Although thermoplastic components can address the cure cycle roadblock that slows down thermoset parts production because their linear polymer chains do not crosslink and therefore do not require a cure cycle . There is a need to develop and innovate assembly systems, aligned with components’ supply chain production rate. As demonstrated earlier, assembly tools and techniques used for thermosets and metallic materials are either not feasible or limit thermoplastics potentials, i.e. adhesive bonding and mechanical fastening. Hence, the weldability of the thermoplastics is exploited with the aim to develop an assembly system that demonstrates industrial scalability and integration to increase the production rate.
Several challenges drive the development of the assembly system in this project. These challenges are divided into four main categories.
Scale of components and assemblies. The demonstrator is a full-scale fuselage section with a length of around 8 m and a varying radius between 2 and 2.5 m, with many of its components having significantly large dimensions, such as the stringers.
Assembly space complexity. In addition to its size, the demonstrator will be assembled under a defined gantry envelope, limiting the available picking space surrounding the demonstrator and systems manoeuvrability above the skin while placing and welding components. Besides, some processes such as short welds are sequenced after the placement of frame and floor beam assemblies in a very confined space.
Process compatibility. The systems defined for the assembly of this demonstrator must work together seamlessly as a single solution, integrating much of their design and functionalities from earlier stages of the project. The designed systems should also satisfy the requirements for other out-of-scope operations such as clips, permanent welding and access for cargo door surrounding structure.
Quality requirements. This challeng is primarily driven by the demonstrator functional requirements presented earlier in “Demonstrator functional requirements.”
The contents of the above categories are presented in more detail in Fig. 5. This figure also outlines the developed solutions to address these challenges in the context of three robotic end-effector systems. The detailed development of these systems within the boundaries of this study is presented in the following sections, while referencing the solutions shown in Fig. 5 where applicable.
Long welds adaptive assembly tool
The lower half of the multifunctional fuselage demonstrator is treated as a modular unit which will couple with the upper half, forming the fuselage body that will be assembled with other fuselage bodies. Therefore, the components of this system must be housed effectively during the assembly process. To achieve this, a cradle tool that supports the assembly is developed by The Welding Institute (TWI) . The cradle tool is equipped with a welding beam to complete the assembly of stringers to the skin; see Fig. 6. The development and function of both the cradle and welding beam systems are detailed as follows.
The cradle (Solution 1.1) is designed to provide stiff support for the fuselage skin and the fully assembled lower section of the fuselage demonstrator, resisting the transferred large forces applied to the demonstrator because of stringers and clips welding. The cradle assembly consists of the following components.
The cradle contains 34 different styles, longitudinally positioned, backing anvils to suit the different lengths and locations of stringers within the demonstrator. These backing anvils are supported along their length by the lateral support frames. Each backing anvil is designed to support the skin during the stringer and clip welding processes. They are also required to keep the demonstrator such that other operations can be carried out without exceeding the defined quality tolerances; see Table 1.
The width of the backing anvils is larger than stringers’ flange weld width to support two operations (Solution 1.2): (1) The welding of stringers. (2) In combination with specially designed clip support fixtures (see Fig. 6(a)), the backing anvils incorporate the support required for the out-of-scope clips’ permanent ultrasonic spot welding, which extend beyond stringers’ flange width. On the other hand, the depth and thickness of the backing anvils sections are defined based on the stress analysis to meet the maximum allowable deflection requirement at all locations.
Cradle indexing units and plungers
At the ends of each backing anvil, there is an indexing unit and plunger assemblies shown in Fig. 6(b). The indexing unit is used as the datum for the conduction welding head such that the weld occurs precisely over the backing anvils and the corresponding welding interfaces (Solution 1.3). This is achieved using a manually operated plunger assembly that index and lock the anvil indexing with the welding beam indexing unit (see “Welding beam indexing”).
The cradle contains 5 types of support frames necessary to support the backing anvils (Solution 1.1). The support frames are connected using support frame connecting beams. These are necessary to space the support frames the correct distance from one another (see Fig. 6).
On the other hand, each support frame has accurately machined cut-outs to locate the backing anvils profile when inserted accurately. These cut-outs have a datum face that the backing anvils are pressed against to aid the backing anvils’ positioning. This is done using a backing anvil clamp (grub screw) to push firmly and fix the backing anvil against the datum face once inserted (see Fig. 6(c)).
As for the cargo door opening, the support frame incorporates removable backing anvils to support the stringer above the cargo door during the welding operation. Additionally, the support frame below the cargo door opening is modified to allow access for this out-of-scope activity, as shown in Fig. 6(d) (Solution 1.4).
A welding beam that runs longitudinally across the demonstrator’s length is developed to carry the 1 m conduction weld head developed by GKN-Fokker Aerospace  using a carriage assembly (Solution 1.5). The welding operation which this beam will conduct follows positioning and tack welding stringers in place by the jig-less assembly end-effector presented in “Jig-less assembly end-effector.” The components and functions of this welding beam system are detailed in the following sections.
Carriage guide beams
The welding beam system is mainly supported on two beams that run across the demonstrator’s length. This assembly, the welding beam, is moved from one stringer welding interface to another using a crane system and the manual indexing operation (see “Cradle indexing units and plungers” and “Welding beam indexing”). Due to the length and function of the two guide beams, hollow sections are chosen to provide high stiffness to weight ratio which in turn lessens load requirements for the crane (Solution 1.7). The selected section allows a maximum elastic deflection of 9 mm when the carriage is positioned halfway along with the guides during the welding operation, i.e. pressing against the cradle.
A carriage system is developed to accommodate moving the 1 m conduction weld head along each stringer’s length and apply vertical pressure at the desired positions to perform welding. The longitudinal movement is driven by a rack attached to both square guide beams that engage with a pinion on the carriage to form the automated drive mechanism, which translates the carriage along the welding beam (Solution 1.6), as can be seen in Fig. 6(e).
Whereas the vertical movement needed to deploy and retract the conduction welding head is accomplished using four pneumatic cylinders with a configuration that allows the weld head to pivot (Solution 1.6); see Fig. 7. This pivoting feature supports the welding head’s surface adaptivity to buffer the step changes in the skin (“Thickness-varying composite fuselage skin”) so that it is normal to the stringer flange surface.
Welding beam indexing
To ensure accurate perpendicular placement of the welding head to each welding interface, i.e. each stringer flange, an indexing plate attached to the end of the welding beam is designed to lock with the plunger (“Cradle indexing units and plungers” and Fig. 6(b)). The indexing plate contains several holes, each corresponding to the unique position of a stringer flange weld. This allows the operator to rotate the welding beam to the required angle necessary for the conduction weld head to be normal to the welding surface (Solution 1.3). To ease welding beam manual rotation, the axis of rotation lies on the beam’s assembly centre of gravity. After rotation, the indexing rod is inserted through the indexing plate and into the lifting bar to lock the welding beam at the correct angle; see Fig. 6(f). For the welding beam to be directly aligned with each stringer flange, the indexing plunger assembly is deployed through both the anvil end and indexing plate.
Jig-less assembly end-effector
Accurate positioning of stringers and clips is a critical and exhausting step for the manufacturing of aircraft fuselage due to their quantity and function. Therefore, to decrease assembly time, a multifunction jig-less assembly end-effector supported by a gantry system is needed to conduct two main activities: picking and placing both stringers and clips, respectively. To achieve these activities, the Advanced Center for Aerospace Technologies (CATEC)  developed and integrated several systems forming the required jig-less end-effector (Solution 2.1); an overview of this end-effector can be seen in Fig. 8. The development and functionality of the individual systems in this end-effector are presented in the following sections.
Stringer picking and placing
Since the geometry of the stringer presented in “Composite stringers” has flat faces, and its surface is smooth, vacuum technology (suction cups) has been chosen for picking and positioning the stringers on the skin. To define the dimensions of the end-effector, the establishment of picking points and the distance between the suction cups were based on the deflection analysis of the stringers presented in “Stringers anti-bending fixtures.” Additionally, to buffer the deflection of the stringers and to have better contact with the stringer’s surface, spring plungers have been added to the vacuum suction cups. On the other hand, two machined centring elements are used to guide stringers in position with respect to the vacuum suction cups. The operation of the vacuum suction cups and spring plungers of the vacuum system while performing the stringers’ picking is illustrated in Fig. 9.
Stringers anti-bending fixtures
The deflection analysis of the 8 m stringer shows that the combination of stringers stiffness and the distributed four picking points presented in “Stringer picking and placing” will not entirely eliminate the defection of these stringers based on the analysis results seen in Fig. 10, where a maximum deflection of 17.5 mm and 9.78 mm is occurring at stringer ends in the case of horizontal and vertical picking configurations, respectively.
To avoid the deflection of the 8 m stringers and support their weight before conducting the tack welds, the anti-bending fixtures solution is designed to ensure precise placement and subsequent welding (Solution 2.2). These are stringers’ housing fixtures positioned on the corresponding anvil indexing unit employing the plunger at the cradle (“Cradle indexing units and plungers”) to fix them in place; see Fig. 11.
Stringers tack welding
The period between stringers’ placement by jig-less end-effector and performing the conduction welding at each flange will require a temporary hold. Several thermoplastic joining technologies have been considered to provide this temporary hold, such as adhesive tape, adhesive spray, glue, welding and magnets. It was concluded that ultrasonic welding is an optimum option for this application as it meets reliability, scalability and production rate requirements (Solution 2.3).
The proposed ultrasonic welding system consists of an SPA20 Rinco ultrasonic welding actuator, acoustic stack formed by the converter, the booster and the sonotrode, and ADG 20 Rinco ultrasonic generator as seen in Fig. 12. The ultrasonic welding tool is positioned at the centre of the jig-less assembly end-effector, with a slight lateral offset position that allows it to perform the spot weld following the placement of each stringer. As this completes, the end-effector transits to the corresponding weld spots to secure the stringer adequately.
Following several trials, the energy and force parameters required to conduct a successful ultrasonic weld were identified, avoiding the risking damaging the laminate, nor having a weak weld that can lead to disbond as the end-effector releases the stringer and move to the next tack weld position. The microscopic images in Fig. 13 show an example of an optimum ultrasonic tack weld conducted using the jig-less assembly end-effector on trial sections of the stringer and skin.
Clips picking and placing
The jig-less assembly end-effector employs a pneumatic technology to pick the clips illustrated in “Composite frame clips” (Solution 2.1). The system consists of a pneumatic cylinder that provides the gripping force and a machined part that guides the clip to its proper position while picking it. This can be seen in Fig. 14.
Clips temporary fastening
Similar to the stringers, the clips will require a temporary fix before conducting the permanent weld. A simplified approach was adopted to address this requirement (Solution 2.4). The solution uses a fir tree fastener inserted in pre-drilled holes at the crown of the clip. This in place will be pushed all the way down by the end-effector as it picks the clip. When the clip reaches its destination, the clip with the fir tree fastener will be pushed into the corresponding pre-drilled holes at the crown of the stringer. This operation is illustrated in Fig. 14. It is important to note that holes in both clips and stringers do not exceed the allowable open-hole size.
To ensure meeting the tolerances required in the positioning of stringers and clips within the assembly, four perpendicularity proximity sensors fixed at the far ends of the jig-less assembly end-effector are used (Solution 2.5); see Fig. 8. The measurements collected by all the sensors must be equal, so the end-effector will be appropriately positioned and ready to perform the placement and tack weld of the stringer, also the operation of fastening clips in place. It is important to note that this sensing system is not a substitute for the general positioning system supported by the overhead gantry.
Short welds end-effector
Frame sub-assemblies and floor grid assembly (see “Composite frame and floor beam assemblies”) require joining at several hard-to-reach spots because this operation is sequenced following the assembly of stringers, clips, frames to clips, and and a fully equippedfully equipped floor grid, creating a relatively restricted manoeuvring space. Joining the interfaces at these spots is achieved using an end-effector developed by London South Bank University  which facilities and transits a scaled-down version of the same conduction welding technology developed by GKN-Fokker Aerospace  that is used in the long welds adaptive assembly tool.
The main development drives of this short welds end-effector are the ability to apply the required pressure to the weld head, manoeuvrability to allow precise positioning for welding, and extended reach capabilities. These targets are achieved in a design that integrates the weld head with a gripping system attached to a robotic arm, as shown in Fig. 15. The development and functionality of these individual systems are presented in the following sections.
To reach all welding spots required for the floor beam to frame and fuselage assembly, this end-effector uses a robotic arm attached to the gantry system from one end, and to the gripper system from the other end. The robotic arm selected is the small footprint Universal UR16e  which provides a reach of 900 mm, six rotating joints degrees of freedom with a working range of 360° and having 16 kg payload capability. These capabilities, combined with machined aluminium grippers’ design, allow reaching all welding locations (Solution 3.1); see Fig. 16.
On the other hand, the control boxes of the arm and the heating weld head are supported at the base of the robotic arm (the interface with the gantry system). This can be seen in Fig. 15.
Gripper and welding system
Conduction short welds require applying heat and pressure to the external surface of the welding interface. The thermal energy generated transfers from the laminate’s outer surface to the joining interface, melting the thermoplastic matrix to form the join. Like the long welds, GKN-Fokker Aerospace  has developed a welding head consisting of two parts, a heating stamps unit and a corresponding anvil. The stamps heat the interface area using the energy provided from the remote heating box. In contrast, the anvil provides the complementary support to obtain the required welding pressure for the defined welding duration; see Fig. 18(a).
One of the main challenges in designing gripper arms that will accommodate the welding head is the ability to clamp and clear off all welding locations inside the fuselage without clashing. This was addressed by starting with a simple gripper arms design, then optimised to have the minimum weight possible while ensuring collision-free clamping operation at all positions shown earlier in Fig. 15. The final geometry of these gripper arms is shown in Fig. 17 and Fig. 18.
While the welding temperature can exceed 400°C at the joint interface, the maximum temperature at the weld head interface with the gripping system average around 175°C. Therefore, the stress analysis is conducted at 200°C, applying the mechanical properties of Aluminium alloy 6082 T6 tested at a high temperature according to ASTM:E21-09 . The results show that the maximum elastic deformation with the reduced modulus of elasticity at 200°C is 0.15 mm and 0.36 mm for each gripper arm, while von Mises yield stress remains below the lower yield strength at 200°C, ensuring an approximate safety factor of 3, as illustrated in Fig. 17. This stress analysis concludes that the grippers will withstand the applied forces without sustaining excessive elastic or any plastic deformation.
On the other hand, as the system is intended to weld various interface thicknesses, there is a need to ensure that the minimum welding pressure is applied. This is addressed by attaching the heating unit to the gripper arms through a spring pack solution (Solution 3.2). The spring pack consists of four springs housed in two components sliding tool, as seen in Fig. 18(b). Conversely, the welding force via the gripper arms will be applied using Schunk PGN-plus-P 2-finger gripper  powered through 6 Barg compressed air supply. This gripper can produce 5000 N of gripping force as a sum for the two claws providing sufficient welding force.
The gripper will be complemented with a tolerance compensation accessory unit Schunk TCU-P  that mates with the Schunk PGN-plus-P to provide the flexibility required in the case of gripper misalignment (Solution 3.3), while this unit is attached to the robotic arm interface plate from the other side; see Fig. 15.