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Modular Tensegrity Robotic Arm Design Review: December 9 th, 2010.

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Presentation on theme: "Modular Tensegrity Robotic Arm Design Review: December 9 th, 2010."— Presentation transcript:

1 Modular Tensegrity Robotic Arm Design Review: December 9 th, 2010

2 Kyle Brown Jared Garrison Chris Edwards George Korbel Sean Wagoner Andrew Smith Andy Wixom Team Members: 2

3 Sponsors: Vytas SunSpiral Dave Atkinson Intelligent Robotics Group NASA – Ames Research Center Mentors: Dave Gardner Bryce Winterbottom Idaho Space Grant – RLEP Fellows Jay McCormack 3

4  Problem  Tensegrity  Overall Concept  Project Goal  Mechanical System Design  Control System Design  One-Bar Testing Platform  Future Plans  Cost Estimate 4

5  The goal of our project is to design and test the feasibility of a robot based on a special class of structure known as tensegrity. This robot will provide a movable stage with six degrees of freedom between the top and bottom platforms. Also, this robot must be able to interface with other tensegrity modules as well as other devices. 5

6  Tensegrity defines a class of structures where all members are strictly in either tension or compression. Type I tensegrity structures have the additional requirement that no two compression members connect to one another. Type II structures allow rod-to-rod connections as long as the tension/compression condition is still met. 6

7  Six-bar tensegrity structure  Shown to give six degrees of freedom to the top stage  2 three-bar stages stacked on top of each other  Control accomplished through controlling tendon lengths 7

8 6 Degrees of Freedom Interface with other tensegrity modules Interface with other tooling Control of multiple modules as well as outside tooling Determination of Ranges (position, velocity, force) Stacking modules provides increased range of motion (electrical and mechanical connection) Provide for communication between modules Electrical (USB and RS-232) and mechanical connection Accepts positional input Provides visual feedback on motion Motion Modularity User Interface 8

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10  Base Plate  Pivot  String Routing  Machining and Production 10

11  Allows for a pivot-to-pivot distance of 5.18” and therefore maximum range of movement for arm.  Extra material was added for securing the servos. Also, material was removed where not in use  Depending on modular connections, more material may be removed.  Mounting servos on top allows for base to base connection of modules with little gap in between. Holes for Pivots Holes for Servos 11

12 Universal joint Pros: Easier to machine Cons: Rod can move away from pivot Two bends in wire Ball and Socket Pros: Compact Cons: Limited range of motion Rotating Pivot Pros: Only one bend in wire Full range of motion Cons: Harder to Machine 12

13 Specifications Route tensile members (strings) from servo motors to ends of rods without interference with other components. OptionProsCons Route directly from servo to end of rod (straight line) Does not require channels, very easy to connect Will most likely interfere with other rods and restrict movement Thread string through machined groves and center of rod without sheath Will not interfere with motion, more streamlined look Difficult to plan routing (groves and holes for string), string might fray or cause resistance on sharp surfaces. Thread string through machined groves and center of rod with sheath Will not interfere with motion, string will be protected from friction and damage. Difficult to plan routing, More room will be required in channels and holes for routing. 13

14 14 Characteristic Dimensions  Bar length = 12”  Pivot height =~1.5”  Pivot-to-Pivot distance = 5.18”

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16  Control Scheme  AX-12 Servos  Microcontroller – Parallax Propeller 16

17  Based on control through local condition of individual tendons  Eliminates need for overarching global control laws  Model independent  Given desired lengths and tensions of tendons, it moves until it reaches the desired state  Movement through states of quasi-Static equilibrium 17

18 i=i+1 mod 6  L>L_d TT_min err max? err min? err max? err min? L↑L↑ L↓L↓ err Otherwise  L<=L_d T=L_d T>T_min L↓L↓ L↑L↑ err 18

19 Selector Block: This block selects the next tendon in sequence (starting over when it gets to the end) Case Checker: Given a tendon, this block decides which case it falls into, one of the two operable cases (top and bottom), or neither. Operate until Error Occurs: This block changes the length of the given tendon as indicated until an error occurs (some tendon reaches the maximum or minimum tension). Previous Error Checker: This block checks the previous error to ensure that the possible operation doesn’t exacerbate an existing problem. Tendon Finder: Finds a tendon that meets the conditions specified in the adjacent block. i=i+1 mod 6  L>L_d TT_min Otherwise L↓L↓ err err max? err min?  condition 19

20  The Propeller Microcontroller is used to read data from the computer and control the servos.  Pros  Quick compile and upload time.  Easy to program  8 cores that can act like peripheral devices.  Cons  Interpreted language that’s slow.  Limited peripherals.  Difficult to program complex or computationally intensive tasks, since it must be written in assembly. 20

21  The AX-12 Servo will be used to control wire lengths.  Pros  Current state of the servos can be set and read, such as torque, current angle, and speed.  Can daisy chain the servos so several can be controlled on a single wire  Powerful, up to 13 lb-in of torque  Cons  Inaccurate state data 21

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23  Concept  One-Bar Unit  One-Bar Testing Run  Conclusions 23

24  Test aspects of design  Mechanical components perform as expected  Control scheme moves bar as desired while keeping tension in all tendons  Propeller and AX-12’s work as desired  Begin to understand intricacies of user interface and communication between GUI and Propeller 24

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26 Click this one first Then this one really quickly Here is a movie of both the actual One-Bar and the Matlab GUI running in real time. 26

27  Alternate tension sensing necessary  AX-12 tension and position measurements are coupled  Indicates that the control scheme is feasible  Mechanical components work well  Dacron fishing line as tendons  Strings need to be able to slide around top of bar  Communication between Matlab and Propeller accomplished 27

28  Tension Sensing  Six-bar unit  Modularity ▪ Electrical and mechanical connections  Visualization ▪ Given feedback (lengths and tensions), provide visual representation  User Interface 28

29 RLEP TensegriTeam Budget Starting BalanceCurrent Balance $6,000.00$5, Expensed $ ItemCostNotes 9/13/10 Tensegritoy-ebay-mysweetcharlotte$ /15/10 Tensegritoy-ebay-jil112$17.99 PSoC board-Purchased from Cypress$ AX-12 Servo-Purchased from Robot shop$64.43 PSOC Parts-Purchased from Digikey$3.30 Mechanical parts-Purchased from McMasterCarr$147.33(See spreadsheet on next page) 5 AX-12 Servos--Purchased from Crust Crawler$ Propeller proto board-Purchased fromPropeller$ Metric Tap (M2x0.4P)--Purchased from McMasterCarr$18.03

30 30 Mechanical parts-- McMasterCarr quantity limiting dimensions what we want to buypart numberprice eachqtysub total base plate27.57 across8x8x k base plate bushing dia length12x k pivot bushing61.11 dia.5 length taken care of with pivot material 0 threaded studs thread9634k tubes63/8x.1458'x3/8x t end caps6.5x.5x.5.5x.5x.5 acrylic8680k screws length counter sink 8-32 x.5 socket cunter pack of a shoulder screws dia shoulder 4-40 thread91829a string 50 lb fishing line.028 dia 825 feet9442t spool6 taken care of with pivot material 0 McMaster-Carrtotal127.06

31  Current State  All raw materials purchased for Six-Bar unit  Servos and microcontroller purchased  Expected Expenses  Tension measurement sensors  Mechanical and electrical connections to accomplish modularity  Dacron fishing line for tendons  (Possibly a second Six-Bar unit to demonstrate modularity) 31

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