This paper addresses the problem of adaptive stabilization of MIMO systems whose constant high-frequency gain matrix is nonsingular but has completely unknown sign structure. Unlike most existing approaches, no positivity, symmetry, or diagonal dominance assumptions on the gain matrix are required. Building upon the unit-vector framework and the concept of unmixing sets, a modified discontinuous adaptive control strategy is proposed. The key novelty consists in decoupling the mechanism that cycles through the elements of the unmixing set from the adaptation of the control input magnitude. In particular, the adaptive gain is used exclusively to govern the switching among candidate gain matrices, while the control input magnitude is fixed a priori. A Lyapunov-based analysis proves global asymptotic stabilization for driftless systems. The results are further extended to systems affected by matched bounded disturbances under mild additional assumptions. Simulation studies illustrate the effectiveness of the proposed approach and highlight its advantages in terms of reduced control authority and alleviated transient peaking.
Wave propelled platforms represent a promising technology for sustainable ocean monitoring and marine energy harvesting, yet achieving precise position regulation under persistent and time-varying wave disturbances remains a significant control challenge. This paper presents a Super-Twisting Algorithm (STA) controller for position regulation of multi-blade wave-propelled platforms, requiring only platform position measurements and eliminating the need for blade angle sensing or full state feedback. A comprehensive dynamic model captures blade rotation, platform translation, and wave–structure interactions for an N-blade system. The proposed controller achieves robust regulation while eliminating chattering, supported by Lyapunov-based stability analysis. Comparative simulations against conventional PID and standard SMC with boundary layer are conducted under three scenarios: constant wave forcing, target position switching, and time-varying wave disturbances. Results confirm that the STA approach achieves superior steady-state accuracy and tracking performance while maintaining robust regulation under time-varying wave conditions.
Soft landing in the presence of hazardous terrain remains a critical and challenging problem in a spacecraft landing mission. The problem is more daunting when environmental uncertainties/disturbances are considered. Further, the presence of terrain features such as boulders, craters, and uneven topography in the vicinity of the designated landing location pose challenges. This paper explores a sliding-mode-based landing guidance for robustness against matched bounded uncertainties, encountered during terminal descent. To provide safety guarantees in a hazardous terrain, a Control Barrier Function (CBF) based safety filter that minimally modifies the nominal Sliding Mode (SM) based guidance law is developed while honoring the thrust constraints. A time-varying altitude barrier function is used as an upper bound for the vertical motion, thereby ensuring that the landing mission is completed within a predefined time. Simulation results demonstrate robustness and safety of the proposed guidance framework.
To address plants with high relative degree, sliding-mode control has evolved toward higher-order techniques. Yet, in control applications these approaches still rely on discontinuous inputs. To mitigate the resulting chattering and to handle unbounded disturbances, discontinuous integral control has recently been proposed. This method combines a continuous homogeneous state-feedback controller with a discontinuous integrator, enabling stabilization of high–relative-degree plants in the presence of smooth but unbounded disturbances. Building on this idea, we introduce a controller with a double discontinuous integrator for systems of relative degree two. This structure further smooths the control action and allows to compensate faster-growing disturbances. The paper provides Lyapunov-based stability guarantees and offers tuning guidelines to facilitate implementation.
Fully actuated UAVs provide enhanced maneuverability and accurate six-degree-of-freedom (6-DoF) control, making them well-suited for demanding tasks such as aerial manipulation, operation in confined environments, and fault-tolerant missions. However, the resulting over-actuation poses challenges in control allocation and robustness to external disturbances. This paper develops a sliding-mode control strategy integrated with a control-allocation framework to achieve precise maneuvering of fully actuated UAVs. The approach is demonstrated on a quadrotor equipped with dual-axis tilting propellers. The proposed guidance framework leverages the vehicle’s full actuation to enable a decoupled design for translational and rotational dynamics. Under the assumption of symmetric actuation across all rotors, thrust vectors are systematically constructed to meet the prescribed tracking objectives. These vectors are then employed in an inverse control-allocation problem to compute the required thrust magnitudes, along with the associated servo and tilt angles for each rotor. Extensive simulations are presented to validate the performance of the proposed scheme across a variety of dynamic reference trajectories.
