Showing papers by "Cristina I. Muresan published in 2016"

A novel auto-tuning method for fractional order PI/PD controllers.

Abstract: Fractional order PID controllers benefit from an increasing amount of interest from the research community due to their proven advantages. The classical tuning approach for these controllers is based on specifying a certain gain crossover frequency, a phase margin and a robustness to gain variations. To tune the fractional order controllers, the modulus, phase and phase slope of the process at the imposed gain crossover frequency are required. Usually these values are obtained from a mathematical model of the process, e.g. a transfer function. In the absence of such model, an auto-tuning method that is able to estimate these values is a valuable alternative. Auto-tuning methods are among the least discussed design methods for fractional order PID controllers. This paper proposes a novel approach for the auto-tuning of fractional order controllers. The method is based on a simple experiment that is able to determine the modulus, phase and phase slope of the process required in the computation of the controller parameters. The proposed design technique is simple and efficient in ensuring the robustness of the closed loop system. Several simulation examples are presented, including the control of processes exhibiting integer and fractional order dynamics.

Tuning algorithms for fractional order internal model controllers for time delay processes

Abstract: This paper presents two tuning algorithms for fractional-order internal model control (IMC) controllers for time delay processes. The two tuning algorithms are based on two specific closed-loop control configurations: the IMC control structure and the Smith predictor structure. In the latter, the equivalency between IMC and Smith predictor control structures is used to tune a fractional-order IMC controller as the primary controller of the Smith predictor structure. Fractional-order IMC controllers are designed in both cases in order to enhance the closed-loop performance and robustness of classical integer order IMC controllers. The tuning procedures are exemplified for both single-input-single-output as well as multivariable processes, described by first-order and second-order transfer functions with time delays. Different numerical examples are provided, including a general multivariable time delay process. Integer order IMC controllers are designed in each case, as well as fractional-order IMC...

Theoretical Analysis and Experimental Validation of a Simplified Fractional Order Controller for a Magnetic Levitation System

Abstract: Fractional order (FO) controllers are among the emerging solutions for increasing closed-loop performance and robustness. However, they have been applied mostly to stable processes. When applied to unstable systems, the tuning technique uses the well-known frequency-domain procedures or complex genetic algorithms. This brief proposes a special type of an FO controller, as well as a novel tuning procedure, which is simple and does not involve any optimization routines. The controller parameters may be determined directly using overshoot requirements and the study of the stability of FO systems. The tuning procedure is given for the general case of a class of unstable systems with pole multiplicity. The advantage of the proposed FO controller consists in the simplicity of the tuning approach. The case study considered in this brief consists in a magnetic levitation system. The experimental results provided show that the designed controller can indeed stabilize the magnetic levitation system, as well as provide robustness to modeling uncertainties and supplementary loading conditions. For comparison purposes, a simple PID controller is also designed to point out the advantages of using the proposed FO controller.

A fractional order controller for seismic mitigation of structures equipped with viscoelastic mass dampers

Abstract: In this paper a fractional order (FO) controller is proposed for solving the vibration suppression problem in civil structures. A laboratory scaled steel structure, with one floor, modeled as a single degree-of-freedom system is used as a case study. Two passive control solutions are proposed: a tuned mass damper (TMD) and a viscoelastic damper (VED), the latter being modeled using fractional derivatives. The simulation results show that the VED is able to further reduce the vibrations induced as forced oscillations or due to seismic excitation inputs, as compared to the passive TMD. The FO controller is then tuned using a new approach based on imposing a magnitude condition for the closed-loop system at the structural resonance frequency. The resulting FO active control strategy, together with the VED, ensures an increased seismic mitigation. Structural modeling errors are also considered, with the proposed active FO control strategy behaving robustly in terms of vibration suppression. The novelty of the...

Vector-based tuning and experimental validation of fractional-order PI/PD controllers

Abstract: Although a considerable amount of research has been carried out in the field of fractional-order controllers, a simplified tuning routine has yet to be established. Most of the tuning techniques for fractional-order controllers deal with complex computations and optimization routines. This paper proposes a simple yet efficient methodology based on a vector representation of the fractional-order controllers. This simplifies considerably the computations and derivation of the fractional-order controller parameters. The tuning procedure is exemplified first for a fractional-order PI controller designed for a simple first-order process, as well as for a fractional-order PD controller for a servoing system. In this case, the experimental results are also included, showing that this novel tuning approach is a viable replacement for the more complex tuning procedures currently employed in the design of different fractional-order controllers.

Improvements in Dissolved Oxygen Control of an Activated Sludge Wastewater Treatment Process

Abstract: Nowadays there is great emphasis on the optimization of wastewater treatment plants (WWTP) due to the strict regulations concerning discharged waters. The control techniques currently used in WWTPs include simple PLC-based proportional-integral-derivative control techniques. The most complex part of wastewater treatment is the activated sludge process, where the sewage is biologically treated by means of a microorganism culture in the presence of air or oxygen. This paper shows a simple way of decreasing the operational costs, together with an increase in performance and disturbances rejection by using fractional order Proportional-Integral controllers on the dissolved oxygen concentration and on the air pressure generated by the air blower. Closed loop system performance, using the fractional order Proportional-Integral controller, are compared to those obtained by using the conventional Proportional-Integral controller on the dissolved oxygen concentration. They prove the robustness of the former controller against the parameters variations. The controllers are designed and simulated on the mathematical model of the WWTP of Romanofir, Talmaciu.

A comparison between integer and fractional order pdµ controllers for vibration suppression

Abstract: Along the years, unwanted vibrations in airplane wings have led to passenger discomfort. In this study, the airplane wing is modeled as a cantilever beam on which active vibration suppression is tested. The paper details the tuning of both integer and fractional order Proportional Derivative type controllers based on constraints imposed in the frequency domain. The controllers are experimentally validated and the results prove once more the superiority of the fractional order approach.

Design and analysis of a multivariable fractional order controller for a non-minimum phase system:

Abstract: Two control strategies for multivariable processes are proposed that are based on a decentralised and a steady state decoupling approach. The designed controllers are fractional order PIs. The efficiency and robustness of the proposed strategies is tested and validated using a non-minimum phase process. Previous research for the same non-minimum phase process has proven that simple decentralised or decoupling techniques do not yield satisfactorily results and a multivariable IMC controller has been proposed as an alternative solution. The simulation results presented in this paper, as well as the experimental results, show that the proposed fractional order multivariable control strategies ensure an improved closed loop performance and disturbance rejection, as well as increased robustness to modelling uncertainties, as compared to traditional multivariable IMC controllers.

Vibration suppression with fractional-order PIλDμ controller

Abstract: Cantilever beams have an important role in day to day life in bridges, towers, buildings and aircraft wings, making active vibration suppression a highly researched field. The purpose of this paper is to detail the design of fractional order PID controllers for smart beams. A novel tuning procedure is proposed based on solving a set of nonlinear complex equations that directly aim at reducing the resonant peak. The control parameters are computed through optimization techniques, making sure that the best ones are chosen. The practical stand was realized using magnet-coil approach and not piezoelectric actuators. The experimentally obtained vibration results prove that fractional order PID controllers can be used in practice to significantly reduce the amplitude and settling time of the vibrating system.

A Portable Implementation on Industrial Devices of a Predictive Controller Using Graphical Programming

Abstract: This paper presents an approach for developing an extended prediction self-adaptive controller employing graphical programming of industrial standard devices for controlling fast processes. For comparison purposes, the algorithm has been implemented on three different field-programmable gate arrays (FPGAs) chips. This paper presents research aspects regarding graphical-programming controller design, showing that a single advanced control application can run on different targets without requiring significant program modifications. Based on the time needed for processing the control signal and on the application, one can efficiently and easily select the most appropriate device. To exemplify the procedure, a conclusive case study is presented.

Analysis of a new continuous-to-discrete-time operator for the approximation of fractional order systems

Abstract: One of the methods for discrete-time approximation of fractional order systems consists in an indirect approach in which a continuous-time rational transfer function is firstly derived. Then, using different mapping techniques, the s plane transfer function is converted to the z plane. In this paper a new mapping technique is proposed. The tuning parameter of this continuous-to-discrete-time mapping operator balances the discrete-time approximation between the classical Euler and Tustin rules, ensuring an increased flexibility compared to these classical methods. An analysis of the effects of the changes in this parameter is given, along with some general guidelines for its proper selection. The authors show that for digital approximation of fractional order systems using the indirect approach, Tustin rule can lead to ringing and should be avoided. The inverse discrete-to-continuous time operator is also presented. Numerical examples are provided. The results show that high accuracy of approximation is obtained and that the proposed method can be considered as a suitable solution, compared to classical discretization methods (Tustin).

Experimental results of a fractional order PD λ controller for vibration suppresion

Abstract: Vibration occurs in numerous important domains, such as the aerospace industry, and has negative effects since it can limit the life span of the devices, cause malfunctioning or even system instability. The necessity of research on vibration suppression in aeroplane wings is important from a scientific and technologic point of view: to provide better, more efficient, robust, cheaper solutions to eliminate vibrations effects, which triggers the socioeconomic aspect regarding the increase of flight safety and passenger comfort. Vibration suppression can be achieved using passive techniques, but they come along with certain disadvantages related to efficiency/costs. An alternative solution is to use active control. Several active control strategies have been proposed so far, with aeroplane wings/helicopter blades simulated as smart cantilever beams. The present paper proposes a fractional order Proportional Derivative controller, designed using frequency domain specifications. The case study consists in a smart beam equipped with dedicated sensors and actuators. The experimental results, considering both passive and active control responses of the smart beam, show that the proposed method leads to significant improvement of the closed loop behavior.

Fractional order modeling of diffusion processes: A new approach for glucose concentration estimation

Abstract: This paper presents the application of fractional calculus tools, i.e. fractional order impedance and Cole-Cole elements to detect, measure and estimate glucose concentrations by means of electrochemistry. Fractional calculus can provide a concise model for the description of the dynamic events that occur in biological tissues. The concepts of fractional calculus has been successfully applied in physics, chemistry and material science, electrodes and viscolelastic materials over extended ranges of time and frequency. In this paper, the fractional order impedance model is presented and compared with the measured impedance. The model parameters are related to various physical conditions of the test-cells and a baseline measurements along with blind evaluation are presented. The results indicate that the proposed model is valid in a limited range of frequencies and generalization for higher order models is possible.

Seismic Mitigation in Civil Structures Using a Fractional Order PD Controller

Abstract: In this paper, a systematic tuning procedure for a fractional order PD controller for seismic mitigation is proposed. The tuning is based upon reducing the magnitude of the compensated system at the resonance frequency as compared to the magnitude of the uncompensated structure. For simplicity, a laboratory scale 1DOF (one degree of freedom) steel structure is used as the case study, The simulation results considering the El Centro earthquake accelerograms show that the designed control strategy is highly suitable for solving seismic mitigation of steel structures and ensures improved response in comparison with steel structures equipped with passive protection. 

Autotuning method for a fractional order controller for a multivariable 13C isotope separation column

Abstract: The preferred controller design technique in industrial applications is based on autotuning procedures that do not involve knowledge about an actual mathematical model of the process. In this paper, a novel autotuning method for designing fractional order controllers is addressed. The proposed technique is simple and efficient. Previous research with respect to autotuning methods for fractional order controllers has considered exclusively the case of a single-input-single-output process. However, in this paper, a multivariable case study is preferred. The simulation results demonstrate the validity of the design technique.

Design and experimental validation of an optimal fractional order controller for vibration suppression

Abstract: In this paper, a fractional order optimal controller is designed, tested and validated experimentally for seismic mitigation in a one floor structure. The design is based on a two-step optimization procedure. The first step is concerned with the computation of the classical optimal controller gains. A second step follows that deals with the computation of an optimal fractional order parameter value that minimizes the attenuation level. The designed controller is implemented and tested experimentally on a laboratory scale civil structure. For comparison purposes, the passive seismic mitigation case is also considered, as well as the active case using the traditional optimal controller. The closed loop experimental results show that the designed fractional order optimal controller can ensure an improved attenuation level in the occurrence of a seismic event in comparison to the classical optimal controller.