TechTalks from event: Technical session talks from ICRA 2012

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Redundant robots

  • Motion Control of Redundant Robots under Joint Constraints: Saturation in the Null Space Authors: Flacco, Fabrizio; De Luca, Alessandro; Khatib, Oussama
    We present a novel efficient method addressing the inverse differential kinematics problem for redundant manipulators in the presence of different hard bounds (joint range, velocity, and acceleration limits) on the joint space motion. The proposed SNS (Saturation in the Null Space) iterative algorithm proceeds by successively discarding the use of joints that would exceed their motion bounds when using the minimum norm solution and reintroducing them at a saturated level by means of a projection in a suitable null space. The method is first defined at the velocity level and then moved to the acceleration level, so as to avoid joint velocity discontinuities due to the switching of saturated joints. Moreover, the algorithm includes an optimal task scaling in case the desired task trajectory is unfeasible under the given joint bounds. We also propose the integration of obstacle avoidance in the Cartesian space by properly modifying on line the joint bounds. Simulation and experimental results reported for the 7-dof lightweight KUKA LWR IV robot illustrate the properties and computational efficiency of the method.
  • Priority Oriented Adaptive Control of Kinematically Redundant Manipulators Authors: Sadeghian, Hamid; Keshmiri, Mehdi; Villani, Luigi; Siciliano, Bruno
    In this paper an adaptive multi-priority nonlinear control algorithm for a redundant manipulator system is developed based on the Lyapunov like approach. The method considers the parametric uncertainties in the system and defines a proper filtered error signal to achieve asymptotic stability and convergence in tracking error both for the main task and sub-tasks according to the allocated priority. The performance of the proposed method is studied by some numerical simulations.
  • Resolving the Redundancy of a Seven DOF Wearable Robotic System Based on Kinematic and Dynamic Constraint Authors: Kim, Hyunchul; Li, Zhi; Milutinovic, Dejan; Rosen, Jacob
    According to the seven degrees of freedom (DOFs) human arm model composed of the shoulder, elbow, and wrist joints, positioning of the wrist in space and orientating the palm is a task requiring only six DOFs. Due to this redundancy, a given task can be completed by multiple arm configurations, and there is no unique mathematical solution to the inverse kinematics. The redundancy of a wearable robotic system (exoskeleton) that interacts with the human is expected to be resolved in the same way as that of the human arm. A unique solution to the system's redundancy was introduced by combining both kinematic and dynamic criteria. The redundancy of the arm is expressed mathematically by defining the swivel angle: the rotation angle of the plane including the upper and lower arm around a virtual axis connecting the shoulder and wrist joints which are fixed in space. Two different swivel angles were generated based on kinematic and dynamic constraints. The kinematic criterion is to maximize the projection of the longest principle axis of the manipulability ellipsoid for the human arm on the vector connecting the wrist and the virtual target on the head region. The dynamic criterion is to minimize the mechanical work done in the joint space for each two consecutive points along the task space trajectory. These two criteria were then combined linearly with different weight factors for estimating the swivel angle. Post processing of experimental data collected with a motion capturing
  • Dual-Arm Redundancy Resolution Based on Null-Space Dynamically-Scaled Posture Optimization Authors: Zanchettin, Andrea Maria; Rocco, Paolo
    Dual-arm robotic systems have been intensively studied in the literature. However, in industrial robotics, the resolution of the kinematic redundancy allowed by the coordinated manipulation task is still an open issue. In fact, typical proprietary industrial robotic controllers do not allow the programmer to modify the inverse kinematics algorithm, and thus to solve redundancy following any specified criterion. In this paper a method to enforce an arbitrary redundancy resolution criterion on top of an industrial robot controller is discussed and applied to the execution of a coordinated manipulation task. The extra degrees of freedom are used to maximize the dynamic manipulability measure in order to reduce the needed torque. Simulations and experimental results achieved on an ABB IRC 5 industrial robot controller are presented.
  • Optimal Decentralized Gait Transitions for Snake Robots Authors: Droge, Greg; Egerstedt, Magnus
    Snake robots are controlled by implementing gaits inspired from their biological counterparts. However, transitioning between these gaits often produces undesired oscillations which cause net movements that are difficult to predict. In this paper we present a framework for implementing gaits which will allow for smooth transitions. We also present a method to determine the optimal time for each module of the snake to switch between gaits in a decentralized fashion. This will allow for each module to participate in minimizing a cost by communicating with a set of modules in a local neighborhood. Both of these developments will help to maintain desired properties of the gaits during transition.

High Level Robot Behaviors

  • Automated Feedback for Unachievable High-Level Robot Behaviors Authors: Raman, Vasumathi; Kress-Gazit, Hadas
    One of the main challenges in robotics is the generation of controllers for autonomous, high-level robot behaviors comprising a non-trivial sequence of actions. Recently, formal methods have emerged as a powerful tool for automatically generating autonomous robot controllers that guarantee desired behaviors expressed by a class of temporal logic specifications. However, when there is no controller that fulfills the specification, these approaches do not provide the user with a source of failure, making the troubleshooting of specifications an unstructured and time-consuming process. In this paper, we describe a procedure for analyzing an unsynthesizable specification to identify causes of failure. We also provide an interactive game for exploring possible causes of failure, in which the user attempts to fulfill the robot specification against an adversarial environment. Our approach is implemented within the LTLMoP toolkit for robot mission planning.
  • Backtracking Temporal Logic Synthesis for Uncertain Environments Authors: Livingston, Scott; Murray, Richard; Burdick, Joel
    This paper considers the problem of synthesizing correct-by-construction robotic controllers in environments with uncertain but fixed structure. "Environment" has two notions in this work: a map or "world" in which some controlled agent must operate and navigate (i.e., evolve in a configuration space with obstacles); and an adversarial player that selects continuous and discrete variables to try to make the agent fail (as in a game). Both the robot and the environment are subjected to behavioral specifications expressed as an assume-guarantee linear temporal logic (LTL) formula. We then consider how to efficiently modify the synthesized controller when the robot encounters unexpected changes in its environment. The crucial insight is that a portion of this problem takes place in a metric space, which provides a notion of nearness. Thus if a nominal plan fails, we need not resynthesize it entirely, but instead can "patch" it locally. We present an algorithm for doing this, prove soundness (correctness of output), and demonstrate it on an example gridworld.
  • On the Revision Problem of Specification Automata Authors: KIM, Kangjin; Fainekos, Georgios; Sankaranarayanan, Sriram
    One of the important challenges in robotics is the automatic synthesis of provably correct controllers from high level specifications. One class of such algorithms operates in two steps: (i) high level discrete controller synthesis and (ii) low level continuous controller synthesis. In this class of algorithms, when phase (i) fails, then it is desirable to provide feedback to the designer in the form of revised specifications that can be achieved by the system. In this paper, we address the minimal revision problem for specification automata. That is, we construct automata specifications that are as ``close" as possible to the initial user intent, by removing the minimum number of constraints from the specification that cannot be satisfied. We prove that the problem is computationally hard and we encode it as a satisfiability problem. Then, the minimal revision problem can be solved by utilizing efficient SAT solvers.
  • LTL Robot Motion Control Based on Automata Learning of Environmental Dynamics Authors: Chen, Yushan; Tumova, Jana; Belta, Calin
    We develop a technique to automatically generate a control policy for a robot moving in an environment that includes elements with partially unknown, changing behavior. The robot is required to achieve an optimal surveillance mission, in which a certain request needs to be serviced repeatedly, while the expected time in between consecutive services is minimized. We define a fragment of Linear Temporal Logic (LTL) to describe such a mission and formulate the problem as a temporal logic game. Our approach is based on two main ideas. First, we extend results in automata learning to detect patterns of the partially unknown behavior of the elements in the environment. Second, we employ an automata-theoretic method to generate the control policy. We show that the obtained control policy converges to an optimal one when the unknown behavior patterns are fully learned. We implemented the proposed computational framework in MATLAB. Illustrative case studies are included.
  • Towards Formal Synthesis of Reactive Controllers for Dexterous Robotic Manipulation Authors: Chinchali, Sandeep; Livingston, Scott; Topcu, Ufuk; Burdick, Joel; Murray, Richard
    In robotic finger gaiting, fingers continuously manipulate an object until joint limitations or mechanical limitations periodically force a switch of grasp. Current approaches to gait planning and control are slow, lack formal guarantees on correctness, and are generally not reactive to changes in object geometry. To address these issues, we apply advances in formal methods to model a gait subject to external perturbations as a two-player game between a finger controller and its adversarial environment. High-level specifications are expressed in linear temporal logic (LTL) and low-level control primitives are designed for continuous kinematics. Simulations of planar manipulation with our synthesized correct-by-construction gait controller demonstrate the benefits of this approach.
  • Sequential Composition of Robust Controller Specifications Authors: Le Ny, Jerome; Pappas, George J.
    We present a general notion of robust controller specification and a mechanism for sequentially composing them. These specifications form tubular abstractions of the trajectories of a system in different control modes, and are motivated by the techniques available for certifying the performance of low-level controllers. The notion of controller specification provides a rigorous interface for connecting a planner and lower-level controllers that are designed and refined independently. With this approach, the planning layer does not integrate the closed-loop system dynamics and does not require the knowledge of how the controllers operate, but relies only on the specifications of the output tracking performance achieved by these controllers. The control layer aims at satisfying specifications that account quantitatively for robustness to unmodeled dynamics and various sources of disturbance and sensor noise, so that this robustness does not need to be revalidated at the planning level. As an illustrative example, we describe a randomized planner that composes different controller specifications from a given database to guarantee that any corresponding sequence of control modes steers a robot to a given region while avoiding obstacles.

Biologically Inspired Robotics

  • Estimation of Relative Position and Coordination of Mobile Underwater Robotic Platforms through Electric Sensing Authors: Morel, Yannick; Porez, Mathieu; Ijspeert, Auke
    In the context of underwater robotics, positioning and coordination of mobile agents can prove a challenging problem. To address this issue, we propose the use of electric sensing, with a technique inspired by weakly electric fishes. In particular, the approach relies on one or several of the agents applying an electric field to their environment. Using electric measures, others agents are able to reconstruct their relative position with respect to the emitter, over a range that is function of the geometry of the emitting agent and of the power applied to the environment. Efficacy of the technique is illustrated using a number of numerical examples. The approach is shown to allow coordination of unmanned underwater vehicles, including that of bio-inspired swimming robotic platforms.
  • Localization of Small Objects with Electric Sense Based on Kalman Filter Authors: Lebastard, Vincent; Chevallereau, Christine; GIRIN, Alexis; Boyer, Frédéric; Gossiaux, Pol Bernard
    Electric fish feel the perturbations of a selfgenerated electric field through their electro-receptive skin. This sense allows them to navigate and reconstruct their environment in conditions where vision and sonar cannot work. In this article, we use a sensor bio-inspired from this active sense to address the problem of small objects reconstruction and electrolocation. Based on a Kalman filter, any small object in the surrounding of the motion controlled sensor can be encapsulated into an equivalent sphere whose location is well estimated by the filter. Experimental results illustrate the feasibility of the approach.
  • Non-Visual Orientation and Communication by Fishes Using Electrical Fields: A Model System for Underwater Robotics Authors: von der Emde, Gerhard; Gebhardt, Kristina; Behr, Katharina
    Building autonomous underwater robots is a challenging problem. Different sensory modalities have been employed successfully, some inspired by human and animal senses. The European ANGELS project uses an electric sense inspired by weakly electric fish. These fish have the unique ability to navigate and orient in complete darkness by using self-produced electrical fields. They emit electric signals into the environment, which in turn they perceive with an array of electroreceptor organs in their skin. The fish's whole body serves as an antenna, which shapes the emitted electrical field. As a result, the animals are able to detect, localize and analyze objects in their vicinity and to perceive a 3-dimensional electrical picture of their surroundings. Here, we review biological experimental results highlighting the animal's perceptual abilities, which allow them to navigate in extreme environments where vision can not be used. In addition, electric fishes use electric signals for communication. Behavioral communication strategies such as synchronization of electric signals and fixed-order-signaling can play a role in group coherence. Because of their unique sensory abilities, electric fish can serve as a model system for roboticists building underwater vehicles that can communicate and navigate in extreme environments where vision is not possible. In ANGELS, the electric sense is used to navigate a robot without knowledge of the surroundings, keep multi robots in formation, reco
  • An Underwater Reconfigurable Robot with Bioinspired Electric Sense Authors: Mintchev, Stefano; Stefanini, Cesare; GIRIN, Alexis; Marrazza, Stefano; Orofino, Stefano; Lebastard, Vincent; Manfredi, Luigi; Dario, Paolo; Boyer, Frédéric
    Morphology, perception and locomotion are three key features highly inter-dependent in robotics. This paper gives an overview of an underwater modular robotic platform equipped with a bio-inspired electric sense. The platform is reconfigurable in the sense that it can split into independent rigid modules and vice-versa. Composed of 9 modules, the longer entity can swim like an eel over long distances, while once detached, each of its modules is efficient for small displacements with a high accuracy. Challenges are to mechanically ensure the morphology changes and to do it automatically. Electric sense is used to guide the modules during docking phases and to navigate in unknown scenes. Several aspects of the design of the robot are described and a particular attention is paid to the inter-module docking system. The feasibility of the design is assessed through experiments.
  • Underwater Electro-Navigation in the Dark Authors: Lebastard, Vincent; Boyer, Frédéric; Chevallereau, Christine; Servagent, Noël
    This article proposes a solution to the problem of the navigation of underwater robots in confined unstructured environments wetted by turbid waters. The solution is based on a new sensor bio-inspired from electric fish. Exploiting the morphology of the sensor as well as taking inspiration from passive electro-location in real fish, the solution turns out to be a sensory-motor loop encoding a simple behavior relevant to exploration missions. This behavior consists in seeking conductive objects while avoiding insulating ones. The solution is illustrated on experiments. It is robust and works even in very unstructured scenes. It does not require any model and is quite cheap to implement
  • Electric Sensor Based Control for Underwater Multi-Agents Navigation in Formation Authors: Chevallereau, Christine; Boyer, Frédéric; Lebastard, Vincent; benachenhou, mohamed
    Thanks to an electro-sensible skin, some species of fish can feel the perturbations of a self generated electric field caused by their surroundings variations. Known under the name of "electric-sense", this ability allows these fish to communicate and navigate in confined surroundings wetted by turbid waters where vision and sonar cannot work. Based on a bio-inspired electric sensor recently proposed in [1], this article presents a first attempt to use electric sense for the navigation in formation of a set of rigid underwater vehicles. The navigation strategy combines some behaviors observed in electric fish as well as a follower-leader strategy well known from multi-robot navigation. Being based one the servoing of the electric measurements, these laws do not require the knowledge of the location of the agents as this is usually the case in multi-robot navigation. At the end of the analysis, sufficient convergence conditions of the resulting control laws are given. Moreover, some limits on the possible motion of the leader are exhibited and the importance of the choice of controlled outputs is discussed too. Finally, simulation results illustrate the feasibility of the approach.