Master Thesis
HALMSTAD
UNIVERSITY
Master's in Mechanical Engineering, 60 credits
Design and development of a power seat structure for a sports car
Mechanical Engineering, 15 credits
Halmstad 2021-05-28
Hampus Olsson; Matías Fernández Cranz
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Preface
This report has been performed as the final project of the master’s program (60 credits) in Mechanical Engineering at Halmstad University in collaboration with Koenigsegg
Automotive AB.
The project has been carried out by Hampus Olsson and Matías Fernández Cranz during the spring of 2021 focusing on developing a power seat structure.
We would like to thank out industrial partner, Koenigsegg Automotive AB, as well as our industrial supervisor, Przemyslaw Lamasz, for the opportunity of participating in the development of their vehicles and the support throughout the project.
At last, we would like to thank our university supervisor, Håkan Peterson for the support he has provided throughout the project.
______________________________ ______________________________
Hampus Olsson Matías Fernández Cranz
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Abstract
The purpose of this thesis is to develop a power seat structure that can serve as an alternative to the one developed by Koenigsegg for their upcoming model Gemera and potentially others.
The power seat structure will be designed around an existing chassis and will use it as a reference to create an accommodating power seat structure. It will be designed to be used with a fixed-back seat that is used in the development process of the Gemera.
Koenigsegg requires that the power seat structure allows for horizontal and lifting motion, as well as tilting. These functions will be adapted to the power seat structure designed by the project group.
Due to confidentiality concerns, some parts of this thesis will not be made available to the public.
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1. Table of Contents
Introduction ... 1
1.1. Background ... 1
1.2. Presentation of the client ... 1
1.3. Aim of the study... 2
1.3.1. Problem definition ... 2
1.4. Limitations ... 2
1.5. Individual responsibility, efforts, and study environment ... 3
1.6. Data provided by the industrial partner ... 3
Method ... 4
2.1. Chosen methodology ... 4
2.2. Preparation and data collection ... 5
2.2.1. Data collection ... 5
2.2.2. Literature search... 5
2.2.3. Patent search ... 5
Theory ... 6
3.1. Product development processes ... 6
3.2. Concept selection: Pugh's matrix ... 7
3.3. Production cost analysis ... 7
3.4. Collisions: loads and testing ... 8
3.5. Lightweight materials ... 9
3.5.1. Carbon fiber reinforced polymer (CFRP) ... 9
3.5.2. Aluminium alloys... 9
3.5.3. Advanced high strength steel ... 10
3.6. Manufacturing processes for low-volume production ... 10
3.7. Computer-based structural analysis ... 11
3.8. Technical solutions ... 11
3.8.1. Lead and ball screws ... 11
3.8.2. Rack-and-pinion gear sets ... 12
3.8.3. Flexible shafts ... 12
3.9. DC motors: types and selection ... 13
3.9.1. Types of DC motors ... 13
3.9.2. DC motor selection ... 14
Results ... 15
4.1. Project planning ... 15
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4.2. Customer needs and target specifications ... 15
4.3. Concept generation and selection ... 17
4.3.1. Translation in X-direction ... 17
4.3.2. Translation in Z direction and rotation around Y-axis (tilt) ... 18
4.4. Detail design ... 18
4.4.1. Product description ... 19
4.4.2. Components dimensioning... 21
4.4.3. CAD modelling ... 22
4.4.4. Structural analysis ... 22
4.4.5. Material and process selection ... 22
4.4.6. Life cycle analysis... 24
4.4.7. Cost analysis ... 24
4.4.8. Bill of Materials (BOM) ... 24
Conclusions ... 25
5.1. Recommendations to future activities ... 25
Critical review ... 26
6.1. Efforts ... 26
6.2. Ethical ... 26
6.3. Economics ... 26
6.4. Social... 26
6.5. Environmental ... 27
6.6. Sustainability... 27
References ... 28
Appendix ... 30
1
Introduction
This chapter sets out to provide the reader with a brief explanation of the circumstances under which this paper is written. In this chapter information about the client, aim, problem definition and limitations are included.
1.1. Background
The seating position is critical to the driver's experience of a vehicle. This is emphasized even more in sports cars, such as those produced by Koenigsegg, where drivers require a greater sense of feedback from the vehicle to perform certain manoeuvres. Having a good seating position has proven to significantly affect driver's performance, which is critical when manoeuvring a high-performance sports car.
Koenigsegg has previously used seats with an adjustable backrest but has recently switched to a seat configuration with a fixed backrest. When using a seat configuration with an adjustable backrest, the backrest angle can be adjusted independently. In a fixed-back seat configuration, the angle must be adjusted by changing the angle of the whole seat. According to Koenigsegg, this solution is not readily available in a form that suits the company's requirements.
1.2. Presentation of the client
Koenigsegg Automotive is a Swedish company that focuses on producing high performance sports cars. The cars produced by the company are genuinely variational, reaching from full race cars to true GT's.
The company was founded back in 1994 by Christian von Koenigsegg and was based in Olofström. The prototype, Koenigsegg CC, was launched two years later, in 1996. In 1998 the company relocated to Margretetorp, just outside Ängelholm. This was where the company managed to produce their first production cars, the CC8S. Sadly, in 2003 the workshop burns to the ground, and the company is forced to relocate to their current location at the former air force base, F10, also just outside Ängelholm. The following year the CCR is launched, which turns out to be the most extensive series jet with 14 examples produced. With the CCR, Koenigsegg claims the record as the world's fastest production car with a top speed of 387 km/h.
2006 saw the introduction of the CCX. This model was produced in several versions, such as the CCXR, Tre vita, and Edition. This model was the companies first ever to breach the 1000 hp mark. The CCX base models were in production until 2010, when Agera replaced them.
The introduction of the Agera marked and end of the previously supercharged in favour of turbocharged V8 engines. The Agera was produced in different versions such as the S, R, RS, and Final. Production of the Agera halted in 2016. In 2017 a new record of 0-400-0 km/h was set using the Rs model, clocking in at 36.44 s.
The Regera, introduced in 2015, was the company's latest model to take over after the Agera.
This model is built in a series of 80 examples, making it the company's most extensive series jet. The model features a combination of electric and combustion powerplant topping out at 1500 hp. In 2019 the record of 0-400-0 km/h was improved using a Regera. The time was reduced to 31.49 s, which is 1.8 s faster than the previous record.
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2019 also saw the introduction of a new model, the Jesko. Jesko shares many features with the Agera but has a redesigned aerodynamics and an improved engine with an output of 1600 hp distributed thru a 9-speed transmission. This model was unveiled in an updated version in 2020 called Jesko Absolut with an improved drag coefficient of 0.278 cd. 2020 marked another milestone as the company introduced its first four-seater, the Gemera. The Gemera features a 2-liter, 3-cylinder combustion engine producing 600 hp combined electric power giving a total output of 1700 hp and 3500 Nm of torque.
1.3. Aim of the study
The project here presented aims to find a power seat system proposal counting with three degrees of freedom (translation along two axes, and rotation around one axis). The working principles of the product, its architecture and how it fits the boundary conditions set by the client are the main points to be explored. Besides, the complete development of the design proposal in terms of testing and validation, manufacturing, and materials will be carried out.
Other topics like structural optimizations and motion analysis could be covered to some extent.
1.3.1. Problem definition
The exploration of different proposals that fulfil the requirements set by the company is the main reason for the development of the project. Although the company already have solutions that satisfy this project's requirements, it is looking for innovative approaches that could create higher value among their clients.
1.4. Limitations
Limitations of different nature can potentially affect the course of the project. To allow the two students to be able to complete the project, several limitations are created in agreement with both Koenigsegg and the university supervisor. These limitations are listed below.
• This project is focused on the design and development of a power seat base and track system; thus, the design of other related systems like the seat will not be carried out.
• Geometric boundary conditions provided by the company are, to some extent, a limitation for the generation of new solutions.
• Manufacturing methods should be selected according to the production limitations of the company. Manufacturing methods that are feasible for low production volumes must be taken into consideration.
• In relation to the previous point, the product must be developed bearing in mind the production constraints of the available manufacturing methods.
• The product developed in this project has a close relation to the driver's posture and comfort. Carrying out a complete ergonomic assessment can be a very time-consuming task that is out of the scope of the project. This can be recognized as a design limitation, since an important factor for the product's functionality will not be evaluated.
• Physical testing of the product will not be carried out as the project group will not be able to complete a prototype in the timeframe of this project.
• The project must be complete in a lapse of approximately 17 weeks, which can be scarce for product design and development tasks.
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• The current COVID-19 pandemic will affect, to a certain extent, the development of the project. Although in continuous contact, project group members are not located in the same area, and the access to some facilities at Halmstad University is restricted.
1.5. Individual responsibility, efforts, and study environment
This thesis is written by two students studying at Halmstad University's Master program in mechanical engineering. Due to COVID-19, all meetings will be done online using the Teams and Zoom applications. The two authors have been working from their homes and using the previously mentioned Microsoft Teams applications to discuss the work when ever needed.
The two authors of this thesis have agreed to equally share the workload. All decision has been taken based on thorough discussion between the authors to ensure the quality of the results.
The project group has a time budget of 17 hours in total with the industrial supervisor, Przemyslaw Lamasz and a total of 22 hours with the assigned university supervisor, Håkan Petersson.
1.6. Data provided by the industrial partner
The industrial partner has provided a model that defines limiting surfaces that need to be considered when designing the power seat system. The model consists of a floor, sidewalls, and part of the seat. The model also has some of the features that must be included in the seat rail structure. These are made up of a seat belt retractor located under the seat, a fire extinguisher located at the front of the seat, and a bracket that can support the rear part of the seat rail structure. In Figure 1.1, an image displaying the boundary conditions as received by the industrial partner is shown.
The industrial partner has also provided the project group with the forces that the seat rail structure needs to withstand in the event of a crash (see Table 1.1)
Table 1.1: Occurring forces in a crash situation.
Point Magnitude (N) Direction
Front left 1189 -Z
Rear left 35160 Z
Front right 1189 -Z
Rear right 35160 Z
Figure 1.1. CAD data as received by the industrial partner
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Method
In this chapter, a description of the methodology used to carry out this project is presented.
Information about alternative methods that could be consulted is presented. Besides, data collection methods and results of data research are offered to the reader.
2.1. Chosen methodology
The methodology used for the development of the project is based on the one described by Karl T. Ulrich, Steven D. Eppinger, and Maria C. Yang in the book Product Design and Development. Figure 2.1 depicts the process of the original methodology, and Figure 2.2 the one selected for this project.
Figure 2.1: Product design and development process suggested in the literature Product Design and Development.
Figure 2.2 Product design and development process selected for this project.
The following lines provide a thorough description of each phase depicted in Figure 2.2, their aim, and the tasks involved in each one of them.
1. Project planning. In the first step of the process, a thorough definition of the task is provided, a time plan shaped as a Gantt chart is created, and the available resources are set.
2. Research. Current solutions and patents of similar products will be explored in this phase. Also, current European legislation regarding the creation of machines and mechanical systems will be investigated. This phase includes the exploration of the topics included in Section 3, Theory.
3. Customer needs and target specifications. Potential customer needs will be identified internally and related to different requirements to be fulfilled by the product to achieve them. Numerical values will be assigned to the metrics that will define the characteristics and performance of the product. This set of specifications will be followed during the process, and design decisions will be made according to them.
4. Concept generation. Individual and collective brainstorming sessions will be carried out during this phase. The outcome will be a set of sketches defining the different parts of the product and their interaction.
5. Concept selection. One of the concepts from the previous phase will be selected for further development. This will be done employing Pugh's matrixes, in which the concepts will receive a score by comparing them to another design (usually called DATUM) according to several criteria with varying levels of importance. These criteria are related to the specifications set on stage 3.
6. Detail design. This phase encompasses various processes; however, they could be categorized into CAD and CAE tasks, dimensioning, and material and manufacturing
Planning Concept Development
System-Level
Design Detail Design Testing and Refinement
Production Ramp-Up
Planning Research Customer needs
and target specifications
Concept
generation Concept
selection Detail design
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processes selection. CAD and CAE are intended to be carried out concurrently throughout the process. Specifically, 3D modelling and drawings are tasks related to CAD; structural analysis and motion studies through FEM are part of CAE applications.
Calculations regarding dimensioning of components and selection of suitable material and manufacturing processes will also be carried out in this phase.
2.2. Preparation and data collection 2.2.1. Data collection
The project group has not been able to access the client's customers to perform surveys to understand customer desires better. But in general, surveys are better used for collection quantitative information. Quantitative information is not as desirable to the project group as the client's customer base is considered as small and an eventual survey would not have yielded the desired information. Therefore, data collection has been done considering the client as the customer, and that the project group will have to use the client's requests and suggestions as the "voice of the customer" to obtain the needs and wants. The information received from the client was then complemented by external information.
2.2.2. Literature search
In search of a better understanding of seat rail structures, the project group has utilized the database "Onesearch" provided by Halmstad University. The search terms found below in Table 2.1, where used to locate articles related to the seat rail structure. By reading the individual articles abstracts, articles potentially relevant to this project where chosen. The result of the search can be seen below in Table 2.1. Literature search using "Onesearch".
Table 2.1. Literature search using "Onesearch". Table 2.2. Patent search using” Google Patents”.
2.2.3. Patent search
Patents can be a major source of inspiration as well as a provider of information about how a similar problem have previously been solved. Patents are also a limitation as a desired solution already can be patented. The project group have used the database “Google Patents” to find patents related to the design of the seat rail structures. The result of the search can be seen below in Table 2.2.
Search string Search result
Articles investigated
"seat AND
rail" 292 2
"seat AND
track" 374 3
"seat AND
translation" 73 0
"seat AND
development" 380 2
Search term Search result Patents investigated
"seat AND rail" 34 849 14
"seat AND track" 12 386 6
"power AND seat
AND track" 468 10
"power AND seat
AND rail" 254 2
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Theory
In this section, theories, methods, and tools used throughout the project are presented and explained. The topics here included have been thoughtfully chosen to support the actions carried out during the development of the project. This chapter aims to provide the reader a better understanding of the work carried out to complete the project.
The topics selected can be categorized into four main groups, namely product design and development practices, design requirements, analysis tools and methods, and technical solutions.
The first group of topics, related to product design and development practices, aims to give the reader an insight into various processes and structured methods that can be followed for the systematics development of physical products. A section regarding cost analysis and cost estimation has been included for being one of the concluding tasks of the project. Besides, a common concept selection method, the Pugh's matrix method, is introduced.
Next, topics related to the specific design requirements of the project are presented. Once the needs and limitations of the project have been identified (see Section 1), specific research is done to ensure the feasibility of the solutions. For this matter, authors have identified the study of loads acting on drivers and seats during collisions to be an essential topic for the validation of the solution. In addition, due to the fact that the solution is part of a system in which the overall weight is crucial for performance, a topic specially dedicated to lightweight materials has been included. Finally, given the small number of units that the company is used to produce, a discussion on manufacturing methods suitable for low-volume production has been added.
Specific methods used during the "detail design" phase of the development of the product are covered. These are mainly related to the evaluation and optimization of results, and they include specific ones for structural analysis, performance validation, and shape optimization.
Finally, a set of technical solutions that could be potentially implemented during the project is presented to the reader. They are components or assemblies of parts that have proved to be used in previous similar projects. Also, aspects covering the types of DC motors and their sizing have been included for being a central part of the solution.
3.1. Product development processes
Having a process to stick to throughout the development of a product helps bridging the gap between an idea and its physical realization. Besides, it assures the quality of the outcome, improves the coordination between members, serves as a time plan, and helps identifying problems and opportunities (Ulrich et al., 2019). However, as Magrab et al., (2010) state, "there is no sequence of operations that will always guarantee a result". In addition, not all the information needed is available throughout process, and problem and solution evolve concurrently.
Even though there is not a unique way to approach product development problems that will conclude in an optimal result, several authors have defined methodologies that can serve as a guide throughout the process. Ullman, (2010) ensures that, for every design project, there must be a generic set of phases that have to be accomplished, and proposes the following ones:
product discovery, project planning, product definition, conceptual design, product
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development, and product support. Alternatively, the process suggested by Ulrich et al., (2019) is composed by six phases: planning, concept development, system-level design, detail design, testing and refinement, and production ramp-up. These phases, in turn, can be approached by the application of structured methods. According to Ulrich et al., (2019), structured methods are useful for three reasons: (1) the decisions taken based on them are objective and explicit, (2) they can act as "checklists" (3) they are self-documenting.
Although design and development processes are formulated differently from one another, when it comes to mechanical design and development, similarities can be found in the tasks encompassed in each phase. Considering the ultimate goal of the product development practice to be satisfying the needs of a certain market, some sort of planning must be done ahead of time to reach that goal. A plan in which, among other tasks, the problem to be solved is described, the goals of the project are set, and available time and resources are defined. Since the outcome of the process is a physical model ready for production, it is common to include a phase regarding ideation and conceptualization, which usually results in a set of sketches depicting the features of potential solutions. The concepts are then screened out using different criteria and one or more are selected for further development. The phase in which the product is fully defined and its performance validated is usually known as detail design.
Magrab et al., (2010) suggest that the result of a product development process should present a balance between: development speed, product cost, and product performance and quality (i.e., a balance between resource consumption and customer satisfaction). According to Ulrich et al., (2019), the product development process finishes when the information regarding production and sales is completed.
3.2. Concept selection: Pugh's matrix
To screen generated concepts, a tool can be used to narrow the concepts. One of the most frequently used ones is called the Pugh's matrix (Pugh, 1990) and was developed by Stuart Pugh in the 1980s (Ulrich et al., 2019). The purpose of this tool is to narrow the number of concepts by contrasting them with criteria of very different nature, unlike other selection methods, which focus just on economic or performance criteria.
The application of the method starts by creating a matrix and applying the previously obtained criteria. Now the importance weight can be introduced as a way of declaring the importance of the individual criteria. Then the concepts can be brought in, preferably in the form of both sketches and text. Then a reference, also known as a datum, needs to be chosen. It can either be one of the concepts or a current solution if such exists (Ulrich et al., 2019).
Now the concepts can be compared to the datum. Resulting in a score of a "1" if it is better, a
"-1"if worse, or a "0" if considered as equal. The scores are then added up to a net score which can serve as a guideline when deciding which design to use (Burge, 2009).
3.3. Production cost analysis
Different authors have proposed cost models for estimating the costs involved in the manufacturing of products. Initially, the sources of cost can be divided into direct costs ─those that can be traced to a specific component─ and indirect costs ─those that need to be met regardless of the production volume─ (Ullman, 2010). Alternatively, they can be understood as costs inferred by materials and by manufacturing (Ashby, 2017). However, to aid the
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accounting process and be able to optimize costs, sources are further divided into specific categories. Table 3.1 shows a breakdown of the sources of production costs according to different authors:
Table 3.1 Sources of production costs according to different authors.
According to Kalpakjian and
Schmid (2010) According to (Ståhl et al., 2007) According to Ashby (2017)
Materials Materials Materials
Tooling Tooling Tooling
Fixed costs Production Equipment
Capital costs Downtime Overhead costs
Direct-labour Salary Energy
Indirect-labour Maintenance Information
Ashby (2017) suggests a specific procedure for estimating the costs of production of a component based on the calculations of four variables (see Eq. 3.1).
𝐶𝑇 = 𝐶1+ 𝐶2+ 𝐶3+ 𝐶4 = 𝑚𝐶𝑚 (1 − 𝑓)+𝐶𝑡
𝑛 {𝐼𝑛𝑡(𝑛
𝑛𝑡+ 0,51)} + ( 𝐶𝑐
𝑛̇𝐿𝑡𝑤𝑜) + (𝐶̇𝑜ℎ
𝑛̇ ) [Eq 3.1]
Where: 𝑚 = mass of the component (kg); 𝐶𝑚 = material costing (SEK/kg); 𝑓 = scrap fraction;
𝐶𝑡 = set tooling cost (SEK); 𝑛 = production volume (units); 𝑛𝑡 = units before tooling needs replacement (units); 𝐶𝑐 = capital cost of equipment (SEK); 𝑛̇ = production rate (units/hour);
𝑡𝑤𝑜 = capital write-off time (hours); 𝐶̇𝑜ℎ = overhead rate (SEK/hour).
In Eq. 3.1, the total cost of production is calculated by the sum of four variables. 𝐶1 is related to the costs inferred by the component's material, 𝐶2 by the dedicated cost of tooling, 𝐶3 by the fixed costs of the equipment needed to manufacture the component, and 𝐶4 encompass every other cost not directly related to the manufacturing of the component ─e.g., rents, utilities, insurances─.
3.4. Collisions: loads and testing
The consequences of a collision depend on factors like impact speed, passenger characteristics, car geometry and weight, and obstacle nature. The force exerted on the passengers can be quantified by using the impact force equation (Eq. 3.2) (Tipler and Mosca, 2008), which relates mass, speed and deformation distance (travel distance during a collision) Alternatively, the deformation distance can be substituted by the duration of the collision; in this way, Eq. 3.2 can be expressed as Eq. 3.3.
𝐹 =𝑚𝑣2
2𝑑 [Eq. 3.2] 𝐹 =𝑚𝑣
𝑡 [Eq. 3.3]
Eq. 3.3 is a special case of the impulse and momentum formula (Tipler and Mosca, 2008). The idea of extending the time of impact during a crash is the reason behind many of the existing solutions for keeping passengers safe in car crashes ─e.g., seatbelts and airbags─. This fact is dictated by the work-energy principle, which states that a longer stopping distance decreases
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the impact force. Car bodies, seat belts and airbags all have the function of extending the stopping distance, and thus the impact time.
In the European Union, regulations regarding seat belt anchorages on seat structures are dictated by the Economic Commission for Europe of the United Nations (UN/ECE). The seat belt anchorages test, as described in ECE R14 (EUR-Lex, 2011), consists on the application of a tractive force to a couple of loading devices reproducing the upper and lower geometry of the torso ─so called body blocks─. The seat belt strap is located around the loading devices. For the case relevant to this project (M1 category vehicle), two loads must be applied simultaneously: a tensile load of 13500 N (± 200 N) to the loading device reproducing the upper torso and another of equal magnitude and direction to the loading device reproducing the lower torso. The load must be increased as fast as possible and the anchorages must withstand it for no less than 0,2 seconds.
3.5. Lightweight materials
Lightweight materials are widely used in the supercar industry for their potential to increase performance in both handling and speed. The following subheadings cover some of them and their current use in the automotive industry.
3.5.1. Carbon fiber reinforced polymer (CFRP)
Carbon fiber reinforced polymer (CFRP) is a composite material with excellent strength and stiffness to weight ratio compared to other materials used for manufacturing automotive parts (Mallick, 2010). Therefore, it is commonly used in body panels and monocoques. The material consists of two components: fibers and matrix. The matrix transmits the loads, providing the material with ductility and toughness, and is made of either epoxy or polyester to provide optimum performance while the fibers are what carries the loads exerted on the part (EduPack, 2020).
In the sportscar industry, carbon fiber parts are commonly made from pre-preg fibers, which is fiber that has been impregnated with resin prior to the lay-up, as they as they are easy to work with and produce high quality parts. First the fibers are placed in a mold that’s treated with a release agent. The mold is then placed in a vacuum bag and cured in an oven or autoclave.
Most parts require post-treatment to remove excess material.
Carbon fiber reinforced polymer has a higher price than materials like aluminium or steel and it requires a large amount of energy to produce, generating a higher CO2 footprint (EduPack, 2020).
3.5.2. Aluminium alloys
Aluminium alloys are widely used metals in the automotive industry as they offer low density, good machinability, weldability and castability with some of the most common applications being gearbox-cases, engine blocks, heads, and wheels (Mallick, 2010).
Aluminium is an abundant material in the earth's crust that requires a large amount of energy to extract. Like other metals, it can be recycled, significantly reducing energy consumption and CO2 footprint. Aluminium used in the automotive industry is usually an alloy of copper, zinc, magnesium, or silicon depending on the desired properties (EduPack, 2020).
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Some of the commonly used aluminium alloys in automotive industry are 6000-series, which is an alloy of Magnesium and Silicon, and 7000 series aluminium alloy which is an alloy of Zinc and Magnesium. (Rowe, 2012)
3.5.3. Advanced high strength steel
Advanced high strength steel, also known as AHSS, does not offer as low density as the previously mentioned materials but offers great strength, low cost and lower environmental impact when produced (Geck, 2014). Its great strength (over 500 MPa) allows for designing structures with less material, therefore reducing weight. AHSS has a complex structure in comparison to alloy steels, with a carefully selected chemical composition where multiphase microstructures are precisely controlled through heating and cooling processes. AHSS has lower ductility and formability compared traditional low carbon and high strength steels, although, the ductility (represented by a percentage of elongation) has significantly improved from the first generation AHSS (Rowe, 2012).
3.6. Manufacturing processes for low-volume production
Exploring the spectrum of available manufacturing processes and selecting the one that better fits the characteristics of a project is a task that can be approached in different manners.
According to Ashby (2017), the process selection starts by screening out the available processes regarding the materials and shapes they support. Next, processes are ranked regarding cost and quality. Kalpakjian and Schmid (2010) support this idea by stressing the importance of considering materials, geometric features of the parts, and production rate and quantity. The suitability of a process for low production volumes is strongly related to its economic batch size ─the production volume range in which a process is economically feasible─. The economic batch size of a process is mainly dictated by the tooling costs associated with it. In addition, Ullman (2010) stresses the importance of the volume production in the selection of manufacturing processes and considers it to be the main selection criterion.
Investigations carried out by Ashby (2017) conclude that the manufacturing processes shown in Table 3.2 are suitable for low-volume productions (one to ten parts). They are presented in relation to the materials they support.
Table 3.2 Low production volume manufacturing methods (Ashby, 2017).
Metals Polymers Composites Ceramics
Sand Casting ●
Investment Casting ●
Sheet forming ●
Electro-machining ● ●
Machining ● ●
Filament winding ●
Lay-up methods ●
Vacuum bag ●
Additive manufacturing ● ● ●
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From the classification shown in Table 3.2, additive manufacturing proves to be a versatile process for low-volume production in terms of material compatibility. Besides, it is alongside machining, a process compatible with almost any shape, including prismatic (both circular and noncircular), sheet (flat, folded and dished) and 3D shapes (both solid and hollow).
3.7. Computer-based structural analysis
Structural analysis carried out through the finite element method, relates to numerical approximations of partial differential equations solved with the help of a computer software (Goelke, 2015). These equations can be solved for either a single part or a whole assembly.
The algorithm used by the software in combination with model complexity and number of nodes greatly affect the time it takes to solve. Simple parts can be done in minutes while complex assemblies can take several days.
The aim of a structural analysis is to find out how the estimated loads affect the structure. The result can then be analysed and if need the design altered until a satisfactory result is achieved.
To confirm the solutions given by a software it is recommended that a second calculation is performed using a different software where the most conservative result of the two solutions shall be used (Petersson, 2020).
3.8. Technical solutions
The main function of any seat structure, independently of being manually or electrically driven, is being able to translate at least in one direction. Power seat structures commonly provide linear travel in two directions and rotary or tilting motion around one axis. There are several mechanical systems commonly used to achieve the desired motions. Some of them are introduced in this section.
3.8.1. Lead and ball screws
Lead and ball screws are widely used when linear motion in one direction is sought. Budynas and Nisbett (2015) define them as "devices used in machinery to change angular motion into linear motion, and, usually, to transmit power". Lead screws consist mainly of two elements, a screw and a nut, one travelling along the other when an angular motion is provided. Similarly, ball screws work based on the same principle but balls are added between the nut and spindle, replacing the sliding friction of lead by the rolling motion of balls.
Lead screw dimensioning is done by calculating the torque required to raise and lower the load applied. The torque needed for raising the load results from a binomial function as seen in Eq.
3.4; from which the first addend is related to the geometry of the thread and the nut and screw materials, and the second to the geometry and materials of the collar and its housing (Budynas and Nisbett, 2015).
𝑇𝑅 =𝐹𝑑𝑚
2 (𝑙 + 𝜋𝑓𝑑𝑚𝑠𝑒𝑐𝛼
𝜋𝑑𝑚− 𝑓𝑙𝑠𝑒𝑐𝛼) + (𝐹𝑓𝑐𝑑𝑐
2 ) [Eq. 3.4]
Where: 𝐹 is the force applied to the nut in the direction of the screw's longitudinal axis; 𝑑𝑚 is the average diameter between the screw's outer diameter (𝑑) and inner diameter (𝑑𝑚); 𝑙 is the lead of the screw; 𝑓 is the friction coefficient between the surfaces of the nut and the screw; 𝑓𝑐
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is the friction coefficient between the surface of the collar and its housing; and 𝑑𝑐 is the average diameter of the collar.
The reduced friction in ball screws has shown to provide much higher mechanical efficiency than lead screws. Also, the high starting force required in lead screws to overcome friction is drastically diminished when using ball screws (HIWIN, 2021).
A common application of lead and ball screws is seen in screw jack systems, used for providing vertical motion and lift heavy loads. Jack screws relies on the employment of worm gear sets for achieving high gear ratios and thrust bearings able to withstand high axial loads. Two main types of jack screws can be differentiated based on the motion of the screw: one in which it rotates but remains stationary and other in which it translates (Zimm Maschinenelemente GmbH, 2021).
Patents by Budinski (1981) and Gauger (1995) show the employment of power and ball screws to achieve linear translations in power seat mechanisms.
3.8.2. Rack-and-pinion gear sets
One of the most common and straightforward ways to achieve translational motion from rotational motion is by the use of rack-and-pinion gear sets. They can be understood as those in which one of the gears has an infinite radius that tends to a straight line (Norton, 2020). In rack-and-pinion gear sets, the linear velocity of the rack (𝑣𝑟) is the same as the pitch line velocity of the pinion (𝑣𝑝). Thus, for a desired linear velocity on the rack, the angular velocity of the pinion (𝜔𝑝) can be calculated using Eq. 3.5. Also, a relation between the linear displacement of the rack (𝑠𝑟) with regards of the angular position of the pinion (𝜃𝑝) can be derived following Eq. 3.6 (Mott et al., 2018).
𝜔𝑝 =2 ∗ 𝑣𝑟
𝐷𝑝 [Eq. 3.5] 𝜃𝑝 =2 ∗ 𝑠𝑟
𝐷𝑝 [Eq. 3.6]
Where: 𝐷𝑝= pitch diameter of the pinion.
Rack dimensions can be established based on the diametral pitch of the pinion. In this sense, the thickness, the face width, or the pinion to back of the rack distance can be consulted in tables relating these values.
Depending on the requirements of the application, the rack may be installed horizontally or vertically. In horizontal applications, the force exerted on the rack is that of the moved mass acting against the friction coefficient of the guide rails plus the force resulting from accelerating the mass. In contrast, for vertical applications friction can be neglected (Schmid et al., 2014).
The power seat track patent by (Budinski, 1981) shows the employment of a pinion-and-rack gear set for providing the back and forth motion of the seat.
3.8.3. Flexible shafts
Flexible shafts can be used to transmit rotational motion and power between points that are not aligned (Mott et al., 2018). They are often cheaper than other transmission means like belts or chains, and do not create as much noise (Budynas and Nisbett, 2015). Power seat patent by Gauger (1995) shows the use of flexible shafts to transmit rotational motion and power from DC motors to various lead screws. Besides being used for power seat tracks, they are employed
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in the automotive industry in power sliding door systems, power rear lift gates, and pedal adjusters (S.S. White Technologies, 2021).
For selecting flexible shafts for a certain application, suppliers provide catalogues specifying various characteristics, like the diameter of the cross section of the shaft or the maximum transmittible torque. Special attention must be paid to factors such as the main direction of rotation, the transmittible torque at a certain torsion angle, or the minimum permitted bending radius (BIAX Flexible Power, 2021).
Coatings can be applied to the outer layer of flexible shafts to enhance functionality. In particular, they can be used for diminishing the noise, reduce vibration, increase flexibility or reduce the play between the shaft and its casing (GEMO, 2021).
3.9. DC motors: types and selection
The concept of a DC motor is that it rotates by using the force produced on the current-carrying conductors placed in the magnetic field created by the stator. A DC motor features two static fields where one is the rotor magnetic field produced by the current in the conductors of the rotor. The other field, the stator, is produced by either magnets or a field winding. The interaction between these two fields produces a torque that enables the motor to turn continuously. (Kim, S., 2017)
3.9.1. Types of DC motors
DC motors can be split into two categories, separately excited and self-excited. The difference between the two is that a separately excited motor has both its windings separately excited by a DC power source. Separately excited motors can be split in to two categories, ones with permanent magnets and those with a wound field. In the self-excited motor, the two windings share the same DC power source (Kim, S., 2017). The permanent magnet motors are the most common BDC motors as they are a cost-efficient solution for low torque applications that require precise control.
The self-excited motors are further categorized into shunt, series, and compound motors depending on how the windings are connected to the DC power source. Shunt motors are best used in high horsepower applications that require excellent speed control while series motors are suitable for applications that require high torque in combination with low speeds.
Compound motors, which is a combination of series and shunt motors, delivers higher torque than a shunt motor and better speed control than a series motor. (Condit, 2004)
Brushless motors are more sophisticated than those mentioned above but also more expensive as they require more complex controllers to regulate the speed of the rotor. However, they last longer as there are no brushes that can wear out and are able to operate at higher speeds. These motors have a lighter rotor which enable better speed response and allows for greater heat dissipation which allows the motor to have a better peak torque capability (Kim, S., 2017).
Brushless motors are mainly categorised into stepper motors and servo motors. Servo motors are the most sophisticated of the two as it offers some undeniable performances advantages.
They run faster, enabling the use of a gearbox to deliver higher torque while maintaining a useful speed. These motors also have a better constancy in torque delivery across the speed range. Stepper motors are still worthy opponents, as they are significantly cheaper, generate high torque from zero speed, and are generally more compact.
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3.9.2. DC motor selection
Dimensioning of DC motors can be done by setting up criteria that the motor needs to be able to operate under. Some of the more common criteria that could apply in a selection process could be (Hughes and Drury, 2013):
• voltage availability.
• physical size.
• torque.
• speed.
• duty cycle.
Another matter that shall be considered when selecting a DC motor is weather to use a brushed or a brushless motor.
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Results
Given the practical nature of the project, the results are presented in relation to the outcome of each one of the phases of the product design and development process followed (see section 2, Method). Although there have been various iterations throughout the process, only the ultimate outcome of each phase is presented in this section.
4.1. Project planning
During the project's start-up phase, the authors started with researching necessary information to better understand the required features of a seat rail structure. The results of this research can be found in Table 2.1 and Table 2.2. The authors also received specifications, limitations, and further information on features needed from the industrial partner. Based on this, the authors chose a method and set a time plan. The time plan set, created in the form of a Gantt chart, can be found in Appendix 1: Gantt chart.
4.2. Customer needs and target specifications
The customer needs for the purpose of this project have been identified based on the information given by the client and internal discussion of the team members. They can be seen in Table 4.1. Along with their description, an identification number and a value regarding its importance for the application have been assigned.
Table 4.1 Identified customer needs.
Need Nº Need Importance
(1-5) 1 The mechanism provides linear motion in two axes and rotational motion
around one axis 5
2 The product can be manufactured cost-effectively according to the production
volume 5
3 The structure can withstand crash loads 5
4 The structure incorporates a seat belt buckle 5
5 The structure fits a fire extinguisher, seat belt retractor and various cables 4
6 The mechanism is actioned by three DC motors 4
7 The mechanism works without making much noise 3
8 The structure can be adjusted in a reasonable amount of time 3
9 The structure provides good stability 4
10 The structure is positioned (bolted) in already existing holes 3
11 The structure fits an already existing seat model 4
12 The structure is supported by an already existing bracket 3
13 The product's look fits that of the car 2
14 The product is sustainable 2
15 The product is maintenance-free for the end-user 3
16 The product lasts a long time 2
17 The product is lightweight 4
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18 The product can be maintained with already available tools 3 19 The product's electrical components must be dust and watertight 3
20 The product does not corrode 3
21 The product is easy to assemble/disassemble 4
Once the needs are identified, the voice of the customer is translated into concrete and precise target design specifications. For that purpose, a set of metrics, units and values are presented in Table 4.2, each of them numbered and linked with the need of Table 4.1 that they are meant to fulfil.
Table 4.2 Target product design specifications.
Metric Nº
Need
Nº Metric Units Marginal
Value
Ideal Value
Importance (1-5)
1 1 Travel distance in X direction mm 255 255 5
2 1 Travel distance in Z direction mm 55 55 5
3 1 Rotational travel around Y-axis degrees 9,2 9,2 5
4 2 Unit manufacturing cost (parts that are
not already made) SEK 1000 500 4
5 5 Floor clearance mm >92 100 3
6 7 Maximum level of noise dB 40 25 3
7 8 Maximum horizontal adjusting time s 12 10 4
8 8 Maximum vertical adjusting time s 8 6 4
9 13, 2,
20 Rails surface finish Ra 20 10 4
10 10,
12 Fit already existing bolt-holes Yes/No Yes Yes 5
11 9, 11 Minimum width mm 480 500 3
12 9, 11 Minimum length mm 300 390 3
13 21 Assembly time s 3600 1200 3
14 16 Expected lifetime years 15 40 4
15 17 Total weight kg 10 5 4
16 21 Maximum number of parts parts 50 30 4
17 3, 4 Maximum load N 35500 35500 5
18 3, 4 Maximum displacement mm 2 1,5 5
19 19 IP classification of the motors IP 44 55 3
20 14 Maximum material CO2 footprint kg/kg 15 8 3
21 4 Incorporate a seatbelt buckle Yes/No Yes Yes 5
22 6 Employs double-rotor DC motors Yes/No Yes Yes 4
23 18,
21 Standardized hardware Yes/No Yes Yes 4
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4.3. Concept generation and selection
In the concept generation phase, the project group members decided that all concepts shall be created individually. The project group thinks this allows for greater freedom and, therefore, higher variation among the concepts. Concepts are then divided based on the motion they intend to perform. Thus, the concepts have been created based on the translation along the vehicle's traveling direction (X) and a combination of translation in height and tilt (ZY). For selecting the most relevant concepts of each of the motions, the project group has, as previously mentioned, chosen to use Pugh's matrix as the primary tool for comparing concepts. The criteria used to evaluate the concepts in the matrix have been selected by the project group in collaboration with the industrial supervisor. However, this does not mean that the concept that scores the highest in each of the matrix automatically is the one that is going to be used. All concepts and selections matrix’s can be found in Appendix 2: Concepts.
4.3.1. Translation in X-direction
The translation of the seat in the X-direction, which the project group considers the traveling direction of the car, is considered in five concepts. These concepts mainly consist of two types:
one is made up of a rack and a pinion, and the other of a lead screw. After this, the project group has adapted and redesign these solutions to make them suit the needs of this project.
Table 4.4 shows the concept selection matrix for the one proving the translation in X.
Table 4.3 Concept selection: Pugh's matrix for X-translation solutions.
Importance (1-5)
Relative weight
Concept X1
Concept X2
Concept X3
Concept X4
Concept X5
Cost of production ▼ 4 9,76 D 1 0 1 1
Number of parts ▼ 2 4,88 A 1 1 1 1
Weight ▼ 4 9,76 T 1 0 1 1
Space ● 3 7,32 U 1 1 1 1
Ease of
manufacturing ● 3 7,32 M 1 0 1 1
Need of maintenance ▼ 2 4,88 * 0 0 0 0
Durability ▲ 3 7,32 * -1 0 -1 -1
Passenger safety ▲ 5 12,20 * 0 0 0 -1
Noise ▼ 2 4,88 * -1 0 -1 -1
Ease of
assembly/disassembly ● 2 4,88 * 1 0 1 1
Development time ▼ 3 7,32 * -1 0 -1 -1
Degree of
standardization ▲ 3 7,32 * -1 0 -1 -1
Autolocking ● 3 7,32 * -1 0 -1 -1
Serves as support for the rest of the structure
● 2 4,88 * 0 0 0 -1
TOTAL 41 100 0 9,76 12,20 9,76 -7,32
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4.3.2. Translation in Z direction and rotation around Y-axis (tilt)
For the translation of the seat up and down, which the project group here refers to as the Z- direction, the project group has created six concepts. The six concepts represent a wide variety of solution with inspiration take both from various existing solutions and patents. Table 4.5 shows the concept selection matrix for the ones providing the lifting motion.
Table 4.4 Concept selection: Pugh's matrix for Z translation and Y rotation solutions.
Importan ce (1-5)
Relative weight
Concept ZY1
Concept ZY2
Concept ZY3
Concept ZY4
Concept ZY5
Concept ZY6
Cost of production ▼ 4 10,26 D -1 0 -1 -1 0
Number of parts ▼ 2 5,13 A -1 -1 -1 0 0
Weight ▼ 4 10,26 T -1 -1 -1 -1 0
Space ● 3 7,69 U -1 -1 0 -1 0
Ease of
manufacturing ● 3 7,69 M -1 1 0 0 0
Need of
maintenance ▼ 2 5,13 * -1 0 0 1 0
Durability ▲ 3 7,69 * 0 1 0 1 0
Passenger safety ▲ 5 12,82 * 0 1 1 0 0
Noise ▼ 2 5,13 * 0 0 0 0 0
Ease of
assembly/disassem bly
● 2 5,13 * -1 -1 -1 0 0
Development time ▼ 3 7,69 * -1 0 0 0 0
Degree of
standardization ▲ 3 7,69 * -1 -1 0 -1 0
Autolocking ● 3 7,69 * -1 0 0 -1 0
TOTAL 39 100 0 -74,36 -7,69 -17,95 -30,77 0
4.4. Detail design
In this section, the results of the detail design stage of the product development process are presented. This section includes a variety of tasks for the definition and validation of the product. To start off, a thorough description of the product is presented in section 4.4.1. It is based on various figures to identify the components and help the reader better understand the subsequent sections. Section 4.4.2 includes the calculations carried out for dimensioning the components of the mechanisms providing the desired motions. Section 4.4.3 includes information regarding the results obtained from the CAD modelling task. Next, the results obtained from the structural analysis for verifying the model are presented in section 4.4.4.
Results regarding material and process selection of the components are included in section 4.4.5. Considerations regarding the life-cycle of the product are presented in section 4.4.6 . In section 4.4.7, the results of the cost estimations are included. Finally, a bill of materials (BOM) including information of all the components making up the product is presented in section 4.4.8.
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Notice that, in most of the subsections included in this section, references to section 8.
Appendix are made for consulting the full information of every task.
4.4.1. Product description
The product is fitted in various already-existing holes of the car, ones on the floor and others laying in the same plane but located in a bracket mounted to the sidewall (monocoque) (1). A two-part sliding guide rail system made of extruded metallic profiles provides the horizontal translation of the system and serves as a support for the rest of the parts making up the product.
A bottom rail with a flat middle surface (2) is bolted to various holes located in the car floor and the rear bracket (1). The bottom rail is symmetrical along its longitudinal axis and counts with some protrusions that cause the engagement of the top rail (3). The engagement is achieved thanks to their cross-sections being of opposite shapes. The flat surface in the upper rail (3) has several holes for the rest of the parts of the product to be bolted. The parts that are bolted to the top rails are 4, 5, 6, 7, and 8. Part 4 is a sheet metal bracket in which a fire extinguisher (31), seat belt retractor (32) and three DC motors (33, 34, 35) are mounted to and move together with the assembly as it translates horizontally. Parts 5 and 6 are the housings of the mechanisms providing the vertical lifting motion of the seat. Similarly, part 7 is the housing of the mechanism providing the horizontal translation of the system. Finally, part 8 is a bent sheet metal seat belt anchorage. Thanks to the cross-section geometries of rails 2 and 3, and the seat belt anchorage (8) being bolted to the upper rail (3), a locking action between the rails will occur when a pulling force is exerted on the seat belt; thus, ensuring the position of the system.
The mechanism enclosed by parts 5 and 6 have very similar working principles, with a few parts varying in dimension. Part 9 is a worm that engages with part 10, a teethed-metallic- cylindrical-part similar to a spur gear. Part 10 counts with an inner-threaded-cylindrical-surface with the necessary characteristics to mesh with part 11, a trapezoidal screw. As part 10 rotates, the trapezoidal screw (11) travels up and down. Part 10 is supported by a thrust bearing and a thrust washer, 12 and 13 respectively. The thrust bearing is located on top of a plastic-made cylindrical part (14) that remains in contact with the upper rail and provides guidance to part 11 when travelling up and down. Finally, the tip of part 11 counts with a hole to fit a cylindrical protrusion in part 12, a bracket bolted to the seat. The trapezoidal screw tip sits on a washer (15).
The trapezoidal screw in the front mechanism (16) has a screwed end that passes through a curved slot in the front seat bracket (17). The trapezoidal screw is secured by means of two retaining parts (18 and 19). The tilting motion of the seat will be achieved when one of the trapezoidal screws translates while maintaining the other steady. By means of the curved slot in the front seat bracket, the front trapezoidal screw can be maintained vertical at every point.
The mechanism providing the horizontal motion is formed by a worm gear set that provides the input motion and torque to a rack-and-pinion gear set. The worm 20 mounted to the shaft 21 meshes with a spur gear (22). This spur gear is located in a shaft (23) by means of retaining rings and moves together with a shaft thanks to a squared-key. Spur gear 24 is also mounted to shaft 23, and meshes with a rack (36) adhered to one of the inner vertical faces of the bottom rail (2). Shaft 23 is supported by two ball bearings (26 and 27), as the rest of the shafts of the product. Every shaft element is axially located by retaining rings.
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By means of five flexible shafts ─ from which (28) (29) (30) can be seen in Figure 4.1─
connected to three dual-rotor DC motors (33) (34) (35), the input rotational motion is provided to the shafts rotating the worms.
Figure 4.1. Power seat structure proposal assembly (top); rail system (bottom)
