Chapter 0: INTRODUCTION
In the age of mass production, dedicated assembly lines brought the work to the worker or robot to cut wasted motion and functioned best when identical products were made over and over using the same route and sequence of work. Today, flexible manufacturing systems seek to combine efficient use of individual workstations with flexible transport systems, permitting different routes for different products in the same factory and at the same time.
Assembly is a key process in modern production: only when assembled the components become a functional product for customers. Nevertheless, during product design, assembly is often neglected, especially when the product is based on new technologies.
As Onori states, it is no surprise that many national R&D project frameworks within the EU today are dominated by product development topics, leaving production engineering, robotics and systems sidelined.
Production systems which have truly changed the world placed the production system at the centre of attention, not the product. Assembly system constraints and production system constraints should be placed upon the product design. It is necessary to structure, formalise and control the assembly processes.
The goal is to reach a situation where assembly systems and products are made for each other. Transparency is necessary, in the sense that the influence of every single decision on both product and production system becomes visible. Systems and their architecture usually emerge from long iterations between potential users and the system architects and engineers.
Assembly systems should be designed to fit the structure of modular products – but it is also important to adapt the product design to the assembly possibilities, which we partially call Design for Assembly. A link between the design of products and assembly systems is required, in order to rearrange the system in response to new technologies, or redesign the products when better assembly possibilities have been elaborated.
Of course, traditional methods such as DFX should not be neglected – their usefulness is undisputed. A good example is that the most effective way of increasing the productivity associated with drilling and tapping holes in a finished part may be through the elimination of the need for the holes, rather than through optimisation of drilling and tapping operations.
0. 1 Today’s assembly systems
Assembly has a significant technological retard compared to other production processes. As a consequence of this lack of knowledge, diverse phenomena happen to appear. It is not unusual to have big problems with blocked installations – blocked because of a wrong turned or non-conformal component. But not only every-day problems make the use of automatic assembly difficult, also medium- and longer term aspects bring challenges.
Most of today’s assembly systems are still fully or partially dedicated and only of very limited flexibility. An important point to justify the investment for a flexible assembly system is that dedicated installations can only assemble its “own” product. If something else should be produced, the installation can eventually be adapted – but this is also an expensive and laborious procedure.
On the other hand, flexible installations can treat many kinds of different products with minor changes or only intended setting modification. But also flexibility has its limits.
This is why the elaboration of a method for the subdivision of assembly systems into functional, modular components, resulting in an evolvable installation, is necessary.
Generally, automated assembly systems and operations should be more studied and their development should be strengthened. There is not only need for modern assembly systems, but also for disassembly installations: reasons can be recycling and reverse engineering.
There are two traditional schools of thought for building assembly machines:
each installation could be a unique creation, designed for a specific task and with no or little resemblance to anything that has existed before or will come in future – or one bundles together a selection of known items to perform this specific demanded task, but also offering general flexibility.
The recently elaborated third way of thinking is more systematic and focuses on optimum accuracy, repeatability and performance. Few assembly operations are always the same – but many exist in similar form when compared between several assembly installations. For a modular approach, not only basic assembly tasks should be standardized but also tools for control, supervising of the operation’s completeness / correctness and basic test functions.
A big advantage of modular systems is that most of the usually needed elements already exist and they can be re-arranged and step-by-step expanded when the assembly requirements change.
Working with known and available standard elements significantly decreases the time and monetary investment necessary to elaborate a new installation. The use and reuse of standard elements guarantees also a higher reliability: it is possible to work more on the single modules and optimise them because the same module types will be repeatedly used. An important point when modularising assembly lines is the installations availability. It is important to keep in mind that the whole system is maximally as reliable as its weakest component.
System requirements are based on the current trends in manufacturing:
• assembly on customer order from product modules
• shifting production / assembly volumes according to customer orders, therefore the need for flexible systems
• shorter product life spans, demanding frequent system reconfigurations • high number of product variants
• just-in-time delivery, shorter time to market.
• need for products which assist the production and assembly processes
• giving the product its final identity as late as possible, means towards the end of the assembly process
• final assembly could be KANBAN controlled
The request of modular and re-configurable assembly systems is now more and more pressing . Flexible assembly modules would also be a helpful tool for the production of pre-series because of their fast reconfiguration / retooling and broad usability.
Flexible modules could be used to assist the main installations in time of production peaks. It is nevertheless vital to well analyse and understand the real needs of today’s industry.
0.2 Outsourcing
The European electronic industry, as most other OECD areas, is confronted with major potential challenges in the decades to come. These are varied and range from the ageing population to new ethical requirements, the widening globalisation of markets, information technology progress, as well as the forming of regulations curbing energy usage and pollution control.
To be able to afford manual assembly work, many companies are forced to move their production to low-salary countries, often combined with the collaboration with Contract Manufacturers. But when out-sourcing production, one loses control over the processes which may render one’s products unique. Often the knowledge who to do things are a core-competence of companies.
Unfortunately, in Europe these challenges have been predominantly met by outsourcing, with assembly being the major sector affected. Analyst IDC reported that the total value of the 100 largest European outsourcing deals signed in 2003 increased from $19bn to $44bn. So outsourcing is increasing, and if analysed by sector, governments were found to have committed most heavily to outsourcing, accounting for almost half of all major deals.
Of course, not all outsourcing is detrimental, and in some cases may also be beneficial: if one only looks at the short-term customer aspects, than quicker delivery, cheaper prices, and wider availability might sound compelling. However, one must take a close look at what is being outsourced, in which way, and at which rate.
The outsourcing problem is real: McKinsey & Co., the USA, Europe and Japan are losing approximately 600,000 jobs/year within the manufacturing sector, a fact rendered even more serious by the Gartner Inc. study, which notes that this trend is likely to maintain its course until 2010 and result in the loss of 25% of high-technology jobs to emerging markets in India,
China, and elsewhere. According to Forrester Research, 150,000 IT jobs will have moved outside Europe by 2015.
The current rates of outsourced jobs, as a percentage of the total outsourced in Europe, are given as:
• UK: 35%
• Germany, Switzerland, Austria: 22.8% • France: 12.8%
• Italy: 7.7%
• Nordic regions: 7.2% • Spain & Portugal: 4.6%
The above figures relate to the outsourcing of manufacturing operations. Within these, there may be a series of operational classes, ranging from final assembly to product design. However, outsourcing has now gone as far as to include R&D operations, which is particularly alarming as this may be the cause of losing the core-competencies of the companies.
0.3 Manual vs Automated Assembly
Today, an important decision often has to be made by the companies which is about to produce a new product: to assemble manually or automatically. The choice between manual or automatic assembly must be made before starting the production, since it is still very expensive to make changes later on.
Of course, both manual and automated assembly have advantages and drawbacks. Manual operations allow high flexibility in assembly processes, but on the other hand they bring also high exploitation cost for workers’ salaries, often higher and varying cycle times due to a wide spectrum of individual performance, the risk of diverse errors and incidences. Therefore, the balancing of manual assembly stations is a tough task. Automated assembly, on the other side, has low exploitation cost but involves higher initial investment, depending on the strategy; modular systems allow of course more freedom of choice.
Additionally, when out-sourcing manufacturing and assembly, the difficult low-volume initial series with many variants are most often kept in-house, while the high-volume production series are given away to other companies and/or low-wedge countries. This means high cost for the retained part – another drawback. It is only a question of time until the low-wage countries will take over the whole business, and Europe will lose entire industries, including the employment opportunities.
This means that we have to find ways to be able to keep production including assembly in Europe; automation is inevitable.
Some more reasons for automation:
- product quality can be improved through higher reliability (less assembly defects) than made by manual workers.
- environmental issues: certain tasks should not be executed by humans because of toxic environments or high injury risk.
- lack of workforce because of the increasingly critical view of youth on industry (but this problem will probably stay unique in Japan – in other countries, people are glad to have a job). - minimization of products: there are new trend arising such as the miniaturization of products, which leads to difficulties in manual assembly. If the parts become to small, manual assembly becomes more or less impossible.
- uncoupling of skill and capacity through automation allows to follow the market demand more easily: machines can often more easily be put on higher schedule, stand-by, bought, sold or replaced than human worker force.
The economic or social issues of employing persons or using robots are not addressed here, only performance differences and assembly-relevant questions are considered.
According to the task to be fulfilled, it might be better to assign it to a human worker; especially when asking for complicated movements with coordination of different parts at the same time, no machine will probably ever reach the performance level of human persons.
On the other hand, manipulators or robots are appropriate if repetitive and simple movements have to be executed, if high speed and high repeatability are required.
The consequences of part non-quality are another fundamental difference between manual and automated assembly. To verify part or assembly quality, instead of 100% tests, the AQL (Acceptable Quality Level) concept is often used: AQL defines the acceptable number of defect parts in a lot. If this number is exceeded, the entire lot is rejected.
Typical values are:
Manual assembly AQL = 1
Semi-automatic or mechanized assembly AQL = 0.65 Automatic assembly AQL = 0.4
This means that a certain number of defects parts will be met during assembly, and one of the critical factors is the way a human person, a manipulator or a robot react to them. Some basics are given below.
Manual assembly:
• clearly defect parts are eliminated without any special effort or cost: human workers can focus on correct parts
• defect part elimination is very expensive if 100% verification is necessary • rare defects pass through verification
• defects on non-functional dimensions do not disturb • non-normal situations are easily identified
Automated assembly:
• very sensitive to component quality • integrated test necessary
• sensitive to geometrical defects which have not been registered before
• generally, non-registered defects pass through verification: automated tests search for registered defects, non-registered problems are not recognized
Human Worker Robot Manipulator Arm movements - DOF - Precision of positioning, without feedback - Rigidity, eigenfrequency - approximately 30 per arm - 10 to 50 mm - approximately 0,5 Hz - 4 to 6 - better than 0,2 mm - 10 to 50 Hz - 2 to 4 - better than 0,2 mm - 10 to 50 Hz
Force of the arm - maximal usable force - with controlled position - up to 100 N - approximately 5 N -according to the chosen system -according to the chosen system “Feed-back” - proprioceptive - visual - tactile - rough estimation, fugitive - auto-adaptable, intelligent - auto- adaptable, intelligent - very precise, permanent - possible in easy cases - only verification of limits - “all or nothing” - not used - not used Task flexibility - Adaptability to variant changes -Simultaneous handling of different variants - Resistance to stress - excellent - risk of confusions - limited if the charge is over 80% of the capacity - very good - very good - excellent - limited - very limited - excellent
Quality assurance - traceability - interaction with a measuring device - mastery of procedures
- difficult and low certainty
- slow and low reliability - limited reliability - excellent - excellent - highly repetitive - lack of verification - reliable - highly repetitive Economic criteria - investment - exploiting cost - starting cost - very weak - according to the country - very weak - high - weak - quite high - medium - weak - medium
Table 1. Human Worker vs Robot vs Manipulator
0.4 Micro-assembly
Beside the need for extreme capacity flexibility and stepwise expandability, an additional challenge is today miniaturisation. Micro-assembly brings many difficulties which are specific to small, light parts and their handling. Micro-products and their applications are expected to become a critical strategic industrial sector in the coming years.
Also integration of nanotechnology in mini- and micro-products will become a key issue. Similarly, mini-assembly equipment should be adapted for integration into normal-size systems.
Production systems become ever more responsive and agile. This is particularly relevant to micro-products, since with increasing miniaturisation manual assembly becomes impossible, rendering out-sourcing strategies less effective if note deliberately negative.
In microtechnology, assembly represents up to 80% of the production cost of a product. Nevertheless, when consistently using DFX, cost can be dramatically reduced. Especially micro-assembly has to be process-driven: product design may not be miniaturised without serious consideration of the required assembly processes, since a scaling down of traditional assembly processes is no viable.
In the millimetre and sub-millimetre range, the human hand becomes much less performing than in the range of the centimetre. For reasons of precision and profitability, the use of manual work is limited to 15-20% of the manufacturing cost and automated solutions have to be found to reach high productivity as required for industrial products.
It is important to make the difference between mechanisation, where a machine imitates the human manual movements, and automation, which consists of finding clever solutions to solve the same problems with most often simpler operations than those executed by a human worker.
When talking about semi-automated processes, a rationalisation of manual work is meant; giving the worker tools to improve the repeatability of certain movements and helping them to be faster can be a very effective but simple method.
Today’s micro-assembly systems cannot be considered as industrial products. They are all prototypes designed for a certain task – therefore are assembly systems always very expensive and coupled to a certain risk of failure or unpredictable troubles. Investments have to be well considered and ways to assure their amortisation have to be found. This is one of the reasons why it is so important to find better concepts for assembly installations, like modularity, example given.
Also in microtechnology, when designing and planning a product or product family, people do often not sufficiently consider how the assembly will be done. Design for manufacturing does often not cover the full spectrum of the production – design for assembly should generally be emphasized.
0.5 Flexibility
In the past years, flexibility and many of its different aspects have been discussed in literature and lots of projects tried to implement flexibility in their systems. The opinion about what kind of flexibility is required as evolved as well as the used approaches.
According to Sehti and Sehti [1990] at least 50 different terms for various types of flexibility can be found in manufacturing literature:
• adaptation flexibility • batch flexibility • capacity flexibility • change-over flexibility • design flexibility • expansion flexibility
• geometrical / technical flexibility • interior / exterior flexibility • machine flexibility
• material flexibility • mix flexibility • operation flexibility • part flexibility
• planned / unplanned change flexibility • process flexibility
• product flexibility • ramp-up flexibility
• re-use / re-build flexibility • routing flexibility
• variant flexibility • volume flexibility
Usually there are several terms referring to the same flexibility type.
Many authors like Brown, Dubois, Rathmill, Sethy and Stecke refer to a list of eight different kind of flexibility, that summarize the whole survey:
• Machine flexibility: the ease of making the changes required to produce a given set of part types.
• Process flexibility: the ability to produce a given set of part types, each possibly using different materials, in several ways.
• Product flexibility: the ability to change-over to produce a new (set of) product(s) very economically and quickly.
• Routing flexibility: the ability to handle breakdowns and to continue producing the given set of part types.
• Volume flexibility: the ability to operate an FMS profitably at different production volumes.
• Expansion flexibility: the capability of building a system, and expanding it as needed, easily and modularly.
• Operation flexibility: the ability to interchange tho ordering of several operations for each part type.
• Production flexibility: the universe of part types that the FMS can produce.
Not only do different types of flexibility exist, but flexibility itself also exists on different (vertical classification) levels.
• A company could be flexible by having a lot of factories to choose from when putting the products into production.
• An assembly system could be flexible by allowing different kinds of assembly cells within the system.
• An assembly system could be flexible by allowing different feeders to be used. • A feeder could be flexible by allowing different parts to be fed.
On the high level, it is much a management and strategy issue, whereas on the machine level it is more a question of technology.
Flexibility can also be classified horizontally, e.g. classified as internal/external or classified through the value chain. One important aspect of flexibility is the time horizon. Some situation demand flexibility within second, others may require years. Johansson and Erixon [2001] has identified three different time horizons:
• Daily demands.
• Dealing with new variants.
• Dealing with completely new products.
Tichem [2000] defines flexibility as the ability of a system (e.g. an assembly system) to react to changes that arise in the environment or in the system, and that have been foreseen to at least some degree.
Tichem argues that there exist three main types of flexibility for an assembly system:
• Capability flexibility. • Capacity flexibility. • Error recovery flexibility.
The capability flexibility refers to the system ability to react to changing market demands in terms of product variants. It can further be decomposed into:
• Flexibility in assembling products that belong to one product family.
• Flexibility in assembling products that belong to a number of product families.
• Flexibility in assembling products that do not belong to the product family or families the assembly system was developed for.
The capacity flexibility of an assembly system is its ability to react to changing market demands in terms of quantities. In general, the demand for a product is small from start, then growing, and will fall off again when the product’s economic life-cycle ends.
In addition, the assembly system may have to deal with short-term fluctuations on top of this cycle.
Error recovery flexibility refers of the ability to deal with errors that occur depending on internal disturbance. Tichem [2000]. It is very useful, to understand the issues, dealing by this thesis, to introduce the next classification of the flexibility:
1) Static flexibility
The configured installation remains fixed while a given product mix is assembled and is only changed when new a product family will be produced and another type of system is needed. Reconfiguration through exchange of specific tools, feeders and fixtures or entire rebuilding of the system is done “off-line” and the elaboration of the new installation has to be done while the new products are designed.
2) Dynamic flexibility
In this case, the flexibility is managed “on-line”: there are possibilities to adapt to different tasks while the system is running, through choosing between anticipated and prepared scenarios. This can be activated by a human user or by the system’s self-adaptive control, which would mean that the system has knowledge about its own possibilities and decision algorithms to decide about which modules to use and how to proceed in which case.
The following table summarizes these concepts.
Static or Physical flexibility Dynamic or Logical flexibility
Time to react:
Product life cycle phases
Time to react:
Very short
Why:
• Production volume changes • New variants in the same system • New products in the same system • Demand fluctuation
Why:
• Mass customisation, lot size one, assembly-to-order
• Disturbances, machine break • Repair work, rush orders • Demand fluctuation
How:
• Layout modifications • Size and degree automation • Re-configurability, re-utilization • Modularity, expandability • Scalability • Exchange of system module/submodule How:
• Control of tasks and resources • Use of information technology • Change of control programs, routines • Sorting and routing
• Robotic, flexible automation • Human intelligence and skills
Table 2
Under certain conditions, the two concepts could be combined: in a modular assembly system, one could also integrate robots and adaptive logistic services. For bigger changes, the installation could be stopped and reconfigured. But then for smaller modifications, this would not be necessary. If the normal functionality of the working system is not disturbed by adding modules, the system does not need to be stopped.
By modularising assembly systems, the negative correlation between static and dynamic flexibility, as shown on Figure 1, can be limited to single modules or entirely eliminated if there is no flexibility within a module.
Under certain circumstances, flexibility can also have negative aspects: flexible systems have lower efficiency, they require over-design, generic components, extra interfaces and changeover times. However, advantages and drawbacks have to be weighed out against each other.
In this project, the focus is mainly on static flexibility and reconfiguration. Modules can be exchanged, added or removed and allow the whole system to evolve – which is also called agility.
Certain modules can have different related functions, accessible by automated tool exchange orbselection of other programmes, which can be seen as dynamic flexibility without having the disadvantage of exaggeratedly heavy cost.
When designing “rigid-flexible” assembly systems, scenarios have to be anticipated, planned and realized at the beginning, when the system is installed. Of course, also the entire costs for all scenarios have to be paid immediately.
When working with exchangeable modules at contrary only the immediately needed modules have to be installed and paid – if the requirements change, modules can be added and paid whenever needed.
This means that through modularisation, automated assembly systems become affordable also for smaller companies with lower production volumes.
Another advantage is that when keeping the same base elements, companies can learn from their experiences and directly apply them whenever modifying the system by exchanging modules.
Figure 1
Ideally, a new product, variant or volume fluctuation could be managed without any adaptation to a given assembly system. In reality, this is of course not possible; the re-engineering phase is often heavy and expensive. Evolvability and modularity are concepts which aim at reducing this. The idea is that assembly systems should go through an evolution rather than adaptation. One should try to change the strategic perspective from system installation (known product launch) to re-engineering (future product launch).
