| 1. |
- Hult, Robert, 1984, et al.
(author)
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An Interior Point Algorithm for Optimal Coordination of Automated Vehicles at Intersections
- 2019
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Journal article (other academic/artistic)abstract
- In this paper, we consider the optimal coordination of automated vehicles at intersections under fixed crossingorders. We state the problem as a Direct Optimal Control problem, and propose a line-search Primal-Dual Interior Point algorithm with which it can be solved. We show that the problem structure is such that most computations required to construct the search- direction and step-size can be performed in parallel on-board the vehicles. This is realized through the Schur-complement of blocks in the KKT-matrix in two steps and a merit-function with separa- ble components. We analyze the communication requirements of the algorithm, and propose a conservative approximation scheme which can reduce the data exchange. We demonstrate that in hard but realistic scenarios, reductions of almost 99% are achieved, at the expense of less than 1% sub-optimality.
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| 2. |
- Hult, Robert, 1984, et al.
(author)
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An MIQP-based heuristic for Optimal Coordination of Vehicles at Intersections
- 2018
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In: Proceedings of the IEEE Conference on Decision and Control. - 2576-2370 .- 0743-1546. ; December 2018, s. 2783-2790
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Conference paper (peer-reviewed)abstract
- The problem of coordinating automated vehicles at intersections can be formulated as an optimal control prob- lem which is inherently difficult to solve, due to its combinato- rial nature. In this paper, we propose a two-stage approximation algorithm based on a previously presented decomposition. The procedure (a) first solves a Mixed Integer Quadratic Program (MIQP) to compute an approximate solution to the combinatorial part of the problem, i.e. the order in which the vehicles cross the intersection; then (b), solves a Nonlinear Program (NLP) for the optimal state and control trajectories. We demonstrate the performance of the algorithm through extensive simulation, and show that it greatly outperforms the natural First-Come-First-Served heuristic.
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| 3. |
- Hult, Robert, 1984, et al.
(author)
-
Energy-Optimal Coordination of Autonomous Vehicles at Intersections
- 2018
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In: 2018 European Control Conference (ECC). - 9783952426982 ; , s. 602-607
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Conference paper (peer-reviewed)abstract
- The problem of coordinating automated vehicles at intersections is naturally posed within the optimal control framework, using objectives such as minimization of energy consumption. In this paper we extend previous work to include relevant nonlinearities in the vehicle models and propose a cost function that directly captures both energy consumption and travel time. The problem is a so-called Economic MPC (EMPC) problem, which entails both numerical and theoretical chal- lenges. To address these issues, we propose to use a previously presented procedure to tune a MPC with a quadratic objective to approximate the EMPC. We evaluate the performance of both linear and nonlinear approximating MPC controllers in simulation. In particular, we demonstrate that a standard linear MPC can be tuned to so that the losses with respect to the EMPC is below 1%.
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| 4. |
- Hult, Robert, 1984, et al.
(author)
-
Experimental validation of a semi-distributed sequential quadratic programming method for optimal coordination of automated vehicles at intersections
- 2020
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In: Optimal Control Applications and Methods. - : Wiley. - 1099-1514 .- 0143-2087. ; 41:4, s. 1068-1096
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Journal article (peer-reviewed)abstract
- In this article, we study the optimal coordination of automated vehicles at intersections. The problem can be stated as an optimal control problem (OCP), which can be decomposed as a bi-level scheme composed by one nonlinear program (NLP) which schedules the access to the intersection and one OCP per vehicle which computes the appropriate vehicle commands. We discuss a practical implementation of the bi-level controller where the NLP is solved with a tailored semi-distributed sequential quadratic programming (SQP) algorithm that enables distribution of most computation to the vehicles. Results from an extensive experimental campaign are presented, where the bi-level controller and the semi-distributed SQP are implemented on a test setup consisting of three automated vehicles. In particular, we show that the vehicle-level controller can enforce the scheduled intersection access beyond the accuracy admitted by the sensor system, and that the bi-level controller can handle large perturbations and large communication delays, which makes the scheme applicable in practical scenarios. Finally, the use of wireless communication introduces delays in the outer control loop. To allow faster feedback, we introduce a real-time iteration (RTI) like variation of the bi-level controller. Experimental and simulated results indicate that the RTI-like variation offers comparable performance using less computation and communication.
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| 5. |
- Hult, Robert, 1984, et al.
(author)
-
Optimal Coordination of Automated Vehicles at Intersections: Theory and Experiments
- 2019
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In: IEEE Transactions on Control Systems Technology. - 1063-6536 .- 1558-0865. ; 27:6, s. 2510-2525
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Journal article (peer-reviewed)abstract
- With the introduction of Cooperative Automated Vehicles, traffic lights can be replaced by coordination algorithms. In this paper, we present a bi-level, model predictive controller for coordination of automated vehicles at intersection. The bi- level controller consists of a coordination level, where intersection occupancy timeslots are allocated, and vehicle-level controllers, where the control commands for the vehicles are computed. We establish persistent feasibility and stability of the bi-level controller under some mild assumptions, and derive conditions under which closed-loop collision avoidance can be ensured with bounded position uncertainty. We thereafter detail an implemen- tation of the coordination controller on a three-vehicle test bed, where the intersection-level optimization problem is solved using a distributed Sequential Quadratic Programming (SQP) method. We present and discuss results from an extensive experimental campaign where the proposed controller was validated. The experimental results indicate the practical applicability of the proposed controller, and validates that safety can be ensured for large positioning uncertainties.
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| 6. |
- Hult, Robert, 1984, et al.
(author)
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Optimal Coordination of Automated Vehicles at Intersections with Turns
- 2019
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In: 2019 18th European Control Conference, ECC 2019. - 9783907144008 ; , s. 225-230
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Conference paper (peer-reviewed)abstract
- In this paper we address the problem of co- ordinating automated vehicles at intersections, with a spe- cial focus on turning maneuvers. The inclusion of rear-end collision avoidance constraints into the problem is decided during turning maneuvers by a smooth function of the vehicle state, rather than integer variables. Moreover, curvature-based acceleration constraints are introduced, which limit the velocity of the vehicle during the turn, and a term in the objective function accounts for passenger comfort. We discuss how the coordination problem is formulated as a nonlinear program and show though simulations that for practical problem instances the proposed approximation is either exact or introduces very little conservativeness.
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| 7. |
- Hult, Robert, 1984, et al.
(author)
-
Optimisation-based coordination of connected, automated vehicles at intersections
- 2020
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In: Vehicle System Dynamics. - : Informa UK Limited. - 1744-5159 .- 0042-3114. ; 58:5, s. 726-747
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Journal article (peer-reviewed)abstract
- In this paper, we analyse the performance of a model predictive controller for coordination of connected, automated vehicles at intersections. The problem has combinatorial complexity, and we propose to solve it approximately by using a two stage procedure where (1) the vehicle crossing order in which the vehicles cross the intersection is found by solving a mixed integer quadratic program and (2) the control commands are subsequently found by solving a nonlinear program. We show that the controller is persistently safe and compare its performance against traffic lights and two simpler optimisation-based coordination schemes. The results show that our approach outperforms the considered alternatives in terms of both energy consumption and travel-time delay, especially for medium to high traffic loads.
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| 8. |
- Hult, Robert, 1984, et al.
(author)
-
Primal decomposition of the optimal coordination of vehicles at traffic intersections
- 2016
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In: 2016 IEEE 55th Conference on Decision and Control, CDC 2016. - 9781509018376 ; 2016:55, s. Art no 7798649, Pages 2567-2573
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Conference paper (peer-reviewed)abstract
- In this paper we address the problem of coordinating automated vehicles at intersections, which we state as a constrained finite horizon optimal control problem. We present and study the properties of a primal decomposition of the optimal control problem. More specifically, the decomposition consists of an upper problem that allocates occupancy time-slots in the intersection, and lower-level problems delivering control policies for each vehicle. We investigate the continuity class of the upper problem, and show that it can be efficiently tackled using a standard sequential quadratic programming and that most computations can be distributed and performed by the participating vehicles. The paper is concluded with an illustrative numerical example.
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| 9. |
- Zanon, Mario, 1985, et al.
(author)
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An Asynchronous Algorithm for Optimal Vehicle Coordination at Traffic Intersections
- 2017
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In: IFAC-PapersOnLine. - : Elsevier BV. - 2405-8963. ; 50:1, s. 12008-12014
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Conference paper (peer-reviewed)abstract
- In this paper we propose an algorithm for vehicle coordination at intersections in order to avoid collisions within the intersection area while optimising an objective given as the sum of individual costs associated with each agent. Extending the results presented in Hult et al. (2016), we develop an algorithm with asynchronous sensitivity updates in order to reduce the time spent in communication. We select which sensitivities to update in order to minimise an upper bound on the contraction of the inexact Newton iterates and introduce a projection of the inexact Newton steps in order to ensure feasibility of the local problems. We prove convergence of our algorithm and test it on a numerical example in order to validate its effectiveness.
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| 10. |
- Zanon, Mario, 1985, et al.
(author)
-
Experimental Validation of Distributed Optimal Vehicle Coordination
- 2018
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In: 2018 European Control Conference (ECC). - 9783952426982 ; , s. 1511-1516
-
Conference paper (peer-reviewed)abstract
- In this paper we solve the problem of coordinating autonomous vehicles approaching an intersection in experi- ments. We cast the problem in the distributed optimisation framework and use the algorithm proposed in [10], [14] to solve it in real time. We compare two variants of the algorithm in simulations and test our algorithm in experiments using real cars on a test track. The experimental results demonstrate the applicability and real-time feasibility of the algorithm and show that the underlying assumptions are justified.
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