Stephen M. Rock

Randomized Kinodynamic Motion Planning with Moving Obstacles

This paper presents a novel randomized motion planner for robots that must achieve a specified goal under kinematic and/or dynamic motion constraints while avoiding collision with moving obstacles with known trajectories. The planner encodes the motion constraints on the robot with a control system and samples the robots state × time space by picking control inputs at random and integrating its equations of motion. The result is a probabilistic roadmap of sampled state ×time points, called milestones, connected by short admissible trajectories. The planner does not precompute the roadmap; instead, for each planning query, it generates a new roadmap to connect an initial and a goal state×time point. The paper presents a detailed analysis of the planners convergence rate. It shows that, if the state×time space satisfies a geometric property called expansiveness, then a slightly idealized version of our implemented planner is guaranteed to find a trajectory when one exists, with probability quickly converging to 1, as the number of milestones increases. Our planner was tested extensively not only in simulated environments, but also on a real robot. In the latter case, a vision module estimates obstacle motions just before planning starts. The planner is then allocated a small, fixed amount of time to compute a trajectory. If a change in the expected motion of the obstacles is detected while the robot executes the planned trajectory, the planner recomputes a trajectory on the fly. Experiments on the real robot led to several extensions of the planner in order to deal with time delays and uncertainties that are inherent to an integrated robotic system interacting with the physical world.

Precise measurements of the proton and deuteron structure functions from a global analysis of the SLAC deep inelastic electron scattering cross sections

Abstract We report new values of the proton and deuteron structure functions F2(x, Q2) based on a global analysis of eight SLAC experiments on deep inelastic e-p and e-d scattering. These functions were determined over the entire SLAC kinematic range: 0.06⩽x⩽0.9 and 0.6⩽Q2⩽30.0 (GeV/c)2. The data are compared with high-Q2 measurements of F2(x, Q2) made in deep inelastic μ-p and μ-d scattering experiments at CERN. New results for the ratio F n 2 F p 2 are also reported.

A precise extraction of R=σL/σT from a global analysis of the SLAC deep inelastic e-p and e-d scattering cross sections

Abstract We report the extraction of R = σ L / σ T from a global analysis of eight SLAC deep inelastic experiments on e-p and e-d scattering performed between 1970 and 1985. Values of R p , R d , and R d − R p are determined over the entire SLAC kinematic range: 0.1⩽ x ⩽0.9 and 0.6⩽ Q 2 ⩽20.0 (GeV/ c ) 2 . We find that R p = R d . Measured values of R ( x , Q 2 ) are larger than predictions based on perturbative QCD and on QCD with the inclusion of kinematic target mass terms, indicating that dynamical higher twist effects may be important in the SLAC kinematic range.

Measurements of the Q**2 dependence of the proton and neutron spin structure functions g(1)**p and g(1)**n

Abstract The ratio g 1 F 1 has been measured over the range 0.03 g 1 F 1 to be consistent with no Q2-dependence at fixed x in the deep-inelastic region Q2 > 1 (GeV/c)2. A trend is observed for g 1 F 1 to decrease at lower Q2. Fits to world data with and without a possible Q2-dependence in g 1 F 1 are in agreement with the Bjorken sum rule, but Δq is substantially less than the quark-parton model expectation.

Motion planning for multiple mobile robots using dynamic networks

A new motion planning framework is presented that enables multiple mobile robots with limited ranges of sensing and communication to maneuver and achieve goals safely in dynamic environments. To combine the respective advantages of centralized and de-centralized planning, this framework is based on the concept of centralized planning within dynamic robot networks. As the robots move in their environment, localized robot groups form networks, within which world models and robot goals can be shared. Whenever a network is formed, new information then becomes available to all robots in this network. With this new information, each robot uses a fast, centralized planner to compute new coordinated trajectories on the fly. Planning over several robot networks is decentralized and distributed. Both simulated and real-robot experiments have validated the approach.

Kinodynamic motion planning amidst moving obstacles

This paper presents a randomized motion planner for kinodynamic asteroid avoidance problems, in which a robot must avoid collision with moving obstacles under kinematic, dynamic constraints and reach a specified goal state. Inspired by probabilistic-roadmap (PRM) techniques, the planner samples the state x time space of a robot by picking control inputs at random in order to compute a roadmap that captures the connectivity of the space. However, the planner does not precompute a roadmap as most PRM planners do. Instead, for each planning query, it generates, on the fly, a small roadmap that connects the given initial and goal state. In contrast to PRM planners, the roadmap computed by our algorithm is a directed graph oriented along the time axis of the space. To verify the planners effectiveness in practice, we tested it both in simulated environments containing many moving obstacles and on a real robot under strict dynamic constraints. The efficiency of the planner makes it possible for a robot to respond to a changing environment without knowing the motion of moving obstacles well in advance.

Experiments in the Coordinated Control of an Underwater Arm/Vehicle System

The addition of manipulators to small autonomous underwater vehicles (AUVs) can pose significant control challenges due to hydrodynamic interactions between the arm and the vehicle. Experiments conducted at the Monterey Bay Aquarium Research Institute (MBARI) using the OTTER vehicle have shown that dynamical interactions between an arm and a vehicle can be very significant. For the experiments reported in this paper, a single-link “arm” was mounted on OTTER. Tests showed that for 90-degree, two-second repetitive slews of the arm, the vehicle would move as much as 18 degrees in roll and 14 degrees in yaw when no vehicle control was applied.Using a new, highly accurate model of the arm/vehicle hydrodynamic interaction forces, which was developed as part of this research, a coordinated arm/vehicle control strategy was implemented. Under this model-based approach, interaction forces acting on the vehicle due to arm motion were predicted and fed into the vehicle controller. Using this method, station-keeping capability was greatly enhanced. Errors at the manipulator end point were reduced by over a factor of six when compared to results when no control was applied to the vehicle and by a factor of 2.5 when compared to results from a standard independent arm and vehicle feedback control approach. Using the coordinated-control strategy, arm end-point settling times were reduced by a factor three when compared to those obtained with arm and vehicle feedback control alone. These dramatic performance improvements were obtained with only a five-percent increase in total applied thrust.

Measurement of the deuteron spin structure function g1d(x) for 1 (GeV/c)2 < Q2 < 40 (GeV/c)2

New measurements are reported on the deuteron spin structure function g_1^d. These results were obtained from deep inelastic scattering of 48.3 GeV electrons on polarized deuterons in the kinematic range 0.01<x<0.9 and 1<Q^2<40 (GeV/c)^2. These are the first high dose electron scattering data obtained using lithium deuteride (6Li2H) as the target material. Extrapolations of the data were performed to obtain moments of g_1^d, including Gamma_1^d, and the net quark polarization Delta Sigma.Abstract New measurements are reported on the deuteron spin structure function g1d. These results were obtained from deep inelastic scattering of 48.3 GeV electrons on polarized deuterons in the kinematic range 0.01 6 Li 2 H) as the target material. Extrapolations of the data were performed to obtain moments of g1d, including Γ1d, and the net quark polarization ΔΣ.

Automatic visual station keeping of an underwater robot

This paper presents a method for drift-free station keeping of an underwater robot using computer vision. The sensing problem is simplified by assuming an active control system can be used to keep positional errors small. Robot position is obtained by tracking texture features using image filtering and correlation. Errors in four degrees of freedom (translation and yaw) are determined in real time and are fed into a robot control system to accomplish the task of station keeping. Experimental results demonstrating sensing quality and robot station keeping are presented.<<ETX>>

Precision measurement of the proton and deuteron spin structure functions g2 and asymmetries A2

We have measured the spin structure functions g{sub 2}{sup p} and g{sub 2}{sup d} and the virtual photon asymmetries A{sub 2}{sup p} and A{sub 2}{sup d} over the kinematic range 0.02 {le} x {le} 0.8 and 0.7 {le} Q{sup 2} {le} 20 GeV{sup 2} by scattering 29.1 and 32.3 GeV longitudinally polarized electrons from transversely polarized NH{sub 3} and {sup 6}LiD targets. Our measured g{sub 2} approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3 reduced matrix elements d{sub 2}{sup p} and d{sub 2}{sup n} are less than two standard deviations from zero. The data are inconsistent with the Burkhardt-Cottingham sum rule if there is no pathological behavior as x {yields} 0. The Efremov-Leader-Teryaev integral is consistent with zero within our measured kinematic range. The absolute value of A{sub 2} is significantly smaller than the A{sub 2} < {radical}(R(1+A{sub 1})/2) limit.