INA reprint from “Werkstatt und Betrieb”No 9, September 2001Carl Hanser Verlag, Munich

Parallel Kinematics:Ready for Mass Production

Frank Dürschmied

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Parallel Kinematics: Ready for Mass ProductionFrank Dürschmied

It’s not easy for new technologies.First there is a great deal ofenthusiasm: engineers are involvedfrom the very beginning, lookforward to good sales and some-how hope to eliminate problems,most of which they’ve alreadythought about. After this though,a stagnation phase sets in.

This was probably no different inthe case of parallel kinematicsmachines (PKMs). The idea waspromising, and designers got readyto quickly sift through 100 years ofconventional machine-tool history.

The first prototypes were completedas early as 1997 and exhibited at

the EMO trade show. Just twoyears later, 16 different exhibitsfrom this area were displayed, andan article submitted by INA about aproduct line specially developed forPKM appeared in the special EMOissue of the German trade maga-zine Werkstatt und Betrieb. Whatwas missing though was the “real”mass-produced machines –machine tools with big benefits forend customers and in high volumesthat make PKMs more interestingfrom an economic standpoint.That will all change at this year’sEMO, because the first production-ready machine tools will be onexhibit.

Big – Strong – FastEADS, previously known as DASA, is oneof the precursors in PKM implementation.In March 2000, “Ecospeed”, the firstTripod, manufactured by DS-Technolo-gie, was put into operation at the EADSplant in Augsburg. EADS requirementswith regard to dynamics, 5-axis machin-ing with spindle pivot angles up to +/–40°as well as a significant increase in totalproductivity reached their limits usingconventional designs. DS-Technologie’ssolution was a tripod that is used as amodular component of a completemanufacturing cell and combined withconventional technology. A special effortis made to utilize the advantages of PKMtechnology and, at the same time, workaround disadvantages inherent to thedesign.The tripod concept is based on a designincorporating constant-length “struts”and driven “end points” that are traversedusing conventional technology, i.e. ballscrew drives. In the selection of PKMcomponents such as joints and pivot jointbearings, INA is DS-Technologie’sengineering partner. For instance, U-joints with a stiffness of 500 N/mm playa decisive role for the design of the workplatform. The joints used are specialsolutions for DS-Technologie that arebased on standard component designs.

Figure 1 Tripod parallel kinematic Ecospeed supplied by DS-Technologie, machine tools for theaviation industry

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A common view about PKMs holds thatonly tension and compression forces aretransmitted, but bending stresses arealso present in the rod end for this kind oftripod kinematics. The U-joints wereoptimized accordingly to handle thesestresses . DS-Technologie also relies onINA’s precision components for conven-tional feed axes such as guides and ballscrew support bearings.The result was that the new designallowed a significant increase inperformance to be achieved. The tripodallows feed speeds of up to 50 m/min.and accelerations up to 1 g in the linearaxes. Up to 7,000 cm3 of aluminum canbe cut per minute, which corresponds toapprox. 20 kg of aluminum. At the sametime, manufacturing times are reduced by 30–50%. [1]As mentioned above, the “Ecospeed” isa type of hybrid kinematics. The tripodmodule is not the only componentresponsible for motion along the axes inspace. To reach a larger work area,i.e. for milling frames, the tripod head isadded to two conventional axes.Because of this arrangement, the tripodcan be moved toward the machiningarea. This configuration represents asignificant improvement on the ratio ofmachine size with respect to work area,which is often unfavorable in the case of

pure PKMs. The machining head usedby DS-Technologie also fills the gapfound with conventional machines for thisapplication. Ecospeed ensures reliablemanufacturing for honeycomb structureswith wall thicknesses less than onemillimeter.

Blazing New TrailsWho is not familiar with the dynamic workcenters supplied by Heckert Werkzeug-maschinen, especially the CWK 400D,which is regarded as one of the fastest inits class? And now they want to take thisone step further: the “end of massmovement” has been announced for theSKM 400 tripod. The goal is to achievesimilar accuracy and more dynamics forthe same machining space, all atsignificantly reduced costs. The concept is a tripod with axis modulesspaced 120° apart as well as a doublecoupling link. A rotary table was addedfor 5-axis machining, making the machinevery flexible for the applications involved.Regardless whether 5-axis machining isto be used for mold making, palletchanging or to be included in a manu-facturing line as a 3-axis machine, theSKM 400 is equipped to do the job. [2]Heckert blazed new trails with INA inworking out a concept for the drive.Neither the almost classic telescopic struts

nor the constant-length struts with linearmodules were used, but rather axismodules with driven nut & bearing supportand direct drive. The idea was to place theentire weight of the powertrain about thepivoting axis of the cardan suspension.The inertial torque around the suspensionaxis is greatly minimized for this configura-tion, which allows high jerk values withoutdisturbing transverse vibration. This is alsopromoted by the high natural frequency ofthe ball screw, which has a significantlylarger diameter than telescopic struts forinstance due to the driven nut and bearingsupport. In contrast to many PKMprototypes, the SKM 400 can thusexhaust all potential in terms of dynamicsand can reach maximum accelerationvalues very quickly. In addition, the ballscrew and the motor are cooled tominimize thermal effects.An important aspect here is that INAsupplied Heckert a complete system forthe SKM 400, i.e. no time and effort arerequired for mounting drives, measuringsystems, etc. The design also means thatthe time-consuming work needed toprepare guide raceways and to alignguides is eliminated. This allows mount-ing times for machines used in volumeproduction to be decreased significantly.An approximate cost savings of 10 % isplanned for the long term. [2]

Figure 2 High dynamic feed unit for PKMs

There are currently several users who areinterested in a pilot version of thismachine, and they intend to use theSKM 400. In the start-up phase, volumesare of course still low, which is not easyfor the supplier cost-wise. On the otherhand, the idea of being accepted as asystem supplier is very important to INA.

Precision Tops the ListThe topic of accuracy plays a key role incurrent PKM developments. With regard tospeeds for rapid motion, acceleration andjerk, there are already quite a few thingsthat speak for PKMs. The critics though liketo compare the accuracy of a PKM withthat of conventional machining centers.

One important factor for PKM accuracy iscalibration, in other words the adjustmentof the mathematical model to themachine geometry. Just as important arethe influencing factors in operation thatchange machine geometry. Such factorshere include thermal effects, deforma-tions, vibration, etc. Finally, accuracy willalso depend on whether a tripod or ahexapod is used. In general, forhexapods, machine manufacturingaccuracy does not represent an influenc-ing factor with respect to the machiningaccuracy to be achieved. The only thingthat is decisive here is the determinationof the actual geometry by means ofcalibration. In the case of tripods,however, machine manufacturingaccuracy plays a decisive role due tothe different system.The topic of calibration is also animportant issue in the “InnovativeMachine-Tool with Parallel Kinematics”project, which is sponsored by theBaden-Württemberg Ministry of Tradeand Commerce and includes participantsfrom the machine-tool and supplierindustry. For the first time, the calibrationprocedure developed at the Zentrumfür Fertigungstechnik Stuttgart(ZFS = Stuttgart Manufacturing Techno-logy Center) allows a decisive increasein hexapod accuracy by calibration only.What must be emphasized here is thatthis accuracy refers to a real-life workarea. Researchers have been able toreach the defined accuracy target of less

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Figure 3 Swivel bearing support for PKMEcospeed

Figure 4 Disturbing factors that interact withcalibration

Figure 5 SKM 400 dynamic machining center in parallel kinematics for machining housing-shaped workpieces made from light metal and steelPhoto: courtesy of Heckert Werkzeugmaschinen

than 10 µm on a 300-mm circle anddemonstrate that in terms of accuracyand calibration, this kind of kinematics issuitable for practical applications. [3]The best way to record the effects ofdisturbing factors such as deformationand temperature would be to perform adirect measurement on the tool centerpoint (TCP). However, there is still nomeasuring system or measuring strategyfor this application. As a manufacturer ofprecision components, INA decided todevelop an integrated measuring systemfor telescopic struts that can be used todetermine the distance between theswiveling axes between the frame andthe platform during operation. In this way,geometric changes due to thermal effectsand deformations can be obtained,which leads to a significant increase inaccuracy. In principle, this idea is notnew. Measuring via the strut axis wasimplemented for some prototypes,e.g. via laser measurement. However,INA wanted to find a practical solutionusing conventional length-measurementsystems. Initial studies performed incooperation with Heidenhain will beintroduced at the 2001 EMO trade show.Together with the RWTH-Aachen(Aachen Polytechnic), a trial run will beheld within the scope of the EuropeanUnion’s Mach 21 project with the aim tomarket a mass-produced product by thebeginning of 2002.

Future ProspectsBesides the Heckert SKM 400D and DS-Technologie’s Ecospeed, there aresurely other machine designs (e.g. theIndex V100) that look promising. Manydesign studies such as the mobile repairunit developed at the University ofHanover, have potential for industrialapplication. These examples demonstratethat there is a turn away from theresearch object and that more emphasisis being placed on the product. Mainlykey industrial customers and automobilemanufacturers see benefits in thistechnology. INA will be involved in the development ofother PKMs ready for mass production,and these will be on exhibit at the 2001EMO trade show. New components forthese applications will also be presented.The trend is definitely heading towardhybrid kinematics – be it tripod configura-tions or plane PKMs – in conjunction withconventional axes. In my view, this willhave a positive influence on the relation-ship between machine volume and thework area, which is certainly an importantaspect for mass-production users.This concept is obviously more accept-able for customers than the still rather“exotic” hexapod. It’s this customeracceptance that will be decisive forfurther developments and above all allowparallel kinematics to spread.

The Autor:Frank Dürschmied is an applicationengineer in the Manufacturing Machines and Systems Division at INA Wälzlager Schaeffler oHG in Herzogenaurach (Germany).

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Bild 6 Circular interpolation test (Source: ZSF)a before calibration b after calibration

References:[1] Lilla A. (DS-Technologie)

Volles Tempo in der Black Boxmaschine + werkzeug 06/2000

[2] Schoppe E. (Heckert WZM)Bearbeitungszentrum der besonderen Art.

[3] Heisel U. / Hestermann J.O. /Böhler H. / Pliesch N.Verfahren der Positionsmessung undKalibrierung bei PKMConference documentation FTK09/2000

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