John O'Reilly

A nonlinear disturbance observer for robotic manipulators

A new nonlinear disturbance observer (NDO) for robotic manipulators is derived in this paper. The global exponential stability of the proposed disturbance observer (DO) is guaranteed by selecting design parameters, which depend on the maximum velocity and physical parameters of robotic manipulators. This new observer overcomes the disadvantages of existing DOs, which are designed or analyzed by linear system techniques. It can be applied in robotic manipulators for various purposes such as friction compensation, independent joint control, sensorless torque control and fault diagnosis. The performance of the proposed observer is demonstrated by the friction estimation and compensation for a two-link robotic manipulator. Both simulation and experimental results show the NDO works well.

Short-Circuit and Ground Fault Analyses and Location in VSC-Based DC Network Cables

The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

A Series-Dynamic-Resistor-Based Converter Protection Scheme for Doubly-Fed Induction Generator During Various Fault Conditions

This paper proposes a new converter protection method, primarily based on a series dynamic resistor (SDR), that avoids the doubly-fed induction generator (DFIG) control being disabled by crowbar protection during fault conditions. A combined converter protection scheme based on the proposed series dynamic resistor and conventional crowbar is analysed and discussed. The main protection advantages are due to the series topology when compared with crowbar and DC-chopper protection. Various fault over-current conditions (both symmetrical and asymmetrical) are analysed and used to design the protection in detail, including the switching strategy and coordination with crowbar, and resistance value calculations. PSCAD/EMTDC simulation results show that the proposed method is advantageous for fault over-current protection, especially for asymmetrical faults, in which the traditional crowbar protection may malfunction.

Multiterminal DC Wind Farm Collection Grid Internal Fault Analysis and Protection Design

The multiterminal dc wind farm is a promising topology with a voltage-source inverter (VSI) connection at the onshore grid. Voltage-source converters (VSCs) are robust to ac-side fault conditions. However, they are vulnerable to dc faults on the dc side of the converter. This paper analyzes dc faults, their transients, and the resulting protection issues. Overcurrent faults are analyzed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in the multi-VSC dc wind farm collection grid and VSC-based high-voltage direct current (HVDC) offshore transmission systems.

Multivariable control by individual channel design

Abstract A new approach—individual channel design (ICD)—to an enduring problem— multivariable feedback control—is presented. The approach is applications-oriented: it starts from the engineering premise that feedback control design is interactive; it involves an interplay between customer specification, uncertain plant characteristics, and the multivariable feedback design process itself. It is shown that individual signal transmission channels arise naturally from customer specification on selected plant outputs with no loss of structural (loop interaction) information. By invoking customer performance specification on different channels, highly successful single-input single-output classical (Nyquist-Bode) design is made possible. ICD is not a design method per se per se; rather it is a global structural framework wherein the possibilities and limitations for local-loop-shaping design (e.g. Bode or Nichols) of a particular plant are made apparent from the outset. Also, the conditions are established whe...

Enhanced Power System Stability by Coordinated PSS Design

A step-by-step coordinated design procedure for power system stabilisers (PSSs) and automatic voltage regulators (AVRs) in a strongly coupled system is described in this paper. It is shown that it is possible to separate the design of individual PSSs and separate the design of individual AVRs. Thereby, the designs of AVR and PSS devices at a given machine can be coordinated to achieve near-optimal overall power system stability performance, including oscillation stability performance and transient stability performance. The proposed coordinated PSS/AVR design procedure is established within a frequency-domain framework and serves as a most useful small-signal complement to established large-signal transient simulation studies.

The Effective Role of AVR and PSS in Power Systems: Frequency Response Analysis

Two tradeoffs in the effectiveness of automatic voltage regulators (AVRs) and power system stabilizers (PSSs) are investigated together for the first time. The first is the effect of a high-gain fast response AVR on decreasing power system oscillation stability as well as increasing transient stability, and vice versa. The second is that a PSS can reduce transient stability by overriding the voltage signal to the exciter as well as increasing oscillation stability, and vice versa. In essence, the actions of the AVR and PSS devices are dynamically interlinked. A novel Bode frequency response framework for dynamic analysis of AVR and PSS performance and tradeoffs is presented. Bode frequency response also assists with the determination of suitable generator locations for PSSs and the assessment of robustness under changing power system operating conditions.

Trajectory generation for road vehicle obstacle avoidance using convex optimization

Abstract This paper presents a method for trajectory generation using convex optimization to find a feasible, obstacle-free path for a road vehicle. Consideration of vehicle rotation is shown to be necessary if the trajectory is to avoid obstacles specified in a fixed Earth axis system. The paper establishes that, despite the presence of significant non-linearities, it is possible to articulate the obstacle avoidance problem in a tractable convex form using multiple optimization passes. Finally, it is shown by simulation that an optimal trajectory that accounts for the vehicles changing velocity throughout the manoeuvre is superior to a previous analytical method that assumes constant speed.

A series dynamic resistor based converter protection scheme for doubly-fed induction generator during various fault conditions

This paper proposes a new converter protection method, primarily based on a series dynamic resistor (SDR) that avoids the doubly-fed induction generator (DFIG) control being disabled by crowbar protection during fault conditions. A combined converter protection scheme based on the proposed SDR and conventional crowbar is analyzed and discussed. The main protection advantages are due to the series topology when compared with crowbar and dc-chopper protection. Various fault overcurrent conditions (both symmetrical and asymmetrical) are analyzed and used to design the protection in detail, including the switching strategy and coordination with crowbar, and resistance value calculations. PSCAD/EMTDC simulation results show that the proposed method is advantageous for fault overcurrent protection, especially for asymmetrical faults, in which the traditional crowbar protection may malfunction.

Frequency domain based control design for an HVdc converter connected to a weak ac network

A small-signal frequency domain model of an HVDC system connected to a weak AC network is used for an operating point-based rectifier direct current control design. The frequency response of the open-loop system is systematically shaped, clearly showing the controller design tradeoffs involved in achieving the objectives of damped system oscillations and disturbance rejection. The performance of the controller is verified by time-domain simulation of the CIGRE Benchmark HVDC test system. This validates the use of small-signal frequency response models for HVDC system control analysis and design. The further developments necessary for a practical controller for large signal excursions are discussed.