Reduced Control Effort Research Articles

Control of gas turbine power plants by means of the weighted one-step-ahead adaptive technique

The one-step-ahead controllers represent a branch of minimum prediction error adaptive controllers. They combine the parameter estimation of the controlled system model with the control scheme; therefore, they are especially suitable for non-linear and time-varying systems. Since the estimated parameters are updated at each time step (by using the sampled data), these methods can be adopted for non-linear applications. Consequently, the one-step-ahead controllers do not require knowledge of the dynamic characteristics of the controlled system (e.g. state-space systems or transfer functions). Sometimes, in the gas turbine field, the control system could produce an excessive control effort, due to sudden variations of the electric load. In order to reduce this control action, the weighted one-step-ahead adaptive (WOSAA) control algorithm considers a penalty associated with the control effort using an appropriate cost function. In this way, the control variable does not assume values that are too large, even when the gas turbine undergoes sudden changes in the external load. As a consequence, the robustness and stability features of the WOSAA control system are increased. The results show that the WOSAA control technique, applied to both the double-shaft aero-derivative gas turbine and the single-shaft heavy-duty gas turbine, effectively counteracts the load reduction with limited overshoot in the controlled variables with reduced control effort.

Arbitrarily fast and robust tracking by feedback

The output of a singe-input-single-output linear feedback system with more than one pole in excess over the zeros in the loop transmission cannot track arbitrarily fast its input (by the root locus). In this work we extend the linear feedback so that some of the open loop poles may depend on the open loop gain; we call this new class quasi-linear feedback systems. We then derive time domain, pole-zero, and frequency domain conditions which ensure arbitrarily fast and robust tracking by quasi-linear feedback, for an arbitrary number of poles in excess over the zeros. We prove that in a particular case these conditions are equivalent, and that the boundedness in frequency of the closed loop transfer function is no longer necessary for achieving arbitrarily fast tracking. The robustness is to external disturbances and initial conditions, and the open loop has to be minimum phase. Some examples are presented which illustrate these results. They also show that this good performance can be obtained with a reduced control effort, and that quasi-linear feedback can alleviate the limitation on performance of non-minimum phase open loops.

Implications of using colocated strain-based transducers for output active structural acoustic control.

Piezoceramic transducers have become popular elements of smart structures that are used for active vibration control and active structural acoustic control. A spatial differentiation is performed by the piezoceramic transducers since they couple into the strain field of the piezostructure. This differentiation causes higher-frequency modes to be emphasized more heavily, causing the effective compliance of the structure as seen by the piezoceramic transducer to increase with frequency. This nonuniform compliance has significant impact on the performance that can be achieved through colocated direct rate feedback control. It is shown that the rectangular piezoceramic transducer is a low-pass wave number filter with a cutoff frequency inversely proportional to the aperture size. Thus DRFB performance can be greatly improved simply by making the size of the piezoceramic transducer large relative to the size of the structure. The resulting increase in coupling to the lower-frequency modes, which are generally targeted by the control system, results in a much reduced control effort. In the event that a large aperture is not practical, it is shown that dynamic compensation can be used to obtain good performance at the cost of much increased computational complexity. Analytical open and closed loop results for an acoustically radiating simply supported plate piezostructure are presented.

Robust adaptive output-feedback control of a class of uncertain nonlinear systems

With an attempt to simplify the controller design and to reduce the control effort, a new robust output-feedback controller is designed to solve the tracking problem of a class of nonlinear uncertain systems by using observer plus backstepping. In the controller design, a simple observer is constructed, by which only a coarse estimate of the unknown states is provided, and a positive function used to generate a new normalization signal and the positive nonlinear functions used in nonlinear damping terms are all left to be chosen freely. Thanks to these techniques, the new controller can be designed with a simpler structure and reduced control effort under weaker assumptions on both the uncertainties and the reference signal. The global boundness of all closed-loop signals can be guaranteed, and the output tracking error can be made as small as possible

Adaptive control of parametric-strict-feedback systems with reduced control effort

In order to reduce the large control effort needed by the adaptive controller proposed in [1] in the transient period, the regulation problem of parametric-strict-feedback system is restudied and resolved by reformulating it as a tracking problem which aims to asymptotically track the output of a chosen linear stable model. By choosing the model properly, the control effort needed by the adaptive controller proposed in [1] can be greatly reduced while similar or better performance can be achieved. This can be seen clearly in our simulations.

Terminal sliding mode control for rigid robots

This paper presents a new design of terminal sliding mode control for n-link rigid robot manipulators. By exploring thoroughly some properties of robot manipulators, the proposed scheme simplifies significantly the controller designs given in Venkataraman and Gulati (1989) and Man et al. (1994). Compared to the linear sliding mode control, the main features of this terminal sliding mode control are the reduced tracking error in the steady state and reduced control efforts in the transient state. Experiment results are included to show the performance of the proposed controller.

Discrete-time quasi-sliding-mode control strategies

In this paper, discrete-time quasi-sliding-mode control systems are considered. A new definition describing the quasi-sliding mode as a motion of the system, such that its state always remains in a certain band around the sliding hyperplane, is introduced. Then, two novel reaching laws satisfying conditions of the definition are proposed and applied to the design of appropriate linear control strategies which drive the state of the controlled system to a band around the sliding hyperplane. Consequently, the undesirable chattering and high-frequency switching between different values of the control signal are avoided. The strategies, when compared with previously published results, guarantee better robustness, faster error convergence, and improved steady-state accuracy of the system. Furthermore, better performance of the system is achieved using essentially reduced control effort.

Manifold based feedback linearization of a 1-DOF beam with one-sided spring

Vibration reduction in a harmonically excited 1-DOF beam with one-sided spring is realized by control-ling the system state from a stable large amplitude 1/2 subharmonic response towards a coexisting unstable small amplitude harmonic response using feedback linearization. At the unstable harmonic response no control effort is needed because the unstable harmonic response is a long term solution of the uncontrolled system. To reduce control effort when stabilizing the unstable harmonic response, the stable manifold can be used within the control design because at the stable manifold the system state approaches the unstable harmonic response without control effort. Unfortunately, the calculation of this manifold acquires much off-line computational effort while its usage complicates the on-line control design. Therefore, the stable manifold is approximated by the stable eigenvectors of the monodromy matrix. Due to the local validity of the approximation, a two-stage control approach is used. In the first stage, the system state is controlled towards the unstable harmonic response to reach the region where the stable manifold can be approximated accurately by the stable eigenvectors. In the second stage, the system state is controlled towards the stable eigenvectors and approaches the unstable harmonic response with hardly any control effort

Self-Tuning Power System Stabilizers Based on Pole Assignment

An analysis of the use of Pole Assignment control in Self-Tuning Power System Stabilizers (STPSS) is presented in this paper. Based on this analysis a new STPSS is proposed, which uses a Partial Pole Assignment control strategy and the Extended Least Squares algorithm. The controller aims at obtaining improved identification capabilities and reduced control effort. Simulation results of a benchmark multimachine system support this claim and show that the proposed control scheme provides the system with considerably improved dynamic performance

Reduced Control Effort for Self-Tuning Regulators via an Input Window

Self-tuning regulators (STRs) have proven themselves to be well suited to many process control environments, especially in the case of unknown or slowly time varying parameters. However, for processes containing moderate to excessive noise levels the basic minimum variance type STRs may prove inadequate as the resulting control action may be excessive and costly. This paper presents a method of limiting the control action via an input window without unnecessarily exceeding output variance limitations.