CERES :: Browsing by Author "Cho, Namhoon"

Browsing by Author "Cho, Namhoon"

Now showing 1 - 20 of 23

Open Access
A passivity-based method for accelerated convex optimisation
(IEEE, 2024-12-16) Cho, Namhoon; Shin, Hyo-Sang
This study presents a constructive methodology for designing accelerated convex optimisation algorithms in continuous-time domain. The two key enablers are the classical concept of passivity in control theory and the time-dependent change of variables that maps the output of the internal dynamic system to the optimisation variables. The Lyapunov function associated with the optimisation dynamics is obtained as a natural consequence of specifying the internal dynamics that drives the state evolution as a passive linear time-invariant system. The passivity-based methodology provides a general framework that has the flexibility to generate convex optimisation algorithms with the guarantee of different convergence rate bounds on the objective function value. The same principle applies to the design of online parameter update algorithms for adaptive control by re-defining the output of internal dynamics to allow for the feedback interconnection with tracking error dynamics.
Open Access
Analysis of guidance laws with non-monotonic line-of-sight rate convergence
(IEEE, 2021-09-20) Lee, Seokwon; Cho, Namhoon; Shin, Hyosang
This study presents analyses of guidance laws that involve non-monotonic convergence in heading error from a new perspective based on an advanced stability concept. Pure proportional navigation with range-varying navigation gain is considered, and the gain condition to guarantee asymptotic convergence to the collision course is investigated while allowing the heading error to exhibit patterns that involve intermediate diversion. The extended stability criterion considered in this study allows local increase of the function in some finite intervals, which is less conservative than the standard stability theorem. The existing guidance laws involving intentional modulation of the heading error as well as the design of the navigation gain are discussed with respect to the new stability criterion.
Open Access
Analytic approach to impact time guidance with look angle constraint using exact time-to-go solution
(American Society of Civil Engineers, 2023-12-12) Lee, Seokwon; Kim, Jinrae; Kim, Youdan; Cho, Namhoon
This paper proposes an analytic approach for impact time control guidance laws against stationary targets using biased proportional navigation. The proposed guidance scheme realizes the impact time control in two different ways: the first approach directly uses the exact time-to-go error to satisfy both the impact time control and the field-of-view constraint, while the second approach adopts a look angle tracking law to indirectly control the impact time, with the reference profile of the look angle generated using the exact time-to-go solution. The stability properties of the proposed guidance laws are discussed, and numerical simulations are carried out to evaluate their performance in terms of accuracy and efficiency.
Open Access
Approximation of achievable robustness limit based on sensitivity inversion
(AIAA, 2023-11-07) Cho, Namhoon; Lee, Hae-In
Introduction: The sensitivity function, defined as the closed-loop transfer function from the exogenous input to the tracking error, is central to the multi-objective design and analysis of a feedback control system. Its frequency response determines many performance characteristics of the closed-loop system, such as disturbance attenuation, reference tracking, and robustness against uncertainties and noise. It is well known that the nominal sensitivity peak, i.e., the H∞ -norm of the sensitivity function, is a direct measure of stability robustness, because the sensitivity magnitude quantifies both the attenuation of the effect of external disturbances on the closed-loop output and the variations of the closed-loop system with respect to the plant perturbations.
Open Access
Collision-geometry-based optimal guidance for high-speed target
(Elsevier, 2021-04-30) Kim, Boseok; Kim, Young-Won; Cho, Namhoon; Lee, Chang-Hun
This paper proposes a new unified form of guidance law based on the collision geometry that can be applied to both head-on (HO) or head-pursuit (HP) engagements for intercepting a target faster than an interceptor. To this end, two possible collision courses for a high-speed target and corresponding nonlinear heading errors are first investigated. The proposed guidance is then determined in a way to specify the desired heading error dynamics that ensures an optimal decreasing pattern. The characteristics of the proposed method are also investigated compared to existing methods. The favorable features are that the engagement geometries between HO or HP can be flexibly selected, and the optimality of the guidance command can be addressed. Moreover, since the proposed guidance law is directly derived from nonlinear collision geometry, the working mechanism is clearly explained, and the nonlinear nature is preserved. Finally, numerical simulations are performed to support our findings.
Open Access
Composite model reference adaptive control under finite excitation with unstructured uncertainties
(IEEE, 2024-01-19) Cho, Namhoon; Shin, Hyosang; Kim, Youdan; Tsourdos, Antonios
This paper presents an online parameter update algorithm in the context of composite model reference adaptive control based on intermittent signal holding to improve convergence properties of the parameters representing the unstructured uncertainties in the absence of persistent excitation. The present study extends the algorithm which was previously developed by considering only the structured uncertainties for which the basis functions are known a priori. The proposed extension utilises the Gaussian radial basis function neural network as the model for the uncertainty assuming appropriate placement of the local basis functions in the state space. A notable distinction from the case with full knowledge of the features constituting the linearly-parameterised uncertainty model is that the extended algorithm introduces a robustifying modification in the earlier phase of operation to deal with the inevitable learning residual.
Open Access
Composite model reference adaptive control with parameter convergence under finite excitation
(IEEE, 2017-08-09) Cho, Namhoon; Shin, Hyosang; Kim, Youdan; Tsourdos, Antonios
A new parameter estimation method is proposed in the framework of composite model reference adaptive control for improved parameter convergence without persistent excitation. The regressor filtering scheme is adopted to perform the parameter estimation with signals that can be obtained easily. A new framework for residual signal construction is proposed. The incoming data is first accumulated to build the information matrix, and then its quality is evaluated with respect to a chosen measure to select and store the best one. The information matrix is built to have full rank after sufficient but not persistent excitation. In this way, the exponential convergence of both tracking error and parameter estimation error can be guaranteed without persistent oscillation in the external command which drives the system. Numerical simulations are performed to verify the theoretical findings and to demonstrate the advantages of the proposed adaptation law over the standard direct adaptation law.
Open Access
Data-efficient active weighting algorithm for composite adaptive control systems
(IEEE, 2022-08-09) Kim, Seong-hun; Lee, Hanna; Cho, Namhoon; Kim, Youdan
We propose an active weighting algorithm for composite adaptive control to reduce the state and estimate errors while maintaining the estimation quality. Unlike previous studies that construct the composite term by simply stacking, removing, and pausing observed data, the proposed method efficiently utilizes the data by providing a theoretical set of weights for observations that can actively manipulate the composite term to have desired characteristics. As an example, a convex optimization formulation is provided, which maximizes the minimum eigenvalue while keeping other constraints, and an illustrative numerical simulation is also presented.
Open Access
Design framework for optimizing waypoints of vehicle trajectory considering terminal velocity and impact angle constraints
(Taylor and Francis, 2021-05-17) Kim, Youngil; Cho, Namhoon; Park, Jongho; Kim, Youdan
Ballistic missiles often require the terminal velocity and impact angle to be confined to a certain region around a desired value considering a wide variety of uncertain initial conditions and operational ranges. This study presents a design framework to determine optimum waypoints that satisfy constraints for given launch conditions and mission profiles. As a systematic approach to this kinodynamic motion planning problem, the proposed framework deterministically samples the waypoints. The trajectory generated by the determined waypoints in consideration of various conditions satisfies the terminal constraint. Numerical simulations are performed to demonstrate the effectiveness of the proposed method.
Open Access
Generalized analysis of biased proportional navigation guidance with fractional power error feedback
(AIAA, 2022-06-03) Cho, Namhoon; Kim, Jinrae; Lee, Seokwon; Kim, Youdan
This study presents a comprehensive analysis of biased proportional navigation guidance laws with variable gain in the bias command that achieve stationary target interception with a desired impact angle under the restriction of permissible look angle. More specifically, this study investigates the bias shaping approach, which takes the bias gain function given by a product of factors in range and look angle. The linear feedback and fractional power feedback of impact-angle error are considered for command structures. For each command form, this study examines the conditions that should be satisfied by the gain functions to guarantee mission accomplishment without violation of the constraints. The analysis particularly focuses on estimating the set of feasible initial or final conditions that ensure compliance with the constraints along the trajectory. Numerical simulation is performed for a range of initial and final flight-path angles to verify the analytical results. The findings of this study provide a general foundation for the design of a nonswitching guidance law for impact-angle control with limited look angle through shaping of gain profiles.
Open Access
Generalized formulation of linear nonquadratic weighted optimal error shaping guidance laws
(AIAA, 2019-12-29) Cho, Namhoon; Kim, Youdan; Shin, Hyosang
This study presents a novel extension to the theory of optimal guidance laws represented by the nontraditional class of performance indices: nonquadratic-type signal Lp" role="presentation">Lp norm for the input weighted by an arbitrary positive function. Various missile guidance problems are generally formulated into a scalar terminal control problem based on the understanding of the predictor–corrector nature. Then, a new approach to derive the optimal feedback law, minimizing the nonquadratic performance index, is proposed by using the Hölderian inequality. The proposed extension allows a more general family of formulations for the design of closed-form feedback solutions to various guidance problems to be treated in a unified framework. The equivalence between the proposed approach and other design methodologies is investigated. In general, the type of input norm mainly determines the variability of input during the engagement while trading off against the rate of error convergence. The analytic solution derived in this study is verified by comparison with the solution from numerical optimization, and the effect of the exponent p" role="presentation">p in the performance index on the trajectory and command is demonstrated by numerical simulations.
Open Access
Generalized time-to-go inversion guidance with impact time and angle constraints
(AIAA, 2025-03) Wang, Pengyu; Lee, Chang-Hun; Cho, Namhoon; Wie, Bong
A major challenge in homing guidance is that accurately predicting the time to go for missiles is generally difficult, which limits the practical application of a certain class of guidance laws with impact-time constraints or cooperative capabilities. To address this issue, a generalized method referred to as the time-to-go inversion guidance is presented, where the time to go is first specified as an explicit function and then the corresponding guidance law is inversely derived. By employing this new method, a variety of guidance laws and their globally accurate time-to-go solutions can be developed, regardless of whether the impact-angle constraint is considered or not. Different from all existing studies, the proposed method addresses the problem of time-to-go estimation without relying on any linearizing approximations or numerical iterations. To enhance the practicality of the proposed method, it is augmented with bias commands to result in either the impact time control guidance law or the impact time and angle control guidance law. Numerical simulation results demonstrate the higher accuracy, larger feasible solution region, and lower computational demand of the proposed guidance laws even for the missiles with nonconstant speeds.
Open Access
Guidance of gliding vehicles with energy management based on approximate prediction of speed
(2022-05-05) Cho, Namhoon; Kim, Youngil; Shin, Hyosang; Kim, Youdan
This study presents a guidance method for flight vehicles gliding in the vertical plane to achieve desired position and velocity at the final time. The proposed guidance algorithm combines two decoupled elements to plan future flight trajectories satisfying the given constraints at each guidance update cycle: i) parametric path generator, and ii) approximate speed predictor. The parametric path generator is capable of producing an altitude profile as a parametric function of downrange by solving a convex optimisation problem considering only the shape properties of a flight path. An approximate method for predicting the future speed history endows the proposed guidance algorithm with the capability to address energy management objectives in trajectory planning. Provided that an altitude profile is specified by the parametric path generator and the lift-to-drag ratio model is known, the approximation neglecting gravitational acceleration turns the speed dynamics along the given path into a scalar linear first order ordinary differential equation, the form which admits a closed-form solution that can be represented by definite integrals. In this way, the proposed method opens a possibility to update the trajectory in flight to achieve the desired final speed by reducing the computational load due to speed prediction task, although the predicted speed contains approximation errors of certain degrees.
Open Access
Inverse optimality of pure proportional navigation guidance for stationary targets
(AIAA, 2021-11) Lee, Seokwon; Cho, Namhoon
The main contribution of this study is the optimality analysis of the PPNG performed in full generality. The new theoretical findings can explain the result of the former analysis in which the PPNG is derived as the minimum effort solution [5] and also describe a comprehensive design framework including the observability-enhanced guidance laws developed for the dual homing guidance problem. Furthermore, this study provides several examples illustrating how the PPNG with various navigation gain functions can be understood as optimal control solutions.
Open Access
Look-angle-constrained control of arrival time with exact knowledge of time-to-go
(AIAA, 2021-05-26) Cho, Namhoon; Lee, Seokwon
The capability to control the time of arrival at a goal position as desired endows a single vehicle or a coalition of many of them with the strategic advantage to perform time-critical missions. Arrival time coordination can be used as an element to solve multi-agent, multidepot routing and task planning problems in cooperative unmanned aerial robots. The tactic known as Salvo, which either designates or synchronizes the impact times across multiple missiles to enhance their collective survivability as well as attack effectiveness, strongly depends on control of arrival time. In principle, control of arrival time is essentially adjustment of the arc length of the vehicle’s flight path through manipulation of the curvature, provided that most vehicles flying in the atmosphere often prefer not to change their speeds excessively. On the other hand, the capability to take measurements of the target with onboard sensors provides a higher degree of autonomy to the vehicle and hence allows a more intelligent behavior. Modern autonomous vehicles acquire information about the designated destination or the surrounding environment with imaging sensors, in particular. An onboard sensor that collects emission or reflection from the target is usually not likely to be omni-directional yet possesses only a finite field-of-regard. The requirement to ensure continuous acquisition of target-originated signals necessitates a measure to keep the information source inside the sensor’s field of view that spans over a solid angle of limited range. That is, a box constraint is imposed on the look angle.
Open Access
Nonlinear optimal missile guidance for stationary target interception with pendulum motion perspective
(IEEE, 2021-07-28) Cho, Namhoon
This study outlines the set of equations constituting the necessary conditions that should be solved to determine the optimal guidance command for a missile to intercept a stationary target along a desired impact direction at a prespecified final time. Unlike the earlier studies on nonlinear optimal guidance problems, the present study formalises the optimal control problem with both final time and final state fixed. The pure control effort quadratic norm is considered as the performance index to be minimised. A noticeable finding from the study of the necessary conditions is that the flight path angle of the optimal trajectory obeys the simple pendulum dynamics. Full characterisation of the exact optimal solution requires numerically solving a set of four nonlinear algebraic equations with respect to four unknowns.
Open Access
Online corrections to neural policy guidance for pinpoint powered descent
(AIAA, 2024-02-03) Cho, Namhoon; Shin, Hyo-Sang; Tsourdos, Antonios; Amato, Davide
This study presents incremental correction methods for refining neural network parameters or control functions entering into a continuous-time dynamic system to achieve improved solution accuracy in satisfying the interim point constraints placed on the performance output variables. The proposed approach is to linearize the dynamics around the baseline values of its arguments and then to solve for the corrective input required to transfer the perturbed trajectory to precisely known or desired values at specific time points, in other words, the interim points. Depending on the type of decision variables to adjust, parameter correction and control function correction methods are developed. These incremental correction methods can be used as a means to compensate for the prediction errors of pretrained neural networks in real-time applications where high accuracy of the prediction of dynamical systems at prescribed time points is imperative. In this regard, the online update approach can be useful for enhancing overall targeting accuracy of finite-horizon control subject to point constraints using a neural policy. A numerical example demonstrates the effectiveness of the proposed approach in an application to a powered descent problem on Mars.
Open Access
Online trajectory replan for gliding vehicle considering terminal velocity constraint
(IEEE, 2022-08-08) Kim, Youngil; Cho, Namhoon; Park, Jongho; Kim, Youdan
Controlling the terminal velocity can improve the effectiveness of guided missiles. In particular, a ballistic missile propelled by solid rocket motors can successfully accomplish its mission when it hits the target at an appropriate speed. In this study, a method for modifying the trajectory of gliding vehicle, i.e., gliding ballistic missiles is proposed to meet the terminal velocity constraint by reflecting the effects of the environment during a flight. The proposed framework consisting of trajectory generation and dynamic propagation compensates for errors due to uncertainties in real time. The trajectory generation step provides various trajectories that satisfy the given constraints based on information about the current state. The dynamic propagation step efficiently predicts the terminal velocity for each of the generated trajectories and finds the trajectory with the lowest terminal speed error. A numerical simulation is performed considering various conditions to demonstrate the performance of the proposed method.
Open Access
Recursive least squares with log-determinant divergence regularisation for online inertia identification
(IEEE, 2024-05-13) Cho, Namhoon; Lee, Taeyoon; Shin, Hyo-Sang
This study presents a recursive algorithm for solving the regularised least squares problem for online identification of rigid body dynamic model parameters with emphasis on the physical consistency of estimated inertial parameters. One of the geometric approaches is to use a regulariser that represents how close the pseudo-inertia matrix is to a given reference on the feasible manifold in the regression problem. The proposed extension enables memory-efficient online learning in addition to the benefits of geometry-aware convex regularisation using the log-determinant divergence of the pseudo-inertia matrix. Also, the recursive version endows the estimator with the capability to deal with time-variation of parameters by introducing an optional forgetting mechanism. The characteristics of the recursive regularised least squares algorithm is demonstrated using the MIT Cheetah 3 leg swinging experiment dataset and compared to the existing batch optimisation method.
Open Access
Three-dimensional biased proportional navigation guidance based on spatial rotation of predicted final velocity
(IEEE, 2022-08-17) Cho, Namhoon; Lee, Seokwon; Shin, Hyosang; Kim, Tae-Hun
This study presents the design of three-dimensional biased proportional navigation guidance laws for arrival at a stationary target along a desired direction based on spatial rotation of predicted final velocity vector. The focus is on full constructive derivation using vector-form expressions without introducing local representation of rotation such as Euler angles or quaternions. The proposed approach synthesises the bias command in the form of an angular velocity vector through realisation of the predictive control design philosophy, the direction which has been unexplored in a three-dimensional setting. The proposed approach avoids heuristic choices and approximations in the design process and hence overcomes the limitation of earlier studies. The vector-form design approach provides theoretical and practical advantages including rigour in derivation, clear geometric understandings about the problem provided by identification of the most effective direction for rotation of final velocity, independence from selection of a fixed coordinate system, avoidance of singularities in local representations, more direct trajectory shaping, and simple implementation.