As researchers have pushed the limits of what can be accomplished by a single robot operating in a known or unknown environment, a greater emphasis has been placed on the utilization of mobile multi-robotic systems to accomplish various objectives. In transitioning from a robot-centric approach to a system-centric approach, considerations must be made for the computational and communicative aspects of the group as a whole, in addition to electromechanical considerations of individual robots. This paper reviews the state-of-the-art of mobile multi-robotic system research, with an emphasis on the confluence of mapping, localization and motion control of robotic system. Methods that compose these three topics are presented, including areas of overlap, such as integrated exploration and simultaneous localization and mapping. From these methods, an analysis of benefits, challenges and tradeoffs associated with multi-robotic system design and use are presented. Finally, specific applications of multi-robotic systems are also addressed in various contexts.

In this paper we consider the problem of planning paths for articulated bodies operating in workplaces containing obstacles and regions with preferences expressed as degrees of desirability. Degrees of desirability could specify danger zones and desire zones. A planned path should not collide with the obstacles and should maximize the degrees of desirability. Region desirability can also convey search-control strategies guiding the exploration of the search space. To handle desirability specifications, we introduce the notion of flexible probabilistic roadmap (flexible PRM) as an extension of the traditional PRM. Each edge in a flexible PRM is assigned a desirability degree. We show that flexible PRM planning can be achieved very efficiently with a simple sampling strategy of the configuration space defined as a trade-off between a traditional sampling oriented toward coverage of the configuration space and a heuristic optimization of the path desirability degree. For path planning problems in dynamic environments, where obstacles and region desirability can change in real time, we use dynamic and anytime search exploration strategies. The dynamic strategy allows the planner to replan efficiently by exploiting results from previous planning phases. The anytime strategy starts with a quickly computed path with a potentially low desirability degree which is then incrementally improved depending on the available planning time.

An algorithm for the tele-operation of mobile-manipulator systems with a focus on ease of use for the operator is presented. The algorithm allows for unified, intuitive, and coordinated control of mobile manipulators. It consists of three states. In the first state, a single 6-degrees-of-freedom (DOF) joystick is used to control the manipulator's position and orientation. The second state occurs when the manipulator approaches a singular configuration, resulting in the mobile base moving in a manner so as to keep the end-effector travelling in its last direction of motion. This is done through the use of a constrained optimization routine. The third state is entered when the operator returns the joystick to the home position. Both the mobile base and manipulator move with respect to one another keeping the end-effector stationary and placing the manipulator into an ideal configuration. The algorithm has been implemented on an 8-DOF mobile manipulator and the test results show that it is effective at moving the system in an intuitive manner.

For decades Robotica has had a characteristic look and page layout that has served our readership well. However, recent advances in publishing technology make now an excellent time to experiment with some new ideas. From January 2014 we will be using an exciting new cover design concept that will include unique artwork featuring one or more of the topics covered in each issue. Moreover, we will be changing from our traditional double-column format to a new single-column one that will facilitate larger figures, tables and equations. At the end of this month's issue, we will include a sneak peek at the new page format via an article typeset in this new single-column style. We hope that our readership embraces this change. And we look forward to continuing to publish papers of high quality and broad international appeal.

Over the past two decades, robots have been increasingly used in biomedical applications such as bone cutting. Traditional automated manufacturing processes are often unable to meet the safety and accuracy requirements for such applications, particularly for cutting inhomogeneous constitutions of bone. In this case, human–robot cooperation may prove to be an effective approach. In this paper, we demonstrate that a hybrid parallel manipulator under cooperative force control can achieve accurate bone cutting with sufficient safety guaranteed. First, a hybrid parallel manipulator was constructed to provide the required rigidity for bone cutting. Then a two-loop controller was designed to implement the human–robot cooperation in bone cutting. The position control loop of adaptive fuzzy control is responsible for achieving high-tracking performance by overcoming varying friction forces from the mechanism. The force control loop of the cooperative force control adjusts the feed rate of the cutter according to the bone slicing conditions and operator's supervisory commands. The experimental results show that the proposed controller can effectively achieve the required accuracy in bone cutting with required safety.

Walking with a maximum speed is an interesting subject in the field of biped motion. Giving an answer to the question of “what is the maximum achievable speed of a certain biped walking with a physically acceptable pattern?” is the main objective of this work. In this paper, minimum time motion of biped was studied during one step that consists of single support phase (SSP) and double support phase (DSP). The minimum time problem is formulated with stability and non-slip conditions along with actuator limits expressed as some inequality constraints. In addition, certain kinematic constraints in terms of hip joint position are considered that ensure an acceptable walking pattern. A phase-plane technique is used to find the minimum time solution. A numerical simulation is given to shed some light on how the proposed method works. Validity and effectiveness of the method are verified by comparing the results with those of other researches.

Tangential motion on a finger pad is a promising method of transmitting directional tactile information to human users. This study examined the identification and discrimination of tangential force motion on an index finger pad. An experimental device was built to automatically and randomly move a small probe in eight radial directions (45° apart) and two distances (0.5 and 1.5 mm). Index fingers of 62 subjects were tested. The results showed that moving the probe at 1.5 mm was detected with more accuracy than the 0.5 mm one. And, the absolute direction was not a statistically significant variable affecting accuracy for 1.5 mm distance, but was a significant effect for 0.5 mm distance. Implications of these results are discussed and future developments are offered within the context of a proposed Braille design with tangential actuators.

In this paper a new sliding mode controller for set-point control of robot manipulators is proposed. The controller does not use any part of the robot dynamics in the control law. Thus, it is structurally simpler than other sliding mode controllers where the control law uses a nominal model of the robot dynamics. The controller uses a new nonlinear Proportional-Integral-Derivative (PID) sliding surface. The stability of the controlled robot dynamics is proved. On applying the boundary-layer approach to remove chattering, a nonlinear PID controller exists inside the boundary layer. This PID controller ensures that the error tend to zero asymptotically if there is no disturbances applied to the robot dynamics.

A novel 3-leg 5-DoF parallel manipulator (PM) with a UPU-type composite active constrained leg is proposed and its kinematics and statics are analyzed systematically. First, the formulae are derived for solving the inverse/forward displacements, inverse/forward velocities, and active/constraint forces. Second, the formulae are derived for solving inverse/forward accelerations. Third, a simulation mechanism of this PM is created and its workspace is constructed and analyzed. Finally, the analytic results are verified by its simulation mechanism.

We present a review of recent activities in swarm robotic research, and analyse existing literature in the field to determine how to get closer to a practical swarm robotic system for real world applications. We begin with a discussion of the importance of swarm robotics by illustrating the wide applicability of robot swarms in various tasks. Then a brief overview of various robotic devices that can be incorporated into swarm robotic systems is presented. We identify and describe the challenges that should be resolved when designing swarm robotic systems for real world applications. Finally, we provide a summary of a series of issues that should be addressed to overcome these challenges, and propose directions for future swarm robotic research based on our extensive analysis of the reviewed literature.

This paper presents the preimpact configuration of a dual-arm space manipulator with a prismatic joint for capturing an object based on the momentum conservation principle. A unique precapture configuration “generalized straight-arm capture” (GSAC) is proposed based on the dual-arm space manipulator with a prismatic joint and the corresponding angular relation is obtained. The configuration satisfies GSAC and can reduce the effect of system's angular momentum caused by the impact force during the capture operation and the burden of postimpact control, so it avoids the limitation of joint velocity and actuator torque when controlling the compound (a manipulator with a prismatic joint and an object) and guarantees the stability of the system. Finally, the effectiveness of the method is demonstrated by numerical simulations.

This work presents the experimental validation and updating of a flexible multibody model ideated for taking into account installation conditions of industrial serial planar manipulators without resorting to cumbersome modeling. The flexibility of the frame, the manipulator is fixed, is modeled over the flexibility of joints, which is introduced as lumped stiffness. In particular, the flexible frame is included in the model by using the Component Mode Synthesis methodology, in which only the natural modes of vibration and the static constrain modes are accounted. The flexible multibody model has been developed because these commercial machines are mainly used to perform low-speed tasks, and they are designed by taking into account their flexibility at most in the joints. Unfortunately, there are particular installation conditions in which even low-speed tasks can generate low-frequency vibrations that highly interfere with the task. This aspect is considered here, and how to manage this problem is explained by using the developed multibody model. The model is validated through experimental measurements. The experimental tests consist of several modal analyses, together with acceleration and laser Doppler measurements in operational conditions. This methodology takes into account the installation conditions through the model of flexible frame, and gives a tool for studying ad hoc solutions which prevent the occurrence of unwanted low-frequency vibrations.

Experiments with a nonlinear trajectory-tracking controller for marine unmanned surface vessels are reported. The tracking controller is designed using a nonlinear robust model-based sliding mode approach. The marine vehicles can track arbitrary desired trajectories that are defined in Cartesian coordinate as continuous functions of time. The planar dynamic model used for the controller design consists of 3 degrees of freedom (DOFs) of surge, sway, and yaw. The vessel only has two actuators, so the vessel is underactuated. Therefore, only two outputs, which are functions of the 3-DOF, can be controlled. The Cartesian position of a control point on the vessel is defined as the output. The orientation dynamics is not directly controlled. It has been previously shown that the orientation dynamics, as the internal dynamics of this underactuated system, is stable. The result of field experiments show the effectiveness of tracking control laws in the presence of parameter uncertainty and disturbance. The experiments were performed in a large outdoor pond using a small test boat. This paper reports the first theoretical development and experimental verification of the proposed model-based nonlinear trajectory-tracking controller.

This paper considers the lateral undulation motion of snake robots. The first contribution of the paper is a model of lateral undulation dynamics developed for control design and stability analysis purposes. The second contribution is an analysis of the simplified model that shows that any asymptotically stabilizing control law for the snake robot to an equilibrium point must be time varying. Furthermore, the analysis shows that a snake robot (with four links) is strongly accessible from almost any equilibrium point, except for certain singular configurations, and that the robot does not satisfy sufficient conditions for small-time local controllability. The third contribution is based on using averaging theory to prove that the average velocity of the robot during lateral undulation will converge exponentially fast to a steady-state velocity which is given analytically as a function of the gait pattern parameters. From the averaging analysis, we also derive a set of fundamental relationships between the gait parameters of lateral undulation and the resulting forward velocity of the snake robot. The paper presents simulation results and results from experiments with a physical snake robot that support the theoretical findings.

This paper presents a new approach to matching occupancy grid maps by means of finding correspondences between a set of sparse features detected in the maps. The problem is stated here as a special instance of generic image registration. To cope with the uncertainty and ambiguity that arise from matching grid maps, we introduce a modified RANSAC algorithm which searches for a dynamic number of internally consistent subsets of feature pairings from which to compute hypotheses about the translation and rotation between the maps. By providing a (possibly multi-modal) probability distribution of the relative pose of the maps, our method can be seamlessly integrated into large-scale mapping frameworks for mobile robots. This paper provides a benchmarking of different detectors and descriptors, along extensive experimental results that illustrate the robustness of the algorithm with a 97% success ratio in loop-closure detection for ~1700 matchings between local maps obtained from four publicly available datasets.

This paper proposes a novel dynamic stable grasping method of an arbitrary polyhedral object for a hand-arm system with hemispherical fingertips. This method makes it possible to satisfy the force/torque equilibrium condition for the immobilization of the object without knowledge of the object. Two control signals are proposed which generate grasping forces normal and tangential to an object surface in a final state. The dynamics of the overall system is modeled and analyzed theoretically. We demonstrate the stable grasping of an arbitrary polyhedral object using the proposed controller through numerical simulations and experiments using a newly developed mechanical hand-arm system.

This paper introduces a probabilistic model for collision risk assessment between moving vehicles. The uncertainties in the states and the geometric variables obtained from the sensory system are characterized by probability density functions. Given the states and their uncertainties, the goal is to determine the probability of collision in a dynamic environment. Two approaches are discussed: (1) The virtual configuration space (VCS), and (2) the rates of change of the visibility angles. The VCS is a transformation of observer that reduces collision detection with a moving object to collision detection with a stationary object. This approach allows to create simple geometric collision cones. Error propagation models are used to solve the problem when going from the VCS to the configuration space. The second approach derives the collision conditions in terms of the rate of change of the limit visibility angles. The probability of collision is then calculated. A comparison between the two methods is carried out. Results are illustrated using simulation, including Monte Carlo simulation.

We investigate the controlled realization of a stable circular pursuit model in a multi-agent robotic system described by double-integrator dynamics with homogeneous controller gains. The dynamic convergence of the system starting from a randomly chosen, non-overlapping initial configuration to a sustained, stable pursuit configuration satisfying velocity matching and uniform inter-agent separation is demonstrated using the proposed control framework. The cyclic pursuit configuration emerges from local, linear, inter-agent interactions and is shown to be robust under stochastic perturbations of small and moderate intensities. The stability criterion discussed in this work is independent of the number of agents, permitting dynamic addition/deletion of agents without affecting overall system stability. Experimental results that validate the key theoretical results are also presented. Potential applications of the results obtained include cooperative perimeter tracking and resource distribution applications such as border patrol and wildfire monitoring.

Hazardous terrains pose a crucial challenge to mobile robots. To operate safely and efficiently, it is necessary to detect the terrain type and modify operation strategies in real-time. Fast analytical models of wheeled locomotion on deformable terrains are thus important. Based on classic terramechanics, a closed-form wheel–soil interaction model was derived by quadratic approximation of stresses along the wheel–soil interface. The bulldozing resistance and the effects of grousers were also added for more accurate prediction of wheel contact forces. A non-iterative method was proposed to estimate the entry angle, by using approximated vertical pressure acting on the wheels. The computational efficiency was improved by avoiding traditional recursive search. Real-time computation of the wheel contact forces is achieved by the terramechanics-based formula (TBF), which was developed by integrating the wheel–soil interaction model and the entry angle estimator. In addition, an automotive-inspired approach was used to integrate the TBF and the simplified vehicle dynamics model for fast simulation of mobile robots. Stability problems in numerical simulation could be avoided by this method. The above models were verified by comparing simulation results and experiment data, including single-wheel experiments and full-vehicle experiments.

This work presents an approach to motion planning for robotic manipulators that aims at improving path quality in terms of safety. Safety is explicitly assessed using the quantity called danger field. The measure of safety can easily be embedded into a heuristic function that guides the exploration of the free configuration space. As a result, the resulting path is likely to have substantially higher safety margin when compared to one obtained by regular planning algorithms. To this end, four planning algorithms have been proposed. The first planner is based on volume trees comprised of bubbles of free configuration space, while the remaining ones represent modifications of classical sampling-based algorithms. Several numerical case studies are carried out to validate and compare the performance of the presented algorithms with respect to classical planners. The results indicate significantly lower danger metric for paths obtained by safety-oriented planners even with some decrease in running time.