Discrete Control Scheme Research Articles

Discrete control of resonant wave energy devices

Aiming at amplifying the energy productive motion of wave energy converters (WECs) in response to irregular sea waves, the strategies of discrete control presented here feature some major advantages over continuous control, which is known to require, for optimal operation, a bidirectional power take-off able to re-inject energy into the WEC system during parts of the oscillation cycles. Three different discrete control strategies are described: latching control, declutching control and the combination of both, which we term latched-operating-declutched control. It is shown that any of these methods can be applied with great benefit, not only to mono-resonant WEC oscillators, but also to bi-resonant and multi-resonant systems. For some of these applications, it is shown how these three discrete control strategies can be optimally defined, either by analytical solution for regular waves, or numerically, by applying the optimal command theory in irregular waves. Applied to a model of a seven degree-of-freedom system (the SEAREV WEC) to estimate its annual production on several production sites, the most efficient of these discrete control strategies was shown to double the energy production, regardless of the resource level of the site, which may be considered as a real breakthrough, rather than a marginal improvement.

Fully Discrete Control Scheme of the Energy-Performance Tradeoff in Embedded Electronic Devices

AbstractA voltage scalable device is known to be interesting for energy saving. It enables to reduce the general speed of the device and, therefore, its consumption. We already proposed a fast predictive control strategy to deal with this power-performance tradeoff in an electronic device supplied by two voltage levels and a continuously varying frequency. In this paper, the approach is extended to a fully discrete scheme with M possible voltage levels and N frequency levels. The proposed approach clearly gives an important reduction of the energy consumption with a very low control computational cost. Moreover, the control strategy is highly robust to tackle variability since it is not based on any parameters of the system.

Digital Control of a Wind Turbine Supplying an Isolated Load in Cooperation with a Hydro Power Unit

This paper presents different discrete control strategies for a variable speed pitch-controlled wind turbine (WT) which supplies an autonomous system in cooperation with a small hydro power unit. The WT consists of a synchronous generator (SG) with two three-phase stator windings displaced by 300, so as the torque distortions due to the wind fluctuations to be limited. The SG operates in variable speed mode in order to maximize the power absorbed from the wind and minimize the amount of water consumed by the hydro power unit. The requirements from the control schemes are to deliver the total power from the WT to the isolated load under constant voltage and frequency and adjust suitably the power from the hydro unit at the same time. There are two controllers: the rectifier controller, which drives the WT at such value of mechanical velocity that maximum power from the wind is absorbed and the inverter controller, which passes this amount of power to the load under constant voltage. There are two schemes presented in this paper: the first includes two PID controllers and the second includes a robust controller for the rectifier pulsation and a state-feedback controller for the inverter pulsation.

CONTROL SCHEME BASED ON INTERNAL PREDICTION FOR UNSTABLE LINEAR TIME-DELAY SYSTEMS

This work considers the regulator problem for unstable linear input-delay systems. A discrete time control strategy intended to compensate the effects of the involved time delay and to stabilize the overall closed loop system, is designed. The proposed control strategy provides a prediction of “ all” the internal information in the system, which is used to solve the considered regulation problem in a similar way that it is used in a classical Smith predictor compensator

Natural and non-natural spacecraft formations near the L1 and L2 libration points in the Sun–Earth/Moon ephemeris system

Space-based observatory and interferometry missions, such as Terrestrial Planet Finder (TPF), Stellar Imager, and MAXIM, have sparked great interest in multi-spacecraft formation flight in the vicinity of the Sun–Earth/Moon (SEM) libration points. The initial phase of this research considered the formation keeping problem from the perspective of continuous control as applied to non-natural formations. In the present study, closer inspection of the flow corresponding to the stable and centre manifolds near the reference orbit, reveals some interesting natural relative motions as well as some discrete control strategies for deployment. In addition, some implementation issues associated with discrete formation keeping of natural versus non-natural configurations are addressed in the present study.

Using Perceptual Boundaries to Control Braking Actions

Flach, Smith, Dittman, & Stanard's (2003) hypothesis that a linear weighting of optical angle and expansion rate are used as a criterion for braking is evaluated in a simple driving task. The task required participants to drive from an initial stationary position to a full stop at a comfortable distance in front of a stationary obstacle. The simulated vehicle was controlled with an accelerator and a brake. Acceleration and deceleration were proportional to displacement of these controls. Results showed that people's critical actions (releasing the accelerator and initiating the brake) corresponded with a Tau criterion (constant ratio of angle and expansion rate). Time histories suggest that people were adopting non-proportional, discrete control strategies that approached time optimal, bang-bang control.

A novel discrete control strategy for independent stabilization of parallel three-phase boost converters by combining space-vector modulation with variable-structure control

We propose a discrete nonlinear controller, developed in a synchronous frame, for a parallel three-phase boost converter consisting of two modules. The basic idea, however, can be extended to a system with N modules. Each of the closed-loop power-converter modules operates asynchronously without any communication with the other modules. The controller stabilizes the currents on the dq-axes and limits the flow of the pure-zero sequence current. It combines the space-vector modulation scheme with a variable-structure control, thereby keeping the switching frequency constant and achieving satisfactory dynamic performance.

Predicting and adapting satellite channels with weather-induced impairments

Efficiency improvements using predictive and adaptive methods over satellite channels with weather-induced impairments are presented. Scintillation and rain attenuation are the two dominant factors for signal fading over satellite-Earth paths at operating frequencies over 10 GHz. We develop statistical and spectral analyses of these processes, and obtain simple linear predictors for received signal attenuation using autoregressive (AR) models. For adaptation, we propose changing signal transmission power, modulation symbol size, and/or code rate as the state of the channel changes. In particular, we introduce a continuous power control and discrete rate control strategy. Quantitative analyses of power consumption and channel capacity indicate that there can be a substantial gain in performance with such adaptive schemes.

A Combined Model for Supervisory Control of Continuous Processes

AbstractUsing Hybrid Petri Nets (HPNs) together with a continuous mathematical model, a combined modeling method for continuous‐process dynamic scheduling and supervisory control is presented. This method can describe both continuous characteristics of a continuous process and its discrete control strategies on the same level. An ideal binary mixture distillation column is used as an example to illustrate the proposed method. Based on the combined model, a supervisory control scheme is also provided.

A review of the stochastic lot scheduling problem

This paper surveys the current research literature on the stochastic lot scheduling problem which deals with scheduling production of multiple products with random demand on a single facility with limited production capacity and significant change-overs between products. The deterministic version of this problem has received significant coverage in the literature; however, the stochastic problem has been addressed only recently. Furthermore, a range of distinctly different analytical methods have been applied to this problem. This paper provides a unifying framework for discussing these approaches and offers some explanation and clarification of the different analytical methods for this problem. After discussing some of the modeling and managerial implications of this problem, a detailed review of both continuous and discrete time control strategies is given, and areas for further research are outlined.

Controlling Belousov–Zhabotinsky–continuous stirred tank reactor chaotic chemical reaction by discrete and continuous control strategies

Discrete and continuous control strategies are applied to regulate the Belousov–Zhabotinsky–continuous stirred tank reactor (BZ–CSTR) chaotic chemical reaction onto the stabilized states. The orderly intrinsic states of the chaotic system, such as unstable periodic-1 orbit (UPO-1), unstable periodic-2 orbit (UPO-2) and unstable periodic-4 orbit (UPO-4) are obtained by the discrete nonlinear perturbation feedback method from within the whole chaotic region (NPF-WCR). The non-intrinsic orderly states, such as period-4, period-2, period-1 and steady state can be maintained by the continuous linear and nonlinear self-interacting feedback control strategy. The differences between the two methods are also discussed. An '‘extremum space control’' method is suggested.

Synthesizing a Supervisory Control Scheme for the Start-Up Procedure of a Distillation Column - An Approach Based on Approximating Continuous Dynamics by DES Models

This paper addresses the problem of synthesizing a supervisory control scheme for the start-up procedure of a distillation column. We use techniques from the areas of hybrid and discrete event systems theory to solve the problem: First, the detailed plant model is approximated by an automaton. Then, a discrete control scheme is generated for the abstract (automaton) model. By construction, the automaton behaviour covers the detailed model behaviour. Hence, the controller not only forces the abstraction but also the underlying detailed plant model to obey the specifications.

98/01306 Discrete control of series compensation for stability improvement in power systems: Padiyar, K. R. and Rao, K. U. Electrical Power & Energy Systems, 1997, 19, (5), 311–319

Changes in the network such as connecting a shunt resistor, inserting a series capacitor, reducing of generation, load etc., when accomplished under a suitable control scheme can improve transient stability. Switching of series capacitors in a transmission line can also help to damp oscillatory power transients. In this paper a detailed analysis and study of a discrete control strategy for the Thyristor Controlled Series Capacitor (TCSC) to improve stability is presented. A single machine infinite bus system is considered to illustrate the development of the control strategy. The energy function is used in determining the switching instants. The control philosophy is later extended to the multimachine system where the energy of the transmission line is used in determining the control strategy.

Model-based control of mold level in a continuous steel caster under model uncertainties

In tackling the problem of liquid steel level regulation in the mold of a continuous steel caster, the values of some system parameters, such as the clogging degree and the discharge coefficients of nozzles, are seldom precisely known. The discrete control laws developed in this work employ on-line estimated values of these parameters, to produce a stable regulation of mold and tundish levels for an industrial-type two-strand continuous slab caster. The estimation procedure is delayed by one time step (the sampling time) in order to become realizable; when the sampling time is small enough, the discrete control scheme gives stability and convergence properties that are analogous to those of an ideal control scheme with accurate knowledge of the parameter values.

Discrete control of series compensation for stability improvement in power systems

Changes in the network such as connecting a shunt resistor, inserting a series capacitor, reducing of generation, load etc., when accomplished under a suitable control scheme can improve transient stability. Switching of series capacitors in a transmission line can also help to damp oscillatory power transients. In this paper a detailed analysis and study of a discrete control strategy for the Thyristor Controlled Series Capacitor (TCSC) to improve stability is presented. A single machine infinite bus system is considered to illustrate the development of the control strategy. The energy function is used in determining the switching instants. The control philosophy is later extended to the multimachine system where the energy of the transmission line is used in determining the control strategy.

Zero-residual-energy, single-axis slew of flexible spacecraft using thrusters - Dynamics approach

By examining the phase plane response of an elastic mode to a sequence of step torques, the paper first shows that if the half-slew time is an integer times the modal period, a flexible spacecraft can be slewed with zero residual energy in one critical mode. Because this stipulation cannot be met for an arbitrary slew angle and slew acceleration, two intervening pul: 3S of equal width for undamped mode, and of unequal width for damped mode, are shown to be required to achieve two goals at once: 1) slew the spacecraft by the desired angle, and 2) have zero residual energy in a critical elastic mode. Using elementary structural dynamics, easily solvable analytical relationships are developed among the rest-to-rest maneuver angle, slew acceleration, slew time, widths of the intervening pulses, natural frequency of the critical mode, and its damping coefficient. This minimum-time, zero-residual-energy, single-axis slew is illustrated on a spacecraft with two solar arrays. In addition, the assumption of only one elastic mode being critical is carefully examined. To bring cohesion between this work and the works in the past, the place of this contribution is identified. Finally, to bring an arbitrary rigid and elastic modai state of a flexible spacecraft to the origin, a conceptual, discrete control scheme using thrusters is suggested.

Generator resynchronization by a knowledge-based discrete controller

An explanation is presented of the fundamental mechanism of the entire resynchronization control process and the sufficient conditions for the resynchronization of generators. Based on the knowledge of this mechanism, a discrete control scheme is deduced. Simulation studies and scaled model experiments show that reliable and fast automatic resynchronization is realized by this control scheme after loss of synchronism occurs. The attractive advantages of this control scheme are its effectiveness, simplicity, and adaptability to various operating conditions. >

A real-time calculation method for optimal reactive power compensator

A real-time calculation method is presented to determine the optimal reactive power compensator for power systems. The reactive power compensators are expressed as both the continuous control scheme and the discrete control scheme. The optimization problem is written in terms of the minimization of power line loss by minimizing the line currents. Three-phase current balancing is also considered in this problem. By an integerization method, suboptimal solutions are approached for utilizing the discrete-tap compensators. Theoretical derivations and computation procedures are formulated by including digital techniques for real-time data acquisition. Applications to radial-type systems are described and discussed. Examples of real-time simulations and experiments are discussed to show the performance of the method and to support the application capability. >

Discrete-Tap Control Scheme for Capacitive Compensation of Distribution Feeders

Substation-based computers for control of reactive power on distribution primary feeders are being included within the distribution automation schemes now undergoing installation. This paper develops the algorithmic basis for the optimal control strategies necessary to capacititively compensate distribution feeders having capacitors with multipletap settings. The theory underlying these discrete-control strategies is presented along with simple numerical examples which show how to minimize the kilowatt power losses as reactive load varies on the feeders.

Optimal point-wise discrete control and controllers' allocation strategies for stochastic distributed systems

The design of point-wise discrete controllers for a class of stochastic distributed-parameter systems is considered. Assuming a fixed set of controllers' positions, the optimal feedback control is derived using a direct approach in which the infinite dimensional space is approximated using a set of orthonormal functions. The resulting optimal cost is minimized again w.r.t. this set of positions, using gradient techniques, to get the optimal locations for the controller. A one-dimensional diffusion process is used to demonstrate the algorithm.