Disturbance Observer-based Robust Control and Its Applications: 35th Anniversary Overview

Disturbance Observer has been one of the most widely used robust control tools since it was proposed in 1983. This paper introduces the origins of Disturbance Observer and presents a survey of the major results on Disturbance Observer-based robust control in the last thirty-five years. Furthermore, it explains the analysis and synthesis techniques of Disturbance Observer-based robust control for linear and nonlinear systems by using a unified framework. In the last section, this paper presents concluding remarks on Disturbance Observer-based robust control and its engineering applications.Comment: 12 pages, 4 figure

A survey on fractional order control techniques for unmanned aerial and ground vehicles

In recent years, numerous applications of science and engineering for modeling and control of unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs) systems based on fractional calculus have been realized. The extra fractional order derivative terms allow to optimizing the performance of the systems. The review presented in this paper focuses on the control problems of the UAVs and UGVs that have been addressed by the fractional order techniques over the last decade

Multi - objective sliding mode control of active magnetic bearing system

Active Magnetic Bearing (AMB) system is known to inherit many nonlinearity effects due to its rotor dynamic motion and the electromagnetic actuators which make the system highly nonlinear, coupled and open-loop unstable. The major nonlinearities that are associated with AMB system are gyroscopic effect, rotor mass imbalance and nonlinear electromagnetics in which the gyroscopics and imbalance are dependent to the rotational speed of the rotor. In order to provide satisfactory system performance for a wide range of system condition, active control is thus essential. The main concern of the thesis is the modeling of the nonlinear AMB system and synthesizing a robust control method based on Sliding Mode Control (SMC) technique such that the system can achieve robust performance under various system nonlinearities. The model of the AMB system is developed based on the integration of the rotor and electromagnetic dynamics which forms nonlinear time varying state equations that represent a reasonably close description of the actual system. Based on the known bound of the system parameters and state variables, the model is restructured to become a class of uncertain system by using a deterministic approach. In formulating the control algorithm to control the system, SMC theory is adapted which involves the formulation of the sliding surface and the control law such that the state trajectories are driven to the stable sliding manifold. The surface design involves the transformation of the system into a special canonical representation such that the sliding motion can be characterized by a convex representation of the desired system performances. Optimal Linear Quadratic (LQ) characteristics and regional pole-clustering of the closed-loop poles are designed to be the objectives to be fulfilled in the surface design where the formulation is represented as a set of Linear Matrix Inequality optimization problem. For the control law design, a new continuous SMC controller is proposed in which asymptotic convergence of the system’s state trajectories in finite time is guaranteed. This is achieved by adapting the equivalent control approach with the exponential decaying boundary layer technique. The newly designed sliding surface and control law form the complete Multi-objective SMC (MO-SMC) and the proposed algorithm is applied into the nonlinear AMB in which the results show that robust system performance is achieved for various system conditions. The findings also demonstrate that the MO-SMC gives better system response than the reported ideal SMC (I-SMC) and continuous SMC (C-SMC)

Suppression of line voltage related distortion in current controlled grid connected inverters

The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost–effect mechanisms for achieving improved output current quality

A time delay controller for magnetic bearings

The control of systems with unknown dynamics and unpredictable disturbances has raised some challenging problems. This is particularly important when high system performance needs to be guaranteed at all times. Recently, the Time Delay Control has been suggested as an alternative control scheme. The proposed control system does not require an explicit plant model nor does it depend on the estimation of specific plant parameters. Rather, it combines adaptation with past observations to directly estimate the effect of the plant dynamics. A control law is formulated for a class of dynamic systems and a sufficient condition is presented for control systems stability. The derivation is based on the bounded input-bounded output stability approach using L sub infinity function norms. The control scheme is implemented on a five degrees of freedom high speed and high precision magnetic bearing. The control performance is evaluated using step responses, frequency responses, and disturbance rejection properties. The experimental data show an excellent control performance despite the system complexity

Design and practical implementation of a fractional order proportional integral controller (FOPI) for a poorly damped fractional order process with time delay

One of the most popular tuning procedures for the development of fractional order controllers is by imposing frequency domain constraints such as gain crossover frequency, phase margin and iso-damping properties. The present study extends the frequency domain tuning methodology to a generalized range of fractional order processes based on second order plus time delay (SOPDT) models. A fractional order PI controller is tuned for a real process that exhibits poorly damped dynamics characterized in terms of a fractional order transfer function with time delay. The obtained controller is validated on the experimental platform by analyzing staircase reference tracking, input disturbance rejection and robustness to process uncertainties. The paper focuses around the tuning methodology as well as the fractional order modeling of the process' dynamics

Accurate Tracking of Aggressive Quadrotor Trajectories using Incremental Nonlinear Dynamic Inversion and Differential Flatness

Autonomous unmanned aerial vehicles (UAVs) that can execute aggressive (i.e., high-speed and high-acceleration) maneuvers have attracted significant attention in the past few years. This paper focuses on accurate tracking of aggressive quadcopter trajectories. We propose a novel control law for tracking of position and yaw angle and their derivatives of up to fourth order, specifically, velocity, acceleration, jerk, and snap along with yaw rate and yaw acceleration. Jerk and snap are tracked using feedforward inputs for angular rate and angular acceleration based on the differential flatness of the quadcopter dynamics. Snap tracking requires direct control of body torque, which we achieve using closed-loop motor speed control based on measurements from optical encoders attached to the motors. The controller utilizes incremental nonlinear dynamic inversion (INDI) for robust tracking of linear and angular accelerations despite external disturbances, such as aerodynamic drag forces. Hence, prior modeling of aerodynamic effects is not required. We rigorously analyze the proposed control law through response analysis, and we demonstrate it in experiments. The controller enables a quadcopter UAV to track complex 3D trajectories, reaching speeds up to 12.9 m/s and accelerations up to 2.1g, while keeping the root-mean-square tracking error down to 6.6 cm, in a flight volume that is roughly 18 m by 7 m and 3 m tall. We also demonstrate the robustness of the controller by attaching a drag plate to the UAV in flight tests and by pulling on the UAV with a rope during hover.Comment: To be published in IEEE Transactions on Control Systems Technology. Revision: new set of experiments at increased speed (up to 12.9 m/s), updated controller design using quaternion representation, new video available at https://youtu.be/K15lNBAKDC

Culture, Efficacy, and Outcome Expectancy in Teacher Preparation;How Do the Beliefs of Pre-Service Interns, Mentor Teachers, University Supervisors and Teacher Educators Compare?

Researchers agree that teachers\u27 attitudes and efficacy beliefs play a significant role in student achievement (Armor et al., 1976 Hoy & Spero, 2005 Szabo & Mokhtari, 2004 Woolfolk & Hoy, 1990). Teachers with high self-efficacy: 1) believe they can positively influence students\u27 learning and achievement despite environmental conditions (Armor et al., 1976 Ashton & Webb, 1986 Gibson & Dembo, 1984) and 2) assume accountability for student learning (Gibson & Dembo, 1984 Ross, 1998 Siwatu, 2007). On the contrary, teachers with low teacher efficacy have minimal expectations for and fewer interactions with minority students. They are also more likely to feel teacher burnout and abandon the profession (Betoret, 2006 Friedman, 2004 Guerra, Attar, & Weissberg, 1997). Various studies have confirmed that even after successfully completing multicultural courses and field experiences, interns\u27 negative beliefs and low outcome expectancies for minority students remained (Garmon, 1996, 2004 Easter, Schultz, Neyhart & Reck, 1999). Although little is known about how teaching efficacy develops, it is possible that programs incorporating efficacy-building opportunities assist to create and maintain a pool of quality teachers in culturally diverse schools (Clark & Wegener, 2009 Garcia, 2004 Milner, 2005 Santoro & Allard, 2005 Tucker et al., 2005 Siwatu, 2007 Swearingen, 2009). The purpose of this study will be to examine patterns in culturally responsive teaching self efficacy and outcome expectancies between interns, mentor teachers, university supervisors, and teacher educators in an urban teacher education progra

Culture, Efficacy, and Outcome Expectancy in Teacher Preparation;How Do the Beliefs of Pre-Service Interns, Mentor Teachers, University Supervisors and Teacher Educators Compare?

Researchers agree that teachers\u27 attitudes and efficacy beliefs play a significant role in student achievement (Armor et al., 1976 Hoy & Spero, 2005 Szabo & Mokhtari, 2004 Woolfolk & Hoy, 1990). Teachers with high self-efficacy: 1) believe they can positively influence students\u27 learning and achievement despite environmental conditions (Armor et al., 1976 Ashton & Webb, 1986 Gibson & Dembo, 1984) and 2) assume accountability for student learning (Gibson & Dembo, 1984 Ross, 1998 Siwatu, 2007). On the contrary, teachers with low teacher efficacy have minimal expectations for and fewer interactions with minority students. They are also more likely to feel teacher burnout and abandon the profession (Betoret, 2006 Friedman, 2004 Guerra, Attar, & Weissberg, 1997). Various studies have confirmed that even after successfully completing multicultural courses and field experiences, interns\u27 negative beliefs and low outcome expectancies for minority students remained (Garmon, 1996, 2004 Easter, Schultz, Neyhart & Reck, 1999). Although little is known about how teaching efficacy develops, it is possible that programs incorporating efficacy-building opportunities assist to create and maintain a pool of quality teachers in culturally diverse schools (Clark & Wegener, 2009 Garcia, 2004 Milner, 2005 Santoro & Allard, 2005 Tucker et al., 2005 Siwatu, 2007 Swearingen, 2009). The purpose of this study will be to examine patterns in culturally responsive teaching self efficacy and outcome expectancies between interns, mentor teachers, university supervisors, and teacher educators in an urban teacher education progra