This paper presents a novel fast fixed-time nonsingular sliding mode controller designed to enhance target tracking accuracy and reduce vibrations in flexible-link manipulators. The core innovation lies in simultaneously addressing model uncertainties, external disturbances, and actuator saturation by incorporating them into the system’s dynamics, thereby overcoming the inherent complexity and design challenges. Beyond this simultaneous handling, the proposed sliding surface itself incorporates new functions that improve upon previous sliding mode designs. Furthermore, actuator saturation, uncertainties, and disturbances are all explicitly considered within the dynamic equations themselves, not added as afterthoughts. The proposed paper integrates three key contributions. First, a new nonsingular fast terminal sliding surface is designed, which uses improved functions to ensure rapid convergence of tracking errors while avoiding singularity issues. Second, a nonlinear extended state observer (NESO) is combined with an auxiliary function to actively estimate and compensate for aggregated disturbances, model uncertainty, and input saturation. Third, an adaptive mechanism is embedded to eliminate the requirement for prior knowledge of uncertainty bounds, making the controller more practical and robust in real-world applications. The proposed method is compared with intelligent control strategies. Closed-loop stability is guaranteed using Lyapunov theory, with theoretical proof of fixed-time tracking error convergence to zero, independent of initial conditions. Effectiveness and robustness are validated via comparative simulations against a state-of-the-art benchmark and experiments on a Speedgoat real-time machine.
This paper describes preliminary results on a Proportional plus Adaptive Disturbance Observer (P+ADOB) controller applied to velocity regulation tasks in a servo system. Adaptation law is obtained to estimate the servo system input gain, which is subsequently employed in the design of a Disturbance Observer. Compared with previous approaches, this feature relaxes the assumption on exact knowledge on the input gain, and only upper and lower bounds on this term are assumed known. A stability proof assuming constant disturbances allows concluding that the estimate of the input gain is bounded, and the velocity tracking error converges to zero. Real-time experiments illustrate the performance of the proposed controller.
Accurate and robust control methodologies are critical to the reliable and safe operation of engineering systems. Sliding mode control (SMC) is a form of variable structure control and is regarded as one of the most effective nonlinear robust control approaches. The control law is designed so that the system state trajectories are forced towards the sliding surface and stays within a region of it. The switching gain in the control signal brings an inherent amount of stability to the control process. However, the controller is only as effective as the knowledge of critical system states and parameters. Estimation strategies, such as the Kalman filter or the smooth variable structure filter (SVSF), may be employed to improve the quality of the state estimates used by control methods. A recently developed SVSF formulation, referred to as the second-order SVSF, offers robustness and chattering suppression properties of second-order sliding mode systems. It produces robust state estimation by preserving the first- and second-order sliding conditions such that the measurement error and its first difference are pushed towards zero. This paper aims to combine the SMC with the second-order SVSF in an effort to develop and offer an improved control strategy. It is proposed that this controller will offer an improvement in terms of controller accuracy without affecting its inherent stability and robustness. An electro hydrostatic actuator will be used for proof of concept, and future work will extend the application to automotive power trains.
Adaptive control techniques are often avoided in aerospace systems due to stringent plant structural requirements and validation difficulties. This dissertation seeks to broaden the range of aerospace engineering applications that can utilize an adaptive controller through the development of an extended model reference adaptive control (MRAC) design. First, a partitioned control framework is presented that permits the combined use of an adaptive control law and a nonadaptive control law. The partitioned framework is used to shift full control authority away from the adaptive portion of the system. Next, two MRAC variations that can accommodate the nonminimum phase zeros often seen in aerospace applications are discussed for use as the adaptive system. The parallel feedforward compensator approach proposes inclusion of a user--defied fictitious model in parallel with the plant that is designed to make the plant appear nonminimum phase. The surrogate tracking error approach modifies the typical MRAC structure to handle nonminimum phase plants by requiring knowledge of its nonminimum phase zeros. A tracking error convergence proof is provided for this continuous-time MRAC variant. The partitioned design using the surrogate tracking error approach is applied to the control tasks of an experimental, flexible wing aircraft. A simulation is used to demonstrate much improved flight path angle command tracking when compared to use of the aircraft's existing nonadaptive control law, even in the presence of large--scale modeling error. A second simulation is used to show the design applied to flexible motion control of the same aircraft model and exhibits similarly improved performance.
This study investigates the robust fault-tolerant attitude control of an orbiting spacecraft with a combination of unknown actuator failure, input saturation and external disturbances. A fault-tolerant control scheme based on variable structure control is developed that is robust to the partial loss of actuator effectiveness, where the actuators experience a reduced actuation but are still active. The results are then extended to the case in which some of the actuators fail completely, although some redundancy in actuation is assumed. In contrast to traditional fault-tolerant control methods, the proposed controller does not require knowledge of the actuator faults and is implemented without explicit fault detection, separation and accommodation processes. Moreover, the designed controller rigorously enforces actuator saturation constraints. The associated stability proof is constructive and develops a candidate Lyapunov function that shows the attitude and the angular velocities converge asymptotically to zero. Simulation studies are used to evaluate the closed-loop performance of the proposed control solution and illustrate its robustness to external disturbances, unknown actuator faults and even input saturation.
Control system design is considered for attitude control and vibration suppression of flexible space structures. The problem addressed is that of controlling both the zero-frequency rigid-body modes and the elastic modes. Model-based compensators, which employ observers tuned to the plant parameters, are first investigated. Such compensators are shown to generally exhibit high sensitivity to the knowledge of the parameters, especially the elastic mode frequencies. To overcome this problem, a class of dynamic dissipative compensators is next proposed which robustly stabilize the plant in the presence of unmodeled dynamics and parametric uncertainties. An analytical proof of robust stability is given, and a method of implementing the controller as a strictly proper compensator is described. Methods of designing such controllers to obtain optimal performance and robust stability are presented. Numerical and experimental results of application of the methods are presented, which indicate that dynamic dissipative controllers can simultaneously provide excellent performance and robustness.>
Control system design is considered for attitude control and vibration suppression of flexible space structures. The problem addressed is that of controlling both the zero-frequency rigid-body modes and the elastic modes. Model-based compensators, which employ observers tuned to the plant parameters, are first investigated. Such compensators are shown to generally exhibit high sensitivity to the knowledge of the parameters, especially the elastic mode frequencies. To overcome this problem a class of dynamic dissipative compensators is next proposed, which robustly stabilize the plant in the presence of unmodeled dynamics and parametric uncertainties. An analytical proof of robust stability is given, and a method of implementing the controller as a strictly proper compensator is given. Methods of designing such controllers to obtain optimal performance and robust stability are presented. Numerical and experimental results of application of the methods are presented, which indicate that dynamic dissipative controllers can simultaneously provide excellent performance and robustness.
A complete proof is given of global convergence to zero of the prediction error and asymptotic optimality for a direct adaptive controller is established based on multistep quadratic cost minimization with a receding horizon philosophy. The only substantial assumptions are that the controller designed when the parameters are known, stabilizes the plant and the exact knowledge of the first N y (N y -optimization horizon) impulse response coefficients of the plant. The technical hurdles for the development of a comprehensive convergence theory of this type of controller are also highlighted.