ModRED: A Modular Self-Reconfigurable Robot for Autonomous Exploration

Carl Nelson*, Khoa Chu*, Prithviraj (Raj) Dasgupta**, Zachary Ramaekers**University of Nebraska

*: Mechanical Engineering, University of Nebraska, Lincoln**: Computer Science, University of Nebraska, Omaha

IntroductionModular self-reconfigurable robots (MSRs) are

robots consisting of identical programmable modules capable of reconfiguration.

To enable long-term robotic support of space missions, MSRs needed for:unstructured environmentschanging tasksself-repair

MSR capabilities can result in savings in:timemoneylives

Design MotivationTypes of MSR

Mobile – CEBOT & S-botChain – CONRO, Polypod, & PolyBotLattice – Telecube, Molecule, & StochasticHybrid – Superbot & MTran II

Advanced chain-type MSRs have up to three degrees of freedom (DOF)

More tasks are possible with higher numbers of DOF

Existing MSRsFocusing on chain-

type (as opposed to lattice-type)

Desire light, small package with high task adaptability and dexterity

System Class DOFMotionSpace

YaMor chain 1 2-D

Tetrobot chain 1 3-D

PolyBot chain 1

3-D

Molecube chain 1

3-D

CONRO chain 2

3-D

Polypod chain 2 3-D

MTRAN II hybrid 2 3-D

Superbot hybrid 3 3-D

Design Motivation

4-DOF Architecture

KinematicsToroidal position

workspace of onemodule end w.r.t.the other

Some embedded orientation workspace

1 2 3 1 3 1 2 3 1 3 1 2 1 2 4 3 1 4 3 1 3 2 1

1 2 3 1 3 1 2 3 1 3 1 2 1 2 4 3 1 4 3 1 3 2 1 105

2 3 2 3 2 2 4 3

( ) ( ) ( )

( ) ( ) ( )

( )

0 0 0 1

c c c s s c c s s c c s c c d s s d c s d P d s

s c c c s s c s c c s s s c d s c d c c d P d c dT

s c s s c s d s

Transmission2 motorsSolenoids (dis)engage DOF

Reconfiguration and LocomotionIntended to handle unstructured

environmentsNeeds to be able to form useful

configurations for task accomplishment as well as locomotion (multi-module or single-module)

Prototype System

Webots Robot Simulator: Simulated ModRED modulesAccurate models for environments, robots

Physics engine can be used to simulate external forcesSimulations in real or accelerated time

2-module Inchworm Gait Pattern Step 1: Initial configuration Step 2a: Raise rear joint of

posterior module Step 2b: Raise forward joint of

anterior module Step 2c: Extend posterior

module Step 3a: Lower connected

section Step 3b: Raise posterior rear

module and adjust angle Step4a: Lower posterior rear

module Step 4b: Raise connected section Step 4c: Contract posterior

module

Step 4d: Extend anterior module Step 5a: Lower connected

section Step 5b: Raise anterior front

module and adjust angle Step 6a: Lower anterior front

module Step 6b: Raise front joint of

anterior module Step 7a: Contract front module Step 7b: Lower rear joint of

posterior module Step 7c: Lower front joint of

anterior module

The series of steps that have to be done by 2 modules of ModRED:

2-module Inchworm Gait Pattern: Simulated on Webots

Currently the gaits of ModRED are configured by hand

Research Challenges in Designing Autonomous MSRsTo enable long-term robotic support of space

missions, MSRs can encounter:unstructured environmentschanging tasksself-repair

These require autonomous, dynamic reconfiguration among the modules

Issues involved:What is the best module or set of modules to pair

with?What is the best set of connections to have with

neighboring modules?

Operational Issues and Robot CapabilitiesDistributed – no shared memory or map of the

environment that the robots can use to know which portion of the environment is covered

Each ModRED module is frugal...limited storage and computation capabilitiesCan’t store map of the entire environment

Other challenges: Sensor and encoder noise, communication overhead, localizing robots

Learn from our research on multi-robot team formation to get directions for investigating these issues

Multi-robot Team Formation with Fixed Size Teams

Coverage with Multi-robot TeamsSquare

Corridor

Office

• Larger robot team sizes in environments with many obstacles reduces the efficiey of exploration• Lesson: Robot team sizes cannot remain fixed – must be adapted dynamically based on operation conditions

Dynamic Reconfigurations in ModREDHaving chains of modules is efficient for

explorationHaving large chains of modules doing

frequent reformations is inefficient for exploration

Can we make the modules change their configurations dynamicallyBased on their recent performance: If a large

chain is doing frequent reformations (and getting bad exploration efficiency), split the chain into smaller chain and see if exploration efficiency improves

Structured Way to Form Modules:Coalition GamesCoalition games provide a theory to divide a

set of players into smaller subsets or teamsWe have used a form of coalition games called

weighted voting games (WVG)

20

Layered Approach

Weighted Voting Game

Robot ControllerLayer

Mediator

Works with agent utility, agent strategies,

equilibrium points, etc.

Works with physical characteristics such

as wheel speed, sensor reading,

pose, etc.

Map from agent strategy to robot

action, sensor reading to agent

utility, maintain data structure for

mapping

Ongoing and Future WorkFurther develop the prototype of ModRED

Sensors, actuators, comms, processorAdapt the results from multi-robot team

formation to chain robot formation using ModRED

Terrain simulationTest hand-crafted and autonomous gait

patterns Testing motion algorithms in variety of

terrains on prototype ModRED

AcknowledgementsWe are grateful to NASA Nebraska Space

Grant Consortium for their continued support in this project

Students involvedZachary Ramaekers, UNOKhoa Chu, UNL

Supervising FacultyRaj Dasgupta, Computer Science, UNOCarl Nelson, Mechanical Engineering, UNL/

Dept. of Surgery, UNMC

Ke Cheng, UNO 23

Thank You!For more information:

Dr. Nelson’s lab at UNL: http://robots.unl.edu/Nelson/www/index.htm

Dr. Dasgupta’s lab at UNO: http://cmantic.unomaha.edu