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The present document exposes the current state of the art in the application ofparallel kinematics mechanisms to the machine tool field and presents one solutiondeveloped by Tekniker Research Centre that introduces important innovations.

Parallel kinematics structures present very important advantages in comparison withthe serial kinematics structures: less inertia to be moved, reduction of mechanicalcomponents (linear scales, sliders, etc.), higher stiffness (for same inertia) andtheoretically can reduce the positioning error. If considered from the economicalpoint of view, all these arguments are even important because they can suppose abig reduction in the price of the whole mechanism [GOS 95].On the other hand, parallel kinematics machines present some importantdisadvantages, compared to conventional machines, like the difficulty due to 6 axiscontrol, the inexistence of inherent Cartesian axes and the lack of previousexperience outside the field of robotics.

The complex control explains why these mechanisms had not been consideredbefore: the mechanics needed improved electronics that were not available until afew years ago. The new generation of improved numeric controls has made possiblethe development of parallel kinematics based mechanisms able to move with therequired precision, speed and acceleration.

Regarding the lack of experience outside robotics, many other sectors are investingbig amounts in the R&D of parallel kinematics based positioners, milling machines,etc.

The first prototypes were based on configurations used for other purposes: flightsimulators, wheel test benches, etc. and used 6 drives to control 5 axes. Latterprototypes have reduced the number of drives and the number of axes to becontrolled trying to find new solutions for the machine tool industry and the lasttendency shown in the last machines mixes parallel and serial kinematics trying toextract the best properties of both architectures.

The prototypes developed until present moment present excellent design solutionsand have brought out very important innovations, but there are some drawbacks thatthey have not solved up to now: the shape of their workspace is very far from beingprismatic and the volume of the machine is very big in comparison with theprismatic workpiece that they can machine.

SEYANKA project tries to improve the architecture of the parallel kinematicsmachines applied to the machine tool industry facing the main problems of machinesize and workspace shape.

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All the current parallel kinematics mechanisms are inspired by one mechanismcalled Stewart Platform. This configuration was developed in 1949 by 9*RXJK thatconstructed a pneumatics test bench based on parallel kinematics. Later '6WHZDUW[STE 65] adapted the mechanism to the development of flight simulators and thisapplication was so important that his name has been adopted for that mechanism.

The parallel kinematics machines are quite known in other areas outside themachine-tool field: robotics, flight simulators, etc. In these areas the group ofreachable points of the space, that is the working volume or workspace, is essentialto perform any action and that supposes that it must comply with specific conditionsfor each application.

The application of parallel kinematics mechanisms inside the machine tool sectorwasnt possible until just a few years ago due to the lack of speed enough in theelectronics at that time. The improvement experimented in the field of electronics inthe last years has originated the development of new faster and cheapermicroprocessors, regulators, etc. So the latest numeric controls perform a very fastcontrol of 6 axes that makes possible the development of the new parallel kinematicsbased milling machine prototypes and that is the reason why the introduction of theparallel kinematics into the machine tool sector is being performed in the last years.

The first public presentations of parallel kinematics based milling machines wereperformed in the International Machine Tool Show of Chicago in 1994. Ingersolland Giddings & Lewis presented their first prototypes named Octahedral andVariax. Since that moment many companies entered in the research anddevelopment of new improved prototypes (18 of them presented in EMO MachineTool Show in Hannover, 1997).

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In the field of flight simulators the working volume must be such that themovements of a plane in the air must be simulated, that is the X, Y and Zmovements and the three rotations: roll, pitch and jaw. For this purpose, the mostextended configuration is the Stewart platform (also known as Gough platform).Such configuration shows very advantageous characteristics for that application, inwhich the optimum shape of the workspace would be a sphere. The front and top ofthe working volume for a conveniently dimensioned Stewart Platform are hexagons(not straight sides but pieces of arc, see fig. 1) that can be almost regular. Theconfined spherical volume can be very big and that means that the configuration isgood for flight simulation.

In the field of robotics, the parallel kinematics machines are used for severalapplications. They are used as motion systems in robots, as controllers in roboticarms, etc. applications in which the workspace should be almost spherical to presentlimitations similar to the real physical process. Stewart platform complies with such

characteristics and so, it is also the most popular parallel structures in theseapplications.

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Inside the Machine tools sector, the Stewart Platform has been also used in manyprototypes of milling machines [ARO 97]: Hexel, Geodetics, Itia, Okuma, etc. In allthese designs the main spindle is assembled on the moving platform while theworkpiece is fixed to the floor table.

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The kinematics and the control for such systems have been widely studied bydifferent authors (all the developments performed in the field of Robotics weredirectly suitable to the new machines) being probably the most standardisedconfiguration.

Anyway, analysing the characteristics of this platform, some machine-toolcompanies have appreciated that there could be other configurations whoseproperties could suit better the needs of a milling machine. The Stewart platformconfiguration, used in a conventional milling machine, can carry many advantagesbut presents some disadvantages that must be solved to get a competitive millingmachine.

The first disadvantage is the size of the machine: all the prototypes based on theStewart Platform present huge dimensions in comparison with the workspace theyprovide. The second weak point of these designs is due to the stiffness, speed andacceleration variations that these structures present along the working volume, thatare much bigger than in serial kinematics designs.

Another important drawback for this architecture is the existence of singular points(axes or planes) in which the platform has got null stiffness in some direction. Thesepoints are very dangerous because the parallel kinematics machine can be damaged.Every configuration can show such positions that are usually avoided because otherlimitations like actuator length, joint angle or actuator crosses appear before.The first attempts to solve these problems are directed to increase theworkspace/machine-volume ratio and reduce the variations. Some designs proposedin this direction were performed by important American Companies like Ingersolland Giddings & Lewis. The design of Giddings & Lewis [KIE 95] was based onother configuration used in flight simulation and basically was very similar to theStewart Platform. The actuators are longer and so the maximum stiffness is reducedbut the physical properties are much more balanced. In this architecture the workingvolume is reduced and that is the reason why other configurations have beenadopted in the last prototypes.

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The configuration proposed by Ingersoll [ROM 98] (see fig. 4) or by IFW (+H[DFWsupported by Siemens and INA) [PAG 97] are some of the configurations that reachbetter workspace/machine volume ratios, due to the division of the actuators joints attwo heights in the spindle and two heights in the structure.

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Both solutions are still being improved because they dont count with the supportthat was available for the Stewart platform. The first analysis claim that, apart fromimproved working volume, the physical properties are also more balanced all overthe volume and the maximum stiffness is not reduced.

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The prototype developed by IFW (fig. 5) is very interesting from any point of view.The workspace is quite big, although the actuators cross part of it, reducing theuseful working volume.

The shape of the working volume in these designs is almost similar to the volumeshown above for the Stewart platform, they show a 120 symmetry (see figs. 1, 2, 4,5) and the workspace is much bigger in these configurations than in the Stewartplatform. Inside this volume, the new parallel kinematics machines reduce and eveneliminate the singular points.

All mentioned machines have served to stand out very important advantages thatthese architectures can supply. From the economical point of view, they dont uselinear scales, linear guides or expensive sliders moving at high speed, and so theprize of mass production could be very reduced. Moreover, the mechanical anddynamical properties of all these designs are really interesting and quite a lot ofinvestigators think that these architectures could reach the highest levels of stiffness,speed and acceleration.

All these machines are excellent designs and the performance of them all isexcellent but still they are not commercial solutions. Compared to the usual serial-kinematics milling machines, the precision is much lower [NEU 97] (10 timeslower), for the same workspace the machine is much bigger (3 or 4 times bigger)and the stiffness, though being very high in some areas, is not acceptable to performany machining in the extremes of the working volume.

All these facts have make many people think that parallel-kinematics based millingmachines are not suitable for the machine tool sector at present. So, most of themachine tool manufacturers have not invested much money in the development ofnew parallel kinematics machines and just a few research centres and Universitieshave gone on their research. One exception to this tendency is Japan: in Japan themachine tool manufacturers are investing big amounts in the R&D of parallelkinematics based machines and the first commercial solutions are already available(Okuma, Hitachi Seiki, Honda).

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Trying to keep the same architecture and solve all the problems of precision andstiffness all over the volume, there are some prototypes that have incorporated linearguides in the design of the actuators.

The model of Mikromat (fig. 6) has served to analyse the performance of thesearchitectures when there are thermal charges. The thermal problems are verycommon in all kind of milling machines and the analysis of the error produced dueto these charges is very important.

Using encoders, the thermal deformations cant be measured, but using scales andlinear actuators, these effects can be measured and even reduced. On the other hand,all the economical advantages of the machines disappear in this design, because itsupposes very important costs.

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There are some prototypes (Triaglide, Linapod, Urane SX, etc., see fig. 7) thatcontrol just 3 d. o. f., fixing the other 3. These solutions are very interesting althoughthey dont provide solutions for 5 axes milling.

The main target of all these models is to take advantage of the low inertia moved inthese architectures and the reduction of the costs due to all the elements avoided:sliders, linear guides, scales, and so on.

There are many models involved in this tendency. The problem in the design of suchconfigurations resides in the way to fix the value of the 3 non-controlled degrees offreedom. Usually the 3 searched degrees of freedom are the three Cartesiantranslations, so that the milling machine produced can be independent from otherserial mechanisms (sliders and so on). The first model based on this idea was theLinapod [PRI 97] (see fig. 7)

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The Linapod fixed 3 d. o. f. just joining the actuators by pairs. And each couple ofactuators was controlled by just one actuator moving in the Z direction. The idea isvery good because the design was very stiff and also the control was easier.

The prototype seems very interesting and many other prototypes based on the sameidea are being introduced by many companies, universities and research centres. Thedifference in all these prototypes is the way to fix the movement. The modelpresented by Renault in the EMO of Paris 99 (see fig. 7) and the model PA35 ofHitachi Seiki (see fig. 8) could be included in the same group.Hitachi Seiki has taken this concept to a final product that they offer in theircatalogues. The machine is able to reach accelerations o 1.5 G and speeds of 100m/min.

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According to the exposed problems, the current investigation efforts are focalised inthe increase of precision, reduce of machine size and increase of mean stiffness.

Most of the newest designs reduce the number of actuators to control, so the CNCcan be fed faster and the precision can be improved. Also, fixing some degrees offreedom with very stiff kinematics chains, the stiffness can be higher than 6 d. o. f.designs.

Some machines reduce even 4 d. o. f., but that suppose that there are only 2 d. o. f.and the mechanism cant be used as a 3 axes milling machine, unless other

movement is introduced. So, these designs combine the parallel kinematics (fixingsome degrees of freedom) with the serial kinematics in order to improve thecapabilities of the machine.

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Such combination can be performed by many ways and so there are manyprototypes that are very different whose capabilities are not completely studied inmost cases.

Most of mentioned designs fix the value of the three rotations and sometimes one ofthe Cartesian translations, conforming high speed milling machines that can move inthree (see fig. 9) or two (pantographs) Cartesian directions: Honda (fig. 10), Dyna-M [WZL 97] (fig. 9), Ulises (Fatronic), Tricept [NEU 99] (Neos Robotics AB), etc.

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These parallel kinematics machines present very reduced workspace (the parallelkinematics machines with 2 d. o. f. Present just a planar workspace) and dontsuppose real solutions to perform the milling process. These prototypes are usuallycomplemented by other serial-kinematics mechanisms (see figs. 9 and 10) that serveto feed and/or position the workpiece and constitute 3D and 5D milling machines. Inall these prototypes, the workspace/machine-size ratio is not very advantageous.

Apart from these hybrid systems, there is another group of parallel kinematicsmachines that mix the serial and the parallel kinematics to achieve 3 or 6 d. o. f.trying to combine all the available techniques to control the serial kinematicssystems and incorporate the advantages that can be supplied by the parallelkinematics. The hexaglide (see fig. 11) developed by IWF [HEB 97] controls 6linear axes to provide the 6 d. o. f.

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Seyanka is a high-speed, 5 axes milling machine designed by Tekniker as aprototype to demonstrate the suitability of the parallel kinematics structures to thedevelopment of high speed milling machines.

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The project started in April 1998, and the machine was finished in February 2000. Inthis time kinematics and dynamics of the machine were widely analysed and theprototype was designed and manufactured. The public presentation of the machinewas performed in the machine tool show of Bilbao: BIEMH 2000 (March, 13th 18th).Seyanka is a new concept of design, trying to work out some of the drawbacks foundin parallel kinematics machines.

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Taking into account all the arguments exposed about the current problems of thefirst parallel kinematics machines used as milling machines, Seyanka is one of thenew prototypes that tries to apply the parallel kinematics to the machine toolindustry, solving the problems of past attempts. The design proposed by Tekniker isbased on two important ideas that must be pointed out:

The milling machine sector is based on Cartesian Co-ordinate Systems and allthe hardware, software, programming languages, machining operations, etc. assumea prismatic shape working volume for the machine that must be considered. Themost common operations are straight machinings and the usual workpieces areprismatic. So the milling machine must be programmable in Cartesian co-ordinatesand must supply a prismatic-shaped workspace [MER 95]. Any parallel kinematics machine controls 6 axes while any milling operation canbe done controlling only 5 axes: three translational axes and two rotational axes(azimuth and elevation). The rotation of the moving platform around the axis of thespindle doesnt have to be controlled because the spindle performs that rotationduring the machining process, that degree of freedom can always be fixed to reducethe number of axes to be controlled and increase the stiffness.

The first idea is very important because any structure presenting a 120 symmetry,will not make use of big part of the working volume if only the inscribed prismaticshape is considered in order to use the mechanism as a milling machine.

The second idea implies that one of the actuators is not necessary to use a parallelkinematics machine as a milling machine. If the rotation of the platform around thespindle axis is fixed somehow, the parallel kinematics machine can still be used asmilling machine to produce 5 axes milling processes.

The main targets sought by Tekniker in the development of Seyanka are thefollowing:

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So, the first innovation introduced by Tekniker in Seyanka is the configuration ofthe actuators in the initial position: in other designs, with 120 symmetry, but inSeyanka with a 90 symmetry. This configuration is much more suitable for amilling machine because so the Working Volume is almost prismatic shaped (notplanar sides but parts of ellipsoids, see fig. 14).As can be seen in the figure (fig. 14), the prismatic volume inscribed in theworkspace of Seyanka covers most of it [MER 93], being very suitable for themilling process because nearly all the workspace can be used when programming inthe prismatic shape inscribed.

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Seyanka is a 5 axes milling machine, and only needs 5 actuators to control themovements in these axes, so there is one d. o. f. that has been fixed. As wasmentioned above the d. o. f. that was removed is the rotation of the spindle grouparound the spindle axis. And that constraint was performed using a very rigidconstant-length, scissors-shape kinematics chain that avoids the rotation andsupports efforts and torsional torques. This chain has been dimensioned to avoid anydisturbance on the prismatic workspace and to avoid the existence of singular points.

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Figure 15 shows the position and number of actuators. In the initial position, four ofthe actuators are disposed in two perpendicular planes in the X and Y directions andthat means that during the movement they will reach the positions limited by thearcs shown in fig. 14. The vertical actuator is the most solicited because it has tosupport always the weight of the spindle group and is positioned in the Z-axis. Thekinematics chain is disposed trying to keep the symmetry with respect to a planedisposed at 45 of the XZ plane.

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Parallel kinematics machines have smaller workspace than serial machines with thesame machine size. SEYANKA prototype has also smaller workspace thanconventional serial machines, but its workspace is bigger than typical workspace inparallel kinematics machines. Following table shows table-area / machine-floor-arearatio and workspace-volume/machine-size ratio for different kind of machines.

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Most of the prototypes of parallel kinematics milling machines are not seriousattempts directed to industrialisation and so the stiffness of the machine is notconsidered essential while the kinematics and dynamics are studied widely. Only themachine-tool manufacturers have considered the structure as a very important pointto be considered. In the development of Seyanka Tekniker intends to provide a realmilling machine that will be close to the industrialisation phase.

The structure was conformed welding UPN beams and the stiffness of the wholestructure was studied using FEM techniques (NASTRAN), the stiffness of thestructure itself studied with these tools foresaw a minimum stiffness of 77 N/m inthe Z direction, that was very good to support the rest of the mechanism.

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slash effect is advantageous to provide a softest transmission, but they are notsuitable for in a parallel kinematics machine because that effect produces errors andloss of stiffness. Some manufacturers (i. e. INA) have designed a special series ofjoints for parallel kinematics that provide high stiffness and high precision. Evenusing these components, the joints are one of the weakest points of any parallelkinematics machine. The stiffness they provide rounds 50 N/m what supposes animportant decrease in the stiffness of the whole structure.

Seyanka counts with special joints developed in Tekniker, whose stiffness is biggerthan commercially supplied ones. The stiffness of the U-joints manufactured inTekniker is close to 70 N/m.

Finally, the actuators (also named legs or arms) are the other weak points in aparallel kinematics structure. Usually they consist of ballscrews, bearings, pulleysand the motor is usually supported on the exterior structure of the assembly. Thestiffness of the actuator depends on the quality of the assembly, and the stiffness ofeach of the components. Inside the group, the joint ballscrew-nut is one of theweakest points.

Seyanka counts with tailor-made ballscrews supplied by manufacturers of theBasque Country and the assembly and guidance have been done using pre-loadedcomponents of the highest quality and precision available in the market. Althoughbeing pre-loaded transmissions the stiffness of these components is the lowest of thewhole structure and reduces around 8 times the stiffness of the parallel kinematicsmachine.

The stiffness of the structure depends on the position and orientation in which themoving platform is placed [GOS 90]. Those positions in which the actuators arelongest present lowest stiffness while positions in which the length of the actuatorsis smallest show highest stiffness (see fig. 18).

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As can be seen in fig. 18, the stiffness measured in the tip of the spindle axis, wherethe tool will be assembled is much bigger in those regions in which the X and Yactuators are shortest. This can be appreciated in the stiffness measured in any otherdirection. Measuring the stiffness in the worse position (outer extreme in the lowestsection), the values foreseen using FEM were the following:.[ 1P

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The stiffness of Seyanka is similar to that of 3 axis high speed milling machines forlight materials while allowing 5-axis control at high speed and acceleration.

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Parallel kinematics machines provide new solutions for the machine tool sector.These structures can provide improved capabilities, not reachable with current serialarchitectures, but it will be necessary to make a very important effort in the researchof this kind of machines so that the current levels of precision and stiffness can besatisfied.

The Stewart platform, though being the most documented parallel kinematicsmechanism, has served to study the application of this kind of structures to themachine tool sector but is not easily suitable for machining labours because its sizeand the shape of the workspace it can reach are not easily defined by means ofCartesian co-ordinate systems.

The new designs based only on parallel kinematics are allowing a better knowledgeof these mechanisms and are achieving excellent results and specifications but theyare still far from being market competitive solutions. Those configurations thatcombine parallel and serial kinematics are very interesting from the mechanicalpoint of view but currently their cost (apart from other properties like precision,homogeneity, etc.) is still very high to consider them as valid solutions for marketintroduction.

The main objectives that Tekniker considers as strategic in the development of thenew designs for industrialisation are the following: UHGXFWLRQ RI PDFKLQH FRVWPDFKLQH IRRWSULQWZRUNVSDFH DUHD UDWLR LPSURYHPHQW LQVFULEHG SULVPDWLF

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In order to comply with these objectives, the main innovations that the prototypedeveloped by Tekniker claims are the usage of only five drives, the achievement ofan almost prismatic workspace, the improvement in the workspace/machine-sizeratio and the easy access to the work table.

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[STE 65] STEWART, D., A Platform with Six Degrees of Freedom, UK Institution ofMechanical Engineers Proceedings vol. 180, pt. 1, n 15, 1965-1966.

[HEB 97] HEBSACKER, M., HONEGGER, M., The IWF Hexaglide A NewConcept for High Speed Machining, IWF, ETH-Zentrum, April 1997.

[MER 95] MERLET, J. P., "Designing a parallel robot for a specific workspace", INRIAResearch Report n 2527, April 1995.

[GOS 90] GOSSELIN, C., Stiffness mapping for parallel manipulators. IEEE Trans. Onrobotic and Automation, vol. 6, n 3, p. 377-382, June 1990.

[MER 93] MERLET, J. P., "Orientation Workspace of a Parallel Manipulator with aFixed Point", ICAR, p. 141-146, November 1993.

[WZL 97] WECK, M., "Dyna-M compact, stiff, highly dynamic", WZL RWTHAachen, September 1997.

[PRI 97] PRITSCHOW, G., WURST, K. H., "Linapod A Concept for Modular ParallelLink Machines", ISW University of Stuttgart, 1997.

[NEU 99] NEUMANN, K., "Tricept times", Neos Robotics AB, n 1, 1999.[ROM 98] ROMBERG, J., Machines for die and mold making Trends and outlook.Colloquium Tool and Die Making for the Future, 98.

[GOS 95] GOSSELIN, C., RICARD, R., A Comparison of Architectures of ParallelMechanisms for Workspace and Kinematic Properties. Design Engineering TechnicalConferences, ASME, vol. 1, DE-Vol. 82, p. 951-958, 1995.

[ARO 97] ARONSON, R., Hexapods: Hot or Ho Hum?, Manufacturing Engineering, p.60-64, October 1997.

[PAG 97] PAGE, M., Hexapods: still st the development stage?, MetalworkingProduction, p. 32-33, November 1997.

[NEU 97] NEUGEBAUER, R., SCHWAAR, M., WIELAND, F., Accuracy of Parallel-Structured Machine Tools. Fraunhofer Institut Werkzeugmaschinen und Umformtechnik,paper n 78, 1997.

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