Parallel Cascade Control Structure Research Articles

Analytical Design of Fractional-Order PI Controller for Parallel Cascade Control Systems

The fractional-order proportional-integral (FOPI) controller tuning rules based on the fractional calculus for the parallel cascade control systems are systematically proposed in this paper. The modified parallel cascade control structure (PCCS) with the Smith predictor is addressed for stable, unstable, and integrating process models with time delays. Normally, the PCCS consists of three controllers, including a stabilized controller, for a class of unstable and integrating models, a disturbance rejection controller in the secondary loop, and a primary servomechanism controller. Accordingly, the ideal controller is obtained by using the internal model control (IMC) approach for the inner loop. The proportional-derivative (PD) controller is suggested for the stabilized controller and is designed based on a stability criterion. Based on the fractional calculus, the analytical tuning rules of the FOPI controller for the outer loop can be established in the frequency domain. The simulation study is considered for three mentioned cases of process models and the results demonstrate the flexibility and effectiveness of the proposed method for the PCCS in comparison with the other methods. The robustness of the proposed method is also justified by perturbed process models with ±20% of process parameters including gain, time constant, and delay time.

Smith predictor based parallel cascade control strategy for unstable and integrating processes with large time delay

This manuscript presents a modified parallel cascade control structure (PCCS) with Smith predictor for open loop unstable and integrating process models with large time delay. The proposed PCCS consists of a secondary disturbance rejection controller, a primary stabilizing controller and a primary setpoint tracking controller. Parameters of the setpoint tracking controller are obtained by equating the first and second derivatives of desired and actual closed-loop transfer functions at s=0 whereas the secondary disturbance rejection controller is designed using internal model control approach. Routh–Hurwitz stability criterion is used to design the stabilizing controller of the primary loop. An analytical expression for computing the primary closed-loop time constant in terms of known plant model parameters is obtained. Moreover, a suitable value is recommended for the secondary closed-loop time constant based on extensive simulation studies. This is an advantage of the present work over the contemporary Smith predictor based parallel cascade control schemes where the authors provide suitable range of values for the closed-loop time constants. Simulation results illustrate that the proposed method yields significant improvement in closed-loop performance compared to the recently reported tuning strategies for nominal and perturbed process models.

Modified parallel cascade control strategy for stable, unstable and integrating processes

This manuscript presents a modified parallel cascade control structure (PCCS) for a class of stable, unstable and integrating process models with time delay. The proposed PCCS consists of three controllers. Internal Model Control (IMC) approach is used to design the disturbance rejection controller in the secondary loop. Parameters of the proportional-integral (PI) controller which is used for setpoint tracking is obtained by equating the first and second derivatives of desired and actual closed loop transfer functions at the origin of s-plane. Routh Hurwitz stability criterion is used to design the proportional-derivative (PD) controller which stabilizes the unstable/integrating primary process model. An analytical expression is proposed for computing the desired closed loop time constant of the primary loop in terms of plant model parameters so as to achieve an user-defined maximum sensitivity. Based on extensive simulation studies, a suitable value for the secondary closed loop time constant is also recommended. This is an advantage of the present work over the reported parallel cascade control schemes where authors provide a suitable range of values for the closed loop time constants. The proposed tuning strategy requires tuning of four/six controller parameters for stable/unstable and integrating process models which is less compared to the reported strategies. Simulation results illustrate that the proposed method yields significant improvement in closed loop performance compared to some of the recently reported tuning strategies for both nominal and perturbed process models.

An improved parallel cascade control structure for processes with time delay

Parallel cascade control strategies, to improve the dynamic performance of a control system, have been proposed earlier mainly for control of stable processes. In this paper, further results are presented for a new parallel cascade control structure and controller design for controlling stable, unstable or integrating processes with time delay. The design of the disturbance rejection controllers and the setpoint filter are based on loop shaping and ISE performance measures, respectively. A modified Smith predictor scheme is used in the primary loop to enhance the closed-loop performance of the system. The stabilization, robustness and performances of time delay processes are analyzed. The disturbance rejection capability of the proposed scheme is superior as compared to some existing methods. Examples are given to illustrate the usefulness of the proposed method and its superiority over some parallel cascade control schemes.

Synthesis of PID tuning for a new parallel cascade control structure

Most of the published papers on parallel cascade control strategies present improvement of dynamic performance of stable processes. In this paper, a new parallel cascade control scheme is proposed for controlling stable and unstable processes with time delay. The two main features of the proposed scheme are: the primary process output completely tracks the primary setpoint and the servo response decouples the regulatory response in the nominal system. The proposed structure has only two controllers. The inner loop controller is designed based on IMC approach. The outer loop controller is a PID controller in series with lead/lag filter which is designed based on the desired complementary sensitivity function. Significant improvement in the load disturbance rejection performances are obtained when compared to some recent methods in the literature. Simulation results show the superiority and usefulness of the proposed control method over the existing ones.

Modified Smith Predictor Based Parallel Cascade Control Strategy for Non-Self Regulating Time Delay Processes

Modified Smith predictor based parallel cascade control strategy is proposed for enhanced control of non-self regulating processes involving time delays. Modified Smith predictor is integrated in the primary loop of the parallel cascade control structure to enhance the closed loop performance. The proposed structures consist of two control loops, a secondary inner loop and a primary outer loop. The secondary loop controller is designed using the internal model control (IMC) method. Then, the primary loop controllers are designed after identifying the overall primary process model. The primary loop has two controllers meant for set point tracking and good disturbance rejection and designed using direct synthesis method. The primary set point tracking controller is designed as a PID with lag filter and the primary disturbance rejection controller is designed as a PID with lead-lag filter. For comparison, a simple parallel cascade control is also applied to non-self regulating time delay processes. Simulation studies have been carried out on various unstable and integrating processes. The proposed method provides enhanced closed loop performances compared to that of simple parallel cascade control.

Temperature Control System with Multi-closed Loops for Lithography Projection Lens

Image quality is one of the most important specifications of optical lithography tool and is affected notably by temperature, vibration, and contamination of projection lens(PL). Traditional method of local temperature control is easier to introduce vibration and contamination, so temperature control system with multi-closed loops is developed to control the temperature inside the PL, and to isolate the influence of vibration and contamination. A new remote indirect-temperature-control(RITC) method is proposed in which cooling water is circulated to perform indirect-temperature-control of the PL. Heater and cooler embedded temperature control unit(TCU) is used to condition the temperature of the cooling water, and the TCU must be kept away from the PL so that the influence of vibration and contamination can be avoided. A new multi-closed loops control structure incorporating an internal cascade control structure(CCS) and an external parallel cascade control structure(PCCS) is designed to prevent large inertia, multi-delay, and multi-disturbance of the RITC system. A nonlinear proportional-integral(PI) algorithm is applied to further enhance the convergence rate and precision of the control process. Contrast experiments of different control loops and algorithms were implemented to verify the impact on the control performance. It is shown that the temperature control system with multi-closed loops reaches a precision specification at ±0.006 ℃ with fast convergence rate, strong robustness, and self-adaptability. This method has been successfully used in an optical lithography tool which produces a pattern of 100 nm critical dimension(CD), and its performances are satisfactory.

Indirect feedforward control: multivariable systems

Based on the parallel cascade control structure, the concept of indirect feedforward control is proposed for multivariable systems. The indirect feedforward control uses the fast secondary measurements to sense the load changes and the secondary controllers are designed to take necessary actions to overcome the load disturbances. In the multivariable systems, an interaction measure array (Γ) is defined. The secondary controllers are designed according to Γ to provide indirect feedforward actions. Quantitative results are derived which serve as a measure of robustness for indirect feedforward control when uncertainties are presented in the load transfer functions. A distillation example is used to illustrate the design and performance of the indirect feedforward control. Simulation results show that the proposed method provides an attractive alternative for disturbance rejection.

Design of parallel cascade control for disturbance‐rejection

AbstractA new approach is presented to overcome load disturbance problems. Based on the parallel cascade control structure, the primary controller is designed for the servo response, and the secondary controller is designed for the disturbance‐rejection purpose. A perfect disturbance‐rejection controller is derived for the secondary loop. Simulation studies show that improvement in load responses can be achieved for some cases. However, for some other cases, parallel cascade control simply worsens the load responses as a result of interaction between primary and secondary loops. A new interaction measure for load disturbance (γ) is proposed to determine whether improved load responses is achievable using cascade control structure. More importantly, the interaction measure also quantifies the margin of improvement that can be achieved over conventional single‐loop control. A design procedure for improved load responses was proposed. The simulation results show that the new approach offers a simple and effective alternative for disturbance‐rejection.