Hassan Karimi

Input/output characterization of highly nonlinear multivariable systems

This paper presents a software in the MATLAB environment for input/output characterization of highly nonlinear multivariable systems. The software is based on obtaining the sinusoidal-input describing function (SIDF) models of the nonlinear multivariable plant. The mathematical development of the input/output characterization approach is outlined, and the printout of the nucleus of the characterization software is given. The method and the associated software have no restriction on nonlinearity type, configuration, and arrangement, system order as well as the number of inputs and outputs; the only requirement is that system output(s) for sinusoidal input(s) must be obtainable or available by either simulation or experiments. The software has a variety of applications in such industries as robotics, aerospace, automotive, and chemical processes. The method and the associated software are demonstrated by solving a problem of the sort encountered in aerospace.

Nonlinear controller synthesis based on inverse describing function technique in the MATLAB environment

In this research, a new software based on a systematic nonlinear controller design technique for nonlinear systems is developed. This software is in the MATLAB environment, and it is based on inverse describing functions in order to arrive at nonlinear gains of the controller. The software may be used to synthesize nonlinear controllers for nonlinear systems without restriction on system order, the number, type, and arrangement of nonlinear terms. The goal is to design an output feedback system that is a solution to a robust design such that sensitivity of the feedback control system with respect to amplitude of the excitation signal and the operating regimes would be at a minimum.

Dynamic and Nonlinear Simulation of Liquid-Propellant Engines

A new dynamic and nonlinear simulation method for liquid-propellant engines is presented, and the corresponding software for a particular engine is developed. The logic of the simulation method and the software is based on following the liquid(s). The dynamic equations of motion are composed of implicit nonlinear algebraic equations, as well as nonlinear and time-varying differential equations. The implicit nonlinear algebraic equations are solved using a number of nested Newton-Raphson loops, and the nonlinear and time-varying differential equations are solved using a first-order Euler technique. The simulation results are compared with experimental results. The software language is FORTRAN, and it is composed of approximately 200 subroutines for a total of approximately 4000 lines.

Mixture ratio control of liquid propellant engines

Purpose – Development and application of a new systematic approach for design of a control system in order to control the mixture ratio of liquid propellant engines.Design/methodology/approach – The design approach is based on a full nonlinear dynamic model of the engine, and the controller design method is based on describing function models of the engine coupled with the factorization theory. The presented systematic design procedure is comprised of five primary steps. The developed software for the design approach is in the MATLAB environment.Findings – It is found that the presented design approach may successfully be used to control the mixture ratio of a class of liquid propellant engines whose control loops are decoupled. The performance and robustness of the designed controller is found to be satisfactory.Research limitations/implications – At present, the research is limited to liquid propellant engines whose control loops are decoupled.Practical implications – The major outcome of this research ...

Closed-form solution for design of lead-lag compensators

A closed-form solution for the design of lead-lag compensators based on an exact model-matching criterion is developed. Using the developed design tool, students are able to obtain lead-lag compensators in a fraction of the time that is required by traditional design methods.

A new software tool for synthesis of linear PID controllers

A new versatile software utility for synthesis of linear PID controllers is described, and the software listing is presented. The software is in the MATLAB environment. Closed-form PID controller gain design equations are developed. The design approach is systematic, and it is based on frequency matching technique with a model matching criteria. The objective is to design a closed-loop feedback system with a PID controller whose dynamic and static behavior would mimic a user-defined reference linear model. The design procedure is automated via a new MATLAB command. The software also has applications in synthesis of nonlinear PID controllers. Because the design equations are of a closed form, the speed of calculations is high; therefore, design software may be used in designing self-tuning adaptive PID controllers.

Application of a simulation algorithm to a specific liquid propellant engine with experimental verification

Purpose – The purpose of this paper is to apply a new systematic simulation approach to an existing liquid propellant engine.Design/methodology/approach – The simulation approach is based on following the liquids (oxidizer and fuel) in their respective paths. The nonlinear dynamic model of the engine is composed of implicit nonlinear algebraic equations coupled with a set of differential equations. The model is solved by placing the implicit nonlinear algebraic equations in a set of nested Newton‐Raphson loops followed by numerical integration of the differential equations using a first‐order Euler technique.Findings – It is found that the simulation algorithm may successfully be applied to an operating point model to predict the steady‐state values with errors under 10 percent. These results indicate that such engine models may be used to design reiable robust engine control systems because a robust control system design would allow for about 20 percent discrepancy between the model and the actual case.R...

Modeling and simulation of a two combustion chambers liquid propellant engine

Purpose – The purpose is to develop and apply a systematic simulation approach for dynamic analysis in order to study a two combustion chambers liquid propellant engine.Design/methodology/approach – The logic of the simulation method and the software is based on following the liquids. The implicit nonlinear algebraic equations are solved using a number of nested Newton‐Raphson loops, and the nonlinear and time varying differential equations are solved using a first‐order Euler technique.Findings – It is found that the developed simulation code predicts the steady‐state values with errors under 5 percent, and this code has the capability to be used in studying the effect of various elements and subsystems parameters on the forecasting the performance and operation of the engine system.Research limitations/implications – At present, the research is limited to a specific liquid propellant engine. Development of a general purpose software package for simulation of liquid propellant engines, based on the devel...

Numerical investigation on thermo-acoustic effects and flow characteristics in semi-conical Hartmann–Sprenger resonance tube

Hartmann–Sprenger tube is a device in which an underexpanded jet enters a closed end tube that is placed in a specific distance from the nozzle. In specific conditions, a standing shock is built in front of the tube, which oscillates based on the tube resonance frequency and creates oscillatory flows with periodic shock motions along the tube. In these conditions, intensive temperature rise could be observed near the tube end wall. Considering these thermal effects, the device could be used as a combustion starter in the space propulsion systems. The present study focuses on flow analysis in various phases of the oscillatory process in a semi-conical PTFE resonance tube by the aim of numerical simulation results. An experimental test is also performed for validation purposes. The T–S diagram is plotted to describe the thermal effects in detail during the oscillatory processes. Various modes of shock contact with flow front are described. In order to follow up the shock traveling process, the diagrams of changes related to major flow properties inside the tube are used. Generation of small turbulences at the moments of combination of compression waves and beginning of flow entrance is also detected. According to the results, traveling of shock waves through the trapped gas was found to be the major mechanism for heat generation inside the tube. The thermal effects are also compared in the conical and cylindrical tubes. The flow analysis will lead to increase in insight for shock motion and heat generation mechanism in a semi-conical Hartmann–Sprenger tube.

Dynamic Simulation and Parametric Study of a Liquid Propellant Engine

Liquid propellant rocket engines form an important part of aerospace systems. They are complicated systems and simulation task of these systems is important, because if the simulation software of LPRE is available, designers may easily optimize the engine.This paper presents application of a simulation algorithm to dynamic and nonlinear analysis of a specific liquid propellant engine. In this algorithm the mathematical model is solved by placing the implicit nonlinear algebraic equations in a set of nested Newton-Raphson loops followed by numerical integration of the differential equation using a first-order Euler technique. The simulation approach is based on following the liquids (fuel and oxidizer) in their respective paths. Comparison of the nominal values obtained from simulation with actual design values is presented. Typical simulation outputs of primary engine variables are also given. The developed simulation code based on this algorithm has the capability to be used in studying the effect of various elements and subsystem parameters on operation of the engine system. With this capability, parametric study of a liquid propellant engine is presented.