ScienceDirect
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect
Procedia CIRP 00 (2017) 000–000
www.elsevier.com/locate/procedia
2212-8271 © 2017 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
28th CIRP Design Conference, May 2018, Nantes, France
A new methodology to analyze the functional and physical architecture of existing products for an assembly oriented product family identification
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
* Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected]
Abstract
In today’s business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach.
© 2017 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
Keywords: Assembly; Design method; Family identification
1. Introduction
Due to the fast development in the domain of communication and an ongoing trend of digitization and digitalization, manufacturing enterprises are facing important challenges in today’s market environments: a continuing tendency towards reduction of product development times and shortened product lifecycles. In addition, there is an increasing demand of customization, being at the same time in a global competition with competitors all over the world. This trend, which is inducing the development from macro to micro markets, results in diminished lot sizes due to augmenting product varieties (high-volume to low-volume production) [1].
To cope with this augmenting variety as well as to be able to identify possible optimization potentials in the existing production system, it is important to have a precise knowledge
of the product range and characteristics manufactured and/or assembled in this system. In this context, the main challenge in modelling and analysis is now not only to cope with single products, a limited product range or existing product families, but also to be able to analyze and to compare products to define new product families. It can be observed that classical existing product families are regrouped in function of clients or features.
However, assembly oriented product families are hardly to find.
On the product family level, products differ mainly in two main characteristics: (i) the number of components and (ii) the type of components (e.g. mechanical, electrical, electronical).
Classical methodologies considering mainly single products or solitary, already existing product families analyze the product structure on a physical level (components level) which causes difficulties regarding an efficient definition and comparison of different product families. Addressing this
Procedia CIRP 93 (2020) 137–142
2212-8271 © 2020 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems 10.1016/j.procir.2020.04.038
© 2020 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems
53rd CIRP Conference on Manufacturing Systems
ScienceDirect
Procedia CIRP 00 (2019) 000–000
www.elsevier.com/locate/procedia
2212-8271 © 2019 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems
53rd CIRP Conference on Manufacturing Systems
Flexible Production Concept of a Low-Cost Battery Pack Housing for Electric Vehicles
Günther Schuh a , Georg Bergweiler a , Falko Fiedler a *, Marcel Koltermann a
a
Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany
* Corresponding author. Tel.: +46 160 917 94 274, E-mail address: [email protected]
Abstract
Fixture and clamping technologies of automated production lines for structural components in vehicles turned out as the main cost driver with up to 29 % of total investment costs of the production equipment. Additionally, they limit the product flexibility for product variants and change requests during the product development process. Flexible production concepts, like jigless joining with component-integrated jig features (CJF) might be a promising approach to reduce component specific jig technology. Technical as well as economical aspects are investigated on a low- cost battery pack housing for electric vehicles and are compared to conventional concepts concerning annual production volumes.
© 2019 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems
Keywords: body shop; fixture; jig technology; jigless, laser welding; component-integrated jig features; battery system; battery production; BEV, PHEV; CJF
1. Introduction
New competitors in the automotive market like E- Mobility Start-Ups, Original Equipment Manufacturer (OEM), and supplier facing new production challenges [1]
in terms of shorter product life cycles with a simultaneously increasing number of product variants [2, 3]. For example, the fleet of the premium manufacturer BMW covered 73 vehicle model variants in 2012 and the volume manufacturer Volkswagen even more than 200 model variants [4]. These production related challenges are intensified by uncertain sales forecasts, which can deviate up to ± 40 % from the final sales [3]. An efficient industrialization process, cost efficiency, and flexible production are essential, especially for companies in triad markets, to remain competitive in the global market [1].
The battery system has a particularly strong influence on the production costs with 35 - 50 % of the entire electric vehicle cost structure [2]. The main cost driver of a battery system are the battery cells as the smallest unit. Many battery cells are connected and mounted in a battery
module. Several battery modules are connected inside a battery pack housing with the battery management and cooling system and form the battery pack [2, 5].
Kampker approximates investment costs of 57 million € for a battery pack assembly line [6], which have almost identical machine and system technology than the automated production lines in conventional automotive body shops. Hence, body shop departments of OEM and automotive suppliers are mostly in charge of battery production since they have experience for decades in the necessary production technologies. These automated production lines are designed for mass production without product flexibility and consist of many rigidly linked robots, jigs, gripper, and joining machines. Most of the investment costs with up to 44 % are caused by robots including joining technologies and secondly by jig and clamping technology with up to 29 % [3, 7].
This research paper analyzes the technical and economic potential of production concepts for a low-cost battery pack housing (LCBPH) to meet the challenges of nowadays development and production processes.
Available online at www.sciencedirect.com
ScienceDirect
Procedia CIRP 00 (2019) 000–000
www.elsevier.com/locate/procedia
2212-8271 © 2019 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems
53rd CIRP Conference on Manufacturing Systems
Flexible Production Concept of a Low-Cost Battery Pack Housing for Electric Vehicles
Günther Schuh a , Georg Bergweiler a , Falko Fiedler a *, Marcel Koltermann a
a
Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University, Campus-Boulevard 30, 52074 Aachen, Germany
* Corresponding author. Tel.: +46 160 917 94 274, E-mail address: [email protected]
Abstract
Fixture and clamping technologies of automated production lines for structural components in vehicles turned out as the main cost driver with up to 29 % of total investment costs of the production equipment. Additionally, they limit the product flexibility for product variants and change requests during the product development process. Flexible production concepts, like jigless joining with component-integrated jig features (CJF) might be a promising approach to reduce component specific jig technology. Technical as well as economical aspects are investigated on a low- cost battery pack housing for electric vehicles and are compared to conventional concepts concerning annual production volumes.
© 2019 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 53rd CIRP Conference on Manufacturing Systems
Keywords: body shop; fixture; jig technology; jigless, laser welding; component-integrated jig features; battery system; battery production; BEV, PHEV; CJF
1. Introduction
New competitors in the automotive market like E- Mobility Start-Ups, Original Equipment Manufacturer (OEM), and supplier facing new production challenges [1]
in terms of shorter product life cycles with a simultaneously increasing number of product variants [2, 3]. For example, the fleet of the premium manufacturer BMW covered 73 vehicle model variants in 2012 and the volume manufacturer Volkswagen even more than 200 model variants [4]. These production related challenges are intensified by uncertain sales forecasts, which can deviate up to ± 40 % from the final sales [3]. An efficient industrialization process, cost efficiency, and flexible production are essential, especially for companies in triad markets, to remain competitive in the global market [1].
The battery system has a particularly strong influence on the production costs with 35 - 50 % of the entire electric vehicle cost structure [2]. The main cost driver of a battery system are the battery cells as the smallest unit. Many battery cells are connected and mounted in a battery
module. Several battery modules are connected inside a battery pack housing with the battery management and cooling system and form the battery pack [2, 5].
Kampker approximates investment costs of 57 million € for a battery pack assembly line [6], which have almost identical machine and system technology than the automated production lines in conventional automotive body shops. Hence, body shop departments of OEM and automotive suppliers are mostly in charge of battery production since they have experience for decades in the necessary production technologies. These automated production lines are designed for mass production without product flexibility and consist of many rigidly linked robots, jigs, gripper, and joining machines. Most of the investment costs with up to 44 % are caused by robots including joining technologies and secondly by jig and clamping technology with up to 29 % [3, 7].
This research paper analyzes the technical and economic
potential of production concepts for a low-cost battery pack
housing (LCBPH) to meet the challenges of nowadays
development and production processes.
2. Concept of the Low-Cost Battery Pack Housing Due to the high energy and chemicals of battery cells, battery systems are subject to various legal, product, and production-related requirements. Kampker et al. [8]
analyzed these requirements for the battery system and assumed for a concept, that current battery pack housings, battery modules, and the car body structure partly redundantly fulfill these requirements. The concept of the LCBPH is therefore just focusing on requirements resulting from the mechanical influences during driving and the assembly into the body-in-white [8].
Agile product development methods, such as daily scrums or burn-down charts for workload visualization, seem to be an excellent way to meet today's work challenges of variant diversity, short life cycles, and flexible production. Companies rate their success higher than the industry average when they implement agile working methods in their daily business [9]. The E- Mobility Start-Up e.GO developed the LCBPH for early prototypes of its electric city car “e.GO Life” by using agile development methods in combination with suitable cost- efficient and flexible manufacturing technologies. The construction was designed for sheet metal cutting, bending, and rivet joining technologies to reduce the high prototype costs and ability to react to component change requests in the ongoing agile development process [10]. The components are flexibly manufactured by laser cutting and bending. The previous enables agile product development, necessary for rapid change requests in the product architecture, and the best technology for a scalable annual production volume of up to 20,000 [11].
The production concept for this early prototype was designed for manual production with a total of 308 blind rivets to reduce expenditure in jigs, automation, and joining technology and is an example for the later explained component-integrated jig feature (CJF). In this case, laser- cut holes for the following riveting process determine the position of the components to each other. The cycle time for manufacturing a LCBPH was set by one worker to 38 minutes without complex production equipment, such as specific geometric jigs [8]. The advantages of this production concept are the low investment costs, the small footprint, low-technologies, and the ability for fast development, which makes it perfect for prototype series.
With respect to the target production volume for serial production, the big disadvantages are high cycle time and less scalability of annual production volume. For example, seven parallel manufacturing lines with one worker each are required for an annual production volume of 20,000, which results in high personnel costs and uneconomic production.
Thereof, a production concept for serial production with respect to the demand of agile product development, flexible production, cost-efficiency, and volume scalability needs to be development.
3. Serial Production Concept Development
Many battery pack housings are produced in automated production lines with specific jig technology and resistance spot welding (RSW) in automotive body shops [10]. In addition to this established conventional production concept, an innovative jigless production concept using remote laser welding (RLW) will be analyzed.
Bergweiler [12] invented an innovative “interlock connection” with a twisting tab for car body structures with the aim to reduce component specific jig technology and to increase product flexibility. This production concept is called component-integrated jig feature (CJF) and is realized by the integration of one or more jig features, such as positioning, orienting, or fixing from a jig into car body components [12]. Figure 1 shows the procedure and functions of the CJF with twisting tab for jigless joining and RLW. The twisting tab is a geometric feature of the CJF and is used to mount (I) two components (a and b) manually together, while being automatically oriented (II) in x, y, and z dimensions with comparable accuracies of pin/hole connections of a conventional jig. Twisting off the tab is part of the third step (III) and fixes the assembly by plastic deformation of the tab and resulting force lock. This force lock fulfills the requirement of the following joining process and prevent dislocation as well as thermal distortion. The last step (IV) shows the RLW process of the force locked components to create the structural strength of the body-in-white [12, 13].
Joining Fixing
IVIII
a b
Remote Laser welding
xyz-Orienting
IIx y
1y
2z
0 5 mmMounting
IFig. 1: Component-integrated jig feature (CJF) with twisting tab.
According to Bergweiler [12]
Kampker et al. investigated and defined the production concept of CJF with the potential of reducing investment costs and increasing product flexibility [14]. The main part of past research activities on CJF was the development of a methodology to describe the requirements of the components to be joined, choosing the best geometry for the CJF, and rating of the potential [7, 15]. Several studies have been made to analyze the technical proof of concept. A rear- frame with aluminum tubes of an electric assisted cart was designed with CJF to demonstrate jigless joining with the conventional TIG-Welding process [16]. The concept of the LCBPH was presented, compared to conventional battery pack housings, and the first experiments have been made to determine the cycle time by using CJF + RLW [8]. In addition to these investigations, experiments for RLW process development for both steel [13] and aluminum [11, 17] have been performed to ensure requirements on mechanical strength and process cycle time. Current results on the application of CJF on the LCBPH shows, on the one hand, the technical proof of concept and, on the other hand, the demand for investigating the economic potential in comparison to state of the art.
3.1. Design with CJF for Serial Production
The prototype version of the LCBPH was designed for rivet joining technology with overlapping flanges and has been re-designed for the use of CJF with twisting tab (cf.
fig.1). The use of CJF made it possible to eliminate overlapping flanges with material doublings, which were required for riveting. Figure 2 shows the CAD-Model of the LCBPH with CJF, the three colored component variants, and a detailed view on the application of the CJF itself.
Component-integrated jig feature (CJF) A
B C
0 500 mm Components
(A) Floor pan (B) Cross beam (C) Longitudinal beam
Fig. 2: Product concept of the low-cost-battery pack housing [8]
The two floor pans form the basic structure of the LCBPH and have 10 slots for inserting the cross beams by two semicircular twisting tabs in the assembly process.
Thus, eight compartments are created between each cross beam, where later, the battery modules are mounted and fixed by longitudinal beams.
In comparison to the rivet design, the total weight of the newly developed LCBPH with CJF was reduced from 12.4 to 8.2 kg, which is 34 % in mass reduction. The result was mainly achieved by eliminating overlapping flanges and reducing from 26 to 15 components by 42 % [8].
3.2. Development of Production Concepts and Layouts The production concepts and block layouts of CJF + RLW and RSW are presented in the following two subchapters. The product architecture of the concept with RSW has not changed in comparison to the rivet design, since both joining technologies need overlapping flanges, and thus, all 308 rivet joints are replaced by weld spots.
Some assumptions have been taken for development and are described for both concepts. The block layouts are designed in such a way. that the required area is kept as small as possible. The width of the routes for worker is at least 1 m and for logistic at least 2,5 m. Furthermore, the working area is without dead ends for worker to ensure two exits on every point and accessibility from two sides for forklifts. The working area of the robots are limited by safety fences and can only be entered through locked doors.
The robots selected are industrial standard with a footprint of 1,006 x 1,006 mm² and a max. range of 2,700 mm, so that the maximum possible working area has a diameter of 5,400 mm. The external dimensions of the universal load carriers are 835 x 1,240 mm². The two-carrier principle ensures continuous production without interruptions due to load carrier exchange, although the load carrier footprint is two times higher.
3.3. Production Concept with CJF + RLW
This production concept is semi-automated and combines the advantages of a versatile worker for the manual jigless assembly of the LCBPH and automated RLW for a flexible and fast welding process without geometric jigs. Figure 3 shows the block layout of the CJF + RLW production concept and is divided into assembly cell (cf. right side of fig.3) and the remote laser welding cell (cf. left side of fig.3).
Safety fence Industrial robot
Material flow Laser cell Laser source Working area Universal carrier Special carriers Floor
Support jig Balancer
LB
BPH LC
C B1 CB2 FP 1 FP 2
0 1 m
LCBPH = low-cost battery pack housing CB = cross beam
FP = floor pan LB = laser beam robot
Fig. 3: Block layout of the CJF + RLW production concept The respective process steps are described in this paragraph with respect to the block layout and the cycle time in figure 4. The assembly and welding process run parallel at the same time and are separated in two cells by safety fences. The worker starts the production process by rotating the turntable of the LCBPH, which connects the laser and the assembly cell. After removing the welded LCBPH with a balancer into one of the two special load carrier, the worker manually assembles the LCBPH on a support jig. Two floor pans (cf. fig. 2) are manually transported from the universal load carrier and are mounted on the support jig. Then, one cross beam after the other is transported from the universal load carrier and mounted on the floor pans. After each mounting, the twisting tabs of the CJF of the cross beams are twisted off to fix the components for the following joining process. At the same time, only one robot is required inside the laser welding cell for welding the LCBPH. The robot takes about 3 s to move from its home position to the first joint and vice versa to return back. All 20 weld seams of the LCBPH are welded in a meandering pattern with a total process time of 132 s. After mounting all cross beams on the LCBPH, the worker transports and put the LCBPH on the turntable.
1.2 Removing BPH 1.3 Assembling BPH 1.4 Positioning BPH 2.1 Rotating table 2.2 Starting from home 2.4 Returning to home 1.1 Starting & table rotation
2.3 Welding seams (20 x)
40 80 120 160 200
0 Time [s]
100 s
132 s 3 s
3 s
170 s cycle time
Laser welding cell 15 s
25 s
25 s Assembly cell 20 s
Fig. 4: Process steps and cycle time for CJF + RLW
2. Concept of the Low-Cost Battery Pack Housing Due to the high energy and chemicals of battery cells, battery systems are subject to various legal, product, and production-related requirements. Kampker et al. [8]
analyzed these requirements for the battery system and assumed for a concept, that current battery pack housings, battery modules, and the car body structure partly redundantly fulfill these requirements. The concept of the LCBPH is therefore just focusing on requirements resulting from the mechanical influences during driving and the assembly into the body-in-white [8].
Agile product development methods, such as daily scrums or burn-down charts for workload visualization, seem to be an excellent way to meet today's work challenges of variant diversity, short life cycles, and flexible production. Companies rate their success higher than the industry average when they implement agile working methods in their daily business [9]. The E- Mobility Start-Up e.GO developed the LCBPH for early prototypes of its electric city car “e.GO Life” by using agile development methods in combination with suitable cost- efficient and flexible manufacturing technologies. The construction was designed for sheet metal cutting, bending, and rivet joining technologies to reduce the high prototype costs and ability to react to component change requests in the ongoing agile development process [10]. The components are flexibly manufactured by laser cutting and bending. The previous enables agile product development, necessary for rapid change requests in the product architecture, and the best technology for a scalable annual production volume of up to 20,000 [11].
The production concept for this early prototype was designed for manual production with a total of 308 blind rivets to reduce expenditure in jigs, automation, and joining technology and is an example for the later explained component-integrated jig feature (CJF). In this case, laser- cut holes for the following riveting process determine the position of the components to each other. The cycle time for manufacturing a LCBPH was set by one worker to 38 minutes without complex production equipment, such as specific geometric jigs [8]. The advantages of this production concept are the low investment costs, the small footprint, low-technologies, and the ability for fast development, which makes it perfect for prototype series.
With respect to the target production volume for serial production, the big disadvantages are high cycle time and less scalability of annual production volume. For example, seven parallel manufacturing lines with one worker each are required for an annual production volume of 20,000, which results in high personnel costs and uneconomic production.
Thereof, a production concept for serial production with respect to the demand of agile product development, flexible production, cost-efficiency, and volume scalability needs to be development.
3. Serial Production Concept Development
Many battery pack housings are produced in automated production lines with specific jig technology and resistance spot welding (RSW) in automotive body shops [10]. In addition to this established conventional production concept, an innovative jigless production concept using remote laser welding (RLW) will be analyzed.
Bergweiler [12] invented an innovative “interlock connection” with a twisting tab for car body structures with the aim to reduce component specific jig technology and to increase product flexibility. This production concept is called component-integrated jig feature (CJF) and is realized by the integration of one or more jig features, such as positioning, orienting, or fixing from a jig into car body components [12]. Figure 1 shows the procedure and functions of the CJF with twisting tab for jigless joining and RLW. The twisting tab is a geometric feature of the CJF and is used to mount (I) two components (a and b) manually together, while being automatically oriented (II) in x, y, and z dimensions with comparable accuracies of pin/hole connections of a conventional jig. Twisting off the tab is part of the third step (III) and fixes the assembly by plastic deformation of the tab and resulting force lock. This force lock fulfills the requirement of the following joining process and prevent dislocation as well as thermal distortion. The last step (IV) shows the RLW process of the force locked components to create the structural strength of the body-in-white [12, 13].
Joining Fixing
IVIII
a b
Remote Laser welding
xyz-Orienting
IIx y
1y
2z
0 5 mmMounting
IFig. 1: Component-integrated jig feature (CJF) with twisting tab.
According to Bergweiler [12]
Kampker et al. investigated and defined the production concept of CJF with the potential of reducing investment costs and increasing product flexibility [14]. The main part of past research activities on CJF was the development of a methodology to describe the requirements of the components to be joined, choosing the best geometry for the CJF, and rating of the potential [7, 15]. Several studies have been made to analyze the technical proof of concept. A rear- frame with aluminum tubes of an electric assisted cart was designed with CJF to demonstrate jigless joining with the conventional TIG-Welding process [16]. The concept of the LCBPH was presented, compared to conventional battery pack housings, and the first experiments have been made to determine the cycle time by using CJF + RLW [8]. In addition to these investigations, experiments for RLW process development for both steel [13] and aluminum [11, 17] have been performed to ensure requirements on mechanical strength and process cycle time. Current results on the application of CJF on the LCBPH shows, on the one hand, the technical proof of concept and, on the other hand, the demand for investigating the economic potential in comparison to state of the art.
3.1. Design with CJF for Serial Production
The prototype version of the LCBPH was designed for rivet joining technology with overlapping flanges and has been re-designed for the use of CJF with twisting tab (cf.
fig.1). The use of CJF made it possible to eliminate overlapping flanges with material doublings, which were required for riveting. Figure 2 shows the CAD-Model of the LCBPH with CJF, the three colored component variants, and a detailed view on the application of the CJF itself.
Component-integrated jig feature (CJF) A
B C
0 500 mm Components
(A) Floor pan (B) Cross beam (C) Longitudinal beam
Fig. 2: Product concept of the low-cost-battery pack housing [8]
The two floor pans form the basic structure of the LCBPH and have 10 slots for inserting the cross beams by two semicircular twisting tabs in the assembly process.
Thus, eight compartments are created between each cross beam, where later, the battery modules are mounted and fixed by longitudinal beams.
In comparison to the rivet design, the total weight of the newly developed LCBPH with CJF was reduced from 12.4 to 8.2 kg, which is 34 % in mass reduction. The result was mainly achieved by eliminating overlapping flanges and reducing from 26 to 15 components by 42 % [8].
3.2. Development of Production Concepts and Layouts The production concepts and block layouts of CJF + RLW and RSW are presented in the following two subchapters. The product architecture of the concept with RSW has not changed in comparison to the rivet design, since both joining technologies need overlapping flanges, and thus, all 308 rivet joints are replaced by weld spots.
Some assumptions have been taken for development and are described for both concepts. The block layouts are designed in such a way. that the required area is kept as small as possible. The width of the routes for worker is at least 1 m and for logistic at least 2,5 m. Furthermore, the working area is without dead ends for worker to ensure two exits on every point and accessibility from two sides for forklifts. The working area of the robots are limited by safety fences and can only be entered through locked doors.
The robots selected are industrial standard with a footprint of 1,006 x 1,006 mm² and a max. range of 2,700 mm, so that the maximum possible working area has a diameter of 5,400 mm. The external dimensions of the universal load carriers are 835 x 1,240 mm². The two-carrier principle ensures continuous production without interruptions due to load carrier exchange, although the load carrier footprint is two times higher.
3.3. Production Concept with CJF + RLW
This production concept is semi-automated and combines the advantages of a versatile worker for the manual jigless assembly of the LCBPH and automated RLW for a flexible and fast welding process without geometric jigs. Figure 3 shows the block layout of the CJF + RLW production concept and is divided into assembly cell (cf. right side of fig.3) and the remote laser welding cell (cf. left side of fig.3).
Safety fence Industrial robot
Material flow Laser cell Laser source Working area Universal carrier Special carriers Floor
Support jig Balancer
LB
BPH LC
C B1 CB2 FP 1 FP 2
0 1 m
LCBPH = low-cost battery pack housing CB = cross beam
FP = floor pan LB = laser beam robot
Fig. 3: Block layout of the CJF + RLW production concept The respective process steps are described in this paragraph with respect to the block layout and the cycle time in figure 4. The assembly and welding process run parallel at the same time and are separated in two cells by safety fences. The worker starts the production process by rotating the turntable of the LCBPH, which connects the laser and the assembly cell. After removing the welded LCBPH with a balancer into one of the two special load carrier, the worker manually assembles the LCBPH on a support jig. Two floor pans (cf. fig. 2) are manually transported from the universal load carrier and are mounted on the support jig. Then, one cross beam after the other is transported from the universal load carrier and mounted on the floor pans. After each mounting, the twisting tabs of the CJF of the cross beams are twisted off to fix the components for the following joining process. At the same time, only one robot is required inside the laser welding cell for welding the LCBPH. The robot takes about 3 s to move from its home position to the first joint and vice versa to return back. All 20 weld seams of the LCBPH are welded in a meandering pattern with a total process time of 132 s.
After mounting all cross beams on the LCBPH, the worker transports and put the LCBPH on the turntable.
1.2 Removing BPH 1.3 Assembling BPH 1.4 Positioning BPH 2.1 Rotating table 2.2 Starting from home 2.4 Returning to home 1.1 Starting & table rotation
2.3 Welding seams (20 x)
40 80 120 160 200
0 Time [s]
100 s
132 s 3 s
3 s
170 s cycle time
Laser welding cell 15 s
25 s
25 s Assembly cell 20 s
Fig. 4: Process steps and cycle time for CJF + RLW
Thereof, the production concept with CJF + RLW has a cycle time for manufacturing the LCBPH of 170 s and the proposed block layout has a footprint of 48,79 m².
However, one of the advantageous of this production concept is the fact, that no component and variant specific production equipment, such as geometric jigs or welding guns are required.
This was mainly achieved by the application of CJF on the LCBPH and is described in detail below. Two CJF with twisting tab orients and fixes each cross beam on both sides to the floor pans of the LCBPH for the subsequent joining process. One CJF with twisting tab eliminates at one point all degrees of freedom (DOF) in translation. In particular, the shallow gap of 0,1 mm around the slot of the floor pan and the tab of the cross beam blocks the x and y-direction (cf. step 2 in fig.1) in the assembled step. Twisting off the twisting tab of the cross beam blocks the z-direction (cf.
step 1 & 3 in fig.1) by plastic deformation and resulting force locking. The second CJF with twisting tab on the other side blocks the z-direction to ensure planar contact of the entire component and a much longer slot eliminates just the DOF in y-direction. Two additional tab slot connections are used without the jig features orienting and fixing, but as optimal joining geometry for remote laser welding. Hence, only the mounted LCBPH need to be placed and oriented on the universal working table of the laser welding cell.
In addition to the jigless concept, RLW has been primarily used as a flexible joining technology and secondly for the potential of economic production for larger series.
Joining with RLW is mainly realized by heat conduction of the absorbing laser beam without direct contact of the production equipment to the workpiece. The only changes for adapting the RLW process to other products or variants is programming the laser parameter and welding geometry.
Furthermore, high process speeds of up to several m/s at high-energy spots can realize short cycle times and the potential of an economic production.
3.4. Concept with Automated Production and RSW In contrast to the presented production concept with CJF + RLW (cf. 3.3), an established conventional production concept with RSW has been developed for the LCBPH as a benchmark. This concept runs completely automated with robots and welding guns. Figure 5 shows the developed block layout and indicates the automated robot cell in the upper part (surrounded by a safety fence).
LTB2 LTB1
FP1 FP2 CB1 CB2
6,47 m BPH LC
LTB CB FP
GR
RSW
7, 26 m
Magazines
Industrial robot Material flow Working area
Universal carrier Special carriers Floor
Geometry jig
0 1 m
CB = cross beam FP = floor pan GR = gripper
RSW = resistance spot welding LCBPH = low-cost battery pack
housing Safety fence
Fig. 5: Block layout of the concept with automated production and RSW
The block layout and the cycle time in figure 6 describes the production process with RSW. The process starts with robot-guided pick and place of cross beams from magazines to the geometric jig with a gripper, which is assumed to take 15 s. A worker loads the respective workpiece magazines at the robot cell by picking the floor pans, the cross beams, and the longitudinal beams manually from the universal load carriers. The opening and closing of the jig, as well as the approach and departure of the welding robot takes about 2 s. The process time for setting a welding spot with the spot-welding gun is assumed to be 2.5 s. After the cross beams are welded, the flor pans are inserted to be welded with the cross beams. The gripper robot has another gripper on the backside for placing the manufactured LCBPH in one of the two special load carriers. Two geometric jigs and two parallel running robots are required to manufacture the cross beams as well as LCBPH.
200 400 600 800 1000
0 2.1 Closing jig
8.0 Closing jig 6.0 Opening jig
12.0 Opening jig 3.0 Starting RSW robot
9.0 Starting RSW robot 11.0 Ending RSW robot 5.0 Ending RSW robot
2.2 Removing BPH 10.0 Welding spots (212 x) 4.1 Welding spots (96 x) 4.2 Inserting 2 FP & 2 CB
7.0 Inserting 3 CB & 8 LTB 1.0 Inserting CB in jig (6 x)
2 s 2 s 2 s 2 s 90 s
2 s 60 s
530 s 2 s 2 s 2 s
20 s
Cycle 1041 s time 240 s
165 s RSW-cell
Time [s]
Fig. 6: Process steps and cycle time of the production concept with RSW The total cycle time for the automated production concept with RSW of a LCBPH is 1041 s and the total area of the block layout is 46.97 m².
4. Analysis and Discussion
Both production concepts of the LCBPH have been developed for annual production volume of 20,000.
Analysis of the production concepts with regard to technical and economic aspects have been made to determine the potential for a cost-efficient and flexible production.
4.1. Technical Comparison
The technical comparison considers both product and production by focusing on the implication on the design, the joining technology, and the technical equipment of the production cells. The main differences of the two production concepts are the three joining technologies riveting, RSW, and CJF + RLW, which can be compared at the joint spot level. Figure 7 shows the cross-section and the top view of the three joining technologies, which has been used for prototype production as well as the serial production concept.
Rivet CJF + RLW RSW
Cr oss se ct io n Top vi ew
rivet welding lens
0 10 mm welding seam
Fig. 7: Differences in joints of the production concepts The design of the production concept with rivets as well as RSW needs overlapping flanges for the joining process.
The two columns on the left side in figure 7 show that the overlapping flange needs twice as much material as the CJF + RLW joint, due to material doubling. Moreover, reducing material doublings results in an increase of useable volume, for example, for battery modules and to a reduction of total mass. Another important criterion is the accessibility of the joint itself. RSW requires access from both sides for setting the welding spots, which affects the installation space of the LCBPH and restricts the production process additionally.
To achieve the same strength as utilizing rivets or RSW, CJF + RLW has the advantage of requiring significantly fewer joining points, due to the more significant and two sided weld seam (cf. middle column in fig. 7). The CJF + RLW joint results in a reduction of joining process time and increase of strength per joining spot.
Moreover, the cycle time of RSW can be reduced by 54.34 % to 1,041 s in comparison to the production time of the rivet version (2,280 s). The CJF + RLW cycle time achieved a reduction by 92.54 % to 170 s and in comparison to RSW by 83,67 %. The footprint of the CJF + RLW concept is slightly larger with 1.8 m ² (3.87 %).
4.2. Economical Comparison
To determine the unit costs concerning the annual production volume, the machine costs of one year were calculated and divided by the annual production volume.
The machine costs are calculated in formula 1 by the sum of amortization costs C , interest costs C , maintenance costs C , area costs C , energy costs C , and labor costs C [18]. Finally, the unit cost of a LCBPH is calculated by dividing the annual machine cost by the number of units.
, = = (1)
Some assumptions were made for the calculation of the individual cost types. The useful life of the assets is assumed to be five years, the interest rate is assumed to be 2 %, and the maintenance rate for machine tools can be assumed to be 3 % [19]. The personnel costs for the employer are 62,200.00 € per year, which corresponds to the qualification of a journeyman [20]. The required space is taken from the developed block layouts. The investment costs, as well as the room costs, are taken from quotes.
Figure 8 shows the unit costs as a function of annual production volume, the break-even point of 6,916 for the CJF + RLW concept, and each cost saving potential.
0 20 40 60 80 100 120 140
0 20 40 60 80 100 120 140 160 180
U ni t c os t [€ ]
Annual production volume [in k] 10
30 50 70
5 6 7 8 9 10 11 12 13
Break-Even-Point at 6916 units per year
Rivets CJF + RLW RSW
Fig. 8: Unit costs of the production concepts for different annual production volume with parallel production
The curve of the RSW concept runs almost identical to the CJF + RLW until the break-even point, whereby the rivet concept has the lowest unit costs until the break-even point. For larger production volume above the break-even point, the unit costs of the CJF + RLW concept are lower than for the other two production concepts. The hatched area between the curves highlights the economic potential of the CJF + RLW concepts over the annual production volume. The curve progression from an annual production volume of 50,000 to 200,000 shows that the unit costs for all production processes approach the minimum value.
After each production concept exceeds its capacity utilization of 100 %, the max. production volume is reached for a three shift production. In this study, parallelization of the proposed production cells has been used for the scalability from prototype series to serial production without initial planning of large scales from beginning and interrupting for adaption of current production. Thus, after each exceeding, a new parallel production cell is added. Up to 40,000 LCBPHs can be yearly produced with one CJF + RLW cell by being cost-efficient starting from 6,916, which facilitates the flexibility for annual production volume.
Figure 9 shows the unit cost, cost structure, and the capacity utilization of the three production concepts for an annual production volume of 20,000. The unit cost for a LCBPH manufactured by manual riveting is the highest with 33.60 € and requires seven parallel cells with an overall capacity utilization of 90.48 %. The high share of labor costs is significant and indicates that the production process is not well suited for high production volumes.
0,42 % 52,01 % 59,60 %
0,02 %
2,60 % 2,98 %
3,99 %
3,48 % 5,80 %
0,06 %
7,80 % 8,94 %
84,23 %
33,17 %
19,13 % 11,28 %
0,94 % 3,55 %
33,60
12,19
21,14
0 20 40 60 80 100
0 5 10 15 20 25 30 35 40
Niet Laser WPS
C ap ac ity u til iz at ion [% ]
U ni t c ost [€]
Cost structure [%]
Rivet RSW
Amortization
Area Interest
Maintenance Labor Energy/Rivets
CJF + RLW Production concepts
- 63,70 % - 37,00 %
Fig. 9: Cost structure of the production concepts at annual production
volume of 20,000
Thereof, the production concept with CJF + RLW has a cycle time for manufacturing the LCBPH of 170 s and the proposed block layout has a footprint of 48,79 m².
However, one of the advantageous of this production concept is the fact, that no component and variant specific production equipment, such as geometric jigs or welding guns are required.
This was mainly achieved by the application of CJF on the LCBPH and is described in detail below. Two CJF with twisting tab orients and fixes each cross beam on both sides to the floor pans of the LCBPH for the subsequent joining process. One CJF with twisting tab eliminates at one point all degrees of freedom (DOF) in translation. In particular, the shallow gap of 0,1 mm around the slot of the floor pan and the tab of the cross beam blocks the x and y-direction (cf. step 2 in fig.1) in the assembled step. Twisting off the twisting tab of the cross beam blocks the z-direction (cf.
step 1 & 3 in fig.1) by plastic deformation and resulting force locking. The second CJF with twisting tab on the other side blocks the z-direction to ensure planar contact of the entire component and a much longer slot eliminates just the DOF in y-direction. Two additional tab slot connections are used without the jig features orienting and fixing, but as optimal joining geometry for remote laser welding. Hence, only the mounted LCBPH need to be placed and oriented on the universal working table of the laser welding cell.
In addition to the jigless concept, RLW has been primarily used as a flexible joining technology and secondly for the potential of economic production for larger series.
Joining with RLW is mainly realized by heat conduction of the absorbing laser beam without direct contact of the production equipment to the workpiece. The only changes for adapting the RLW process to other products or variants is programming the laser parameter and welding geometry.
Furthermore, high process speeds of up to several m/s at high-energy spots can realize short cycle times and the potential of an economic production.
3.4. Concept with Automated Production and RSW In contrast to the presented production concept with CJF + RLW (cf. 3.3), an established conventional production concept with RSW has been developed for the LCBPH as a benchmark. This concept runs completely automated with robots and welding guns. Figure 5 shows the developed block layout and indicates the automated robot cell in the upper part (surrounded by a safety fence).
LTB2 LTB1
FP1 FP2 CB1 CB2
6,47 m BPH LC
LTB CB FP
GR
RSW
7, 26 m
Magazines
Industrial robot Material flow Working area
Universal carrier Special carriers Floor
Geometry jig
0 1 m
CB = cross beam FP = floor pan GR = gripper
RSW = resistance spot welding LCBPH = low-cost battery pack
housing Safety fence
Fig. 5: Block layout of the concept with automated production and RSW
The block layout and the cycle time in figure 6 describes the production process with RSW. The process starts with robot-guided pick and place of cross beams from magazines to the geometric jig with a gripper, which is assumed to take 15 s. A worker loads the respective workpiece magazines at the robot cell by picking the floor pans, the cross beams, and the longitudinal beams manually from the universal load carriers. The opening and closing of the jig, as well as the approach and departure of the welding robot takes about 2 s. The process time for setting a welding spot with the spot-welding gun is assumed to be 2.5 s. After the cross beams are welded, the flor pans are inserted to be welded with the cross beams. The gripper robot has another gripper on the backside for placing the manufactured LCBPH in one of the two special load carriers. Two geometric jigs and two parallel running robots are required to manufacture the cross beams as well as LCBPH.
200 400 600 800 1000
0 2.1 Closing jig
8.0 Closing jig 6.0 Opening jig
12.0 Opening jig 3.0 Starting RSW robot
9.0 Starting RSW robot 11.0 Ending RSW robot 5.0 Ending RSW robot
2.2 Removing BPH 10.0 Welding spots (212 x) 4.1 Welding spots (96 x) 4.2 Inserting 2 FP & 2 CB
7.0 Inserting 3 CB & 8 LTB 1.0 Inserting CB in jig (6 x)
2 s 2 s 2 s 2 s 90 s
2 s 60 s
530 s 2 s 2 s 2 s
20 s
Cycle 1041 s time 240 s
165 s RSW-cell
Time [s]
Fig. 6: Process steps and cycle time of the production concept with RSW The total cycle time for the automated production concept with RSW of a LCBPH is 1041 s and the total area of the block layout is 46.97 m².
4. Analysis and Discussion
Both production concepts of the LCBPH have been developed for annual production volume of 20,000.
Analysis of the production concepts with regard to technical and economic aspects have been made to determine the potential for a cost-efficient and flexible production.
4.1. Technical Comparison
The technical comparison considers both product and production by focusing on the implication on the design, the joining technology, and the technical equipment of the production cells. The main differences of the two production concepts are the three joining technologies riveting, RSW, and CJF + RLW, which can be compared at the joint spot level. Figure 7 shows the cross-section and the top view of the three joining technologies, which has been used for prototype production as well as the serial production concept.
Rivet CJF + RLW RSW
Cr oss se ct io n Top vi ew
rivet welding lens
0 10 mm welding seam
Fig. 7: Differences in joints of the production concepts The design of the production concept with rivets as well as RSW needs overlapping flanges for the joining process.
The two columns on the left side in figure 7 show that the overlapping flange needs twice as much material as the CJF + RLW joint, due to material doubling. Moreover, reducing material doublings results in an increase of useable volume, for example, for battery modules and to a reduction of total mass. Another important criterion is the accessibility of the joint itself. RSW requires access from both sides for setting the welding spots, which affects the installation space of the LCBPH and restricts the production process additionally.
To achieve the same strength as utilizing rivets or RSW, CJF + RLW has the advantage of requiring significantly fewer joining points, due to the more significant and two sided weld seam (cf. middle column in fig. 7). The CJF + RLW joint results in a reduction of joining process time and increase of strength per joining spot.
Moreover, the cycle time of RSW can be reduced by 54.34 % to 1,041 s in comparison to the production time of the rivet version (2,280 s). The CJF + RLW cycle time achieved a reduction by 92.54 % to 170 s and in comparison to RSW by 83,67 %. The footprint of the CJF + RLW concept is slightly larger with 1.8 m ² (3.87 %).
4.2. Economical Comparison
To determine the unit costs concerning the annual production volume, the machine costs of one year were calculated and divided by the annual production volume.
The machine costs are calculated in formula 1 by the sum of amortization costs C , interest costs C , maintenance costs C , area costs C , energy costs C , and labor costs C [18]. Finally, the unit cost of a LCBPH is calculated by dividing the annual machine cost by the number of units.
, = = (1)
Some assumptions were made for the calculation of the individual cost types. The useful life of the assets is assumed to be five years, the interest rate is assumed to be 2 %, and the maintenance rate for machine tools can be assumed to be 3 % [19]. The personnel costs for the employer are 62,200.00 € per year, which corresponds to the qualification of a journeyman [20]. The required space is taken from the developed block layouts. The investment costs, as well as the room costs, are taken from quotes.
Figure 8 shows the unit costs as a function of annual production volume, the break-even point of 6,916 for the CJF + RLW concept, and each cost saving potential.
0 20 40 60 80 100 120 140
0 20 40 60 80 100 120 140 160 180
U ni t c os t [€ ]
Annual production volume [in k]
10 30 50 70
5 6 7 8 9 10 11 12 13
Break-Even-Point at 6916 units per year
Rivets CJF + RLW RSW
Fig. 8: Unit costs of the production concepts for different annual production volume with parallel production
The curve of the RSW concept runs almost identical to the CJF + RLW until the break-even point, whereby the rivet concept has the lowest unit costs until the break-even point. For larger production volume above the break-even point, the unit costs of the CJF + RLW concept are lower than for the other two production concepts. The hatched area between the curves highlights the economic potential of the CJF + RLW concepts over the annual production volume. The curve progression from an annual production volume of 50,000 to 200,000 shows that the unit costs for all production processes approach the minimum value.
After each production concept exceeds its capacity utilization of 100 %, the max. production volume is reached for a three shift production. In this study, parallelization of the proposed production cells has been used for the scalability from prototype series to serial production without initial planning of large scales from beginning and interrupting for adaption of current production. Thus, after each exceeding, a new parallel production cell is added. Up to 40,000 LCBPHs can be yearly produced with one CJF + RLW cell by being cost-efficient starting from 6,916, which facilitates the flexibility for annual production volume.
Figure 9 shows the unit cost, cost structure, and the capacity utilization of the three production concepts for an annual production volume of 20,000. The unit cost for a LCBPH manufactured by manual riveting is the highest with 33.60 € and requires seven parallel cells with an overall capacity utilization of 90.48 %. The high share of labor costs is significant and indicates that the production process is not well suited for high production volumes.
0,42 % 52,01 % 59,60 %
0,02 %
2,60 % 2,98 %
3,99 %
3,48 % 5,80 %
0,06 %
7,80 % 8,94 %
84,23 %
33,17 %
19,13 % 11,28 %
0,94 % 3,55 %
