Transcript TensegriTeam

Modular Tensegrity Robotic Arm
Design Review: December 9th, 2010
Team Members:
Kyle Brown
Jared Garrison
George Korbel
Sean Wagoner
Chris Edwards
Andrew Smith
Andy Wixom
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Sponsors:
Vytas SunSpiral
Dave Atkinson
Intelligent Robotics Group
NASA – Ames Research Center
Mentors:
Dave Gardner
Bryce Winterbottom
Idaho Space Grant – RLEP Fellows
Jay McCormack
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Problem
Tensegrity
Overall Concept
Project Goal
Mechanical System Design
Control System Design
One-Bar Testing Platform
Future Plans
Cost Estimate
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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 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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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
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Control accomplished
through controlling
tendon lengths
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Determination of Ranges
(position, velocity, force)
6 Degrees
of Freedom
Motion
Interface with
other tensegrity
modules
Modularity
Interface with
other tooling
User Interface
Control of multiple modules
as well as outside tooling
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
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Base Plate
Pivot
String Routing
Machining and Production
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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.
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Holes for Servos
Holes for Pivots
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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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• 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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Characteristic Dimensions
 Bar length = 12”
 Pivot height =~1.5”
 Pivot-to-Pivot distance
= 5.18”
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Control Scheme
AX-12 Servos
Microcontroller – Parallax Propeller
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Based on control through local condition of
individual tendons
 Eliminates need for overarching global control
laws
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Model independent
 Given desired lengths and tensions of tendons, it
moves until it reaches the desired state
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Movement through states of quasi-Static
equilibrium
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L>L_d
T<T_max
i=i+1
mod 6
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err max?
L↓
err max?
err
Otherwise
L<L_d
T>T_min
err min?
err min?
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
err min?
L↑
err
err max?
L<=L_d
T<T_max
L↓
err
L>=L_d
T>T_min
L↑
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Selector Block:
i=i+1
mod 6
This block selects the next tendon in sequence (starting over when it gets to the
end)
L>L_d
T<T_max
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Case Checker:
Given a tendon, this block decides which case it falls into, one of the two operable
cases (top and bottom), or neither.
Otherwise
L<L_d
T>T_min
Operate until Error Occurs:
L↓
err
err min?
err max?
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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:
condition
Finds a tendon that meets the conditions specified in the adjacent block.
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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.
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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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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
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Cons
 Inaccurate state data
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Concept
One-Bar Unit
One-Bar Testing Run
Conclusions
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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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Here is a movie of both the actual One-Bar and the Matlab GUI running in real time.
Click this one first
Then this one really quickly
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Alternate tension sensing necessary
 AX-12 tension and position measurements are
coupled
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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
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Communication between Matlab and
Propeller accomplished
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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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2010 RLEP TensegriTeam Budget
Starting Balance
$6,000.00
Current Balance
$5,097.43
Expensed
$902.57
Item
9/13/10 Tensegritoy-ebay-mysweetcharlotte
9/15/10 Tensegritoy-ebay-jil112
PSoC board-Purchased from Cypress
1 AX-12 Servo-Purchased from Robot shop
PSOC Parts-Purchased from Digikey
Mechanical parts-Purchased from McMasterCarr
5 AX-12 Servos--Purchased from Crust Crawler
Propeller proto board-Purchased fromPropeller
1 Metric Tap (M2x0.4P)--Purchased from
McMasterCarr
Cost
$34.52
$17.99
$274.00
$64.43
$3.30
$147.33
$295.72
$47.25
Notes
(See spreadsheet on next page)
$18.03
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Mechanical parts-McMasterCarr
quantity
base plate
base plate
bushing
pivot bushing
threaded studs
tubes
end caps
screws
shoulder screws
limiting
dimensions
2 7.57 across
1.11 dia 1.375
6 length
12x1.25
taken care of
with pivot
6 1.11 dia .5 length material
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6 3/8x.145
6 .5x.5x.5
8-32 .47 length
counter sink
12
string
spool
what we want to
buy
part number
8x8x.25
9246k11
6
McMaster-Carr
8-32 thread
8'x3/8x.145
8974k161
price each
qty
sub total
16.38
2
32.76
8.86
2
17.72
0
9634k22
1658t43
.5x.5x.5 acrylic 8680k24
8-32 x.5 socket
cunter pack of 25 91263a524
.125 dia shoulder
4-40 thread
91829a517
50 lb fishing line
.028 dia 825 feet 9442t4
taken care of
with pivot
material
2.98
6.16
12
1
35.76
6.16
0.33
10
3.3
6.25
1
6.25
1.16
12
13.92
11.19
1
11.19
0
total
127.06
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Current State
 All raw materials purchased for Six-Bar unit
 Servos and microcontroller purchased
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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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