Huabo Yang

Robust missile longitudinal autopilot design based on equivalent-input-disturbance and generalized extended state observer approach

This paper presents a new robust design for the longitudinal autopilot of a tail-controlled, skid-to-turn missile based on equivalent-input-disturbance (EID) and generalized extended state observer (GESO) approach. Firstly, the nonlinear missile longitudinal dynamics is modeled as a linear formulation for angle-of-attack tracking with uncertainties and disturbances. Next, as a new method, EID-GESO-based control is proposed for disturbance rejection. The method regards the uncertainties and disturbances as a lumped disturbance, named as EID, and simultaneously attenuates it through estimation of linear GESO and simple feedback. Closed-loop stability of the method is also proved. The most important feature of the proposed method over the others is that there is no requirement for system transformation, differentiation of the measured output, full system state available, exact plant model or any information about uncertainties and disturbances. Subsequently, the method is applied to robust longitudinal autopilot design and the feasibility is illustrated by numerical simulations. Finally, a performance comparison with some notable designs under considerable uncertainties and disturbances is implemented to demonstrate the effectiveness and robustness of the proposed design.

Kalman Filtering for Relative Spacecraft Attitude and Position Estimation: A Revisit

I N [1], an extended Kalman filter (EKF) was formulated to estimate the relative position and attitude between the chief and deputy spacecraft using the vision-based-navigation (VISNAV) system coupled with gyro measurements from each spacecraft. The VISNAV system consists of an optical sensor combined with specific light sources (beacons) to achieve a selective vision. In general, the known beacon locations are defined in the chief’s body frame, whereas the relative position vector is expressed in its local vertical/ local horizontal (LVLH) frame. In [1], it was implicitly assumed (but not clearly stated) that the absolute position and attitude of the chief were known, and only the relative quantities needed to be estimated. Therefore, a simplified assumption that the chief body frame coincides with its LVLH frame was made in [1] to construct the lineof-sight (LOS) observations for convenience. Unfortunately, this assumption is not valid under all situations, or in some rigorous sense, it is only a special case. One approach to solve this problem is to formulate the relative equations ofmotion in the chief body frame [2], and thus, the beacon-location vectors and relative position vector between the chief and deputy spacecraft are described in the same coordinates. However, this approach has two main disadvantages in practice: 1) the angular velocity of the chief body frame generally varies rapidly, and its measured value is contaminated by the gyro measurement error, which may cause large computation errors in the relative equations of motion; and 2) the linear relative dynamics are not obtained, and relative rotational and translational motions are interlaced in highly nonlinear and coupled fashions. The objectives of this Notewere to revisit this relative position and attitude estimation problem, and to design a novel navigation filter without the assumption. Two relative quaternions thatmap the chief’s LVLH frame to the chief body frame and to the deputy body frame are involved. The corresponding attitude matrices are used to construct the LOS observations, and thus, the assumption that both the chief’s body and the LVLH frames are the same can be removed. An EKF is derived to estimate the mentioned relative quaternions, relative position, and velocity of two spacecraft, as well as gyro biases. The relative attitude between two spacecraft is obtained by using these two relative quaternions. This Note extends the previous work [1] to additionally estimate the attitudes of both spacecrafts relative to the LVLH frame, in which the relative orbit equations are written, so that the attitude of the chief need not be known a priori.

Electron beam-associated symmetric electrostatic solitary waves on the separatrix of magnetic reconnection: multi-spacecraft analysis

We present an in situ evidence of electron beam-associated symmetric bipolar electrostatic solitary waves (ESWs) on the current sheet-side of the separatrix of the magnetic reconnection in the near-Earth magnetotail by multi-spacecraft observation of Cluster. Within one spin period, 42 cases of symmetric ESWs are continuously observed during 2 s by SC2 while other spacecrafts do not “detect” them. And the Plasma Electron and Current Experiment (PEACE) spinPAD mode data exhibits unidirectional electron beam antiparallel to the ambient field, and no electron beam-like distribution is found by other spacecrafts without ESW observation. Though the electron beam is strongly associated with the ESWs in observation by multiple spacecraft differentiation, however, the relationship between the counter-directed electron beam and the simultaneously observed ESWs remains unclear and open to the next study.

Enhanced predictive functional control based on equivalent input disturbance and generalized extended state observer for nonlinear systems with fast dynamics

An approach for improving tracking performance of the predictive functional control (PFC) based on equivalent input disturbance (EID) and generalized extended state observer (GESO) is presented in this paper. Control structure of the proposed method includes two parts, a PFC controller and a state observer, and both of them can be designed separately. The PFC controller is employed to optimize the tracking control of a nonlinear system and its tracking performance heavily depends on the accuracy of the predictive model. Whereas, various internal perturbations and external disturbances always make the predictive model significantly deviate from the nominal model. To address this problem, a state observer is introduced into the proposed approach. All of the above-mentioned uncertainties are regarded as a lumped disturbance, and the lumped disturbance is estimated by EID–GESO as well as the system states in an integrated manner. The estimated disturbance can be eliminated in a negative feedback loop and then, a relatively accurate predictive model for the PFC controller is offered. Closed-loop stability of the composite control is also provided. Compared with previously related work, the notable feature of the proposed design is that the observer-based PFC is extended to nonintegral chain systems subject to mismatched uncertainties and better performance is obtained. Finally, the proposed approach is applied to a missile longitudinal autopilot design, and a comparison with some prominent methods in the presence of significant uncertainties demonstrates the robustness and effectiveness of the proposed design.

Unscented Kalman Filtering for Relative Spacecraft Attitude and Position Estimation

A novel relative spacecraft attitude and position estimation approach based on an Unscented Kalman Filter (UKF) is derived. The integrated sensor suite comprises the gyro sensors on each spacecraft and a vision-based navigation system on the slave spacecraft. In the traditional algorithm, an assumption that the masters body frame coincides with its Local Vertical Local Horizontal (LVLH) frame is made to construct the line-of-sight observations for convenience. To solve this problem, two relative quaternions that map the masters LVLH frame to the slave and master body frames are involved. The general relative equations of motion for eccentric orbits are used to describe the positional dynamics. The implementation equations for the UKF are derived. A modified version of the UKF is presented based on the averaging-quaternion algorithm. Simulation results indicate that the proposed filters provide more accurate estimates of relative attitude and position than the Extended Kalman Filter (EKF).

Observability of self-calibration and self-alignment for inertially stabilized platform:

The observability of the self-calibration and self-alignment system for an inertially stabilized platform is of vital importance, because it determines the solution existence of the system states. This article provides a straightforward and comprehensible method to investigate the observability of the nonlinear inertially stabilized platform’s self-calibration and self-alignment system. The proposed method is based on a principle that a parameter is observable only if it has a unique solution from the system outputs. The effect of the platform coordinates frame on the system observability is discussed in detail. The demonstration results indicate that the system is completely observable if the platform frame is defined based on the input axes of accelerometer triad. Besides, the analysis processes show that a high performance self-calibration and self-alignment can be accomplished if the inertially stabilized platform is kept stationary with the Earth at different positions and alternately rotated around its each axis. The validation of those results is checked by simulations, and the achieved conclusions make outstanding contributions to the development of the optimal torqueing schemes for the inertially stabilized platform’s self-calibration and self-alignment system.

Improved disturbance rejection control based on H∞ synthesis and equivalent-input-disturbance for aircraft longitudinal autopilot design

Disturbance rejection control has been developed over decades attracting wide interest and research attention. Aiming at providing a potential engineering application of disturbance rejection control to aircraft control system design following traditional frequency-domain methods, this paper presents an improved disturbance rejection control of two degrees of freedom based on H ∞ synthesis and equivalent-input-disturbance for an aircraft longitudinal autopilot design. The mismatched disturbance is transformed as a “total disturbance” in the input channel for compensation through the establishment of an equivalent-input-disturbance system. The H ∞ synthesis based on classical frequency-domain analysis is applied to a disturbance filter and a composite feedback controller design. In terms of the controller design, the system including the filter is considered as a whole in H ∞ optimization process without separate design to guarantee the stability of the overall system. Furthermore, the proposed method is successfully implemented on the autopilot design by modeling nonlinear aircraft longitudinal dynamics as a linear equivalent-input-disturbance formulation of angle of attack. The simulation of tracking performance in comparison with existing renowned methods is conducted in the presence of aerodynamic uncertainties, gust disturbance, actuator characteristics and sensor noise. The results verify the effectiveness of the proposed method with excellent performance and practical prospects.