Moshe Idan

Sliding-Mode Control for Integrated Missile Autopilot Guidance

A sliding-mode controller is derived for an integrated missile autopilot and guidance loop. Motivated by a differential game formulation of the guidance problem, a single sliding surface, defined using the zero-effort miss distance, is used. The performance of the integrated controller is compared with that of two different two-loop designs. The latter use a sliding-mode controller for the inner autopilot loop and different guidance laws in the outer loop: one uses a standard differential game guidance law, and the other employs guidance logic based on the sliding-mode approach. To evaluate the performance of the various guidance and control solutions, a two-dimensional nonlinear simulation of the missile lateral dynamics and relative kinematics is used, while assuming first-order dynamics for the target evasive maneuvers. The benefits of the integrated design are studied in several endgame interception engagements. Its superiority is demonstrated especially in severe scenarios where spectral separation between guidance and flight control, implicitly assumed in any two-loop design, is less justified. The results validate the design approach of using the zero-effort miss distance to define the sliding surface.

Integrated Sliding Mode Autopilot-Guidance for Dual-Control Missiles

An integrated autopilot and guidance algorithm is developed, using the sliding mode control approach, for a missile with forward and aft control surfaces. Based on guidance considerations, the zero efiort miss (ZEM), encountered in difierential games guidance solutions, is used as one of the sliding variables in the proposed control scheme. The dual control conflguration provides an additional degree of freedom in the integrated design. This degree of freedom is exploited by introducing a second sliding surface, selected based on autopilot design considerations. Restraining the system to the ZEM surface guarantees zero miss distance, while remaining on the second surface provides a damped response. The performance of the integrated dual controller is evaluated using a two-dimensional nonlinear simulation of the missile lateral dynamics and relative kinematics, assuming flrst order dynamics for the target evasive maneuvers. The simulation results validate the design approach of using ZEM and the ∞ight-control based sliding surfaces to attain high accuracy interceptions.

Adaptive Neural Control of Deep-Space Formation Flying

A novel nonlinear adaptive neural control methodology is presented for the challenging problem of deep-space spacecraft formation flying. When the framework of the circular restricted three-body problem with the sun and Earth as the primary gravitational bodies is utilized, a nonlinear model is developed that describes the relative formation dynamics. This model is not confined to the vicinity of the Lagrangian libration points but rather constitutes the most general nonlinear formulation. Then, a relative position controller is designed that consists of an approximate dynamic model inversion, linear compensation of the ideal feedback linearized model, and an adaptive neural-network-based element designed to compensate for the model inversion errors. The nominal dynamic inversion includes the gravitational forces, whereas the model inversion errors are assumed to stem from disturbances such as fourth-body gravitational effects and solar radiation pressure. The approach is illustrated by simulations, which confirm that the suggested methodology yields excellent tracking and disturbance rejection, thus, permitting submillimeter formation keeping precision.

H2/H∞ filtering theory and an aerospace application

In many filtering problems of practical interest, some of the noise signals satisfy the assumptions of H 2 (KalmanBucy) filtering, while others can be more accurately modeled as bounded energy signals (hence more amenable to an H, filtering approach). These problems may be addressed by considering a mixed H 2 I ? i , filtering problem. In this paper we present a novel theory which solves the mixed problem ezac t ly and in a computationally efficient way. The applicability of the theory is illustrated by designing a filter to estimate the states of an aircraft flying through a downburst.

Hierarchical Approach to Adaptive Control for Improved Flight Safety

Following failures of primary aerodynamic actuators, safe flight can be maintained by introducing alternative actuation systems, such as analytically redundant secondary aerodynamic surfaces and propulsion, for higher-priority stability and control augmentation tasks. An intelligent hierarchical flight control system architecture is presented that is designed using nonlinear adaptive synthesis techniques and online learning neural networks to enhance flight safety. Pseudocontrol hedging is used for proper adaptation in the presence of actuator saturation, rate limits, and failure. The hierarchical structure incorporates nonactive secondary actuation channels that are engaged after a failure of a primary control surface is encountered. The methodology requires only the knowledge that a failure in a specific actuator has occurred. A model of the failed aircraft, the failure type, and the failure size need not to be known: The neural network element of the secondary channel will adapt to the failed actuator effect. The secondary control channels are designed to account for the typically lower authority and degraded performance that can be expected with secondary actuation systems. The proposed hierarchical flight control architecture is attractive, in particular, as a retrofit to existing certified flight control systems for enhanced flight safety. The proposed flight control architecture is evaluated in a nonlinear flight simulation environment, demonstrating its retrofit features.

Integrated Sliding Mode Guidance and Control for a Missile with On-Off Actuators

A sliding mode controller was recently introduced for integrated guidancecontrol loops of agile missiles. The sliding surface was chosen to be the zeroeffort miss-distance. The current work extends this result to address nonlinear on-off actuators commonly used in such interceptors. The performance of the integrated design is compared with a two-loop design, i.e., separate guidance and autopilot loops. The simulation includes a detailed pneumatic model of the aerodynamic surface actuators. Compared to the results obtained with a linear first order actuation system, it is shown that the advantages of the integrated design are more significant when tested with the on-off actuator. The proposed integrated algorithm is effective especially for the endgame phase of the interception. However, its high interception accuracy can be attained only if engaged from a limited range of initial conditions within the so called region of attraction, thus posing performance requirements to the midcourse guidance system. The paper presents the regions of attraction for a sample interception setup.

Optimal planar interception with terminal constraints

In this paper, planar interception laws for maneuvering targets with known trajectories are presented. Optimal interception problems are defined, which include constraints on the initial and final flight-path angles of the interceptor. For cases where the initial flight-path angle can be freely assigned, it is included in the optimization problem. Analytical solutions for the planar interception problems are derived. Numerical examples that demonstrate the optimal trajectories are presented showing also the effect of the interceptor initial flight-path angle on the interception characteristics. It is shown that when the interceptor initial conditions can be optimized superior performance is obtained.

Cauchy estimation for linear scalar systems

An estimation paradigm is presented for scalar discrete linear systems entailing additive process and measurement noises that have Cauchy probability density functions (pdf). For systems with Gaussian noises, the Kalman filter has been the main estimation paradigm. However, many practical system uncertainties that have impulsive character, such as radar glint, are better described by stable non-Gaussian densities, for example, the Cauchy pdf. Although the Cauchy pdf does not have a well defined mean and does have an infinite second moment, the conditional density of a Cauchy random variable, given its linear measurements with an additive Cauchy noise, has a conditional mean and a finite conditional variance, both being functions of the measurement. For a single measurement, simple expressions are obtained for the conditional mean and variance, by deriving closed form expressions for the infinite integrals associated with the minimum variance estimation problem. To alleviate the complexity of the multi-stage estimator, the conditional pdf is represented in a special factored form. A recursion scheme is then developed based on this factored form and closed form integrations, allowing for the propagation of the conditional mean and variance over an arbitrary number of time stages. In simulations, the performance of the newly developed scalar discrete-time Cauchy estimator is significantly superior to a Kalman filter in the presence of Cauchy noise, whereas the Cauchy estimator deteriorates only slightly compared to the Kalman filter in the presence of Gaussian noise. Remarkably, this new recursive Cauchy conditional mean estimator has parameters that are generated by linear difference equations with stochastic coefficients, providing computational efficiency.

Aeroservoelastic Structural and Control Optimization Using Robust Design Schemes

The procedure for structural and robust control design of multi-input/multi-output aeroservoelastic systems presented contains modeling of uncertainties, synthesis of a robust controller, and a unified structural and control optimization process under stress, flutter, and control performance constraints. Robust control design techniques can enhance the preliminary structural design for cases where certain system parameters are not known exactly or are uncertain. An aeroservoelastic interaction module is used for mediation between the structural optimization code and the control synthesis tools in an iterative coupled process. It provides the reduced-size state-space aeroelastic models needed for control synthesis, including model sensitivities to structural uncertainties, and integrates the resulting control model in structural optimization that includes robust-control considerations in terms of singular value constraints. The efficient uncertainty modeling developed is based on the readily available structural sensitivity data and the linear fractional transformation tools. Modeling of inertial uncertainties of the controlled structure was performed to account for relatively large inertial deviations of a fighter aircraft wing-tip missile. The numerical example clearly demonstrates the effectiveness of the proposed design scheme and its usefulness at preliminary design stages of aircraft with multiple external store loads.

Estimation of Rodrigues parameters from vector observations

A recursive minimum variance algorithm for attitude determination is presented. The attitude is expressed by the Rodrigues vector, where only three parameters are needed to describe orientation. The advantage of using the Rodrigues parameters is computational. Attitude calculations involve only relatively simple algebraic expressions, which is advantageous especially in real-time applications. The efficiency of the derived algorithm is demonstrated using Monte-Carlo computer simulation results and in an initial alignment aerospace application.