Flexapods - Flexible tooling at SAAB for building the NEURON Aircraft

2010-01-1871
Flexapods - Flexible tooling at SAAB for building the NEURON
Aircraft
Henrik Kihlman
DELFOi
Magnus Engström
SAAB Aeronautics
Copyright © 2010 SAE International
ABSTRACT
Building prototype aircrafts is costly in tooling especially since only one aircraft is being built. Today’s most
common tooling strategy is to weld together a beam framework. Welded framework solutions have long lead
times both in design and manufacturing and once the aircraft is assembled the tool becomes obsolete. Flexible
tooling strategy uses non-welded tooling thus it can be changed and re-used for future products. Early version of
a new aircraft model is always hampered by frequent changes in its design, which is cumbersome to handle in a
welded framework solution. This paper presents a flexible assembly tooling solutions based on Flexapods and
BoxJoint. The Flexapods are commercialized reconfigurable tooling units that are manually adjusted in junction
with a laser tracker to a final positional accuracy of +/- 0,05 mm absolute accuracy. An operator software
program called the Flexapod control panel collect metrology data in real-time and an operator screen show
graphics on how to manually jog the Flexapod joints to reach the final Cartesian 3D-coordinate. The Flexapods
are installed in a modular steel based framework solution called BoxJoint. A complete PLM package has been
developed for the solution where the Flexapods are configured in CATIA using an add-on package to CATIA
called the Flexapod configurator. All CATIA data is stored in ENOVIA. Once the Flexapod fixture is designed
in CATIA a file, containing all Cartesian coordinates of the Flexapods, is exported and loaded into the Flexapod
control panel on the workshop floor. A previous paper on the Flexapod as an early concept and a paper on
BoxJoint have been presented at SAE Aerofast. This paper follows up on these results and presents a case study
at SAAB Aeronautics for implementing the first industrial solution of Flexapods to build the military unmanned
aerial vehicle - nEURON.
INTRODUCTION
The assembly process in aircraft manufacturing is mainly done by drilling followed by fastening. In this process
the airframe parts are located and clamped using assembly tooling. The most common strategy is to tailor the
tooling to suite the parts to be located in the tooling. The tooling is normally made up of welded steel
framework structures and machined pick-ups unique for one assembly. There have been many initiatives
undertaken to move over to flexible and reconfigurable tooling but many times these solutions turns out to be
very expensive since moving flexible parts are complex with all the built in servos and encoders that is required
to perform the reconfiguration of the tooling.
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The tooling and robotics research at Linköping university together will SAAB Aeronautics has in ten years
focused the research to find alternative solution that are flexible and reconfigurable and affordable. One strategy
presented by Kihlman 2005 [1], was a solution based on using a robot to reconfigure flexible fixturing modules
called Flexapods. Using a robot to perform the manipulation of the flexible units resulted in low cost for each
flexible unit since servos and encoders did not have to be incorporated in each flexible unit; hence a flexible,
and reconfigurable fixture solution with low cost. The idea of using robots was based on the idea that the robot
was already in place to automate drilling and fastening; hence the investment of the robot was not part of the
tooling cost. This new paradigm was called ART - Affordable Reconfigurable Tooling [1] and there where
many more papers presented on this solution. Back in 2005 a real industry project came about at SAAB that
could benefit from the ART idea. SAAB was going to build an unmanned aerial vehicle (UAV) called the
nEURON aircraft. The nEURON program had one major challenge in a tooling perspective. The nEURON
aircraft was to be built as one off prototype; hence there was not going to be a robot in place to automate the
drilling and fastening. In the ART concept the robot was the key technology to avoid high cost servos and
controller systems to manipulate the configurable tooling. The paradigm came in a crisis. There we had a
concrete industrial project perfect for flexible tooling since the nEURON aircraft could benefit having
reconfigurable tooling to avoid expensive tailor made tooling that would take long lead time and in the end of
the day give small margins on the project. This crisis caused the paradigm of ART to take a very important turn.
The research team had to quickly find a solution to manipulate the tooling by hand without having to start all
over again with a new concept. In the ART concept the robot was "guided" by a 6D-metrology system to
converge on a absolute accuracy of +/- 0,05 mm [2]. The modification to the ART concept was to have an
operator instructed to manipulate the tooling by looking on an operator screen and adjust the tooling with small
increments. From a draft sketch a quick demonstrator was built to evaluate the concept to manually manipulate
the Flexapods. There were a 50% chance the Flexapod final position would never converge to an accuracy
better than +/- 0,2 mm which was SAAB’s requirements at this time. The demonstration was built by using
simple jack screws attached on the legs of the Flexapods and a MATLAB hack to calculate the coordinate
transforms. The test was a success. The datum attachments on the Flexapod TCP's converged almost all the
time. From these lab test in the robotic lab at Linköping University SAAB decided to build an industrial
demonstrator at SAAB. DELFOi got the contract to build an industrial prototype of the Flexapod and high
quality jack screws to show for the people at SAAB that it would work for the nEURON Aircraft. This first
demonstrator out of two demonstrators also included: a kinematic model of the Flexapods to work with the
them in CATIA; a design tool for BoxJoint to work in CATIA; FEM calculator for BoxJoint; Line-of-sight tool
for CATIA to evaluate line of sight for the laser beam, a control panel for the physical Flexapod, physical tests
of Flexapods an BoxJoint. After this first demo with one Flexapod it was decided to build a second
demonstrator with four Flexapods and a framework based on BoxJoint. In the second prototype a typical aircraft
part representing the nEURON aircraft was installed in the aircraft coordinate system. After the second
prototype installation SAAB decided to build the nEURON aircraft with Flexapods and BoxJoint. The rest of
this paper will present each key technology of the manual ART concept.
THE NEURON AIRCRAFT
The nEURON aircraft is a €405 million contract experimental Unmanned Combat Air Vehicle being developed
by the French company Dassault Aviation. nEURON is a delta wing stealth UCAV. The nEURON
development, originally planned by Dassault evolved to a European cooperation including Swedish SAAB,
Greek EAB, Swiss RUAG Aerospace, Spanish EADS CASA and Italian Alenia. Dassault's plan is primarily to
collect data from the flight tests to produce derived UCAVs. SAAB who joined the project in December 22,
2005 is responsible for the overall design, equipped fuselage, avionics, fuel system and flight testing. SAAB
claimed 25% of development and is also the coordinator for the other Swedish corporations involved. First
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flight was scheduled in 2011. Information gathered from Wikipedia online [3]. Figure 1 show the work package
for SAAB. The fuselage will be built in seven separate building steps.
Figure 1: In courtesy to Jonas Askgren at NyTeknik, 2010
ENABLING TECHNOLOGIES
This section of the paper explains each existing technology that is used at SAAB to build the nEURON aircraft.
BOXJOINT
BoxJoint is a patented tooling framework kit in steel. Box plates, screws and bolts are used to joint beams and
welding the beams can be avoided; hence joined beams can be dismantled for adjusting or for re-use in future
aircraft programs (Figure 2). BoxJoint are based on four module sizes (see one of the modules in Figure 3).
BoxJoint M16 uses 25 mm box plates and M16 bolts to joint box beams from 400x400 mm to 100x50 beams.
Each bolt are tightened to 320 Nm giving each bolt 100.000 N pressure, which for the four bolt in one box gives
a total plate pressure of 400.000 N. This pressure to joint two beams will be equivalent to welding the two
beams together. BoxJoint M10 is based on 16 mm box plates and M10 screws to join box beams in the range
200x200 mm to 50x50 mm. BoxJoint M8 is based on 8 mm box plates and M8 screws to joint box beams from
200x100 to 25x25 mm. Due to the smaller sizes of the BoxJoint components in the M8 range this system is
preferred to build tooling framework closer to the parts. BoxJoint M5 is based on box plate with a thickness of
only 5 mm and this system uses M5 screws. BoxJoint M5 is preferred for very small products or for attaching
tooling clamps and locators. BoxJoint M16, M10, M8 and M5 can be combined in any way since it uses the
same section size 400-200-100-50-25 mm.
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Figure 2: Four plates joint two beams
Figure 3: One module kit for BoxJoint
BoxJoint has been implemented in various industry projects. The most successful installation yet is for
prototype manufacturing of wheel loader
loaderss at Volvo CE. They saved 42% in lead time of assembling there wheel
loader chassis when they started using BoxJoint compared to their previous method. Airbus Operations Limited
installed BoxJoint for a wing box assembly and laser scanning cell in 2009 [[4].
FLEXAPOD 6
In the ART paradigm Flexapod is the group name for configurable tooling units that can be adjusted. Flexapods
can be reconfigurable but configurable is the more general term since they can very well be beneficial for only
one product where no reconfiguration takes place. The research team has designed and evaluated many different
solutions on Flexapods. For the nEURON project SAAB decided to go for the Flexapod 6 model. The Flexapod
6 unit has a base plate and a top plate. Six legs join the bas
basee plate and the top plate. Each leg has a universal
joint in each end and each leg length can be changed by a telescopic movement. Locking the telescopic
cylinders will lock the Flexapod in all six degrees of freedom. The locking sleeve on the legs is a standard
st
component from the company ETP Transmissions. The locking sleeves have a very good quality to ensure the
Flexapods does not move no matter how long they stay locked. The base plate has eight holes for M16 screws.
Using a BoxJoint M16 400x200 plate the
the Flexapod can be attached on a 400x400, 400x200 or a 200x200 mm
box beam. The top plate has a universal tool changing component – the Capto C4 from Sandvik Coromant.
The Flexapod 6 comes in three different sizes. Flexapod 6M is the medium sized Flexapo
Flexapod. It has a range
between 1135 and 693 mm in height. The Flexapod 6S is the small
small Flexapod. It has a range of 717 and 474 mm
in height. Both Flexapod 6M and Flexapod 6S has a base plate to be attached on box beams. The Flexapod 6P
model has a different basee plate. Its base plate is designed to be attached on Pedestals. Flexapod 6P has a range
between 733 and 500 mm.
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STANDARD PICK-UPS
The pick-up is the only part in the system that is customized for a single purpose. The Pick-ups are the tooling
components that engage the product. These can be clamps or locators that locate and hold the aircraft parts
during assembly. SAAB decided to design one single Pick-up (see Figure 4). This Pick-up has a Sandvik
Coromant Capto adaptor installed. This adaptor is then used to attach the Pick-ups to the Flexapod top plates.
The Pick-ups has a machined surface to engage the part. A tooling pin screw is used to lock the part to the Pickup datum point (see Figure 5). The Pick-ups also have interfaces to attach the T-MAC (see Figure 5)
Figure 4 Pick-ups installed on Flexapod top plates
Figure 5: Left, pin screw for attaching part, Right a metrology adaptor is attached to the Pick-up
T-MAC
The T-MAC is a commercial metrology probe from Hexagon, which works together with their 6D laser trackers
(see Figure 6). The T-MAC has a prism to measure the position with laser interferometry and LEDs that via a
camera on top of the tracker measures the orientation. The tracker provides an absolute measurement of +/0,025 mm and a orientation accuracy of +/- 0,02 degrees. The T-MAC can be attached either directly on the
Flexapod top plate or on the Pick-up datum surface (see Figure 7). This is done by using the tool changer
component Capto from Sandvik Coromant. The T-MAC is originally designed to use a Capto C4 100 for
attaching the probe to machine equipment for online measuring. The reason the T-MAC can be installed on the
Pick-up itself is because the accuracy requirement of the tooling is on the datum points where the aircraft
component is installed. The T-MAC was used in the project since SAAB has good experience in using Leica
trackers. It could be an easy task to change the system to use any other 6D metrology system on the market.
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Figure 6: The T-MAC
Figure 7: T-MAC installed on a Pick-up
MANUAL MANIPULATORS
As presented in the introduction of this paper no robot was available in this project to configure the Flexapods.
To configure the Flexapods manually the Manual Manipulators (MM) was developed (see Figure 8). The MMs
are based on the same principal as jack screws. By rotating an ergonomic cylinder the length of the MMs is
increased or decreased. One MM is installed on each Flexapod leg (see Figure 9). Changing the length of the
MMs the leg length of the Flexapod is changed. The mathematical proof is shown in section Coordinate system
transforms further in the paper. SAAB ordered two sets of MMs. Six for the Flexapod 6M and six for the
Flexapod 6S and Flexapod 6P. The MMs was attached once the Flexapod had been positioned roughly. Rough
configuration means the top plate was pushed and pulled by hand a couple of centimeters distance from the goal
position. After the rough configuration the MMs are attached and the operator uses the MMs to fine tune the
Flexapod top plate to within high accuracy. This activity is called fine positioning. Final accuracy was proved to
provide accuracy bellow +/- 0,08 mm at all times.
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Figure 8:
Figure 9:
FLEXAPOD HANDLE
The MMs described earlier was engaged once the rough position of the Flexapod top plate was set. To simplify
the pushing and pulling of the top plate a Flexapod Handle
andle was developed purposed to ergonomically hold the
top plate (see Figure 10).. Rough positioning
positi
may appear a bit tough but it proved to be a rather intuitive
approach to save time. Since the MMs are for fine tuning moving top plate with the MMs it would be very time
consuming. The MMs are only to adjust the top plate to the last centimeter. Whenn the rough positioning takes
place the operator looks on a 3D model on a computer screen to see how to move the top plate to get close to
the final position.
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Figure 10: The Flexapod Handle
PEDESTALS
During the planning of the tooling design there was an
an issue how to gain a nice ergonomic access under the
aircraft during aircraft build. Preliminary design with beams was not appreciated since there would be a risk that
personal would stumble on the beams at the lower section. Therefore Pedestals where developed to cope with
this issue (see Figure 11).. The Pedestals are purposed to lift up the Flexapods
Flexapods to a working environment for the
assembly workers. The Flexapod 6Ps utilizes tooling pins to enable the Flexapod to be removed and re-attached
without losing accuracy. The Pedestals was delivered in three different heights: 930 mm, 850 mm and 770 mm.
These
ese three different heights took up the different heights for the Flexapods. The final adjusting of Height was
achieved by the Flexapods leg extension (see Figure 12).
Figure 11: Three different Pedestals
Figure 12: Flexapods installed on the Pedestals
Pedestal
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PLM SYSTEMS
The Flexapod technology is not just about hardware. A very important instrument to reduce lead-time is the
systems behind the design and planning of the tooling build process. Flexapods is a rather complicated
mechanism to work out in a CAD environment. Moreover the major cost savings for this project was not in the
hardware but in the time savings in the design, planning and build process. SAAB used something called the
BoxJoint Design Tool to define how beams are jointed together with BoxJoint components. Furthermore the
measuring of all Flexapods in a complex fixture environment with a large amount of components that may
break the free line of sight from the laser tracker head and the probe. Adjusting the Flexapods is also a rather
complex procedure unless having support from software to calculate leg lengths. The project used a paperless
work instructions solution delivered by DELFOi. Further in this section of the paper all these sub systems are
presented.
Enovia
All CAD components for Flexapods and BoxJoint were stored in Enovia at SAAB. One CAD library
component was stored for each module. For example all components for Flexapods were stored in one
component. When the designer wanted to import a Flexapod to the tooling assembly in CATIA V5 the design
engineer first had to load this component first. The Flexapod configurator software then copies each CAD
component and structures what to do with each CATPart. In the Flexapod case the configurator dresses up a
mathematical model of the Flexapod. If the design engineer wants to build tooling with BoxJoint the BoxJoint
components must first be load to the active CAD session from Enovia to CATIA in order to use that data.
Loading all components like this was a way to speed up the processing of loading parts from the BoxJoint and
Flexapod libraries and having all CAD data stored in one database. If each part to be loaded in the session was
to be loaded from Enovia each time the loading times would be very frustrating. Using Enovia instead of a file
based solution was important for the BoxJoint and Flexapod to be successful in a large project like the
nEURON program.
Flexapod Configurator
The Flexapod configurator is a kinematic module in CATIA V5 to calculate the leg configuration between the
Flexapod top and base plate. The software runs as an add-on on CATIA V5. All CAD data was stored in the
Enovia database as explained in the previous section. Beside the kinematics it managed different coordinate
systems, adding and deleting Flexapod. It made sure to locate Flexapod centre on beams or in the exact location
on a Pedestal. One very important feature was the possibility to snap the top plate to the Pick-ups. Snapping top
plates would be very complicated without the kinematic calculations inside the Flexapod configurator. Using
the Flexapod configurator the designer can see that the top plate is within the defined work envelop of the
Flexapod. The Flexapod configurator also has a Finite Element Calculator to feed back information to the
designer the stiffness of the current configuration of a Flexapod. Finally besides the CAD model the
configurator also output a “Flexapod file”, which is a text file containing the positions of all top and base plates.
This file was later used as input to the Flexapod Control Panel used on the workshop floor. A screen shot on the
working environment for the Flexapod Configurator is shown in Figure 25.
BoxJoint Design Tool
The BoxJoint Design Tool is purposed to simplify the design work when designing a BoxJoint fixture. The
BoxJoint Design Tool runs on-top of CATIA V5 as an add-on. The design tool contains rules how beams are
jointed together. For example the designer could click on a beam and select to insert a BoxJoint box. In this case
the design tool asks the designer to select only the possible selection of boxes that are available for this
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particular beam section. Once the designer had inserted the box the box plates and bolts are located correctly in
the CAD model. Furthermore the design tool could very easy duplicate sections and manipulates the tooling
components in different coordinate systems. Using default CAD functions to design tooling like this would
require special skills. In this case if the designer understands BoxJoint the system ask questions to the designer
and design the tooling for him. After only a two days training course any engineer could easily design systems
rather complex BoxJoint fixture projects. Once the design work is finished the designer makes a dress-up
procedure to define which bolts and nuts to be used for the beam joining. When the dress-up is done the
designer can click one button to get an exact BOM list and a price tag for the complete tooling design. The
design tool had also a Finite Element Analysis utility. Using this utility all the boxes that join beams were
automatically converted to FEM elements and the designer could discover possible sections of the fixture that
had high tension, which for example could require more supporting structure.
Line-of-sight
Line-of-sight is a another CATIA V5 add-on that was used by SAAB to analyze where to put the metrology
system on the workshop floor in order to have a free line of sight between the tracker head and the probing
positions. The T-MAC, which is used to measure the top plate positions have a +/-45 degree range for how the
probe can be oriented relative the metrology system. If this angle is exceeded the measurement flow from the
tracker controller will stop. Discovering this by just looking at the CAD model and guess a position for the
metrology system on the workshop floor is very complicated. The line-of-sight software runs all possible
locations and colors these locations green on the virtual shop floor. The software was also used to make a
deeper analysis of one given location of the laser tracker. This analysis showed the designer which of the datum
points that caused a broken beam or to high angular value of the probe. If a datum point resulted in a broken
beam the datum point was colored red in the CAD model. Discovering these kinds of issues before starting to
build the cell eliminated a lot of hassle with metrology access issues on the workshop floor.
Flexapod Control Panel
The last step when configuring the virtual Flexapods in CAD using the Flexapod configurator was to generate
the Flexapod datum points file. The Flexapod file contains the 6D positions of all top and base plates. The first
thing for the operators did was to load this file into the Flexapod Control Panel at the computer standing on the
workshop floor (see Figure 13). After importing the file a drop down list is populated and the fixture builder
selects which Flexapod to configure. After selecting a Flexapod in the software the control loop was initiated. A
3D model showing the goal position of the top plate and a second 3D model will show the current position of
the top plate (see Figure 14). The operator moved the physical top plate until the 3D model of the current
position was more or less merged with the goal position top plate on the screen. This was the rough
configuration activity.
Figure 13
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Figure 14
Figure 15
The next step was to attach the Manual Manipulators to the Flexapod legs. Once that was done the operator
looked at a window of the operator screen called the Leg window (see Figure 15, 16 and 17).
Figure 16
Figure 17
The leg window displayed how much each leg had to be changed. If the leg was to short the software showed a
negative value on the column representing that leg. In figure 16 the first leg was to short and the fourth leg was
to long. As the operator extracted or retracted the legs using the MMs the leg columns were continuously
updating on the screen. As the top plate came close to the goal position the columns changed colors and the leg
column window continuously scaled down to display smaller values. Once the leg columns were green the
operator looked on the total accuracy of the system. Figure 17 shows a top plate position very close to its goal
value (+/- 0,036 mm in this case). Once the operator was happy with the accuracy, which normally was less
than +/- 0,06 mm the leg locks were tightened and the MMs were dismounted and attached on the next
Flexapod. All measurements were made in the aircraft zero coordinate system. This coordinate system was first
created in the Hexagons software Metrolog. This coordinate system was a homogeneous 4x4 matrix that
corresponded to the relative 6D position between the aircraft zero coordinate system and the current position of
the metrology system. Before starting the measurement this coordinate system was imported to the Flexapod
Control Panel.
Process planning and work instructions
All the process planning was done in DELMIA V5 at SAAB. The process planning solution included: the
planning which Flexapods to use in each step of the process; which airframe part to be brought in and to decide
which holes to be drilled in each operation step. There were also a lot of geometry simulation being done to
discover issues in design and build. The holes and engineering requirements was stored in a DELMIA software
database called Process Engineer. The 3D data, work instructions, holes to be drilled and engineering
requirements was loaded into a computer on the workshop floor. This computer used a DELMIA package called
DPM Shop floor viewer. This solution eliminated 2D drawings.
TIME STUDIES
SAAB had the requirement that the time for a complete configuration cycle of one Flexapod was allowed to
take 40 minutes maximum. This activities included: to attach a Flexapod on the BoxJoint framework; to install
the T-MAC; to roughly adjust the top plate; to install manipulators; to fine adjust; to lock the leg locking
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sleeves; and finally to remove the equipment. The cycle was tested several times and after some training an
average fixture builder could easily complete the cycle on 24 minutes. Table 1 show a summary of the average
times.
Table 1: Time studies on a configuration cycle for one Flexapod
BUSINESS CASE
The most important thing for introducing a new technology is to have a good business model. SAAB put an
experienced assembly engineer on the time and cost comparison to compare the approach of using conventional
welded tooling and the approach of using BoxJoint and Flexapods. The exact figures are not public information
but the percentage comparison is shown in figure 18 and figure 19.
MAN HOURS COMPARISON
1.
2.
3.
4.
5.
6.
Concept man hours for welded tooling
Detailed design man hours with welded tooling
Man hours to build a welded fixtures
Concept man hours for using Flexapods and BoxJoint
Detailed design man hours for Flexapods and BoxJoint
Man hours for building the fixture with Flexapods and
BoxJoint
Figure 18: Man hours for conventional welded tooling compared to Flexapods and BoxJoint
The concept work is the work carried out to investigate different tooling concepts. This is the typical work done
by an assembly engineer. In welded tooling most of the tooling components have to be modeled from scratch.
Using BoxJoint and Flexapods the designer uses a standard kit of components to build the fixture. The BoxJoint
Design Tool and Flexapod Configurator enable rapid design. SAABs conclusion was 36% time savings using
Flexapods and BoxJoint. Going from a concept to detailed design welded tooling requires a vast amount of 2DPage 12 of 20
drawings. Using BoxJoint and Flexapods no 2D-drawings is necessary and very little adjusting needed to be
done from the tooling concept to detailed design. SAAB estimated time savings of up to 81% for this work. The
actual physical build up is much shorter with Flexapods and BoxJoint. A typical BoxJoint fixture seldom takes
more than a week to build and the Flexapods took 24 minutes per unit to be set up so SAAB estimated time
savings of 58% for this work.
COST COMPARISON
1.
2.
3.
4.
Cost for the concept work for welded tooling
Cost for the detailed design work for welded tooling
The hardware cost for a welded tooling solution
Cost for the concept work for Flexapods and
BoxJoint
5. Cost for the detailed design work with Flexapods
and BoxJoint
6. The hardware cost for a fixture with Flexapods and
BoxJoint.
Figure 19: The cost for conventional welded tooling compared to Flexapods and BoxJoint
The cost drivers for welded tooling seamed to follow the man hours. Using Flexapods and BoxJoint had
equivalence between man hours and cost for the concept phase and detailed design. That was probably because
it was purely a man hour based entity. The hardware cost for Flexapod and BoxJoint was a bit higher than
conventional tooling. The hardware investment for Flexapod and BoxJoint was 25% more expensive than the
conventional welded assembly tooling. Note however that even if the hardware investment is higher the total
capital investment for Flexapods and BoxJoint tooling is 13% cheaper than the conventional solution. Moreover
assume that in the next project all the flexible Flexapods and BoxJoint are re-used. In this scenario the project
only pays for the concept, and man hour cost. This would give capital cost savings of 74% by re-using the
BoxJoint and Flexapods from a previous project.
COORDINATE TRANSFORM CALCULATIONS
One very important challenge in the project was to make the operators to accept the technology. If it would be
too complicated to adjust the Flexapods they would never accept the technology. Therefore a very user friendly
operator panel was developed by DELFOi to show the 3D model top plates and the leg length window that has
been presented earlier in this paper. This section will explain some of the mathematical theory behind that
functionality.
One important assumption was made: “For one Cartesian 6D position of the top plate in space there is only one
deterministic leg length configuration of the Flexapod”. Going deeper in the theory of kinematics one realizes
that there are other Cartesian positions for one leg length configuration, but these are not likely to occur as long
as the top plate is position in the positive direction above the base plate.
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The practical approach is to continuously measure the position of the top plate with the T-MAC and then
calculate the leg length. This was the key to keep the cost down. No measuring gauges had to be installed on the
Flexapod legs to measure the leg length. The leg length was calculated! This means that since the goal position
for the top plate is given from CAD (from the Flexapod file). The required leg length to reach the goal position
was calculated theoretically. The difference in leg length from the current (measured) position and the leg
lengths to reach the goal position for the top plate is then easily calculated. This leg length delta is what is
shown on the operator computer screen.
Current top plate position
Figure 20: The coordinate systems and transforms for the current position
Tr – Metrology tracker origin
Prism – The T-MAC prism origin
Tcp – The Flexapod top plate origin
Base – The Base plate origin
TrPrism – The 4x4 matrix that comes for each taken measurement
ActualTcp – Transform between Tracker origin and top plate origin
PrismTcp – The transform between Prism and the Capto female interface in the T-MAC
BaseTcp – The transform between the base plate and the top plate
TrBase – The transform between the Tracker origin and the base plate origin
To calculate the leg current length the BaseTcp transform is required and has to be calculated in the control
loop. TrPrism is given for each measurement, PrismTcp is static and was calibrated by Hexagon. TrBase is
static since this is provided from the Flexapod file generated from The Flexapod Configurator. The
transformation equation theory is taken from Craig, 1989 [5]. Let us start to find out the ActualTcp transform:
ActualTcp = TrPrism x PrismTcp
(Eq. 1)
ActualTcp = TrBase x BaseTcp
(Eq. 2)
Let us combine Eq. 1 and Eq. 2:
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TrBase x BaseTcp = TrPrism x PrismTcp
BaseTcp TrBase1 x TrPrism x PrismTcp
BaseTcp TrBase1 x TrPrism x PrismTcp (Eq. 3)
Goal top plate position
The goal value of the top plate that is taken from the Flexapod file from CAD.
Figure 21: The coordinate systems and transforms for the goal and current position
TargetTcp = TrBase x TargetBaseTcp
(Eq. 4)
Here the TrBase, which is the base plate positions, is given from the Flexapod file and the TargetTcp, which is
the top plate goal positions, is also given from the Flexapod file. Equation 4 requires no further derivation, since
all values are known to calculate the top plate relative the base plate (TargetBaseTcp). Now we have the top
plate current position from Equation 3 and the top plate goal position from Equation 4. What is left to calculate
the leg length is the transformation from Cartesian space to vector length of each leg. This is a simple operation
since the leg attachments for a leg on the base plate and the top plate is found in the CAD model of the base
plate and the top plate.
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Leg length calculation
Once the Cartesian positions for the top plate current and goal is calculated in the control loop it is a rather
simple task.
Figure 22: The coordinate systems and transforms for the leg length calculation
BaseTcp0 x Tcp0T0 = BaseB0 x B0T0
(Eq. 5)
Deriving B0T0 according to transformation equation theory:
B0T0 BaseB01 x BaseTcp0 x Tcp0T0 (Eq. 6)
The scalar X, Y and Z values are taken from the fourth column in the homogeneous 4x4 position matrix. The
scalar length of the vector B0T0 is then given from:
L0 X2 x Y2 x Z2 (Eq. 9)
L1 is calculated the same way and the delta length is taken from L1-L0. The delta length is then populated on
the leg columns in the leg window.
SUMMARY/CONCLUSIONS
DELFOi delivered 35 Flexapod 6 and a number of BoxJoint components to build a configurable fixture at
SAAB Aeronautics during the spring of 2009. When this paper was written the building process was in the
middle of the third stage out of a total of seven. From the delivery date there has been no issues with the
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hardware and software installation. This was accomplished partly by utilizing high quality standard components
for locking and the pick-up tool changing mechanism. The people at SAAB learnt very quickly to configure the
Flexapods using the Flexapod Control Panel. The software is an intuitive user interface to manually configure
the Flexapod using Manual Manipulators. Avoiding servos, encoders and other advanced automation equipment
the capital investment was smaller for a totally flexible system compared to building the system with welded
tooling frameworks, which is the most common tooling strategy today. Much of the cost savings came from the
man hour savings by using CATIA add-on design software developed by DELFOi. Using the DELFOi's
experience in digital manufacturing the design process was very lean and 2D drawings was eliminated since the
tooling data was embedded in a Flexapod file and by utilizing DELMIA PLM system program portfolio.
REFERENCES
1. Kihlman, H., Affordable Automation for Airframe Assembly – Development of Key Enabling
Technologies, Dissertation Thesis No. 953, Linköpings universitet, ISBN 91-85299-59-6, 2005
2. Kihlman, H. and Loser, R., 2003 "6DOF Metrology-integrated Robot Control", Aerospace Automated
Fastening Conference & Exhibition (Aerofast); September 8-12; Palais des Congrès; Montreal, Quebec;
Canada)
3. Wikipedia, Web page visited 2010-03-25, http://en.wikipedia.org/wiki/Dassault_nEUROn
4. Alison, M. and Kihlman, H. (2009) Reconfigurable Flexible Tooling for Aerospace Wing Assembly,
Proceedings of the 2009 SAE Aero Tech Congress & Exhibition, Seattle, USA
5. Craig J.J. (1989) Introduction to Robotics Mechanics and Control, Addison-Wesley, ISBN-0-201-09528-9.
CONTACT INFORMATION
Henrik Kihlman is a project manager and application specialists at DELFOi based in Gothenburg, Sweden. He
can be reached at henrik.kihlman@delfoi.com
Magnus Engström is a project manager at SAAB Aeronautics based in Linköping, Sweden. He can be reached
at magnus.engstrom@saabgroup.com
ACKNOWLEDGMENTS
The authors first would like to acknowledge Linköping university and professor Mats Björkman that has been a
great support for the project. The University has acted as a kind of skunk works division to evaluate alternative
concepts to investigate routes that have longer applicative horizon. Special thanks also to Gilbert Ossbahr at
BoxJoint AB for his enthusiasm and willingness to brainstorm on how to maximize the performance of
BoxJoint. Johan Millenberg at SAAB made a great contribute in the design work and without him this project
would not have had such great success. Finally the authors would like to acknowledge the metrology division at
SAAB who showed great enthusiasm from start giving the Flexa Technology a fair chance to prove itself and
the important input they gave throughout the industrialization of the Flexapods.
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APPENDIX
Figure 23: An overview of Flexapods and BoxJoint for the Stage 1 build.
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Figure 24: An overview of the Flexapods on Pedestals.
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Figure 25: A screen shot from working with the Flexapod Configurator
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