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

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

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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?