Robust Feedback Control Law Research Articles

Life-extending control of fossil fuel power plants

The objective of life-extending control is to achieve a trade-off between structural durability and dynamic performance. This paper focuses on structural durability of the main steam header under load following to illustrate how the life-extending control of fossil fuel power plants can be achieved via feedforward/feedback. This concept is potentially applicable to both new and aging power plants under a variety of operational modes such as hot start-up, scheduled shutdown, and load following where the plant power is rapidly maneuvered to meet the varying load demand. The feedforward control policy is synthesized via nonlinear optimization of a multi-objective cost functional of dynamic performance and service life under the constraints of actuator saturation, operational limitations, and allowable structural damage, including thermomechanical fatigue and plastic deformation. A linear robust feedback control law that is superimposed on the feedforward sequence is synthesized based on induced L 2-norm techniques. The results of simulation experiments are presented to demonstrate that the proposed feedforward/feedback control policy is potentially capable of ramping the plant power up at a rate of 10% of the full load per minute while maintaining the specified performance and satisfying the damage constraints.

Robust crossfeed design for hovering rotorcraft

AbstractControl law design for rotorcraft fly‐by‐wire systems normally attempts to decouple the angular responses using fixed‐gain crossfeeds. This approach can lead to poor decoupling over the frequency range of pilot inputs and increase the load on the feedback loops. In order to improve the decoupling performance, dynamic crossfeeds should be adopted. Moreover, because of the large changes that occur in the aircraft dynamics due to small changes about the nominal design condition, especially for near‐hovering flight, the crossfeed design must be ‘robust’. A new low‐order matching method is presented here to design robust crossfeed compensators for multi‐input, multi‐output (MIMO) systems. The technique minimizes cross‐coupling given an anticipated set of parameter variations for the range of flight conditions of concern. Results are presented in this paper of an analysis of the pitch/roll coupling of the UH‐60 Black Hawk helicopter in near‐hovering flight. A robust crossfeed is designed that shows significant improvement in decoupling perfomance and robustness over the fixed‐gain or single point dynamic compensators. The design method and results are presented in an easily used graphical format that lends significant physical insight to the design procedure. This plant precompensation technique is an appropriate preliminary step to the design of robust feedback control laws for rotorcraft.

Modelling and Simulation of Large-Scale Multiparameter Computer Aided Dynamical Control Systems

Modelling and simulation of large-scale Multidimensional multiparameter dynamical systems require the use of large-scale computers to generate feed-back control laws, especially when model uncertainties exist. Vow robust control theory is concerned with the problem of analyzing and synthesizing control systems that provide and acceptable level of performance where many model parameters or uncertainties may exist, since mathematical models of physical systems are usually never exact due to the presence of such parameters.The need to be able to design robust feedback control laws is very important in such systems. Usually a physical model will have significant structural information about the interconnection of components and subsytems but leas information concerning their integrated system performance. Hence, many variations of parameters must be carried out on supercomputers in order to determine the more significant and sensitive parameters which must be adjusted very rapidly to accomplish a desired level of performance.Dynamical systems of the form: Eθxt, θ=Aθxt, θ+Bθut, θ;xt, θ=dxt, θdtyt, θ=Cθxt, θ+Dθut, θ;t ≥0 are considered.The elements of matrices E(θ), A(θ). B(θ), C(θ), D(θ) belong to the ring of polynomials R[θ] where θ = (θ1, θ2, θ3,...,θq) is a multiparameter and the coefficients of the polynomials belonging to C[θ] have elements belonging to the field C of complez numbers, or the elements of such multiparameter matrices may be the form f(θ) = a(θ)∣b(θ) where the polynomials a(θ). b(θ). & c(θ). for b(θ) ≠ 0. E(θ) may be a singular matrix for possible parameter values of θ. The parameter θ may also be holomorphic function of a single complex variable z for z belonging to simply-connected bounded regions in the z-plane. The basic question of stabilization, controllability, observability, etc., in the presence of changes in subsystems as regards the overall dynamical systems needs to be treated in response to changing parameters in subsystems.Fast numerical methods requiring parallel proceasing are necessary to compute adjustments as time t changes. Transfer function matrices, controllability matrices, observability matrices, feedback control laws need to be recomputed as parameters change. Such matrices may be multiparameter matrices and may allow for improvement of control laws such as in cases where E(θ) may become singular matrix and the dynamical systems require considerable fast changes in feedback control laws.Use is made of recent results o£ J. Jones. Jr. concerning generalized inverses of such multiparameter matrices to aid in computer aided changes to carry out modelling, simulation and analysis of such dynamical systems.

Improvement of helicopter handling qualities using H∞-optimisation

In the paper, we present the results of a study into the use of H∞-optimisation for the design of robust feedback control laws for improving the handling qualities of high-performance helicopters. A low-speed controller was designed to give accurate control of pitch and roll attitude, yaw rate and heave velocity for a typical combat helicopter. The design was based on a linearised model obtained from a six-degree-of-freedom nonlinear model of a helicopter provided by the Royal Aerospace Establishment (RAE), Bedford, UK. Robustness was a major issue because rotor dynamics were not included in the design model. Numerical compliance of the design with handling qualities requirements was assessed by testing simulated responses against specifications from a proposed airworthiness design standard. Robust stability was assessed from singular-value plots. In addition, the qualitative features of the design were assessed in a piloted flight simulation trial held at RAE, Bedford.

Modelling and vibration control of a flexible manipulator with three axes by using accelerometers

Industrial robots have lumped flexibility at joints as well as distributed flexibility due to the elasticity of their arms. Because of the flexibility, undesirable vibration occurs during motion. In this paper, an equivalent spring which represents the flexibility is introduced, and the equations of motion of robot links and equations of vibration of the end-effector are derived by using Lagrangian dynamics. Motor torques should be controlled in such a way that the required response of the motor positions is achieved and at the same time the vibration disappears as quickly as possible. For measuring the vibration, three accelerometers are installed at the tip of the forearm. A robust feedback control law is derived by using the root locus technique. Several successful experimental results are given.