Reachable Velocity Research Articles

Multi-robot navigation based on velocity obstacle prediction in dynamic crowded environments

PurposeThe purpose of this paper is to propose a new velocity prediction navigation algorithm to develop a conflict-free path for robots in dynamic crowded environments. The algorithm BP-prediction and reciprocal velocity obstacle (PRVO) combines the BP neural network for velocity PRVO to accomplish dynamic collision avoidance.Design/methodology/approachThis presented method exhibits innovation by anticipating ahead velocities using BP neural networks to reconstruct the velocity obstacle region; determining the optimized velocity corresponding to the robot’s scalable radius range from the error generated by the non-holonomic robot tracking the desired trajectory; and considering acceleration constraints, determining the set of multi-step reachable velocities of non-holonomic robot in the space of velocity variations.FindingsThe method is validated using three commonly used metrics of collision rate, travel time and average distance in a comparison between simulation experiments including multiple differential drive robots and physical experiments using the Turtkebot3 robot. The experimental results show that our method outperforms other RVO extension methods on the three metrics.Originality/valueIn this paper, the authors propose navigation algorithms capable of adaptively selecting the optimal speed for a multi-robot system to avoid robot collisions during dynamic crowded interactions.

Regulation aware dynamic path planning for intelligent ships with uncertain velocity obstacles

Risk assessment and collision avoidance are extremely important for the safe navigation of surface ships. In this paper, we propose a regulation aware dynamic path planning method based on uncertain velocity obstacles for intelligent ships. Specifically, we first use the fuzzy logic theory to evaluate the real-time collision risk of the own ship. Based on the closest point of approach criterion, considering the influences of the relative bearing, the distance, and ship speed ratio, the collision risk is used to activate the path planning process. Moreover, we consider the various ship encountering scenarios including crossing, overtaking and head-on. To deal with the uncertainties in the target ship information, the relative cone angle of the traditional velocity obstacles are systematically expanded using a robust tube idea. The reachable velocity set of the own ship is accordingly divided into three subsets that meet the rules specified by COLREGs in crossing, overtaking and head-on scenarios. Real automatic identification system (AIS) data in an area of the Yangtze River in Wuhan, China, is used to demonstrate the effectiveness of the proposed approach.

A newly developed corner smoothing methodology based on clothoid splines for high speed machine tools

Continuous linear commands are widely executed in computer numerical control (CNC) machining. The tangential discontinuity at the junction of consecutive segments restricts the machining efficiency and deteriorates the surface quality. Corners of linear segments have been successfully blended by inserting parametric splines. There still exists challenges when the common methods are employed in the line-segment commands due to part of the following restrictions: (1) the stringent computation for iteratively calculating the arc-length; (2) the unwanted feedrate fluctuation; (3) the oversize contour deviation for separately completing curve fitting and velocity planning. A novel smoothing method based on a clothoid pair to synchronously accomplish planning of geometry blending and speed scheduling is proposed, the spline parameter of which is arc-length-parameterized. The arc-length, curvature extreme, and geometric shape of the transition curve are analytically expressed by the transition length. On these bases, the transition curve and the velocity profile are concurrently constructed based on the predefined approximation error, the reachable velocity, and normal kinematic constraints in the look-ahead stage. Then, a real-time interpolation scheduling is developed to overcome the crossing difficulties between the linear and parametric segments. Compared with existing methods, the proposed method can analytically calculate the length of transition curves for the arc-length-parameterized expression form. Furthermore, the feedrate fluctuation is eliminated in the fine interpolation. Moreover, the overlarge contour derivation produced by corner smoothing is significantly avoided. It is friendlier to the CNC system for the on-line executing smooth motion since more computing resources can be released to handle other tasks, smoother motion can be achieved and higher contour accuracy can be obtained. The experimental results also demonstrate its practicability and reliability.

Improving stand-on ship's situational awareness by estimating the intention of the give-way ship

The lack of situational awareness is a major cause of ship collisions. Thus, enhancing the situational awareness of the stand-on ship is a key for navigational safety, where the intention estimation of the give-way ship is crucial. According to COLREGs, the stand-on ship is not allowed to take evasive actions until the give-way ship does not take proper actions timely. The stage that needs the stand-on ship to take actions plays as the second protective layer for the ship, which is named as ‘Stand-on Ship as Second Line of Defense’ (SLoD). A method to estimate the intention of the give-way ship and to trigger SLoD is proposed in this article. Four modules of the proposed method include: “data pre-processing” collects all traffic information and determines the ships' obligations; “action identification” pinpoints the turning points; “action uncertainty” generates a bounded reachable velocity considering the give-way ship's maneuverability; “conflict assessment” judges potential collision by using non-linear velocity obstacle algorithm. Several typical encounter scenarios are simulated to demonstrate the feasibility of the proposed method. The results show that intention estimation of the give-way ship improves the situational awareness of the stand-on ship, which can support the stand-on ship to make collision avoidance decisions.

Time-Varying Risk Measurement for Ship Collision Prevention.

We propose an innovative time-varying collision risk (TCR) measurement for ship collision prevention in this article. The proposed measurement considers the level of danger of the approaching ships and the capability of a ship to prevent collisions. We define the TCR as the probability of the overlap of ships' positions in the future, given the uncertainty of maneuvers. Two sets are identified: (1) the velocity obstacle set as the maneuvers of the own ship that lead to collisions with target ships, and (2) the reachable velocity set as the maneuvers that the own ship can reach regarding its maneuverability. We then measure the TCR as the time-dependent percentage of overlap between these two sets. Several scenarios are presented to illustrate how the proposed measurement identifies the time-varying risk levels, and how the approach can be used as an intuitively understandable tool for collision avoidance.

Dynamic Obstacle Avoidance for Unmanned Underwater Vehicles Based on an Improved Velocity Obstacle Method.

In view of a dynamic obstacle environment with motion uncertainty, we present a dynamic collision avoidance method based on the collision risk assessment and improved velocity obstacle method. First, through the fusion optimization of forward-looking sonar data, the redundancy of the data is reduced and the position, size and velocity information of the obstacles are obtained, which can provide an accurate decision-making basis for next-step collision avoidance. Second, according to minimum meeting time and the minimum distance between the obstacle and unmanned underwater vehicle (UUV), this paper establishes the collision risk assessment model, and screens key obstacles to avoid collision. Finally, the optimization objective function is established based on the improved velocity obstacle method, and a UUV motion characteristic is used to calculate the reachable velocity sets. The optimal collision speed of UUV is searched in velocity space. The corresponding heading and speed commands are calculated, and outputted to the motion control module. The above is the complete dynamic obstacle avoidance process. The simulation results show that the proposed method can obtain a better collision avoidance effect in the dynamic environment, and has good adaptability to the unknown dynamic environment.

Muscle fascicle shortening behaviour of vastus lateralis during a maximal force-velocity test.

Muscle fascicles-tendon interactions are the main determinant in production of high joint velocity. Currently, no study has investigated the muscle fascicles behaviour of knee extensor muscles until the highest reachable velocity (e.g., unloaded knee extension). We aimed to track the changes in vastus lateralis fascicles length during knee extensions to quantify muscle fascicles and tendinous tissues contributions to muscle-tendon unit shortening and to determine maximal muscle fascicles shortening velocity. Fifteen participants performed isokinetic and isoinertial knee extensions, and ultrafast ultrasound imaging was used to observe the vastus lateralis fascicles from low to very high joint velocity. The muscle fascicles shortening velocity increased linearly with the increase in knee joint velocity up to the maximal joint velocity (mean R 2=0.93±0.08). Muscle fascicles contribution to muscle-tendon unit shortening velocity was almost constant regardless of the condition (83±23%). Using Hill's equation, the maximal velocity of knee joint and muscle fascicles was determined at 1000±489°s-1 and 5.1±2.0 L0s-1 (47.4±18.7cms-1), respectively. Contribution of muscle fascicles to the muscle-tendon unit shortening velocity was much higher for the vastus lateralis in this study compared to the gastrocnemius medialis in two previous studies. Moreover, this contribution of muscle fascicles shortening velocity was constant whatever the velocity condition, even at the highest reachable velocity. Thus, the vastus lateralis fascicles shortening velocity increases linearly with the knee joint velocity until high velocities and its behaviour strongly accorded with the classical Hill's force-velocity relationship.

Admissible velocity propagation: Beyond quasi-static path planning for high-dimensional robots

Path-velocity decomposition is an intuitive yet powerful approach to addressing the complexity of kinodynamic motion planning. The difficult trajectory planning problem is solved in two separate, simpler steps: first, a path is found in the configuration space that satisfies the geometric constraints (path planning), and second, a time-parameterization of that path satisfying the kinodynamic constraints is found. A fundamental requirement is that the path found in the first step must be time-parameterizable. Most existing works fulfill this requirement by enforcing quasi-static constraints during the path planning step, resulting in an important loss in completeness. We propose a method that enables path-velocity decomposition to discover truly dynamic motions, i.e. motions that are not quasi-statically executable. At the heart of the proposed method is a new algorithm – Admissible Velocity Propagation – which, given a path and an interval of reachable velocities at the beginning of that path, computes exactly and efficiently the interval of all the velocities the system can reach after traversing the path, while respecting the system’s kinodynamic constraints. Combining this algorithm with usual sampling-based planners then gives rise to a family of new trajectory planners that can appropriately handle kinodynamic constraints while retaining the advantages associated with path-velocity decomposition. We demonstrate the efficiency of the proposed method on some difficult kinodynamic planning problems, where, in particular, quasi-static methods are guaranteed to fail.

VCS-based motion planning for distributed mobile robots: collision avoidance and formation

This paper addresses decentralized motion planning among a homogeneous set of feedback-controlled mobile robots. It introduces the velocity obstacle, which describes the collision between robot and obstacle, and the hybrid interactive velocity obstacles are designed for collision checking between interacting robots. The (sub)goal selection algorithm is also studied for formation control, then the preferred velocity is designed for robot tracking its desired (sub)goal. Furthermore, the rules for the size regulation of obstacle are presented to avoid conservative motion planning and enhance the safety. Then, we establish a novel Velocity Change Space (VCS), map the velocity obstacles, the desired (sub)goal and the reachable velocity change window before collision in this space, and directly get the new velocity by a multi-objective optimization method. We apply VCS-based motion planning methods to distributed robots, and simulation is used to illustrate the good performances with respect to the un-conservative, foresighted and multi-objective optimal motion planning, especially the successful application in the formation control of the multi-robot system.

Path Generation and Control for Unmanned Multirotor Vehicles Using Nonlinear Dynamic Inversion and Pseudo Control Hedging

This paper presents a new structure for path following control that allows multirotor UAVs to fly on predefined paths with high velocity. In contrast to pure trajectory tracking, it prioritizes spatial over temporal errors in case of actuator saturation. Path generation is achieved by a unit quaternion curve and an associated parallel transport frame in an interactive process, which leads to feasible paths of class C3. The suggested control approach is composed of a nonlinear dynamic inversion trajectory controller and a path-based reference model with in-flight adjustable path velocity. By adopting the concept of pseudo control hedging, the tangential path progression is slowed down to the physically reachable velocity. Yaw rotation remains as an additional degree of freedom. Experimental flight tests are performed on a quadrotor with GPS receiver and low cost inertial sensors for state estimation. They demonstrate high velocity path following in the presence of thrust saturation, external disturbances and modeling uncertainties.

Impact controllability of an air hockey puck

In this paper, we investigate the controllability of an air hockey puck sliding on a frictionless table and subject to impacts from a mallet. This problem is a special case of the more general problem of impulsive manipulation. We first characterize the set of velocities reachable with arbitrary impulsive inputs. In reality, one cannot specify impulsive forces as inputs but rather only the velocity of a mallet impacting the puck. Therefore, we refine the characterization to the case that the impulsive forces arise through impacts with a mallet whose velocity can be arbitrarily controlled. The characterization of the reachable puck velocities then depends on the material properties of the impacting objects. We use the two-dimensional Routh impact model to determine these reachable velocities in terms of the coefficients of friction and restitution. We show that the system satisfies an accessibility-like property as long as the coefficient of friction is nonzero.