Search

Many processes operated in chemical process industries show time-varying and highly nonlinear characteristics. This paper proposes an enhanced nonlinear PID (NPID) controller for the improvement of setpoint tracking or disturbance rejection responses and new tuning formulas for a FOPTD process model. The NPID controller has a structure with a first-order filter in the derivative term to avoid possible Derivative Kick. The parameters of the NPID controller are expressed in terms of the ratio L/τ of the time delay L to the time constant τ in the process by using the dimensionless approach. Repeated optimizations are performed for each value over the ranges of 0.01 to 1 and 1 to 3 of L/τ and over the ranges of 5 to 30 of the filter parameter N to obtain the average of optimal parameter values that minimize the integral of absolute error performance criterion. By using the least-squares method with together the calculated optimal values and the rule formulas, the tuning rules are obtained. A set of simulation works on the five processes are carried out to demonstrate tracking and disturbance performance and robustness against the noise of this approach. [ABSTRACT FROM AUTHOR]

Relay in the feedback loop produces a stable oscillation whose cyclic steady state response contains process information of ultimate gain and ultimate period. By measuring these process data and Ziegler-Nichols type tuning rules, PID controllers can be designed. Due to its simplicity and performances, this earlier relay feedback method becomes one of the standard methods for autotuning of PID controllers. Later the first-order plus time delay (FOPTD) models are used for the relay feedback method with additional process data such as the process steady-state gain, improving autotuning performances. Like other FOPTD model-based methods, this FOPTD model-based relay feedback method is very popular in the field. However, for some high-order processes, the FOPTD model-based method shows oscillatory closed-loop responses that are not acceptable. For such processes, the critically damped second-order plus time delay (C2PTD) identifiable with the same three process information of steady state gain, ultimate gain and ultimate period can be used. Unfortunately, the C2PTD model-based relay feedback method cannot cover the whole range of processes. One solution is the selective use of FOPTD and C2PTD models. For this purpose of selection, a shape factor that uses the process measurement of average residence time is proposed here. [ABSTRACT FROM AUTHOR]

This work aims to introduce a tuning-free PID controller for stable FOPTD and FOLIPD systems. The objective was achieved using a partial model-matching strategy; matching the coefficients of corresponding powers of s of the closed loop response of the model to the desired closed loop transfer function. Closed-loop performance is evaluated by considering a unit step change in the process model and compared the results with other methods reported earlier in the literature. Input and output performances were evaluated using error analyses and total variation. The robustness of the proposed method was assessed via incorporation of uncertainties in the process model parameters. The proposed method was also experimentally validated for FOPTD model using a temperature controlled lab. It was concluded that the method was analytically derived and model based with a key advantage that the resultant PID required no tuning parameter.

In our previous work, the authors presented an effective nonlinear proportional-integral-derivative (PID) controller by incorporating a nonlinear function. The proposed controller is based on a conventional PID control architecture, wherein a nonlinear gain is coupled in series with the integral action to scale the error. Three new tuning rules for processes represented as the first-order plus time delay (FOPTD) model were obtained by solving an optimization problem formulated to minimize three performance indices. The main feature of the proposed controller is that it preserves the numbers of tuning gains even though nonlinearity is introduced and remains easy implementation in real applications. However, due to the introduction of a nonlinear element, the stability problem of the proposed controller may be raised. This paper presents one sufficient condition in the frequency domain for the absolute stability of the nonlinear PID controller, based on circle stability theory. It is proved that the nonlinear gain used is in the sector [0, 1]. The condition of the linear block F(s) is derived for the overall feedback system to be stable. Checking the stability and the effectiveness and robustness of the feedback system for setpoint tracking are demonstrated through a set of simulation works on three processes with uncertainty. [ABSTRACT FROM AUTHOR]

In this work, a design approach of proportional integral-fractional filter (PI-FF) controller for first order plus time delay system (FOPTD) is proposed in order to enhance the feedback control system performance characteristics. The controller design method is drawn up such that the transfer function of the overall closed-loop system is equivalent to the transfer function of the general fractional Bagley–Torvik reference model whose behaviour ranges from relaxation to oscillation for different values of the fractional order derivative and the damping ratio-like parameter. The tuning parameters of the PI-FF controller are derived analytically from the FOPTD process model and the general fractional Bagley–Torvik reference model parameters. Illustrative examples were presented to test the effectiveness and the usefulness of the proposed PI-FF controller on the feedback control system performance characteristics enhancement. [ABSTRACT FROM AUTHOR]

In this work, a design approach of proportional integral-fractional filter (PI-FF) controller for first order plus time delay (FOPTD) system is proposed in order to enhance the feedback control system performances characteristics. The controller design method is drawn up such that the transfer function of the overall closed-loop system is equivalent to the transfer function of the general fractional Bagley-Torvik reference model whose behaviour ranges from relaxation to oscillation for different values of the fractional order derivative and the damping ratio-like parameter. The tuning parameters of the PI-FF controller are derived analytically from the FOPTD process model and the general fractional Bagley-Torvik reference model parameters. Illustrative examples were presented to test the effectiveness and the usefulness of the proposed PI-FF controller on the feedback control system performance characteristics enhancement. [ABSTRACT FROM AUTHOR]

First order plus time delay (FOPTD) models obtained from the cyclic steady state of the relay feedback oscillations can suffer from poor performances for some processes such as first and second order processes with fast parasitic dynamics. Poor FOPTD models are due to high frequency oscillations that activate parasitic dynamics. Dynamic elements such as hysteresis and integrator added in the feedback loop can reduce the oscillation frequencies but they increase the experimental times. Here, to relieve these problems without increasing the experimental time and computational complexity, relay feedback autotuning methods that apply the method of weighted moments to relay feedback transients are proposed. The proposed methods use the same relay feedback tests as the conventional relay feedback methods and are simple to use computationally, providing conservative PI controllers for all the test batch processes and the above first and second order processes with fast parasitic dynamics. [ABSTRACT FROM AUTHOR]