Hexapod

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Hexapod
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• 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. 

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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.
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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.
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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
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• 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.

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• flight simulator by D. Stewart
• Stewart platforms

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Applications

• Flight Simulators
• Cock pit as Platform
• Enhances realism of the experience 
• Virtually eliminates motion sickness
• Increases safety during training

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• Robotics
• Walking Mechanism
• Routers

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• PRECISION MACHINING
• high speed, accuracy, stiffness, and multi-axial
  capabilities
• Machining tool is connected to platform
•  free access to the work zone

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• 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

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•   Fine Positing device for the mirrors of high
  resolution telescopes.  
• Positioning of optics, electron guns, Lasers,
  and other energy resources. 

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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

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Number Interface Constraints DOF DOF Types
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
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• 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

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The Six Axes Positioner

• six strutshydraulic
• The platform gets the 6 DOF.
• Both ends of the hydraulic struts are connected
  to either the platform or the base using
  universal joints.

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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 

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Quantity Occurrences Constraints DoF Description
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.
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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