Browsing by Author "Cho, Namhoon"
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Item Open Access Analysis of guidance laws with non-monotonic line-of-sight rate convergence(IEEE, 2021-09-20) Lee, Seokwon; Cho, Namhoon; Shin, Hyo-SangThis 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.Item 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, NamhoonThis 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.Item Open Access Approximation of achievable robustness limit based on sensitivity inversion(AIAA, 2023-11-07) Cho, Namhoon; Lee, Hae-InIntroduction: 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.Item Open Access Collision-geometry-based optimal guidance for high-speed target(Elsevier, 2021-04-30) Kim, Boseok; Kim, Young-Won; Cho, Namhoon; Lee, Chang-HunThis 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.Item Open Access Composite model reference adaptive control with parameter convergence under finite excitation(IEEE, 2017-08-09) Cho, Namhoon; Shin, Hyo-Sang; Kim, Youdan; Tsourdos, AntoniosA 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.Item 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, YoudanBallistic 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.Item Open Access Guidance of gliding vehicles with energy management based on approximate prediction of speed(2022-05-05) Cho, Namhoon; Kim, Youngil; Shin, Hyo-Sang; Kim, YoudanThis 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.Item Open Access Inverse optimality of pure proportional navigation guidance for stationary targets(American Institute of Aeronautics and Astronautics, 2021-07-21) Lee, Seokwon; Cho, NamhoonThe 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.Item Open Access Look-angle-constrained control of arrival time with exact knowledge of time-to-go(AIAA, 2021-05-26) Cho, Namhoon; Lee, SeokwonThe 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.Item Open Access Online corrections to neural policy guidance for pinpoint powered descent(AIAA, 2024-02-03) Cho, Namhoon; Shin, Hyo-Sang; Tsourdos, Antonios; Amato, DavideThis 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.Item Open Access Online trajectory replan for gliding vehicle considering terminal velocity constraint(IEEE, 2022-08-08) Kim, Youngil; Cho, Namhoon; Park, Jongho; Kim, YoudanControlling 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.Item 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, Hyo-Sang; Kim, Tae-HunThis 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.Item Open Access Three-dimensional guidance method with course modification for altitude shaping in endoatmospheric interception(IEEE, 2023-07-18) Cho, NamhoonThis study presents a three-dimensional guidance law for the interception of an endoatmospheric target. The proposed method takes an empirical design approach which first specifies the structure of the lateral acceleration command as that of a linear optimal guidance law for zero-effort-miss nullifcation. Then, the direction of pursuit and the guidance gain are designed in accordance with the physical understandings of the motion characteristics of an aerodynamically-controlled interceptor. More specifically, the proposed method induces an intentional increase in the flight altitude around the initial phase while respecting the maximum altitude constraint, all of which are realised through modification of the desired flight path angle in the vertical plane. The proposed guidance method does not rely on explicit definition of design elements such as engagement planes, guidance phases, complicated time-to-go estimation, and waypoints. Moreover, the proposed design approach of modifying the desired course based on the collision courses naturally facilitates smooth handover to the terminal phase near the collision condition. Numerical simulation shows that the proposed guidance method is effective in intercepting a nonmanoeuvring target over a wide range of engagement conditions to the target in comparison to the existing guidance laws developed for homing and midcourse flight.Item Open Access Unified control parameterization approach for finite-horizon feedback control with trajectory shaping(IEEE, 2022-03-22) Cho, Namhoon; Park, Jongho; Kim, Youdan; Shin, Hyo-SangThis study presents control parametrisation as a unifying framework for designing a linear feedback control law that achieves finite-time transfer of output as well as trajectory shaping. Representing control input as a linear combination of independent basis functions allows wide variability in the resultant feedback control laws through selection of the number and types of basis functions. Given an array of basis functions that meets the trajectory shaping necessities, the unified design approach proceeds with determination of the coefficients so that the predicted trajectory attains the desired output at the final time. The input evaluated with the coefficients found at each instance essentially turns out to be a linear state feedback policy with an additional feedforward term and time-dependent gains which is appropriate for practical use. The unified control parametrisation approach lends itself well to missile guidance applications with the expandability and direct trajectory shaping capability that it provides. To emphasise expandability of the framework, this study revisits the trajectory shaping guidance laws from the control parametrisation viewpoint and shows how the notion of specifying input basis functions not only generalises various existing methods but also enables further extensions. Furthermore, an application to integrated guidance and control illustrates the strength of design process in handling the shaping requirements more directly through construction of appropriate basis.