Modular Tensegrity Robotic Arm Design Review: December 9 th, 2010.
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Transcript of Modular Tensegrity Robotic Arm Design Review: December 9 th, 2010.
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TensegriTeam
Modular Tensegrity Robotic Arm
Design Review: December 9th, 2010
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Kyle Brown Jared Garrison Chris Edwards
George Korbel
Sean Wagoner
Andrew Smith
Andy Wixom
Team Members:
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Sponsors:
Vytas SunSpiralDave AtkinsonIntelligent Robotics GroupNASA – Ames Research Center
Mentors:
Dave GardnerBryce WinterbottomIdaho Space Grant – RLEP Fellows
Jay McCormack
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Presentation Overview
Problem Tensegrity Overall Concept Project Goal Mechanical System Design Control System Design One-Bar Testing Platform Future Plans Cost Estimate
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Problem
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.
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Tensegrity
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.
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Overall Concept
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
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Project Goals
6 Degrees of FreedomInterface 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
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Mechanical System Design
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Mechanical Breakdown
Base Plate Pivot String Routing Machining and Production
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Base Plate configuration
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
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Pivot Configuration
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
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String Routing• Specifications• Route tensile members (strings) from
servo motors to ends of rods without interference with other components.
Option Pros Cons
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.
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Machining and Production
Characteristic Dimensions Bar length = 12” Pivot height =~1.5” Pivot-to-Pivot distance
= 5.18”
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Control System Design
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Control Systems Breakdown
Control Scheme AX-12 Servos Microcontroller – Parallax Propeller
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Control Scheme
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
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Control Scheme
i=i+1mod
6
L>L_dT<T_max
L<L_dT>T_min
err max?
err min?
err max?
err min? L↑
L↓ err
err
Otherwise
L<=L_dT<T_max
err max?
err min?
L>=L_dT>T_min
L↓
L↑
err
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Control Scheme
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+1mod 6
L>L_dT<T_max
L<L_dT>T_min
Otherwise
L↓ err
err max?
err min?
condition
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Microcontroller – Parallax Propeller
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.
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AX-12 Servos
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
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One-Bar Testing Platform
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One-Bar Breakdown
Concept One-Bar Unit One-Bar Testing Run Conclusions
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Concept
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
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One-Bar Unit
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One-Bar Testing Run
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.
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Conclusions
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
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Next Steps
Tension Sensing Six-bar unit
Modularity▪ Electrical and mechanical connections
Visualization▪ Given feedback (lengths and tensions),
provide visual representation User Interface
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Budget
2010 RLEP TensegriTeam Budget
Starting Balance Current Balance$6,000.00 $5,097.43
Expensed$902.57
Item Cost Notes9/13/10 Tensegritoy-ebay-mysweetcharlotte $34.529/15/10 Tensegritoy-ebay-jil112 $17.99PSoC board-Purchased from Cypress $274.001 AX-12 Servo-Purchased from Robot shop $64.43PSOC Parts-Purchased from Digikey $3.30Mechanical parts-Purchased from McMasterCarr $147.33 (See spreadsheet on next page) 5 AX-12 Servos--Purchased from Crust Crawler $295.72Propeller proto board-Purchased fromPropeller $47.251 Metric Tap (M2x0.4P)--Purchased from McMasterCarr $18.03
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BudgetMechanical parts--McMasterCarr
quantitylimiting dimensions
what we want to buy part number price each qty sub total
base plate 27.57 across 8x8x.25 9246k11 16.38 2 32.76base plate bushing 6
1.11 dia 1.375 length 12x1.25 8974k161 8.86 2 17.72
pivot bushing 61.11 dia .5 length
taken care of with pivot material 0
threaded studs 12 8-32 thread 9634k22 2.98 12 35.76tubes 63/8x.145 8'x3/8x.145 1658t43 6.16 1 6.16
end caps 6.5x.5x.5 .5x.5x.5 acrylic 8680k24 0.33 10 3.3
screws 8-32 .47 length counter sink
8-32 x.5 socket cunter pack of 25 91263a524 6.25 1 6.25
shoulder screws 12 .125 dia shoulder 4-40 thread 91829a517 1.16 12 13.92
string
50 lb fishing line .028 dia 825 feet 9442t4 11.19 1 11.19
spool 6
taken care of with pivot material 0
McMaster-Carr total 127.06
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Budget
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)
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Questions?