18.2 Microsystem enhanced machine tool structures to support sustainable production in value creation networks

Bernd Peukert1, Jan Mewis1, Mihir Saoji1, Eckart Uhlmann1 Stephan Benecke2, Rolf Thomasius², Nils F. Nissen³, Klaus-Dieter Lang2,3

1 Institute for Machine Tools and Factory Management, Technische Universität Berlin, Germany 2 Research Centre for Microperipheric Technologies, Technische Universität Berlin, Germany

3 Fraunhofer Institute for Reliability and Microintegration, Berlin, Germany

Abstract

The modularization of machine tool frames is a promising approach to support sustainable manufacturing in global value creation networks. The idea of designing single versatile lightweight and accuracy optimized (LEG²O) modules allows for innovative concepts with respect to mobility, configurability and adaptability. This contribution focuses on possible use-case scenarios that involve modular machine tool frames equipped with microsystems providing enabling functionalities, e.g. self-identification and provision of additional sensor data. The study provides a profound overview of potential capabilities and limitations of the proposed concept. As replacement, reuse and upgrade of single parts become critical issues when considering the complete product lifecycle, the question on how electronics integration can successfully contribute to a sustainable usage is in-vestigated. Keywords: Microsystem, Modular Machine Tool, Sustainable Manufacturing, System Life Cycle

1 INTRODUCTION

Accuracy, productivity and reliability are key attributes of machine tools in order to meet market demands of today’s

manufacturing environments. Global economic growth is strongly connected to employment and therefore to the wealth of people [1]. More than a quarter of the national output of Germany in 2012 can be attributed to the producing industry [2] and the biggest volume of sales in the engineering sector in 2011 was created within the section of machine tools [3]. Easily reconfigurable equipment meeting high production capacities [4] is evolving into a strongly requested technology to even further stimulate the market and cope with rising production volumes. This is of special concern with respect to flexibility and mobility of modern production systems. Some final products are available in countless possible combina-tions [5]. Together with shorter innovation cycles, this will lead to a rising need for an even more intensive, efficient usage of machine tools or parts of it. Previous attempts have been made with the focus on con-structing stiff and accurate but passive modular structures like ball-and-rod systems [6]. Those systems tend to be difficult to assemble or lack the required stiffness and dynamic behavior of conventional machine tools. The fundamental approach proposed within this work is to replace monolithic machine tool frames by a set of modular building blocks including active components. Therefore, the system will eventually be able to readjust and eliminate positioning errors. This is achieved through fusion of machine tool structures with micro system technology to bring features of distributed sensing and identification into the single modules, Figure 1. Moreover, active mechanical building blocks being part of the mechani-cal frame serve as compensating structures to account for

static displacements. By integrating autonomous sensors, additional data can be provided to the machine tool’s internal

control loop resulting in a much higher local resolution to improve working accuracy.

Figure 1: LEG²O concept for milling machine

Along with advanced interconnect technologies for electronics miniaturization comes an increased flexibility towards possible sensing locations and least interference with the mechanical frame, that would otherwise be difficult to obtain with conven-tional wired sensors. This leads to significant advantages with respect to potential system upgrades in the future, e.g. condi-

G. Seliger (Ed.), Proceedings of the 11th Global Conference on Sustainable Manufacturing - Innovative Solutions ISBN 978-3-7983-2609-5 © Universitätsverlag der TU Berlin 2013

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tion monitoring or modifications as part of the component re-use activities. This contribution outlines how the proposed concept of modu-larized machine tools can conduce towards more sustainable solutions whilst meeting market demands. Issues of sustaina-bility have to be tackled on different abstraction levels [7]. First, reduction of environmental impacts directly associated with the technologies and materials is in the focus of re-search. This comprises the complete life-cycle of the system at a whole. Second, effects on the superordinate system hosting the proposed technological solution need to be taken into account (e.g. effects of micro system technology imple-mentation on energy-efficiency or accuracy of machine tool). On further levels of abstraction substantial influences of revolutionized technology paths on general sustainability issues of manufacturing have to be analyzed. This includes the behavioral changes in machine tool use through techno-logical innovations. Being crucial part of mechanics and electronics design phase, first and second order effects are of primary concern within this work at the current stage. Funda-mental requirements towards system design of electronics and mechanics are derived from possible use cases for the proposed concept in order to meet the overall goal of support-ing sustainable production. 2 HISTORY OF MODERN MACHINE TOOLS AND LEG²O

EVOLUTION

Machine tool systems have evolved rapidly during the last 70 years as a result of various technological developments in the manufacturing world. The study of significant innovations helps to understand the evolution of machine tools with re-spect to the demand for accuracy and flexibility in the prod-ucts being manufactured. Fehler! Verweisquelle konnte

nicht gefunden werden. presents the timeline from the 1940s to the present, demonstrating improvements in achiev-

able machining accuracy along with a broad overview of the developments in production and micro system technology. Achievable machining accuracy is a valid representation of the level of technological advancement of the respective era [6]. The 19th century saw inventions of different machine tools and the development of the basic fundamentals of machining. Until the invention and development of numerical control (NC), most of the machines were controlled mechanically. Competition for products was local and there was no demand for variations in the product [8]. In early 20th century was the start of the automotive industry development and the con-cepts of mass production and tight dimensions which led to the improvement in the known machines [6]. The automatic control systems developed for military in the Second World War found practical applications in engineering and technolo-gy [8]. As a result, numerical control was developed during the late 1940s and 1950s with the help of servomechanisms and punched tapes for input. With the development of electronic calculators leading to the first electronic computer using vacuum tubes in 1946, a new era for manufacturing began. It is noticed, that the use of computers in the early phases of development of NC ma-chines was considered but because of the high cost of com-puters, special purpose control units were used instead [9]. Following the concept of NC for machines was the beginning of development of programming languages like APT, the invention of machining centre, expansion of large-scale as-sembly lines and mass production. After the advent of mini computers in 1960s, computer numerical control was intro-duced which dramatically affected manufacturing by in-creased production rates, improved quality and accuracy, more accurate control and easier integration. The 1970s saw the growth in computing power with developments like the programmable logic controller (PLC), first microprocessor by Intel (4004) in 1971 and the beginning of development of

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Figure 2: Timeline of machine tool and microsystem technology

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geometric modelling and computer aided design and manu-facturing (CAD/CAM) techniques [8]. Personal computers (PC) also came into existence through Radio Shack, Com-modore and Apple. This development in computing was implemented in manufacturing in the 1980s. Digital control was introduced for peripheral equipment, speciality machines and PCs were used to track the machining parameters along with in-house machinability databases [10]. Mehrabi, Ulsoy and Koren have reviewed the literature re-garding the development period of manufacturing technolo-gies and divided it in three epochs viz. pre-CNC (pre-1960s), CNC (1960 - 1990) and knowledge epochs (post-1990) [8]. Fehler! Verweisquelle konnte nicht gefunden werden. shows, that the pre-CNC epoch was an era when the funda-mentals of machining were developed and the new concept of numerical control was born. This idea of automation proves to be the first node for development of integrated electronic systems into machine tools and leads the way for further research and developments in the CNC epoch (1960 - 1990). Consequently, by the end of that era, the invention of flexible automation or flexible manufacturing systems changed the outlook on manufacturing systems as a whole. A flexible manufacturing system (FMS) is a machining system with fixed hardware and fixed, but programmable, software to handle changes in work orders, production schedules, part-programs and tooling for different parts [8]. Thus, flexible manufacturing was the first step in giving a degree of freedom to the manufacturing system so that it can be adapted to produce different parts that change over time with required volume and quality. The next development, reconfigurable manufacturing systems (RMS) which focus on modularity, can be seen as the next level of flexibility. Those systems propose a library of machine elements which could be assembled according to the specific requirements. The last 20 years have been characterized by intense global competition and progress in computer, information technolo-gy, management of information systems, advances in com-munication systems and penetration of computer technology in various fields. This has led to a market of high global com-petition and fluctuating demands which is reflected in the manufacturing paradigms that are currently being developed. The introduction of first universally applicable wireless sensor platforms [11], able to operate autonomously with a set of conventional batteries for many years, was a logical conse-quence of the developments in the microelectronics field during the early 1990s. Integrated circuits had advanced to a degree that power aware routing along with significantly reduced device startup power and improved sleep modes made average power consumption in the microamps range possible. Breakthroughs in micro-electro-mechanical system (MEMS) technology provided sensor concepts, which allowed for hybrid system integration of multiple sensing principles regarding component size, cost and energy requirements. By transferring advanced interconnect technologies onto the class of autonomous microsystems it was shown, that com-plete functionality (sensing, processing, wireless data trans-fer, energy supply) would comply with system dimensions in the millimetre-range [12]. These developments along with then available, energy densities of lithium-based batteries and wireless communication standards lead to first commercial applications, e.g. building automatization, of wireless sensor nodes (WSN). WSN were introduced into machine tools in [13] for tool parameter optimization in existing structures.

Challenges regarding harsh environmental conditions and prolonged lifetime through energy harvesting from the ambi-ent environment were approached in [14]. Microelectronic condition monitoring systems were adapted to a paper mill with the focus on housing all functional components within a single, wireless system. Main innovation hubs at the current level of machine tool (MT) and microsystem technology (MST) fusion (MT-MST) can be summarized as follows:

Distributed sensing of temperature and/or forces at the frame for accuracy optimization by contribution of additional data to the control loop of the machine tool

Identification of machine parts and their history in flexible and modular design approaches

Sensing and/or transmission of consumption relat-ed data to improve efficiency of manufacturing equipment

Additional functionality through condition monitoring capabilities at selected spots and hardware at loca-tions that are difficult to reach with conventional, cabled sensors and/or at spots that are objected to steady adjustments due to tool reconfiguration.

Figure 3 comprises the development path described: Now that the implementation of WSN even in harsh environmental conditions has successfully been proven from a technical perspective, strategic research questions concerning their sustainable usage arise. Positive second level effects on sustainability in manufacturing environments (improvements in yield, efficiency and effective usage of equipment) are undisputed and have to be quantified by adequate environ-mental indicators in further periods of the project. Direct, first level effects of mechanical and micro system parts have to be evaluated against the background of environmental impacts reduction during the complex LEG²O life-cycle. The particular importance for the case of modular machine tools is predicat-ed in the increasing number of potential sensor interfaces and therefore quantity of electronics. Moreover effects of aging, wear and obsolescence have to be considered on different technological levels as these will ultimately determine post production services. Considering the latter already at the design phase of the tool, will contribute to its sustainable usage of the tool along the complete life cycle.

Figure 3: Development of wireless sensor networks (WSN)

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3 ANALYSIS OF USE CASES

3.1 Compatibility of LEG²O with conventional tools

The LEG²O vision is to replace conventional monolithic ma-chine tool frames with microsystem enhanced, universal modules. As every structure can be theoretically approximat-ed, some machine tools benefit less from LEG²O, since their process simply does neither require reconfigurability nor flexibility. In this section, the authors will evaluate the compat-ibility of the LEG²O concept under consideration of both, feasibility and specific advantages towards conventional machine tools. For this purpose, a comprehensive list of the most common produced machine tools of Germany in 2003 is used to evaluate their compatibility with the LEG²O modules [15]. They divide into cutting machines and forming machines. Data is derived from a broad recherché of current market and the grading bases on the comparison of different machine tools. At the end of the evaluation, adequate use-cases for modular machine tools are discussed on the basis of the tables’ grades. The results can be found in Table 1.

In Table 1, technical feasibility defines the ability of replacing conventional machine tools and building types with LEG²O modules. The grading ‘+’ suggests, that the modules can resemble most parts of the machine tool whilst ‘-‘ indicates, that the machine tool requires a high number of application-specific add-ons besides the reconfigurable, standardized modules. Therefore, a high technical feasibility implies a good use case for LEG²O modules. Configurations define the process’s demand towards a high degree of possible machine tool configurations, e.g. a milling machine tool has different spindle orientations and hence synergies with the LEG²O approach. Accuracy compares the overall tolerances of the

named machine tools which are an indicator for feasible usage of the accuracy optimizing strategies like wireless sensor data and active blocks. Flexibility is about future de-mands towards adaption of the modularized machine tool to new product requirements [16]. It motivates efforts to increase intelligence in the single LEG²O modules, i.e. identification, enabling them to be part of a steadily evolving system within the context of global manufacturing environments. 3.2 Results from compatibility analysis and develop-

ment of use-case scenarios for LEG²O modules

As it can be seen in Table 1, the class of forming machine tools has generally a lower accuracy and flexibility when compared to cutting machine tools. These processes are primarily used for less complicated tasks that do not require high precision tolerances, e.g. punching, bending, edging. Another broad application field is the preprocessing of semi-finished workpieces for later processing like milling or turning. Also there is only a low demand for flexibility considering today’s market. In the field of cutting machine tools, the conditions differ as there is a strong dependence upon the specific cutting pro-cess. The trend towards more flexible and productive tools results in a variety of different machines [17]. Cutting as a manufacturing process is separated into six groups which in turn subdivide into 39 sub-categories [18]. Most of the con-ventional machine tools are specialized in one of the sub-categories but with machining centers, FMS and RMS the foundations are laid for flexible manufacturing systems. As cutting accuracy is directly bound to the tolerances of the machine tool itself, there is a strong need for fitting the re-quirements of dimensional accuracy and surface conditions in an economic way. The class of cutting machine tools is often distinguished not only in terms of the specific construction, e.g. horizontal or vertical milling spindles, but also in accuracy and processing power. In this context, new terms like high precision machine tools and high performance machine tools have established. As a result of the comparisons made in Table 1, Laser-, Ions- and Ultrasonic machine tools, machining centers and milling machines tend to be a good use case for the LEG²O frame building set. All mentioned machine tools are highly accurate, flexible and available in different configurations. Furthermore, processes similar to milling like gear cutting or boring as well as lathes provide feasible scenarios. 4 RESULTING SYSTEM REQUIREMENTS FOR MT-MST

STRUCTURES

4.1 General requirements towards machine tool layout

In [17] it is mentioned, that the most important task for ma-chine tool frames is the correct positioning of functional parts as well as in loaded and unloaded condition. Therefore, gen-eral requirements for machine tool frames like static, dynamic and thermal stiffness exist. Depending on the machining type, the environment and application field, additional demands have to be satisfied. The LEG²O modules try to adopt the form and functionality of conventional machine tool frames. Therefore, when assem-bled, they must behave like conventional frames in terms of stiffness and dynamics. Besides operating conditions, ma-chine tool workloads need also to be considered, as they determine local forces and temperatures within the frame.

Table 1: Comparison of machine tools

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Electrical discharge machining + o + o Laser-, Ions- and Ultrasonic-MT + + + +

Machining center, flexible systems + + + +

Transfer machines o + + o Lathe + o + +

Drill machine + o o o Boring machine + o + o Boring machine comb. with milling + + + o Milling machine + + + + Sawing and separating machines o - - -

Honing and lapping machine + o + o Grinding and polishing machine + o + o Gear cutting machine + + + o

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Shearing machine + o o o Punching machine + o o o Corner notcher o - o -

Bending machine o o o o Edging machine o o o -

Straightening machine + + o -

Press + + o -

Forging press + + o -

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The blocks themselves can physically resist only limited forces before they exceed the maximal yield stress and de-flect permanent or even break. That is why considerations of base metal alloy and geometry of the modules are primarily focused on lightweight, high static stiffness at moderate tem-perature dependence and dynamic behaviour. The application field prescribes the basis functionality, which has to be offered by the modules and the periphery. As there are various machining processes applied in modern manufac-turing, requirements towards the specific machine tool frames differ significantly from process to process. For example, high precision milling tools need a stiffer machine tool frame than an electrical discharge machine tool or the frame weight of high speed cutting machines must be low due to the dynamic behaviour of the whole structure. To make up a complete machine tool, guidelines, spindles, actuators and more have to be connected to the LEG²O structure. Due to the modular approach, the tolerances of each block add up when it comes to assemble the structure. Additional components like linear guidelines need at least flatness and parallelism of a few micrometers. Therefore, the modules tolerances have to compete with requirements in terms of dimensions, angularity and parallelism. When it comes to assemble, re-use or exchange of single blocks, the mechanical connections becomes important. Although bolting one block on another is an easy method to make up LEG²O structures, this is rather unfavorable for disassembling. Permanent connections like gluing and weld-ing complicate the reuse and recycling as well. Detachable connections like quick-clamping, magnetic or pneumatic fixing could solve the problem and contribute to easy and time-efficient assembling. The intermediate environment has a major influence on making design choices. As the LEG²O blocks have a maximum weight of 30 kg, there is a great potential for simple manual transportation. Hence, new pro-duction strategy like building machine tools near the applica-tion field becomes possible. A mobile open-air production is exposed to sun and therefore large deflections result from temperature changes. In a laboratory, the temperature is almost constant and the frame is only deflecting due to the actuators and process temperatures. Dirt might pose chal-lenges to moving parts of the machinery if operated outside a clean machine hall. Other environmental factors such as humidity or moisture require modified surfaces to protect the material from corrosion. Implementation of this new technolo-gy could lead to serious drawbacks when considering differ-ent locations and backgrounds of service personnel. Along with maintenance provided by the manufacturer of the tool, support for correct assembly, modification and operation should be part of the overall service concept. Besides intuitive design, software-tools could simplify planning and usage of the LEG²O system. Use of MST for in-situ monitoring of mod-ule handling (e.g. analysis of three-axial acceleration) and provision of feedback to the worker is another valid option. 4.2 Implementation of micro system technology in

modular machine tools

Increased functionality at low costs and minimum back cou-pling to the superordinate mechanical system is the main motivation for micro system technology implementation into the frame. General criteria describing the requested system functionality as well as the environmental conditions can be directly derived from the requirements catalogue of the ma-chine tool itself. Most common tasks will cover the distributed

sensing of temperature values, forces or humidity with strong-ly varying requirements towards spatial resolution and accu-racy. The context of the application (data provision to the control loop of the machine tool, condition monitoring of sub-systems or acquisition of tool handling related data) will con-sequently reduce the set of technical options regarding choice of components, achievable degree of miniaturization and operating time without battery exchanges. Nevertheless, the environment of machine tools provides particular challenges that must not be neglected within system design (Figure 4).

Given the particular task of the targeted application, minimiza-tion of energy consumption will be the primary focus to pro-long wireless operating time. Design decisions will most often be contradictory to further performance criteria, e.g. accuracy, data rates or duty-cycle, which should therefore be carefully chosen to avoid overdesign. Testability, adaptability and modification will become more challenging with every step in the design phase of highly integrated MST solutions. This is due to the nature of application specific realizations for WSN that are tailored to the specific measurement tasks in order to reduce energy demand to a minimum. For a precise definition of micro system functionality in the context of distributed sensing for accuracy optimization, a modular test platform based on commercially available components was imple-mented. A modular test platform based on commercially available components was implemented. Main components are a 3-axis acceleration sensor for handling support of the blocks and a temperature sensor to access temperature distribution within the frame. Processing is done using an ATmega328. Evaluation of sensing concepts as well as rou-tines for data processing is carried out during the conception-al phase of the LEG²O frame. The system can currently be addressed via Bluetooth or standardized I²C and SPI interfac-es for rapid sensor evaluation. Design guidelines for the final demonstrator are then derived from the results of the analysis after coupling of electronics with currently developed prototyp-ical mechanical building blocks. 4.3 Challenges towards sustainable development -

Multi life cycle approach

The most crucial aspect in MST design for machine tools evolves from the fusion of two complex technical domains with individual life cycles. Machine tools exhibit use times estimated around 20 years and more [19] in combination with high workloads. Depending on the environmental loads occur-ring at the specific mounting location these lifetimes might be challenging for WSN. Concepts for electronics condition monitoring in combination with design rules for robust system setup can prolong necessary service intervals. However, interruptions for battery exchange will be necessary in order to achieve adequate system dimensions. Furthermore, MST is objected to explicitly shorter innovation cycles.

Figure 4: Design criteria for MST in LEG²O modules

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A comparison of functional improvement rates for core com-ponents in electronics equipment is shown in Figure 5. In addition to effects of aging and wear, aspects of obsoles-cence need to be considered. Within the targeted lifespan it is rather likely that production of technically superseded compo-nents is discontinued. Moreover, supporting technologies within the infrastructure of the device, e.g. wireless communi-cation using standardized protocols, might no longer be avail-able. There will be break-even points for replacement of efficiency improved parts from an environmental point of view that lead to exchanges even before end of life. The key to sustainable solutions lies in the alignment of measures for repair, exchange or upgrade of MST with regu-lar service intervals of the machine tool structure. Therefore, the ability for multiple modifications of MST during the lifespan of the machine tool (multi life cycle of MST) has to be consid-ered already in the design phase of the system. This includes design for recycling of electronics devices or single functional units, accessibility to mounting locations and removable connections. In terms of upgradability, interfaces between MST and MT need to be precisely defined.

5 SUMMARY

In this contribution, the evolution of the LEG²O concept as a result of latest trends in manufacturing equipment and wire-less sensor systems was described. Driven by the need to define adequate use-cases that involve LEG²O equipment, an analysis of compatibility with conventional machine tools was carried out. The identification of possible applications for modular machine tools will lead to a further concretization of requirements towards the prototypical implementation of the proposed concept. However, from analysis of the special class of wireless sensor systems wedded to machine tool frames, a core set of general criteria for system layout was identified. Beside the technical realization of distributed sens-ing in LEG²O modules, main focus lies on the development of routines for sustainable implementation of MST-MT structures in the context of aligning multiple life cycles of its subsystems. 6 ACKNOWLEDGMENTS

This work was funded by the Deutsche Forschungsgemein-schaft (German Research Foundation) within the Collabora-tive Research Centre (SFB) 1026.

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Figure 5: Development rates of microelectronic components

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