Showing papers by "Hassan K. Khalil published in 2004"

Performance recovery under output feedback sampled-data stabilization of a class of nonlinear systems

Abstract: This paper studies sampled-data output feedback control of a class of nonlinear systems. It is shown that the performance of a stabilizing continuous-time state feedback controller can be recovered by a sampled-data output feedback controller when the sampling period is sufficiently small. The output feedback controller uses a deadbeat discrete-time observer to estimate the unmeasured states. Two schemes are proposed to overcome large initial transients when the controller is switched on.

Sensorless speed control of induction motors

Abstract: We consider field-oriented speed control of induction motors without rotor position sensors. We augment the traditional approach with flux and speed observers and derive a sixth-order nonlinear model that describes the motor in field-oriented coordinates. The model takes into consideration the error in flux estimation. The flux regulation problem is a simple one and we follow the traditional approach of using PI controllers. For the speed regulation problem, we simplify the model by assuming that flux regulation takes place relatively fast and by using a (high-gain) PI controller to regulate the q-axis current to its command. This results in a third-order nonlinear model in which the speed and two flux estimation errors are the state variables, the q-axis current is the control input and a speed estimate (provided by the high-gain observer) is the measured output. This nonlinear model is the main contribution of this paper because it enables us to perform rigorous analysis of the closed-loop system under different controllers. In the current paper, we limit our analysis to the design of PI controllers via linearization. The linearized model is used to study when a PI controller can stabilize the nonlinear third-order model at the desired equilibrium point. The analysis reveals an important role played by the steady-state product of the flux frequency and the q-axis current in determining the control properties of the system.

Output feedback control using high-gain observers in the presence of measurement noise

Abstract: Output feedback control using high-gain observers in the presence of measurement noise is considered for a class of nonlinear systems. We show that the closed-loop system with high-gain observer converges to a closed-loop system with ideal differentiation as the gain of the observer is increased. This ideal differentiation system is developed to reference the convergence properties. Analytical and simulation results are presented.

Output feedback sampled-data stabilization of nonlinear systems

Abstract: This paper studies sampled-data output feedback control of a class of nonlinear systems. It is shown that the performance of a stabilizing continuous-time state feedback controller can be recovered by a sampled-data output feedback controller when the sampling period is sufficiently small. The output feedback controller uses a deadbeat discrete-time observer to estimate the unmeasured states. Two schemes are proposed to overcome large initial transients when the controller is switched on.

Measurement and Control of Microphonics in High Loaded-Q Superconducting RF Cavities

Abstract: INTRODUCTION Table 1: Beam loading requirements for 400 kW, 400 MeV/u uranium in RIA elliptical cavities. Superconducting radio frequency (SRF) linacs with light beam loading, such as the Rare Isotope Accelerator (RIA), S-DALINAC, CEBAF upgrade, and energy recovery linacs, operate more efficiently with loaded-Q, QL, values greater than 10. The resulting narrow bandwidth puts stringent limits on acceptable levels of vibration, also called microphonics, which detune the SRF cavities and require additional rf power to maintain amplitude and phase. Type 6-cell 6-cell 6-cell βg 0.47 0.61 0.81 Va(MV) 5.12 8.17 13.46 Pbeam(W) 166

State feedback regulation of nonlinear systems using conditional integrators

Abstract: This paper studies the regulation of nonlinear systems using conditional integrators. Previous work introduced the tool of conditional integrators that provide integral action inside a boundary layer while acting as stable systems outside, leading to improvement in transient response while achieving asymptotic regulation in the presence of unknown constant disturbances or parameter uncertainties. The approach, however, is restricted to a sliding mode control framework. This paper extends this tool to a fairly general class or state feedback control laws, with the stipulation that we know a Lyapunov function for the closed-loop system. Asymptotic regulation with improvement in transient response is done by using the Lyapunov redesign technique to implement the state feedback control as a saturated high-gain feedback and introducing a conditional integrator to provide integral action inside a boundary layer. Improvement in the transient response using conditional integrators is demonstrated with an experimental application to the Pendubot.

Universal integral controllers with non-linear integral gains

Abstract: We consider a single-input single-output minimum-phase non-linear system which has a normal form. In previous work, integral control and high-gain observers were used to achieve robust regulation under output feedback. To improve the transient performance, we propose the use of integrators with non-linear gains. We prove that the controller achieves regional and semiglobal regulation. Simulation results are presented showing the improved overshoot and settling time.

Comparison of logic-based switching control designs for a nonlinear system

Abstract: We consider an example of a second order nonlinear system with large parametric uncertainties. The two parameters of the system are assumed to belong to a finite set. The goal is to guarantee (practical) convergence of the system output to a given constant reference signal. Feedback linearization-based candidate controllers with pole placement are designed for each possible set of parameters. After that, we consider the design of a high-level logic-based supervisor to organize switching between these candidate controllers. Three different approaches are used and compared.