Enhanced Modified Smith Predictor for Delay Dominated Unstable Processes

A number of tuning rules have been reported in literature for unstable first order plus time delay processes. However, at least to the author knowledge most of the reported methods fail to give robust performance for process models with time delay equal to or greater than the process time constant. In this work, a robust tuning method based on the modified Smith predictor is proposed for unstable first order plus time delay process models. Set point tracking controller is designed using direct synthesis approach. The PD controller which is used for load disturbance rejection is designed so as to satisfy the Routh Hurwitz's stability criterion. Suitable value of the tuning parameter is recommended after studying its effect on the system performance and robustness. This is also an advantage of the present work over the other recently reported strategies where suitable ranges of the tuning parameters' values are provided.

Set point weighted modified Smith predictor for integrating and double integrating processes with time delay

The modified Smith predictor is well known as an effective time-delay compensator for a plant with large time delays, and several papers on the modified Smith predictor have been published. The parametrization of all stabilizing modified Smith predictors for minimum-phase time-delay plants is obtained by Yamada and Matsushima. In addition, Yamada et al. expanded the result by Yamada and Matsushima and proposed the parametrization of all stabilizing modified Smith predictors for non-minimum-phase time delay plants. However, no paper examines the parametrization of all stabilizing 2-degree-of-freedom modified Smith predictors that can specify the input-output characteristic and the feedback characteristic separately. From the practical point of view, it is desirable that the input-output characteristic and the feedback characteristic are specified separately. In this paper, we propose the parametrization of all stabilizing 2-degree-of-freedom modified Smith predictors for time-delay plants that can specify the input-output characteristic and the feedback characteristic separately.

Smith Predictor is very sensitive to uncertain system with variable time-delay. Using dual-locus diagram method, we first analyse the essence of Smith Predictor's sensitivity and present a proposition to illustrate the system stability criteria with relation to system gain and time-delay variation. Then a new modified Smith Predictor is proposed to compensate the variable time-delay system with uncertain model for set-point tracking. After that, we discuss the system internal stability and give two-degree freedom controller self-adaptive to eliminate the impact of system uncertainty with varying time-delay. Examples show the improved performance compared to previous Modified Smith Predictor structures.

Three factors that complicate the position and orientation control of unmanned autonomous vehicles are the hardware-induced delays, delays caused by teleoperated modes of operation, and persistent disturbances such as wind. In this article, we study how to overcome these challenges by controlling a twin rotor multi-input, multi-output system. This nonlinear system has two degrees of freedom and significant cross-coupling, making its control particularly challenging. Time delay and external disturbances contribute to system instability and complicate flight control. The effects of time delay can be mitigated using the standard Smith predictor controller. However, this approach performs poorly when persistent disturbances are present. Our contribution is a modified Smith predictor that effectively mitigates disturbances. Additionally, a decoupler is employed to transform the system into two single-input, single-output models, enabling precise control of both the horizontal and vertical planes. The proposed method is implemented on a prototype and compared with the original Smith predictor-based control scheme. Experimental results validate the effectiveness of the proposed approach, showing that our approach reduces the steady-state tracking error to less than 2%, decreases settling time from 30 s to 20 s, and improves disturbance rejection under wind disturbance conditions by 35% compared to the standard Smith predictor. These results demonstrate the superior set-point tracking performance and enhanced disturbance rejection of the proposed approach.

Leveling operation is a very important step in the installation process of offshore jacket platform and it is performed by leveling tool. In this paper, the hydraulic lifting subsystem model of leveling tool is established, and a modified Smith predictor is used to address the time delay problem caused by long pipeline. A composite control strategy combining identical and master slave control mode is adopted to solve synchronization control problem of the multi-cylinder hydraulic lifting system. A fuzzy self-tuning PI controller between the master and slave cylinder is designed to compensate the synchronization error. The simulation results show that the modified Smith predictor has better control effect than conventional Smith predictor for the hydraulic system with long pipelines. The synchronous approach has high precision and can meet the leveling requirements.

This paper studies anti-windup control design for process with dead time and integrator. Based on the two degrees of freedom (2DOF) control schemes presented by Yu-Chu Tian, et al, the effects of actuator saturation are studied. An anti-windup scheme for the modified Smith predictor is proposed. Some problems in stability analysis are presented. Computer simulations show the significant performance improvement when the set point or disturbance is large enough.

In this paper, a new modified Smith predictor using a rapid load estimator scheme is proposed. The advantage of the design is that the load estimator does not involve the solution of a closed-loop equation that contains dead time, and this can achieve a better load disturbance response. This scheme can be easily extended to the case of double integrators. Robust stability of the proposed method is analyzed, and a simple and effective robust controller design rule is derived. It is demonstrated by simulations that the new controller provides a similar or better setpoint response and a faster load disturbance rejection than the dead-time compensator in other papers.

The distillation columns are widely used in various chemical industries. It is basically used for separating the components of a liquid mixture. The More volatile component is vaporized, and the less volatile one is left back in the liquid. The plant of distillation column that is considered in the present work is a SOPDT transfer function model imported from an online source. Due to the impact of present dead time, the stability in the output variable is a challenging task. So, the improved control performance is achieved by the dead time compensation using the Smith predictor. The PI control systems are designed, using MATLAB, for this plant with and without Smith predictor, and their performance is compared.

In this paper, a new modified Smith predictor using a rapid load estimator scheme is proposed. The advantage of the design is that the load estimator does not involve the solution of a closed-loop equation that contains dead time, and this can achieve a better load disturbance response. This scheme can be easily extended to the case of double integrators. Robust stability of the proposed method is analyzed, and a simple and effective robust controller design rule is derived. It is demonstrated by simulations that the new controller provides a similar or better setpoint response and a faster load disturbance rejection than the dead-time compensator in other papers.

A novel control structure with PID-type controllers is proposed in this paper to deal with first-order unstable processes with dead time (UFOPDT). The PD controller in the inner loop is aimed to stabilize the unstable process while the purpose of the PID controller at the outer loop is to improve the performance of the overall system. Stability analysis is based upon the Nyquist stability criterion. And the tuning schemes arc the classic Ziegler-Nichols step response methods as well as the tuning rules for stable second-order systems with time delay. It is shown that the proposed method is able to control the UFOPDT process satisfactorily and has made significant improvements over the existing methods in terms of performance and simplicity.

Optimal dead-time compensator design for stable and integrating processes with time delay

A new recurrent neural-network predictive feedback control structure for a class of uncertain nonlinear dynamic time-delay systems in canonical form is developed and analyzed. The dynamic system has constant input and feedback time delays due to a communications channel. The proposed control structure consists of a linearized subsystem local to the controlled plant and a remote predictive controller located at the master command station. In the local linearized subsystem, a recurrent neural network with on-line weight tuning algorithm is employed to approximate the dynamics of the time-delay-free nonlinear plant. No linearity in the unknown parameters is required. No preliminary off-line weight learning is needed. The remote controller is a modified Smith predictor that provides prediction and maintains the desired tracking performance; an extra robustifying term is needed to guarantee stability. Rigorous stability proofs are given using Lyapunov analysis. The result is an adaptive neural net compensation scheme for unknown nonlinear systems with time delays. A simulation example is provided to demonstrate the effectiveness of the proposed control strategy.

To improve a vehicle's anti-roll performance while consuming less energy, an active/passive dual-mode switched control method based on a hydraulically interconnected suspension (HIS) is proposed. This method has two key components as follows: modelling the “mechanical-liquid-gas” coupled dynamic system consisting of hydraulic cylinders, a hydraulic pump and accumulators; and robustly controlling a time-delayed system with uncertain model and lag time. In this study, the nonlinear characteristics of the underlying actuators were considered, and a four degrees-of-freedom (4DOF) roll-plane half-car model was employed. The backstepping algorithm was adopted to maintain the body in the desired posture. Subsequently, a modified Smith predictor and a Taylor expansion of lateral acceleration were applied simultaneously to compensate for the time delay. Finally, simulations with uncertain parameters and lag time were performed. The results indicate that the proposed method is robust in the case of an uncertain control system and can effectively improve the vehicle's anti-roll performance.

Generalized Pattern Search Methods for control of stable, unstable and integrating systems with unknown delay under step input