Young Ok Lee

A modified nonlinear damping of zero — Dynamics via feedback control for a STATCOM

Recently linearization via feedback control law for a STATCOM to minimize the internal dynamics oscillations was proposed. The ripples of the internal dynamics correspond to the oscillation of the current ripple on the DC side capacitor so that it affects the lifecycle of the capacitor. In this paper, a modified nonlinear damping of zero dynamics via feedback control for a STATCOM is proposed. The proposed nonlinear feedback controller improves the stability of the internal dynamics. The controller gives the damping on zero-dynamics via the nonlinear feedback controller optimized at each operating point. It guarantees internal stability in all operation regions by moving the poles of the internal dynamics away from the imaginary axis. This effect is validated through the root locus analysis. Simulation results show that the oscillation of the internal dynamics is effectively reduced using the averaged and the topological model.

Multirate active steering control for autonomous vehicle lateral maneuvering

A multirate steering control scheme is developed for autonomous vehicle lateral maneuvering. The proposed scheme consists of a multirate extended Kalman filter and a state feedback control. The multirate extended Kalman filter is to estimate the vehicle states at a fast rate of the car ECU using multirate sensing - a slow vision-based lane detection by a camera and fast motion detection by inertia sensors. A method to design the multirate decentralized extended Kalman filter is presented. Through application results, the proposed multirate steering control scheme is shown to exhibit significantly improved control performance.

Proximate model predictive control strategy for autonomous vehicle lateral control

A proximate model predictive control strategy is proposed applied to autonomous vehicle lateral control. A simple and fast method to compute an approximate optimal solution is developed from the interpolation between the pre-computed optimal solutions, which is used for warm-start on-line optimization to reduce iterations in finding optimal solutions. The proposed proximate model prediction control exhibits proximate optimality in very few on-line iterations, which can become arbitrarily close to its optimality with further iterations. The results of computational and real-time experiments for autonomous vehicle lateral control demonstrate the utility of the proposed proximate model predictive control.

Output tracking control with enhanced damping of internal dynamics and its output boundedness

This paper investigates an important and challenging problem in nonlinear systems: achieving both output tracking control with a tolerable bound and enhanced damping control of internal dynamics. This problem is solved by using a damping controller in an input-output linearized STATCOM system. The proposed controller provides improved damping for lightly damped internal dynamics with degraded but tolerable output tracking performance. It ensures that the tracking error of output stays within a bound. Finding a parameter-dependent Lyapunov function for the zero dynamics proves the stability of the damped internal dynamics for time invariant/varying trajectories. Stability of the closed-loop system is established by designing a Lyapunov function.

Stability Analysis of an Electric Parking Brake (EPB) System with a Nonlinear Proportional Controller

In this paper, an Electric Parking Brake (EPB) system is modelled as a state-dependent switched system. The model involves screw friction which varies depending on the operation region. A new nonlinear proportional (P) controller is proposed and its stability is analyzed via Lyapunov and LaSalles theory. It is shown that the equilibrium point is locally uniform and ultimately bounded.

Passivity-based control with nonlinear damping for STATCOM system

This paper describes a nonlinear controller for a static synchronous compensator (STATCOM). A passivity-based controller (PBC) using the Euler-Lagrange form is proposed for this system. The proposed PBC consists of a feedforward part and a feedback part. The feedforward part is generated from the dynamics equation of the STATCOM system. The feedback part is designed based on the Lyapunov function, thus guaranteeing the exponential stability of the equilibrium point. In inductive operating mode, the system with the PBC experiences undesirable transient response oscillations. To achieve improved performance in terms of the time-response, a PBC with nonlinear damping is proposed. The simulation results show that the proposed control method has the effect of reducing the oscillatory responses. Furthermore, the global exponential stability of the system is guaranteed by using the Lyapunov theorem.

Clamping Force Control for an Electric Parking Brake System: Switched System Approach

This paper addresses the modeling, controller design, and stability analysis of an electric parking brake system in which a screw-nut self-locking mechanism is used. The system is modeled as a state-dependent switched system according to the operating mode. A nonlinear proportional (P) clamping force controller using the measured force is proposed to enhance the performance specifications. We show the uniform stability of the state-dependent switched system with the nonlinear P controller using a common Lyapunov theorem and LaSalles invariance principle. We derive the conditions to assure stable self-locking operation of the system. We also show the existence of the largest invariant set depending on the target braking force. This analysis offers a guideline as to how a nonlinear controller can be designed in view of the self-locking stability and control performance. Through simulation and experimental results, we confirm that the solution is locally uniformly ultimately bounded. Through the experimental results, we show that the nonlinear P controller outperforms a simple on/off controller in terms of the average and deviation of the braking force error. Furthermore, it is experimentally verified that the system is also able to function as a pseudo-antilock braking system.

A nonlinear control for a BTB STATCOM system with asymmetrically structured converters

This paper describes a new BTB STATCOM system with asymmetrically structured converters and develops its mathematical model. The BTB STATCOM employs a PWM technology for a rectifier station and a multipulse technology for an inverter station. We propose a control strategy for the following three control variables: active and reactive rectifier power and DC voltage. In the rectifier station, input-output linearization via the feedback control law is applied for independent control of the active and reactive power. Conversely, the input-output linearization controller is not applicable for maintaining the DC voltage. We design an alternative indirect damping controller for the reactive current within the inverter station. The proposed controller provides an improved stability margin and a decreased oscillation for DC voltage with a degraded reactive current tracking performance of the inverter. The effectiveness of the proposed controller is validated via the local stability at each operating point and the performance of the time response. Parameter-dependent Lyapunov functions for zero dynamics and the closed-loop system prove the stability of the damped internal dynamics and closed-loop system, respectively.

Uniform output regulation via approximated input–output linearisation for lightly damped internal dynamics

We present a general framework for the design of a damping controller to tame lightly damped internal dynamics and to assess the stability of an approximate input–output linearised closed-loop system. In nonlinear systems, conclusions cannot be drawn regarding the stability of internal dynamics even though the zero dynamics is globally stable. Here, we investigated the sufficient conditions with a parameter-dependent Lyapunov (PDL) function for semi-globally exponential stability of damped internal dynamics. Using a composite Lyapunov function based on the PDL function, the uniform boundedness of the output regulation error is established within a region of interest. Controllability and observability analyses offer a theoretical guideline as to how a damping controller can be designed in the view of control effort and transient performance.

Novel passivity-based controller design for Back-to-back STATCOM with asymmetrically structured converters

This paper describes a nonlinear controller for a type of voltage-source converter based high voltage direct current called Back-to-back static synchronous compensator (BTB STAT-COM) system with asymmetrically structured converters. The BTB STATCOM employs a pulse width modulation technology for a rectifier station and a multipulse technology for an inverter station. To improve robustness and simplify controller design, a passivity-based controller (PBC) using Euler-Lagrange form is proposed for this system. The PBC, which consisted of the desired control input and a new control input considering the performance of all states, is designed based on error dynamics. The control input based on the Lyapunov theorem guarantees the exponential stability of equilibrium point of the system. The proposed method is validated through simulation and its effectiveness is compared with an input-output linearization controller. Simulation results show that PBC enhances the performances of the states of the inverter and DC voltage.