Related Work

  Recently, modular robots have been intensively investigated mainly by universities and research institutes in Japan and USA since around 1990 as the robots' versatility, flexibility and fault-tolerance has been attracting researchers' interest.

  In early years, two-dimensional (2D) modular robots were studied. Recently, these years research on three-dimensional (3D) model has made remarkable progress and many types of robots have been proposed.  

  The related work can be classified from several aspects:

• Homogeneity:                   Whether the robot is composed of identical modules or not

• Self-reconfigurability :       Whether the robot can reconfigure itself or do it manually

• Control system:                The robot is controlled in a distributed manner through 
                                         modules' autonomy or is controlled by a central coordinator 

• Dynamic robotic motion:   Whether dynamic robotic motions can be generated
   Static structure                 Whether static structure can be constructed

The related work are listed below with respect to this classification 

2D Modular Robots  Homogeneity/ Heterogeneity Self- reconfigurablity Static Structure Dynamic Motion Control System Comments CEBOT (Nagoya Univ.) Hetero Yes Yes No Centralized A modular robot proposed first Fractum (AIST) Homo Yes Yes No Distributed First exmeriment of self-repair Mirco-module (AIST) Homo Yes Yes No Centralized Micro-sizing into 2cm cube using SMA Metamorphic Robot (Johns Hopkins Univ.) Homo Yes Yes No Centralized Hexagonal link mechanism Modular robot of Riken Homo Yes Yes No Centralized Cube-shape, can be stuck vertically

3D Modular Robots

	Homogeneity/ Heterogeneity	Self- reconfigurablity	Static Structure	Dynamic Motion	Control System	Comments
3D Self-reconfigurable Structure (AIST)	Homo	Yes	Yes	No	Centralized	First 3D self-reconfigurable homogeneous modular structure
Crystaline (Dartmouth)	Homo	Yes	Yes	No	Centralized	Shrinkable module to build 2D and 3D structures
Moledule (Dartmouth)	Homo	Yes	Yes	No	Centralized	Composed of a link and two connecting parts
I-Cube (CMU)	Hetero（bipartite）	Yes	Yes	No	Centralized	Consists of two types of modules, links and cube. The link is actuated
CONRO (USC)	Homo	Yes (Difficult)	No	Yes	Centralized	Serially connected modules with small degrees of freedom for dynamic motion
Polybot (Xerox PARC)	Homo	Yes (Difficult)	No	Yes	Centralized	Serially connected modules with small degrees of freedom for dynamic motion
3D Modular Structure (TiTech)	Homo	Yes	Yes	No	Centralized	Reconfiguration performed by pneumatic actuator
RBR (TiTech)	Hetero	Yes	No	Yes	Centralized	Reconfigurable robotic arm for space applications
"Modular Transformer" MTRAN (AIST)	Homo	Yes	Yes	Yes	Centralized	Succeeded in moving by changing its locomotion mode using 9 modules
SkyWalker  (CMU)*	Hetero	No (Manual)	No	Yes	Centralized	An example of modular space robot
Tetrobot (Renssaeler Polytech. Inst.)*	Hetero (Links, Hinges)	No (Manual)	Yes	Yes	Centralized	An example of a modular robot with many possible configurations

*Although this is not self-reconfigurable modular robot, we give an descriptive example of a robot that has modular structure