Linköping University | Department of Management and Engineering Master’s thesis, 30 credits| Master of Science in Applied Physics and Electrical Engineering Autumn 2019| LIU-IEI-TEK-A--20/03638—SE
Design of a mechatronic
locking system in a rotating
gear shifter
– How a knowledge-intensive approach can be utilized in a
product development project
________________________________________________
Marcus Jackson
Supervisor: Magnus Sethson Examiner: Robert Braun
Abstract
Gear shifters are currently undergoing a shift of technology forcing companies within the industry to develop new concepts and new designs. An example of this is Kongsberg Automotive that is developing one new innovative design; a rotating gear shifter. An important part of the rotating gear shifter is the locking system that control whenever rotation (shifting of gear) should be possible.
The purpose and objective of this thesis is to show how the design of a mechatronic locking system can be implemented in the rotating gear shifter being developed by Kongsberg Automotive to fulfill the desired function (lock-ing the rotat(lock-ing gear shifter), as well as optimiz(lock-ing it with regards to size, cost and energy consumption. The thesis is investigating how a knowledge-intensive approach can be adopted in order to deliberately and systematically build understanding of the basic principles involved in the operation of the new technology.
The thesis presents six different design concepts that has been studied and, based on different design aspects, they have been compared to each other. The design concept that is deemed the most suitable to the application is a locking system based on a magnetic circuit.
The physics of the magnetic circuit has been studied and a mathematical model of the system has been derived that identifies the design parameters that affect the size, cost and energy consumption. Simulations and experiments has been performed to compare how well the theory correlate to reality.
A mathematical model is presented which has been derived from conven-tional physics. The thesis also presents simulations which has been performed using state-of-the-art software. Finally, results from experiments are presented which have been conducted on prototypes of the system. All results show the same behaviour which is taken as a verification and validation of the mathemat-ical model, motivating its continued use in further design efforts. A knowledge-intensive approach can thus be used in the development of new designs.
Acknowledgments
I would like to thank my supervisor at Link¨oping University, Magnus Sethson, and my supervisor at Kongsberg Automotive, Alexander Fransson, for all their help and support in writing this master thesis. Thanks also to all the employees of Kongsberg Automotive for showing great interest and answering any ques-tions I have had. Special thanks to Tim Sk¨oldberg for all the help with making simulations, Ismael Dobon and Marc Taken for helping me with the experiments on prototypes I did, to Andreas Kammensj¨o for the discussions, and to Johan Samuelsson, my manager at Kongsberg Automotive, for always being support-ive and giving me the responsibility of working in such an interesting project. I also want to thank my parents, Mats and Maria Jackson, for letting me stay in their home while I was working with this thesis, and specially my father for always taking the time to discuss any problems I encountered. Lastly I want to thank my examiner Robert Braun and my opponent Kristian Fodor for all their valuable reflections and feedback.
Marcus Jackson
Contents
1 Introduction 1
1.1 What is a gear shifter system? . . . 1
1.2 Shift of technology . . . 2
1.3 Company background . . . 3
1.4 Product development project . . . 4
1.5 Purpose and objective . . . 6
1.6 Research questions . . . 7
1.7 Delimitations . . . 7
1.8 Report outline . . . 8
2 Theoretical framework 9 2.1 Relation between chord and arc of a circle . . . 9
2.2 Potential energy and conservative force . . . 9
2.3 Force of a spring . . . 10 2.4 Moment of force . . . 11 2.5 Magnetism . . . 12 2.6 Permeability . . . 13 2.7 Electromagnetism . . . 14 2.8 Magnetic circuits . . . 16 2.9 Dimensional analysis . . . 17 2.10 One-factor-at-a-time method . . . 18 2.11 Gaussian distribution . . . 18 3 Methodology 19 3.1 Research method . . . 19
3.2 Literature study of design concepts . . . 20
3.3 Model of the locking system . . . 21
3.4 Simulation of locking system . . . 21
3.5 Testing of prototypes . . . 21
4 Literature study of design concepts 22 4.1 Problem description . . . 22
4.2 Screw . . . 22
4.3 Rack and pinion . . . 23
4.4 Magnetic lock . . . 24
4.5 Solenoid bolt . . . 25
4.6 Magnetic circuit . . . 27
4.7 Magnetorheological fluid . . . 28
4.8 Summary . . . 29
5 Model of the locking system 31 5.1 Problem description . . . 31
5.2 Model of the electromagnet . . . 32
5.4 Calculating time to lock system . . . 39
5.5 Dimensional analysis . . . 42
5.6 Sensitivity analysis . . . 43
5.7 Summary . . . 50
6 Simulation of locking system 52 6.1 Problem description . . . 52
6.2 Simulation model . . . 53
6.3 Simulation results . . . 54
6.4 Comparison with model . . . 57
6.5 Summary . . . 61 7 Proof of concept 62 7.1 Problem description . . . 62 7.2 Design of experiment . . . 63 7.3 Conducting experiment . . . 65 7.4 Result of experiment . . . 67 7.5 Summary . . . 67 8 Discussion 68 8.1 Discussions in relation to the case studies . . . 68
8.2 Knowledge-intensive approach . . . 70
9 Conclusions 72
10 Future developments 73
List of Figures
1 A size comparison of a shift by wire system (left) and a shift by cable system (right). Note that the black plates that the shifters are mounted on are of the same size. . . 1 2 A collection of different gear shifter designs from different car
manufacturers available on the market [1]. . . 2 3 Kongsberg Automotive ASA logotype. . . 3 4 Prototype of a rotating gear shifter system being developed by
Kongsberg Automotive. . . 4 5 Average portion of total development cost at different Technology
Readiness Levels compiled from data in Linick [8]. . . 6 6 Illustration of how a chord between two points on the
circumfer-ence of a circle relates to the arc between the same two points. . 9 7 An illustration of Hooke’s law and the actual force of a spring as
functions of the spring deformation dx. a) A spring compressed by some distance dx. b) A spring in its relaxed position, where dx = 0. c) A spring extended by some distance dx. . . 10 8 Example of a system in balance due to the two weights exerting
the same moment of force. . . 11 9 a) Magnetic domains of a material canceling each other out. b)
Magnetic domains of a material aligning in response to a magnetic field. . . 12 10 BH curve of a ferromagnetic material. a) The initial
magnetiza-tion curve. b) Positive saturamagnetiza-tion. c) Residual magnetism (posi-tive). d) Negative saturation. e) Residual magnetism (nega(posi-tive). 13 11 A coil with N turns conducting a current I, enclosed by a
mag-netic field, H. . . 15 12 Schematic research process by Fagerstr¨om [22] in Eriksson [23]. . 19 13 Illustration of how a screw converts a torque generated by an
electric motor, ωm, to a linear motion, v. . . 22 14 Drawing of a water screw used to transport water designed by
Archimedes during antiquity [28]. . . 23 15 Illustration of how a rack and pinion converts a torque generated
by an electric motor, ωm, to a linear motion, v. . . 24 16 Depiction of how a magnetic lock works. When a current is
flow-ing through a coil the core become magnetic and attracts the rotating shifter, locking it. . . 24 17 An engineer walking upside down across the bottom of a steel
beam using boots with built in magnetic locks developed by NASA in 1967 [34]. . . 25 18 Depiction of a normally locked solenoid bolt. a) When no current
is flowing through the coil the spring pushes the bolt into a locking position. b) When a current is coursing through the coil the magnetic bolt moves in the direction of the magnetic field H and unlocks the system. . . 26
19 One-Inch Stroke Shot Bolt Solenoid made by TLX Technologies
[37]. . . 27
20 Depiction of how a relay works. . . 27
21 Illustration of how MRF works. a) No magnetic field is applied and the particles are evenly distributed in the carrier fluid. b) A magnetic field is applied and the particles align along the mag-netic flux, Φ, increasing viscosity. . . 28
22 CAD sketch of the locking system. . . 31
23 Simplified model of the electromagnet. . . 32
24 Simplified model of the latch. . . 36
25 Total moments of force exerted on the system. a) When the electromagnet is turned off and the system is locked. b) When the electromagnet is turned on and the system is unlocked. . . . 38
26 The length of the air gap as a function of time (equation 5.4.18) with the locking time, tlmarked in the graph. . . 41
27 Force as a function of latch length. . . 43
28 Force as a function of distance between spring and center of ro-tation. . . 44
29 Force as a function of length of the air gap. . . 44
30 Force as a function of maximum distortion of the spring. . . 45
31 Force as a function of the spring constant. . . 45
32 Force as a function of coil current. . . 47
33 Force as a function of number of wraps of the coil. . . 47
34 Force as a function of core permeability. . . 48
35 Force as a function of core cross-section area. . . 48
36 Force as a function of length of the air gap. . . 49
37 Force as a function of core length. . . 49
38 Force as a function of latch length. . . 50
39 COMSOL Multiphysics R being used to simulate the locking system. 52 40 Mesh of the locking system in COMSOL Multiphysics R. . . . 53
41 Simulation results for the force exerted by the electromagnet as a function of the current in the coil. Force as predicted by the model using the same parameters as in the simulation is included for comparison. . . 54
42 Simulation results for the force exerted by the electromagnet as a function of the number of wraps of the coil. Force as predicted by the model using the same parameters as in the simulation is included for comparison. . . 55
43 Simulation results for the force exerted by the electromagnet as a function of the permeability of the core and latch. Force as predicted by the model using the same parameters as in the sim-ulation is included for comparison. . . 55
44 Simulation results for the force exerted by the electromagnet as a function of the length of the core. Force as predicted by the model using the same parameters as in the simulation is included for comparison. . . 56
45 Simulation results for the force exerted by the electromagnet as a function of the length of the air gap. Force as predicted by the model using the same parameters as in the simulation is included
for comparison. . . 56
46 Simulation results for the force exerted by the electromagnet as a function of the current in the coil. Force as predicted by the model modified by the mean using the same parameters as in the simulation is included for comparison. . . 59
47 Simulation results for the force exerted by the electromagnet as a function of the number of wraps of the coil. Force as predicted by the model modified by the mean using the same parameters as in the simulation is included for comparison. . . 59
48 Simulation results for the force exerted by the electromagnet as a function of the permeability of the core and latch. Force as predicted by the model modified by the mean using the same parameters as in the simulation is included for comparison. . . . 60
49 Simulation results for the force exerted by the electromagnet as a function of the length of the core. Force as predicted by the model modified by the mean using the same parameters as in the simulation is included for comparison. . . 60
50 Simulation results for the force exerted by the electromagnet as a function of the length of the air gap. Force as predicted by the model using the same parameters as in the simulation is included for comparison. . . 61
51 Prototype of the locking system. . . 62
52 Schematic model of the test setup. . . 63
53 Photograph of the test setup. . . 64
54 Zoomed in view of the test setup. . . 65
55 Prototype of the locking system mounted in a casing to allow testing. . . 65
56 The laser sensor [51] from multiple viewpoints. . . 66
57 Close up on the load cell [52]. . . 66
58 Force as a function of the length of the air gap as measured in experiments. Forces estimated by the model and in simulations are included for comparison. . . 67
59 Force as a function of the length of the air gap as measured in experiments. Forces estimated by the model, in simulations and by the modified model are included for comparison. . . 70
List of Tables
1 Technology Readiness Levels as defined in Horizon 2020 [7]. . . . 5 2 Maxwell’s equations. . . 14 3 Variables of magnetic circuits and their electric circuit analogues. 16 4 Description of studies performed to answer the research questions. 20 5 A comparison of the investigated design concepts. . . 30 6 Design parameters of the magnet. . . 33 7 Design parameters of the latch. . . 36 8 Relevant units expressed in SI base units with dimension symbols
[19]. . . 42 9 Nominal values for design parameters of the latch. . . 43 10 Nominal values for design parameters of the electromagnet. . . . 46 11 Summary of all identified design parameters of the locking system. 51
Nomenclature
• SBC - Shift By Cable • SBW - Shift By Wire • KA - Kongsberg Automotive • TRL - Technology Readiness Level • RQ - Research Question
• MMF - Magnetomotive Force • MRF - Magnetorheological fluid • OFAT - One-Factor-At-a-Time • CAD - Computer-Aided Design • PDE - Partial Differential Equation • FEM - Finite Element Method
1
Introduction
Gear shifter systems are currently undergoing a shift of technology and new innovative designs are becoming more common, forcing companies within the industry to develop new technologies. An example of this is Kongsberg Au-tomotive that is developing one such new design; a rotating gear shifter. An important part of the rotating gear shifter is the locking system that control whenever rotation (shifting of gear) should be possible.
1.1
What is a gear shifter system?
Traditionally the gear shifter system (often referred to as the ”gear lever”) is the system used by the driver of a vehicle with a manual transmission to shift gear. When a combustion engine is used, the transmission is crucial since the engine can not work at low speeds. In an electrical car, or one with an automatic transmission, the shifting is not done between gears as was originally referred to but rather between different driving modes (reverse, parking etc), but the system is still refereed to as the gear shifter system.
Figure 1: A size comparison of a shift by wire system (left) and a shift by cable system (right). Note that the black plates that the shifters are mounted on are of the same size.
Traditional shifter systems are purely mechanical and have a cable connect-ing the gear shifter to the transmission and are referred to as shift by cable (SBC) systems. Shifting by cable requires there to be a mechanical link be-tween the shifting system and the transmission and this limits the design of car interiors as well as requires extra force from the driver to shift gears. Many manufacturers are because of this replacing the classic shift by cable systems with shift by wire (SBW) systems instead. In a SBW system, the gear shifter
is an electrical control system that sends a signal through an electrical wire to an actuator mounted on the transmission that does the actual mechanical gear shifting. SBW systems are thus mechatronic; combining mechanical com-ponents with electronics. Shifting by wire allows for new design opportunities since the system can be built smaller (see figure 1), requires less force to shift and opens up the possibility that the gear shifter does not necessarily need to have the shape of a lever.
Figure 2: A collection of different gear shifter designs from different car manu-facturers available on the market [1].
While the traditional lever is still the most common gear shifter design (also in SBW systems) new innovative designs are becoming more common. Examples of gear shifters that are not levers include paddle shifters, button shifters and rotary shifters. A couple of examples that are available today can be seen in figure 2. The lack of standardization within the industry is a clear sign that gear shifters are undergoing a shift of technology. This is exemplified by Jay Joseph, American Honda’s assistant vice president for product planning, who when asked about differing gear shifter designs states; ”I think over time we’ll begin to see standardization, we’re not seeing it yet because I don’t think anybody is convinced that they’ve seen the best solution yet” [2].
1.2
Shift of technology
In an ever changing market the need for manufacturing companies to react to new trends and be quick to develop new products is key to staying competitive.
This is summarized well by the World Economic Forum which states; ”In the 21st century manufacturing environment, being able to develop creative ideas, addressing new and complex problems and delivering innovative products and services to global markets will be the capabilities most coveted by both countries and companies” [3].
To be able to constantly develop new ideas and products, a manufacturing company in the 21st century must be proficient with both creating and assimilat-ing new knowledge. Ever increasassimilat-ing demands and expectations from customers force companies to focus on knowledge-intensive product development to stay ahead of the competition. The need for flexibility and ability to change cannot be understated, and an example of this is a quote from Mikael Damberg, the former Swedish minister for enterprise and innovation, who stated ”Sweden’s prosperity is built on innovative and successful export companies that time and again have managed to renew and reorganise production and products to keep pace with changing markets” [4].
This is perhaps most apparent when the products designed and manufac-tured by a company is going through a shift of technology : knowledge that pre-viously has not been relevant may suddenly, due to demands from customers, become a core part of what the company do. When a shift of technology occurs, there are often a number of alternate designs competing with each other until a dominant design emerges. The dominant design is the solution that the market chose as a de facto standard. The ability to pick up on new trends and adjust accordingly is thus key to success for a manufacturing company [5].
1.3
Company background
Kongsberg Automotive ASA (KA) provides world-class products to the global vehicle industry. It has its origins in Kongsberg V˚apenfabrikk, a defence and weaponry company, which began producing brakes and driveshafts for Volvo in the late 1950’s. Since then KA has developed from a Scandinavian automotive parts supplier to become a global leader in one of the most competitive and complex industries in the world.
Figure 3: Kongsberg Automotive ASA logotype.
Kongsberg Automotive AB Mullsj¨o originates from two companies: Mekania, established in 1946, and Scandia, established in 1950. Forsheda acquired the two companies in the 1980s, and they became Scandmec. In 1996 Scandmec was acquired by Kongsberg Automotive.
KA has three business segments: Interior, Powertrain and Chassis, and Speciality Products. KA Mullsj¨o is serving KA’s Powertrain and Chassis, and Interior business segments.
KA Mullsj¨o is a full service plant with revenues exceeding EUR 100 million. A new global (EU) tech center for Driveline and Interior was innaugurated in late 2014. KA Mullsj¨o produces head restraints, crash brackets, expansion tanks, and gear shifter systems for both passenger cars and heavy vehicles.
Figure 4: Prototype of a rotating gear shifter system being developed by Kongs-berg Automotive.
1.4
Product development project
An example of a product development project where a shift of technology has made a company develop new technology is when KA, after an inquiry from a major automobile manufacturer, was chosen as an advanced development part-ner to develop the next gepart-neration SBW. The shifter system in question is a
rotating gear shifter system and a prototype built by KA is displayed in figure 4. By the year of 2025 the automobile manufacturer expect the technology to be present in up to 2,5 million of their cars annually.
The automobile manufacturer requires the gear shifter to only be able to be turned at certain times, for example when the car is standing still and the break is applied. This creates the need of a mechatronic locking system that can control whether shifting gears should be allowed or not.
The technology needed to design and manufacture the mechatronic locking system is new to KA and the need, and willingness, to assimilate this knowl-edge can be seen as an example of a company that is renewing itself to stay competitive in a changing market.
To measure the maturity of a technology the Technology Readiness Level (TRL) scale is used. The TRL scale was originally developed by the National Aeronautics and Space Administration (NASA) [6] and has been adopted by a multitude of organizations and industries and exists in many different ver-sions. Table 1 showcase TRL as defined in the European Union’s Research and Innovation programme Horizon 2020 [7].
Table 1: Technology Readiness Levels as defined in Horizon 2020 [7].
Level Description
TRL 1 Basic principles observed
TRL 2 Technology concept formulated
TRL 3 Experimental proof of concept
TRL 4 Technology validated in lab
TRL 5 Technology validated in relevant environment TRL 6 Technology demonstrated in relevant environment TRL 7 System prototype demonstration in operational environment TRL 8 System complete and qualified
TRL 9 Actual system proven in operational environment
When a technology is mature, and a company has experience developing similar products, the lower TRL scale might be omitted. When a shift of tech-nology occurs, however, what is needed is a knowledge-intensive approach; to deliberately and systematically move through the TRL scale. This means that in order to develop new advanced products a company must first understand the basic principles involved in the operation of the technology.
Figure 5 relates the costs involved in moving through the TRL scale ex-pressed as portions of the the total development cost. From the figure it is clear that development and changes of the product concept in the early levels of the scale is much cheaper than in the higher. This is another argument for the knowledge-intensive approach; if a greater understanding can be built at the lower levels at a lower cost, the total cost of the product development project might be reduced.
Figure 5: Average portion of total development cost at different Technology Readiness Levels compiled from data in Linick [8].
1.5
Purpose and objective
The purpose of this thesis is to show how the design of a mechatronic locking system can be implemented in the rotating gear shifter being developed by Kongsberg Automotive to fulfill the desired function to lock the rotating gear shifter. The thesis is investigating how a knowledge-intensive approach in the early development phase (approximately TRL 1 − 4) can be adopted in order to deliberately and systematically build understanding of the basic principles involved in the operation of the new technology.
To fulfill this purpose, the objective of the thesis is to study how a mecha-tronic locking system can be designed as well as how it can be optimized with regards to:
• Size: a smaller locking system allows for a more compact gear shifter system design.
• Cost: the cheaper the locking system is to manufacture, the more prof-itable the gear shifter system will be.
• Energy consumption: this is often of utmost importance in the auto-motive industry where all (in modern cars many) electrical components in the car must be supplied by a single car battery.
In order to fulfill the objective, four research questions has been formulated and are presented in section 1.6.
1.6
Research questions
The objective of the thesis has been broken down into a number of research questions (RQ’s) that the thesis aims to answer. Thus, to fulfill the objective the following research questions have been formulated:
1. Which mechatronic locking solutions exists on the market and what design concept should be used?
The first research question is posed to help choose (and motivate the choice of) locking system solution. It is thought to exist many possible solutions on the market that would fulfill the requirements and the challenge is thus to chose the one best suited in this particular case.
2. What design parameters will affect the performance, size, cost and energy consumption of the locking system?
The second research question is posed to identify the design parameters as well as how they will affect the final performance of the chosen locking system. It is important to build an understanding of how the system works in order to optimize it and by answering this questions a foundation for further design efforts is lain.
3. Can the system be simulated with regards to the identified design param-eters?
The third research question is posed to study the system in a simulation envi-ronment to validate how the different design parameters affect the performance of the system.
4. How will a prototype validate and verify the proposed solution?
The fourth and final research question is posed to answer if prototype locking systems will be well modeled by previous efforts. It is of interest to see how the system will perform in practice and the design of prototypes is deemed to be the best way to do this.
1.7
Delimitations
The focus of this thesis is to investigate the locking system of the rotary shifter and not the shifter as a whole.
In the literature study no hydraulic or pneumatic design concepts will be studied.
The mathematical model will not take into account how performance might be affected by external aspects, such as temperature and vibrations.
1.8
Report outline
The chapters of the thesis and their purpose is listed below: • Chapter 2 presents the theoretical framework.
• Chapter 3 describes the research methodology.
• Chapter 4 contains a literary study of locking system design concepts. • Chapter 5 presents a mathematical model of the system.
• Chapter 6 contains results from simulations of the system. • Chapter 7 describes tests performed on prototypes.
• Chapter 8 presents a summary of the results and a discussion of these. • Chapter 9 returns to the original RQ’s and presents the conclusions. • Chapter 10 describe suggestions for further research.
2
Theoretical framework
This chapter contains a theoretical framework of concepts and subjects of in-terest used later in the thesis.
2.1
Relation between chord and arc of a circle
The distance between two points on the circumference of a circle is called a chord (denoted Lc) and the distance along the circumference of a circle between two points is called an arc (denoted La) [9].
Figure 6: Illustration of how a chord between two points on the circumference of a circle relates to the arc between the same two points.
By studying figure 6 it becomes clear that when the angle ϕ is small the length of the chord is approximately the same as the length of the arc. The length of an arc of a circle can easily be calculated by multiplying the angle with the radius and the length of the chord can thus be approximated as equation 2.1.1.
Lc≈ La = r · ϕ (2.1.1)
2.2
Potential energy and conservative force
If the energy of a system only depends on its position or configuration it is said to be a potential energy. The most obvious example of this type of energy is gravitational energy, where (on Earth) you most often calculate the potential en-ergy of an object as a function of the distance from the ground: the enen-ergy only depends on the position of the object relative to the ground. Other examples of potential energy include elastic energy of an extended spring, electrostatic energy and magnetic energy [10].
If a force acting on an object only depends on its position it is said to be a conservative force: the work done in moving between two states is independent of the path taken. In a three-dimensional Euclidean space this means that the
conservative force, Fc, satisfies the derivative condition displayed in equation 2.2.1, where Eprepresents a potential energy. Note that the negative sign affects the direction of the force: if the potential energy increases in a direction, the force tends to move in the opposite direction to decrease the potential energy [10]. Fc= − 5 Ep= −( dEp dx , dEp dy , dEp dz ) (2.2.1)
2.3
Force of a spring
The force of a spring is proportional to the distance by which it is deformed (compressed or extended ). The relation between the force and the deformation is nonlinear, but can be described by a linear function when the deformation is small in comparison to the length of the spring. This was first done by Robert Hooke in the 17th-century and the linear relation, known as Hooke’s law, is presented in equation 2.3.1. Here F is the force, dx the deformation, and k is a constant characteristic of the stiffness of the spring (the ”spring constant”) [11].
Figure 7: An illustration of Hooke’s law and the actual force of a spring as functions of the spring deformation dx. a) A spring compressed by some distance dx. b) A spring in its relaxed position, where dx = 0. c) A spring extended by some distance dx.
F = k · dx (2.3.1) That Hooke’s law is a good approximation of the force when the deformation is small can be illustrated by figure 7, where an ”actual”, measured nonlinear, force of some spring is plotted with the approximation of Hooke’s law. It is clear from the figure that Hooke’s law is a good approximation as long as the deformation of the spring is small. Note that the spring is relaxed when dx = 0 and that the spring forces are always directed towards the point of relaxation.
2.4
Moment of force
Moment of force, often shortened to moment, sometimes also called torque, can be thought of as the ”turning effect” or ”twist” of an object. The moment of force at some point in a system is calculated according to equation 2.4.1 as the force at that point multiplied by the distance between the point and the center of rotation [12].
M = F · d (2.4.1)
Moment of force thus accounts for how forces exerted on the system are arranged and is used to explain how a rotating system will behave as a result of this. A simple example can be studied in figure 8, where the rotating system, in this case a seesaw, is in balance (the system is stationary and will not tip in either direction) due to two forces, of different magnitude, exerting the same moment of force as a result of their arrangement. The principle of moment of force also explains how a lever can be used to lift heavy objects and was first derived by Archimedes during antiquity [13].
Figure 8: Example of a system in balance due to the two weights exerting the same moment of force.
2.5
Magnetism
Magnetism is a class of physical phenomena mediated by the electromagnetic force; one of the four fundamental forces of nature. Magnetism exists due to the spin of electrons and knowledge of quantum physics is needed to accurately explain how a magnet works. This thesis will however focus on the design of an electromagnet and not explore the underlying quantum physics, as described in section 1.7.
All materials are in some sense magnetic since the fundamental particles they are made of are magnetic. What is usually meant with magnetism though is when the net magnetism of a material is enough to attract other materials. As mentioned the atoms in the material are magnetic; groups of atoms with the same alignment is often called magnetic domains, and can be thought of as small magnets within the material. Usually these domains are aligned in such a way that they cancel each other out, and the material will thus not be magnetic (it will have no net magnetism), as can be seen in figure 9a. However when an external magnetic field is applied, in some materials these domains align in response and the material will now become magnetic, as is illustrated in figure 9b.
Figure 9: a) Magnetic domains of a material canceling each other out. b) Magnetic domains of a material aligning in response to a magnetic field.
How easy the magnetic domains are to align depends mainly on the struc-ture of the material; how the different atoms are bound to each other. In some materials, such as iron, the domains easily align in response to a magnetic field, while in other materials, such as plastic, it is very hard to align the domains. In some materials the magnetic domains are naturally aligned without an ex-ternal magnetic field (or more accurately, once the domains are aligned they do not unalign again when the external field is removed) and these are called permanent magnets. In some materials the domains unalign again when the external magnetic field is removed. This is what is taken advantage of in an
electromagnet (it will only be a magnet as long as an external magnetic field is applied) [14].
An important difference between magnetism and electricity is that in an electric circuit there are actually electrons moving, while in a magnet there are no ”flows” or movement of anything; what happens is that magnetic domains align in response to magnetic fields.
2.6
Permeability
Permeability is a property of materials that measure how well they support the formation of magnetic fields. A higher permeability means that a material re-ceives a higher degree of magnetization when exposed to an external magnetic field. This property is often expressed as relative permeability, which is a mea-sure of how much higher (or lower) the permeability of the material is compared to the permeability of vacuum (which is a constant of nature and is defined as µ0= 4 · π · 10−7). For example a material with a relative permeability µr= 500 has permeability µ = 500 · µ0 [15].
Figure 10: BH curve of a ferromagnetic material. a) The initial magnetization curve. b) Positive saturation. c) Residual magnetism (positive). d) Negative saturation. e) Residual magnetism (negative).
The permeability of a material is generally not constant and is calculated as µ = HB (magnetic flux density divided by magnetic field strength). In the case of so called ferromagnetic materials (generally materials mainly composed of iron, cobalt or nickel; materials that will be used in the core), the plot of B as a function of H (a so called BH curve; the magnetic flux density B of the magnet
as a function of an external magnetic field H) results in a hysteresis curve (see figure 10).
The BH curve of figure 10 may at first glance be hard to interpret, but what it says is that when you increase H, B will increase to a certain point, and then stop increasing; the material will saturate. This follows logically if magnetism, as described in section 2.5, is considered: once all magnetic domains in the material are aligned the material can simply not become more magnetic than it already is [15].
Ferromagnetic materials saturate around 0.2-0.5 T (high permeability iron alloys used in transformers may saturate around 1.6-2.2 T ). Tesla, T , is a ”big” unit; if a magnet is the size of a coin and has a flux density of 1 T it can lift more than 9 kg. For example a typical refrigerator magnet has a flux density of about 0.005 T . Note that saturation is a property of the magnetic flux density and thus the size of the core does not matter: a larger core wont saturate slower or at a higher level than a smaller core of the same material would.
By studying figure 10 it becomes clear that as long as the material is not sat-urated the curve is almost a straight line. If the line is straight it indicates that the fraction BH is constant, and thus the permeability will be almost constant as long as the material is not saturated.
2.7
Electromagnetism
Electricity and magnetism are two phenomenon that has been studied separately since antiquity, but during the 19th-century physicists realised that the two phenomena were aspects of the same fundamental force: the electromagnetic force. The research culminated with James Clerk Maxwell’s 1873 treatise A Treatise on Electricity and Magnetism, in which he unified all the previous research into a single theory where the behaviour of the electromagnetic field can be described by four equations collectively called Maxwell’s equations. Maxwell’s equations, formulated as differential equations, can be studied in table 2.
Table 2: Maxwell’s equations.
Name Equation
Gauss’ law 5 · E =ρ
0 Gauss’ law for magnetism 5 · B = 0 Faraday’s law of induction 5 · E = −dB
dt Amp`ere’s circuital law 5 · B = µ0(J + 0dEdt)
Faraday’s law of induction states how a magnetic field will interact with an electric circuit to create a voltage. This phenomenon is referred to as induction and is an important principle used within a multitude of applications. Fara-day’s law of induction can, like all of Maxwell’s equations, be formulated in a multitude of ways and can for a tightly wound coil of wire, with N identical turns and an evenly distributed magnetic flux Φ, be stated as equation 2.7.1
[16]. The direction of the induced voltage will, according to Lenz’s law (formu-lated by Emil Lenz in 1834), be such that it opposes the direction of the current (expressed by the negative sign in equation 2.7.1) [16].
U = −N · dΦ
dt (2.7.1)
Amp`ere’s circuital law relates the current passing through a loop to the magnetic field around the loop. It can be expressed, formulated as an integral, as equation 2.7.2, where the free and enclosed current (If,enc) can be calculated by integrating the magnetic field (H) with a line integral around a closed curve. This is an extension of the original law by James Clerk Maxwell called the Maxwell-Amp`ere equation. The free and enclosed current is also equal to the magnetomotive force, F (see section 2.8).
If,enc= I
(H)dl = F (2.7.2)
In the case of a current conducting coil of wire, an alternative way to view a free current enclosed by a magnetic field is illustrated in figure 11. If a coil of wire with N turns is conducting a current I, the total current enclosed by the magnetic field H is equal to the number of turns multiplied with the current (since every turn conducts the current I). The free and enclosed current can thus in this case also be expressed as equation 2.7.3.
If,enc= N · I = F (2.7.3)
Figure 11: A coil with N turns conducting a current I, enclosed by a magnetic field, H.
Table 3: Variables of magnetic circuits and their electric circuit analogues.
Magnetic circuit Electric circuit
Symbol Parameter Unit Symbol Parameter Unit
F Magnetomotive force At U Voltage V
R Reluctance 1/H R Resistance Ω
Φ Magnetic flux W b I Current A
2.8
Magnetic circuits
A magnetic circuit is a model used to describe the workings of a magnet in a way that is an analogue to an electric circuit. This allows the use of methods and techniques developed to analyse electrical circuits to be applied to the magnet; significantly simplifying the calculations needed to describe otherwise complex magnetic systems [17].
A magnetic circuit is a closed loop containing a magnetic flux. The concept use a one-to-one relation between the equations of a magnet and that of a conventional electric circuit to calculate the magnetic fields of complex devices. The relationship between the phenomena in a magnetic circuit corresponds to the same phenomena in electric circuit. The well known Ohm’s law is displayed in equation 2.8.1.
U = R · I (2.8.1)
the analogue of this in magnetic circuits is called Rowland’s law (sometimes also known as Hopkinson’s law or Ohm’s law for magnetic circuits) and is dis-played in equation 2.8.2, where the variables are explained in table 3 along with their electric counterparts. Note that the analogue is purely mathemat-ical, meaning that the quantities described only fulfill the same mathematical role. The physical qualities, as well as the physics of the two theories, are very different (see section 2.5).
F = R · Φ (2.8.2)
Magnetic flux is an analogue of current and is defined as the surface integral of the normal component of the magnetic field B passing through the surface, described in equation 2.8.3. Using the simplified model of magnetic field lines discussed in section 2.5 the magnetic flux can be described as ”the number of magnetic field lines passing through a surface” [17].
Φ = Z
(B)dA (2.8.3)
Reluctance is an analogue of resistance and represents the opposition to magnetic flux in a material. Reluctance can be calculated if the dimensions and the permeability (see section 2.6) of the material is known as equation 2.8.4, where L is the length of the material, A is the cross-section area, and µ is the permeability. Using the simplified model of magnetic field lines, in a magnetic
circuit the field lines will follow the ”path of least resistance”, meaning the path with the lowest reluctance. As previously discussed the reluctance of vacuum is not close to infinity, as is the case of resistance of vacuum when discussing electrical circuits. A magnetic circuit will thus have a lot of leakage (magnetic flux that do not move within the circuit) when compared to the electric case where all the current are moving within the circuit [17].
R = L
µ · A (2.8.4)
If there are several materials with different reluctance in series the total reluctance of the circuit can be calculated by summing the reluctances of the different parts. This phenomenon is summarized in equation 2.8.5.
Rtot= R1+ R2+ ... (2.8.5)
Magnetomotive force (MMF) is an analogue of electromotive force (com-monly known as voltage) in an electric circuit. It represents a ”pressure” that ”pushes” electromagnetic flux through the circuit. The name may be misleading since a magnetomotive force is neither a force, nor is there any motion; there is no magnetism ”moving” through the circuit as previously discussed in section 2.5. The unit of MMF is ampere, A, but is frequently measured in ampere-turns, At, to avoid confusing it with electrical current which is also measured in amperes. MMF is defined as free and enclosed current, If,enc, and can be calculated by Amp`ere’s circuital law either as equation 2.7.2 or equation 2.7.3 (see section 2.7 for details) [17].
2.9
Dimensional analysis
Dimensional analysis is a method developed by Joseph Fourier in 1822 [18]. By comparing the dimensions of different physical quantities dimensional analysis can be used to check if an equation is meaningful: any equation should have the same dimension on both sides of an equality. This is called dimensional homogeneity.
The dimension of a physical quantity can be identified from its unit of mea-sure. As an example a physical quantity has the dimension ”length” (with dimension symbol L) if it is measured in metres, inches, and many more units of measure. Equation 2.9.1 is an example of a simple dimensional analysis: by adding two quantities with dimension length the result will be a new length: the equation is meaningful. If two units with different dimensions, for example a distance and a time, were added the equation would not be meaningful.
L + L = L (2.9.1)
Base quantities used in dimensional analysis and their corresponding dimen-sion symbols include length L, mass M , time T , and electric current I. More complex units of measure are expressed in base quantities, so for example the
unit newton (N ), expressed in base SI-units N = kg · m · s−2 [19], has the dimension M · L · T−2.
2.10
One-factor-at-a-time method
The one-factor-at-a-time (OFAT) method, also known as the one-variable-at-a-time method or monothetic analysis, is a method based on testing factors one at a time, rather than multiple factors simultaneously.
The OFAT method is a simple method that is preferred by many engineers due to being easy to implement and that the results are easy to interpret. In certain cases the OFAT method, even if it is much less complex than many other methods, tend to achieve greater gains than the more complicated methods [20].
2.11
Gaussian distribution
A Gaussian distribution, also known as a Gauss distribution, a Gauss-Laplace distribution, or a normal distribution (sometimes also informally called a bell curve), is generally given by equation 2.11.1, where µ is the mean and σ is the standard deviation of a random variable.
f (x) = 1 σ√2πe −1 2( x−µ σ ) 2 (2.11.1) Gaussian distributions are often used when the true distribution of a ran-dom variable is unknown. This is in part because of what in mathematics is known as the central limit theorem, which states that when several independent random variables are added, the distribution of the sum will approximately be a Gaussian distribution, even if the original variables did not have Gaussian distributions [21].
If the mean and standard deviation of a random variable that is normally distributed is known, the probability that the value of the random variable will be lower than some chosen value can easily be calculated using a Z-table (see Appendix A).
3
Methodology
The purpose of this thesis as stated in section 1.5 is to show how a knowledge-intensive approach can be adapted in product development projects. The thesis will thus combine theory with practice. The research process has included the-oretical as well as practical investigations and aims to produce results useful both to the scientific (academic) and industrial communities. This research process can be described by the model developed by Bj¨orn Fagerstr¨om [22] that is available in figure 12.
Figure 12: Schematic research process by Fagerstr¨om [22] in Eriksson [23]. The research presented in the thesis has followed a product development project through different stages of development. The different stages in the project relates to different Technology Readiness Level (TRL) scales (presented in table 1) with different maturity of the technology. The research started in the early phases, e.g. TRL 1-2 and has ended with a laboratory experiment, e.g. TRL 4.
3.1
Research method
The method used to best answer the research questions, described in section 1.6, is to perform four unique research studies that each aim to answer a specific question: one literature study and three case studies performed at Kongsberg Automotive.
Case studies can be used in five different applications: to explain, to describe, to illustrate, to explore and for a meta-evaluation. Each case study can also be categorized in three different ways: explanatory, exploratory and descriptive case studies [24].
used to explain phenomenons.
• Exploratory case studies are used to explore the situations in which the intervention being evaluated has no clear, single set of outcomes.
• Descriptive case studies are used for describing an intervention and the real-life context.
The studies that will be performed to answer each question as well as the type of study are listed in table 4.
Table 4: Description of studies performed to answer the research questions. RQ Basic method to answer RQ Type of study
1 Literature study of design concepts Literature study 2 Model of the locking system Explanatory case study 3 Simulation of locking system Exploratory case study 4 Testing of prototypes Exploratory case study
3.2
Literature study of design concepts
To answer research question 1; Which mechatronic locking solutions exists on the market and what design concept should be used? a literature study will be performed according to the following methodology:
1. Possible design concepts with the potential to solve the task at hand are identified.
2. The identified design concepts are studied with regards to how they func-tion and what applicafunc-tions they have been/are used in and evaluated with regard to how well they fit this application.
3. The different design concepts are compared and evaluated against each other and the concept that is deemed most promising is chosen to move forward with.
The first step of identifying design concepts will be done by studying a number of applications that use locking systems of different kinds and identifying which of these might be applied in this context. Note that it is believed to exist an uncountable number of solutions that could be modified to be used in the locking system so a judgement call must be made by the author on which solutions should be studied.
The second step of studying function and applications and evaluating the different concepts will be done by consulting literature on the relevant areas.
The third and final step of identifying a preferred solution will be done by comparing the pros and cons of different solutions with the demands on the
system defined by the customer (the automobile manufacturer). Note that this step also involves a judgment call by the author.
This is a practical product development project and the focus is not on finding the optimal solution; a task that may be impossible and that may take a lifetime to complete. The focus is instead on finding a solution that solve the problem. This mentality is well captured by a quote attributed to the former president of the United States Theodore Roosevelt, who said In any moment of decision, the best thing you can do is the right thing, the next best thing is the wrong thing, and the worst thing you can do is nothing [26].
A literature study aimed at identifying design concepts can be considered an activity aimed at making the technology reach technology readiness level one; basic principles observed (see table 1).
3.3
Model of the locking system
To answer research question 2; What design parameters will affect the perfor-mance, size, cost and energy consumption of the locking system? an explanatory case study should be performed where known physics should be identified and applied to the design concept chosen after conducting the first literature study. This should result in a mathematical model where the different variables rep-resent the different design parameters as well as identifying how they relate to each other. The mathematics and physics that will be applied is based on what is presented in the theoretical framework (chapter 2).
3.4
Simulation of locking system
To answer research question 3; Can the system be simulated with regards to the identified design parameters? ; an exploratory case study will be conducted at Kongsberg Automotive where the state-of-the art simulation software COM-SOL Multiphysics R will be used to create a simulations model of the locking system and simulations of the performance as a function of the different design parameters will be run according to the one-factor-at-a-time (OFAT) method (described in section 2.10).
The results from the simulations should also be compared to what the model derived in the explanatory case study predicted.
3.5
Testing of prototypes
To answer research question 4; How will a prototype validate and verify the proposed solution? an exploratory case study should be conducted at Kongs-berg Automotive where prototypes of the locking system should be created and subjected to experiments designed to give values that are comparable with the results from the other case studies.
4
Literature study of design concepts
The aim and scope of this case study is to answer the first research question in section 1.6: Which mechatronic locking solutions exists on the market and what design concept should be used?.
4.1
Problem description
There are many design concepts available on the market that could be adjusted to solve the problem of locking the rotating gear shifter. To find the design concept best suited to this application a literature study is conducted where a couple of different possible design concepts are studied. No design concepts based on pneumatic, hydraulic or thermal solutions are considered (see section 1.7) and the study will focus on electro-mechanical and electromagnetic con-cepts. To lock the system both linear motions used to plug the rotation and holding forces that hold back a blocker are considered.
4.2
Screw
Using a screw is a simple way of converting the torque of an electric motor to a linear motion (building a linear actuator ) that can be used to lock a system. If a screw is mounted in a worm gear, when the screw is turned the threads of the screw push up on the worm gear. A reaction force from the worm gear pushes back on the screw threads and in this way the screw moves forward, even though the screw is turning [27]. A screw-based design will lock and unlock the system by moving the screw, and will thus consume power when switching state (locking/unlocking the system) but require no power to maintain a state.
Figure 13: Illustration of how a screw converts a torque generated by an electric motor, ωm, to a linear motion, v.
The screw is considered one of five simple machines known to antiquity and were studied by Archimedes (ca 287 –212 BC) who greatly contributed to knowledge concerning its function [13]. The screw has a myriad applications,
besides use as a simple linear actuator, and has for example through history been used to transport water (= a water screw, see figure 14) and to hold building materials, for example wooden planks, together [27].
Figure 14: Drawing of a water screw used to transport water designed by Archimedes during antiquity [28].
The main advantage of using a screw-based design concept is the simplicity of the design, making it easy to understand and cheap to manufacture. The big problem with a screw design concept is high friction (which is the main reason screws are widely used to hold things together) reducing the performance and increasing the power consumption of a locking system. Another problem is that the design displayed in figure 13 would mean that the electric motor also is moving when the screw is turned, spending unnecessary power. This could be fixed with additional gear-based mechanisms but this would increase friction (and reduce performance/increase power consumption) even further.
4.3
Rack and pinion
A rack and pinion (or rack-and-pinion) is a simple linear actuator using a system where a cog (= a round metal part with small teeth) turns against a long bar that also has small teeth, and makes it move [29]. Combining a rack and pinion with an electric motor, the torque of the electric motor can be converted to a linear motion that could be used to lock and unlock a system. A rack and pinion generally consumes power when switching state between locked and unlocked and require no power to hold a state, but could be modified with a spring to only require power when switching in one direction (locking or unlocking) but require constant power to hold the other state.
Use of the rack and pinion is described in literature as early as 1621 [30] and despite (or maybe because of) the simple design of the rack and pinion it is used in a great many different applications still today. It has even been proposed that nanoscale rack and pinions could be used in nanoelectromechanical systems [31]. One nanometre is a billionth of a metre (1nm = 10−9m) and the nanoscale is used when discussing nanotechnology that usually measure around 1 –100 nanometres [32].
A rack and pinion would be simple to design and easy to implement. Another advantage is that an electric motor can be mounted orthogonal to the direction of the lock, which is good since in the gear shifter there is more room upwards and downwards than to the sides. The rack and pinion seems like a solid choice
Figure 15: Illustration of how a rack and pinion converts a torque generated by an electric motor, ωm, to a linear motion, v.
of design concept but a downside is that there will be many moving parts that will require good tolerances to manufacture, may be complicated to assemble, and that might wear over time.
4.4
Magnetic lock
A magnetic lock, sometimes called a maglock or an electromagnetic lock, is an electromagnet used to hold a device in place with a magnetic field when current are coursing through a coil (see section 2.5 and section 2.7). The function of a magnetic lock can be illustrated in figure 16.
Figure 16: Depiction of how a magnetic lock works. When a current is flow-ing through a coil the core become magnetic and attracts the rotatflow-ing shifter, locking it.
Magnetic locks are widely used to lock doors. In the United States of Amer-ica it was prior to the 1970’s illegal to lock perimeter exit doors from the interior side due to fire life safety issues. This was problematic in that it left facilities
vulnerable to breaking and entering. Magnetic locks would release in the case of a power outage or by a signal from the fire life safety system and were thus approved by the state fire marshals [33].
A future application for magnetic locks that is often used in science fiction is magnetic locks built into boots to allow ”walking” across metal surfaces (like the hull of a spaceship) in space. The technology has never been used in real life but NASA built prototypes as early as 1967 (see figure 17) [34].
Figure 17: An engineer walking upside down across the bottom of a steel beam using boots with built in magnetic locks developed by NASA in 1967 [34].
A magnetic lock could be a good solution due to no moving parts. In a magnetic lock there will be no physical latch that will stop the rotation; only the holding force of the magnet. The problem with the magnetic lock is that to get a good enough holding force the power consumption and/or the size of the magnetic lock might be to big.
4.5
Solenoid bolt
A solenoid is a coil of wire usually in cylindrical form that when carrying a current acts like a magnet so that a movable core is drawn into the coil when a current flows and that is used especially as a switch or control for a mechan-ical device (such as a valve) [35]. A solenoid bolt (sometimes called a locking solenoid) is a mechatronic locking system and take advantage of a solenoid to move a bolt of magnetic material into a position where it locks/unlocks a system. How a solenoid bolt works can be understood by studying figure 18. A coil of wire (the solenoid) is wrapped around a bolt of magnetic material. When a current is coursing through the coil a magnetic field will be created (see section 2.7). The magnetic field will move the bolt in the direction of the magnetic field until it reaches an end position. When the current is turned off the bolt will be pushed back by the spring to the original position. A solenoid bolt can be
designed to be either unlocked when a current is applied and locked otherwise (a normally locked solenoid bolt) or locked when a current is applied and unlocked otherwise (a normally unlocked solenoid bolt) [36].
Figure 18: Depiction of a normally locked solenoid bolt. a) When no current is flowing through the coil the spring pushes the bolt into a locking position. b) When a current is coursing through the coil the magnetic bolt moves in the direction of the magnetic field H and unlocks the system.
Solenoid bolts are manufactured and sold by many companies and are used in a large variety of locking and safety applications [37]. A solenoid bolt closely resembles a solenoid valve which is used to control the flow of liquids. Both solenoid bolts and solenoid valves are examples of electromagnetic reluctance actuators. Electromagnetic reluctance actuators have many applications, in-cluding in aerospace applications as electromagnetic brakes, in the manufactur-ing industry as valves that perform fast sortmanufactur-ing tasks and in the automotive industry to achieve variable valve timing in camless engines [38].
A solenoid bolt is a simple solution used in a number of applications on the market. It has the pros of being simple to design, cheap and with a good performance. The cons of the solenoid bolt is that, since it will move back to the original position by the spring if power is cut, requires a constant supply of electricity to function, something that may be hard to realize since power
Figure 19: One-Inch Stroke Shot Bolt Solenoid made by TLX Technologies [37].
consumption is an important factor when designing the locking system of the gear shifter. It is also somewhat limited in design possibilities since the motion is linear, the solenoid bolt requires space in the locking direction, and this space is limited.
4.6
Magnetic circuit
One way to build a mechatronic locking system is to design a magnetic circuit (see section 2.8). The principle of the solution is described in figure 20. When an electromagnet is supplied with a current, it attracts a latch that is used to lock the system. This is basically the working principle of an electromagnetic relay, with the difference of using the latch to lock a system instead of making contacts.
Figure 20: Depiction of how a relay works.
The most common device utilizing the principles of magnetic circuits are the electromagnetic relays. Electromagnetic relays are simple devices that were invented in the early 19th century. Today there are other types of relays that
is not based on electromagnetism but have the same function. Samuel Morse included an electromagnetic relay in his patent for a telegraph in 1840 to be used as an amplifier [39]. In 1937 Claude Shannon proved that relays could be used to perform the basic operations of Boolean combinatorial logic [40] and electromagnetic relays were used to build the first computers before the more effective transistors were invented. Today electromagnetic relays are still widely used to protect more vulnerable circuits (protective relays) when a fault is detected [41] and whenever a stronger current must be controlled by a weaker one.
The pros of a relay is that it can be designed with different form factors since the ”circuit” may simply be any loop of magnetic material. This greatly increases the flexibility when designing the locking system. It is also a very simple solution that is believed to be easy and cheap to design and implement and have few moving parts. The cons is a believed lack of performance when compared to other solutions.
4.7
Magnetorheological fluid
Magnetorheological fluid (MRF) is a so called ”smart” fluid, consisting of mi-croscopic magnetic particles suspended in a carrier fluid (usually an oil). When the fluid is subjected to a magnetic field the particles begin to align increasing the viscosity of the fluid [42]. A locking system design could be created where the shifter rotate in a MRF, and when locking should occur, an electromagnet might be activated, increasing the viscosity of the fluid, thus locking the shifter.
Figure 21: Illustration of how MRF works. a) No magnetic field is applied and the particles are evenly distributed in the carrier fluid. b) A magnetic field is applied and the particles align along the magnetic flux, Φ, increasing viscosity. An illustration of how a MRF works is displayed in figure 21. A magnetic field can be generated by an electromagnet and by adjusting the strength of
the magnetic field different viscosity (and thereby different rotating resistances of the gear shifter) can be obtained. The particles in the MRF will settle over time and the fluid must thus be changed at regular intervals.
MRF is a relatively new invention and a very active area of research. The predicted applications in the future are many. Possible applications include use in various medical equipment [43], robotics [44], brakes [45], transmissions [46], and many more.
A great advantage is that the viscosity of the fluid, and thus the resistance of the gear shifter, can be controlled by the strength of the magnetic field. It is noted that this function is not needed in this application and that everything else with MRF, due to the experimental state of the technology, is negative in this application. A concept based on MRF would be expensive, ineffective, complicated and require constant maintenance.
4.8
Summary
There are many mechatronic design concepts that could be applied to solve the problem of locking the rotary gear shifter and a number of solutions have been studied in this chapter.
Using a screw (section 4.2) seems inferior due to the friction which is thought to be high with the screw, and problems with designing a system where the elec-tric motor is not in the same line as the line of locking motion. This could be addressed with more gears but that would increase friction and reduce perfor-mance even further.
A rack and pinion solution (section 4.3) seems like a solid choice of design concept due to it being simple and effective. The downsides are that there will be many moving parts that will require good tolerances to manufacture, may be complicated to assemble, and that might wear over time.
A magnetic lock discussed in section 4.4 is eliminated from consideration due to the extra force that is deemed to be required to lock the system when compared to the other solutions that use a mechanical bolt or latch to lock rotation.
A solenoid bolt (section 4.5) seems like a solid choice of design concept due to ’being simple to design, cheap and with a good performance. The main problem with a solenoid bolt is that, since it will move back to the original position by the spring if power is cut, it requires a constant supply of electricity to function, which will be limited in the gear shifter.
Another solid choice of design concept is a magnetic circuit (section 4.6) The pros of a magnetic circuit is that it is a very simple solution that is believed to be easy and cheap to design and implement, have few moving parts, and can be designed with different form factors. The cons is a lack of performance when compared to other solutions.
It is easy to conclude that MRF (section 4.7) is utterly unsuitable in this ap-plication. MRF seems like a promising technology that may be critical in many future mechatronic systems but due to the lack of maturity of the technology, high cost, high power consumption, and need of regular maintenance it is today
not realistic to use. The main advantage of the technology is to have varying viscosity and thereby varying turning resistance but in this application this is not needed: the shifter system only needs to be locked/unlocked.
To compare the six investigated design concepts with each other, they will all be given scores in the range 1 − 5 (where low scores are indications of suitability to this application) based on five aspects that are all deemed to be important to consider when choosing a design concept. The five aspects that will be considered when comparing the design concepts are:
• Size: how large would the locking system be? – Low score: the system should be small.
• Complexity (Comp.): how complicated would the system be to manu-facture?
– Low score: the system should be easy to manufacture.
• Cost: how expensive would it be to manufacture the locking system? – Low score: the system should be cheap.
• Energy consumption (Energy.): how much electricity would the lock-ing system require?
– Low score: the system should have a low energy consumption. • Performance (Perf.): how fast would the design concept lock the
sys-tem?
– Low score: the system should lock fast.
The comparison is done in table 5. When the scores based on the five different aspects are summed up for each of the design concept it is clear that the magnetic circuit is the design concept with the lowest total score, and thus the preferred design concept of the six investigated.
Table 5: A comparison of the investigated design concepts. Design concept Size Comp. Cost Energy. Perf. Total
Screw 3 1 1 4 5 14
Rack and pinion 2 2 2 2 3 11
Magnetic lock 4 2 1 5 5 17
Solenoid bolt 3 3 2 2 1 11
Magnetic circuit 1 3 2 2 2 10
5
Model of the locking system
The aim and scope of this case study is to answer the second research question in section 1.6: What design parameters will affect the performance, size, cost and energy consumption of the locking system?.
5.1
Problem description
The chosen mechatronic system will include a magnetic circuit (section 4.6) with a latch that will be held in place by an electromagnet when the system should be unlocked. A spring will be used to apply a force opposite to the force of the magnet, and when the electromagnet is turned off the spring should move the latch into the locked position. Figure 22 shows a CAD sketch of the locking system.
Figure 22: CAD sketch of the locking system.
To model the system, and in that way identify design parameters and how they will affect final performance, the locking system will be divided into two parts: the magnet and the latch. The first part involves calculating the force the electromagnet will exert on the latch as a function of the electromagnets design parameters (section 5.2). The second part is the latch that will be a
balancing act of the spring force and the magnetic force (section 5.3). Together these two models will be able to describe the full locking system.
5.2
Model of the electromagnet
To calculate the force the electromagnet exert on the latch a simplified model is used to describe the system. The simplified model is presented in figure 23 and the corresponding design parameters is described in table 6.
Figure 23: Simplified model of the electromagnet.
To calculate the force with which the magnet pulls on the latch the first step is to calculate the total energy contained in the magnetic field. The law of conservation of energy state that energy can not be created nor destroyed; it can only be converted to other types, and thus the energy of the magnetic field is equated with the electrical energy needed to establish the coil current (assuming no losses).
