Collective pitch control of wind turbines using stochastic disturbance accommodating control

Fidelity of a plant's dynamic model is a concern in any controller design process. In this context, fidelity refers to which dynamics of the plant needs to be included in the control model and which dynamics can be left out or approximated. Studies on wind turbine control have shown that modelling error due to the unmodeled dynamics can lead to unstable closed-loop dynamics. This paper investigates the use of Kalman estimator to design the Stochastic Disturbance Accommodating Control (SDAC) scheme to stabilize the system in the presence of the unmodeled dynamics. Performance of the presented control scheme is investigated through simulations on two different wind turbine configurations under turbulent wind conditions with different mean wind speeds and turbulence intensities using FAST (Fatigue, Aerodynamics, Structures, and Turbulence) aero-elastic tool. The generator speed regulation, drivetrain load, and control effort of the presented control scheme are compared with those of the baseline Gain Scheduled Proportional Integral (GSPI) controller. The results indicate better speed regulation and lower drivetrain load for the presented SDAC under the tested wind conditions

Correlation-based estimation of cutting force coefficients for ball-end milling

This article presents a methodology to estimate cutting force coefficients based on the least squares approximation using correlation factor between the estimated and measured cutting forces in order to determine the corresponding tool angular position. This method can be applied on measured cutting force data over any small interval of time that need not contain information of the time instant when the cutting tool enters the workpiece, which has been the main requirement in the conventional method. This allows a quick estimation of the cutting force coefficients regardless of the chosen cutting conditions and tool-workpiece material, which is often the case in industrial machining processes. This proposed method has been validated by comparison of cutting force coefficients obtained using conventional estimation technique for a slot ball-end milling test. Besides being useful for predictive evaluation of forces, such estimation of cutting force coefficients of the cutting force model can be useful for understanding variations in cutting process over the tool life and can assist in online monitoring and process optimization

Identification and control of stick–slip vibrations using Kalman estimator in oil-well drill strings

Excessive stick–slip vibrations of drill strings can cause premature component failures and inefficient drilling operations. Previous research works employ real-time measurements of all states of the drill string as feedback to the controllers to suppress such vibrations. While real-time measurements are readily available for components at the surface, only limited measurements are practically available for the downhole states. To address the requirement for downhole states, this paper proposes the utilization of Kalman estimator to estimate the downhole drill bit position and velocity based on the measurements at the surface. In the design of the estimator, the nonlinear downhole friction torque is approximated by a linear persistent disturbance model. A linear–quadratic–Gaussian (LQG) control strategy is then applied on the estimated states to mitigate the unwanted vibrations. Performance of this control scheme is investigated through numerical simulations where the dynamics of the drill string is modeled using a high fidelity lumped parameter model considering torsional stick–slip and lateral motions as well as a nonlinear friction model. The dynamic response from the high-fidelity model is demonstrated to have a close qualitative agreement with the stick–slip vibrations observed in field. The simulated results demonstrate the capability of the proposed Kalman estimator in identifying the stick–slip vibrations and estimating the downhole friction torque. The effectiveness of the proposed controller in avoiding such vibrations is also verified.Accepted versio