Hexapod

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					Hexapod
• A hexapod is a new design of six- legged
  parallel mechanism structure.
• parallel links (struts) joining between the its
  base and its platform, or the output piece.
      The Hexapod - A Brief History of
                 Design




(a)                                  (b)

Fig.1. 1949-2000 (a)The original Dunlop tire testing machine invented by Eric
Gough, (b) The modern tire testing machine.
      The Hexapod - A Brief History of
                 Design
(a)




                                     (b)
Fig.2. 1965 -1970 (a)The original Stewart Platform for aircraft simulation, (b)
later incorporating the design of an octahedral hexapod.
         The Hexapod - A Brief History of
                    Design

                                   Universal Joints - offer 2
                                   rotational DoF




Linear Hydraulic Actuators -
offer 2 DoF: 1 translation and 1
rotation                                              Source: Marks’ Standard Handbook
                                                      for Mechanical Engineers
• several prismatic joints or legs (struts) acting
  in parallel
• legs are linear actuators such as hydraulic
  cylinders
• six degree of freedom
• movable platform is capable of moving in
  three linear directions and three angular
  directions singularly or in any combination.
• flight simulator by D. Stewart
• Stewart platforms
                Applications
• Flight Simulators
  – Cock pit as Platform
  – Enhances realism of the experience
  – Virtually eliminates motion sickness
  – Increases safety during training
• Robotics
  – Walking Mechanism
  – Routers
• PRECISION MACHINING
  – high speed, accuracy, stiffness, and multi-axial
    capabilities
  – Machining tool is connected to platform
  – free access to the work zone
• PRECISION SURGICAL ROBOTS
  – high precision and high forces to reach very
    complicated zones
  – Robot helps the surgeon operate on a sub-micron
    accuracy
  – surgeon will be sitting in the operating cockpit
  – fly through the patient
• Fine Positing device for the mirrors of high
  resolution telescopes.

• Positioning of optics, electron guns, Lasers,
  and other energy resources.
     BASIC STEWART PLATFORM
• six extensible struts, with only one DOF
• ball-socket joints
• 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
Number   Interface          Constraints DOF   DOF Types


6         Base / Struts:    3          3      RRR
         ball and socket
         joint
6        Base struts /      5          1      T
         extensible strut
         lower end
6        Strut upper end / 3           3      RRR
         Platform: ball
         and socket joint
• TOTAL CONSTRAINTS: 2 X 6 X 3 + 6 X 5 = 66
  System DOF = 78-66= 12 DOF
• six local mobilities
• Calculated DOF = Required/Basic DOF + Local
                     mobilities
         12        =        6 +6
       The Six Axes Positioner
• six struts—hydraulic
• The platform gets the 6 DOF.
• Both ends of the hydraulic struts are
  connected to either the platform or the base
  using universal joints.
•
 Degrees of Freedom Calculation:
• 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
   Quantity                   Occurrences                   Constrai   #     Descripti
                                                                nts        D      on
                                                                           o
                                                                           F
       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        4       2        RR
                   Joint (Y4)
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.
 Development of Surgical Applications
Hexapod vs. Nonapod
  – Extra legs contain
    redundant sensors
  – Insures against failure of
    standard measuring
    system
  – Reliability increase is of
    the essence

				
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posted:10/27/2011
language:English
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