Parallel Kinematics: Ready for Mass Production
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Parallel Kinematics:
Ready for Mass Production
Frank Dürschmied
INA reprint from “Werkstatt und Betrieb”
No 9, September 2001
Carl Hanser Verlag, Munich
Parallel Kinematics: Ready for Mass Production
Frank Dürschmied
It’s not easy for new technologies. the EMO trade show. Just two Big – Strong – Fast
First there is a great deal of years later, 16 different exhibits
EADS, previously known as DASA, is one
enthusiasm: engineers are involved from this area were displayed, and of the precursors in PKM implementation.
from the very beginning, look an article submitted by INA about a In March 2000, “Ecospeed”, the first
forward to good sales and some- product line specially developed for Tripod, manufactured by DS-Technolo-
how hope to eliminate problems, PKM appeared in the special EMO gie, was put into operation at the EADS
most of which they’ve already issue of the German trade maga- plant in Augsburg. EADS requirements
thought about. After this though, zine Werkstatt und Betrieb. What with regard to dynamics, 5-axis machin-
a stagnation phase sets in. was missing though was the “real” ing with spindle pivot angles up to +/–40°
mass-produced machines – as well as a significant increase in total
This was probably no different in machine tools with big benefits for productivity reached their limits using
the case of parallel kinematics end customers and in high volumes conventional designs. DS-Technologie’s
machines (PKMs). The idea was that make PKMs more interesting solution was a tripod that is used as a
promising, and designers got ready from an economic standpoint. modular component of a complete
to quickly sift through 100 years of manufacturing cell and combined with
That will all change at this year’s conventional technology. A special effort
conventional machine-tool history. EMO, because the first production- is made to utilize the advantages of PKM
The first prototypes were completed ready machine tools will be on technology and, at the same time, work
as early as 1997 and exhibited at exhibit. around disadvantages inherent to the
design.
The tripod concept is based on a design
incorporating constant-length “struts”
and driven “end points” that are traversed
using conventional technology, i.e. ball
screw drives. In the selection of PKM
components such as joints and pivot joint
bearings, INA is DS-Technologie’s
engineering partner. For instance,
U-joints with a stiffness of 500 N/mm play
a decisive role for the design of the work
platform. The joints used are special
solutions for DS-Technologie that are
based on standard component designs.
Figure 1 Tripod parallel kinematic Ecospeed supplied by DS-Technologie, machine tools for the
aviation industry
2
Figure 2 High dynamic feed unit for PKMs
A common view about PKMs holds that pure PKMs. The machining head used nor the constant-length struts with linear
only tension and compression forces are by DS-Technologie also fills the gap modules were used, but rather axis
transmitted, but bending stresses are found with conventional machines for this modules with driven nut & bearing support
also present in the rod end for this kind of application. Ecospeed ensures reliable and direct drive. The idea was to place the
tripod kinematics. The U-joints were manufacturing for honeycomb structures entire weight of the powertrain about the
optimized accordingly to handle these with wall thicknesses less than one pivoting axis of the cardan suspension.
stresses . DS-Technologie also relies on millimeter. The inertial torque around the suspension
INA’s precision components for conven- axis is greatly minimized for this configura-
tional feed axes such as guides and ball Blazing New Trails tion, which allows high jerk values without
screw support bearings. Who is not familiar with the dynamic work disturbing transverse vibration. This is also
The result was that the new design centers supplied by Heckert Werkzeug- promoted by the high natural frequency of
allowed a significant increase in maschinen, especially the CWK 400D, the ball screw, which has a significantly
performance to be achieved. The tripod which is regarded as one of the fastest in larger diameter than telescopic struts for
allows feed speeds of up to 50 m/min. its class? And now they want to take this instance due to the driven nut and bearing
and accelerations up to 1 g in the linear one step further: the “end of mass support. In contrast to many PKM
axes. Up to 7,000 cm3 of aluminum can movement” has been announced for the prototypes, the SKM 400 can thus
be cut per minute, which corresponds to SKM 400 tripod. The goal is to achieve exhaust all potential in terms of dynamics
approx. 20 kg of aluminum. At the same similar accuracy and more dynamics for and can reach maximum acceleration
time, manufacturing times are reduced the same machining space, all at values very quickly. In addition, the ball
by 30 – 50 %. [1] significantly reduced costs. screw and the motor are cooled to
As mentioned above, the “Ecospeed” is The concept is a tripod with axis modules minimize thermal effects.
a type of hybrid kinematics. The tripod spaced 120° apart as well as a double An important aspect here is that INA
module is not the only component coupling link. A rotary table was added supplied Heckert a complete system for
responsible for motion along the axes in for 5-axis machining, making the machine the SKM 400, i.e. no time and effort are
space. To reach a larger work area, very flexible for the applications involved. required for mounting drives, measuring
i.e. for milling frames, the tripod head is Regardless whether 5-axis machining is systems, etc. The design also means that
added to two conventional axes. to be used for mold making, pallet the time-consuming work needed to
Because of this arrangement, the tripod changing or to be included in a manu- prepare guide raceways and to align
can be moved toward the machining facturing line as a 3-axis machine, the guides is eliminated. This allows mount-
area. This configuration represents a SKM 400 is equipped to do the job. [2] ing times for machines used in volume
significant improvement on the ratio of Heckert blazed new trails with INA in production to be decreased significantly.
machine size with respect to work area, working out a concept for the drive. An approximate cost savings of 10 % is
which is often unfavorable in the case of Neither the almost classic telescopic struts planned for the long term. [2]
3
Figure 3 Swivel bearing support for PKM Figure 4 Disturbing factors that interact with
Ecospeed calibration
There are currently several users who are Precision Tops the List One important factor for PKM accuracy is
interested in a pilot version of this calibration, in other words the adjustment
machine, and they intend to use the The topic of accuracy plays a key role in of the mathematical model to the
SKM 400. In the start-up phase, volumes current PKM developments. With regard to machine geometry. Just as important are
are of course still low, which is not easy speeds for rapid motion, acceleration and the influencing factors in operation that
for the supplier cost-wise. On the other jerk, there are already quite a few things change machine geometry. Such factors
hand, the idea of being accepted as a that speak for PKMs. The critics though like here include thermal effects, deforma-
system supplier is very important to INA. to compare the accuracy of a PKM with tions, vibration, etc. Finally, accuracy will
that of conventional machining centers. also depend on whether a tripod or a
hexapod is used. In general, for
hexapods, machine manufacturing
accuracy does not represent an influenc-
ing factor with respect to the machining
accuracy to be achieved. The only thing
that is decisive here is the determination
of the actual geometry by means of
calibration. In the case of tripods,
however, machine manufacturing
accuracy plays a decisive role due to
the different system.
The topic of calibration is also an
important issue in the “Innovative
Machine-Tool with Parallel Kinematics”
project, which is sponsored by the
Baden-Württemberg Ministry of Trade
and Commerce and includes participants
from the machine-tool and supplier
industry.
For the first time, the calibration
procedure developed at the Zentrum
für Fertigungstechnik Stuttgart
(ZFS = Stuttgart Manufacturing Techno-
logy Center) allows a decisive increase
in hexapod accuracy by calibration only.
What must be emphasized here is that
this accuracy refers to a real-life work
area. Researchers have been able to
reach the defined accuracy target of less
Figure 5 SKM 400 dynamic machining center in parallel kinematics for machining housing-
shaped workpieces made from light metal and steel
Photo: courtesy of Heckert Werkzeugmaschinen
4Bild 6 Circular interpolation test (Source: ZSF)
a before calibration b after calibration
than 10 µm on a 300-mm circle and Future Prospects References:
demonstrate that in terms of accuracy [1] Lilla A. (DS-Technologie)
and calibration, this kind of kinematics is Besides the Heckert SKM 400D and
DS-Technologie’s Ecospeed, there are Volles Tempo in der Black Box
suitable for practical applications. [3] maschine + werkzeug 06/2000
surely other machine designs (e.g. the
The best way to record the effects of Index V100) that look promising. Many [2] Schoppe E. (Heckert WZM)
disturbing factors such as deformation design studies such as the mobile repair Bearbeitungszentrum der
and temperature would be to perform a unit developed at the University of besonderen Art.
direct measurement on the tool center Hanover, have potential for industrial
point (TCP). However, there is still no [3] Heisel U. / Hestermann J.O. /
application. These examples demonstrate Böhler H. / Pliesch N.
measuring system or measuring strategy
that there is a turn away from the Verfahren der Positionsmessung und
for this application. As a manufacturer of
research object and that more emphasis Kalibrierung bei PKM
precision components, INA decided to
is being placed on the product. Mainly Conference documentation FTK
develop an integrated measuring system
key industrial customers and automobile 09/2000
for telescopic struts that can be used to
manufacturers see benefits in this
determine the distance between the
technology.
swiveling axes between the frame and
the platform during operation. In this way, INA will be involved in the development of
geometric changes due to thermal effects other PKMs ready for mass production,
and deformations can be obtained, and these will be on exhibit at the 2001
which leads to a significant increase in EMO trade show. New components for
accuracy. In principle, this idea is not these applications will also be presented.
new. Measuring via the strut axis was The trend is definitely heading toward
implemented for some prototypes, hybrid kinematics – be it tripod configura-
e.g. via laser measurement. However, tions or plane PKMs – in conjunction with
INA wanted to find a practical solution conventional axes. In my view, this will
using conventional length-measurement have a positive influence on the relation-
systems. Initial studies performed in ship between machine volume and the
cooperation with Heidenhain will be work area, which is certainly an important
introduced at the 2001 EMO trade show. aspect for mass-production users.
Together with the RWTH-Aachen This concept is obviously more accept-
(Aachen Polytechnic), a trial run will be able for customers than the still rather
held within the scope of the European “exotic” hexapod. It’s this customer
Union’s Mach 21 project with the aim to acceptance that will be decisive for
market a mass-produced product by the further developments and above all allow
beginning of 2002. parallel kinematics to spread.
The Autor:
Frank Dürschmied is an application
engineer in the Manufacturing
Machines and Systems Division
at INA Wälzlager Schaeffler oHG
in Herzogenaurach (Germany).
5Sach-Nr. 010-671-480/PKS US-D 09011 ● · Printed in Germany INA Wälzlager Schaeffler oHG 91072 Herzogenaurach (Germany) Phone +49 / 9132 / 82-0 Fax +49 / 9132 / 82-49 50 info@ina.com www.ina.com
You can also read
