Barrier Lyapunov function-based robust flight control for the ultra-low altitude airdrop under airflow disturbances

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This paper develops a new method to control uncertain robot manipulators by using only position measurements. The controller is designed based on a combination of a computed torque controller (CTC) with a higher-order sliding-mode observer and a fuzzy compensator. First, three higher-order sliding-mode (SM) observers (second-order SM, third-order SM and third-order SM linear (TOSML) observers) are designed and compared to verify whether the TOSML observer is the best for observing velocity and identifying uncertainty. A combined CTC-TOSML controller was then designed. Although this controller scheme can overcome the drawbacks of conventional CTCs, its tracking performance can still be improved. To enhance capability of the tracking performance, a CTC-TOSML controller plus fuzzy compensator called a CTC-TOSML-Fuzzy controller is proposed. The proposed controller increases the potential of the CTC for real robot applications. Finally, computer simulation results on a PUMA560 robot are discussed to verify the effectiveness of the proposed strategy.

In this paper, we present the design of observer based event-triggered sliding mode control (ETSMC) for a chain of integrator system in the presence of matched uncertainties. In the scenario, where only output information is available, the observer is needed to observe all the states of the system. If a Luenberger observer is used the robustness property cannot be achieved, so it is appealing to use sliding mode observer (SMO). But conventional SMO does not give finite time convergence of all the observed states to their true values. Consequently, we use a higher order SMO (HOSMO) to observe all the states of the system in finite time. Also, to reduce computational effort, the behaviour of the system is analysed using this observer in the event-triggered environment. For this an ETSMC is proposed using these observed states and stability of the system is proved using Lyapunov analysis. It is also proved that the triggering instants generated using the proposed triggering rule do not lead to Zeno behaviour. Finally, the effectiveness of the proposed method is shown by simulation of a third order chain of integrator system.

Aiming at the robust attitude control issue on reentry vehicle subject to uncertainties and disturbances. A novel optimal predictive attitude control method integrating with a high order sliding mode observer is proposed in this paper. On the one hand, the proposed optimal one-step predictive control approach can guarantee good dynamic performance of the control system, on the other hand, a high order sliding mode observer is applied to estimating the uncertainties and disturbances exactly, which can strengthen the robustness of the control system. System stability is strictly proved by Lyapunov theory and the domains of convergence are given exactly with/without the observer. Finally, an attitude predictive control law is presented for a reentry vehicle with dynamic characteristic of the actuators. Numerical simulations demonstrate the effectiveness of the proposed control law.

This work presents a nested super-twisting second-order sliding mode speed controller for a brushless direct current motor with a high order sliding mode observer used for back electromotive force (back-EMF) estimation. Due to the trapezoidal nature of the back-EMF, a modified Park transformation is used in order to achieve proper field orientation. Such transformation requires information from the back-EMF that is not accessible. A second-order sliding mode observer is used to estimate the back electromotive forces needed in the modified transformation. Sliding mode control is known to be robust to matched uncertain disturbances and parametric variations but it is prone to unmatched perturbations that affect the performance of the system. A nested scheme is used to improve the response of the controller in presence of unmatched disturbances. Simulations performed under similar conditions to real-time experimentation show a good regulation of the rotor speed in terms of transient and steady-state responses along with a reduced torque ripple.

The paper pertains to model validation and novel higher order sliding mode observer design for estimating precursor concentration of a nuclear research reactor. The linear and non-linear models of Pakistan Research Reactor-1 have been tuned and validated with experimental data. These models are subsequently used for higher order sliding mode observer design and performance evaluation. After lab experimentations, the non-linear and linear models of PARR-1 have been validated by giving desired motion to the control rod and recording output power change at the actual reactor. The mathematical model of the overall system, from control rod movement to output power change is validated with different sets of data at different operating points. Based on the validated model a robust nonlinear observer for estimating precursor concentration has been developed by using second order sliding mode technique. The observer estimates this variable with only the measurement of neutron flux available. The estimated value is in close agreement with the theoretically calculated value.

In this paper, the robust longitudinal flight control is developed for the fighter using the backstepping technique. To improve the robust control performance for the unknown external disturbance, the sliding mode disturbance observer is employed to estimate the unknown external disturbance. Utilizing the disturbance estimate output, the robust backstepping flight control scheme is proposed for the fighter with the unknown external disturbance. Finally, simulation results are given to show the effectiveness of the proposed robust flight control scheme for longitudinal flight dynamics.

In this paper, an experimental validation of vertical forces estimation is presented. An original method using high order sliding mode observer is developed. The test bench is a static vehicle excited vertically by hydraulic jack. It's equipped by different sensors in order to measure its dynamics. Real time tests are done and results are presented in this paper.

A robust finite-time control (FTC) framework using continuous terminal sliding-mode control (SMC) and high-order sliding-mode observer (HOSMO) is discussed to realize the trajectory tracking of flexible-joint robots in this article. Control performances of the robots always suffer from unknown matched and mismatched time-varying disturbances. Traditional SMC exists with a chattering phenomenon and cannot cope with mismatched time-varying disturbances due to its inherent structure property. For this reason, two HOSMOs are devised to estimate the time-varying disturbances on the link and motor side, respectively. Then, by fusing the states and disturbance estimations into a novel terminal sliding-mode surface, a continuous robust FTC scheme is developed. The proposed control strategy can not only handle both matched and mismatched time-varying disturbances but also obtain a finite-time convergence performance. The rigorous finite-time stability analysis of the closed-loop system under the proposed control method is guaranteed. The results are illustrated to verify the effectiveness and robustness of the proposed design approach.

Although reconfigurable flight control has been well demonstrated on fixed-wing aircraft using existing control surface redundancies, such failure accommodations were widely believed to be impossible for single main rotor helicopters. In fact, on any such existing helicopter, the failure of a main rotor actuator is catastrophic, resulting in a complete loss of control. An actuator geometry that enables reconfigurable helicopter flight control is presented, and, from this, reconfigurable flight-control methods are developed to accommodate main rotor actuator failures. The geometry provides control axes coupling that is key to solving the reconfigurable flight-control problem. Such coupling enables the development of a swashplate reconfiguration strategy, where any two of the three control axes can be controlled at a time, despite a failure in any one of the main rotor actuators. Of most interest is the particular swashplate reconfiguration solution where attitude control (pitch and roll) is retained, whereas vertical control is sacrificed. Vertical control is then achieved by one of two methods: flight to a longitudinal velocity that supports the desired vertical velocity or the use of rotor speed to change the main rotor's thrust to perform limited closed-loop vertical control. These methods are combined to form a robust reconfigurable flight-control method. All of these methods are independent of the underlying flight-control system. Such independence is demonstrated by evaluation of the reconfigurable flight-control methods for two different flight-control systems: a classic proportional-integral-derivative controller and neural dynamic programming, a neural network based controller, All designs are tested by the use of a sophisticated nonlinear validated model of the Apache helicopter.

Nonlinear Dynamic Inversion(NDI) control has excellent rapidity and decoupling ability, unfortunately it lacks the essential robustness to disturbance. From the perspective of enhancing the robustness, an adaptive NDI method based on L1 adaptive structure is proposed. The L1 adaptive structure is introduced into the NDI control to enhance its robustness, which also guarantees the stability and expected dynamic performance of the system suffering from the disturbance influence. Secondly, the flight control law of the advanced aircraft is designed based on the present method to improve the robustness and fault tolerance of the flight control system. Finally, the effectiveness of the flight control law based on the present approach is verified under the fault disturbance. The results showed that the flight control law based on L1 adaptive NDI has excellent dynamic performance and strong robustness to parameter uncertainties and disturbances.

This paper considers the problem of farming tractors working in agriculture fields. Agricultural fields are usually uneven and have low friction. Thus tractors undergo more lateral disturbances and slippage than traveling on hardened surfaces. In such scenarios, kinematic models are not adequate to describe the vehicle's motion due to the lateral slip, thus may lead to poor performance. In this paper, a dynamic tractor model is established considering vehicle side slip and lateral force disturbance. A sliding mode observer is proposed to estimate the disturbances and vehicle states which are difficult for direct measurement (especially for the lateral force). In order to attenuate the influence of unknown lateral force disturbance and parameter perturbations, the H∞ robust controller based on linear matrix inequality (LMI) approach is designed to reduce the lateral offset of the tractor. Simulation results verified the effectiveness of the proposed method and showed superior performance than LQR controllers.

This paper presents the design and implementation of incremental backstepping (IBS) flight control laws for the attitude control and stabilization on a fixed-wing aircraft. The design consists of multiple functionalities such as command-filtered backstepping, angle of attack control and body attitude control, that are based around an incremental control inner loop that tracks the angular rates of the aircraft. The results include flight data of an integrated IBS design that validate simulation results of control laws shown previously in literature. The results show that it is possible to implement robust nonlinear flight control laws that are easy to tune and require only little knowledge about the system dynamics parameters.

In this paper, the adaptive robust tracking control scheme is proposed for a class of multi-input and multi-output (MIMO) non-affine systems with uncertain structure and parameters, unknown control direction and unknown external disturbance based on backstepping technique. The MIMO non-affine system is first transformed into a time-varying system with strict feedback structure using the mean value theorem, and then the bounded time-varying parameters are estimated by adaptive algorithms with projection. To handle the possible controller singularity problem caused by unknown control direction, a Nussbaum function is employed, and the dynamic surface control (DSC) method is applied to solve the problem of explosion of complexity in backstepping control. It is proved that the proposed control scheme can guarantee that all signals of the closed-loop system are bounded through Lyapunov stability theorem and decoupled backstepping method. Simulation results are presented to illustrate the effectiveness of the proposed control scheme.

The problem of robust exact control for a Stewart platform with smooth bounded unknown inputs is considered. This platform has three degrees of freedom and it is used as a remote surveillance device. We consider high-order sliding mode observers to provide both theoretical exact observation and unknown input identification. In this paper, a methodology is proposed to select the most adequate control strategy for unknown external perturbation identification. The results obtained are illustrated by simulations.