TechTalks from event: Technical session talks from ICRA 2012

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Trajectory Planning and Generation

  • Optimal Acceleration-Bounded Trajectory Planning in Dynamic Environments Along a Specified Path Authors: Johnson, Jeffrey; Hauser, Kris
    Vehicles that cross lanes of traffic encounter the problem of navigating around dynamic obstacles under actuation constraints. This paper presents an optimal, exact, polynomial-time planner for optimal bounded-acceleration trajectories along a fixed, given path with dynamic obstacles. The planner constructs reachable sets in the path-velocity-time (PVT) space by propagating reachable velocity sets between obstacle tangent points in the path-time (PT) space. The terminal velocities attainable by endpoint-constrained trajectories in the same homotopic class are proven to span a convex interval, so the planner merges contributions from individual homotopic classes to find the exact range of reachable velocities and times at the goal. A reachability analysis proves that running time is polynomial given reasonable assumptions, and empirical tests demonstrate that it scales well in practice and can handle hundreds of dynamic obstacles in a fraction of a second on a standard PC.
  • Robot Excitation Trajectories for Dynamic Parameter Estimation Using Optimized B-Splines Authors: Rackl, Wolfgang; Lampariello, Roberto; Hirzinger, Gerd
    In this paper we adressed the problem of finding exciting trajectories for the identification of manipulator link inertia parameters. This can be formulated as a constraint nonlinear optimization problem. The new approach in the presented method is the parameterization of the trajectories with optimized B-splines. Experiments are carried out on a 7 joint Light-Weight robot with torque sensoring in each joint. Thus, unmodeled joint friction and noisy motor current measurements must not be taken into account here. The estimated dynamic model is verified on a different validation trajectory. The results show a clearly improvement of the estimated dynamic model compared to a CAD-valued model.
  • On-Line Trajectory Generation: Nonconstant Motion Constraints Authors: Kroeger, Torsten
    A concept of on-line trajectory generation for robot motion control systems enabling instantaneous reactions to unforeseen sensor events was introduced in a former publication. This previously proposed class of algorithms requires constant kinematic motion constraints, and this paper extends the approach by the usage of time-variant motion constraints, such that low-level trajectory parameters can now abruptly be changed, and the system can react instantaneously within the same control cycle (typically one millisecond or less). This feature is important for instantaneous switchings between state spaces and reference frames at sensor-dependent instants of time, and for the usage of the algorithm as a control submodule in a hybrid switched robot motion control system. Real-world experimental results of two sample use-cases highlight the practical relevance of this extension.
  • Setpoint Scheduling for Autonomous Vehicle Controllers Authors: Au, Tsz-Chiu; Quinlan, Michael; Stone, Peter
    This paper considers the problem of controlling an autonomous vehicle to arrive at a specific position on a road at a given time and velocity. This ability is particularly useful for a recently introduced autonomous intersection management protocol, called AIM, which has been shown to lead to lower delays than traffic signals and stop signs. Specifically, we introduce a setpoint scheduling algorithm for generating setpoints for the PID controllers for the brake and throttle actuators of an autonomous vehicle. The algorithm constructs a feasible setpoint schedule such that the vehicle arrives at the position at the correct time and velocity. Our experimental results show that the algorithm outperforms a heuristic-based setpoint scheduler that does not provide any guarantee about the arrival time and velocity.
  • A Real-Time Motion Planner with Trajectory Optimization for Autonomous Vehicles Authors: Xu, Wenda; Wei, Junqing; Dolan, John M.; Zhao, Huijing; Zha, Hongbin
    In this paper, an efficient real-time autonomous driving motion planner with trajectory optimization is proposed. The planner first discretizes the plan space and searches for the best trajectory based on a set of cost functions. Then an iterative optimization is applied to both the path and speed of the resultant trajectory. The post-optimization is of low computational complexity and is able to converge to a higherquality solution within a few iterations. Compared with the planner without optimization, this framework can reduce the planning time by 52% and improve the trajectory quality. The proposed motion planner is implemented and tested both in simulation and on a real autonomous vehicle in three different scenarios. Experiments show that the planner outputs highquality trajectories and performs intelligent driving behaviors.
  • Improved Non-Linear Spline Fitting for Teaching Trajectories to Mobile Robots Authors: Sprunk, Christoph; Lau, Boris; Burgard, Wolfram
    In this paper, we present improved spline fitting techniques with the application of trajectory teaching for mobile robots. Given a recorded reference trajectory, we apply non-linear least-squares optimization to accurately approximate the trajectory using a parametric spline. The fitting process is carried out without fixed correspondences between data points and points along the spline, which improves the fit especially in sharp curves. By using a specific path model, our approach requires substantially fewer free parameters than standard approaches to achieve similar residual errors. Thus, the generated paths are ideal for subsequent optimization to reduce the time of travel or for the combination with autonomous planning to evade obstacles blocking the path. Our experiments on real-world data demonstrate the advantages of our method in comparison with standard approaches.

Stochastic Motion Planning

  • An Incremental Sampling-Based Algorithm for Stochastic Optimal Control Authors: Huynh, Vu Anh; Karaman, Sertac; Frazzoli, Emilio
    In this paper, we consider a class of continuous-time, continuous-space stochastic optimal control problems. Building upon recent advances in Markov chain approximation methods and sampling-based algorithms for deterministic path planning, we propose a novel algorithm called the incremental Markov Decision Process (iMDP) to compute incrementally control policies that approximate arbitrarily well an optimal policy in terms of the expected cost. The main idea behind the algorithm is to generate a sequence of finite discretizations of the original problem through random sampling of the state space. At each iteration, the discretized problem is a Markov Decision Process that serves as an incrementally refined model of the original problem. We show that with probability one, (i) the sequence of the optimal value functions for each of the discretized problems converges uniformly to the optimal value function of the original stochastic optimal control problem, and (ii) the original optimal value function can be computed efficiently in an incremental manner using asynchronous value iterations. Thus, the proposed algorithm provides an anytime approach to the computation of optimal control policies of the continuous problem. The effectiveness of the proposed approach is demonstrated on motion planning and control problems in cluttered environments in the presence of process noise.
  • Stochastic Distributed Multi-Agent Planning and Applications to Traffic Authors: Lim, Sejoon; Rus, Daniela
    This paper proposes a method for multi-agent path planning on a road network in the presence of congestion. We suggest a distributed method to find paths for multiple agents by introducing a probabilistic path choice achieving global goals such as the social optimum. This approach, which shows that the global goals can be achieved by local processing using only local information, can be parallelized and sped-up using massive parallel processing. The probabilistic assignment reliably copes with the case of random choices of unidentified agents or random route changes of agents who ignore our path guidance. We provide the analytical result on convergence and running time. We demonstrate and evaluate our algorithm by an implementation using asynchronous computation on multi-core computers.
  • Navigating between People: A Stochastic Optimization Approach Authors: Rios-Martinez, Jorge; Renzaglia, Alessandro; Spalanzani, Anne; Martinelli, Agostino; Laugier, Christian
    The objective of this paper is to present a strategy to safely move a robot in an unknown and complex environment where people are moving and interacting. The robot, by using only its sensor data, must navigate respecting humans’ comfort. To obtain good results in such a dynamic environment, a prediction on humans’ movement is also crucial. To solve all the aforementioned problems we introduce a suitable cost function. Its optimization is obtained by using a new stochastic and adaptive optimization algorithm (CAO). This method is very useful in particular when the analytical expression of the optimization function is unknown but numerical values are available for any state configuration. Additionally, the proposed method can easily incorporate any dynamical and environmental constraints. To validate the performance of the proposed solution, several simulation results are provided.
  • Probabilistic Path Planning for Multiple Robots with Subdimensional Expansion Authors: Wagner, Glenn; KANG, MINSU; Choset, Howie
    Probabilistic planners such as Rapidly-Exploring Random Trees (RRTs) and Probabilistic Roadmaps (PRMs) are powerful path planning algorithms for high dimensional systems, but even these potent techniques suffer from the curse of dimensionality, as can be seen in multirobot systems. In this paper, we apply a technique called subdimensional expansion in order to enhance the performance of probabilistic planners for multirobot path planning. We accomplish this by exploiting the structure inherent to such problems. Subdimensional expansion initially plans in each individual robot's configuration space separately. It then couples those spaces when robots come into close proximity with one another. In this way, we constrain a probabilistic planner to search a low dimensional space, while dynamically generating a higher dimensional space where necessary. We show in simulation that subdimensional expansion enhanced PRMs can solve problems involving 32 robots and 128 total degrees of freedom in less than 10 minutes. We also demonstrate that enhancing RRTs and PRMs with subdimensional expansion can decrease the time required to find a solution by more than an order of magnitude.
  • Stochastic Receding Horizon Control for Robots with Probabilistic State Constraints Authors: Shah, Shridhar; Pahlajani, Chetan; Lacock, Nicholaus; Tanner, Herbert G.
    This paper deals with the problem of receding horizon control of a robot subject to stochastic uncertainty within a constrained environment. We deviate from the conventional approach that minimizes expectation of a cost functional while ensuring satisfaction of probabilistic state constraints. Instead, we reduce the problem into a particular form of stochastic optimal control where the path that minimizes the cost functional is planned deterministically and a local stochastic optimal controller with exit constraints ensures satisfaction of probabilistic state constraints while following the planned path. This control design strategy ensures boundedness of errors around the reference path and collision-free convergence to the goal with probability one under the assumption of unbounded inputs. We show that explicit expressions for the control law are possible for certain cases. We provide simulation results for a point robot moving in a constrained two-dimensional environment under Brownian noise. The method can be extended to systems with bounded inputs, if a small nonzero probability of failure can be accepted.
  • High-Speed Flight in an Ergodic Forest Authors: Karaman, Sertac; Frazzoli, Emilio
    Inspired by birds flying through cluttered environments such as dense forests, this paper studies the theoretical foundations of high-speed motion through a randomly-generated obstacle field. Assuming that the locations and the sizes of the trees are determined by an ergodic point process, and under mild technical conditions on the dynamics of the bird, it is shown that the existence of an infinite collision-free trajectory through the forest exhibits a phase transition. In other words, if the bird flies faster than a certain critical speed, there is no infinite collision-free trajectory, with probability one, i.e., the bird will eventually collide with some tree, almost surely, regardless of the planning algorithm governing its motion. On the other hand, if the bird flies slower than this critical speed, then there exists at least one infinite collision-free trajectory, almost surely. Lower and upper bounds on the critical speed are derived for the special case of a Poisson forest considering a simple model for the bird's dynamics. Moreover, results from an extensive Monte-Carlo simulation study are presented. This paper also establishes novel connections between robot motion planning and statistical physics through ergodic theory and the theory of percolation, which may be of independent interest.

Medical Robotics II

  • Automatic Extraction of Command Hierarchies for Adaptive Brain-Robot Interfacing Authors: Bryan, Matthew; Nicoll, Griffin; Thomas, Vibinash; Chung, Mike; Smith, Joshua R.; Rao, Rajesh P. N.
    Recent advances in neuroscience and robotics have allowed initial demonstrations of brain-computer interfaces (BCIs) for controlling wheeled and humanoid robots. However, further advances have proved challenging due to the low throughput of the interfaces and the high degrees-of-freedom (DOF) of the robots. In this paper, we build on our previous work on Hierarchical BCIs (HBCIs) which seek to mitigate this problem. We extend HBCIs to allow training of arbitrarily complex tasks, with training no longer restricted to a particular robot state space (such as Cartesian space for a navigation task). We present two algorithms for learning command hierarchies by automatically extracting patterns from a user's command history. The first algorithm builds an arbitrary-level hierarchical structure (a "control grammar") whose elements can represent skills, whole tasks, collections of tasks, etc. The user "executes" single symbols from this grammar, which produce sequences of lower-level commands. The second algorithm, which is probabilistic, also learns sequences which can be executed as high-level commands, but does not build an explicit hierarchical structure. Both algorithms provide a de facto form of dictionary compression, which enhances the effective throughput of the BCI. We present results from two human subjects who successfully used the hierarchical BCI to control a simulated PR2 robot using brain signals recorded non-invasively through electroencephalography (EEG).
  • Powered Wheelchair Navigation Assistance through Kinematically Correct Environmental Haptic Feedback Authors: Vander Poorten, Emmanuel B; Demeester, Eric; Reekmans, Eli; Huntemann, Alexander; De Schutter, Joris
    This article introduces a set of novel haptic guidance algorithms intended to provide intuitive and reliable assistance for electric wheelchair navigation through narrow or crowded spaces. The proposed schemes take hereto the nonholonomic nature and a detailed geometry of the wheelchair into consideration. The methods encode the environment as a set of collision-free circular paths and, making use of a model-free impedance controller, ‘haptically’ guide the user along collision-free paths or away from obstructed paths or paths that simply do not coincide with the motion intended by the user. The haptic feedback plays a central role as it establishes a fast bilateral communication channel between user and wheelchair controller and allows a direct negotiation about wheelchair motion. If found unsatisfactory, suggested trajectories can always be overruled by the user. Relying on inputs from user modeling and intention recognition schemes, the system can reduce forces needed to move along intended directions, thereby avoiding unnecessary fatigue of the user. A commercial powered wheelchair was upgraded and feasability tests were conducted to validate the proposed methods. The potential of the proposed approaches was hereby demonstrated.
  • A Haptic Instruction Based Assisted Driving System for Training the Reverse Parking Authors: Hirokawa, Masakazu; Uesugi, Naohisa; Furugori, Satoru; Kitagawa, Tomoko; Suzuki, Kenji
    The accident probability of beginner drivers is significantly higher than that of experienced drivers. It can be assumed that this is due to lack of driving skills which lead to making wrong decisions according to cognition and operating in correct way. In this paper, we propose a novel assisted driving system intended to help drivers to improve their skills for the reverse parking. The system is able to assist the driver by haptic instruction on the steering wheel in order to induce the driver to make the adequate operation. For the validation, we developed a 1/10 scale car simulator as a simulation environment on which we installed the proposed assistance method and conducted reverse parking experiment by using the simulator. According to the experiment, we validated that the parking accuracy and the trajectory similarity of subjects assisted by proposed system significantly increased compare to subjects unassisted. Consequently, the proposed assisted driving system could accelerate the learning of humans' driving skills.
  • Remote Palpation to Localize Tumors in Robot-Assisted Minimally Invasive Approach Authors: Talasaz, Ali; Patel, Rajnikant V.
    This paper presents a new tactile-force integrated method to localize tumors minimally invasively using robotic assistance. This method relies on using a capacitive sensor at the tip of a Tactile Sensing Instrument (TSI) which can be inserted into a patient's body in a minimally invasive manner. In this work, the operator palpates tissue containing tumors in a minimally invasive surgical (MIS) training box, representing the patient's body, through a master-slave teleoperation system which consists of a 7 degrees-of-freedom (DOF) haptic interface, used as the master, and a Mitsubishi PA10-7C robot as the slave. Using the proposed method, the operator would be able to palpate the tissue consistently, observe the pressure distribution over the tissue by a color contour map on a screen and feel the tumor on his/her fingers through a grasping mechanism of the haptic interface as a result of higher stiffness of the tumor. The tissue used for the experiments was ex vivo bovine lung and seven participants were asked to locate artificial tumors embedded in the lungs. The results show an accuracy of 93% in tumor localization using the proposed method while the average force applied to the tissue was 3.42 N and the force never exceeded 6 N.
  • Improvements in the Control of a Flexible Endoscopic System Authors: Bardou, Berengere; Nageotte, Florent; zanne, Philippe; de Mathelin, Michel
    The use of flexible cable-driven systems is common in medicine (endoscope, catheter...). Their flexiblity allows surgeons to reach internal organs through sinuous and constrained ways. Unfortunately these systems are subject to backlash due to their internal mechanism. These non linearities raise many difficulties when robotizing and controlling such systems. In this article we propose an approach to improve the cartesian control of a four ways flexible endoscopic system with strong and unknown backlash-like non linearities. The method is based on an automatic off-line hystereses learning. We show that, despite coupling between degrees of freedom, it is possible to extract information from the hystereses which allow to improve cartesian control. Experiments on a real endoscopic system show the validity and the interest of the approach.