Nominal Operating Points Research Articles

Reduced‐order modeling for the control of selective noncatalytic reduction of nitrogen monoxide

The design of a reduced‐order model is discussed to help in the control of selective noncatalytic reduction (SNCR) of nitrogen monoxide. Instead of relying on a look‐up table of nominal operating points, it is proposed to solve for the time evolution of a set of stochastic particles interacting through a model for turbulent mixing and a reduced chemistry. Each particle is representative of a fraction of the mass flowing in the system in gaseous or liquid form. To calibrate and validate the reduced‐order model, which runs in a few minutes on a desktop computer, reference three‐dimensional and unsteady large‐eddy simulation is performed in the complex geometry of a real incinerator. This is done solving the full set of conservation equations of mass, momentum, and energy over a 162 million cells mesh. The results confirm the possibility of real‐time SNCR optimization from the solving of partial differential equations. © 2015 American Institute of Chemical Engineers AIChE J, 62: 928–938, 2016

Multi-Degree-of-Freedom Stabilization of Large-Scale Photonic-Integrated Circuits

We present and elaborate the principles of a novel control and calibration (C&C) systematic approach, aiming at feedback stabilization of large scale photonic-integrated circuits (PIC) to their nominal operating points in D -dimensional spaces of tuning parameters, based on refinement of extremum-seeking (ES) real-time optimization techniques. The novel methodology enables, in principle, stabilizing a large number D of tuning degrees of freedom (DOF) of PICs, with just one optical power monitoring (probe) point. The proposed C&C digital controller ports multidimensional ES stabilization concepts to photonics for the first time and further introduces novel improvements to known ES techniques, proposing a new frame-based discrete-multitone method of actuation signals generation and probe signals detection, akin to an orthogonal-frequency-division multiplexing modulation format. Another novel element is that the iterative digital algorithm adaptively selects between gradient- versus Newton-based descent and between various methods of line search. This approach enables real-time application of unconstrained optimization techniques for C&C of photonic circuits with multiple tuning DOFs. The new technique is exemplified by numeric simulations of the stabilization of a Silicon photonic microring modulator with D = 2 tuning DOFs, concurrently tuning both resonant phase and a second coupling phase parameter, optimizing critical coupling into the microring based on observing the optical power at single low-frequency monitoring point.

Simultaneous heating and cooling production devices composed by reverse cycle systems under variable loads

Energy and exergy analysis leads to integrated solutions for process design. However, if the overall process design is optimized its robustness and its operability could be major weaknesses. This paper focuses on an integrated process where a reverse cycle system answers both heating and cooling demands. For variable heat demands, the aforementioned reverse cycle system may be inadequate. Therefore, one option is to add flexibility to the device by splitting it into two systems. In this paper, in addition to the initial reverse cycle system, five more solutions are proposed and studied: three uncoupled and two partially coupled solutions. First, simplified models are used to compare the different proposed solutions for several nominal operating points. Then, more detailed models are used to consider heating or cooling demand variation. Results show that the flexible solutions designed can be relevant options when heating or cooling demands vary.

Design of a multivariable neural controller for control of a nonlinear MIMO plant

Abstract The paper presents the training problem of a set of neural nets to obtain a (gain-scheduling, adaptive) multivariable neural controller for control of a nonlinear MIMO dynamic process represented by a mathematical model of Low-Frequency (LF) motions of a drillship over the drilling point at the sea bottom. The designed neural controller contains a set of neural nets that determine values of its parameters chosen on the basis of two measured auxiliary signals. These are the ship’s current forward speed measured with respect to water and the systematically calculated difference between the course angle and the sea current (yaw angle). Four different methods for synthesis of multivariable modal controllers are used to obtain source data for training the neural controller with parameters reproduced by neural networks. Neural networks are designed on the basis of 3650 modal controllers obtained with the use of the pole placement technique after having linearized the model of LF motions made by the vessel at its nominal operating points in steady states that are dependent on the specified yaw angle and the sea current velocity. The final part of the paper includes simulation results of system operation with a neural controller along with conclusions and final remarks.

ADAPTACYJNY REGULATOR LQR W UKŁADZIE STEROWANIA KĄTEM KURSOWYM I PRĘDKOŚCIĄ STATKU OPISANEGO NIELINIOWYM MODELEM DYNAMICZNYM MIMO

In the paper an adaptive control system for the nonlinear 4-DoF model of a container vessel is presented. The considered model of the ship includes input signals saturations and dynamics of actuators. Main goal of the presented control system is control of the course angle and speed of the ship relative to water. The system synthesis is carried out by means of LQ-optimal control method. Sets of controller parameters are designed using linearization of the considered model in the nominal steady-state operating points of the ship. The final part of the paper includes simulation results of control system operation with full nonlinear MIMO model of the container vessel.

Multivariable Adaptive Controller for the Nonlinear MIMO Model of a Container Ship

The paper presents an adaptive multivariable control system for a Multi-Input, Multi-Output (MIMO) nonlinear dynamic process. The problems under study are exemplified by a synthesis of a course angle and forward speed control system for the nonlinear four-Degrees-of-Freedom (4-DoF) mathematical model of a single-screw, high-speed container ship. The paper presents the complexity of the assumed model to be analyzed and a synthesis method for the multivariable adaptive modal controller. Due to a strongly nonlinear nature of the ship movements equations a multivariable adaptive controller is tuned in relation to changeable hydrodynamic operating conditions of the ship. In accordance with the given operating conditions controller parameters are chosen on the basis of four measured auxiliary signals. The system synthesis is carried out by linearization of the nonlinear model of the ship at its nominal operating points in the steady-state and by means of a pole placement control method. The final part of the paper includes results of simulation tests of the proposed control system carried out in the MATLAB/Simulink environment along with conclusions and final remarks.

Simulation of near surge instabilities onset in a turbocharger compressor

This study focuses on numerical simulations of a small automotive turbocharger compressor stage. Two medium speed characteristics were reproduced from nominal operating points to surge and were compared with experimental measurements. The aim of this study is to analyze the flow unsteadiness occurring near the surge line. The complete geometry of the impeller is meshed; hence, no spatio-temporal hypothesis is done during simulations. The main flow patterns are investigated to identify structures that might be responsible of surge inception. Results show that both impeller and diffuser are affected by stall. Blade channels are affected by a complete shroud recirculation extending from upstream of the impeller inlet to the diffuser inlet. Three radial recirculation zones are detected on the vaneless diffuser walls, strongly influenced by the two-pike asymmetric pressure field induced by the volute tongue. Its influence is observed at the inlet of the compressor, increasing the inter-blade flow unsteadiness.

Two degrees of freedom PID multi-controllers to design a mathematical driver model: experimental validation and robustness tests

This paper proposes a mathematical driver model based on PID multi-controllers having two degrees of freedom. Each PID controller making up this model is synthesised by the Ziegler–Nichols oscillation method, using the linear time invariant models which are obtained around their nominal operating points. Different PID controllers are combined using nonlinear optimisation and the H ∞ constraint. To demonstrate its robustness, it was tested on two models: a linear parameter variant model and a nonlinear four-wheel model. It was also tested in situations of high dynamic demand. The driver model showed good performance, stability and trajectory tracking. The performance tests were carried out using experimental data acquired by a Laboratory Peugeot 307 developed by INRETS-MA. This driver model was developed for an application known as ‘Itinerary Rupture DIagnosis’ (DIARI), which aims to evaluate the physical limits of a vehicle negotiating a bend. DIARI requires a tool to determine the steering commands to be applied to a vehicle model, making extrapolations with respect to speed.

Robust proportional–integral–derivative controller design for an electrostatic micro-actuator with measurement uncertainties

In this study the effects of unaccounted modal dynamics within the control scheme of an electrostatic micro-actuator (µ-A) are presented. The µ-A is composed of a µ-capacitor, whose one plate is clamped on the ground whereas its other plate is floating on the air. The dynamic model of the µ-A allows both lateral and angular movements of the upper plate. The feedback controller is designed based on the single-mode (lateral) linearised model. The reduced non-linear model (RnM) is linearised at multiple operating points, and the designed proportional–integral–derivative (PID)-controller, tuned via linear matrix inequalities (LMIs)-theory, stabilises all linear modes within the polytopic defined by the vertices of the linearised systems. The overall scheme comprises: (a) a feedforward controller that stabilises the µ-A around its nominal operating points and (b) a robust PID controller that handles deviations from the operating points. The resulting overall control scheme is applied to the non-linear (bimodal structure) of a µ-A, and the simulation results derived are used to investigate the efficacy of the suggested control architecture.

Operability and Flexibility of a Milk Production Line

The operability and flexibility of an existing milk treatment process are investigated through flowsheet modelling and simulation method. From the flowsheet simulation, a process operating region is determined using incoming milk flow viscosity and heat exchanger pressure drop as characteristic parameters. The operability and flexibility of the nominal operating points are evaluated with a minimization of the process pasteurization and cooling temperatures through vertex enumeration method. The milk treatment process simulation and process analysis provide a quantitative analysis method for the development of a flexible process in dairy industrial applications. The established process operability and flexibility analysis framework and flowsheet simulation can be adapted to other liquid food productions.

Logic-based switching controllers – A stepwise safe switching approach

The present work proposes a new logic-based safe switching technique, called here Stepwise Safe Switching. This technique is based on the description of the process with linear approximants which are valid in operating areas around a finite number of nominal operating points of the process. Switching takes place between common controllers for adjacent nominal operating points. Moreover, the switching between the controllers is allowed to take place only when the process has reached an operating point. The proposed technique guarantees stability and good performance for all transitions between operating points that lie in the neighborhoods of the nominal operating points of the process. This way a simple to design and easy to implement solution that provides good performance despite the drawback of lack of knowledge of the process dynamics can be derived. The proposed technique is successfully applied to control an isothermal CSTR process, described by three linear approximants around corresponding nominal operating points.

A novel power splitting drive train for variable speed wind power generators

In this paper a novel electrically controlled power splitting drive train for variable speed wind turbines is presented. A variable speed wind turbine has many advantages, mainly it can increase the power yield from the wind, alleviate the load peak in the electrical-mechanical drive train, and posses a long life time, also, it can offer the possibility to store the briefly timely wind-conditioned power fluctuations in the wind rotor, in which the rotary masses are used as storages of kinetic energy, consequently, the variable speed wind turbines are utilized in the wind power industry widely. In this work, on the basis of a planetary transmission a new kind of drive train for the variable speed wind turbines is proposed. The new drive train consists of wind rotor, three-shafted planetary gear set, generator and servo motor. The wind rotor is coupled with the planet carrier of the planetary transmission, the generator is connected with the ring gear through an adjustment gear pair, and the servo motor is fixed to the sun gear. By controlling the electromagnetic torque or speed of the servo motor, the variable speed operation of the wind rotor and the constant speed operation of the generator are realized, therefore, the generator can be coupled with the grid directly. At the nominal operation point, about 80% of the rotor power flow through the generator directly and 20% through the servo motor and a small power electronics system into the grid. As a result, the disadvantages in the traditional wind turbines, e.g. high price of power electronics system, much power loss, strong reaction from the grid and large crash load in the drive train will be avoided.

Selection of Actuators and Sensors for Surge Control

For surge control in compression systems, a suitable combination of actuators and sensors must be selected. The goal is to identify the combinations for which there exists a controller that locally stabilizes the linearized compression system model and meet additional control objectives. The latter are quantified in terms of a bound on an H-infinity norm. With this we aim to stay close to the nominal operating point in the face of disturbances, noise, and actuator limitations. The considered selection method is more rigorous than previously found in literature, in the sense that multivariable, dynamic output feedback is the focus and that different performance specifications are handled simultaneously. Though the method was originally developed fot linear systems, it can be used for the nonlinear compression system model by linearizing the model on a grid of nominal operating points. Among the proposed actuators and sensors, the close-coupled valve and mass flow sensor are identified as the most promising. The results are sustained by the physics of the problem.

Reactor experiments on type-I and type-II BWR stability

Investigations on Type-I (hydrostatic head) and Type-II (density wave) BWR stability characteristics are reported. Experimental data are obtained from the Dodewaard natural-circulation BWR at conditions far away from nominal operating points. The Type-I stability boundary was reached; the Type-II stability boundary was crossed. At this latter unstable condition the reactor power showed peculiar oscillation modulations. Experimentally obtained decay ratios and resonance frequencies are put into context using a physical model. Flashing is shown to be an important phenomenon for driving the coolant flow at start-up conditions and for Type-I stability features. From low-pressure experiments, it appears that the decay ratio is an unsafe measure for Type-II stability close to the stability boundary.

Global stabilization of axial compressors using nonlinear cancellation and backstepping designs

Issues concerning the global stabilization of axial compressors are presented. Instead of locally stabilizing the bifurcated rotating stall equilibria to avoid suddern performance drop, control schemes are proposed to globally stabilize the unstalled nominal operating points. These are achieved by employing nonlinear cancellation and backstepping designs via either throttle or direct mass flow rate control if the compressor characteristic satisfies the ‘left-tilt’ properly. The designs not only guarantee a safe operation close to the maximum achievable pressure rise, but the bifurcated system equilibria are also found to be stabilized due to the elimination of the jump phenomenon. Though both throttle and direct mass flow rate control schemes can be implemented for global stabilization, numerical simulations demonstrate that direct mass flow rate control achieves better post-stall system behaviour.

Robust controller design for a parallel resonant converter using /spl mu/-synthesis

DC-to-DC resonant power converters have been the subject of much attention recently. These power converters have the potential to provide high-performance conversion without some of the problems associated with classical pulse-width modulation (PWM)-based converters, thus allowing for smaller, lighter power supplies. However, in order to achieve this, a suitable control circuit, capable of maintaining the desired output voltage under different operating conditions, is required. In the past, small-signal models obtained around the nominal operating points were used to design controllers that attempted to keep the output voltage constant in the presence of input perturbations. However, these controllers did not take into account either load or components variations, and thus could lead to instability in the face of component or load changes. Moreover, the prediction of the frequency range for stability was done a posteriori, either experimentally or by a trial-and-error approach. In this paper, the authors use /spl mu/-synthesis to design a robust controller for a conventional parallel resonant power converter. In addition to guaranteeing stability for a wide range of load conditions, the proposed controller rejects disturbances at the power converter input while keeping the control input and the settling time within values compatible with a practical implementation. These results are validated by means of detailed nonlinear circuit simulations obtained using PSpice.

Robust controller design for a series resonant converter

Because of their reduced switching losses, allowing a higher operating frequency, dc-to-dc resonant converters have been used extensively in the design of smaller size and lighter weight power supplies. The steady state and dynamic behavior of both the conventional series and parallel resonant converters have been thoroughly analyzed and small-signal models around given nominal operating points have been obtained. These models have been used in the past to design controllers that attempted to keep the output voltage constant in the presence of input perturbations. However, these controllers did not take into account either load or components variations, and this could lead to instability in the face of component or load changes. Moreover, prediction of the frequency range for stability was done a posteriori, either experimentally or by a trial and error approach In this paper we use /spl mu/-synthesis to design a robust controller for a series resonant converter (SRC). In addition to robust stability the design objectives include rejection of disturbances at the converter input while keeping the control input and the settling time within values compatible with a practical implementation.

DESIGN OF A ROBUST LINEAR OPTIMAL REGULATOR FOR SYNCHRONOUS GENERATOR EXCITATION CONTROL

ABSTRACT This paper presents a technique for designing generating units linear optimal regulators with extended stability region around nominal operating points. The technique uses a stability robustness criterion for the selection of the relative weights between the control effort and the system state regulation In the linear regulator quadratic performance index. The resulting regulator is stability robust in the sense of tolerating a specified set of unit parameter variations. By proper specification of the parameter variations, a robust linear optimal regulator Is obtained which guarantees unit stability in an extended region of operation around a given operating point. Example results are presented to substantiate the proposed technique by application to the design of a linear optimal regulator for a generating unit excitation system.

Integration Compatibility of Ion Engine Power Conditioner

T HE design of a power conditioner and its compatibility with an ion engine are described. The sequencing control system is designed and analyzed by a computer simulation using mathematical thermal and plasma models. The design requirements for the discharge and beam power supplies are investigated. The capacitance of the output filter is the main factor causing undesired intermittent discharge. The slow start characteristic is employed for beam power supply to reduce harmful influences on the system. Contents The 5-cm-diam mercury electron bombardment ion engine system (IES) has been developed for the space test in Japan.1 The power conditioner consists of a power supply unit (PSU) and a command interface unit (CIU). The PSU converts the satellite bus voltage (28 V dc) to the appropriate voltages for the engine which requires ten power supplies. The CIU receives command signals and controls the engine through the PSU automatically. It also sends out telemetry data to the satellite. The CIU, PSU and engine2 developed are shown from left to right in Fig. 1. A. Modeling and Control Logic Analysis The control sequence is designed under the constraints of the startup time, input power, and current. The possible combinations of control sequence and power levels are so many that the numerical simulation is basically applied for optimization. The operation characteristics and dynamics of the engine depend on the thermal condition of several components, such as cathodes and vaporizers. The ten-nodes mathematical thermal model is derived as discussed in Ref. 3. Most static plasma models of the engine are expressed as the form of rational polynomials for generalization. The main parameter, which varies in the wide range and influences the characteristics strongly, is selected as the independent variable. And the effects of other parameters are accounted for by the second-order polynomial approximation around the nominal operating points. The characteristic of each power supply in the PSU is also modeled mathematically by the polynomial or logarithmic expressions. The sequencing control flow of the CIU can be coded to the simulation program directly. The number of model expressions for the engine is about 30 and the total unknown parameters are about 150. These parameters are identified by the three step computer aided parameter tuning method.3 The sequencing logic for starting up the engine is derived from the simulation analysis. Details can be found in Ref. 3. The turnoff delay time of the heater power supply after the cathode ignition is introduced in the neutralizer start routine. The slow cooling-down procedure of the main cathode after the discharge ignition, which is accomplished conventionally