Loop Transfer Recovery Method Research Articles

Load control optimization method for offshore wind turbine based on LTR

A simple superposition method of two separate pitch control loops is adopted in the traditional tower load control and output power control, ignoring the coupling of rotor rotation and tower movement under aerodynamic force. Besides, tower load control performance is dependent on the accuracy of the tower motion sensor, resulting in a decrease in system reliability. Therefore, a pitch controller without sensor is proposed to achieve the coordinated control of tower load and output power. First, a low-order wind turbine state–space model is derived which is suitable for the design of pitch controller and compared with the linearized model from GH Bladed to verify the rationality of the deduced model. Then, based on the theory of multi-input and multi-output (MIMO) control, a linear quadratic Gaussian (LQG) pitch controller is designed integrated with disturbance accommodation controller (DAC). The Loop transfer recovery (LTR) method is used to restore dynamic characteristics of closed-loop to improve the performance of the pitch controller. The feasibility of the LTR method is proved by detailed theoretical analysis. Finally, compared with traditional controllers, it is verified that the proposed LTR pitch controller can further reduce tower load while stabilizing output power in different wind conditions.

Design and Flight Evaluation of Primary Control System for Learjet-25B Aircraft

In this paper, a multi-input/multi-output servomechanism primary flight controller is presented. Also discussed are flight data related to the system performance and pilot assessments of handling qualities. The data were collected during flight-test experiments (March 2018) on the Calspan Variable Stability In-Flight Simulator Learjet-25B aircraft. Aircraft body angular rates and linear accelerations are used to enable decoupled command tracking in pitch, roll, and yaw axes. Angle of attack and angle of sideslip measurements are not required for the system to operate. The developed primary flight controller translates pilot controls into the corresponding pitch, roll, and yaw commands for the aircraft to track. The system consists of a gain-scheduled robust linear baseline augmented with a direct adaptive model reference output feedback. The baseline is designed via the Observer-Based Loop Transfer Recovery method. The adaptive component incorporates the closed-loop reference model, whereby a state observer from the baseline Observer-Based Loop Transfer Recovery system also acts as the reference model for the adaptive augmentation. In March 2018, the Observer-Based Loop Transfer Recovery controller was successfully flight verified on the Calspan Learjet-25B aircraft, at the U.S. Air Force Test Pilot School, Edwards Air Force Base, California. Flight-test results are discussed, and a brief summary is given at the end.

On Guaranteeing Convergence of Discrete LQG/LTR When Augmenting It With Forward PI Controllers

Using the loop transfer recovery (LTR) method to recover the linear quadratic Gaussian (LQG) robustness properties is a well-established procedure, as well as augmenting the system with integrators at the plant input to deal with steady-state error. However, when using the discrete version of the LQG/LTR controller, simply using integrators discretized by the forward Euler method does not guarantee recovery convergence. This paper presents a solution: augmenting the system with a PI controller. A control moment gyroscope is used to apply this technique, and its modeling process is showed, along with its linearization and discretization. Particularly, it presents a resonance due to nutation frequency, which is damped in an inner loop prior to the robust control design by simple velocity feedback. Particle swarm optimization is applied aiming to shape the target open loop and to guarantee set point, disturbance and measurement noise robustness. At last, real experiments are conducted to corroborate the presented method.

Design of optimal disturbance cancellation controllers via modified loop transfer recovery

The authors proposed a modified loop transfer recovery (LTR) method for designing the disturbance cancellation controllers where the disturbances were assumed to be step functions and the optimality of the controllers was not considered. This paper discusses the extension of their work to a more general class of disturbances with optimality consideration. It is assumed that the plant is minimum phase and the disturbances entering the plant input side are generated by function generators with unknown initial conditions. A quadratic performance index explicitly representing the disturbance cancellation requirement is introduced. The optimal disturbance cancellation controller minimizing the performance index is constructed by the separation principle. As a target for the LTR design, the optimal disturbance cancellation controller based on the measurement of the plant state is chosen. It is shown that the target feedback property can be recovered in the output feedback controller by a simple modification of the standard LTR procedure. A numerical example is presented to illustrate the effectiveness of the proposed optimal design.

Lateral Flight Technical Error Estimation Model for Performance Based Navigation

Flight technical error (FTE) combined with navigation system error (NSE) is the main part of total system error (TSE) in performance based navigation (PBN). The implementation of PBN requires pre-flight prediction and en-route short-term dynamical prediction of the TSE. Once the sum of predicted lateral FTE and NSE is greater than the specified PBN value, the PBN cannot operate. Thus, accurate modeling and thorough analysis of lateral FTE are indispensible. Multiple-input multiple-output (MIMO) lateral track control system of a transport aircraft is designed using linear quadratic Gaussian and loop transfer recovery (LQG/LTR) method, and the lateral FTE of a turbulence disturbed approach operation is analyzed. The error estimation mapping function of latera FTE and its bound estimation algorithm are proposed based on singular value theory. According to the forming mechanism of lateral FTE, the algorithm considers environmental turbulence fluctuation disturbance, aircraft dynamics and control system parameters. Real-data-based Monte-Carlo simulation validates the theoretical analysis of FTE. It also shows that FTE is mainly caused by turbulence fluctuation disturbance when automatic flight control system (AFCS) is engaged and would increase with escalating environmental turbulence intensity.

Two-step design of critical control systems using disturbance cancellation integral controllers

An efficient critical control system design is proposed in this paper. The key idea is to decompose the design problem into two simpler design steps by the technique used in the classical loop transfer recovery method (LTR). The disturbance cancellation integral controller is used as a basic controller. Since the standard loop transfer recovery method cannot be applied to the disturbance cancellation controller, the nonstandard version recently found is used for the decomposition. Exogenous inputs with constraints both on the amplitude and rate of change are considered. The majorant approach is taken to obtain the analytical sufficient matching conditions. A numerical design example is presented to illustrate the effectiveness of the proposed design.

Optimal control of fuel processing system using generalized linear quadratic Gaussian and loop transfer recovery method

This paper proposes an optimal control system that consists of both feedforward and state-feedback controllers designed using a generalized linear quadratic Gaussian and loop transfer recovery (GLQG/LTR) method for a fuel processing system (FPS). This FPS uses natural gas as fuel and reacts with atmospheric air through a catalytic partial oxidation (CPO) response. The control objective is focused on the regulatory performance of the output vector in response to a desired stack current command in the face of load variation. The proposed method provides another degree of freedom in the optimal control design and gives the compensated system a prescribed degree of stability. Finally, the numerical simulations of compensated FPS reveal that the proposed method displays better performance and robustness properties in both time-domain and frequency-domain responses than those obtained by the traditional LQ Method. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society

LTR design of disturbance cancellation integral controllers for time-delay plants

For time-delay plants, the loop transfer recovery (LTR) technique for the integral controller design based on disturbance cancellation is discussed. Since the design requires an unstabilizable extended plant, the standard LTR method cannot be applied. The new LTR method proposed by the authors for lumped plants is extended to time-delay plants by introducing appropriate predictors. A simple design example is presented to illustrate the proposed design method.

Optimal control design of fuel processing system by linear quadratic Gaussian and loop transfer recovery method

This paper presents an optimal control system which consists of both feedforward and state‐feedback controllers designed using a well‐developed linear quadratic Gaussian and loop transfer recovery (LQG/LTR) method for a fuel processing system (FPS). This FPS uses natural gas as fuel and reacts with atmospheric air through a catalytic partial oxidation (CPO) response. The control objective is focused on the regulatory performance of output vector in response to a desired stack current command in face of load variation. First, a Kalman filter is designed to provide an optimal estimation of state variables and to shape the target feedback loop function. And then an optimal two‐degree‐of‐freedom controller is designed subject to a linear quadratic performance index in the LTR process. Finally, the numerical simulations of compensated FPS reveal that the proposed method achieves better performance and robustness properties than presently accepted methods, in both time‐domain and frequency‐domain responses.

Optimal Control of Fuel Processing System Using Generalized Linear Quadratic Gaussian and Loop Transfer Recovery Method

This paper originally proposes an optimal control system which consists of both feedforward and statefeedback controllers using a generalized linear quadratic Gaussian and loop transfer recovery (GLQG/LTR) method. The control objective is focused on the regulatory performances of output vector in response to a desired stack current command in face of load variation. The proposed method provides another degree-of-freedom in optimal controller design and makes the compensated system have a prescribed degree of stability. Finally, the numerical simulations of a compensated fuel processing system reveal that the proposed method achieves better performance and robustness properties in both time-domain and frequency-domain responses than those obtained by the traditional LQ Method.

Integral controller design based on disturbance cancellation: Partial LTR approach for non-minimum phase plants

For non-minimum phase plants, the loop transfer recovery (LTR) design of integral controllers based on the disturbance cancellation is considered. Since the design requires an unstabilizable extended plant, the standard LTR method cannot be applied. A new partial LTR method is proposed to overcome the difficulty. The target of the proposed design is a fictitious controller including a disturbance estimator based on the measurement of the minimum phase state. It is shown that the Riccati equation including the gain matrix of the disturbance estimator in the target can be used to recover the target feedback property in the output feedback controller. A simple design example is presented to show the effectiveness of the proposed method.

Active Roll Control of Single Unit Heavy Road Vehicles

Summary Strategies are investigated for controlling active anti-roll systems in single unit heavy road vehicles, so as to maximise roll stability. The achievable roll stability improvements that can be obtained by applying active anti-roll torques to truck suspensions are discussed. Active roll control strategies are developed, based on linear quadratic controllers. It is shown that an effective controller can be designed using the LQG approach, combined with the loop transfer recovery method to ensure adequate stability margins. A roll controller is designed for a torsionally flexible single unit vehicle, and the vehicle response to steady-state and transient cornering manoeuvres is simulated. It is concluded that roll stability can be improved by between 26% and 46% depending on the manoeuvre. Handling stability is also improved significantly.

AE–Automation and Engineering Technologies: Optimal Header Height Control System for Combine Harvesters

Automatic control of header height has been employed in combine harvesters as a means to reduce harvest losses, operator fatigue and the risks of equipment damage. State-of-the-art combine harvesters usually incorporate on–off controllers to keep the header at the desired height. The fixed control signals and the relatively broad dead-bands required for stabilization of this type of controller impose serious limitations on the performance of such systems. This paper describes an alternative control system aiming to improve the performance of combine harvesters in following the soil profile. The linear quadratic Gaussian with loop transfer recovery (LQG/LTR) method has been used to design a robust and optimal header height controller. The results obtained indicate that the use of the LQG/LTR controller can significantly improve the disturbance rejection capacity of the system, while keeping its energy expenditure levels unchanged with respect to the conventional on–off system.

Load-Following Voltage Controller Design for a Static Space Nuclear Power System

The reliability of static space nuclear power systems (SNPSs) could be improved through the use of backup devices in addition to shunt regulators, as currently proposed for load following. Shunt regulator failure leading to reactor shutdown is possible, as is the possible need to deliver somewhat higher power level to the load than originally expected. A backup system is proposed in SNPSs to eliminate the possibility of a single-point failure in the shunt regulators and to increase the overall system power delivery capability despite changing mission needs and component characteristics. The objective of this paper is to demonstrate the feasibility of such a backup device for voltage regulation in static SNPSs that is capable of overcoming system variations resulting from operation at different power levels. A dynamic compensator is designed using the Linear Quadratic Gaussian with Loop Transfer Recovery method. The resulting compensators are gain scheduled using the SNPS electric power as the scheduling variable, resulting in a nonlinear compensator. The performance of the gain-scheduled compensator is investigated extensively using an SNPS simulator. The simulations demonstrate the effects of the fuel temperature reactivity coefficient variations on the load-following capabilities of the SNPS. Robustness analysis results demonstrate that the proposed controller exhibits significant operational flexibility, and it can be considered for long-term space mission requiring significant levels of autonomy.

An Application for Tank System and its Analysis in Frequency Domain of Two-Degree-of-Freedom Optimal Type-1 Servo System

In this paper, we do an experimental study of two-degree-of-freedom optimal type-1 servo system based on the LQ (linear quadratic) method using a tank system. First, the experiments for the state feedback system which takes into account of a trade-off between low frequency sensitivity and guaranteed stability margin by using an integral gain are carried out. Next, we extend to the output feedback case and carry out the experiments to show the effectiveness of the LTR (loop transfer recovery) method or the integral gain. As the results of these experiments and its analysis in frequency domain, the influence of an observation noise which appears in the control input, the effect of disturbance rejection and robust stability for a plant perturbation are clarified.

Active Control of Store-Induced Flutter in Incompressible Flow

An active method for enhancing the performance and improving the robustness of a decoupler pylon-mounted store flutter suppression system is presented. The proposed active decoupler pylon involves the use of a piezoceramic wafer strut as an actuator that acts as a soft spring between the wing and the store. A two degree-of-freedom typical section of an airfoil is used to represent the structural model of the wing. The circulatory component of the incompressible aerodynamic loads is modeled using an approximation to the Theodorsen function. The GBU-8/B store configuration of an F-16 aircraft is used for the analysis. A classical robust control algorithm called the loop transfer recovery method is applied to design a controller for the linear wing/store model. Some simulations are presented using singular-value Bode plots for robust stability and nominal performance analysis.

Multivariable Neural Network Based Controllers for Smart Structures

This paper details identification and robust control of smart structures using artificial neural networks. To demonstrate the use of artificial neural networks in the control of smart structural systems, two smart structure test articles were fabricated. Active materials like piezoelectric (PZT), polyvinylidene (PVDF) and shape memory alloys (SMA) were used as actuators and sensors. The Eigensystem Realization Algorithm (ERA), a structural identification method has been utilized to determine a minimal order discrete time state space model of the test articles. The ERA requires the Markov parameters of the physical system. A neural network based method has been developed to estimate the Markov parameters of a multi input multi output system from experimental test data. The accelerated adaptive learning rate algorithm and the adaptive activation function were utilized to improve the learning characteristics of the network and reduce the learning time. The identified models were used to design a robust controllers for vibration suppression of smart structures using a modified Linear Quadratic Gaussian with Loop Transfer Recovery (LQG/LTR) method. This control design methodology has better loop transfer recovery properties while accommodating the limited control force available from the SMA and the PZT actuators. This controller was copied into a feedforward neural network using the connectionist approach. This neural network controller was implemented using a PC based data acquisition system. The closed loop performance and robustness properties of the conventional and the neural network based controller are compared experimentally.

Further development of an approximate loop transfer recovery methodology for fixed order dynamic compensator design

AbstractThis paper further develops a methodology for designing fixed order compensators. The formulation uses an approximate loop transfer recovery (LTR) method to achieve performance and stability subject to a disturbance and unstructured uncertainty at either the plant input or the plant output. The approximate LTR methodology has the advantage that the quadratic performance index weighting matrices are uniquely defined. To reduce sensitivity to real parameter uncertainty, the performance index is modified to include a penalty on the closed‐loop sensitivity dynamics. The formulation avoids the explicit introduction of sensitivity states into the performance index and thus does not increase the overall dimension of the problem. An example design for the coupled mass benchmark control problem is provided to demonstrate this methodology.

Crushing mill control for sugar cane - a robust, nonadaptive LQG/LTR strategy

Abstract Milling of sugar cane is concerned with the extraction of sugar bearing juice by crushing in a cascaded series of mills. The problem dealt with in this paper is the design and testing of a mill control system to provide good extraction in spite of large variations in the milling properties of the cane. The control strategy adopted is based on a Linear Quadratic Gaussian with Loop Transfer Recovery (LQG/LTR) method to reject the disturbances due to the cane changes and modified to accommodate plant constraints.

Crushing Mill Control for Sugar Cane - A Robust, Nonadaptive Strategy

Milling of sugar cane is concerned with the extraction of sugar bearing juice by crushing in a cascaded series of mills. The problem dealt with in this paper is the design and testing of a mill control system to provide good extraction in spite of large variations in the milling properties of the cane. The control strategy adopted is based on a Linear Quadratic Gaussian with Loop Transfer Recovery (LQG/LTR) method toreject the disturbances due to the cane changes and modified to accommodate plant constraints. By judicious selection of the disturbance model, a robust linear control strategy is developed in which the variety changes are followed using a state estimator. This is a strategy which is widely applicable in place of fully adaptive approaches. The ensuing controller was tested on the operating mill with some qualified success. The results and conclusions are presente