TechTalks from event: Technical session talks from ICRA 2012

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Embodied Intelligence - Complient Actuators

  • A Versatile Biomimetic Controller for Contact Tooling and Tactile Exploration Authors: Jarrasse, Nathanael; burdet, etienne; Ganesh, Gowrishankar; Haddadin, Sami; Albu-Schäffer, Alin
    This article presents a versatile controller that enables various contact tooling tasks with minimal prior knowledge of the tooled surface. The controller is derived from results of neuroscience studies that investigated the neural mechanisms utilized by humans to control and learn complex interactions with the environment. We demonstrate here the versatility of this controller in simulations of cutting, drilling and surface exploration tasks, which would normally require different control paradigms. We also present results on the exploration of an unknown surface with a 7-DOF manipulator, where the robot builds a 3D surface map of the surface profile and texture while applying constant force during motion. Our controller provides a unified control framework encompassing behaviors expected from the different specialized control paradigms like position control, force control and impedance control.
  • Passive Impedance Control of a Multi-DOF VSA-CubeBot Manipulator Authors: Mancini, Michele; Grioli, Giorgio; Catalano, Manuel; Garabini, Manolo; Bonomo, Fabio; Bicchi, Antonio
    This work presents an example of the application of passive impedance control of a variable stiffness manipulator, which shows the actual benefits of variable stiffness in rejecting disturbances without resorting to the closure of a high level feedback loop. In the experiment a 4-DOF manipulator arm, built with the VSA-CubeBot platform, is controlled to hold a pen and draw a circle on an uneven surface. The control is designed calculating joint and stiffness trajectories with a Cartesian approach to the problem, thus designing the optimal workspace stiffness at first. Then, the joint stiffness yielding the closest workspace stiffness is searched for. Experimental results are reported, which agree with the theoretical outcomes, showing that the sub-optimal joints stiffness settings allow the arm to follow the circular trajectory on the uneven surface at best.
  • Optimality Principles in Stiffness Control: The VSA Kick Authors: Garabini, Manolo; Belo, Felipe; Salaris, Paolo; Passaglia, Andrea; Bicchi, Antonio
    The importance of Variable Stiffness Actuators (VSA) in safety and performance of robots has been extensively discussed in the last decade. It has also been shown recently that a VSA brings performance advantages with respect to common actuators. For instance, the solution of the optimal control problem of maximizing the speed of a VSA for impact maximization at a given position with free final time is achieved by applying a control policy that synchronizes stiffness changes with link speed and acceleration. This problem can be regarded as the formalization of the performance of a soccer player’s free kick. In this paper we revisit the impact maximization problem with imposing a new constraint: we want to maximize the velocity of the actuator link at a given position and fixed terminal time - applicable e.g. to maximize performance of a first-time kick. We first study the problem with fixed stiffness and show that under realistic modeling assumptions, there does exist an optimal linear spring for a given link inertia, final time and motor characteristics. Results are validated with experimental tests. We then study optimal control of VSA and show that varying the spring stiffness during the execution of the kick task substantially improves the final speed.
  • Optimal Control for Exploiting the Natural Dynamics of Variable Stiffness Robots Authors: Haddadin, Sami; Huber, Felix; Albu-Schäffer, Alin
    In contrast to common rigid or actively compliant systems, Variable Stiffness Arms are capable of storing potential energy in their joint and convert it into kinetic energy, respectively speed. This capability is well known from humans and is a good example for the outstanding performance of biological systems. However, only since some years intrinsic compliance is considered as a key feature and not a drawback in robot design. Therefore, only very little work has been carried out for exploiting the natural dynamics of elastic arms for such explosive motion sequences. In this paper, we treat the problem of how to optimally achieve maximum link velocity at a given final time for Variable Stiffness Arms. We show that solutions to this problem lead to excitation motions, which enable the robot to move on the link side at much higher speed on the motor side. In particular, the robot uses the dynamic transfer of elastic joint energy into link side kinetic energy for further acceleration. In our work we consider the practically relevant input and state constraints, and give experimental verification of the developed methods on the new DLR Hand-Arm system.
  • The vsaUT-II: A Novel Rotational Variable Stiffness Actuator Authors: Groothuis, Stefan S.; Rusticelli, Giacomo; Zucchelli, Andrea; Stramigioli, Stefano; Carloni, Raffaella
    In this paper, the vsaUT-II, a novel rotational variable stiffness actuator, is presented. As the other designs in this class of actuation systems, the vsaUT-II is characterized by the property that the output stiffness can be changed independently of the output position. It consists of two internal elastic elements and two internal actuated degrees of freedom. The mechanical design of the vsaUT-II is such that the apparent output stiffness can be varied by changing the transmission ratio between the elastic elements and the output. This kinematic structure guarantees that the output stiffness can be changed without changing the potential energy stored internally in the elastic elements. This property is validated in simulations with the port-based model of the system and in experiments, through a proper control law design, on the prototype.
  • pVEJ: A Modular Passive Viscoelastic Joint for Assistive Wearable Robots Authors: Accoto, Dino; Tagliamonte, Nevio Luigi; Carpino, Giorgio; Sergi, Fabrizio; Di Palo, Michelangelo; Guglielmelli, Eugenio
    In complex dynamical tasks human motor control notably exploits the possibility of regulating joints mechanical impedance, both for stability and for energetic optimization purposes. These biomechanical findings should translate in design requirements for wearable robotics joints, which are required to produce adaptable intrinsic viscoelastic behaviors. This paper describes the design of a purely mechanical, rotary, passive ViscoElastic Joint (pVEJ), functionally equivalent to a torsional spring connected in parallel to a rotary viscous damper. The device has a modular design, which allows to modify the stiffness characteristics by replacing cam profiles. Damping coefficient can be also regulated off-line, manually acting on a valve. Prototype performances are characterized using a custom-developed dynamometric test-bed. Results demonstrate the capability of the system to render both the desired stiffness and damping values, in a range of impedance and peak torque compatible to that of wearable robotics for gait assistance.

Grasping: Modeling, Analysis and Planning

  • On the Caging Region of a Third Finger with Object Boundary Clouds and Two Given Contact Positions Authors: Wan, Weiwei; Fukui, Rui; Shimosaka, Masamichi; Sato, Tomomasa
    This paper presents a caging approach which deals with planar boundary clouds collected from a laser scanner. Given the boundary clouds of a target object and two fixed finger positions, our aim is to find potential third finger positions that can prevent target from escaping into infinity. The major challenge in working with boundary clouds lies in their uncertainty in geometric model fitting and the failure of critical orientations. In this paper, we track canonical motions according to the rotational intersection of Configuration space fingers and rasterize Work space with grids to compute the third caging positions. Our approach can generate the capture region with max(O(np),O(h^2))<=O(n^2) cost where n denotes the resolution of grid rasterization, p denotes the resolution of canonical rasterization and h denotes the resolution of boundary rasterization or the number of boundary cloud points. Moreover, we propose a rough approximation which measures a subset of the possible positions by contracting rotations, indicating computational complexity of max(O(n),O(h^2)). In the experimental part, our proposal is compared with state-of-the-art works and applied to many other objects. The approach makes caging fast and effective.
  • Independent Contact Regions Based on a Patch Contact Model Authors: Charusta, Krzysztof Andrzej; Krug, Robert; Dimitrov, Dimitar Nikolaev; Iliev, Boyko
    The synthesis of multi-fingered grasps on non-trivial objects requires a realistic representation of the contact between the fingers of a robotic hand and an object. In this work, we use a patch contact model to approximate the contact between a rigid object and a deformable anthropomorphic finger. This contact model is utilized in the computation of Independent Contact Regions (ICRs) that have been proposed as a way to compensate for shortcomings in the finger positioning accuracy of robotic grasping devices. We extend the ICR algorithm to account for the patch contact model and show the benefits of this solution.
  • A Grasping Force Optimization Algorithm for Dexterous Robotic Hands Authors: Lippiello, Vincenzo; Siciliano, Bruno; Villani, Luigi
    The problem of grasping force optimization for a robotic system equipped with multi-fingered hands is considered in this paper. This problem is cast in a convex optimization problem, considering also joint torque constraints. A solution suitable for an online implementation, which allows a substantial reduction of the computational load by dynamically decreasing the number of active torque constraints is proposed. Moreover, for the case of a bimanual manipulation system, a sub-optimal single-hand optimization algorithm is presented and compared with the optimal one. The effectiveness of the described methods has been tested in a simulation case study.
  • Local Force Closure Authors: Kruger, Heinrich; Rimon, Elon; van der Stappen, Frank
    We introduce the concept of Local Force Closure. We define a local force closure grasp as a grasp which is capable of resisting some given external wrench as well as (through local variation in contact wrenches) any wrench in some neighborhood of the given wrench, with grasp quality exceeding some given threshold. Local force closure is useful in applications where a grasp only needs to resist some given external wrench, rather than fully constraining object, but where there is some uncertainty regarding the exact external wrench that needs to be resisted, or where there is a possibility of having to cope with some (relatively small) unknown disturbance forces. We show that by allowing disc-shaped fingers in contact with convex vertices of a polygonal object, any given wrench can be resisted by just two frictionless fingers. For a given polygonal object with <i>n</i> vertices and an external wrench <i>w</i><sub>ext</sub>, we show how to find all pairs of features of <i>P</i>, that admit grasps capable of resisting <i>w</i><sub>ext</sub> with grasp quality greater or equal to some threshold <i>Q</i>, in <i>O(n<sup>3/2+&#949;</sup>+K)</i> time, where <i>K</i> is the number of pairs in the output and <i>&#949;</i> is some arbitrarily small, positive constant. We then show how to adapt our algorithm to guarantee that the features reported, admit local force closure grasps.
  • Two-Fingered Caging of Polygons Via Contact-Space Graph Search Authors: Allen, Thomas F; Rimon, Elon; Burdick, Joel
    Based on a novel contact-space formulation, this paper presents a new algorithm to find two-fingered caging grasps of planar polygonal objects. We show that the caging problem has several useful properties in contact space. First, the critical points of the cage representation in the hand’s configuration space appear as critical points of an inter-finger distance function in contact space. Second, the critical points of this distance function can be simply characterized. Third, the contact space admits a rectangular decomposition where the distance function is convex in each rectangle, and all critical points lie on the rectangle boundaries. This property leads to a natural “caging graph,” which can be readily searched to construct the caging sets. An example, constructed from real-world data illustrates and validates the method.
  • Object Categorization and Grasping by Parts from Range Scan Data Authors: Aleotti, Jacopo; Lodi Rizzini, Dario; Caselli, Stefano
    Object category recognition and localization in 3D range data is of great importance in robot manipulation. In this work we propose a novel approach for object categorization and grasping that is focused on topological shape segmentation. The method allows generation of watertight triangulated models of the objects and their shape segmentation into parts. This segmentation provides meaningful information about grasp affordances. An efficient technique for encoding proximity data from range scans is also presented as well as an advanced strategy for manipulation of object sub-parts. Experiments are reported in a real environment using a robot arm equipped with eye-in-hand laser scanner and a parallel gripper.

Learning and Adaptive Control of Robotic Systems I

  • RTMBA: A Real-Time Model-Based Reinforcement Learning Architecture for Robot Control Authors: Hester, Todd; Quinlan, Michael; Stone, Peter
    Reinforcement Learning (RL) is a paradigm for learning decision-making tasks that could enable robots to learn and adapt to their situation on-line. For an RL algorithm to be practical for robotic control tasks, it must learn in very few samples, while continually taking actions in real-time. Existing model-based RL methods learn in relatively few samples, but typically take too much time between each action for practical on-line learning. In this paper, we present a novel parallel architecture for model-based RL that runs in real-time by 1) taking advantage of sample-based approximate planning methods and 2) parallelizing the acting, model learning, and planning processes in a novel way such that the acting process is sufficiently fast for typical robot control cycles. We demonstrate that algorithms using this architecture perform nearly as well as methods using the typical sequential architecture when both are given unlimited time, and greatly out-perform these methods on tasks that require real-time actions such as controlling an autonomous vehicle.
  • Sensorimotor Learning of Sound Localization from an Auditory Evoked Behavior Authors: Bernard, Mathieu; PIRIM, Patrick; de Cheveigné, Alain; Gas, Bruno
    A new method for self-supervised sensorimotor learning of sound source localization is presented, that allows a simulated listener to learn online an auditorimotor map from the sensorimotor experience provided by an auditory evoked behavior. The map represents the auditory space and is used to estimate the azimuthal direction of sound sources. The learning mainly consists in non-linear dimensionality reduction of sensorimotor data. Our results show that an auditorimotor map can be learned, both from real and simulated data, and that the online learning leads to accurate estimations of azimuthal sources direction.
  • Path-following Control of a Velocity Constrained Tracked Vehicle Incorporating Adaptive Slip Estimation Authors: Burke, Michael
    This work presents a model predictive path-following controller, which incorporates adaptive slip estimation for a tracked vehicle. Tracked vehicles are capable of manoeuvring in highly variable and uneven terrain, but difficulties in their control have traditionally limited their use as autonomous platforms. Attempts to compensate for slip in environments typically require that both the forward and rotational velocities of a platform be determined, but this can be challenging. This paper shows that it is possible to estimate vehicle traction using only a rate gyroscope, by providing a suitable adaptive least squares estimator to do so. An approach to generating slip compensating controls when platform velocity constraints are applied is also presented. The approach is controller independent, but we make use of a model predictive controller, vulnerable to the effects of model-plant mismatch, to highlight the efficacy of the proposed estimation and compensation. Path following results using a mixture model to generate feasible slip values are presented, and show a significant increase in controller performance.
  • Direct Yaw Moment Control for Four Wheel Independent Steering and Drive Vehicles Based on Centripetal Force Detection Authors: Lam, Tin Lun; Xu, Yangsheng
    In this paper, a deterministic yaw moment controller for four wheel independent steering and drive vehicles is proposed to enhance driving stability and controllability. Different to conventional methods that track a desired yaw rate, the proposed controller stabilizes a vehicle by additionally tracking the heading angle of a vehicle which is more efficient and robust. The heading angle of a vehicle is obtained by a novel method which is based on centripetal force detection. It eliminates the prerequisite knowledge of the characteristics between wheels and road surface which are time varying and difficult to be measured in real time. The proposed system only requires low cost sensing equipment such as wheel speed sensor and accelerometer that makes the system practical to be utilized. The proposed heading angle detection method can be generally applied to any kind of vehicle. The deterministic yaw moment controller is also applicable to any type of four wheel independent drive vehicles.
  • Predictive Control of Chained Systems: A Necessary Condition on the Control Horizon Authors: Courtial, Estelle; Fruchard, Matthieu; Allibert, Guillaume
    This paper deals with state feedback control of chained systems based on a Nonlinear Model Predictive Control (NMPC) strategy. Chained systems can model many common nonholonomic vehicles. We establish a relation between the degree of nonholonomy and the minimum length of the control horizon so as to make the NMPC feasible. A necessary condition on the control horizon of NMPC is given and theoretically proved whatever the dimension of the chained system consid- ered. This relation is used to design a NMPC-based control strategy for chained systems. One of the advantages of NMPC is the capability of taking into account the constraints on state and on control variables. The theoretical results are illustrated through simulations on a (2,5) chained system, describing a car-like vehicle with one trailer. Difficult motion objectives that require a lateral displacement are considered.
  • Xbots: An Approach to Generating and Executing Optimal Multi-Robot Plans with Cross-Schedule Dependencies Authors: Korsah, G. Ayorkor; Kannan, Balajee; Browning, Brett; Stentz, Anthony; Dias, M. Bernardine
    In this paper, we present an approach to bounded optimal planning and flexible execution for a robot team performing a set of spatially distributed tasks related by temporal ordering constraints such as precedence or synchronization. Furthermore, the manner in which the temporal constraints are satisfied impacts the overall utility of the team, due to the existence of both routing and delay costs. We present a bounded optimal offline planner for task allocation and scheduling in the presence of such cross-schedule dependencies, and a flexible, distributed online plan execution strategy. The integrated system performs task allocation and scheduling, executes the plans smoothly in the face of real-world variations in operation speed and task execution time, and ensures graceful degradation in the event of task failure. We demonstrate the capabilities of our approach on a team of three pioneer robots operating in an indoor environment. Experimental results demonstrate that approach is effective for constrained planning and execution in the face of real-world variations.