WO2012022972A2

WO2012022972A2 - An apparatus and method for the layup of sheets to form a composite article and composite articles made thereby

Info

Publication number: WO2012022972A2
Application number: PCT/GB2011/051547
Authority: WO
Inventors: Andrew Long; Hua Lin; Paul Taylor
Original assignee: University Of Nottingham
Priority date: 2010-08-19
Filing date: 2011-08-16
Publication date: 2012-02-23
Also published as: GB201013885D0; WO2012022972A3

Abstract

This invention concerns a method and system for manufacturing a composite article. The method comprises applying sheets (108) of prepreg material to an article being made or to a former using a robotic end effector (102, 104, 106) under the control of a processor (114). The processor (114) controls the end effector (102, 104, 106) to apply each sheet (108) in a manner that adjusts the application of the sheet (108) to compensate, at least in part, for variations in an actual position of the sheet (108) from a desired position stipulated by an article formation model. The actual position is determined from signals derived from a sensing system (111, 112) sensing attributes of each sheet (108) as the sheet is applied. The invention also concerns a method and system for assisting manual lay-up of sheets.

Description

AN APPARATUS AND METHOD FOR THE LAYUP OF SHEETS TO FORM A COMPOSITE ARTICLE AND COMPOSITE ARTICLES

MADE THEREBY

Field of Invention

This invention relates to an apparatus and method for the layup of sheets for forming composite articles and composite articles made thereby. The invention has particular application to the layup of carbon fibre reinforced polymer (prepreg) plies in sheet form in the manufacture of composite articles, especially but not exclusively in complex shapes .

Background

Composite articles based on thermoset prepreg are typically produced by sequential layup of material plies in sheet form onto a former or over a former. Whilst automation is possible for tows or narrow tapes , in many cases the process is entirely manual, especially for more complex shaped articles. If the sheets are placed incorrectly or move during the process then it is very easy to produce an unacceptable composite article. Composite articles made in this way are usually inspected and typically there is a 5% rejection rate, which is wasteful in materials and time. Offline planning can be used to determine the optimal technique, but in practice there will be real variations in planned and actually experienced geometries, material properties and environment conditions, which can cause defects to occur. This poses particular challenges in replacing the skilled manual layup by an automated system or manual layup using less skilled personnel, particularly for complex geometries .

Summary of the Invention

According to one aspect of the invention there is provided a method of manufacturing a composite article comprising applying sheets to an article being made or to a former using a robotic end effector under the control of a processor, the processor controlling the end effector to apply each sheet in a manner that adjusts the application of the sheet to compensate, at least in part, for variations in an actual position of the sheet from a desired position stipulated by an article formation model , wherein the actual position is determined from signals derived from a sensing system sensing attributes of each sheet as the sheet is applied .

The method of the invention may cause the article being created to return closer to the desired shape and/or sheet layup pattern as stipulated by the article formation model than would have been the case had the feed-back from the sensing system not been used to compensate for variations in the actual and desired positions . In one embodiment, the sheets of material are prepreg sheets and the invention will be discussed in terms of prepreg sheets from hereinafter. However, it will be understood that the invention may have wider applications not necessarily limited thereto. For example, the invention may be used for the lay-up of dry fabric, for example, in wind turbine blade manufacture.

To sense the position of prepreg sheets as the article is made it is feasible to continually monitor the progress of the assembly as layup progresses. If this functionality is combined with predictive modelling and control, then the process can be made to take appropriate corrective action before minor changes become significant. Such an approach also offers intimate knowledge of all stages of assembly, which would be a major benefit in verifying quality , assuring correct placement and the absence of defects. It is believed that the method of the invention can provide an automated method for forming composite articles of complex geometry with an acceptable level of accuracy and therefore, a low enough rejection rate. The processor may determine from the signals from the sensing system the actual position of points on the sheet that lie within and away from the edges of the sheet as (he sheet is applied and may adjust the application of the sheet to compensate, at least in part, for variations in the actual position of these points on the sheet from a desired position stipulated by an article formation model. For complex geometries it may not be sufficient to only determine the actual position of edges of the sheet and the position of points on the sheet away from the edges may need to be determined to identify the corrections that are required.

In one embodiment, the processor may determine from the signals from the sensing system a position of a boundary line between a portion of the sheet that is stuck down and a portion of the sheet that is not stuck down and a displacement of one or more points on the boundary line from a desired position stipulated by the article formation model and may control the end effector based on the determined displacement. Limiting considerations to the points along the boundary line may keep the processing to a level that allows for rapid enough analysis.

The end effector is controlled to compensate for the displacement and may be based on a sheet application model that predicts where unstuck points on the sheet will be positioned during later steps of applying the sheet. In this way, the article formation model may remain unchanged throughout layup of the sheets, with continuous corrections being made such that the actual position of the sheets lies within acceptable tolerances of the desired position stipulated by the model.

However, in an alternative embodiment, the article formation model is recalculated during application of a sheet based on the variations in the actual position of the sheet from a desired position stipulated by the article formation model before recalculation. Alternatively, the article formation model may be recalculated after each sheet has been laid such that the application of further sheets can be adjusted to compensate for any variations on the actual position of the laid sheet(s) to the desired position stipulated by the article formation model before recalculation. Such an arrangement may be desirable as an optimum pattern of sheets may change with variations in the actual position of laid sheets. In a further embodiment, the article formation model may only be recalculated if it is determined that the actual position of a sheet is so far away from the desired position that sufficient corrections cannot be made through later adjustments to the application of the sheet. In such a scenario, a reworking of the pattern of sheets may result in a re-positioning of later sheets that compensates for the inaccuracy of the laid sheet.

Adjustments of the application of the sheet to compensate, at least in part, for variations in the actual position of the sheet from the desired position may be determined from a set of predicted results for available adjustments . For example, in advance of the process or during lay-up of the sheet, effects of a variety of manipulations of the end effector may be mathematically simulated and the results of these simulations may be used to determine the adjustments/manipulations to be made to compensate for variations in the actual position of the sheet to the desired position.

The sensing system may comprise optical devices , such as lasers, cameras or the like, for capturing images of each sheet as it is applied, the signals from these optical devices being used to determine the actual position of the sheet relative to sheel(s) that have already been laid and/or the former. Alternatively, the sensing system may comprise sensors for determining the position of the end effector(s) , the actual position of the sheet that is being applied being determined from the position of the end effector(s) . For example, the boundary line of the sheet being applied may be determined by an end effector, such as a roller, that pushes the sheet onto another sheet/article/former below . The angle of the unstuck portion of the sheet may be determined from the boundary line and the position of end effectors holding edges of the unstuck portion of the sheet.

In one embodiment, a surface of each sheet has a pattern thereon, preferably a visual pattern for example a grid of dots or lines, the sensing system identifying the pattern and determining the actual position of the sheet relative to the other laid sheets and/or former from the pattern. Furthermore, distortion of the pattern can also be used to determine corrective action to take.

The sheet before it is laid may comprise a removable membrane, the removable membrane having the pattern thereon. The method may comprise removing the membrane from the sheet once the sheet has been or as the sheet is laid-up.

Alternatively, each sheet comprises identifiable fibres that are distinguishable from the remaining fibres of the sheet, possibly the fibres having a direction of orientation (for example parallel to or perpendicular to the edge) , these identifiable fibres providing the pattern from which the actual position of the sheet can be determined. By colour is meant differential electromagnetic reflection characteristics compared to the remainder of the sheet (e .g. infrared absorption/reflection) . The method comprises using the sensing system to detect the identifiable fibres and determining the actual position and/or distortion of the sheet from the identified fibres.

Alternatively or additionally , the sensing system may be sensitive enough to detect the orientation of the fibres themselves without providing certain fibres with a distinguishable characteristic and an actual position and/or distortion of the sheet may be determined from the position and/or orientation of the fibres . This may be especially suitable when the fibres of each sheet have directionality.

Determination of fibre position and/or orientation from the pattern or from imaging the fibres themselves allows the fibre positions and/or orientations to be verified against requirements and tolerances . Accordingly, in one embodiment, the method comprises determining a position and/or orientation of fibres of the sheet and comparing the orientation with a required orientation, for example stipulated by the article formation model . If a variation between the determined position and/or orientation and the required position and/or orientation is outside of a predetermined threshold, a warning may be generated or the sheet may be removed and re-laid or replaced with a new sheet. One embodiment comprises a system capable of automated layup of composite components with verified quality . Aspects of the invention lie in sensing techniques , advances in predictive modelling for real time control , and handling techniques for manipulation and consolidation of composite materials .

Part of the elegance of the present invention is to predict the sequential assembly of many pieces of prepreg material and to observe how the reality of manufacturing really pans out, and to revise the future planned prepreg applications positions to take into account perturbations/deviations observed from the actual compared with predicted shapes . This interactive self-correction is made fast enough to be real time by simplified mathematical modelling and predictive modelling . By "realtime" it is meant that the sensing and analysis of the actual positions of each sheet are performed during layup of the sheet in time to allow alterations in the further application of the sheet. In a preferred embodiment, the actual positions of the sheet are determined at least once per minute. In this way, a sheet can be laid in a reasonable time period whilst allowing for corrections. For example, the actual position of the sheet may be determined at least 5 and more preferably at least 10 times during the laying of the sheet.

Preferably , the end effector has a pressure sensor associated with it so as to sense the pressure and/or force applied to the sheet by the end effector.

Preferably , the pressure or force applied by the end effector on the sheet being applied is modified at at least one point of its travel when applying the sheet. According to another aspect of the invention we provide an automated composite article manufacturing system comprising:

a robotic end effector adapted to apply a sheet of material to an article being made or to a former;

a sensing system for sensing attributes from which an actual position of a sheet can be determined;

a control processor arranged from controlling the end effector; and computer readable memory storing an article formation model that models an application of the sheets to form the article and instructions which, when executed by the processor, cause the processor to control the end effector, based upon the article formation model and the signals from the sensing system, to apply each sheet in a manner that adjusts the application of the sheet to compensate, at least in part, for variations in the actual position from a desired position stipulated by the model. As well as providing a system that achieves adequate results for the laying of prepreg sheets, the invention also facilitates quality assurance and control as the system will have a good manufacturing history of the article so that there is greater confidence that it meets mechanical design requirements and shape tolerances . It may be possible, due to the increased predictability of the quality of product to omit some post- manufacturing quality assurance test. For example, at present all aircraft parts made from composite materials are x-rayed/tested for voids or delaminations . That may not be necessary if the reliability of manufacture can be improved . Perhaps a representative sample could be tested.

The robotic end effector may comprise one of more end effectors selected from the group of an applicator that grips an edge of the sheet, a probe/finger for applying force to the sheet, a roller and a heater and/or cooler or other means to vary the viscosity of resin on the sheet.

According to another aspect of the invention there is provided a data carrier having instructions thereon, which, when executed by a processor connected with a robotic end effector and a sensing system, causes the processor to carry out the above described method.

The data carrier may be a laser disc, computer memory , for example, RAM or ROM , memory stick, optical or electronic signal or any other suitable medium for conveying or storing data.

According to a further aspect of the invention, there is provided a method of manufacturing a composite article comprising applying sheets of prepreg material to an article being made or to a former using a robotic end effector under the control of a processor, the processor controlling the end effector to apply each sheet in accordance with the article formation model, the article formation model being recalculated after the sheet has been laid based on an actual position of the sheet to compensate, at least in part, for variations in an actual position of the sheet from a desired position, wherein the actual position is determined from signals derived from a sensing system sensing attributes of each sheet as the sheet is applied ,

According to a further aspect of the invention there is provided a method of identifying points on a boundary line between a portion of a sheet that is stuck to an application surface and a portion of the sheet that is not stuck to the application surface comprising capturing images of the sheet and the application surface, processing the images to identify a curve of a bottom surface of the sheet and determining an intersection of the curve with the application surface, the intersection defining one of the points on the boundary line. In one embodiment, the curve of the sheet may be determined by identifying at least three points on the bottom surface of the sheet and fitting a curve to the identified points. In another embodiment, the curve is determined by identifying from the images a curvature of a top surface of the sheet, and calculating the curve of the bottom surface of the sheet by assuming the bottom surface is displaced from the top surface by a predetermined/estimated thickness .

According to yet another aspect of the invention there is provided a method of manufacturing a composite article formed from a plurality of a sheets comprising determining from an article formation model a desired laying operation for a sheet of material, projecting onto an article being made or a former target indicia indicating the desired laying operation and manually applying a sheet to the article being made or to the former using the indicia as guidance. In this way, the user is guided as to how to lay the sheet in accordance with the predetermined article formation model. This may result in a better composite article than if the user is not provided with guidance. For example, if the user uses his experience to determine where and how to lay the sheet. Furthermore, the use of such a method may allow less skilled labourers to lay the sheets because of the guidance provided by the indicia .

The desired laying operation may be a desired position of at least a portion of the sheet . For example, the indicia may be a grid of points or lines to which points or lines on at least a portion of the sheet are matched or an outline of the sheet projected onto the article being formed or former in the required position. Additionally or alternatively, the desired laying operation may be a process, eg a force, to be applied to the sheet (ie an action to be completed) . For example, the indicia may provide an indication of which portions of the sheet should be laid first and/or the direction that pressure should be applied to the sheet during lay-up. For example, the indicia may be arrows or colour coding indicating the direction that pressure should be progressively applied to the sheets .

The method may comprise monitoring the manual application of the sheet and changing the projected indicia in response to the monitoring step. The indicia may be altered to correspond with the stage of the manual application of the sheet. For example, the arrows or colour coding may be altered as the sheet is applied to indicate changes in the direction of pressure and application of the sheet. With the projected indicia changing dynamically , only indicia relevant to the immediate manual steps to be taken need to be projected, which may result in clearer instructions than projecting indicia representing all steps to be taken at one time. In one embodiment , the method comprises using a sensing system to measure an actual position of the sheet, determining variations in the actual position of the sheet from a desired position and altering the indicia projected onto the article or former to indicate a manual application of the sheet that compensates, at least in part, for the determined variations. This may have the advantage that the person laying the sheet will be provided with updates on the application of the sheet to correct for previous misalignments and mistakes. The method may comprise recording images of the manual lay-up of the sheet. Such images may be later analysed for quality assurance.

Such a method may be used for training of operators as well as of the actual formation of articles .

In a further aspect of the invention there is provided a system for manufacturing a composite article formed from a plurality of a sheets comprising projection apparatus , a processor and memory having an article formation model stored therein, the processor arranged to determine from the article formation model a desired laying operation for a sheet of material and cause the projecting apparatus to project onto an article being made or a former target indicia indicating the desired laying operation. The system may comprise a sensing system for sensing attributes of each sheet as the sheet is applied and the processor is arranged to determine from the attributes an actual position of the sheet, determine from the actual position indicia to be projected and cause the projecting apparatus to project the determined indicia. Determining the indicia to be projected may comprise using the actual position to determine from the predetermined article formation model a current stage of application of the sheet and identifying indicia that are suitable for the next stage of application of the sheet in accordance with the article formation model.

Alternatively or additionally, the processor may be arranged to determine variations in the actual position of the sheet from a desired position as stipulated by the article formation model , determine modifications for future stages of applying the sheet that compensate, at least in part, for determined variations and cause the projecting apparatus to project indicia that embody these modifications.

The system may comprise at least one image capturing device for capturing images of the sheet as it is applied. The processor may be arranged to store these images in memory for later analysis . According to yet another aspect of the invention there is provided a data carrier having instructions thereon which, when executed by a processor connected with projection apparatus , causes the processor to determine from the article formation model a desired laying operation for a sheet of material and cause the projecting apparatus to project onto an article being made or a former target indicia indicating the desired laying- operation.

Description of the Drawings

Embodiments of the invention will now be described , by example only with reference to the accompanying drawings, in which :-

Figure 1 is a diagrammatic view of a system in accordance with an embodiment of the invention; Figure 2 is flowchart of a method in accordance with an embodiment of the invention; Figures 3A to 3D show the application of a sheet to a former by end effectors ;

Figures 4A to 4C show a setup for determining points on a boundary line using side images of the sheet as the sheet is applied to an application surface;

Figures 5A to 5D show a setup for determining a boundary line using images of the sheet from above as the sheet is applied to an application surface;

Figure 6 shows a system for the manual lay-up of sheets in accordance with an embodiment of the invention;

Figure 7 shows a projection pattern of indicia in accordance with an embodiment of the invention; and

Figure 8 is a flowchart of a method in accordance with an embodiment of the invention.

Description of Embodiments of the invention

Referring to Figure 1 , an automated composite article manufacturing system comprises robotic end effectors 102, 104, 106 adapted to apply a sheet of prepreg material 108 to an article being made or to a former 110, a sensing system comprising optical sensors 1 1 1 , 1 12 and a control processor 1 14 arranged to receive signals from the sensing system 1 12 and control the end effectors 102 to 106.

In this embodiment, the robotic end effectors comprise an applicator 102 that grips an edge of the sheet 108, a roller 104 for pressing the sheet onto the article being made or to the former 110 and a heater 106 to vary the viscosity of resin on the sheet 108. The end effectors 102 to 106 are controlled by suitable motors (diagrammatically represented by box 116) under the control of processor 114.

The sensing system is arranged to sense attributes from which an actual position of a sheet 108 c an be determined. In this embodiment, the optical sensors are a plurality of cameras (represented by 111 , 1 12) that obtain images of the sheet 108 , the processor 114 processing these images to determine an actual position of the sheet 108. To adjust for corrections, the positions of edges of the sheet 108 are determined together with the positions of points on the sheet 108 within and away from the edges (ie on the planar surfaces) .

To aid in this identification, a surface of each sheet 108 that is exposed whilst the sheet 108 is being laid has a pattern 1 18 thereon that can be detected by the sensing system. In this embodiment, the pattern 118 is a grid of dots or lines that can be imaged by the cameras 111 , 112. The pattern is provided on a removable membrane covering the exposed surface of the sheet 108 such that the membrane can be removed when the grid pattern 118 is no longer required.

In an alternative embodiment, such a removable membrane may not be required and the pattern 118 could be printed directly on the surface of the sheet 108 or the sheet 108 may have coloured fibres from which the processor 114 can determine the actual position of the sheet 114. For a system in which the images captured by the sensing system are of a high enough resolution to identify individual fibres of the sheet, a pattern or coloured fibres may not be required , as the actual position of the sheet, may be determined from the orientation of the fibres . The control processor 114 is connected to memory 120 having stored therein a computer program 122, which , when executed by the processor 114, causes the processor 114 to carry out the method as described with reference to Figure 2. Also stored in memory 120 is an article formation model 124 describing a pre-planned application of the prepreg sheets to form the article and a sheet application model 126 from which predictions can be made as to how a sheet will be laid based on a current set of parameters, such as measured actual position, applied forces and the known geometry of the article being made/the former.

In general, the computer program 122 causes the processor 114 to control the end effectors 102 to 104, based upon the article formation model 124 and the signals from the sensing system to apply each sheet 108 in a manner that compensates at least in part for variations in an actual position of the sheet 108 from a desired position as stipulated by the article formation model 124. In this embodiment, this is achieved by the method that will now be described with reference to Figure 2.

At step 201 , the control processor 114 determines from the article formation model 124 where the next prepreg sheet 108 is to be applied to the article being made or former 110. The processor 114 then sends signals to the motors 116 that control the end effectors 102 to 106 such that the end effectors move and position the sheet 108 to be applied to the desired position. This is shown in Figures 3A and 3B . End effectors (not shown) grip the sheet 108 at the locations marked with the crosses and position the sheet 108 such that a point A on the sheet 108 is adhered as close as possible to a point A ' on the former 110. Placing sheet 108 on the former 110 forms a tacked portion 140 of the sheet 10.8 adhered to the former 110 and an untacked portion 141 of the sheet 108 that is not adhered to the former 1 10. As shown in Figure 3C , the roller 118 may then be operated to press on the sheet to increase the area of the tacked portion 140 of the sheet 108 onto the article/former 110. This step may also comprise the heater 106 heating the portion of the sheet 108 that is to be applied to soften an adhesive resin on the bottom of the sheet 108. During pressing of the sheet 108 onto the article/former 1.10, one or more applicators 102 that grip the edge of the sheet 108 may hold the sheet 108 under tension to apply a shear strain in a particular direction in order that the initial portion of the sheet 108 is applied in accordance with the article formation model .

Once the initial portion is applied, in step 202 the processor 114 receives signals from the sensors 111 , 112 and determines an actual position of the sheet 108 from these signals. In this embodiment, the processor 114 determines a boundary line 142 between the tacked portion 140 of the sheet 108 that is stuck down and the untacked portion 141 of the sheet 108 that is not stuck down and a position of points along this boundary line. The boundary line may be determined in a number of ways. Referring to Figures 4A to 4C, cameras 111 a to 111d may be located to capture a side view of the sheet 108 as the sheet is being laid-up such that both an underside of the sheet 108 and the top-surface of the former 110 are imaged . In this embodiment, each sheet 108 has a pattern 118 of dots or lines 143 thereon. Processor 114 receives the images from cameras 111a to 11 Id and identifies the dots or lines on the sheet 108. The processor 1 14 fits a curve to these dots/lines, for example using a least squares fit algorithm .

The surface on which the sheet is being laid may also have indicia 144 thereon, for example, the pattern 118 of dots or lines on sheets that have already been laid-up. The processor 1 14 may determine from these indicia 144 the location of part of the surface by fitting a curve to these indicia.

The processor identifies a point on the boundary line 142 by determining an intersection of the two curves, the boundary point being a point where the two curves meet. Multiple boundary points may be determined in this manner.

In an alternative embodiment, shown in Figures 5A to 5D, a 3-D camera 130, for example, a range camera or a stereo camera , is controlled to first image, from above, the surface, such as the surface of the former 110, on which the sheet 108 is to be laid . From this image, the processor 114 determines topography of the surface. During lay-up of the sheet 108 on the surface, the camera 130 captures images of the sheet 108. At certain times during the lay-up process, the processor 114 determines topography of an upper surface 150 of the sheet 108. Stored in memory 120 is a predetermined/estimated thickness of the sheet 108 and the processor 114 uses this predetermined/estimated thickness and the topography of the upper surface 150 to calculate topography of a bottom surface 151 of the sheet 108. The processor 114 determines a boundary line 142 by determining where the surface on which the sheet 108 is being laid first intersects the estimated bottom surface 151 of the sheet 108.

Referring back to Figure 2, in step 203, the processor 114 determines a displacement of the actual position of points on the boundary line 142 to a desired position stipulated by the article formation model 124. At this stage, the processor 114 may also determine distortion of the sheet from the grid pattern 118. In step 204, this measurement of displacement is used in sheet application model 126 to predict where later points on the sheet will be stuck and any adjustments that need to be made to the planned process of applying the sheet 108 to compensate for variations in the actual position of the sheet 108 to the desired position.

In step 205 and as shown in Figure 3D , the processor 114 controls the end effectors, in this embodiment, a finger like probe end effector 160, to apply the next portion of the sheet 108 with the adjustments determined in step 204 made to the application stipulated by the article formation mode. Such an end effector can apply downwards and tangential force to the sheet 108. As a simple example, if the comparison in step 203 finds that the points along the boundary line are shifted slightly to the right compared to the desired position, in step 204 it may be determined that a shear strain to the left needs to be applied to the sheet 108 and applicator 102 controlled accordingly. In a real-life scenario, the considerations are likely to be more complex, with the calculation having to take account of the geometry of the article/former, properties of the sheet, and the effect of external forces on the sheet and compensation for variations requiring adjusting a combination of the end effectors , 102 to 106 , for example an angle and path of the roller 104 as well as a position of the applicator 102.

In step 206 , the processor determines whether the sheet 108 is now completely stuck down . If not, then the processor 1 14 proceeds back to step 102 and repeats the steps for the next portion of the sheet 108 to be stuck down. If the complete sheet 108 has been stuck down, the processor 114 moves onto the next sheet, using the article application model to determine where the next sheet needs to be applied 207. In this embodiment, each sheet 108 is stuck down in more than 5 and preferably more than 10 iterations. After the sheet 108 has been stuck down, the membrane is removed from the sheet 108. in addition to or as an alternative to the compensation method described above, the method may comprise recalculation of the article formation model based on the actual position of the sheet once it has been completely stuck down. In this way, adjustments can be made to the plan to compensate for any variations in the actual position from the desired position .

On completion, if the sheets have been formed on a former, the former would normally be removed.

The processor 114 may store in memory 122 the final position of the sheets and the steps that had to be carried out to lay the sheets in these positions. In this way, this data can then be used for verifying the quality of the resultant article, tolerance checking and the like.

Referring to Figure 6, a system for guiding the manual lay-up of sheets in the formation of a composite article is shown. The system comprises projection apparatus 301 , a processor 302 and memory 304 having an article formation model 305 and sheet application model 311 stored therein. The system further comprises a sensing system for sensing attributes of each sheet as the sheet is applied, in this embodiment, cameras 306, 307 for capturing images of the sheets as they are laid-up.

The system is used in the manual lay-up of sheets, in this embodiment sheets of prepreg material, to form a composite article. Referring to Figure 8, a method carried out by the processor 302 will now be described . The system is set running by an appropriate input, such as a mouse click or keyboard input, from a user. In step 401 , the processor 302 determines from the article formation model 305, stored in memory 304, the laying operation for the next/first sheet to be laid. The article formation model may specify a position where the sheet should be laid together with the steps that should be taken to lay the sheet in that position. From the laying operation the processor 302 determines indicia 330 to project onto the article/former 310 that indicate (provide instructions) to the person who will lay the sheets how the sheet should be applied.

In step 402, the processor causes the projector 301 to project these indicia onto the article/former 310. An example of indicia that may be projected is shown in Figure 7. In this embodiment , the indicia comprises an outline 501 against which an outline of the sheet should be matched and a pattern of grid points 502 to which grid points on the sheet should be matched. Together indicia 501 and 502 define a desired position for the sheet. The indicia further comprise coloured sections 506, 507, 508, 509, 510 , illustrated by border lines 503a, 503b, 503c and 503d, and arrows 504a and 504b. Together indicia 504 and 506 , 507 , 508, 509 and 510 define instructions on processes (ie actions) to apply to the sheet when laying the sheet. In particular, sections 506 , 507, 508, 509 , 510 represent a sequence in which different portions of the sheet should be laid. In this embodiment, the sections 506 , 507 , 508, 509, 510 are colour coded to indicate the order in which sections 506 , 507, 508, 509 , 510 should be laid. For example, section 506 is colour coded green (as indicated by the letter G) to indicate that this section should be stuck to article/former first, sections 507 and 508 are colour coded amber (as indicated by the letter A) to indicate that these sections 507 , 508 should be stuck to the article/former after section 506 and sections 509 and 510 are colour coded red (as indicated by the letter R) to indicate that these sections 509, 510 should be stuck to the article/former last. The arrows 504a and 504b indicate the direction in which a tensioning force should be applied to the sheet.

It will be understood that other indicia could be used. For example, a gradient of colour, rather than blocks of colour, from a portion of the sheet that is to be stuck to the article/former first to a portion of the sheet that is to be stuck to the article/former last. Rather than coloured indicia, finger prints of the like could be projected onto the article/former to indicate where the person should press on the sheet to stick the sheet to the former/article .

Images of the article being formed are captured by cameras 306 , 307 and the processor 302 processes these images to determine an actual position of the sheet as it is manually laid-up (step 404) . This may be achieved in similar ways as discussed with reference to Figures 3 A to 5D. In step

405 , the processor determines variations in the actual position from a desired position as stipulated by the article formation model 305. In step

406 , the processor 302 uses the sheet application model to predict where later points on the sheet will be stuck and determines adjustments that need to be made such that these points are located closer to the desired position. The processor 302 can also use the actual position of the sheet to determine at what stage the person is at in applying the sheet.

In step 407, the processor 302 determines changes that need to be made to the indicia for the next stage in the application of the sheet taking into account the adjustments that need to be made to compensate for variations between the actual position and the desired position. For example, the indicia indicative of the forces to be applied to the sheet may be altered. In particular, a direction of arrows 504a and 504b may be altered to indicate that tension forces applied to the sheet should be in a different direction and the colour of sections 506, 507, 508, 509 and 510 may be altered to represent the progression of the lay-up process. For example, sections 507 and 508 may be changed to green once section of the sheet equivalent to section 506 has been stuck down. In this way, the projected indicia change dynamically as the laying operation progresses.

In certain circumstances , indicia may be displayed to indicate that a portion of the sheet should be unstuck for re-laying . This may occur when the variations in the actual position to the desired position are so great that adjustments to later stages of the laying operation cannot sufficiently compensate for the variations . For example, a cross or exclamation mark may be projected onto a mislaid portion of the sheet to indicate that this portion needs to be unstuck and re-laid .

In step 408 , the processor 302 determines from the captured images whether the laying operation has been completed . If further portions of the sheet need to be stuck down, the processor returns to step 403. If the sheet is completely stuck down, the processor moves onto the laying operation for the next sheet. If there are no further sheets to be laid , the process ends.

Images captured by cameras 306 , 307 during the lay-up process are stored in memory 304, These images may be used during later analysis of the completed composite article.

It will be understood that modifications and alterations can be made to the above-described embodiment without departing from the invention as defined herein. For example, the system for projecting indicia onto the article/former as a guide to the lay-up of sheets may not adjust for variations in the actual position to the desired position. The system may not display different sets of indicia as lay-up of a sheet progresses but: may display a single set of indicia for the complete lay-up of each sheet. Rather than automatic detection of completion of the lay-up of a sheet, the user may provide an input to the processor 302 to indicate when a stage of the lay-up of a sheet has been completed , the processor 302 causing the display of the next set of indicia on receiving such an input.

Claims

1. A method of manufacturing a composite article comprising applying sheets of prepreg material to an article being made or to a former using a robotic end effector under the control of a processor, the processor controlling the end effector to apply each sheet in a manner that adjusts the application of the sheet to compensate, at least in part, for variations in an actual position of the sheet from a desired position stipulated by an article formation model , wherein the actual position is determined from signals derived from a sensing system sensing attributes of each sheet as the sheet is applied.

2. A method according to claim 1 , wherein the processor determines from the signals from the sensing system the actual position of points on the sheet that lie within and away from the edges of the sheet as the sheet is applied and adjusts the application of the sheet to compensate, at least in part, for variations in the actual position of these points on the sheet from a desired position stipulated by an article formation model .

3. A method according to claim 2 , wherein the processor determines from the signals from the sensing system, one or more points on a boundary line between a portion of the sheet that is stuck down and a portion of the sheet that is not stuck down and a displacement of the one or more points on the boundary line from a desired position stipulated by the article formation model and controls the end effector based on the determined displacement.

4. A method according to any one of the preceding claims, wherein the end effector is controlled to compensate for the variation and is based on a sheet application model that predicts where unstuck portions on the sheet will be positioned during later steps of applying the sheet.

5. A method according to any one of the preceding claims wherein the article formation model is recalculated during or after application of a sheet based on the variations in the actual position of the sheet from the desired position .

6. A method according to any one of the preceding claims, wherein the sensing system comprises optical devices for capturing images of each sheet as it is applied, the signals from these optical devices being used to determine the actual position of the sheet relative to sheet(s) that have already been laid and/or the former.

7. A method according to any one of the preceding claims, wherein the sensing system comprises sensors for determining the position of the end effector(s) , the actual position of the sheet that is being applied being determined from the position of the end effector(s) .

8. A method according to any one of the preceding claims, wherein a surface of each sheet has a pattern thereon, the sensing system identifying the pattern and determining from the pattern the actual position of the sheet relative to the other laid sheets and/or the former.

9. A method according to claim 8, comprising determining a distortion of the pattern and determining corrective action to take from the distortion.

1 0. A method according to claim 8 or claim 9 , wherein the sheet before it is laid comprises a removable membrane, the removable membrane having the pattern thereon, the method comprising removing the membrane from the sheet once the sheet has been or as the sheet is being laid-up.

11 . A method according to claim 8 or claim 9, wherein each sheet comprises identifiable fibres that are distinguishable from the remaining fibres of the sheet, these identifiable fibres providing the pattern from which the actual position of the sheet can be determined and the method comprises using the sensing system to detect the identifiable fibres and determine the actual position and/or distortion of the sheet from the identified fibres .

12. A method according to any one of the preceding claims, comprising detecting the orientation of the fibres and determining an actual position and/or distortion of the sheet from the fibres .

13. An automated composite article manufacturing system comprising: a robotic end effector adapted to apply a sheet of material to an article being made or to a former;

a control processor arranged for controlling the end effector; and computer readable memory storing an article formation model that models an application of the sheets to form the article and instructions which, when executed by the processor, cause the processor to control the end effector, based upon the article formation model and the signals from the sensing system, to apply each sheet in a manner that adjusts the application of the sheet to compensate, at least in part, for variations in the actual position from a desired position stipulated by the model.

14. A system according to claim 13, wherein the robotic end effector comprises one of more end effectors selected from the group of an applicator that grips an edge of the sheet, a probe/finger for applying force to the sheet, a roller and a heater and/or cooler or other means to vary the viscosity of resin on the sheet.

15. A data carrier having stored thereon instructions which, when executed by a processor connected with an end effector and a sensing system, cause the processor to operate in accordance with the method of claims 1 to 12.

16. A method of identifying points on a boundary line between a portion of a sheet that is stuck to an application surface and a portion of the sheet that is not stuck to the application surface comprising capturing images of the sheet and the application surface, processing the images to identify a curve of a bottom surface of the sheet and determining an intersection of the curve with the application surface, the intersection defining one of the points on the boundary line.

17. A method according to claim 16, wherein the curve of the sheet is determined by identifying at least three points on the bottom surface of the sheet and fitting a curve to the identified points .

18. A method according to claim 16, wherein the curve is determined by identifying from the images a curvature of a top surface of the sheet, and calculating the curve of the bottom surface of the sheet by assuming the bottom surface is displaced from the top surface by a predetermined/estimated thickness .

19. A method substantially as described herein with reference to the accompanying drawings.

20. A system substantially as described herein with reference to the accompanying drawings.

21 . A data carrier having instructions thereon substantially as described herein with reference to the accompanying drawings.

22. A method of manufacturing a composite article formed from a plurality of a sheets comprising determining from an article formation model, a desired laying operation for a sheet of material, projecting onto an article being made or a former target indicia indicating the desired laying operation and manually applying a sheet to the article being made or to the former using the indicia as guidance.

23. A method according to claim 22, wherein the desired laying operation is a desired position of at least a portion of the sheet.

24. A method according to claim 22 or claim 23 , wherein the desired laying operation is a process to be applied to the sheet.

25. A method according to any one of claims 22 to 24, comprising monitoring the manual application of the sheet and changing the projected indicia in response to the monitoring step.

26. A method according to claim 25 , comprising using a sensing system to measure an actual position of the sheet, determining variations in the actual position of the sheet from a desired position and altering the indicia projected onto the article or former to indicate a manual application of the sheet that compensates , at least in part, for the determined variations .

27. A method according to any one of claims 22 to 26, comprising recording images of the manual lay-up of the sheet.

28. A system for manufacturing a composite article formed from a plurality of a sheets comprising projection apparatus , a processor and memory having an article formation model stored therein, the processor arranged to determine from the article formation model a desired laying operation for a sheet of material and cause the projecting apparatus to project onto an article being made or a former target indicia indicating the desired laying operation.

29. A system according to claim 28, comprising a sensing system for sensing attributes of each sheet as the sheet is applied and the processor is arranged to determine from the attributes an actual position of the sheet, determine from the actual position indicia to be projected and cause the projecting apparatus to project the determined indicia.

30. A system according to claim 29 , wherein determining the indicia to be projected comprises using the actual position to determine from the predetermined article formation model a current stage of application of the sheet and identifying indicia that are suitable for the next stage of application of the sheet in accordance with the article formation model.

31. A system according to claim 29, wherein determining the indicia to be projected comprises determining variations in the actual position of the sheet from a desired position as stipulated by the article formation model, determining modifications that need to be made in future stages of applying the sheet to compensate, at least in part, for the variations and indicia that embody these modifications .

32. A system according to any one of claims 28 to 31 , comprising at least one image capturing device for capturing images of the sheet as it is applied.

33. A data carrier having instructions thereon which, when executed by a processor connected with projection apparatus, causes the processor to determine from the article formation model a desired laying operation for a sheet of material and cause the projecting apparatus to project onto an article being made or a former target indicia indicating the desired laying operation.