Sequential Loop Closure Research Articles

Bayesian Optimization for automatic tuning of digital multi-loop PID controllers

In recent years, the use of Bayesian optimization (BO) for efficient automatic tuning of general controller structures through iterative closed-loop experiments, has been attracting increasing interest. However, its potential for tuning interactive multi-loop PID controllers in Multi Input Multi Output (MIMO) processes remains largely unexplored. Even though the optimization domain greatly affects closed-loop performance and safety, it is usually defined manually, through expert knowledge or experimentation. This paper presents a novel systematic methodology for defining the optimization domain for automatic multi-loop PID tuning using BO. Sequential loop closing, system identification and tuning relations are used to constrain the bounds on controller parameters to meaningful ranges, including gains and sampling times. This provides an effective way to improve the convergence of BO and secure process safety during closed-loop experiments, without requiring a MIMO process model or extensive prior knowledge. The methodology can be applied ”as is” to single-loop PID controllers.

Constrained state feedback pole placement of coupled lateral plant dynamics of RLV during the reentry phase

Stability and dynamic behaviour of a linear multivariable system mainly depend on the pole locations of the plant in the complex s plane. For general linear time-invariant multi-input multi-output (MIMO) systems, static (constant gain) output feedback pole-placement is an open nondeterministic polynomial time (NP) hard problem. This problem has attracted much attention from the control community for the last four decades. Even today, the general problem has not been solved analytically because of its highly nonlinear nature. In this paper, we have addressed the issue of finding gains without using the procedure of sequential loop closure. We have proposed an iterative numerical algorithm to solve the pole placement problem with a minimum number of feedback gains. The proposed method is used to compute the state feedback gain matrix for coupled yaw-roll dynamics of a typical reusable launch vehicle (RLV) during the reentry phase.

Controller Design of UPQC for Enhancing Power Quality in Distribution System

This paper presents the procedure of PI controller design of Unified Power Quality Conditioner (UPQC) for improving power quality in the distribution system. A mathematical model of the distribution system with the UPQC is thoroughly explained in this paper. The mathematical model that is founded has been used for designing the PI controller. In order to investigate the dynamic performance and design the controller, linear approximation technique is used. Linearization of the systems that is around the operating point provides a set of linear equations, and then multi-input multi-output (MIMO) transfer functions of the systems are obtained. Two controller design techniques are presented and compared. One is the sequential loop closure controller design with parameter tuning through the Bode diagram (SLC-BD). The other one is the controller optimization design with the genetic algorithms (CO-GA). The performances of these techniques are verified and compared by using MATLAB program. Simulation results reveal that the power quality in the distribution system can be enhanced by using UPQC with the proposed controller design.

Practical Methods for Aircraft and Rotorcraft Flight Control Systems: An Optimization-Based Approach [Bookshelf

This book provides information covering the fundamentals of control theory and optimization applied to real-world flight control systems. Covers the following topics: the challenges inherent in flight control system design; an overview of their core approach to flight control system design utilizing multiobjective parametric optimization; the specific software developed called Control Designer’s Unified Interface (CONDUIT); detailed explanations of the tools, plots, language, and four key components involved in defining the problem using CONDUIT; the XV-15 hover/forward flight case study is discussed in sufficient detail for even seasoned veterans to find the presentation informative and useful. Here the authors have done an excellent job of demonstrating many flight control concepts; proprietary specifications; methodologies for developing simulation models of model-based flight control system designs; design of flight control systems; the inner workings of the design optimization approach. The coverage includes specifics of various algorithms, along with their relative advantages and disadvantages. This chapter also provides insights into sequential loop closure strategies, with emphasis on their effectiveness and practical use; nested versus simultaneous multiloop strategies; and design considerations for real-world applications. This book serves as an excellent reference for senior undergraduate and/or graduate students, and it provides a wellbalanced perspective covering the theory, implementation, and practical applications of flight control design.

Sequential Loop Closure Based Adaptive Output Feedback

This paper presents a new, systematic method of synthesizing an output feedback adaptive controller for a class of uncertain, non-square multi-input/multi-output systems. The control design process consists of first designing an inner-loop controller for a reduced order plant model to enforce command tracking of selected inner-loop variables, with an adaptive element used to accommodate parametric uncertainties in the plant. Once this inner-loop control design is complete, an outer-loop is then designed, which prescribes the inner-loop commands to enforce command tracking of selected outer-loop variables. The main challenge that needs to be addressed when designing the inner-loop controller is the determination of a corresponding square and strictly positive real transfer function. This is accomplished by appropriate selection of two gain matrices that allow the realization of such a transfer function, thereby allowing a globally stable adaptive output feedback law to be generated. The outer-loop controller is designed around the plant with existing adaptive inner-loop controller such that global stability of the closed-loop system is guaranteed. The design of the outer-loop uses components of a closed-loop reference model in a judicious manner which enables a modular approach, without requiring any re-design of the inner-loop controller. In addition, this architecture facilitates the use of an additional state-limiter to enforce desired limits on the state variables. A numerical example based on a scramjet powered, generic hypersonic vehicle model is presented, demonstrating the efficacy of the proposed control design.

Constant speed control of four-stroke micro internal combustion swing engine

The increasing demands on safety, emission and fuel consumption require more accurate control models of micro internal combustion swing engine (MICSE). The objective of this paper is to investigate the constant speed control models of four-stroke MICSE. The operation principle of the four-stroke MICSE is presented based on the description of MICSE prototype. A two-level Petri net based hybrid model is proposed to model the four-stroke MICSE engine cycle. The Petri net subsystem at the upper level controls and synchronizes the four Petri net subsystems at the lower level. The continuous sub-models, including breathing dynamics of intake manifold, thermodynamics of the chamber and dynamics of the torque generation, are investigated and integrated with the discrete model in MATLAB Simulink. Through the comparison of experimental data and simulated DC voltage output, it is demonstrated that the hybrid model is valid for the four-stroke MICSE system. A nonlinear model is obtained from the cycle average data via the regression method, and it is linearized around a given nominal equilibrium point for the controller design. The feedback controller of the spark timing and valve duration timing is designed with a sequential loop closing design approach. The simulation of the sequential loop closure control design applied to the hybrid model is implemented in MATLAB. The simulation results show that the system is able to reach its desired operating point within 0.2 s, and the designed controller shows good MICSE engine performance with a constant speed. This paper presents the constant speed control models of four-stroke MICSE and carries out the simulation tests, the models and the simulation results can be used for further study on the precision control of four-stroke MICSE.

Design of remotely located and multi-loop vibration controllers using a sequential loop closing approach

In some applications, vibration control objectives may require reduction of levels at locations where control system components cannot be sited due to space or environmental considerations. Control actuators and error sensors for such a scenario will need to be placed at appropriate locations which are potentially remote from the points where ultimate attenuation is desired. The performance of the closed loop system, therefore, cannot be assessed simply by the measurement obtained at this local error sensor. The control design objective has to take into account the vibration levels at the remote locations as well. A design methodology was recently proposed that tackles such problems using a single-loop feedback control architecture. The work in this paper describes an extension of this control design procedure to enable the systematic design of multiple decentralised control loops. The approach is based upon sequential loop closing and conditions are provided that ensure that closed loop stability is maintained even in the event of failure in some control loops. The design procedure is illustrated through its application to a laboratory scale slab floor that replicates the problems associated with human induced vibration in large open-plan office buildings. The experimental results demonstrate the efficacy of the approach and significant suppression of the dominant low frequency modes in the floor is achieved using two independent acceleration feedback control loops.

Recent developments on applications of sequential loop closing and diagonal dominance control schemes to industrial multivariable system

With a focus on an industrial multivariable system, two subsystems including the flow and the level outputs are analysed and controlled, which have applicability in both real and academic environments. In such a case, at first, each subsystem is distinctively represented by its model, since the outcomes point out that the chosen models have the same behavior as corresponding ones. Then, the industrial multivariable system and its presentation are achieved in line with the integration of these subsystems, since the interaction between them can not actually be ignored. To analyze the interaction presented, the Gershgorin bands need to be acquired, where the results are used to modify the system parameters to appropriate values. Subsequently, in the view of modeling results, the control concept in two different techniques including sequential loop closing control (SLCC) scheme and diagonal dominance control (DDC) schemes is proposed to implement on the system through the Profibus network, as long as the OPC (OLE for process control) server is utilized to communicate between the control schemes presented and the multivariable system. The real test scenarios are carried out and the corresponding outcomes in their present forms are acquired. In the same way, the proposed control schemes results are compared with each other, where the real consequences verify the validity of them in the field of the presented industrial multivariable system control.

Nonlinear Robust Adaptive Control of Flexible Air-Breathing Hypersonic Vehicles

This paper describes the design of a nonlinear robust adaptive controller for a flexible air-breathing hypersonic vehicle model. Because of the complexity of a first-principle model of the vehicle dynamics, a control-oriented model is adopted for design and stability analysis. This simplified model retains the dominant features of the higher-fidelity model, including the nonminimum phase behavior of the flight-path angle dynamics, the flexibility effects, and the strong coupling between the engine and flight dynamics. A combination of nonlinear sequential loop closure and adaptive dynamic inversion is adopted for the design of a dynamic state-feedback controller that provides stable tracking of the velocity and altitude reference trajectories and imposes a desired set point for the angle of attack. A complete characterization of the internal dynamics of the model is derived for a Lyapunov-based stability analysis of the closed-loop system, which includes the structural dynamics. The proposed methodology addresses the issue of stability robustness with respect to both parametric model uncertainty, which naturally arises when adopting reduced-complexity models for control design, and dynamic perturbations due to the flexible dynamics. Simulation results from the full nonlinear model show the effectiveness of the controller.

Obtaining multi-loop pursuit-control pilot models from computer simulation

A method for generating simplified pursuit-control pilot models for computer simulation of multi-axis flight control tasks has been developed. The method involves a sequential loop closure synthesis procedure for creating the pilot model and includes handling qualities estimation. The original model formulation previously reported in the literature used frequency-domain techniques, primarily Bode diagrams to select model gains. The present research demonstrates how similar results can be obtained in the time-domain. This latter approach is particularly useful when complex, non-linear aircraft models are being used. The time-domain approach is exercised in a six-degree of freedom rotorcraft control simulation and in a six-degree of freedom tailless fighter simulation, both involving linear models.

Control properties of bottom fired marine boilers

Abstract This paper focuses on model analysis of a dynamic model of a bottom fired one-pass smoke tube boiler. Linearized versions of the model are analyzed and show large variations in system gains at steady state as function of load whereas gain variations near the desired bandwidth are small. An analysis of the potential benefit from using a multivariable control strategy in favor of the current strategy based on single loop theory is carried out and proves that the interactions in the system are not negligible and a subsequent controller design should take this into account. A design using dynamical decoupling showed substantial improvement compared to a decentralized scheme based on sequential loop closing. Similar or better result is expected to be obtainable using a full Multiple input Multiple output scheme. Furthermore closed loop simulations, applying a linear controller to the nonlinear plant model, prove that the model does not call for nonlinear control. However, the results indicate that input constraints will become active when the controller responds to transient behavior from the steam flow disturbance. For this reason an MPC (model predictive control) strategy capable of handling constraints on states and control signals should be considered.

Dynamic Model of a Matrix Converter for Controller Design and System Studies

This paper presents a systematic approach to develop a dynamic model for a matrix converter. The matrix converter interfaces two three-phase ac systems that can be at different voltages and/or frequencies, e.g., a high-speed micro-turbine generator and a utility distribution system. The model is developed by (i) transferring the abc frame equations to a newly defined switching reference frame and then (ii) transferring the resultant equations to a rotating dqo frame. The model is then linearized and the concept of sequential loop closing is used to design a controller for the matrix converter as a multi-input multi-output system. The accuracy of the developed model and performance of the controller are evaluated based on comparing the study results with those obtained from digital time-domain simulations in the PSCAD/EMTDC software environment.

Disturbance attenuation in precise hexapod pointing using positive force feedback

Negative feedback control improves system performance by high loop gain. However, it is well known that the achievable performance is limited theoretically by right half complex plane (RHP) poles, RHP zeros, time delays, etc. of the plant models, and practically by the input constraints as well as model uncertainties. Though positive feedback has the potential to improve the system performance by direct subtraction of the measured external disturbances from the plant inputs, it is notorious for its sensitivity to measurement errors and lack of robustness. Inappropriate use of positive feedback control could deteriorate the performance, or even result in instability. This paper presents a new combined control strategy for two degrees-of-freedom precise hexapod pointing control, in which positive force feedback (PFF) is used to reject payload disturbances, and extra acceleration feedback loops are added to enhance the PFF loop robustness as well as decrease its sensitivity. Also, a new method for avoiding destructive interference in parallel controller design using a sequential loop closure technique is proposed. Since the payload disturbances cannot be measured directly due to sensor mounting difficulties, an estimate is constructed based on the hexapod model and used for PFF. The algorithm is implemented on one of the University of Wyoming (UW) hexapods, and experimental results demonstrate that pointing errors caused by the payload disturbances are decreased, despite residual coupling and disturbance estimation errors.

A Methodology for Autotuning of Multivariable Systems

A methodology for autotuning decentralized proportional-integral-derivative (PID) controllers for multivariable systems is proposed. It combines the autotuning procedure involving sequential loop closing and relay tests with time-domain curve fitting via least squares to identify suitable transfer function models that account for interactions among the loops, using limited response data. Simulation results reveal that the curve fitting is able to identify models using only the first few oscillations of the relay-test response. Thus, there is no need to wait for the process to reach steady oscillations. A controller tuning method developed by Semino and Scali (J. Process Control 1998, 8 (3), 219-227) is adopted to tune the PI controllers. Detailed results on several 2x2, 3x3, and 4x4 multivariable processes show that the proposed methodology gives PI controllers having performances comparable with those tuned by the ATV+ method of Parabita et al. (Sequential Identification and Autotuning by Relay Techniques of Decentralised Controllers for MIMO Processes, ADCHEM 2000, Pisa, Italy, 2000) or Luyben's BLT technique (Ind. Eng. Chem. Process Des. Dev. 1986, 25 (3), 654-660). The major advantage of the proposed autotuning with least-squares is the significant reduction in the duration of relay tests. Computations involved in the methodology can be carried out on a personal computer in reasonable time.

Online Tuning Strategy for Multi-loop SISO PI Control Algorithms in Multivariable Interactive Systems

Tuning of PI control algorithms for coupled multi input multi output (MIMO) systems is a challenging problem. This paper extends a previously developed model-based adaptive tuning method to handle the tuning problem of coupled multivariable systems. The performance of the proposed method is compared to those of existing methods such as Biggest Log Modulus (BLT), Sequential Loop Closing (SLC) and Multi-loop Modified Ziegler-Nichols (MMZN). The proposed method is not limited to any special form of process models and allows the use of a digital control law formulation. The tuning procedure is cast as a constrained least squares problem which can be easily solved enhancing its on-line implementation. Linear constraints are imposed on the least square problem to account for loop interactions. Application of the tuning algorithm to two simulated MIMO examples revealed its ability to perform as well as the other tested methods.

Sequential loop closing identification of multivariable process models

Sequential loop closing (SLC) method is one of the well-known methods to tune multiloop control systems for multivariable processes. In the method, each controller is designed sequentially with single-input single-output methods by finding the transfer function for the paired input and output while former loops have been closed. Utilizing the single-input single-output nature in tuning each controller, autotuning methods can also be applied. However, sometimes iterations are required for better performance. Especially, if pairing is undesirable, the multiloop control system designed with the autotuning SLC method does not show the best performance and tuning should be discarded and repeated totally for the correct pairing. Here, to avoid this, multivariable process models are identified while loops are being tuned. The identified models can be used to correct the pairing and to improve the multiloop control systems. Field experiments needed are just the same as the autotuning SLC method.

Robust feedback control of flow-induced structural radiation of sound

The paper presents the robust multivariable control of noise radiation from panels driven by turbulent flow. The controlled inputs are point forces to the panel and the feedback signals are panel accelerations. Collocated actuator/sensor locations on the panel are chosen such that vibration control will lead to a reduction in the radiated sound pressure levels. The multivariable controller is realized though a sequential loop closure approach. Each controller is designed in the frequency domain where the impact of system uncertainty and closed-loop sensitivity can be analyzed. The proposed frequency domain approach will also facilitate the design of feedback controllers that employ the natural dynamics of the panel for noise control purposes. The proposed design technique is experimentally validated on a panel subjected to turbulent flow in a quiet wind tunnel. Active control of the sound radiation was achieved at frequencies as high as 1000 Hz. Sound pressure level reductions of as much as 15 dB were achieved at the resonant frequencies of the panel modes. Overall reductions over the 100-1000 Hz band were approximately 5 dB.

Sequential Loop Closure in Design of a Robust Rotorcraft Flight Control System

Rotorcrafte ightcontrolsystemspresentmanysignie cantdesignchallenges.First,largevariationsintheresponse characteristics of rotorcraft result from the wide range of airspeeds of typical operation (hover to over 100 kn ). Second, theassumption ofvehiclerigidity often employed in the design of e xed-wing e ight controlsystems israrely justie ed in rotorcraftwhererotordegreesof freedom can havea signie cant impact on the system performance and stability. A methodology is proposed for the design of robust rotorcraft e ight control systems utilizing quantitative feedback theory (QFT). QFT is a technique that accounts for variability in the dynamic response of the controlled element in thedesign of robust control systems. It was developed to address a multiple-input/single-output (MISO) design problem,buttheextension ofthistechniquetoaddressmultiple-input/multiple-output (MIMO)systemswas employed to address the e ight control system (FCS) design of a UH-60 Black Hawk Helicopter. This was accomplished by constructing a set of MISO systems mathematically equivalent to the MIMO system. QFT was applied to each member of the set independently. Inherent conservatism in this design technique leads to limitations in its utility. A second approach utilizing the sequential closure of control loops proved an improved method for designing of robust MIMO FCSs. An analysis of the two FCS design methodologies is presented.

Approximations for Quantitative Feedback Theory Designs

The computational requirements for obtaining the results summarized in the preceding section were very modest and were easily accomplished using computer-aided control system design software. Of special significance is the ability of the PDT to indicate a loop closure sequence for MIMO QFT designs that employ sequential loop closure. Although discussed as part of a 2 x 2 design, the PDT is obviously applicable to designs with a greater number of inputs and system responses.

Frequency Based Multivariable Feedback Controller Design

Presented in this paper is a multivariable feedback controller design methodology for regulating systems required to satisfy hard time domain tolerances despite a vector of external step disturbances. In addition to satisfying time domain tolerances, this design methodology seeks to minimize the bandwidth of each feedback controller so that the cost of feedback is minimized. The two key contributions of this work are (i) the rigorous justification of enforcing time domain tolerances via frequeucy domain amplitude inequalities, and (ii) the design of a fully populated, multivariable controller via sequential loop closure. This multivariable controller design methodology will be demonstrated on the control of an distillation column.