Continuous Time Approximation Method Research Articles

Hunting stability analysis of high-speed train bogie under the frame lateral vibration active control

ABSTRACTIn this paper, we study a multi-objective optimal design of three different frame vibration control configurations and compare their performances in improving the lateral stability of a high-speed train bogie. The existence of the time-delay in the control system and its impact on the bogie hunting stability are also investigated. The continuous time approximation method is used to approximate the time-delay system dynamics and then the root locus curves of the system before and after applying control are depicted. The analysis results show that the three control cases could improve the bogie hunting stability effectively. But the root locus of low- frequency hunting mode of bogie which determinates the system critical speed is different, thus affecting the system stability with the increasing of speed. Based on the stability analysis at different bogie dynamics parameters, the robustness of the control case (1) is the strongest. However, the case (2) is more suitable for the dynamic performance requirements of bogie. For the case (1), the time-delay over 10 ms may lead to instability of the control system which will affect the bogie hunting stability seriously. For the case (2) and (3), the increasing time-delay reduces the hunting stability gradually over the high-speed range. At a certain speed, such as 200 km/h, an appropriate time-delay is favourable to the bogie hunting stability. The mechanism is proposed according to the root locus analysis of time-delay system. At last, the nonlinear bifurcation characteristics of the bogie control system are studied by the numerical integration methods to verify the effects of these active control configurations and the delay on the bogie hunting stability.

Random vibration analysis of time-delayed dynamical systems

This paper presents random vibration analyses of time-delayed linear and nonlinear dynamical systems. The method of continuous time approximation (CTA) is applied to describe the system dynamics in a high dimensional state space without time delay. Spectral analysis of time-delayed linear systems is studied first. An exact solution of the power spectral density function of the linear system is obtained and used to check the validity of the CTA method. The methods of CTA and equivalent linearization are combined to obtain steady-state responses of the system. The analytical results of E[x2] and E[ẋ2] of a Duffing system with time delay are compared with those of extensive Monte Carlo simulations. It is found that when the system is weakly nonlinear under low-level random excitations, the solutions agree well. For the system with strong nonlinearity or under high-level random excitations, the method of equivalent linearization becomes less accurate and its prediction error grows with the nonlinearity and the excitation strength.

Sliding mode control of uncertain dynamical systems with time delay using the continuous time approximation method

A study of sliding mode control of uncertain dynamical systems with time delay is presented in this paper. The systems are assumed to have constant delay time and uncertain parameters with known upper and lower bounds. The method of continuous time approximation is applied to formulate the sliding mode control problem. The proposed treatment of the control delay leads to a higher-order control formulation for a system with additive uncertainties only. An optimal sliding surface is designed such that on the sliding surface, the controlled state variables act like a feedback control to the uncontrolled state variables. Examples of non-linear systems are presented to demonstrate the theoretical work.

Solutions of the FPK equation for time-delayed dynamical systems with the continuous time approximation method

This paper presents a method of finite-dimensional Markov process (FDMP) approximation for stochastic dynamical systems with time delay and numerical solutions of probability density functions of the systems. Solutions of probability density functions of time-delayed systems are rare in the literature. The FDMP method preserves the standard state space format of the system, and allows us to apply all the existing methods and theories for analysis and control of stochastic dynamical systems and to compute the probability density functions efficiently. The solutions of the FPK equation for a linear time-delayed stochastic system are presented. The effects of different spectral differentiation schemes for the FDMP method on the probability density functions are compared.

Control studies of time-delayed dynamical systems with the method of continuous time approximation

This paper presents control studies of delayed dynamical systems with the help of the method of continuous time approximation (CTA). The CTA method proposes a continuous time approximation of the delayed portion of the response leading to a high and finite dimensional state space formulation of the time-delayed system. Various controls of the system such as LQR and output feedback controls are readily designed with the existing design tools. The properties of the method in frequency domain are also discussed. We have found that time-domain methods such as semi-discretization and CTA, and other numerical integration algorithms can produce highly accurate temporal responses and dominant poles of the system, while missing all the fast and high frequency poles, which explains why many numerical methods can be applied to study the stability of time-delayed systems, and may not be a good tool for control design. Optimal feedback controls for a linear oscillator, collocated and non-collocated feedback controls of an Euler beam, and an experimental demonstration are presented in the paper.

A method of continuous time approximation of delayed dynamical systems

This paper presents a new method to analyze response of linear and nonlinear dynamical systems with time delay. The method proposes a continuous time approximation of the delayed portion of the response. This leads to a high and finite dimensional state space formulation of the time-delayed system. The advantage of the current method lies in that the resulting finite dimensional state equations are in the standard state space form, making all the existing analysis methods and control design tools for linear and nonlinear dynamical systems amenable to the current approach. The method can also handle multiple independent time delays in a natural way. One- and two-dimensional dynamical systems with time delay are used to demonstrate the effectiveness of the method.