Collision-free Conditions Research Articles

Multiobjective optimization of collaborative robotic task sequence assignment problems under collision-free constraints

This paper proposes a multiobjective optimization approach to address the challenge of collaborative manufacturing with multiple robot arms. Given the necessity for multiple robot arms to work together, the potential for collisions between robotic motions is a significant concern, and the automated task sequence assignment for robots becomes increasingly complex. Previous research has either simplified the collision-free conditions in a limited working area, or employed a master-slave approach to obtain only a local solution. Consequently, we propose a unified global optimization approach for simultaneously addressing various collaborative manufacturing issues, including robotic task sequence assignment (RTSA), multiple inverse kinematics (IK) selection, joint-space collision-free operations and multiple manufacturing objectives. As the optimal collaborative RTSA problem is a combinatorial optimization problem with non-deterministic polynomial-time hard (NP-hard) complexity, this paper presents a hybrid nondominated sorting genetic algorithm III (NSGA-III) method that integrates a Hamming-distance-based method and a greedy strategy within NSGA-III to improve population diversity and solution quality. To validate the efficacy of the proposed approach, simulation experiments were conducted on cooperative manufacturing scenarios, with two objectives: task completion time and task load balancing. The experimental results demonstrate that the proposed approach is effective in obtaining collision-free Pareto solutions. Furthermore, the proposed hybrid NSGA-III method obtains superior solutions compared to the original method in the studied problem, as measured by two performance indices.

Role of non-statistical effects in deciding the fate of HO3˙ in the atmosphere.

HO3˙ has been postulated as a reservoir for OH˙ in various atmospheric reactions. Under collision-free conditions, experiments indicate that the lifetime of this species should be more than one microsecond. Interestingly, the binding energy of HO3˙ is estimated to be ∼3 kcal mol-1 by recent experimental as well as theoretical works. The value of the binding energy suggests that the lifetime of HO3˙ should be in the picosecond range, and with this lifetime, HO3˙ cannot act as a reservoir for OH˙. In the present work, using on-the-fly semiclassical dynamics, we argue that if non-RRKM effects are included, the lifetime of HO3˙ may be higher than that estimated from the binding energy.

Autonomous Trajectory Tracking and Collision Avoidance Design for Unmanned Surface Vessels: A Nonlinear Fuzzy Approach

An intelligent fuzzy-based control system that consists of several subsystems—a fuzzy collision evaluator, a fuzzy collision avoidance acting timing indicator, a collision-free trajectory generator, and a nonlinear adaptive fuzzy robust control law—is proposed for the collision-free condition and trajectory tracking of unmanned surface vessels (USVs). For the purpose of ensuring that controlled USVs are capable of executing tasks in an actual ocean environment that is full of randomly encountered ships under collision-free conditions, the real-time decision making and the desired trajectory arrangements of this proposed control system were developed by following the “Convention on the International Regulations for Preventing Collisions at Sea” (COLREGs). From the simulation results, several promising properties were demonstrated: (1) robustness with respect to modeling uncertainties and ocean environmental disturbances, (2) a precise trajectory tracking ability, and (3) sailing collision avoidance was shown by this proposed system for controlled USVs.

Vector field-based curved layer slicing and path planning for multi-axis printing

• A new vector field-based curved layer slicing and printing path planning method is proposed. • A quantitative optimization model is established to find the optimal filament orientation and printing orientation vector fields. • Multi-factors including support-free requiement, collision-free condition, and mechanical performance are considered. • A continuous printing path planning algorithm is developed to reduce the number of nozzle retractions. The emergence of multi-axis printing systems provides a new solution to print complex parts. However, the current process planning methods fail to balance the support-free requirement, the collision-free condition, as well as the mechanical performance at the same time when printing parts with complicated features. This paper proposes a new curved layer slicing and continuous printing path planning method that considers these factors. The proposed method makes use of two mutually orthogonal unit vector fields embedded on the tetrahedral mesh of the part: the filament orientation vector field and the printing orientation vector field, which indicate the filament orientation and the nozzle orientation at each position during the printing process respectively. A quantitative optimization model is established to compute the two optimal unit vector fields. Then a monotonically increasing scalar distance field is generated by integrating along the optimal printing orientation vector field, and the isosurfaces of the distance field can be used to naturally slice the part into curved layers. Finally, at each isosurface, a continuous printing path is planned along the optimal filament orientation field by interpolating isolines of a surface embedded scalar distance field. Both the simulation and physical experiments were conducted to confirms the effectiveness of the proposed method and its advantage over the existing methods in balance different factors including support-free condition, collision-free requirement, and mechanical performance.

Flight With Limited Field of View: A Parallel and Gradient-Free Strategy for Micro Aerial Vehicle

In this article, a parallel and gradient-free strategy is presented for autonomous navigation of a quadrotor with a limited field of view (FOV). The FOV constraint is nonlinear and noncontinuous in general and involves higher order dynamics of the vehicle. Planning problems with such constraints are difficult to solve, especially with the real-time requirement. The proposed method first constructs an environment map by integrating the measurement of a limited-FOV sensor in a parallel process. Based on the map, it then selects a feasible local goal from a global path and generates a constraint-free nominal control sequence. Finally, a stochastic optimal control problem is solved to satisfy the limited FOV constraint, the dynamic constraint, and the collision-free conditions at the same time. The objective functions are nonlinear and noncontinuous and involves dynamic of MAV. A parallel-gradient-free algorithm is used to address these problems. Both the sensing and planning problems are solved parallelly on a lightweight graphics processing unit. The effectiveness of the proposed strategy is demonstrated through both simulation and real-flight experiments.

Fast Cooperative Trajectory Generation of Unmanned Aerial Vehicles Using a Bezier Curve-Based Shaping Method

This study uses a Bezier curve-based shaping (BCBS) approach based on the dynamic model to quickly generate 3D cooperative trajectories for unmanned aerial vehicles (UAVs). It can facilitate the solution of the flight trajectory problem that occurs after the reallocation of mission points. Trajectory coordinates are expanded based on the Bezier curves that naturally satisfy boundary conditions (BCs); trajectories with the minimum flight time and those satisfying the dynamic constraints and collision-free conditions can be obtained by adjusting the Bezier coefficients. A three-UAV flight simulation is carried out to verify the effectiveness of the proposed method. Further, the performance of the proposed method is compared with that of the Gauss pseudospectral method (GPM). The simulation results of the BCBS method are used as the initial values for GPM optimization. It is demonstrated that the BCBS method requires a lower computation time than the direct solver, which is only 0.07% of the latter, and obtains similar optimization results (3.34% difference). This is considerably important for the rapid generation of optimized trajectories with the limited computing power of onboard computers. Furthermore, this method is expected to achieve online collaborative trajectory generation for multiple UAVs in view of its high computational efficiency.

Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH2CHO) is a major stable brominated organic intermediate of the bromine-ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH2CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH2CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH2CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm-1 < ⟨ΔEdown⟩ < 450 cm-1, 78-33% of BrCH2CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔEdown⟩ = 300 cm-1 and an excitation wavelength of 320 nm, with 27% for C-C bond fission, 11% for C-Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O-Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C-Br and C-C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation.

Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed "rearrangement in the excited state" (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via C═C bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.

Virtual Electric Dipole Field Applied to Autonomous Formation Flight Control of Unmanned Aerial Vehicles.

The following paper presents a method for the use of a virtual electric dipole potential field to control a leader-follower formation of autonomous Unmanned Aerial Vehicles (UAVs). The proposed control algorithm uses a virtual electric dipole potential field to determine the desired heading for a UAV follower. This method’s greatest advantage is the ability to rapidly change the potential field function depending on the position of the independent leader. Another advantage is that it ensures formation flight safety regardless of the positions of the initial leader or follower. Moreover, it is also possible to generate additional potential fields which guarantee obstacle and vehicle collision avoidance. The considered control system can easily be adapted to vehicles with different dynamics without the need to retune heading control channel gains and parameters. The paper closely describes and presents in detail the synthesis of the control algorithm based on vector fields obtained using scalar virtual electric dipole potential fields. The proposed control system was tested and its operation was verified through simulations. Generated potential fields as well as leader-follower flight parameters have been presented and thoroughly discussed within the paper. The obtained research results validate the effectiveness of this formation flight control method as well as prove that the described algorithm improves flight formation organization and helps ensure collision-free conditions.

Optimal control based coordinated taxiing path planning and tracking for multiple carrier aircraft on flight deck

Coordinated taxiing planning for multiple aircraft on flight deck is of vital importance which can dramatically improve the dispatching efficiency. In this paper, first, the coordinated taxiing path planning problem is transformed into a centralized optimal control problem where collision-free conditions and mechanical limits are considered. Since the formulated optimal control problem is of large state space and highly nonlinear, an efficient hierarchical initialization technique based on the Dubins-curve method is proposed. Then, a model predictive controller is designed to track the obtained reference trajectory in the presence of initial state error and external disturbances. Numerical experiments demonstrate that the proposed “offline planning + online tracking” framework can achieve efficient and robust coordinated taxiing planning and tracking even in the presence of initial state error and continuous external disturbances.

Collision-free flocking for a time-delay system

<p style='text-indent:20px;'>The co-existence of collision avoidance and time-asymptotic flocking of multi-particle systems with measurement delay is considered. Based on Lyapunov stability theory and some auxiliary differential inequalities, a delay-related sufficient condition is established for this system to admit a time-asymptotic flocking and collision avoidance. The estimated range of the delay is given, which may affect the flocking performance of the system. An analytical expression was proposed to quantitatively analyze the upper bound of this delay. Under the flocking conditions, the exponential decay of the relative velocity of any two particles in the system is characterized. Particularly, the collision-free flocking conditions are also given for the case without delay. This work verifies that both collision avoidance and flocking behaviors can be achieved simultaneously in a delay system.

An Efficient Path Planning Methodology Based on the Starting Region Selection

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Automated parking is an efficient way to solve parking difficulties and path planning is of great concern for parking maneuvers [&lt;span class="xref"&gt;1&lt;/span&gt;]. Meanwhile, the starting region of path planning greatly affects the parking process and efficiency. The present research of the starting region are mostly determined based on a single algorithm, which limits the flexibility and efficiency of planning feasible paths. This paper, taking parallel parking and vertical parking for example, proposes a method to calculate the starting region and select the most suitable path planning algorithm for parking, which can improve the parking efficiency and reduce the complexity. The collision situations of each path planning algorithm are analyzed under collision-free conditions based on parallel and vertical parking. The starting region for each algorithm can then be calculated under collision-free conditions. After that, applicable starting regions for parking can be obtained, and each of those regions corresponds to a parking path planning algorithm. However, there always exists overlapped starting regions, which can be applied to multiple parking path planning algorithms. In order to select the most suitable algorithm to plan the parking path, the priority order of algorithms is decided based on the preference criterion function. The collision-free parking path can be generated following the priority order. Based on the modified B-spline curves, a continuous-curvature path is presented. The simulation results based on MATLAB/Simulink and PreScan show that the methodology can smoothly judge the feasibility of automated parking in vehicle’s current position and plan the most suitable parking path. The proposed methodology can calculate the starting region of automated parking rapidly and plan more efficient parking path compared with other methods.&lt;/div&gt;&lt;/div&gt;

A spiral-based inspection path generation algorithm for efficient five-axis sweep scanning of freeform surfaces

Traditional inspection of freeform surfaces via three-axis or three ＋ two-axis CMM mainly works in a point-by-point manner where for each inspection point, the stylus of CMM has to approach to and then retract from the surface of inspection, which inevitably drags down (often substantially) the inspection efficiency. Recently, a new inspection apparatus called five-axis inspection machine has emerged, on which the stylus tip can continuously sweep on the part surface and the inspection efficiency can thus be tremendously improved. The crux in using this new inspection technology is how to plan an efficient five-axis inspection path (including both the position and orientation of the stylus). In this paper, we present a new type of five-axis inspection path generation strategy for the inspection of a general freeform surface. Based on the proposed index called Surface Sweep Rate (SSR), an algorithm is designed to partition the surface into a single spiral strip, which maximizes the overall SSR. Then, for this single spiral strip, a five-axis inspection path is generated that takes into consideration the key constraints such as the required smoothness of the probe head trajectory, the sampling density, and the collision-free condition. Physical inspection experiments of our proposed strategy are conducted on two typical freeform surfaces, and the results convincingly confirm the feasibility, effectiveness, and advantages of our new solution.

Nonstandard second-order formulation of the LWR model

ABSTRACTIn this study, we present an equivalent second-order formulation of the LWR model based on Phillips’ model, in which the acceleration rate equals a relaxation term with an infinitesimal relaxation time. We convert the model into a continuum car-following model and demonstrate its equivalence to that of the LWR model. We demonstrate that the nonstandard second-order model is stable, but the original Phillips model is not. We derive conditions for the nonstandard model to be forward-traveling and collision-free, prove that the collision-free condition is consistent with but more general than the CFL condition, and demonstrate that only anisotropic and symplectic Euler discretization methods lead to physically meaningful solutions. Numerically, the nonstandard second-order model has the same shock and rarefaction wave solutions as the LWR model for both Greenshields and triangular fundamental diagrams; for a non-concave fundamental diagram, the collision-free condition, but not the CFL condition, yields physically meaningful results.

Laser-induced fluorescence spectroscopy of jet-cooled WO in 18, 900–23, 500 cm−1

The laser-induced fluorescence (LIF) excitation spectra of WO molecule have been recorded in the energy range of 18,900–23,500 cm−1 using laser ablation of tungsten target and reaction with free jet expansion of oxygen. Totally 63 vibronic transition bands were observed, and 60 bands were grouped into 10 electronic transition progressions, where 8 electronic transition progressions have the X0+ state as the lower state, as well as 2 progressions have the X1 state as the lower state. The electronic transition systems [21.5]0+–X0+ and [23.3]1–X0+ were newly identified by LIF spectroscopy. The molecular constants, including rotational constant, vibrational frequency, vibrational anharmonic constant, in the electronic excited states were obtained through rotational analysis of the spectra. In addition, the fluorescence lifetimes of the vibronic states were measured under the collision-free condition by exponentially fitting the fluorescence decay.

A Gridmap-Path Reshaping Algorithm for Path Planning

Path planning is a vital function of robotic systems. Different from existing roadmap algorithms which first determine the free space and then determine the collision-free path, researchers recently proposed several convex relaxation based smoothing algorithms which first select an initial path to link the starting configuration and the goal configuration and then reshape this path to meet other requirements (e.g., collision-free conditions) by using convex relaxation. However, convex relaxation based smoothing algorithms often fail to give a satisfactory path, since the initial paths are selected improperly. Moreover, the curvature constraints were not considered in many existing convex relaxation based smoothing algorithms. In this paper, we show that we can first grid the whole configuration space to pick a candidate path and reshape the shortest path to meet our goal. This new algorithm inherits the merit of roadmap algorithms and a convex feasible set algorithm. We further discuss how to meet the curvature constraints by using both the Beamlet algorithm to select a better initial path and an iterative optimization algorithm to adjust the curvature of a path. Theoretical analysis and numerical testing results show that our algorithm can almost surely find a feasible path and require a short computation time.

Longitudinal and Lateral Trajectory Planning for the Typical Duty Cycle of Autonomous Load Haul Dump

With the development of intelligent mining, autonomous driving will be the basic function of intelligent Load Haul Dump (LHD). Trajectory planning is the key part of autonomous driving. Due to the articulated structure, the motion state variables of the front and rear bodies are strongly nonlinear, leading to complex nonlinear collision avoidance constraints. In addition, the articulated angle and angular velocity of the LHD have physically constraints. At the same time, the terminal attitude constraint should be considered since LHD is a mining equipment. All these factors bring great difficulties to LHD trajectory planning and none of the existing methods can deal with these factors directly. In order to solve these problems, according to the most common working scenario, a novel trajectory planning method is proposed for autonomous LHD based on numerical optimization, leading to a safe and feasible time-space trajectory for efficient production. The novelty of this work is the introduction of a longitudinal and lateral trajectory planning for the typical duty cycle of LHD. More importantly, by the ingenious concept for modeling, there are two salient features of the proposed method. Firstly, no angles are used as the decision variable, and secondly, the collision constraints of the rear car are not directly considered. Through this way, the number of nonlinear constraints and the complexity of the model can keep in a reasonable level, which makes the model easy to solve. At the same time, by giving the collision-free condition and limiting the heading angular velocity, the generated trajectory is collision-free and satisfies the physical constraints of articulated angle. Case studies confirm the effectiveness of the proposed method. Adopting the proposed method to generate a spatiotemporal trajectory is beneficial to ensure safety and improve production efficiency.

Distributed Formation Control for Multiagent Systems in the Presence of External Disturbances

This paper analyzes the distributed formation control algorithms that rely on local measurements for second-order multiagent systems under the leader-follower control structure. Here, the reference states are only available to the leader agents, the follower agents have access to the information from the leaders to achieve the control objective, and the target formation is defined by their relative positions. The formation control problem with constant and time-varying reference states is studied, and two classes of practical control system problems in the presence external disturbances are also considered. In the former case, a non-saturated Proportional-integral (PI) controller is proposed to attenuate the constant disturbances, namely, the static errors are eliminated. In the latter case, a saturated distributed control algorithm is first proposed to address the formation control of multiagent systems, which, in many practical applications, is subject to bounded control inputs. To eliminate the effect of chattering caused by external bounded time-varying disturbances, the distributed formation control algorithm is then proposed. The analysis for the proposed control algorithms according to Lyapunov stability theory is provided. A sufficient collision-free condition is also presented. Finally, to demonstrate the effectiveness of the obtained theoretical results, the numerical simulation results of several illustrative examples are presented.

Laser-induced fluorescence spectroscopic study of the [21.2]0+–X0+ and [22.2]0+–X0+ electronic transitions of tungsten monoxide

The laser-induced fluorescence (LIF) excitation spectra of jet-cooled WO molecule have been investigated in the range of 21,000–23,200 cm−1, and two electronic band systems have been observed for the first time and labeled as the [21.2]0+–X0+ and [22.2]0+–X0+ transitions. The vibrational and rotational constants of these two excited electronic states have been derived by rotational analysis of the spectra. The tungsten isotopic structures of these two excited states are clearly resolved, and the analysis of LIF spectra reveals that the isotopic shifts are due to the strong interstate perturbations between different vibronic states. Based on the ab initio calculation results by Ram et al., the primarily attributed configuration of these two newly identified states was discussed. In addition, the lifetimes of the observed vibronic states were measured under the collision-free condition.

Unimolecular Decay of Criegee Intermediates to OH Radical Products: Prompt and Thermal Decay Processes.

Alkene ozonolysis is a primary oxidation pathway for anthropogenic and biogenic alkenes emitted into the troposphere. It is also an important source of atmospheric hydroxyl (OH) radicals, often called the atmosphere's detergent. Alkene ozonolysis takes place through a highly exothermic reaction pathway with multiple intermediates and barriers prior to releasing the OH radical products. This Account focuses on a key reaction intermediate with a carbonyl oxide functional group (-COO), known as the Criegee intermediate, which is formed along with a carbonyl coproduct in alkene ozonolysis reactions. Under atmospheric conditions, the initially energized Criegee intermediates may promptly decay to OH products or be collisionally stabilized prior to thermal decay to OH radicals and other products. Alternatively, the stabilized Criegee intermediates may undergo bimolecular reactions with atmospheric species, including water vapor and sulfur dioxide, which can lead to nucleation and growth of aerosols. The dimethyl-substituted Criegee intermediate, (CH3)2COO, is utilized in this Account to showcase recent efforts to experimentally measure and theoretically predict the rates for prompt and thermal unimolecular decay processes of prototypical Criegee intermediates under laboratory and atmospheric conditions. The experimental laboratory studies utilize an alternative synthesis method to efficiently generate Criegee intermediates via the reaction of iodoalkyl radicals with O2. Infrared excitation is then used to prepare the (CH3)2COO Criegee intermediates at specific energies in the vicinity of the transition state barrier or significantly below the barrier for 1,4-hydrogen transfer that leads to OH products. The rate of unimolecular decay is revealed through direct time-domain measurements of the appearance of OH products utilizing ultraviolet laser-induced fluorescence detection under collision-free conditions. Complementary high-level theoretical calculations are carried out to evaluate the transition state barrier and the energy-dependent unimolecular decay rates for (CH3)2COO using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which are in excellent accord with the experimental measurements. Quantum mechanical tunneling through the barrier, incorporated through Eckart and semiclassical transition state theory models, is shown to make a significant contribution to the unimolecular decay rates at energies in the vicinity of and much below the barrier. Master equation modeling is used to extend the energy-dependent unimolecular rates to thermal decay rates of (CH3)2COO under tropospheric conditions (high pressure limit), which agree well with recent laboratory measurements [ Smith et al. J. Phys. Chem. A 2016 , 120 , 4789 and Chhantyal-Pun et al. J. Phys. Chem. A 2017 , 121 , 4 - 15 ]. Again, tunneling is shown to enhance the thermal decay rate by orders of magnitude. The experimentally validated unimolecular rates are also utilized in modeling the prompt and thermal unimolecular decay of chemically activated (CH3)2COO formed upon ozonolysis of 2,3-dimethyl-2-butene under atmospheric conditions [ Drozd et al. J. Phys. Chem. A 2017 , 121 , 6036 - 6045 ]. Future challenges lie in extension of these spectroscopic and dynamical methods to Criegee intermediates derived from more complex ozonolysis reactions involving biogenic alkenes.