Equivalent Disturbance Estimator Research Articles

Immersion and invariance‐based adaptive pose tracking control under disturbances and input saturation

This paper addresses the simultaneous attitude and position tracking of a rigid body subject to general bounded disturbances and input saturation in the framework of dual quaternions, which provide a compact and integrated description of the coupled rotation and translation. Treating disturbances as unknown parameters, a modular adaptive saturated pose tracking control scheme is then derived which consists of two separately designed parts. One part is the noncertainty equivalent disturbance estimator designed via the immersion and invariance method. Most existing adaptive pose controllers are based on the certainty equivalence principle. Since its parameter adaption process is unpredictable and acts like a forcing disturbance imposed on the deterministic system, it can cause arbitrary degradation of the closed-loop performance. Instead of invoking the certainty equivalence principle, the adaption law designed based on the novel immersion and invariance method counters the effects of the uncertain parameters from a robustness perspective and eliminates the performance degradation by introducing a stable attracting manifold in the adaption process. For both constant and time-varying disturbances, the proposed estimator can effectively attenuate their effects on the closed-loop performance. The other part is the saturated pose tracking controller where the estimated disturbances are directly used. Based on two useful lemmas, it is proved that the saturation constraints can be removed in finite time, which enables the Lyapunov stability analysis of the closed-loop system. Simulation results demonstrate improvements of the proposed control scheme in closed-loop performance and control precision when compared with a certainty equivalent controller and a sliding mode controller.

Finite-Time Terminal Sliding Mode Tracking Control of a VTOL UAV via the Generalized NDOB

In this paper, we propose a finite-time sliding mode trajectory tracking control methodology for the vertical takeoff and landing unmanned aerial vehicle (VTOL UAV). Firstly, a system error model of trajectory tracking task is established based on Rodrigues parameters by considering both external and internal uncertainties. According to the cascade property, the system model is divided into translational and rotational subsystems, and a hierarchical control structure is hence proposed. Then, a finite-time generalized nonlinear disturbance observer (NDOB) is proposed, based on which the finite-time convergence result of equivalent disturbance estimation can be acquired. Finally, by introducing a tan-type compensator into the traditional terminal sliding mode control (SMC), the finite-time convergence result of the closed-loop control system is acquired based on Lyapunov stability analysis. Simulation results show the effectiveness of the proposed methodology.

Robust Output Path-Following Control of Marine Surface Vessels with Finite-Time LOS Guidance

This paper proposes a finite-time output feedback methodology for the path-following task of marine surface vessels. First, a horizontal path-following model is established with unknown sideslip angle, unmeasured system state and system uncertainties. A hierarchical control structure is adopted to deal with the cascade property. For kinematics system design, a finite-time sideslip angle observer is first proposed, and thus the sideslip angle estimation is compensated in a nonlinear line-of-sight (LOS) guidance strategy to acquire finite-time convergence. For the heading control design, an extended state observer is introduced for the unmeasured state and equivalent disturbance estimation, based on which an output feedback backstepping approach is proposed for the desired tracking of command course angle. The global stability of the cascade system is analyzed. Simulation results validate the effectiveness of the proposed methodology.

Antirollback Control for Gearless Elevator Traction Machines Adopting Offset-Free Model Predictive Control Strategy

Direct-drive permanent magnet traction systems have become the trend of modern elevators. Elevator traction machines tend to use low-resolution incremental encoders as position and speed feedback devices because of their low cost. To prevent the occurrence of elevator car rollback for the gearless elevator installed with an incremental encoder with general resolution, a weight-transducerless starting torque strategy based on offset-free model predictive control is proposed. Under the condition of low-resolution incremental encoders, this control strategy can achieve a fast dynamic response and no mechanical vibration during the mechanical brake releasing. The electromagnetic torque generated by the machine can quickly balance the uncertain load torque to minimize the sliding distance of the elevator car, which contributes in achieving superior riding comfort. In order to overcome the mismatch of the predictive model caused by the uncertainty and nonlinearity of the mechanical model of the traction system, as well as intense disturbance, a model corrector is added to adjust the mismatched predictive model. The steady-state speed error can be eliminated by adding the equivalent disturbance estimator to the predictive model during the zero-servo operation at elevator startup. Finally, both simulation and experimental results are provided to verify that the proposed control strategy can achieve shorter sliding distance, smaller sliding speed, and faster dynamic response.

Sliding mode control of a permanent magnet direct drive under non‐linear friction

PurposeThe purpose of this paper is to find a simple structure of motion controller for permanent magnet direct drive. Application of sliding mode controller theory and equivalent disturbance estimator creates proper non‐linear characteristics, which ensures controller robustness against friction.Design/methodology/approachThe position and speed controller is based on robust design methodology introduced by a sliding mode technique. The paper proposes a combination of sliding mode controller and proportional integral (PI) equivalent disturbance estimator. The friction model is Coulomb friction with a large static friction effect. The double boundary layer is used to compensate the effect of stiction. The synthesis is performed using simulation techniques and subsequently the behaviour of a laboratory speed control system is validated in the experimental setup. The control algorithms of the system are performed by a microprocessor floating point DSP control system.FindingsThe proposed sliding mode controller structure with equivalent disturbance estimator guarantees expected robustness against friction. Experimental results show that the control approach can decrease the tracking error, enhance the system's robustness and attenuate high‐frequency chattering in the control signal.Research limitations/implicationsThe proposed controller was tested on a single machine under well‐defined conditions. Further investigations are required before any industrial applications.Practical implicationsThe proposed controller synthesis and its results may be very helpful in robotic systems where non‐linear friction is a characteristic for many industrial robots and manipulators.Originality/valueThe method of sliding mode controller synthesis was proposed and validated by simulation and experimental investigations.