Attaullah Y. Memon

Output regulation of nonlinear systems using conditional servocompensators

This paper studies the output regulation of nonlinear systems using conditional servocompensators. Previous work introduced the conditional servocompensator that acts as a traditional servocompensator in a neighborhood of the zero-error manifold, while acting as a stable system outside a boundary layer, leading to improvement in the transient response while achieving zero steady-state tracking error in the presence of time-varying exogenous signals. The conditional servocompensator tool was introduced for sliding-mode feedback controllers. This paper extends the technique to more general feedback controllers by using Lyapunov redesign and saturated high-gain feedback.

Cyber Security Backdrop: A SCADA testbed

Supervisory Control and Data Acquisition (SCADA) systems are traditionally proprietary and well protected. Due to increasing use of commercial/open source technology and communication protocols, there are growing concerns about the associated security threats. SCADA networks are usually employed in critical infrastructure, therefore, not much technical data of the actual systems is accessible to the research community. Most of the researchers simulate the SCADA functioning through development of testbed. Such projects are usually expensive and requiring financial sponsorship. In this paper we present a simple, inexpensive and flexible approach to develop a SCADA testbed utilizing TrueTime, a MATLAB/ Simulink based tool. The paper describes TrueTime simulation blocks, our control system, simulation of Denial of Service (DoS) attack and its effects. The main aim was to assess the effectiveness and suitability of TrueTime for the intended use in the development of a larger scale SCADA testbed. The results shown reflect that TrueTime can be effectively used for the purpose of SCADA network simulations and collection of necessary data for security analysis.

Full-order high-gain observers for minimum phase nonlinear systems

This paper studies the design of a full-order, high-gain observer for a class of minimum-phase nonlinear systems. The dynamic state feedback design is modified to include the observer for the internal dynamics. It is shown that the performance of such a dynamic partial-state feedback control can be recovered by the output feedback control using a sufficiently fast high-gain observer, in the presence of model uncertainty.

Lyapunov redesign approach to output regulation of nonlinear systems using conditional servocompensators

This paper studies the output regulation of nonlinear systems using conditional servocompensators. Previous work introduced the conditional servocompensator that acts as a traditional servocompensator in a neighborhood of the zero-error manifold, while acting as a stable system outside a boundary layer, leading to improvement in the transient response while achieving zero steady-state tracking error in the presence of time-varying exogenous signals. The conditional servocompensator tool was introduced for sliding-mode feedback controllers. This paper extends the technique to more general feedback controllers by using Lyapunov redesign and saturated high-gain feedback.

Output feedback stabilization of an Inertia Wheel Pendulum using Sliding Mode Control

This paper investigates the application of a novel robust output feedback stabilizing controller to an Inertia Wheel Pendulum (IWP) - a bench mark problem in the control of under-actuated mechanical systems. The proposed scheme uses a Sliding Mode Control (SMC) design for the stabilization of an IWP having unstable zero dynamics. Starting with the system equations of motion in standard form, a normal form representation of the system is derived, and a sliding mode controller is synthesized for robust stabilization of the system. The state feedback control design is extended to an output feedback control using a high gain observer. It is shown that the system response under the output feedback controller converges to that of the state feedback controller and the estimated states converge to actual states arbitrarily fast.

Attitude control of VTOL-UAVs

This paper presents a novel control approach to obtain asymptotic attitude stability of a quadrotor as a representative of Planar Vertical Take Off and Landing (PVTOL) Unmanned Aerial Vehicles (UAVs). The considered quadrotor is a symmetric VTOL-UAV with four rigid mono-directional propellers, which has been modeled based on quaternion representation with taking Coriolis and gyroscopic torques into account. In the proposed approach, two nearly equivalent control laws (model independent as well as model dependent) have been used to obtain exponential stability of attitude angles and asymptotic stability of attitude angular velocity of the quadrotor UAV. The proposed approach also presents how the attitude parameters i.e. attitude angles and attitude angular velocity can be quickly regulated to their desired values as required.

Output regulation of linear systems subject to input constraints

This paper studies the output regulation problem for linear systems subject to input constraints. It presents a new controller that uses a conditional servocompensator. Previous work introduced the conditional servocompensator, which acts as a traditional servocompensator only in a neighborhood of the zero-error manifold, while acting as a stable system outside a boundary layer, leading to improvement in the transient response while achieving zero steady-state regulation error. Starting with a low-gain stabilizing state feedback controller, based on the solution of an algebraic Riccati equation, Lyapunov redesign is used to implement the state feedback control in a saturated high-gain feedback form that includes the conditional servocompensator. The design is extended to output feedback via a full-order high-gain observer.

Integral terminal sliding mode formation control of non-holonomic robots using leader follower approach

Multi-robot formation control has become an important area of research due to its advantages and applications. This paper presents multi-robot formation control using a leader–follower approach without considering the leaders velocity information or estimation. The leader–follower formation is formulated by incorporating the model uncertainties and disturbances. A novel formation controller is presented using integral terminal sliding mode (ITSM) control, which drives the formation tracking error convergence to zero in finite-time. The stability of the close-loop control scheme is verified by using Lyapunov theory. Furthermore, obstacle detection and avoidance are incorporated to avoid collision while maintaining the formation. The effectiveness of the proposed controller is verified and validated using sine and lamniscate curve trajectories. Moreover, the performance of the proposed ITSM formation controller is compared with the standard linear sliding mode (LSM) control.

Robust output feedback linearizing speed sensorless control of PMSM

This work addresses the problem of robust output feedback speed sensorless control of a three-phase permanent magnet synchronous motor (PMSM). A cascaded observer scheme is proposed comprising a continuous sliding mode observer (SMO) and an extended high-gain observer (EHGO) that provides robust estimation of motor position, speed and lumped system uncertainties. For the speed control problem, a robust feedback linearization controller is presented that uses the estimated lumped disturbance provided by EHGO to actively compensate for uncertainties. This observer based scheme helps to recover the motor speed tracking performance achievable under exact feedback linearization. The overall scheme works in output feedback; that is, without speed sensor, requiring motor current as the only output measurement. Simulations and experiments confirm the efficacy of the proposed scheme.

A robust observer and controller design for a DC motor with a low-resolution encoder

This work focuses on the problem of high-performance robust control of a dc motor in the presence of a low-resolution position encoder. An observer assisted feedback control scheme is presented in which a robust extended high-gain observer (EHGO) is designed for simultaneous estimation of motor speed and lumped system uncertainties. The conventional approach of calculating speed by direct differentiation of encoder pulses is superseded by the EHGO technique that allows significant reduction in quantization noise associated with the encoder without requiring a low-pass filter. For the speed control problem, an improved integral sliding mode controller is proposed with a sigmoid boundary layer. The integral action compensates for the boundary layer error thereby guaranteeing tracking error convergence to zero. Stability, robustness and accuracy of the proposed observer and controller are established mathematically. Simulations and experimental tests are carried out that validate the given scheme.