Nano-wireless Communication for Microrobotics: bridging the gap Ludovico Ferranti Francesca Cuomo University of Roma Sapienza University of Roma Sapienza, DIET Rome, Italy Rome, Italy [email protected] [email protected] ABSTRACT will allow MEMS microrobots to be used in many areas of Among all the new robotics technologies, great interest is ris- everyday life. Usually, MEMS nodes size varies from well ing in the field of microrobotics. Micro Electro-Mechanical below one micron on the lower end of the dimensional spec- Systems (MEMS) microrobots are miniaturized electro-me trum to several millimeters. Their applications require a chanical devices that can perform various tasks in a wide massive deployment of nodes, thousands or even millions, area of applications. Despite the low-power and low-memory which define a new concept of Distributed Intelligent MEMS capacity, they are provided with sensors and actuators. Self- (DiMEMS). A DiMEMS system is composed of a compact reconfiguration is a key factor for MEMS microrobots to swarm of MEMS nodes. Several applications need the en- perform their tasks and to optimize their communications in semble shape to be designed in particular topologies there- order to achieve efficiency, parallelism and scalability. Nano- fore efficient communication among nodes is crucial. The transceivers and nano-antennas operating in the Terahertz very limited power supply of microrobots makes it difficult Band o↵er a promising communication paradigm, providing to implement a precise movement to reach a certain desti- nanowireless networking directly integrated in MEMS mi- nation position and it is still one of the major challenges crorobots. Catoms from Claytronics project are an appro- in this field. Although the power consumption for moving priate microrobotics case to explore this novel framework. has been lowered, communication and computation require- Several logical topology shape-shifting algorithms have been ments still represent a challenge in terms of energy. Opti- implemented and tested, along with di↵erent nano-wireless mizing the number of movements and message exchanges of simulations. This paper aims to provide a survey on nano- microrobots is therefore crucial in order to save energy. wireless communication for modular robotics and propose some optimization choices. Special emphasis is given to the use of the nano-wireless communications for topology for- mation and maintenance in microrobotics. Categories and Subject Descriptors Figure 1: Catoms moving principle [5] C.2.1 [Network Architecture and Design]: ;I.2.9[Robotics]: Controlling the self-reconfiguration process is very com- plex because it involves the distributed coordination of large numbers of identical modules connected in time-varying ways. Keywords The range of exchanged information and the amount of dis- MEMS Microrobots; Nano-networks; Terahertz band; Dis- placements determine the communication and energy com- tributed algorithm; Topology Formation plexity of the distributed algorithm. As soon as the informa- 1. INTRODUCTION tion exchange involves close neighbors, the complexity is low and the resulting distributed self-reconfiguration smoothly Micro electro mechanical system (MEMS) is the new fron- scales along with network size. Considering a moderate com- tier of technology that empowers the batch fabrication of plexity in message, execution time, number of movements miniature mechanical devices. MEMS are miniaturized and and memory usage finding an optimal configuration is still low-power devices with the ability to sense and act in their an open issue and the logical topology of the network has environment. High expectation is put in the mass produc- to optimize through rearrangement of the physical topology. tion of MEMS nodes, making them a very a↵ordable tech- Several microrobotics projects are following these assump- nology. Their small size and the batch fabrication process tions. Harvard University Kilobots project aims at achiev- ing a scalable and self-assembly design for microrobotics Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed [21]. Swarms of Kilobots, composed of thousands of tiny for profit or commercial advantage and that copies bear this notice and the full cita- modular microrobots, move through vibration motors on a tion on the first page. Copyrights for components of this work owned by others than lucid plane, which is used to reflect infrared beams as a ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re- mean of communication. Another advanced instance of mi- publish, to post on servers or to redistribute to lists, requires prior specific permission crorobotics is the Claytronics project by Carnegie Mellon and/or a fee. Request permissions from [email protected]. University and Intel. In all these modular robots the possi- NANOCOM15 September 21 - 22, 2015, Boston, MA, USA Copyright 2015 ACM ISBN 978-1-4503-3674-1/15/09 ...$15.00. bility to have wireless communications may facilitate topol- http://dx.doi.org/10.1145/2800795.2800803. ogy formation and maintenance as well as lower the energy Table 1: Catoms parameters and characteristics [1] Macro Micro Nano Dimensions > 1 cm > 1 mm < 10 µm Weight tens of g hundreds of g < 1 mg Power <2W tens of mW tens of nW Locomotive Mecha- Programmable Electrostatics Aerosol nism Magnets or ElectroMagnets Adhesion Mechanism Nanofiber Adhe- Programmable Molecular surface sives or Magnets nanofiber adhe- adhesion and co- sives valent bonds Manufacturing Conventional Micro/Nano- Chemically Methods manufacturing fabrication and directed self- and assembly micro-assembly assembly and fabrication Resolution Low High High Cost $$$/catom $/catom Millicents/catom consumption of the microrobots. In the following we focus electrostatic force and as a direct communication mean. By on Claytronics project as an example and we investigate the actuating their features, catoms are able to move around use of nano-wireless communications framework. The re- their direct neighbors in the same way stepper motors nav- mainder of this paper is organized as follows: in Section II igate their surroundings (Fig. 1 and Fig. 2(a)). However, a brief presentation of Claytronics project is given, under- catoms maintain their ability to communicate. Di↵erent ver- lining how microrobots work and cooperate and the open sions of catoms has been built and dimensions go from 8 cu- challenges. Section III deals with nano-wireless communica- bic meters (Giant Helium Catoms) to 4 centimeters (Planar tions operating in Terahertz band, Section IV will present an Catoms in Figure 2(b)) and to few millimeters (Millime- addressing scheme that is feasible with microrobotics con- ters Scale Catoms) [1]. Bigger catoms are used to evaluate straints and finally Section V will summarize the ideas about forces interaction and physical properties. Table 1 summa- using nanowireless communication for microrobotics. rizes technical parameters of catoms such as weight, power consumption and manufacturing methods. The adoption of wireless communications for catoms has been investigated in 2. CLAYTRONICS MICROROBOTS [4]. Results show that wireless communications in a network Claytronics is a concept that combines nanoscale robotics enhances the way catoms are communicating, improving the and computer science to create individual nanometer-scale communication range and allowing easy way of broadcast- computers called catoms, which can interact with each other ing information. Several catoms form an ensemble, which to form shapes. This idea is more broadly referred to as Pro- is a dense environment in the communication range. Most grammable Matter. Other approaches to modular robotics control algorithms would improve performances from broad- used to create a conglomerate of tens or even hundreds of casting capability, speeding information di↵usion process, small autonomous robots which move in coordination to allowing neighbor discovery and providing ability to com- achieve a global e↵ect not possible by any single unit [1]. municate with non contiguous groups of catoms. Broad- casted messages can also be used as a form of synchroniza- tion, which is very useful when coordinating movements of groups of catoms. In Claytronics energy is provided to each catom from an external power source, and if required can be routed from catom to catom. From a topological point of view, a catoms ensemble can be viewed as a connected undirected graph G =(V ; E) modeling the MEMS network. v V , is a node belonging to the network and, e E is2 a bidirectional edge representing the communication2 be- (a) Node modeling, (b) Two planar tween two physical neighbors. For each node v V , the in each movement catoms set of neighbors of v is denoted as N(v)=u;(u2; v) E. the node travels the 2 same distance Each node v V knows the set of its neighbors in G, N(v). Furthermore,2 the set N(v) is the unique initial and instanta- neous information received by the node. It is assumed that every node systematically updates the set of its neighbors N(v) after a local change. In [7] the following definitions are given: Connectivity: a graph G =(V ; E) is said connected, if • v V ; u V a tuple Cv,u =(ev; ; ...; e( , ); ... (c) Traveled distance 8 2 8 2 9 − − − ; e( ,u)), where ex,y E is a edge from x to y and Cv,u Figure 2: Catoms characteristics and features [7] represents− a path from2 v to u. The main design goal of Claytronics is to scale to mil- Snap-Connectivity:LetT be the full-time running of • lions of micro-scale units. Each Claytronics MEMS micro- a Distributed Algorithm (DA), and t1,t2,...,tn are dif- robot is equipped with structures called “features”. Those ferent time instants of DA execution (rounds). Let the features are used as attachments using electromagnetic or dynamic graph Gt(Vt; Et) the network state at the in- (a) PASC (b) PAUC Figure 3: An example of execution of PASC [7] and PAUC [10] with 9 nodes stant t.