connection mechanisms for modular self-reconfigurable robots

TECHNICAL UNIVERSITY OF CLUJ-NAPOCA
CONNECTION MECHANISMS FOR MODULAR
SELF-RECONFIGURABLE ROBOTS
Dan MÂNDRU, Ion LUNGU, Olimpiu TĂTAR
Abstract: The self-reconfigurable robots are modular systems able to configure various structures. Among other
imposed requirements, there is the connection / disconnection ability of the modules. In this paper, the general aspects
regarding the self-reconfigurable robots are presented and several docking mechanisms are comparatively analysed.
The synthesis criteria of the connection mechanisms are emphasized and a proposed variant of a shape memory
actuated mechanism is described. Key words: modular robot, self-reconfigurable, docking mechanism, shape memory.
1. INTRODUCTION CONCERNING THE
SELF-RECONFIGURABLE ROBOTS
A self-reconfigurable modular robotic
system is composed of identical modules. It can
actively configure various structures using the
same modules and thus can adapt to the
external environment and to a specific task.
Lattice-based reconfigurable robots (Fig. 1a)
are usually homogeneous, they change shapes
by moving into positions on a virtual grid and
may only move to neighbouring positions
within the lattice, [10]. The non-lattice robots
(chain-type robots – Fig. 1b) can split into
several independent parts and then are able to
reassemble in a unique structure.
According to [2] – [9] a variety of
reconfigurable robots have been developed, as
follows:
- Crystalline (Fig. 1c): each module, with a
square cross section, equipped with on-board
CPU, IR sensors and power supply, can expand
its size and connect in plane with another
module;
- Molecube (Fig. 1d): a cube is split into two
parts along a plane that is perpendicular to its
long diagonal; one half of the cube can swivel
about the long axis in increments of 120º , each
time cycling the faces of the cube.
- M-Tran (Fig. 1e): is composed of two
semi-cylindrical boxes (with a servo each)
connected by a link mechanism.
7 - 8 JUNE 2007
a b
c d
e f
g h
Fig. 1 Examples of developed reconfigurable robots
i

- CONRO (Fig. 1f): each module has two
DOF, is 108 mm long and weighs 115g; is
equipped with two motors, batteries, a
microcontroller and IR communication system;
- Molecule (Fig. 1g): each molecule consists
of a pair of two DOF atoms, connected by a
link; twelve movements of each atom can
perform self-reconfiguration;
- Metamorphic (Fig. 1h): the modules,
having a planar hexagonal shape with 3 DOF,
can connect, disconnect and rotate around its
neighbours;
- Polypod and PolyBot (Fig. 1i): are based
on simple and versatile homogeneous systems;
Polypod contains two types of modules: a two
DOF segment with two connection mechanisms
and a passive cubic element with six connection
elements; PolyBot contains one DOF modules
equipped with an on-board computer and
sensing elements.
2. ANALYSIS OF THE DOCKING
MECHANISMS
The reconfiguration ability implies that the
modules contain connection mechanisms
equipped with actuators able to apply forces on
a latching mechanism so that the connector is
able to perform self-connection and
disconnection operations.
b c
Fig. 2 Examples of connection mechanisms
a
d
One problem of the connection mechanisms
is the orientation that must bring the
mechanisms at the right position and
orientation to plug in. Usually a male / female
system is designed to centred the component
during the plugging phase.
Figure 2 presents several docking
mechanisms of the above-presented selfreconfigurable
robots. The connection
mechanism of M-Tran utilizes rare-earth
magnets for attaching and shape memory alloy
coil springs for detaching (Fig. 2a).
Each unit of Fractum (Fig. 2b) has six arms:
three electromagnet male arms and three
permanent magnet female arms. If a neighbour
male has the same polarity of permanent
magnet, the connection occurs.
In [1] an active connector for electrical and
mechanical connection of components of a selfreconfigurable
robot is presented. It can
transmit axial, shear forces and torque due to a
shape memory alloy actuator used to move the
flexible lamellae endpoint and control thus a
connection / disconnection mechanism (Fig. 2c)
Polybot: is composed of one degree of
freedom modules connected end to end through
four pins, four holes and four electrical
connectors actuated with shape memory alloy
actuators. PolyBot has hermaphroditic
connection plates.
CONRO: in its structure there are active and
passive symmetric connectors based on SMA
actuators and permanent magnets. The only
way to disconnect two modules is from the
active side of the connection. Crystalline:
contains a passive and an active connector
based on a channel and rotating key concept.
Most of the above presented mechanisms are
too complicated and provide only electrical or
mechanical connection. Many docking
mechanism include shape memory alloy
elements as actuators of latching components.
3. SYNTHESIS CRITERIA FOR THE
DOCKING MECHANISMS
Any docking procedure implies following
steps: coordinate and align two modules based
on the docking guidance system, overcome the
inevitable errors in the alignment by
coordinated movements of both docking ends

and finally ensure the secure connection. After
docking, the modules must sense the new
connections and thus two connected modules
will move as a single module.
Based on functional and constructive
constrains imposed to the connection
mechanisms of self-reconfigurable robots,
several synthesis criteria, useful in developing
future mechanisms, are identified as follows:
- assuring both mechanical and electrical
connection in order to transmit different types
of forces and electrical signals with a simple
design, small number of components;
- connection / disconnection must be simple,
fast and secure;
- autonomous docking procedure;
- capacity to connect with an identical
connector;
- symmetrical structure to avoid orientation;
- ability for self-alignment;
- convenient and protected sensor placement;
- simplicity and stability of the latching
mechanism;
- low power consumption and no power to
maintain latching;
- impact and load strength, stiffness, protection
from the environment;
- easy access to the small number of moving
parts;
- possibility to be built using CNC machines or
rapid prototyping.
4. THE PROPOSED CONNECTION
MECHANISM
In this paper, a connection mechanism is
proposed. It is based on a semiautomatic
locking mechanism that works like an
automatic mechanism in one direction (locking
is automatically performed) and like a
commanded mechanism in opposite direction
(unlocking is realized through an exterior
command). Constructively, the locking
mechanism contains profiled elements. The
latching mechanism is released by using a
shape memory alloy actuator.
4.1. Shape Memory Actuator
Shape memory alloys represent a new class
of material, capable of transforming thermal
energy into mechanical work. Shape memory
effect is a property of certain materials to
recover some previously shape or size when
subjected to a heating procedure.
Fig. 3 The shape memory effect
In figure 3 the shape memory process is
shown microscopically: austenite is cooled to
form twinned martensite without undergoing a
shape change, and then is deformed by moving
twin boundaries. Heating will return the
originally austenitic structure and shape. The
austenite phase is represented by square lattice,
while the martensite is characterized by
rhombic lattice.
Electrical shape memory alloy actuators are
actuated via direct current (change in
temperature is internally generated by
resistance heating). Designing these shape
memory alloy actuators is an interdisciplinary
approach covering the design of the actuators
components shown in figure 4.
Fig. 4 The structure of shape memory alloy actuators
The power system provides energy to heat
the active elements and to operate the control
and drive circuitry. The control systems
provides “on” and “off” control to operate the
active elements. The driver system limits the
power to the active elements and protects them
from damage due to overheating. The active
elements provide the action. Selection of a
suitable alloy is a function of transformation
temperature, size of memory effect, size of
memory effect, hysteresis, and number of

cycles. Ni-Ti alloy (NITINOL) is most suitable
for applications requiring controllability, high
wok per unit volume, high number of cycles, an
low current for activation. The mechanical
associated structure supports the active
elements, permitting to act in the desired
manner and protects them from overstretching,
sharp bends and other forces, which could
damage or degrade their performance.
The advantages of these actuators are: small
size, light weight, high power to weight ratio,
smooth and silent operation, long life, and
precise controllability. The slow response on
cooling, the restricted energy efficiency and
some non-linear properties are the drawbacks.
For releasing of the latching element, an
actuator based on shape memory wire was
chosen. When heated over the temperature of
phase transformation, the wire will contract
with an amount of 3-5% of the length.
4.2 The 3D model of the mechanism
In figure 5 the geometrical model of the
proposed connection mechanism is presented.
a
b
Fig. 5 The 3D model of the connection mechanism
c
d
e

The male component placed on one face of a
cubic module (Fig. 5a) consists of two special
shaped docking pins. The female connector,
placed on one face of the second cubic module
(Fig. 5b) consists of two holes for accepting
other module’s docking pins.
The female connector has a locking /
releasing mechanism behind the holes. It has
two functioning states. In the non-active state, it
can accept and lock the incoming pins through
an intermediary oscillatory element actuated by
the elastic force given by elastic lamellae (leaf
spring). In the activated state, it can release the
lock due to the action of an actuator based on a
shape memory wire. One end of the fire is fixed
and another one is connected with the mobile
locking element. When a current pass the wire,
it is heating and then contracts and pulls the
oscillatory element. Its turning conducts to
disengaging of the modules. Figures 5 c,d and e
give some details concerning the coupled
modules and the structure of the locking
mechanism. The connector / releasing
mechanism is power efficient and it consumes
no electric energy when in the default state.
Fig. 6 The electrical scheme for the actuator control
A wire made of Ni-Ti alloy called
FLEXINOL was considered in our design. Its
diameter is 150 µm. The recommended current
to heat the wire (up to the transformation
temperature 70ºC) in half of a second is 180
mA, the linear resistance of the wire is 50 Ω/m.
The recovery force developed on heating due to
the shape memory effect is 3,23N and
deformation (or relaxation) force is 0,61N. This
force is give by the leaf spring. The
recommended shape memory effect of 4% was
taken into account in determine the necessary
length of the wire (50 mm, meaning 2 mm
contraction on heating – enough to disengage
the mechanism). The number of operation
cycles depends on the cooling time of the active
wire, less then 0,5 sec in normal environment
conditions. When a docking procedure is
performed, one module must signal its position
to other module and this must sense this signal.
Fig 6 gives a scheme already use by the
authors for controlling shape memory wire
actuators which contains a PWM circuit to
resistively heat the wire.
5. FURTHER RESEARCH
Our future efforts will be focussed on the
design of several connection mechanisms
placed on different faces of the cube, which
allow connecting a module with several similar
modules and making chains, trees and other
structures.
6. CONCLUSION
By changing their configuration, the selfreconfigurable
robots have various potential
applications in extreme environments
inaccessible to humans: in space or deep sea, in
nuclear plants, for urban search and rescue in
damaged buildings, military maintenance and
so on. They respond to client-oriented
production and task-oriented robotic system
requirements.
The docking mechanisms must be simple do
not add extra complexity to an already complex
system. They must respond to several
geometric and latching requirements, physical
robustness, energy transfer, maintenance and
manufacturing conditions.

A docking mechanism has been described. It
presents a self-latching mechanism and shape
memory alloy actuator to disengage. The
proposed variant is characterized by light
weight, small number of mobile parts, simple
actuation system and simple technology.
7. AKNOWLEDGEMENT
This work is supported by CE-EX M1-493
project no. 91/2006, Miniature robotic system
with self-reconfiguring and self-replicating
skills – ROMAR.
8. REFERENCES
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Mecanisme de cuplare-decuplare în structura roboţilor auto-reconfigurabili
Rezumat: Roboţii auto-reconfigurabili sunt sisteme modulare ce îşi pot schimba forma, sau se pot reconfigura, pentru a
adapta propria structură la sarcina de lucru. Printre condiţiile impuse structurilor robotizate auto-reconfigurabile se
numără şi capacitatea cuplare / decuplare a modulelor. În lucrare sunt sistematizate caracteristicile mecanismelor de
conectare, apoi sunt analizate cele mai importante metode de conectare, sunt date criterii de sinteza a mecanismelor de
cuplare / decuplare, după care este prezentată soluţia propusă de autori, având în structura un actuator pe baza de aliaje
cu memoria formei.
Dan Mândru, Professor, Technical University of Cluj-Napoca, Department of Mechanisms, Fine
Mechanics and Mechatronics, email: Dan.Mandru@mmfm.utcluj.ro, tel. +40264-401645
Ion Lungu, PhD Student, Technical University of Cluj-Napoca, Department of Mechanisms, Fine
Mechanics and Mechatronics, email: lungu_ion@yahoo.com, tel. +40264-401645
Olimpiu Tătar, Lecturer, Technical University of Cluj-Napoca, Department of Mechanisms, Fine
Mechanics and Mechatronics, email: olimpiut@yahoo.com, tel. +40264-401681