Robotic woodwork by Marielena Papandreou

Student : Papandreou Maria-Eleni / Tutor: Ruby Law / Design for Manufacture RC101 / The Bartlett School of Architecture, UCL

robotic woodwork automated fabrication of a bridge

Abstract Despite the fact that contemporary timber construction is on the forefront of digital fabrication processes, the current research mainly focuses on subtractive numerically controlled technologies that precisely produce elements which will be later manually assembled into larger components. This results in a crucial interruption of the continuity of the digital chain of production since a great amount of information and precision is lost during the process. There are a few examples in which robotic assembly processes are attained. However, in most cases robots they are mainly used for the precise placement of the individual elements and later manual glue injection or screw fixing is used for the connection of the overall structure. In a case study, computational methods were used for the design and proper data structure of individual timber members that comprise a wooden bridge. For the connection of the beams a set of innovative timber joints was developed. A robotic cell is designed that will enable the prefabrication of large scale timber components on a complete digital workflow. The computational tools are used in conjunction with an industrial robot to carry out a combination of subtractive and additive robotic tasks. A tacking system is integrated to minimize time of production and maximize precision by updating data with regard to material deviations. key words: timber joints, robotic machining, KUKA KR60, assembly processes, motion capture system, digital fabrication

C o

n ts

1. Introduction

1a. Motivation & Context / Mass customization workflows / Industrial robots penetrating the manufacturing sector / Timber structures in the context of robotic fabrication / Present status of research on robotic timber assembly 1b. Focus of the thesis & objectives 1c. Structure & Methodology

01 01 01 02 02 03 03

2. Intricate design problems

2a. Case study overview 2b. Joint systems / Bottom-up design / Self-locking joints / Rack joints 2c. Development of a triangulated truss with integrated structural analysis 2d. Modifying the joint geometry to respond to local conditions / dovetail joint

05 05 06 06 07 08

3. Computational protocol

3a. The establishment of a deterministic planning system 3b. The need for custom computational tools / Data management

11 12

4. Robotic cell design

4a. Cell components 4b. Tool-to-part / Part-to-tool 4c. Manual Calibration 4d. Tracking system / Fast & accurate calibration / Material tracking & evaluation

15 15 17 17 18 19

5. Evaluation of the fabrication process 5a. Robotic machining review 5b. Milling strategies 5c. Assembly process / Tolerances / Robotic movement for the construction of a triangle / Robotic assembly objectives & potentials

21 21 22 23 23 24 24

6. Conclusion

7. Outlook

8. Bibliography

9. List of figures

1. introduction

1a. Motivation & Context mass customization workfLows constantly growing and suggest that in the race for automation across all industries the market will persist increasing henceforth. In the field of architecture, we have witnessed the whimsical utilization of robotic arms conducting various tasks, from simple pick and place paradigms to 3D-printing, machining, bending, folding or welding applications. Although during the last decade these operations had been conducted mostly in academic environments and research facilities, recently we have seen some of this research to be entering the construction industry. From large manufacturing companies to digital fabrication start-ups and fab labs, the demand of skilled robotic technicians is increasing in pursuance of the successful realization of challenging fabrication requests inquired by ambitious contemporary architectural firms. The robotic arms can perform a wide variety of tasks according to the end effector which is mounted on their edge. This multifunctionality makes them even more competitive in comparison with the existing numerically controlled machinery.

Industrial robots penetrating the manufacturing sector

The statistical data that are shown in Figure 2 were published from the Industrial Federation of Robotics in 2018. They show that the number of robotic cells deployed worldwide is

Figure 1 | KADK- Parametric wood (Tamke & Thomsen, 2008)

The established ways of design and production are challenged by tailor-made computational and fabrication tools. The interplay between advanced design tools, mastering the materialâ&#x20AC;&#x2122;s behaviour and limitations and novel methods of production set the ground for innovation. The key to creative advancements in the field of manufacturing is a function of knowledge on materiality and production process, aptitude to vision and skill to address the latest techniques and technologies. Digital solutions for interfacing seem to proliferate, yet we are still far from an established general platform that allows an informed collaboration especially in the field of fabrication. While digital fabrication has empowered the creative employment of mass customization across many design disciplines, the actual implementation of these methods and technologies bears a lot of unresolved questions (Tamke & Thomsen 2008). These vary from technical matters, like the selection of appropriate tools and the potential interfacing between them, over matters of management of manufacturing and assembly, to rather visionary ones, like ways of incorporation of feedback loops in the fabrication process that will allow the update of information about the later steps of the digital operations.

Figure 2 | Worldwide sales of industrial robots from 2004 to 2017 in 1000 units (IFR, 2018)

In the field of machining, contrary to conventional CNC machines which offer a delimited workspace, the robotâ&#x20AC;&#x2122;s working envelope can be way wider. As a result, large pieces can be machined in a single operation rather than conducting multiple operations for a single part or even splitting it in smaller pieces and reattaching them after milling (Padremenos et al, 2011). Furthermore, the flexibility of the robotic arm allows the toolâ&#x20AC;&#x2122;s centre point to approach intricate positions of three dimensional shapes with complex detailing which could not be reached by a conventional milling machine without the conception and construction of special jigs and fixtures. Although the use of industrial robots has numerous advantages, there is a major drawback which has to do with their capability to achieve absolute positioning accuracy. Timber structures in the context of robotic fabrication Wood has been recognised as one of the most sustainable building materials. It is linked with a wide range of diverse techniques of handling and joining. The fact that it allows the easy process of the connection straight from the material itself

is remarkable. A structure that is based on selfaligning timber joints rather than secondary fixings, like bolts and screws, can be very fast to assemble. This is influenced by the rapid generation of individualized elements that parametric models nowadays allow through the easy geometrical adaptability according to specific local conditions. Along with that, the production capacity of numerically controlled manufacturing methods enables the fast processing of unique wooden beams but also the geometrical complexity of unique timber joints which can be quickly put together with little tolerances. The digital fabrication of timber assemblies that consist of short slender beams with self-locking joints in an automated chain of production is one key element of this research. Off-site prefabrication allows the construction of timber members of high precision since it offers the advantages of a controlled fabrication environment. The volume and complexity of the prefabricated components is directly related to their construction quality, sustainability as well as the amount of on-site labour (Eversmann, 2017). In the ongoing production lines of timber elements, we usually encounter three disrupted interfaces. The one involves Computer Aided Design (CAD) platforms, the other is related to Computer Aided Manufacturing (CAM) software and last but not least the manual labourers complete the construction process. CNC machinery is used predominantly for the precise material processing and we could claim that its limited flexibility handicaps this method of production. A continuous automated system of material feeding, material handling and accurately material placing is missing. That means that there is a great amount of unexplored potentials in the field of digital fabrication. Since the manufacturing process is linked to an accurately constructed virtual model, by leveraging the use of robotic automation for the assembly of timber elements the risk of errors could be reduced significantly and the global accuracy could be considerably high (Eversmann, 2017). Present status of research on robotic timber assembly

Figure 3 | EPFL - Integrated Mechanical Attachment for Structural Timber Panels (Robeller & Weinand, 2015)

There have been several research studies which focus on the assembly of timber elements in space rather than just processing of single parts. Gramazio Kohler Research at the ETH Zurich has been investigating during the past

The manufacturing process arises from the concept of exploiting the genericness of the machine and blending it with the variability of the evolving workspace and the imprecision in dimensionality of the input material from which precise distinctive pieces and connections are produced. The following section describes the methodology that is followed in order to achieve the above.

1c. Structure & Methodology Figure 4 | Gramazio Kohler Research & ERNE AG Holzbau, Spatial Timber Assemblies (NCCR Digital Fabrication, 2018)

1b. Focus of the thesis and objectives The main focus of this thesis is built upon these two distinct until now research sectors. The aim is to extend the scope of wood joinery from plain production with NC machinery to volumetric prefabricated assemblies of individual elements using industrial robots. The addition of another level of complexity in the robotic assembly processes, which is the wood joinery, is approached in order to address unresolved issues in the current research status such as eliminating the interval of the human factor. Is it possible for an autonomous robotic cell to accomplish a set of established carpentry tasks such as identifying stock material, processing it in a known manner and placing it into a specific position in a seamless digital workflow?

/ The investigation and review of timber connection techniques / The design and performance of the overall structure / The establishment of a deterministic planning system / The analysis of the tooling and techniques that were adopted / The examination and evaluation of the fabrication process. The general geometric modelling takes place in conventional parametric design software which allows an intuitive approximation of scripting processes. The choice of algorithmic problem solving procedures is essential for the fast adaptation of the model according to updates and new conditions. Since there is an interplay between the physical and the digital world it is necessary for the virtual model to be easily adjusted according to findings or improvements from the physical experiments. Moreover, the deficient interoperability between standard design software packages and fabrication tools led to tailor-made scripting components and data exchange definitions. The development of these tools permit the solution of the diverse facets of the problem within a common programming environment. Consequently, geometric, structural and fabrication properties and constraints can be directly linked within the same context.

years the possibilities of large scale robotic assembly. From the sequential roof (Apolinarska et al., 2016) at the new wing of the faculty to the topology optimized timber structure, which is a project conducted in collaboration with Aarhus Architecture School (SĂ¸ndergaard et al., 2016), the research team has taken great steps in exploring the potential of the method. However, in almost all the papers published complete automation is never achieved since the human factor is always intervening to fix the different elements in place. This happens either with screws and nails (Eversmann, 2017) or with glue injection (Helm et al., 2013). Several researchers in various universities around the world, such as TU Vienna (Braumann & BrellCokcan, 2010) and Carnegie Mellon (Jeffers, 2016) have been engaged to similar research topics trying to tackle problems of automation within the field of timber construction.

The main targets of this research are addressed through a case study, which incorporates the digital production of the geometries in play and the design of the fabrication process. This case study contains:

2. intricate design problems

2a. Case study overview

2b. Joint systems/ bottom-up design The most crucial factor in the design of timber structures is the joint. The stiffness and rigidity of the connectors in a joint define the load-bearing behaviour and performance of the overall

assembly and must be sufficient to transfer the forces between the different members without failing. Correspondingly, another characteristic that we often encounter in timber engineering is that the cross section of an element is determined by the connectorâ&#x20AC;&#x2122;s geometric properties to satisfy adequate structural capacity. Apart from providing enough strength, another function of the joint in woodworking is the ability to offer fast and precise assembly of the different members. Thus, accurate positioning and rigorous alignment define the final quality of the product. As for the length of structural lumber, the span that a structure should cover is typically longer than the standard sizes. Therefore, many timber constructions are either assemblies of many shorter elements or composite systems. A bottom-up design approach is adopted because it assists the process of working with both the material itself and the digital fabrication techniques. Robot control strategies (milling/ assembly operations) become a primordial part of the synthesis and they determine the shapes of the connections from the beginning of the design process. The study of wood joinery happened with both advanced computational methods and physical 1.1 prototypes. Although the overall geometry demonstrates a high degree of complexity, it was necessary to solve the connection in a simple way to facilitate both machining and assembly and at the same time fulfil the structural requirements to transfer

A bridge was chosen as a medium to demonstrate the aforementioned intentions. The general form is based on the concept of a triangulated truss which is created by the conjunction of relatively short timber beams. The goal of this case study is to demonstrate an automated production scheme with a robotic system akin to the setups which are employed in repetitive industrial applications. The process from design to production is controlled through a single software platform in order to maintain an uninterrupted digital workflow. The general measurements are all variable through a parametric virtual model. The relationships between the different elements that comprise the bridge was carefully defined. Consequently, the overall geometry is dependent on certain parameters and their internal ways of interaction. The final shape is just one out of many potential configurations and since there is a direct link to engineering tools, a decision is made with regard to structural requirements. The geometry is additionally solved in a detail level according to fabrication constraints that are introduced also from the beginning of the design process.

triangle is based on the half-lap joint. However, instead of straight cuts the proposed geometry has a hook-like shape which blocks further rotation when it reaches the desired position. In a more detailed description, the assembly process begins in a unique rotational direction. When the first beam(A) is placed on the robotâ&#x20AC;&#x2122;s workbench, the next one(B) approaches the desired position in a specific angle. Only when this position is reached then it slightly rotates and self-aligns with the first beam(A). The shape of this connection is carefully designed so that there is only one possible way for these two beams to dismantle; through exactly the same rotational direction that they were put together. When the last beam(C) of the triangle is led to its final target position the system locks in all directions (Figure 5 & 7). Overall, relative movements between the parts are blocked by the geometry of the joints. The above grant a system that has only one degree of freedom which is also impeded when the adjacent triangles come into position.

b c

Figure 5 | Assembly process of the self locking joints

Figure 6 | Two types of rack joints designed for locking in two directions and self aligning

Rack joints

loads. The two types of structural connections are defined by their ability either to self-lock or transfer forces.

The connection between the different triangles happens at the long side of the beam. If the different members were connected at the short end of the beams then on the nodes of the triangulated structure multiple beams would meet. By creating an edgewise joint we avoid creating a weak point in each node location. This

Self-locking joints

The connection between the beams of the

A b

Figure 7 | 1 : 1 Prototype of four beams

type of connection is based on a system known as finger joints which originally are a planar pair of opposite shapes that have three degrees of freedom. We expanded this planar design in a three dimensional set of complimentary geometries in order to reduce the degrees of freedom to only one. Two guiding rails dictate the toolpath that the robot will follow. The input points to create these rails at each side of the beam have shifted symmetrical patterns and as a result the polylines are similar, but not identical. This leads to a female and male geometry that when paired together permit movement only in Z direction, while the movement in X and Y direction is blocked (Figure 6). Last but not least, the trapezoidal shape of the â&#x20AC;&#x2DC;rackâ&#x20AC;&#x2122;s fingersâ&#x20AC;&#x2122; is developed to facilitate self-aligning.

2c. Development of a triangulated truss with integrated structural analysis

An irregular triangulated mesh occurs after structural optimization strategies through Karamba software. Two levels of structural improvement were defined with regard to self-

The main control curves of the bridge geometry are defined by the general measurements that correspond to site conditions (Figure 8a). The overall shape resembles an arch that has a triangular cross section with variable dimensions along the span of the structure (Figure 8b). The height of this cross section is increased towards the centre and decreased closer to the supports of the bridge in order to satisfy structural requirements. Triangles are chosen as the basic module of the bridge since they are a remarkably stable geometric form. The triangulation is generated by the division of the control curves into smaller segments. If the distance between the points is less than 500 mm then the point is discarded from the set (Figure 8c). This prevents the generation of triangles that are difficult to fabricate due to their small area. The decision to use straight and relatively short elements was made to avoid discrepancies caused by warping of the wood. The size of the beams is within the range of 500 and 2000 mm to facilitate manufacturing and assembly. The utilization of standardised timber members as well as the design of easy connections was decided to satisfy an affordable manufacturing process.

Figure 8 | Parametric definition of the bridge

weight, live loads and dead loads. One has to do with the minimum displacement of the overall structure and the other has to do with the number of elements in play. The final arch as well as the spatial arrangement of the modules is the one that performs best in accordance with the criteria that were set through the structural analysis software. The mechanical behaviour of the arch bridge along with the compression strength of the material permit a longer span way more effectively than in a flat bridge (Kermani & Freedman 2005). Moreover, the truss created by the attachment of the triangles assist the structural capacity of a longer bridge that could alternatively be achieved with far

c d1

e v

Xj+2

Xj+1

Ttop

L0 L2

B Tbottom

Figure 9 | Generation of dovetail geometry according to common edge and rotation between adjacent beams

more pricey composite constructions such as laminated beams. The overall shape together with the arrangement of beams grant a type of construction with increased strength and stiffness.

2d. Modifying the joint geometry to respond to local conditions/ dovetail joint In the bridge prototype the relationship between the adjacent triangles will define the final geometry of the edgewise joints mentioned in section 2b. Traditionally in cabinetmaking these types of connections are used in orthogonal angles between two members. However, in this case study the output mesh contains a great variety of angles between the triangles. Christopher Robeller and Yves Weinand have conducted extensive research on the mechanical performance of edgewise joints on interlocking folded plates of cross laminated timber panels (Robeller et al., 2014; Robeller & Weinand, 2015; Robeller & Weinand, 2016). Although the bridge prototype is not formed by folded timber plates we can acknowledge the fact that our longitudinal timber connections share the same characteristics and have common mechanical behaviour. The rack joints as discussed in section 2b act as finger joints which demonstrate high resistance to shear forces parallel to the edge of the beam and compressive forces perpendicular to the beam (Robeller & Weinand, 2015). Yet bending moments are also transmitted from one beam to another which depend on the rotational stiffness of the joint shape (Robeller & Weinand 2015).

â&#x20AC;&#x2DC;Also, due to the rotation of the plate edge caused by bending, in-plane traction forces perpendicular to the edge line appear and their magnitude increases under asymmetrical loads. Such forces, which occur as a result of out-ofplane loading, cannot be supported only by shear and in-plane compression resistant joints.â&#x20AC;&#x2122; (Robeller & Weinand, 2015) Figure 9 shows the modification of the rack joint in order to correspond to the variable angles between the triangles and at the same time to tackle the structural incapacities of the finger joints. The connection is designed parametrically based on the traditional wood joinery technique known as dovetail. To generate this geometry between each triangle in our case study, first we need to identify the common edge between two triangles. There we place two planes perpendicularly to the normal of each triangle (Figure 9a). The intersection points between the two beams mark the start points of four lines L0, L1, L2 and L3 which will be divided into an even amount of segments (Figure 9b) returning an odd number of points Xj. The division points Xj are moved to both directions along the lines in two different distances d1 and d2. It is important that the exterior distance is always smaller (d1) than the interior one (d2) (Figure 9b). The points are merged in groups of four which are the control vertices of a two trapezoid curves Ttop and Tbottom. When Ttop and Tbottom are lofted, the solid created is subtracted from beam B. After a Boolean operation of beam A with beam B the resultant geometry has a single degree of freedom due to the inclination of the side faces of the trapezoids (Figure 9c). The alignment of

the two parts is achievable in only one possible direction v (Figure 9e). Solely on that direction such prismatic shape can be assembled or dismantled. A great advantage of this geometric definition is that all possible angles can be accommodated. According to Robeller and Weinand the dovetail joints apart from being capable of resisting shear and compressive forces like the rack joints, they can also withstand bending moments as well as traction forces that are not necessarily perpendicular to the beam (Robeller & Weinand, 2015). The mechanical performance of this connection detail is boosted by the rotation of the contact faces between the two timber elements (Figure 9c) but at the same time it is directly related to manufacturing precision. The fitting of the two pieces should be significantly tight in order to achieve maximum structural capacity without adhesive bonding. This poses great challenges to the robotic machining and assembly that were stated as main objectives in the introduction of this thesis. Such challenges will be analysed in further detail in section 5. Figure 10 | 1 ; 1 Prototype of dovetail joint applied on a non-orthogonal fold angle

9 Figure 11 | 1 ; 1 Prototype of dovetail joint , structural performance is dependent on fabrication precision

for (int l = 0 ; l < iLineC.Count ;

l++)

{

double t; if (iLineC[l].ClosestPoint(iPt1[j], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (l); BeamC_DT.AddRange(iBrep1. Branch(j), pth); } } } for (int f = 0 ; f < iPt2.Count ; f++) { for (int i = 0 ; i < iLineA.Count ;

i++)

for (int j = 0 ; j < iPt1.Count ; j++) { for (int i = 0 ; i < iLineA.Count ;

i++)

{

double t; if (iLineA[i].ClosestPoint(iPt1[j], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (i); BeamA_DT.AddRange(iBrep1. Branch(j), pth); } } for (int k = 0 ; k < iLineB.Count ; k++) { double t; if (iLineB[k].ClosestPoint(iPt1[j], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (k); BeamB_DT.AddRange(iBrep1. Branch(j), pth); } }

double t; if (iLineA[i].ClosestPoint(iPt2[f], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (i); BeamA_DT.AddRange(iBrep2. Branch(f), pth); } } for (int k = 0 ; k < iLineB.Count ; k++) { double t; if (iLineB[k].ClosestPoint(iPt2[f], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (k); BeamB_DT.AddRange(iBrep2. Branch(f), pth); } } for (int l = 0 ; l < iLineC.Count ; l++) { double t; if (iLineC[l].ClosestPoint(iPt2[f], out t, doc.ModelAbsoluteTolerance)) { GH_Path pth = new GH_Path (l); BeamC_DT.AddRange(iBrep2. Branch(f), pth); } } }

3. Computanional protocol

DataTree <System.Object> BeamA_DT = new DataTree <System.Object> (); DataTree <System.Object> BeamB_DT = new DataTree <System.Object> (); DataTree <System.Object> BeamC_DT = new DataTree <System.Object> ();

{

for( int m = 0 ; m < iLineA.Count ;

m++)

{

}

GH_Path pth = new GH_Path (m); if (!BeamA_DT.PathExists(pth)) BeamA_DT.EnsurePath(pth); if (!BeamB_DT.PathExists(pth)) BeamB_DT.EnsurePath(pth); if (!BeamC_DT.PathExists(pth)) BeamC_DT.EnsurePath(pth);

3a. The establishment of a deterministic planning system

In particular, the bridge prototype proposed has 5m span and 1m height with a varying triangular cross section of 100 to 500 mm. It includes 78 individual 2’x4’ timber members which carry 3 different types of three dimensional joints - two at each end and one along the edge of the beam. The sculpting process of each beam is composed by the combination of 10 different groups of robotic machining operations excluding transition movements and approach targets. The assembly process of each triangle includes 10 operations such as gripping, approaching and releasing. Depending on the size of the beam the amount of targets for the processing and joining of each part fluctuates between approximately 1000

and 2000. Although the general dimensions of this specific ‘frozen’ structure are relatively small and the topology of the design system is constant along the whole bridge, the data size and data flow within the computational model became a primordial concern. The establishment of a managerial strategy that is easily readable by both human and machine is imperative in order to supervise the virtual model. Through this we can identify the different members and review their specifications while at the same time preserve the basic data structure with the system’s information throughout the whole parametric definition. Interoperability with different software and communication with external hardware is inevitable and implies that specific information should be easily converted and applied to various formats. The programming workflow includes the overall shape of the structure, the connections’ detailing, the production of fabrication data as well as robot control. The general measurements are assigned by the user within Grasshopper and affect the arrangement of a set of points controlling the main curvatures of the bridge in plan and section. Further division produces a triangulated mesh which forms the base of our joint system. From the aforementioned mesh, topological information (such as angles between each triangle, angles between beams, naked/interior edges, lengths of each member) are extracted. The common edges as well as the rotation values between the triangular surfaces form the base for the rack joint modelling, whereas the intersection planes between each

The realization of a rigorous software solution for the production of design and fabrication data is one main difficulty of the project. More specifically a digital model, which can accommodate all the complex relationships of its contained elements, has to be orchestrated properly in order to deliver valid information for every step of the design and construction of the case study. The various dependencies between geometric modelling, structural analysis, detailing and toolpath modelling are interlinked in a linear manner. As stated in section 2c the scheme that is selected as a prototypical demonstrator is just one out of many possible configurations. That means that all possible scenarios of the joint systems and robot programming should be taken into account in order to assure the feasibility of the procedure for different design versions.

BEAM A

BEAM B

BEAM C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure 12 | Data sorting for fabrication (top)

Figure 13 | Parametric workflow to extract fabrication data (left)

beam is used to give shape to the complex selflocking lap joints system. For the production of the fabrication data we used the Robots plugin for Grasshopper where variations in size and length of the joints can be hosted. This section contains the target gripping planes, target milling planes, target placement planes etc. The guiding curves of the machining toolpaths are easily adjustable according to material size and properties as well as tool diameters and milling constraints (e.g. stepover, stepdown). The depth of the cut is also alterable based on the ability to maintain constant load on the tool. The milling and assembly information on each timber beam is dependent on its neighbouring members. The control of the robot is defined by a number of constraints and parameters. These have to do with the workspace limitations, such as robot singularities and collisions, as well as the KUKA Robot Language settings (Figure 13). To transfer data from the real world to the digital model, such as tooling positions and material information, we use a motion capture system called Optitrack. The physical information is acquired by special cameras which recognise the locations of retro reflective spheres inside the workcell. This data is captured within Motive software and is directly assessed within the main Grasshopper definition through a

C# component. This component decodes the numerical values from Motive to 3D points in the Rhino environment.

3b. The need for custom computational tools / Data management The entire algorithmic definition is organised in groups of sub-processes. Each one includes separate aspects of the overall modelling task. This facilitates not only the navigation inside the file, but also the examination of the script as well as the detection, isolation and fixing of problems. In that sense we can freeze and edit parts of the definition if necessary. In addition, we generate and reuse discrete code packages performing specific tasks which remain unchanged and are applicable in various sections of the algorithm. Data sorting, storing and filtering is crucial not only for the trade of information between different software packages but also internally for the temporary modification of the dataflow in order to construct the different toolpaths. Custom computational tools were designed in order to handle and archive the multiplicity of properties and operations that describe each

self-locking joints system described in section 2b (Figure 12). The decision of working in triples will assist the process of assembly in the next fabrication stage. The code which allows the proper grouping from triplets to pairs and vice versa was generated in C# using the RhinoCommon SDK (software development kit) and is assessed in different sections of the overall script. There are two main reasons why scripting becomes inevitable in such perplexing algorithmic design procedures. In order to address multiple scenarios that match diverse local conditions we need to compare, store and recover not only numerical values but also 3D objects and make choices based on this analysis. In this sense recursive loops are essential though not supported by the embedded Grasshopper components. However, the main reason why special tools were constructed is to leverage the simplest acquirement of data for our operations as well as a better control. Since the planning for hundreds of beams simultaneously rises the complexity of the overall model, economizing computational power becomes primary objective. By comparing existing data, we aim at reducing the entanglement of the digital workflow while assuring correct outputs.

data structure for

self-locking joints

dovetail joints

toolpath per beam

a data tree (bridge output-triples)

b List + data tree (with corresponding paths)

c data tree (from pairs back to triples)

(corresponding paths with initial data tree)

Figure 14 | Data structure editing to produce toolpaths for diverse joints while maintaining the order of operations

beam. In Figure 14 the three main data editing actions are described, which were necessary for the generation of the machining toolpaths per part. For the self-locking joints we keep the data structure of the triangular surfaces extracted from the triangulated mesh. That means that we use the data tree where each branch represents one triangle and contains information about the three beams in the right order (Figure 14a). In order to create the edgewise joints we need to regroup the data from triples to pairs according to the neighbouring trianglesâ&#x20AC;&#x2122; common edge. A list that consists of the common edges and a data tree that contains a temporary pair structure is necessary. The list and the data tree should have corresponding paths. That means that the first common edge of the list needs to coincide with the first branch of the data tree (Figure 14b). When the detailing and the toolpaths are obtained from this format we need to restructure the data tree to groups of three and merge it with the initial pattern to have a complete set of operations per beam (Figure 14c). Each beam is assigned with a unique label which includes a number suggesting the location of triangle in the overall structure and a letter (A, B, C) indicating the order in which the beams have to be placed according to the

ROBOTIC CELL - in du strial rob ot K UKA KR 60 - rotary t ab le

- ma terial feedi ng s tati on

- pa rallel p ne um at ic g ri pp ers - sp in dl e

- storag e sh el ves

- optitr ac k ca me ras

- retro- refl ec ti ve s ph eres

4. analysis of the tooling and techniques

- cu stom d es igne d worktabl e

4a. Cell components Another important aspect of the robotic set up is the robotâ&#x20AC;&#x2122;s communication with the range of components described above. The motion capture system uses 10 cameras and 15 retro reflective spheres. The recorded points are sent to an external computer through a hub and the robot program is updated according to the existing situation. Once the updated toolpath is fed back to the robot controller the subtractive and additive operations are executed. Each building process is accurately determined by a series of repeated steps. Information on each timber member is stored in a general cluster defining precise information on milling angles, approach planes, gripping and releasing targets or movement data. The different procedures, like gripper control, can be accessed easily in the visual programming interface of Robots within Grasshopper. However, the spindle is controlled externally by the operator.

4b. Tool-to-part / Part-to-tool Two different robotic setups have been tested namely tool-to-part and part-to-tool. Their difference lies in where the material billet is placed with regard to the robotic arm. In the first set up the material is held on a vice located on the rotary table while the spindle that will perform the subtractive operations is mounted on the robot. The creation of this set up was quick since it uses existing tooling that was available in the workshop. As a result, the fast production of parts was facilitated allowing the immediate understanding of basic machining parameters such as feed rates, cutting forces,

In this section the hardware that is necessary for the manufacturing process is described, as well as the arrangement of the various tools used within the work space of the robot. A robotic cell is created as a multifunctional tool for subtractive and additive operations for timber elements. The design involves the handling and placing of an unprocessed material of various lengths. The main component of this cell is an industrial KUKA KR60 robot which is bolted on the floor. It is equipped with a tool changer for the rapid adjustment of different tools without requiring extra calibration, which is a time consuming process. A custom made feeding station will be developed for the fast supply of 0.5m to 2m long wooden slender beams. These are pre-cut in specific lengths at a table saw outside of the cell. A rotary table is bolted at a certain distance from the robot with a spindle on top of it. This is where the material processing will take place. Different types of cutters are tested of various materials, diameters and geometries. Parallel pneumatic grippers that slide along a rail are mounted on the robot for fixing material. By allowing the movement of the grippers within a distance range of 200 and 1400 mm along the rail we aim at reducing the resonance of the material during machining. A custom made worktable with integrated pins is created in order to fixate the timber elements in the course of the build-up. The installation of cameras in various heights across the four walls of the room allows the tracking of the different elements in play, such as tooling and stock material. This facilitates the calibration process as well as the better control over the exact measurements and the imprecisions of the timber billets.

Figure 15 | Robotic setup with the spindle mounted on the robot and the vice holding the piece of timber on the rotary table

Figure 16 | Robotic setup with parallel pneumatic grippers on a rail. mounted on the robot, a spindle on the rotary table and cameras installed on the walls

quality results of different cutters and distinct milling strategies which will be explained in detail in the next chapter. However, the fact that the wooden beam is clamped only at one location, that coincides with its centre of gravity, results in increased vibrations and the phenomenon of chatter at the edge where it is cantilevering the most. Another disadvantage of the tool-topart setup is that it limits the utilization of the robot to only one set of actions. Since the goal of this research is the robot to complete both subtractive and additive operation, with the described setup this framework is cancelled.

body for high moment carrying capacity and bear holes for the placement of tracking spheres. Other components which connect the different parts together, such as T plates, were manually milled, drilled and taped out of steel. An aluminium plate was designed and waterjet cut that features holes in specific positions for the rigid connection of the spindle body with the external axis, as well as tracker locations for the calibration of the work cell. Additional components such as a tailor-made workbench for assembly and a material feeding station will be placed at strategic positions within the robotâ&#x20AC;&#x2122;s work area. With this setup the robot is programmed to pick the material from a specific position and move it towards the external axis where the machining will take place. The desired geometry is sculptured by the performance of synchronised activities between the spindle on rotary table and the material mounted on robotic arm. When this procedure is completed the part is moved and released on a specially designed holder on the workbench.

All the above constrains led to the advancement of the robotic cell with tailor-made tools in order to designate its multifunctionality and enhance its performance. For this purpose, an end effector was created which consists of a 1.5m robust rail and two parallel pneumatic grippers that feature synchronous jaw movement and a rigid wide bearing design. For the rail a heavy aluminium extrusion of a 60x60mm was used with a T-slot profile structure for easier adjustment of the position grippers. The jaws were accurately designed for best fitting and waterjet cut out of a 25mm aluminium billet. They are bolted at the full length of the actuators

4c. Manual calibration The calibration procedure of the robot and the various components that comprise the cell

Because this method is highly dependent on human precision after the first step (robot’s centre point identification) is completed, a numeric value is indicated on the pendant

which informs us about the level of accuracy of the calibration process. When the error is less than 0.2 of a millimetre, it is within acceptable limits. However, in many cases this error could be higher than the accepted values and lead to inconsistent fabrication results. This happens because we use a false-calibrated device to identify the location of the rest of the hardware and therefore the error accumulates. That is the case that is demonstrated in Figure 19. After completing all the steps of the aforementioned procedure we tried to evaluate the precision results by sending the robot to hover over a specific point on the corner of the vice. The tool’s central axis was 3mm off the sphere’s centre which is a substantial discrepancy for the precision that the manufacturing of this case study requires.

4d. Tracking system The optical capture system called Optitrack uses ten cameras and a set of retro-reflective markers. The cameras are placed in the perimeter of the robotic cell in a way that each of them have a vantage viewpoint. Ideally for better tracking data, their views should overlap since the markers should be detectable by at least two cameras. The cameras capture multiple 2D pictures and rebuild the information into 3D coordinates through Motive software. In order to obtain precise and stable capture results, and

Figure 17 | Tooling and components necessary to create the part-to-tool setup

is a time consuming process and we could claim that the results are often fallible. This is one of the primal reasons why industrial robots fail to achieve absolute positioning accuracy (Padremenos et al., 2011). To initiate this process, we need to mount a teaching spike on the robot and manually jog it using the pendant over a point in space from four different approach orientations. This method is called four-point calibration and the four points describe a unique sphere in space, the centre of which is the robot’s centre point. Only after this first step is finished, then the robot knows where the centre point of the end-effector is in relationship with its body. If we need to know additionally the orientation of the tool’s axis (e.g. the cutter’s centre point and orientation in the tool-to-part setup shown in Figure 15) we need to repeat this process with a short spike and a longer spike (Figure 18) to get the vector we are looking for. Finally, we need to rerun the same process, this time with three points, over a specific point on the rotary table so that the robot ‘understands’ its relationship with the external axis. The fourpoint calibration should be repeated on at least one point of each hardware component that is used within the setup and is a mandatory part before any robotic operation.

(Figure 21). Through this technique, Motive’s origin coincides with the rotary table’s centre plane. The relationship between the rotary table and the robot is constant in the physical world as well as within our Rhino Environment. Fast and accurate calibration using optitrack

Figure 18 | Four-Point Calibration with the long spike. Robot accuracy depends on human precision

In section 4c the sluggishness and ineffectiveness of the manual configuration of the hardware components was manifested. In search for a quicker and utterly reliable method, we decided to use Optitrack for the adequate set up of the relationships between the distinct components within our workcell. For this purpose, retro reflective markers are installed on the robot’s end and rotary table. By performing a circular motion of these two components around their local axis, 3D coordinates are collected that describe 2 circles, the centre of which will be used as their centre planes (Figure 22). Likewise, we will calibrate all our tooling based on the tracker

Figure 19| 3mm mismatch due to calibration error

they have to be correctly focused on the target volume. Within Motive the capture system is first calibrated and configured and afterwards the data is captured and processed. The camera calibration process involves the definition of the position and orientation of the cameras in space as well as the 3D volume within which the tracking process will take place. After this representation of the physical volume is generated into Motive, the next step is to define a ground plane (Figure 20), namely where the origin of our space is. The setting of the origin is of great significance in our case because we need to synchronise data from Motive’s coordinate space to the robot’s coordinate space (which is located at the centre of the robot’s base). This transformation has to be thoroughly controlled, therefore tracker positions imitating Optitrack’s ground plane were integrated in the aluminium plate which is mounted on the rotary table

Figure 20| Optitrack’s ground plane with 3 retroreflective markers. The origin within Motive is set to wherever this device is placed in space. (top left)

Figure 21| We used the same dimensions of the calibration triangle to set location of 3 trackers when creating the rotary table’s aluminium plate.

L W

Figure 24| Warping evaluation according to captured data

positions that were integrated in the design of our hardware, as described in section 4b. The motion capture system has proved to be a solid technique to tackle precision issues. The fact that the calibration procedure is eventually emancipated from human errors is an accomplishment of paramount importance. The overall duration of the set-upâ&#x20AC;&#x2122;s calibration has been reduced to only 5 minutesâ&#x20AC;&#x2122; time which is a supplementary asset.

affected by storage life and moisture (Jeffers, 2013). These must be identified prior to any fabrication action and evaluated with regard to tolerances of the design system. A special jig was designed for material tracking and evaluation of such dimensional irregularities. Six retro-reflective markers are placed at the beams opposite ends (Figure 23). After the data is tracked a special script uses 4 vectors to process the amount of warping the material bears (Figure 24). If this amount is within tolerance limits the piece is approved and the virtual model is adjusted to any alterations. Although the robotic fabrication process is standardized, the possibility of adjustments due to material variabilities was taken into consideration.

material tracking & evaluation

In terms of material variabilities, the timber beams can present dimensional inconsistencies in comparison with the digital model. In the case of wood, the deviations between the anticipated size and the real measurements is considerably

Figure 23| 3d printed jig with retro-reflective markers to detect stock material

Figure 22| Robot and rotary table calibration using trackers in a circular motion to get the 2 centre planes

5. evaluation of the fabrication process

5a. Robotic machining review

In our first milling experiments which were conducted with the tool-to-part setup we

Figure 25| Chatter at the edge of the beam due to robot’s configuration and material’s cantilevering

verified some of the above observations. The phenomenon of chatter often occurred when the robot was performing an operation at the edge of the beam (Figure 26). The two main reasons for that were that on one hand the beam was cantilevering from the clamping position inducing increased vibrations and on the other hand, the robot’s pose was in the maximum extend from its base in order to reach the furthest targets (Figure 25). As soon as we changed to part-to-tool setup chatter was ceased and the

Figure 26| Surface deviations along the edge of the beam due to vibrations and the phenomenon of chatter

During machining, the robotic arm is imposed to periodic forces that are generated between the work piece and the cutter (Padremenos et al., 2011; Iglesias et al., 2015). The low natural frequency of the robot’s articulated body as well as its low stiffness which is directly related to its configuration are the two main factors that generate overall system vibrations or even the phenomenon of chatter (Iglesias et al., 2015). Consequently, these disturbances introduce noise to the quality results and deteriorate the ability to deliver high precision in terms of fitting and finishing. Moreover, many academics believe that another cause of accuracy errors is the gear reducers integrated within the robots’ joints which are able to cause friction losses and backlash (Iglesias et al., 2015). Industrial robots are avoided in machining operations on account of their proneness to vibrations from process forces and their failure to absorb or reject them.

slotting

vibrations were significantly reduced since the clamping took place in two locations across the beam.

5b. Milling strategies

1. path : offset surfaces result: sharp corners

flank milling

2. p ath : chamfered control polylines result: chamfered exterior/ fillet interior corners

3. path : zoning increased to 6 mm result: smooth curve

Figure 27| types of milling strategies that were tested

The toolpath strategies were initially designed according to the position of the cutter in relation to the material. In all scenarios the sidecutting technique is used in order to achieve high material removal rates (Brell-Cokcan & Braumann, 2010). By utilizing the entire cutting length of a three-flute solid carbide cutter, which is especially designed for timber operations, the time of the milling process is reduced. The diagrams in Figure 27 explain these 3 different milling strategies that were initially tested and the geometric results of each operation. The two strategies are based on the flank milling technique. Two control curves are used as rails describing a ruled surface on which the end mill is rolled. The cutter is tangential to both curves at their respective control points. In the slotting strategy the stock material is removed aggressively when the cutter is entering the beam because it is milling it from both sides. All the techniques that were tested offer advantages and disadvantages and the choice of a final milling strategy is a matter of convenience or even preference. When slotting, the main benefit is that the sharp edges give a geometric predictability which facilitates the fitting of two opposite components. However, the tool is imposed to periodic forces that have substantial changes in magnitude. More specifically, in the first cut, when there is not any material removed yet, the contact zone between cutter and workpiece is double comparing to the next cuts when only one side of the end-mill is used. Another cause of increased stress while slotting is that the cutting action takes place perpendicularly to the grain of the wood. Last but not least, the breakouts of the wood when the cutter is exiting the material are far from negligible. In flank milling, due to the cylindrical geometry of the cutter, the resulting shape will have round corners. The main challenge was to match the interior corners of one piece with the exterior corners of its counterpart and for that purpose two different ruling curves were tested. In one experiment we used the control points of a polyline with fillet corners. This method returns smooth

Figure 28 | milling results / slotting - flank milling with zoning and with chamfered edge (top to bottom)

The toolpath modelling decisions were made based on the production of successful interlocking geometries and the accomplishment of minimum disturbances caused by machining. In order to create corresponding round corners in complimentary joints we used the target curve geometry with fillet corners. Instead of just using the control points, as in Figure 27.2, we

divided the curved part in a sufficient amount of points. When the robot interpolates through these vertices, it delivers the expected geometry. In order to maintain constant tool loading the flank milling technique proved to be suitable for most of the operations. However, in some cases we were limited to the slotting method due to one main limitation which was determined by the volume and orientation of the tool holder. Since the stock material should be placed in a specific depth within the jaw’s contact faces, when milling close and perpendicularly to the grippers’ positions collisions might occur. To avoid that, we approach the material with the slotting orientation but we remove material with repeated steps of small depths rather than entering aggressively using the whole flute length. Finally, in order to address the robot’s high sensitivity to resonance and process forces, a lower feed rate was used in comparison with the CNC machines.

5c. Assembly process tolerances

The creation of a solid movement sequence that regulates the order of component placing

interior corners but chamfered exterior edges (Figure 27.2). In the next experiment we used a sharp polyline and changed the robot’s point approximation settings (zoning) to 6mm which corresponded to the radius of the cutting tool. More specifically when the robot approaches a target the trajectory that it will follow is a NURBS curve constructed from 3 points. These 3 points are related to a circle, whose diameter in this case coincides with the diameter of the tool. The NURBS curve is constructed using the centre of the circle as well as the intersection points of the circle with two lines, one between the centre and the previous target and one between the centre and the next target (Figure 27.3). This method returns a smooth edge but inadequate to fit with the geometry’s counterpart. During flank milling the steady contact with the workpiece and the parallel cutting direction to the grain of the wood result in constant tool loading.

and the avoidance of collisions during buildup without exceeding the rotational limits of the robot are two main challenges of the robotic assembly process. However, in this case study the most demanding part is the proper insertion of tight-fitting joints with little tolerances. Both the self-locking joints and the dovetail joints were designed allowing only one possible assembly direction. Nevertheless, during manual assembly the resultant geometries demonstrated different tolerance requirements. The self-locking halflap joints with 0 tolerance during machining permit the easy alignment, fine contact and successful locking between two members. The edgewise dovetail joints require more than 1 mm tolerance and their assembly challenge is proportionate to the length of the timber beam. When the beam is long, more prismatic tails and notches are added to the parts which increase the possibilities of misaligning due to geometric deviations produced by disturbances during machining.

Robotic movement for the construction of a triangle

As described in section 3 the data representing the three beams of a triangle are sorted according assembly sequence (A, B, C) inside each branch of a data tree. The parametric definition of the machining toolpaths is extended to assembly trajectories that are directly linked to the final beam geometry and its centre of gravity. Two assembly jigs with integrated retro-reflective markers are designed. For the assembly process the beam needs to be picked from the opposite side of the one used for the subtractive actions. Therefore the first jig is used for this re-gripping operation which is executed using bisector planes. After the beam is correctly placed within the jaws, it is then moved to a final target on the workbench jig through an approach position that will ensure a safe and exact direction trajectory. The design of single-degree-of-freedom joints guarantee unique insertion vectors that are computed within the parametric definition. The motion planning to carry out both cutting and assembly tasks has to be carefully coordinated to avoid global collisions, robot singularities and axis rotational limits. The multiple tool orientations in order to host all necessary positions for these complex assignments as well as the sensitive orbit range of the robotâ&#x20AC;&#x2122;s axis (especially axis A4 and A5 which have a limited

range) led to a choreographic approach of the robotâ&#x20AC;&#x2122;s movement. This includes the interval of joint movements between the diverse tool orientations through an unwinding position where axis with limited rotational range are reset to zero values. Although this method generates extra movements it grants secure motion course avoiding collisions and kinematic errors for the path planning. robotic assembly potential

objectives

and

In a next phase of the research, branch sorting based on triangle build-up sequence will be developed for the expansion of the process in large-scale timber assemblies. However, one of the most intricate parts is finding the right tolerance balance between the required structural performance and movement flexibility for the assembly of such tight-fitting joints. In terms of insertion direction, thanks to the algorithmic design process there is only a unique guiding vector for the connection of two adjacent parts. Similarly to the self-locking joints, a reference position that hovers over the final target will be generated to establish a secure push normal to the desired location. Another major challenge would be the simultaneous positioning of multiple prismatic geometries along the edge of the beam. Insertion forces should be estimated and controlled. Last but not least, issues such as ensuring stability of the aggregating pieces during build-up should be analysed and solved. The edgewise joints present a great amount of difficulties which increases the complexity of robotic assembly. The points presented are taken into consideration and incorporated in the planning process with the intention to be addressed as much as possible within the tight time-frame of the project.

Figure 29| Simulation frames of the construction process of one triangle

0.86 m 0.40 m

2.60 m

1.20 m

5.00 m

DataTree <Line> perpLineGroups = new DataTree <Line> (); List <Line> tempPerpLineList = new List <Line> (); DataTree <Line> lineGroups = new DataTree <Line> (); List <Line> tempLineList = new List <Line> (); for (int j = 0 ; j < iMidPt.Count ; j++) { for (int i = 0 ; i < iAuxLine.Count ; i++) { Point3d startPt = new Point3d(iAuxLine[i].PointAt(0)); if (startPt == iMidPt[j]) { tempPerpLineList.Add(iAuxLine[i]); tempLineList.Add(iInteriorEdge[i]); if (tempPerpLineList.Count > 1) { GH_Path pth = new GH_Path (j); perpLineGroups.Add(tempPerpLineList[0], pth); perpLineGroups.Add(tempPerpLineList[1], pth); lineGroups.Add(tempLineList[0], pth); lineGroups.Add(tempLineList[1], pth); tempPerpLineList.Clear(); tempLineList.Clear(); } } } }

6. conclusion

oPerpLines = perpLineGroups; oPairs = lineGroups;

Robotic work and automation has seeped in most of the manufacturing sectors. In the building industry most of the research is still exploratory but promises an imaginative future in terms of rapid and accessible production of bespoke and unconventional design solutions. As robotic construction has been embraced worldwide by many institutions and researchers in the field of architecture, the extend of unresolved matters remains immense especially in tasks that require high precision, such as machining. The grounded academic findings become the foundation of more questions to be raised and answered. These emergent questions lead to the detection of additional obstacles to overcome. The presented work seeks to introduce novel ways of thinking and producing through the review of existing findings, the development of software and hardware solutions and the assessment of the results in order to make suggestions. The ground covered, even though it encompasses distinct research topics, was suitable to verify academic concerns of prior studies and to slightly contribute in the current investigation on autonomous manufacturing robotic cells.

This paper discusses the design parameters, the problems and proposed solutions as well as the possibilities of a robotic setup which is developed for the creation of a triangulated timber structure in continuous digital workflow. The research presented shows the potential of the contemporary designer from being simply a user that interacts with the various advanced technologies to become a developer establishing the rules that govern the entire process from design to production. A parametric design platform is used, not only for the geometric definition of a bridge structure along with its timber connections, but also for the fabrication code of the subtractive and additive robotic actions. A bottom-up design approach lead to the formation of a detailing system whose main characteristics are the facilitation of machining and the unique alignment direction of the distinct members. Both the mechanical behaviour of the wooden joints and the exact positioning of the slender beams are conditional to manufacturing precision. The robotic work cell is carefully configured to enable the rapid processing of individual elements. Although it is still a work in progress, early experiments have shown that the estimated time for the fabrication of a medium size triangle takes approximately 45 minutes. The production of all the components that comprise the 5 metre prototype require 20 hours of manufacturing.

7. outlook

Whereas this paper suggests the feasibility of the conceived procedure, there are various aspects of this case study that need advancement.

\ Full automation implies the application of feedback-based processes in order to manage the un-modelled properties of stock material or assembly parts. Instead of having a linear workflow, where we track data, feed them into the algorithmic definition and manually adjust the robot code, a circular adaptive process would be more adequate. In that sense, the capture of data from environment is embedded at the program’s execution and it directly affects the robot’s future actions by updating material deviations or other key variables. \ The geometric complexity and the increased tolerances used for the fitting of the dovetail joints result in overconstrained assembly movements. While at this point we will probe manual solutions, automating this fabrication challenge would be a key feature for the industrial application of the proposed techniques.

\ Cross section optimization is necessary to reduce material redundancies caused by the doubling of 2’x4’ beams when creating the longitudinal joints. Material efficiency was negotiated in this case study but it can be added as a variable without affecting the overall workflow.

8. bibliography

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Robeller, C., Nabaei, S. S. and Yves, W. (2014). Design and fabrication of robot-manufactured joints for a curved-folded thin-shell structure made from CLT. In: W. McGee and M. Ponce de Leon, eds., Robotic Fabrication in Architecture, Art and Design 2014, Switzerland: Springer, pp. 67-81.

Burry, M. (2003). Between intuition and Process: Parametric Design and Rapid Prototyping, In: B. Kolarevic, ed., Architecture in the Digital Age: Design and Manufacturing. Washington DC: Taylor & Francis, pp. 149–162.

Robeller, C. and Weinand, Y. (2015). Interlocking Folded Plate – Integral Mechanical Attachment for Structural Wood Panels, In: International Journal of Space Structures, Vol. 30, Issue 2, pp. 111-122.

González Böhme, L.F., Quitral Zapata, F. and Maino Ansaldo, S. (2017). Roboticus tignarius: robotic reproduction of traditional timber joints for the reconstruction of the architectural heritage of Valparaíso. In: Construction Robotics, Vol. 1, Issue 1-4, New York: Springer, pp. 61-68. Helm, V. et al. (2016). Additive robotic fabrication of complex timber structures. In: A. Menges, T. Schwinn, O.D. Krieg, eds., Advancing Wood Architecture: A Computational Approach, New York: Routledge, pp. 29-43. Iglesias, I., Sebastian, M.A. and Ares, J.E. (2015). Overview of the state of robotic machining: current situation and future potential. In: Procedia Engineering, Vol. 132, pp. 911–917. Industrial Robot Statistics, (2018). The International Federation of Robotics Official Website. [online] Available at: https://ifr.org/ifr-press-releases/news/ industrial-robot-sales-increase-worldwide-by-29percent/ [Accessed 20 Jun 2018]. Jeffers M. (2013). Autonomous robotic assembly with variable material properties. In: D. Reinhardt, R. Saunders and J. Burry, eds., Robotic fabrication in architecture, art and design 2016, Switzerland:

Robeller, C. and Weinand, Y. (2016). Fabrication-aware design of timber folded plate shells with double through tenon joints. In: D. Reinhardt, R. Saunders and J. Burry, eds., Robotic fabrication in architecture, art and design 2016, Switzerland: Springer, pp. 167-177. Robeller, C., Helm, V., Thoma, A., Gramazio, F., Kohler, M. and Yves, W. (2017). Robotic Integral Attachment. In: B. Sheil, A. Menges, R. Glynn and M. Skavara, eds., FABRICATE 2017, London: UCL Press, pp. 92-97. Søndergaard, A., Amir, O., Eversmann, P., Piskorec, L., Stan, F., Gramazio, F., and Kohler, M. (2016). Topology optimization and robotic fabrication of advanced timber space-frame structures. In: D. Reinhardt, R. Saunders and J. Burry, eds., Robotic Fabrication in Architecture, Art and Design 2016, Switzerland: Springer, pp. 191203. Tamke M. and Thomsen, M. R. (2008). Designing Parametric Timber, In: Architecture ‘in computro’ Integrating Methods and Techniques: 26th eCAADe Conference, Antwerpen, Belgium, pp. 609-616 . Tamke M. and Thomsen, M. R. (2009). Digital Wood Craft. In: Joining Languages, Cultures and Visions: 13th International CAAD Futures Conference, Montreal, Canada, pp. 673–683. Willmann, J., Knauss, M., Bonwetsch, T., Apolinarska, A. A., Gramazio F. and Kohler M. (2016). Robotic timber construction expanding additive fabrication to new dimensions. In: Automation in Construction, Vol. 61, pp. 16 – 23.

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9. List of figures

Chapter 1 cover | Tamke ,M., Riiber, J. (2007).

Parametric Wood. [online]. The Royal Danish Academy of Fine Arts Schools of Architecture, Design and Conservation. Available at: https:// kadk.dk/en/case/parametric-wood [Accessed 20 May 2018]

Figure 13 | author’s own image

Figure 1 | Tamke ,M., Riiber, J. (2007). Parametric

Figure 15 | author’s own image

Wood. [online]. The Royal Danish Academy of Fine Arts Schools of Architecture, Design and Conservation. Available at: https://kadk.dk/ en/case/parametric-wood [Accessed 20 May 2018] Figure 2 | IFR Press releases. (2018). Industrial

Figure 14 | author’s own image Chapter 4 cover | author’s own image

Figure 16 | author’s own image Figure 17 | author’s own image Figure 18 | author’s own image

robot sales increase worldwide by 31 percent. [online]. The International Federation of Robotics. Available at: https://ifr.org/ifr-pressreleases/news/industrial-robot-sales-increaseworldwide-by-29-percent [Accessed 20 Jun 2018]

Figure 19 | author’s own image

Figure 3 | Robeller, C., Weinand, Y. (2015) .

Figure 21 | author’s own image

Interlocking Folded Plate – Integral Mechanical Attachment for Structural Wood Panels, International Journal of Space Structures, Vol. 30, No. 2, p.113.

use robots to build complex timber structures, Image copyright: Courtesy NCCR Digital Fabrication/ Roman Keller, [online]. The Architect’s Newspaper. Available at: https:// archpaper.com/2018/05/swiss-researchersrobotic-complex-timber-construction/ [Accessed 20 May 2018] Chapter 2 cover | author’s own image Figure 5 | author’s own image Figure 6 | author’s own image Figure 7 | author’s own image Figure 8 | author’s own image Figure 9 | author’s own image Figure 10 | author’s own image Figure 11 | author’s own image Chapter 3 cover | author’s own image Figure 12 | author’s own image

Figure 4 | Hilburg, J. (2018) Swiss researchers

20 | CS-200 Calibration Square. [online] . Optitrack Official Webshop. Available at: https://optitrack.com/products/tools/ [Accessed 20 Sep 2018] Figure