to the inductor <NUM> in terms of a voltage V/current I with a certain frequency F. These signals may be for instance a frequency F, a current I or an input voltage V, as illustrated in <FIG>. As the workpieces <NUM>, <NUM> are inductively heated and the electromagnetic properties of the workpieces are altered, the inductor <NUM> will experience a change in electromagnetic signals in comparison with the input values generated by the processing means <NUM>. These changes in electromagnetic signals, or parameters P, arising due to the heating of the workpieces, are transmitted through the inductor <NUM> and detected by the processing means <NUM>. For instance, the at least one parameter P measured by the processing means <NUM>, or frequency converter, is at least one of frequency, a phase angle between a voltage and a current, a duty cycle, a resistance, an inductance, peak, mean or a root mean square (RMS) value or similar of a current, a power, and/or energy. An alternative to measuring the phase angle is to measure other electrical signals that give rise to the same phase shift, for example change in resonance frequency.

During welding, the impedance of the inductor changes due to changes of electrical properties, or parameters P, in the workpieces <NUM>, <NUM>. By measuring a difference in for instance resistance and inductance in the presence and absence of the workpiece, respectively, one can obtain an indication of a temperature variation at the weld seam area A. The electric circuit provided in the system <NUM> may have an oscillating behavior which is pronounced at a certain resonance frequency, at which the impedance is purely resistive. The phase angle can be described as the shift between a voltage and a current. At resonance, the phase angle is zero. In <FIG>, the output from the frequency converter is illustrated as the system <NUM> gets closer to resonance. As can be seen, the output voltage is similar to a modified square wave. Conversely, at resonance the current resembles a sinusoidal.

The pressure applied by the pressure means <NUM> to the workpieces <NUM>, <NUM> to ensure proper contact between the two workpieces <NUM>, <NUM> during welding affects both the resistance and the inductance the coil is experiencing.

Other than detecting changes in the at least one parameter P, the processing means <NUM> is configured to evaluate the at least one parameter P by comparing it against predetermined reference values, which may for instance be input values that have been determined previously, based on simulated data, system modelling or AI. , as will soon be described in detail. The value might depend on previous values. Furthermore, the processing means <NUM> is configured to control the operation of the system <NUM> based on this evaluation, for instance by altering the applied voltage, and/or causing a movement of the inductor <NUM>. Other than applying an alternating voltage, the processing means <NUM> is further configured to provide a current I and/or a frequency F to the inductor <NUM> as an input signal.

Knowing for instance the phase angle and the output frequency, one can estimate the temperature in the weld seam area A by comparing the values sensed by the inductor <NUM> with predetermined reference values obtained using a system model or AI. The parameters P, or electrical signals, mentioned previously are detected and measured by the frequency converter to be able to estimate the temperature at the weld seam area A, i.e. a weld zone. Initial values can vary depending on local material properties, geometry, thickness, etc, but the melting of the matrix material typically cause a change in the response, possible to detect, which is beneficial to accurate modelling the temperature close to and around the target/processing temperature.

By measuring these parameters P and estimate the temperature, the process of welding can be controlled since the system <NUM> will know when the appropriate process temperature has been reached at the weld seam area A, i.e. in an interface between the workpieces <NUM>, <NUM>. When the process temperature has been reached, an indication that the workpieces <NUM>, <NUM> have been sufficiently melted will reach the processing means <NUM>. The movement means <NUM> will receive an input signal from the processing means <NUM> to move in relation to the present weld seam area A of the at least two workpieces <NUM>, <NUM>. The pressure means <NUM> may follow the movement of the inductor <NUM>. Potential dwell time can also be integrated in the control means if needed.

In an alternative embodiment, the inductor <NUM> and the pressure means <NUM> are held stationary and the workpieces <NUM>, <NUM> are moved instead. Moreover, the pressure can be varied based on the measured parameters P. When the system <NUM> has determined that the workpieces <NUM>, <NUM> have been appropriately welded at a certain area, the pressure means <NUM> receives input from the processing means <NUM> which controls the pressure means <NUM>, which indicates whether the pressure means <NUM> can stop applying pressure or not.

<FIG> illustrates alternative ways of estimating the temperature. The processing means <NUM> is configured to determine a temperature estimation based on the detected change in parameter(s). The temperature estimation may be based on a neural network, one or more transfer functions, and/or autoregressive model(s). The temperature estimation is based on evaluating the changes in the at least one parameter.

The estimation of the temperature can be performed by a neural network, fed by at least one parameter P. The neural network needs to be trained by vast amount of data to be able to estimate the temperature accurately. The neural network is a good way of implementing a self-learning algorithm in the processing means. The estimation of the temperature in the weld zone can alternatively be performed by using a transfer function model of any type, fed by at least one parameter P. Yet another alternative to estimate the temperature can performed by using an autoregressive model of any type, fed with at least one extra parameter P. All methods are determined by previously recorded data in combination with changes in at least one of the parameters P.

<FIG> is a schematic view of a neural network. In one embodiment, the estimation of the temperature of the at least one surface is obtained through a neural network, where at least one of the parameters P is fed into the model. The specific parameters of the model or neural network can change over time through self-learning algorithms associated with this type om models.

<FIG> is a schematic view of a transfer model structure. In one embodiment, the estimation of the temperature of the at least one surface is obtained through one or several transfer models, fed by at least one parameter P, typically at least one non-linear function is needed for accurate results.

<FIG> is a schematic view of regressive model. In one embodiment, the estimation of the temperature of the at least one surface is obtained through an autoregressive model, fed with at least one extra parameter P, typically at least one non-linear function is needed for accurate results. In this embodiment the regressive model is a non-linear ARX model. The structure of such a models enables models of complex nonlinear behavior using flexible nonlinear functions, such as wavelet and sigmoid networks. A nonlinear ARX model consists of model regressors and a nonlinearity estimator. The nonlinearity estimator preferably comprises both linear and nonlinear functions that act on the model regressors to give the model output.

<FIG> illustrates a method <NUM> for induction welding at least two workpieces <NUM>, <NUM> using the system <NUM> as described above. The method begins by providing <NUM> an inductor <NUM> configured to be arranged in conjunction with at least one of the workpieces <NUM>, <NUM>. A processing means <NUM> is also provided <NUM>. An alternating voltage is applied to the inductor <NUM> by the processing means <NUM> and thereby generates an electromagnetic field. The aim is to inductively heat the at least two workpieces <NUM>, <NUM> so that a weld seam is created in a weld seam area A between the two workpieces <NUM>, <NUM>. The alternating voltage V is an input signal to the inductor <NUM> and is applied via the processing means <NUM>. However, the method is not restricted to applying solely an input voltage to the inductor <NUM>. Other electromagnetic signals are applicable as well, such as a current I or a frequency F. Simultaneously as the inductor <NUM> heats the workpieces <NUM>, <NUM> through the generated electromagnetic field, the inductor <NUM> senses and the processing means <NUM> measures <NUM> at least one parameter P of at least one of the two workpieces <NUM>, <NUM> at least based on the generated electromagnetic field. The parameter P (an electromagnetic signal) is altered when the workpieces <NUM>, <NUM> are subjected to the electromagnetic field. This is due to changes in the material characteristics of the workpieces <NUM>, <NUM> during welding. The processing means <NUM> is configured to detect <NUM> a change of the at least one parameter. One a change in the parameter is detected, the processing means <NUM> is configured to determine <NUM> a temperature estimation of the at least one workpiece based on said change. Hence, the processing means <NUM> is configured to determine a temperature estimation of the at least one workpiece <NUM>, <NUM> by measuring a change of at least one parameter P of the at least one workpiece <NUM>, <NUM> caused by the generated electromagnetic field.

<FIG> illustrates the particular involvement and operation of the processing means <NUM> in the system <NUM> as described previously. As already mentioned, the processing means <NUM> generates <NUM> an electromagnetic field in the workpieces <NUM>, <NUM> by means of the inductor <NUM>. Simultaneously, as the electromagnetic field is generated and the workpieces <NUM>, <NUM> are heated in the weld seam area A where a weld seam is created, the processing means <NUM> measures <NUM> at least one parameter P of at least one of the workpiece(s) <NUM>, <NUM>. Due to changes in the material characteristics of the workpiece(s) <NUM>, <NUM> during welding, the parameter P is altered when the workpieces <NUM>, <NUM> are subjected to the electromagnetic field (the induced current from the inductor <NUM>).

The inductor <NUM> affect the parameter P and transmits the one or more signals to the processing means <NUM>. The processing means <NUM> detects <NUM> a change of the at least one parameter P and evaluates this at least one parameter P, or signal. The detected change of parameter(s) are used to determine <NUM> a temperature estimation of the temperature of the at least one workpiece <NUM>, <NUM>.

Based on the determined <NUM> temperature estimation, the processing means <NUM> controls <NUM> the operation of the system <NUM>. This may be done in one or more of the following ways; by altering <NUM> the applied voltage V, causing a movement <NUM> of the inductor <NUM> through the movement means <NUM>, altering the applied pressure <NUM> through the pressure means <NUM>, and altering <NUM> the applied cooling through the cooling means <NUM>, or a combination thereof. In one embodiment, the current is altered based on the estimated temperature. If the processing means <NUM> determines that the temperature is rising, the current is lowered. On the contrary, if it is determined that the temperature is decreasing, the current is increased.

The time it takes to reach the appropriate process temperature may vary between different materials, geometrics and/or their thickness. The process temperature may for example be in the range of <NUM> and <NUM>, but can also be higher. Therefore, feedback may be used to provide for accurate weld zone temperature estimation. Further, the temperature profile of the workpieces <NUM>, <NUM> may be useful to avoid thermal degradation of the polymer in the layers which are closest to the inductor <NUM>. The welding process may be performed for instance by spot welding where the workpieces <NUM>, <NUM> are pressed and heated momentarily. Alternatively, the method of welding may be performed by a continuous process or by a step-wise static process. In the latter case, the weld seam of the weld seam area A could be for instance <NUM> long, followed by a movement of the inductor about <NUM> before creating the next weld seam of <NUM>. In this case, there would be some overlapping between the weld seams.

Claim 1:
A system for controlled induction welding of at least one weld seam area (A) of at least two surfaces of at least one workpiece (<NUM>, <NUM>, <NUM>) made of a carbon fiber composite material or carbon fiber reinforced plastics (CFRP), the system comprising:
an inductor (<NUM>) configured to be arranged in conjunction with the at least one workpiece (<NUM>, <NUM>, <NUM>) so that the inductor (<NUM>) is electromagnetically coupled to the at least one workpiece (<NUM>, <NUM>, <NUM>),
a processing means (<NUM>) configured to:
generate an electromagnetic field by applying an alternating voltage to the inductor (<NUM>) so as to inductively heat at least one of the surfaces so that the weld seam area (A; A1, A2) is welded together,
characterised in that
the processing means (<NUM>) is also configured to simultaneously as the inductor (<NUM>) heats the at least one workpiece (<NUM>, <NUM>, <NUM>) to a process temperature through the generated electromagnetic field, measure at least one parameter (P) of the at least one workpiece (<NUM>, <NUM>, <NUM>) at least based on the generated electromagnetic field,
detect a change of the at least one parameter (P), and
determine a temperature estimation of the at least one workpiece (<NUM>, <NUM>, <NUM>) based on said detected change of the at least one parameter (P).