Effector for automated direct textile placement in rotor blade production
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The process chain for the production of rotor blades for wind turbines is still mainly characterised by manual process steps. It is planned to reduce the long production times and fluctuations in quality that this causes through automated laying up of dry technical textiles. Scientists from the University of Bremen illustrate the possible development of a suitable effector for the automated direct textile placement.
Automation in Rotor Blade Production
Wind turbine rotor blades are manufactured primarily from fibre reinforced plastics (FRPs), owing to their excellent mechanical properties. The main production method used is the vacuum infusion process, where glass-fibre non-crimp fabrics are inserted manually into the mould. The rotor blade mould with the dry, near-net-shape layer structure, the so-called preform, is then sealed air-tight with a vacuum foil. The space between the mould and the vacuum foil is subsequently evacuated, and the preform impregnated with a resin/hardener mixture and allowed to cure.
A significant quality criterion for the structural properties of the subsequent component is the load-conforming alignment of the non-crimp fabric in the mould. The predominantly manual process step of laying up the fabric requires time and results in reduced quality as a result of the positioning of the fabric, which is often difficult to replicate. Since rotor blade production accounts for a large proportion of the manufacturing costs of a wind turbine — around 21 % — , it is intended to meet the high quality demands and the desire for shorter cycle times through automated production processes.
The Institute for Integrated Product Development (BIK) at the University of
As part of the “BladeMaker” joint project , the BIK is pursuing the alternative strategy of continuous lay-up in a multi curved mould in the sub-project Direct Textile Placement (DTP). It involves handling very large cuttings that generally have to be picked up by an effector, stored, transported, draped and positioned.
The Challenge of Laying Up in Multi Curved Moulds
Automated Direct Laying Up in the DTP Process
Owing to the relatively large dimensions of the cuttings being handled, it is not possible to use the so-called pick-and-place method for picking and positioning the large pieces of textile. Instead, it is necessary to have the effector stores the cutting, which can be as long as the maximum length of the rotor blade. In this situation, it makes sense to make use of a storage mechanism to wind up the cuttings; this can be implemented as a cylindrical reel.
A second draft design shows two safety chucks that can accommodate a winding shaft from above, allowing standard cardboard cores to slide over the winding shaft and be clamped pneumatically, Figure 5 (below). The installation space of the material storage decreases, owing to the smaller diameter of the winding shaft, while the length of the cutting remains the same. This results in the centre of mass shifting towards the robot’s tool centre point. This is also helped by the absence of the internal drive. Theoretically, the drive can be located anywhere within the load-bearing structure, meaning that the resulting bending moment on the structure of the effector can be reduced by locating the motor in a favourable position.
The multi curved rotor blade mould and the defined textile width mean that it is necessary to configure the draping unit very flexibly. For this reason, the draping unit has been split into seven structurally identical draping modules that have one rotational and one translational degree of freedom.
In a second draft design, the draping module is modified such that it only has one CRFP spring, thus increasing flexibility of the draping tip, Figure 6 (right). The gripping technology is integrated into the draping tip, and the draping surface enlarged. Tests show that the second draft design allows greater pressure to be generated at the rotor blade mould.
Both draft designs call for lightweight materials, such as aluminium and carbon-fibre reinforced plastics.
The Support Structure
Finite Element (FE) topology optimisation is used in an initial draft design to develop an attached ideal truss structure in order to increase stiffness. Here, a distinction is made between static and dynamic load cases, working in three spatial directions. The strain in the static load cases results from weight that remains unchanged over time. Additional inertial forces occur in the dynamic load cases as a result of acceleration and braking, which reach their maximum value in the event of an emergency stop.
The acting forces must be defined at the beginning of topology optimisation and the installation space selected for the design of the attached structure. The components used and their possible ranges of movement may not be ignored when defining the installation space available. This applies, for example, to the swapping cycle of the material storage or the rotatory positioning of the draping modules. The aim of topology optimisation is the creation of a structure that is as stiff as possible given the specified volume restriction (percentage of the defined installation space). With regard to the elaborate calculation, the topology is determined for the first load case where only the weight is considered. The topology serves as the basis for developing a bar structure in which high specific stiffness is to be achieved together with very low weight through the use of CFRP-UD tubes. In order to use the optimum stiffness properties of the CFRP tubes, the bar structure is then transformed into a truss structure so that the CFRP tubes are only subjected to tensile and compressive forces. This involves relocating nodes of bars and adding new bars. The calculation of the maximum values for static and dynamic deflection, of the equivalent tensile stress and of the natural frequencies is then performed for all load cases.
At around 21 %, rotor blade production accounts for a large proportion of the manufacturing costs of a wind turbine.
Both draft designs call for lightweight materials such as aluminium and carbon-fibre reinforced plastics.
Use of the DTP Effector in the Mould Segment
The process steps for laying up dry technical textiles can be demonstrated with the help of the working model of the handling effector in the transition section of a rotor blade mould. The concepts of the material storage with internal motor, the draping modules with one CFRP spring and integrated gripping technology as well as a support structure made from aluminium system profiles are implemented, Figure 8.
Both material storages can be separated from the effector and loaded discretely.
Conclusion and Outlook
The handling effector developed as part of the DTP sub-project allows a further step to be taken towards the automated laying up of technical textiles for the production of wind turbine rotor blades. Through the use of various gripping technologies, it is possible for the effector to handle different technical textiles, including those that are unsuitable for conventional manual process steps. This supports material development with regard to the draping properties of technical textiles and therefore with regard to optimising the geometry of the mould. Besides use in the construction of rotor blades, it is conceivable that the DTP handling effector might be used for the automated production of components with similarly large dimensions, such as aircraft wings.
From an automation engineering perspective, many of the project’s goals have already been met. Assembly of the various components and a verification of the process steps will be made by the end of the project. In a final step, the process for continuous laying up will be shown in the “BladeMaker” demonstration centre in Bremerhaven, and all the project partners will demonstrate the entire process chain for the automated production of a rotor blade.
We would like to thank the Federal Ministry for Economic Affairs and Energy, which is funding the “BladeMaker” joint research project based on a resolution of the Bundestag, as well as project lead partner Jülich, for coordinating the scheme.
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