Term Project
Six
DOF Hexapod
Challenge
of Design and Innovation
Presented to:
Dr.
Hani Ali Arafa
Presented by:
Moataz
Mohammad Attallah
Ola
Rashwan
II - History:
· Manipulators and
Hexapods
· Stewart Platform:
first generation of 6 DOF devices
· Early applications
· Limitations and shortcomings
of the early hexapods
· Conclusion
III - Description
of the Hexapod:
· Different designs
· Kinematics: Degrees
of freedom analysis (Over constraints/Local mobility)
· Current research
and modifications into the design.
IV - Applications:
· Flight and motion
simulators
· Manufacturing Technology
· Precision positioning
in medical applications
· Other applications
and current research
V- Conclusion:
· Advantages over other
similar devices
· Disadvantages
· Future developments
and current research
The aim of this term project is to introduce a new family of positional devices, called Hexapods. This relatively new joint is an example for parallel manipulators. The idea of the design was based on a design introduced by D. Stewart for a manipulator that he termed Stewart Platform. The report introduces the conventional Stewart platform and other new designs that are based on the same idea, in addition to calculation of the DOF for these designs. The report also highlights current and future applications for the hexapod. In addition, it traces the limitations and advantages of the new designs over the old ones.
Key Words: Stewart Platform, Manipulators, Hexapod; design, applications, advantages.
HISTORY
A hexapod is a new design of six- legged
parallel mechanism structure. Parallel mechanism structures are those ones
having parallel links (struts) joining between the its base and its platform,
or the output piece. Parallel mechanism generally comprises two platforms
which are controlled by a several prismatic joints or legs (struts) acting
in parallel. The most common configuration comprises six legs, and these
legs are linear actuators such as hydraulic cylinders, or in the case of
positive mechanism they could be spring loaded. The output piece is defined
as the movable platform which has six degree of freedom relative
to the other platform, which is the base. With six degree of freedom
the movable platform is capable of moving in three linear directions and
three angular directions singularly or in any combination.
This type of structure has been known for long time. Around 1800 the mathematician Cauchy studied the stiffness of the so-called " articulated octahedron". More recently in 1949, Gough used similar mechanism for the test of tyres. Then later in 1965, these mechanisms rediscovered and used very widely in the flight simulator by an engineer called D. Stewart. Since that time, any parallel- linkage mechanisms are referred to as "Stewart platforms" ,although Gough discovered this mechanism before him.
A first parallel mechanism device was used in a robotics assembly cell by McCallion in 1979. Then, Parallel manipulators have been under increasing development over the last few decades, so that they are considered attractive alternative to the serial linkage devices, such as the conventional robotics arms.
Since that time many improvements and modifications have been done to that mechanism in order to overcome its restrictive range of motion. Until recently , they created that amazing device which give a wide range of motion besides the advantages of the "Stewart platform".
The initial design of the platform, since
the time it was first adopted, requires many changes. Because, the current
time is the time of computer controlled devices. In addition, with the
advance in the field of robotics, the necessity of inventing positional
devices with very high accuracy (may reach sub-micron) has become necessary.
Moreover, the old design was yet stiff, but it was limiting the ranges
of motions that could be provided from such a design. Therefore,
new techniques of mechanical design; such as: FEA (Finite Elements Method),
was introduced to obtain a new family of positional devices with both stiffness
and large range of motion.
The hexapod is a closed chain kinematic structure. This mechanical component based on the previously explained structure known as Stewart Platform. As clear from its name, the component has to be composed of six struts. However, there are several combinations that fall under the same name.
TYPE A: The Six Axes Positioner:
The hexapod consists of six struts—hydraulic ones. These struts are free to expand and contract between the base at the bottom, and the top platform. The platform is the output element that gets the 6 DOF of the system. The platform receives all six coordinates freedom in motion. Both ends of the hydraulic struts are connected to either the platform or the base using universal joints. Such a system was first introduced in the flight simulators positioning systems. It started to be commercially available for variety of applications that requires sub-micron accuracy.
Unique Characteristics:
· For any change in
the position of the platform along one of the six axis (3 translation,
3 rotation) to be achieved, all the six struts of the hexapod change their
position
· Using software control,
some models of the hexapod can attain sub-micron accuracy.
THE CHALLENGE OF DESIGN:
The hexapod control: as previously mentioned, any change in one of the struts angle or length, there will be a change in all the other struts. Therefore, the most advanced techniques of Finite Elements Analysis (FEA) and Computer Aided Design (CAD) were employed.
Degrees of Freedom Calculation:
Considering the base to be fixed, the degrees
of freedom analysis goes as follow:
· Number of moving
parts: 6 hydraulic struts (hydraulic cylinder), which is actually two parts
with clearance fit inside each others and the platform.
6 Hydraulic strut 12 parts
1 Platform 1 part
Total 13 parts
· Total DOF of the
system = 13 X 6 = 78 DOF
· DOF Analysis:
| Number | Interface | Constraints | DOF | DOF Type |
| 6 | Base: Yoke 1 / Yoke 2 Universal Joint | 4 | 2 | RR |
| 6 | Strut Lower end (Y2)/Strut upper end (Y3) | 4 | 2 | TR |
| 6 | Strut upper end (Y3)/ Platform Universal Joint (Y4) | 4 | 2 | RR |
TOTAL CONSTRAINTS: 3 X 6 X 4 = 72 constraint
System DOF = 78-72= 6 DOF
The SIX DOF systems goes as follows:
· Three Translations
in the three axes (X,Y,Z)
· Three rotations and
swaging motions on the three axes.
· No local mobilities
or kinematic over-constraints.
Absence of kinematic over constraints can
be attributed to the fact that the design does not requires the six struts
to be parallel, hence all the basic DOFs exist.
TYPE B: BASIC STEWART PLATFORM
The basic Stewart Platform is made up of six extensible struts, opposing to the previous design, with only one DOF (the cross section is non-circular, does not allow rotation) . The six struts are fixed to the base and the platform using ball-socket joints. This design is dated up to three decades ago; the first design was introduced in 1965 by D. Stewart for use in air craft simulator. This elementary design had some limitations in its range of motion because of such a design.
Degrees of freedom calculation:
Considering the base to be fixed, the
degrees of freedom analysis goes as follows:
· Number of moving
parts: 13 parts (a platform, six extensible struts and six base struts).
· Total DOF of
the system: 13 X 6 = 78 DOF
· DOF Analysis:
Number Interface
Constraints DOF
DOF Type
6
Base / Struts: ball and socket joint
3
3
RRR
6
Base struts / extensible strut lower end
5
1
T
6 Strut upper end / Platform:
ball and socket joint 3
3 RRR
TOTAL CONSTRAINTS: 2 X 6 X 3 + 6 X 5 =
66
System DOF = 78-66= 12 DOF
· Since the required
DOF of the system is only 6 DOF (TTT, RRR), therefore this implies the
existence of local mobilities. By examining the system, the existence of
six local mobilities is confirmed between the ball and socket joints on
the platform.
· Therefore:
Calculated DOF = Required/Basic
DOF + Local mobilities
12
=
6 + 6
Interface Constraints DOF DOF Type
6 3 3 RRR
6 5 1 T
6 t 4 2 RR
| Number | Interface | Constraints | DOF | DOF Type |
| 6 | Base / Struts: ball and socket joint | 3 | 3 | RR |
| 6 | Base struts / extensible strut lower end | 5 | 1 | T |
| 6 | Strut upper end / Platform: universal join | 4 | 2 | RR |
TOTAL CONSTRAINTS: 6 X 3 + 6 X 5 + 6 X
4 = 72
System DOF = 78-72= 6 DOF
I- Flight Simulators:
After the start of the space era in the
early sixties of this century, simulation of actual conditions passed by
pilots either on planes or space shuttles became a must in order to give
the pilots an adequate and necessary training. This is because planes and
space shuttles are considered to be "expensive". Not only that, but also
because the life of the pilot is equally expensive as the vehicle. The
aim of flight simulators is to simulate man/vehicle interaction, through
measurement of the pilot readiness and performance and putting a scale
for his maneuvering.
The simulator is an actual cockpit mounted
on a hexapod. The hexapod is of the "six axis positioner" type, with six
hydraulic struts and 12 universal joints distributed on both the base and
the platform, which is the cockpit in this case. Such a simulator can be
easily modified to simulate a wide range of both ground-based and flight
vehicles. Through computer system, the simulator gives a quantitative evaluation
for the pilot. From a medical view point, the existence of the body in
a 6 DOF system will produce precise simulation on the body's visual and
tactile sensors. In other words, the biological effects on the inner ear,
and kinesthetic organs ,body's muscle and skin tissue, which are used by
humans to sense motion in all directions.
According to McFadden, a company specialized
in this type of flight simulators, the benefits of a 6 DOF system on simulation
are:
· Enhances realism
of the experience
· Increases rider acceptance
of simulation
· Virtually eliminates
motion sickness
· Improves synchronization
of visual and motion information cues
· Able to accommodate
more realistic and panoramic visual displays
II- PRECISION MACHINING:
With the growing interest in quality of
produced parts, a new generation of machines that combine high speed, accuracy,
stiffness, and multi-axial capabilities started to appear. The basic design
of these structures is based on the conventional Stewart Platform. In this
case, the machining tool is carried on the platform, hence having the ability
to move six degrees of freedom. There are two possible designs. The
first is the one that uses the telescopic struts, hence having universal
joints at the end of the strut. The second design is with the ball and
socket joint at the end of the strut.
These designs have the following advantages:
· It allows free access
to the work zone.
· There is nothing
to impede the motion of the machining tool to the work piece.
· Unlike most industrial
robots, the hexapod design provides stiffness beyond what the design shows.
The struts act longitudinally on the platform, hence producing either tension
or compression on it.
· The machine offers
micron to sub-micron accuracy. This is because of the software package
that is used to control the robot.
· Low friction at the
joint, giving long life time for the robot.
III- PRECISION SURGICAL ROBOTS:
Since most medical disciplines, especially
in the field of surgery, require high accuracy and controllable forces
when working on human organs, the precision controllers were introduced
to the field of medicine as well. Research on the use of hexapod positioners
in this field is done by the support of Siemens Medical Engineering. The
aim of this robot is to help the surgeon operate on a sub-micron accuracy.
This is in order to increase safety in surgeries and prompt further improvement
and give a chance for more complicated surgeries.
The use of the robot with hexapod arms
enables the high precision and high forces to reach very complicated zones,
mostly in the brain because the model was developed for neuro-surgery.
Not only that, on the other side the surgeon
will be sitting in the operating cockpit. This is described as the workspace
of the future surgeon. The cockpit is mounted on a hexapod platform so
that the impression is transmitted to the surgeon for the endoscope going
inside the patient. This helps the surgeon to control the tools and give
him a feeling of the spatial orientation of the scalpel. As they say, they
want the surgeon to "fly through the patient" !
Such a complicated technology is controlled
by a computer simulation to avoid errors, where errors are not originally
possible!
Other Applications and Research:
1 - Fine Positing device for
the mirrors of high resolution telescopes.
2 - Positioning of optics,
electron guns, Lasers, and other energy resources.
The current paradigm in design and manufacturing involved integration of numerous hardware and sophisticated software in order to create An unique product of extremely high accuracy . The objective of this integrated product is to enhance quality and reliability, reduce the cost and overall cycle time, and increase flexibility. Hexapod is a dramatic departure from conventional mechanism design; it offers many new attributes for the most manufacturing processes.
* Six degree of freedom
The hexapod consists of six struts which
expand and contact between the movable platform, which carries a
spindle, and the fixed platform. Coordinated motion of these six
struts enables the spindle to move in any direction. In addition to the
traditional motion in orthogonal axes, X,Y, and Z, this device also is
able to move in the rotary complements of pitch, yaw, and roll. This advantage
allows the spindle to reach unusual angles and geometrical features.
* High precision and accuracy
In contrast to the conventional
multi-axis positioning tools, the hexapod technique requires all six struts
to alter their lengths if a change of the platform in only one axis is
required. On the other hand, if only one strut alters its length, all six
coordinates (X, Y, Z, q, a, j ) will change. The twelve
multi- degree-of-freedom joints must assure precision and zero
backlash.
Unlike other multi-axis positioning devices, in which any change in one
coordinate influences the position of the pivot point and the other coordinates,
the hexapod can compensate itself automatically. Through the sophisticated
software, the coordinate transformations and individual velocity /coordinate
information are transmitted to each motor controller axis.
* Computer visualization
tools
Due to unconventional design of the hexapod,
the control for the hexapod is through visualizations of the products
and the processes by sophisticated software, which is needed to support
extremely detailed visualization. This software package must have " zoom-through"
ability; moreover, it should have the ability to scan the whole workplace
through an individual multi-mode system. The advantages of applying such
numerical visualization tools are copious. The actual hexapod will
not need to be tied up, freeing it for the other tasks. Safety procedure
will be eliminated. A profound set of visual motion data can be attained
with minimum human labor; in other words, this will reduce the human
interference and at the same time the human faults.
* Graphic simulation
and computer animation
Using graphic simulation and animation
of the downstream manufacturing process provides a map for the entire workplace
envelope. Tool velocity and acceleration data will be registered
for different locations in the workspace and for various speeds and accelerations
of the ball- screw struts. Due to extraordinary kinematics of the
hexapod it is predicted that characteristic motion at the tool’s reference
frame will be highly dependent on the location in the workspace. Detailed
kinematics map will be resolved from the obtained data, identifying regions
of the greatest achievable tool velocity and acceleration and those of
the lowest. This will give a clear perception of the optimal regions of
the workspace for achieving that most use of different machining
process. Ultimately this will promote the development of the hexapod in
different manufacturing processes.
* High stiffness
Besides the various advantages stated
above, the hexapod offers another important feature, which is
the high stiffness and rigidity of its components and all moving parts,
such as bearings and joints, and drive screws. This feature results in
extraordinary high natural frequencies (500 Hz@10 kg load);consequently,
this gives very high speed of cutting and the other maching objectives.
In addition, high stiffness of the hexapod’s components will prevent
any bending effect in the six legs or in the platform, and that what
makes the hexapod very advantageous.
*High load/weight ratio
The high nominal load/weight is a very
important advantage of the hexapod. The weight of a load in the platform
is approximately equally distributed on the six parallel links. That means
each link just suffers only from 1/6 of the total weight. Furthermore,
under certain load, the struts on the hexapod act longitudinally and therefore
exerts either tension or compression on the struts in other words, no axial
forces are applied. Consequently, there is no need to design it as massive
as the conventional machines.
* Variety in the size
Rotary hexapods can be as small as soda
can or as big as 3meters in diameter depending upon the objective of the
machine.
Comparing with the conventional
multi-axis machines, the number of parts from which the hexapod is composed
is one third fewer which means lighter weight and lower friction.
* Hexapod is a soft machine
According to Hexel Corporation,
a leading company in hexapod development and production, the hexapod is
"a soft machine ." However, that is not in reference to the machining center’s
stiffness, thrust, or the other traditional descriptions of the machine
tool strength. Soft in the case of the hexapod means that the machine’s
accuracy and repeatability are not dependent on its structural alignments.
Hexel clarified that by saying , instead on relying on the ability
of the skilled assemblers to scrape in way surfaces and other critical
mating points to assure that the base, column, and other components are
square and true, a hexapod does not require that kind of intensive
assembly. Its 25- micron accuracy is derived from
mathematical formulas that are the heart
of the software that coordinates the relative motions of its six struts.
He continued, repeatability
of the hexapod design is about 10 microns, comes from its need to move
a significantly lower amount of mass when machining. Only the spindle motor,
cutting tool, and their carrier exert inertia and momentum beyond the servo
motors themselves.
* High production rate
The hexapod can provide higher production
rate in limited time. Since that needs continuos work processing capability,
the hexapod design can accommodate a pallet shuttle system that can
automatically move work pieces in and out of the work zone. That can be
achieved by designing the machine to be above the worktable.
In essence, the
hexapod design obviously has a great potential in revolutionizing
a host of manufacturing processes. The merits of such machine are
numerous. That providing improved accuracy , stiffness, speed, etc. As
a multi processing system those merits will extend beyond individual processing
improvements and into a system- oriented improvements in the manufacturing
cell. Reduction in set-up and processing time, and consequently the overall
cycle time is one of the most important benefits. High quality and readability
will be easily attained through such advanced systems.
Friction
Friction in the ball joints is a
crucial problem for the hexapod. That the friction coefficient is about
0.8,and that is enough to exert some axial deflection on the struts that
influences the accuracy and repeatability. Using a ceramic coating and
special lubricant, the modified struts are down to 0.2 coefficient
of friction
Length of the struts
The length of the struts affects the accuracy
of the machine. When the length increases, the accuracy decreases dramatically
(possibility of bending). This problem have been overcome by mapping each
screw before installing in the machine.
Dynamic thermal growth
This problem has appeared also in the
serial linkage mechanism. That with the increase in the speed of
the spindle, there is a dramatic increase in the dynamic thermal growth.
One way to overcome that hurdle is by monitoring the struts in real time
employing one dimension finite element analysis that activates an
automatic error compensation routine built into the software and based
on the known growth rate of the struts.
Calibration
The accuracy of the parallel- mechanism
is not only dependent upon an accurate control of the length of its links
but also upon a knowledge of its geometrical characteristics. According
to the fabrication tolerances many factors will play a role in the final
accuracy of the machine. Up to 132 parameters must be specified to describe
the geometrical characteristics of the mechanism which seems to be very
difficult to adjust all parameter. Therefore the calibration of the hexapod
still an open problem.
 An
example for milling machine that uses one kind of hexapod joints. The 6
DOF decreases the machining time required by the machine to perform machining
in very fine positions, with outstanding quality of machining. |
The hexapod joint when used in flight simulators. The above platform acquires 6 DOF, which exactly simulates air craft experience. |
The basic Stewart Platform, with ball and socket joints at both ends of the strut. This system has six local mobilities. |
 The
hexapod initial design put by D. Stewart. |
 Applications
of Hexapods in milling machines. |
 |
 |