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Using the direct synthesis method, a proportional-integral-derivative (PID) controller in series with a leadlag compensator is designed for control of the open-loop unstable second order plus time delay processes with/without a zero. Set-point weighting is considered for reducing the overshoot. The method has two tuning parameters, and guidelines are provided for selecting the two tuning parameters. The method gives significant improvement in load disturbance rejection performances. The designed controller is also robust for uncertainties in the process parameters. Illustrative examples are considered to show the performances of the proposed method. Significant improvement is obtained when compared to recently reported methods.

This paper presents a simple cascade controller in the enhanced modified Smith predictor structure for control of open loop unstable cascade processes with/without zero. The proposed structure consists of two control loops, a secondary inner loop and a primary outer loop. The method has totally three controllers of which the secondary loop has one controller and the primary loop has two controllers. The secondary loop controller is designed using the internal model control (IMC) technique. The primary loop set point tracking and disturbance rejection controllers are designed using the direct synthesis method. The primary set point tracking controller is designed as a proportional, integral, and derivative (PID) controller with lag filter and the primary disturbance rejection controller is designed as a PID with lead-lag filter. Simulation studies have been carried out on various cascade unstable processes with/without zero. The present method gives significant disturbance rejection both in the inner and outer loops. Also, the proposed method shows significant improvement when compared to the recently reported methods in the literature.

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A simple method of designing the controllers for the modified Smith predictor scheme with improved closed-loop performances is proposed for open-loop unstable first-order plus time delay (FOPTD) processes. The proposed method consists of two controllers that are meant for different objectives, namely, the set-point tracking and simultaneous stabilization of the unstable process with time delay and the load disturbance rejection. The design steps of these two controllers are independent. The direct synthesis method is used to design the set-point tracking controller, and simple analytical tuning rules are provided for the load disturbance controller. Robustness studies on the stability and performance are provided, with respect to the uncertainties in the unstable process model parameters. The proposed scheme consists of only one tuning parameter. Good nominal and robust control performances are achieved with the proposed method. Significant improvement in the closed-loop performances are obtained, when compared to the recently reported methods.

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