arXiv subjects

Kenji Fujimoto

Publications and source records attributed to Kenji Fujimoto.

9 recordsLinked to original sources

arXiv2026

Free-Horizon Newton Method for Nonlinear Optimal Control

This paper presents a novel trajectory optimization method for nonlinear l1-optimal control problems in which the control horizon is treated as a free variable, allowing both the control duration and the l1-norm of the control input to be evaluated. Unlike traditional approaches that minimize the l1-norm of the input under a fixed control horizon, the proposed method treats both the control input and control horizon as design variables, enabling joint optimization of energy and temporal efficiency. The method extends a Newton-based algorithm to accommodate this objective, leveraging gradient information with respect to both variables. This formulation enables one to find high-fidelity optimal trajectories without pre-specifying the control horizon. The effectiveness and robustness of the method are demonstrated through numerical simulations, in which the proposed algorithm recovers the analytical solution to the spacecraft Hohmann transfer, and determines an optimal control horizon for an Earth-Moon transfer.

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arXiv2025

System Identification for Virtual Sensor-Based Model Predictive Control: Application to a 2-DoF Direct-Drive Robotic Arm

Nonlinear Model Predictive Control (NMPC) offers a powerful approach for controlling complex nonlinear systems, yet faces two key challenges. First, accurately modeling nonlinear dynamics remains difficult. Second, variables directly related to control objectives often cannot be directly measured during operation. Although high-cost sensors can acquire these variables during model development, their use in practical deployment is typically infeasible. To overcome these limitations, we propose a Predictive Virtual Sensor Identification (PVSID) framework that leverages temporary high-cost sensors during the modeling phase to create virtual sensors for NMPC implementation. We validate PVSID on a Two-Degree-of-Freedom (2-DoF) direct-drive robotic arm with complex joint interactions, capturing tip position via motion capture during modeling and utilize an Inertial Measurement Unit (IMU) in NMPC. Experimental results show our NMPC with identified virtual sensors achieves precise tip trajectory tracking without requiring the motion capture system during operation. PVSID offers a practical solution for implementing optimal control in nonlinear systems where the measurement of key variables is constrained by cost or operational limitations.

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arXiv2025

Stabilizing Optimal Control for Nonlinear Stochastic Systems: A Parametric Gradient-Based Approach

This study proposes a method for designing stabilizing suboptimal controllers for nonlinear stochastic systems. These systems include time-invariant stochastic parameters that represent uncertainty of dynamics, posing two key difficulties in optimal control. Firstly, the time-invariant stochastic nature violates the principle of optimality and Hamilton-Jacobi equations, which are fundamental tools for solving optimal control problems. Secondly, nonlinear systems must be robustly stabilized against these stochastic parameters. To overcome these difficulties simultaneously, this study presents a parametric-gradient-based method with a penalty function. A controller and cost function are parameterized using basis functions, and a gradient method is employed to optimize the controller by minimizing the parameterized cost function. Crucial challenges in this approach are parameterizing the cost function appropriately and deriving the gradient of the cost. This study provides explicit formulations of an optimally parameterized cost and its gradient. Furthermore, a suitable penalty function is proposed to ensure robust stability, even when using the gradient method. Consequently, the gradient method produces a suboptimal feedback controller that guarantees the robust stability. The effectiveness of the proposed method is demonstrated through numerical simulations, highlighting its performance in comparison with other baseline methods.

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arXiv2023

Weighted Stochastic Riccati Equations for Generalization of Linear Optimal Control

This paper presents weighted stochastic Riccati (WSR) equations for designing multiple types of optimal controllers for linear stochastic systems. The stochastic system matrices are independent and identically distributed (i.i.d.) to represent uncertainty and noise in the systems. However, it is difficult to design multiple types of controllers for systems with i.i.d. matrices while the stochasticity can invoke unpredictable control results. A critical limitation of such i.i.d. systems is that Riccati-like algebraic equations cannot be applied to complex controller design. To overcome this limitation, the proposed WSR equations employ a weighted expectation of stochastic algebraic equations. The weighted expectation is calculated using a weight function designed to handle statistical properties of the control policy. Solutions to the WSR equations provide multiple policies depending on the weight function, which contain the deterministic optimal, stochastic optimal, and risk-sensitive linear (RSL) control. This study presents two approaches to solve the WSR equations efficiently: calculating WSR difference equations iteratively and employing Newton's method. Moreover, designing the weight function yields a novel controller termed the robust RSL controller that has both a risk-sensitive policy and robustness to randomness occurring in stochastic control design.

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arXiv2022

Passivity-based sliding mode control for mechanical port-Hamiltonian systems

In this work, we propose a new passivity-based sliding mode control method for mechanical port-Hamiltonian systems. Passivity-based sliding mode control (PBSMC) is unification of sliding mode control and passivity-based control. It achieves sliding mode control and Lyapunov stability simultaneously by employing an energy based Lyapunov function. The proposed method gives a family of stabilizing controllers which smoothly interpolates passivity-based control and sliding mode control with free parameters. The freedom is useful to adjust the trade-off between robustness against external disturbances and undesired chattering vibration. In addition, this paper relaxes the restrictive condition which is required in the authors' former result. As a result, we can apply the proposed PBSMC method to trajectory tracking control problems. Furthermore, the robustness of the proposed controller against matched and unmatched disturbances is investigated. Numerical examples demonstrate the effectiveness of the proposed method.

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arXiv2022

Second Moment Polytopic Systems: Generalization of Uncertain Stochastic Linear Dynamics

This paper presents a new paradigm to stabilize uncertain stochastic linear systems. Herein, second moment polytopic (SMP) systems are proposed that generalize systems with both uncertainty and randomness. The SMP systems are characterized by second moments of the stochastic system matrices and the uncertain parameters. Further, a fundamental theory for guaranteeing stability of the SMP systems is established. It is challenging to analyze the SMP systems owing to both the uncertainty and randomness. An idea to overcome this difficulty is to expand the SMP systems and exclude the randomness. Because the expanded systems contain only the uncertainty, their stability can be analyzed via robust stability theory. The stability of the expanded systems is equivalent to statistical stability of the SMP systems. These facts provide sufficient conditions for the stability of the SMP systems as linear matrix inequalities (MIs). In controller design for the SMP systems, the linear MIs reduce to cubic MIs whose solutions correspond to feedback gains. The cubic MIs are transformed into simpler quadratic MIs that can be solved using optimization techniques. Moreover, solving such non-convex MIs is relaxed into the iteration of a convex optimization. Solutions to the iterative optimization provide feedback gains that stabilize the SMP systems. As demonstrated here, the SMP systems represent linear dynamics with uncertain mean and covariance and other existing systems such as independently identically distributed dynamics and random polytopes. Finally, a numerical simulation shows the effectiveness of the proposed method.

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arXiv2021

Tracking Control foe Multi-Agent Systems Using Broadcast Signals Based on Positive Realness

Broadcast control is one of decentralized control methods for networked multi-agent systems. In this method, each agent does not communicate with the others, and autonomously determines its own action using only the same signal sent from a central controller. Therefore, it is effective for systems with numerous agents or no-communication between agents. However, it is difficult to manage the stochastic action process of agents considering engineering applications. This paper proposes a decentralized control such that agents autonomously select the deterministic actions. Firstly, a non-linear controller with a binary output of each agent including 0 is introduced in order to express stop actions autonomously when the target is achieved. The asymptotic stability to the target is proved. Secondly, the controller can adjust the tendency of actions in order to make it easier to manage the actions. Thirdly, the controller is extended to that with a continuous output in order to reduce the tracking error to the target and the output vibration. Finally, the effectiveness of the proposed control is verified by numerical experiments.

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arXiv2020

Extended balancing of continuous LTI systems: a structure-preserving approach

In this paper, we treat extended balancing for continuous-time linear time-invariant systems, and we address the problem of structure-preserving model reduction of the subclass of port-Hamiltonian systems. We establish sufficient conditions to ensure that the reduced-order model preserves a port-Hamiltonian structure. Moreover, we show that the use of extended Gramians can be exploited to get a small error bound and, possibly, to preserve a physical interpretation for the reduced-order model.

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arXiv2017

Second-Order Sampling-Based Stability Guarantee for Data-Driven Control Systems

This study presents a sampling-based method to guarantee robust stability of general control systems with uncertainty. The method allows the system dynamics and controllers to be represented by various data-driven models, such as Gaussian processes and deep neural networks. For nonlinear systems, stability conditions involve inequalities over an infinite number of states in a state space. Sampling-based approaches can simplify these hard conditions into inequalities discretized over a finite number of states. However, this simplification requires margins to compensate for discretization residuals. Large margins degrade the accuracy of stability evaluation, and obtaining appropriate margins for various systems is challenging. This study addresses this challenge by deriving second-order margins for various nonlinear systems containing data-driven models. Because the size of the derived margins decrease quadratically as the discretization interval decreases, the stability evaluation is more accurate than with first-order margins. Furthermore, this study designs feedback controllers by integrating the sampling-based approach with an optimization problem. As a result, the controllers can guarantee stability while simultaneously considering control performance.

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