José Luis Villarroel

Enforcing Network Connectivity in Robot Team Missions

The growing interest in robot teams for surveillance or rescue missions entails new technological challenges. Robots have to move to complete their tasks while maintaining communication among themselves and with their human operators, in many cases without the aid of a communication infrastructure. Guaranteeing connectivity enables robots to explicitly exchange information needed in collaborative task execution, and allows operators to monitor or manually control any robot at all times. Network paths should be multi-hop, so as not to unnecessarily restrict the team’s range. In this work we contribute a complete system which integrates three research aspects, usually studied separately, to achieve these characteristics: a multi-robot cooperative motion control technique based on a virtual spring—damper model which prevents communication network splits, a task allocation algorithm that takes advantage of network link information in order to ensure autonomous mission completion, and a network layer which works over wireless 802.11 devices, capable of sustaining hard real-time traffic and changing topologies. Link quality among peers is the key metric used to cooperatively move the robots and maintain uninterrupted connectivity, and the basis for novel ideas presented in each subsystem. Simulations and experimental results with real robots are presented and discussed.

Real Time Communications over 802.11: RT-WMP

Ad-hoc networks usually support best-effort traffic and occasionally some kind of quality of service (QoS). However, there are some applications, which generally involve cooperative control, with hard real-time traffic requirements where strict deadlines must be met. To meet deadlines, the communication network has to support the timely delivery of inter-task messages. This is the case, for example, of applications involving cooperative robot teams, such as those used for rescue tasks in hostile environments, emergencies or disaster recovery, where a wired backbone is in-feasible or economically unviable. In this paper, we present RT-WMP, a novel protocol that allows wireless real-time traffic in relatively small mobile ad-hoc networks using the low-cost commercial 802.11 technology. The protocol is based on a token-passing approach and message exchange is priority based. Moreover, support for frequent topology changes is provided through the sharing of a matrix that describes link quality amongst the members of the network.

Using the time Petri net formalism for specification, validation, and code generation in robot-control applications

The main objective of this paper is to show the advantages of using the time Petri net formalism for specification, validation, and code generation in robot-control applications. To achieve this objective, we consider as application the development of a control system for a mobile robot with a rotating rangefinder laser sensor with two degrees of freedom to be used in navigation tasks with obstacle avoidance. It is shown how the use of the time Petri net formalism in the whole development cycle can fulfilll the reliability requirement of real-time systems, make the system development easy and quick, strongly reduce the time for the testing and tuning phases and, therefore, reduce the development cost significantly. It allows verification of functional and temporal requirements, error detection in the early stages of the development cycle, and automatic code generation, avoiding coding mistakes. Experimental tests show that the theoretical results obtained from the analysis of formal system models match the real-time behavior of the robotic system.

Real-time wireless multi-hop protocol in underground voice communication

The underground communication in tunnels and mines is very challenging due to the hostile nature of the environments and to the propagation issues that electromagnetic waves suffer there. Communication is often unidirectional (e.g. in mines) or very costly (e.g. leaky feeder in road tunnels) and hard to install and maintain. This work proposes the use of multi-hop ad-hoc networks to provide multimedia communication between mobile nodes in such a hostile environments, relying on a complete hardware/software, cheap and easy-to-setup platform that can be used both as temporary or fixed infrastructure or as communication backbone in emergency scenarios like mine accidents or a tunnel collapse. The communication is based on the Real-Time Multi-hop Protocol (RT-WMP) and its QoS extension executed over several nodes equipped with specific hardware. This protocol manages delay sensitive messages and the node mobility across the network while the QoS extension is responsible for allowing the end-to-end voice communication. The specific topology and situation have driven to a specialization of RT-WMP to better perform in this type of environments, taking advantage of the a priori (partial) knowledge about the topology. This proposal was tested in a real application in the Somport tunnel, the about 8km-long railroad linking Canfranc, Spain with Pau, France.

Signal-based deployment planning for robot teams in tunnel-like fading environments

Deploying a multi-robot team in confined environments poses multiple challenges that involve task and motion planning, localization and mapping, safe navigation, coordination of robots and also communications among all of them. In recent years, increasing attention has been paid to these challenges by the robotics community, but many problems remain unresolved. In this paper we address a technique for planning the deployment of a robot team in so-called fading environments, such as tunnels or galleries, where signal propagation presents specific characteristics. In order to maintain constant connectivity and high signal quality in the communication network formed by the robots and the base station, the robot deployment is driven by real-time signal measurements. First, an analysis of the signal propagation to obtain the general characteristic parameters of the signals in this kind of environment is carried out. Second, a technique which uses these parameters to drive the deployment is developed. A general strategy for this kind of environment in which the signals exhibit similar behavior is implemented. A complete system involving all of the above-mentioned robotics tasks has been developed. Finally, the system has been evaluated by means of simulation and in a real scenario.

A wireless multi-hop protocol for real-time applications

Mobile Ad-hoc NETworks (MANETs) have been gaining increasing popularity in recent years thanks to their ease of deployment and the low cost of their components. The routing protocols in ad hoc networks face the challenge of establishing and maintaining multi-hop routes while complying with mobility, bandwidth limitation and power constraints. The already demanding problem of offering wireless communication in a MANET becomes more complicated in the case of real-time systems where the loss or late arrival of a single item of data can cause serious problems. In this article, we propose a real-time wireless protocol for MANET capable of timely delivery of data. Taking advantage of a cross-layer design, it includes a novel medium access control mechanism and routing algorithm based on the link-quality among the nodes belonging to the network. The protocol manages message priority and is capable of multi-hop communications. It has been conceived mainly to provide real-time wireless communication for small robot teams, making possible the sharing of information such as kinematics or laser data. The validity of the protocol is proven by an in-depth theoretical analysis of its real-time characteristics and performance and through a set of real-world experiments.

Accurately Locating a Vertical Magnetic Dipole Buried in a Conducting Earth

The use of a buried vertical magnetic dipole to determine the position of a point inside the Earth is based on the geometric structure of the measured field. A new systematic technique that improves the location accuracy of the surface point in the axis of the buried dipole is developed and tested. It is based on the use of state-of-the-art topographic equipment for the null-field direction measurements, that are further processed by two estimation algorithms, namely, the extended information filter and the extended Kalman filter. In addition, emphasis is placed on the study of the sources of error in order to establish the accuracy of the method. This analysis is conducted in a half-space model in which electromagnetic propagation is studied by numerical integration of analytical solutions. The results are confirmed by a direct solution of the problem by the finite-element method. From the computations, simple equations are provided for practical use. Then, an experimental study shows that the uncertainty of the null-field direction measurements can be modeled by a Gaussian probability density function. In addition, the data of the field experiment are used to validate the proposed subsurface location technique.

Robot Teams for Intervention in Confined and Structured Environments

Safety, security, and rescue robotics can be extremely useful in emergency scenarios such as mining accidents or tunnel collapses where robot teams can be used to carry out cooperative exploration, intervention, or logistic missions. Deploying a multirobot team in such confined environments poses multiple challenges that involve task planning, motion planning, localization and mapping, safe navigation, coordination, and communications among all the robots. To complete their mission, robots have to be able to move in the environment with full autonomy while at the same time maintaining communication among themselves and with their human operators to accomplish team collaboration. Guaranteeing connectivity enables robots to explicitly exchange information needed in the execution of collaborative tasks and allows operators to monitor and teleoperate the robots and receive information about the environment. In this work, we present a system that integrates several research aspects to achieve a real exploration exercise in a tunnel using a robot team. These aspects are as follows: deployment planning, semantic feature recognition, multirobot navigation, localization, map building, and real-time communications. Two experimental scenarios have been used for the assessment of the system. The first is the Spanish Santa Marta mine, a large mazelike environment selected for its complexity for all the tasks involved. The second is the Spanish-French Somport tunnel, an old railway between Spain and France through the Central Pyrenees, used to carry out the real-world experiments. The latter is a simpler scenario, but it serves to highlight the real communication issues.

Transversal fading analysis in straight tunnels at 2.4 GHz

In tunnel-like environments the electromagnetic waves propagate differently in comparison to free space and urban scenarios. This is because, depending on the working frequency, tunnels can act as waveguides. As a consequence of this some specific phenomena occur, such as fast and slow fadings, caused by the constructive and destructive interference due to the multipath propagation. Fadings along the longitudinal dimension of tunnels have been widely studied under different geometric conditions, antenna polarization and working frequencies. Nevertheless, fadings also take place in the transversal dimension or cross-section. This issue has been poorly treated in the literature. In this context, our research group has been investigating navigation algorithms for multi-robot teams in tunnel-like environments under communication constraints. As fadings affect the communication among robots, a study about this phenomenon is necessary. In this work we present a complete study about transversal fadings in the Somport tunnel at 2.4 GHz. Results after carrying out a measuring campaign are presented and compared to modal theory based calculations, showing that fadings present a similar behavior in each transversal half of the tunnel (with the same period and phase, and with a phase delay between both halves) but different in the center, where although no deep fadings appear, the mean value of the signal power is lower compared to the fadings maximums. This led us to define a comfort-zone for communications for robot navigation in tunnels.

UHF and SHF Fading Analysis Using Wavelets in Tunnel Environments

The propagation of electromagnetic waves in tunnel- like environments differs from other scenarios, basically because tunnels can act as a waveguide, allowing the communication range to be extended. Nevertheless, phenomena such as strong fadings appear. In this context, our research group has been performing multi-robot navigation under communication constraints in this kind of environments. It is clear that fadings greatly affect communication among robots, and an in-depth study of these phenomena is required. In this paper, we present the results of a measuring campaign carried out in the Somport tunnel (in Spain) at 433 MHz, 868 MHz, 2.4 GHz and 5.2 GHz. Slow and fast fadings, as well as near and far sectors are analyzed using wavelets, a tool never used before to study propagation in tunnels. Results obtained from the wavelets and from classic filtering techniques are compared, showing that the former have the advantage of condensing most of the useful and valid information in a single graph.