The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany

Flexible assembly system for hybrid micro systems using integrated process chains

J.P. Wulfsberg1, A. Kuhn1

1Laboratory of Production Engineering Helmut-Schmidt-University / University of the Federal Armed Forces, Hamburg, Germany

Abstract Taking a look at the strong growing demand for hybrid micro systems, the requirements for micro parts made of engineering materials like for example stainless steel increases. To date hybrid micro systems on stainless steel basis are often built by assembling several single parts. Observing the production of small and middle sized lots, mainly performed by SMBs, only a little automation level stands out: every step during the production cycle is designed and carried out separately, since flexible automated manipulation/ assembly systems are barely available for that kind of production. Combining machining and assembly processes (e.g. building process chains) could lead to cost savings and higher quality. To avoid transportation caused problems like the loss of reference and complex re-measuring procedures before performing the next machining or assembly step several processes can be combined in one working area. If one machine tool provides the working area for several machining operations or processes to be carried out, integrated process chains are an approach. Adding laser support to the machine would provide an excellent option for assembling and fixing several parts of one micro system. For micro welding, fibre lasers are providing the technology for the production of high quality welds.

Keywords: micro machining, micro assembly, integrated process chains

1 INTRODUCTION A lot of specifications meanwhile being demanded from micro systems regarding material and function are incompatible to semiconductor production processes known of micro system technology. As consequence hybrid micro systems, assembled from single subcomponents, have to be produced with in respect to material and geometry expanded manufacturing processes. Therefore assembly operations are essential and play a major role in the whole production of hybrid micro systems. Monolithic production is often theoretically possible, but economically it doesn’t make sense, since most of those hybrid micro systems to date (for example systems in medical technology) are produced in low to midsize volume production. A modular design is being preferred. Different functionalities and fields of application lead to a big number of various products, part geometries and materials. Those parts are product-specific and they often cannot be grouped in several part families. For such products, a high system accuracy is needed to perform assembly operations [1]. Looking at micro systems built of high-alloyed steel like they are applied in the field of medical technology, tolerances are needed in the lower two-digit micron range (in special rare cases higher tolerances up to the one micron range are demanded). To date, the industrially produced systems are assembled manually or semi-automatically, a flexible automated solution cannot be found on the market. In some cases there are automated assembly systems for special part families available. Those systems cannot be applied for more than slightly different parts. They are relatively inflexible and cause high investment costs. Flexibility is a requirement for operating automated

micro assembly systems in low to midsize volume production [2]. The currently available assembly systems for micro technologies are highly based on the use of accurate optical sensors, for example laser sensors or camera based image recognition, providing the necessary resolution to detect the parts position and orientation sufficiently. The high resolution comes along with a small working range of the sensor, for instance a small lens coverage with low depth of sharpness using cameras. Such sensor based assembly systems have to be adjusted and calibrated properly to the application. Regarding the production of complex three-dimensional geometries made of high-alloyed steel, these workpieces are mainly produced by milling and forming processes. During this processes, the workpieces position and orientation within the machining centers coordinate system is well known. This information can also be used for assembly operations. Removing the workpiece out of the machines working space, the position information and the references are getting lost: the workpieces have to be relevelled applying complex measurement procedures during the next production step. A feasible approach to realizing an automated flexible assembly system for micro parts and systems features the integration of manipulation devices into machine tools, known as integrated process chains. The process linking within in one working space allows the usage of the already known workpiece position information for further steps in production without a lot of effort. An intelligent arrangement of the single systems inside the working space and a reasonable production strategy could even lead to parallel execution of production processes [3]. Given that in general all available micro machining

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The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany centers are equipped with position sensing systems, peripheral devices, a field bus system will be attached to providing an accuracy within nanometer range, no the superior control program. Field bus systems provide additional sensors for position recognition have to be several modules like digital and analog input and output built in. By producing the single micro parts for a micro modules or so called intelligent modules with system inside one working space, the exact position and functionalities like serial communication or the analysis of orientation of those parts are known in the range of the strain gauge bridge circuits and more. The possibility of a machine tools system accuracy and can therefore be field bus controller integration makes an outsourcing of used for following operations. computing power additionally possible. Simple

computations or supervising functions concerning particular process variables could be passed to the field2 CONCEPT bus controller. The architecture therefore becomes

By linking of assembly and production processes, a decentralized. micro machining center is being upgraded to perform Later on, the superior control program shell provide aassembly operations. To keep the costs as low as graphical user interface to allow conditional linking ofpossible and to achieve the highest flexibility possible, single processes to complex operational sequences. Foroff-the-shelf components are being used. By reason that this purpose, it can be assumed, that NC-programs for theboth components, the micro machining center and the micro parts production and corresponding programs formanipulation system, have their own well developed the manipulation system being used for handling arecontrol, it is aimed for keeping on using those further on. available. Programs for manipulators as for exampleFor the purpose of linking both controls, they have to robots can be teached or computed via softwareprovide an interface to allow remote controlling and simulations using CAD data. Teaching means pathinformation exchange. An Ethernet interface for example definition by storing several manipulator positions relatingcould fulfill those requirements. To ensure the highest to the manipulators coordinate system including theflexibility possible, the single system components will be orientation. Additionally such manipulators can be connected using a superior control program, which equipped with sensors, offering the potential to implementallows the easy usage of different devices and different sensor controlled movements. To date, force torquecombinations (Figure 1). This control program will be sensors as well as optical sensors are mainly used for this equipped with adequate software drivers providing the purpose.functionality of bi-directional communication as well asfunctions to remote control the attached control systems. Adding a module to the superior control has to be done in

The main functions are the transfer and the allocation of hardware, which means getting all cables and the powerdata, the supervision, the control of the operational supply connected, and in software to give the superior sequences and the handling of status requests [3]. To control program access to the functionalities of the added perform assembly operations, the simple combination of device: the software drivers make process conform production and manipulation processes often doesn’t functions of the related devices available to the superior suffice. For instance it is necessary to perform cleaning control program. Combining several functions of this kind operations after applying milling processes due to the to a sequence of operations leads to a complex program use of metalworking fluids during the process. On the for the overall system. The superior control program can other hand it is required to fix the assembled parts to then execute and supervise those operational sequences. finish the assembly. Technologies for joining the Currently this approach narrows the production to be assembled parts definitively have to be integrated. Such carried out using serial operational sequences. Due to the a technology could be an adhesive dispensing unit to fact that both basic systems (micro machining center and glue the parts together or an integrated welding laser manipulation system) don’t have to perform actions being (see section 7). Simple peripheral devices like dependant to each other, the introduction of parallel dispensing units are operated by rudimental controllers. operations might be possible in some cases. This Digital or analog signals are mainly being used to enhancement is not subject of the current research, operate those devices. Sometimes also a serial interface where first of all the single devices are being connected is provided. For the purpose of integrating such simple via Ethernet.

3 GENERAL MANIPULATION SYSTEM CONDITIONS

A system suitable for micro part assembly has to provide the necessary degrees of freedom and high accuracy concerning the handling operation. If the working space is limited, several system conditions to provide an optimal application arise. Thinking of micro milling of high-alloyed steel, mainly three- to five-axis portal-type milling machines are being used. Those machine tools have a system accuracy within the one-digit micron range and a working space of 0.01 m3 (300 mm * 200 mm * 150 mm) and more. Without loss of generality a three­axis portal-type micro machining center (“MicroGantry GU” built by Kugler GmbH, Salem, Germany) will be used as basic

Figure 1: The concept of the superior control program system. Taking another micro machining center as basis would result in slightly different constraints for the manipulation

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The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany system. The “MicroGantry GU” provides a large space for the installation of peripheral devices and a big working space. This allows the use of big range manipulation systems. Simulations showed, that a standard six-axis articulated arm robot with an arm length of 600 mm (Figure 2) could be used in this case The fact that modular optional systems shell be used and tested leads to the need for a manipulation system with a working space (range) as large as possible to be able to serve all integrated subsystems. In many cases, for example the production of housings for micro systems, the parts to be produced have complex geo­metries, which cannot be produced with two-and-a-half­dimensional milling processes, real three-dimensional milling processes are required. Those processes cannot be operated with three-axis milling machines. To overcome this problem, the manipulation system can also be used as workpiece carrier extending the two­and-a-half dimensional milling processes to three­dimensional milling processes due to the additional degrees of freedom provided by the robot. Especially for laser processes this technique is suitable, since the laser processing result is highly dependent on focus distance and beam angle towards the workpiece. Resulting from this possible application a requirement is a high movability of the manipulator. Six degrees of freedom (three translational in and three rotational ones around the machines X-,Y-,Z-axis) are being preferred. Demanding low tolerances in producing and assembling micro systems, the manipulation system has to fulfil special demands. Thinking of endoscopes as hybrid micro systems in the field of medical technology, assembly tolerances are in the lower two-digit micron range but complex assembly movements are required. Additionally the variety of such systems is huge. Compared to micro systems engineering, clean room environments cannot be found in the production of hybrid micro systems: clean room conditions cannot be achieved for milling applications. For those processes metalworking fluid is often necessary and very small chips are produced. Concerning this, the manipulator doesn’t have to be in particular clean room suitable but it has to be non-sensitive to small chips (metal dust) and spray mist. Regarding complex three­dimensional assembly or workpiece movement, it is preferable to be able to allow direct intervention concerning the current movement. Such an interface can be used to feed the manipulators control system with sensor information to superpose the programmed move­ment with correction values computed out of the sensor data. Real-time force and torque control as well as image processing Figure 2: Stäubli RX60 robot could be realized easily.

4 SYSTEM CONFIGURATIONS Before choosing the optimal manipulation system for the micro machining center, the complete system arrangement has to be taken into account. The configuration presented at the end of this paper is not the final solution for every machining center on the market, but it can be done in a similar way.

As basic system for the prototypic system configuration a micro machining center “MicroGantry GU” built by the Kugler GmbH, Salem, Germany is chosen. This machine tool is a six-axis portal-type milling machine, optimized for micro milling purposes. In addition to the high speed spindle this machine tool is equipped with a pulsed Nd:YAG-Laser, mounted in parallel to the milling spindle. The drives in the XY-plane are high dynamic linear induction motors with an optical position sensing system being able to determine the stators position with a resolution of 10 nm. This results in a system accuracy in the XY-plane of an one-digit micron range (according to the manufacturer: ±0.7 micron). The Z-axis is driven using a high-precise mechanical spindle, reaching a repetitive accuracy below 2 micron [4]. The “MicroGantry GU” is based on a large granite block with an area of 1.5 m2 where the Y-axis is countersunk. This granite block makes a perfect base plate for the manipulation system extension, providing a large and well accessible working space. The already done laser integration allows combinations of milling and laser operations in form of process chains within one working space. Due to the type of laser (currently a Nd:YAG-laser with rectangular pulse shaping), cutting, graving and restricted welding and bending processes can be carried out. Carrying out several milling and laser operations on one workpiece can now be done without transporting the workpiece from one machine tool to another – rechucking is not necessary [5]. At present an extension to the original control software is made available. This extension is an Ethernet interface providing all functions giving full control over the micro machining center including data exchange. In general two basic arrangements for the manipulation system and the micro machining center are imaginable. The manipulation system can be installed inside the machining center and its working space or outside. Outside installation is very common for a robot used to automatically load machine tools with semi-finished products. Installing the manipulation system inside the machine tool can only be found in rare situations, for example special purpose machines where the handling operations are linked in a particular way with other operations (tool changing is not being considered).

Manipulator installation outside the working space Mounting the manipulation system (in the following also called manipulator) outside the machine tool, the installation space can be neglected as limiting factor. The manipulators needed working space to load one machine tool is much smaller than the working space provided. Choosing an intelligent arrangement of one manipulator and a few machine tools, the manipulator can load all machine tools due to the small part of the working space needed to do this operation, but this separated systems design also has negative aspects. Standard micro machining centers are vibration isolated, so both systems wouldn’t be mounted rigidly on one base plate, resulting in stronger possible relative movements. Displacement is therefore not only caused by elastic deformation (swinging/vibrating) of system parts. As a result the system accuracy can be worse in some orders of magnitude compared to the single system accuracy. High accuracy assembling in such an environment can only be performed by tracking the workpieces current position and orientation using external sensors. Secondary the manipulation system being able to load several machine tools from outside the machine have to have a bigger working space to take advantage out of the loading of

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The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany several machine tools than a system mounted inside (Figure 3). In general a larger working space comes together with a decreased accuracy, assuming that the same constructive effort was carried out. Possible corrections in position and orientation using sensor­based closed-loop control are depending on the built-in drives and the kinematics themselves. Correcting the accuracy by using closed-loop control is pretty good for stationary positioning, but for the correction of movements (assembling) such a techniques can only be used in rare cases (optical tracking during movements depends highly on the environmental conditions, the working space and the parts to be tracked).

Figure 3: Configuration with manipulator outside two micro machining centers.

Manipulator installation inside the working space Integrating the manipulation system inside the working space of the machine tool, the manipulation system doesn’t have to have such a big working space as with the “outside”-arrangement, and a smaller working space goes hand in hand with higher system accuracy. Since the manipulator is located inside the machine tool, it can not only be inherent in design used for assembling operations, it can also be used for workpiece handling during the whole production process. Using this technique could extend a three-axis machine tool to be able to perform real three-dimensional milling operations. At this it has to be noticed that the overall system accuracy is worse, compared to the single micro machining center due to the lower stiffness of the workpiece carrier (the manipulator). This kind of application has its advantages especially in low force operations like all kind of laser processes, which could benefit from the gained degrees of freedom. Both kinematics (for manipulation and milling) are mounted on a heavy and stiff base plate with a low thermal expansion coefficient. Neglecting dynamical displacements, the overall accuracy can be computed by simply adding the single system accuracies. Concerning the high accuracy of the micro machining center, the manipulation system accuracy is the one that matters. For assembly operations with tolerances around 20 micron a manipulator accuracy of less than 20 micron would be sufficient. In such a case, additional sensors to determine position and orientation of the workpiece are not essential. Knowing the workpieces position inside the machine tool (the position is known in the range of the machine tools system accuracy) leads to a simple coordinate transformation to get the

coordinates in the manipulators coordinate system. As a result of thermal displacement and other stationary displacement causing influences, a reference check between both machines has to be performed from time to time. Combining this with system calibration, a technique for calibrating robot kinematics developed at our institute can be used. This calibration strategy relies on optical measurements during small movements around one measurement point and could be integrated into the system. Given the reduction of costs and the gained simplicity due to the possibility of sensor abdication and the possible process combinations including assembly processes (integrated process chains), the installation inside the machines working space provides more advantages. Especially the manipulator usage as work carrier extends the possibilities of the basic micro machining center in an interesting way. This arrangement provides high flexibility and makes a modular design realizable (see Figure 4).

Figure 4: Simulation of the micro machining center with integrated articulated arm robot

5 SELECTION OF THE MANIPULATION SYSTEM For the use as manipulation systems the following devices can generally come into consideration. Custom­made products will not be discussed further due to project constraints:

1) Machine Tool kinematics as manipulation system 2) Custom-made manipulation system 3) Robot as manipulation system:

• Parallel-kinematics robot • “Pick & Place” system (e.g. SCARA-robot) • Six-axis articulated arm robot

Looking at the kinematics of a portal-type milling machine the kinematics are similar to the kinematics needed for “Pick & Place” operations. Mounting grippers in parallel to the spindle, the machining center itself can perform handling operations. The dynamics of this solution are quite lower than off-the-shelf “Pick & Place” systems, caused by the low dynamic of the mechanical spindle used to drive the Z-axis. Handling operations can only be carried out in sequence to machining processes, supporting machining processes with additional handling are not possible as well as parallel processing. On the

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The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany other hand, the achieved handling accuracy is equal to the micro machining centers system accuracy. Robots are available in nearly all imaginable sizes and design. They are known to be robust and absolutely reliable systems in the large volume production. Parallel­kinematics robots have a small working space compared to the needed installation space. The movability within the six degrees of freedom (or less if the robot doesn’t provide this number of degrees of freedom) is limited. Otherwise the repetitive accuracy can be below 1 micron [2]. Exemplary systems are the HexaGlide and a HexaPod (Figure 6). As “Pick & Place” systems, SCARA-robots can be considered without loss of generality. Normally these robots provide five or less degrees of freedom and, compared to parallel kinematics (there are also SCARA-robots available designed with parallel kinematics), the stiffness is low. By being able of high dynamic movement, they have a big working space in relation to the needed installation space. The serial link design causes the inaccuracies of the links to sum up. SCARA-robots with non-parallel kinematics can reach repetitive accuracies up to 10 microns [2]. Looking at six-axis articulated arm robots, the stiffness is comparable to SCARA-robots of the same size and working space. The working space is larger in relation to the needed installation space and the system allows dynamic movement. Due to the serial linking of all axis, a high movability is reached in six degrees of freedom going hand in hand with possible summing up of inaccuracies in each link and a reduction of the overall system accuracy. In the market there are off-the-shelf systems with a repetitive accuracy below 20 microns available [2]. Calibrating those kinematics can still lead to an improvement of absolute accuracy. The manipulation system hast to fulfill the following specifications:

1) Small installation space compared to the working space (due to the installation inside the micro machining center)

2) High movability (workpiece carrier function) 3) Accuracy has good as possible, minimum

20 microns repeatability 4) Stiffness as good as possible (the higher the

better) 5) Off-the-shelf system (due to project constraints) 6) Control system with Ethernet interface

(attaching of superior control program) 7) Maximum load approximately 2 kg (gripper and

workpiece for micro parts, 2 kg to be safe)

Given the specifications, a Stäubli six-axis articulated arm robot with a repetitive accuracy of 20 micron

Figure 6: on the left the HexaGlide [8], on the right a HexaPod by Physical Instruments [9].

(manufacturers specification) was chosen, see Figure 5. For the benefit of higher movability less stiffness and accuracy was accepted.

Figure 5: Micro machining center with the integrated Stäubli RX60 six-axis articulated arm robot

6 THERMAL AND DYNAMIC CONSIDERATION Micro production to date is often combined with air conditioning systems controlling the environmental conditions to keep them as stable as possible. Operating the machine tools continually leads therefore to a quasi­static state after some time of operation. Furthermore those machine tools are designed for that kind of application by using low thermal displacement coefficient materials and linear induction motors: First measurements without machining operation but operating drives showed a thermal displacement of less than 3 micron over a period of 2 days. Taking a look at the robot, a stable overall system accuracy can be achieved by referencing the system after reaching the quasi-static state. As a result, there is a translational and rotational coordinate correction computed. Changing thermal influences are not covered with this procedure, but due to the slight change of environmental conditions in time (air conditioned rooms), repetitive referencing can limit this influence quite well. At our institute a suitable referencing / calibration strategy was developed, which allows a software based correction of kinematic parameters leading to a better absolute accuracy of robots or other kinematics [6]. Regarding the robots drives as internal heat sources, those drives could prevent reaching a quasi-static state during operation. Normally they are operated with changing loads and they are switched on and off during the operations. The chosen robot is designed to keep the position by controlling the drives instead of using mechanical brakes. Combined with not necessarily high dynamic movement – the duration to perform micro milling operations is high, and the paths to be followed for assembly are small – the thermal influences caused by the drives can be rated to be small. This has to be verified in more accurate measurements, but first measurements to estimate theses influences are showing this behavior. Further on dynamic loads are rated to be small. The robots overall weight is approximately 60 kg including the gripper and small in relation to the 2 t heavy weight granite base-plate of the micro machining center. During operation approximately two thirds of that mass is being moved as maximum. According to the manufacturer, the robots mounting plate has to stand a momentum of maximal 640 Nm [7]. This load is reduced due to the low dynamic operation and the loads below the robots specification. Assuming such a high momentum, the

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The 1st CIRP-International Seminar on Assembly Systems, 15-17 November 2006, Stuttgart, Germany granite block would experience a displacement in the low one-digit micron range – estimated using a simplified model. Considering the micro machining centers stiffness and the arising angular accelerations caused by the robot, a torsion can also be neglected as first approach. Vibrations of the robot arm on the other hand have to be taken into account for handling operations. First approaches with an applied force control showed good results, even though vibrations could not be avoided.

7 FIBRE LASER QUALIFICATION FOR MICRO PART ASSEMBLY

The currently integrated Nd:Yag-laser is optimized for cutting, and the focus diameter is not the best for micro assembly. For that reason, a fibre laser was tested concerning its use in micro assembly. The size and the easy control of such a laser makes it perfectly usable for the planned micro production system. Due to its good beam quality the laser can be focused down to the one micron range depending on the used optics. Additionally the beam quality is nearly constant over a long period of time compared to a laser which uses flash lamps for laser light generation. The qualification experiments were done with a SPI SP-100C-0012 100W fibre laser. The micro parts to be welded were chosen in respect to a realistic endoscope production. The parts in focus were high alloyed steel tubes with a wall thickness down to 50 microns and a substitution part for a lens holder which is normally attached to the endoscopes tips. In this case the lens holder was replaced by part which functions like a metallic cork. The experiments were performed using tubes with wall thicknesses of 0.3 mm, 0.2 mm, 0.1 mm and 0.05 mm. All tubes have been equipped with a lens holder substitution part to be welded to. Additionally tubes with a wall thickness of 0.1 mm were welded directly together. The welding parameters were changed in power, focus and welding speed. The laser was operated in continuous wave mode and therefore was not pulsed. As cover gas Argon was chosen and applied using two diffusers at the welding spot. In general it was aimed on getting the weld thickness as thick as the tube walls (0.4 mm/0.3 mm/0.2 mm/0.1 mm/ 0.05 mm). This can only be achieved with a deep welding process. The resulting assemblies have first been checked optically using a microscope (magnification up to 1000 times) for weld quality. Additionally the parts were tested for gas tightness by applying hydrogen with a pressure of several atmospheres to the tube, while being dipped into a 130°C oil bath, which is a standard procedure for testing autoclavable medical instruments. As result it could be found, that the process stability was very good over the whole testing period, the weld quality was stable all the time. Even weld thicknesses of 50 microns and less could be reproducible obtained. Furthermore the field of parameters which resulted in good and gas tight welds was big compared to the field of parameters which could be used with Nd:YAG lasers. Hence the fibre laser was found to be very suitable for high alloyed steel micro system production. The easy interface to the control gives a good opportunity for an integration into the planned micro production center described in section 2 sqq. The following picture (Figure 7) show two kind of welds produced in the “MicroGantry GU” machining center. In these pictures, the difference between deep welding and heat conduction welding can be seen. The weld on the left (deep welding) is approximately 80 microns thick and 0.1 mm deep. The weld on the right (heat conduction welding) is about 0.2

Figure 7: left - welded tubes (0.1 mm), right - tube (0.05 mm) welded to lens holder substitution

microns thick and 0.05 mm deep. Both results were gas tight and fulfilled the needed requirements specified for medical endoscopes.

8 CONCLUSION As an approach to build a flexible automated solution for micro part assembly, a standard micro machining center was equipped with a manipulation system. Since the installation space inside the machine tool is limited, a high movability of the manipulator is demanded. The repetitive accuracy achievable in the chosen configuration is below 20 microns and is therefore sufficient for assembling the micro systems at hand. The thermal and dynamic influences of the added manipulation system is being neglected at this state of research by performing referencing / calibration measurements during operation. Accuracy of path, possibly needed for more complex assembly, as well as detailed investigation of thermal and dynamic influences using a more complex model are subject of the current research. Additionally the qualification of a fibre laser in assembling of metallic micro parts was performed. Subject of that research part were micro parts used in medical endoscopes. In that field, the requirements for welds are strong. The welds have to be gas tight, non-sensitive to autoclaving and non-sensitive to corrosion, which could be obtained in the accomplished experiments.

9 REFERENCES [1] Fatikow, S.: „Mikroroboter und Mikromontage“, B.G.

Teubner Verlag, Stuttgart/Germany, 2000 [2] H. Weule, J. Hesselbach et al: “mikroPRO -

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[4] Kugler GmbH: „Produktbeschreibung MicroGantry GU”, Germany, Stand 2005

[5] J.-P. Wulfsberg, A. Kuhn: WT-online „Konzept zur Werkstückhandhabung durch Industrieroboter in der Subfeinwerktechnik“, Germany, 2004, p. 406-409

[6] L. Beyer: Dissertation zum Thema „Genauigkeits­steigerung von Industrierobotern“, Hamburg / Germany, 2004

[7] Stäubli Tec-Systems GmbH: „Produktbeschreibung RX60b“, Germany, Stand 2004

[8] ETH Zürich, Institute of Machine Tools and Manufacturing: http://www.iwf.mavt.ethz.ch, 2005

[9] Physical Instruments: Online Product description http://www.physikinstrumente.de , 2005

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