All-PD control of pure Integrating Plus Time-Delay processes with gain and phase-margin specifications

A unified design approach for control integrating processes with time delay

The article presents a unified control system designing scheme to obtain enhanced performance for processes including integrator and dead time. A simple control structure including two controllers is proposed. Servo performance and disturbance rejection performance can be adjusted independently by introducing a desired transfer function model in the control structure. Servo controller is designed according to the direct synthesis principle and disturbance rejection controller is derived adopting the internal model control (IMC) theory. Simulations have been conducted on four kinds of integrating plants with dead time. The simulation results exhibit that noteworthy enhancement can be achieved by the presented scheme in comparation with the other methods even though there are perturbed dynamics.

Smith-predictor based enhanced Dual-DOF fractional order control for integrating type CSTRs

Continuously Stirred Tank Reactors (CSTR) are one of the widely used reactors in the chemical industry. Controlling such reactors is challenging because many times it demonstrates a model which is having a pole at the origin of the s-plane. Moreover, the presence of a dead time necessitates more effective control measures. This work presents a modified smith predictor-based control for integrating type CSTRs with time delay in order to provide adequate servo and regulatory closed-loop responses. Numerous researches on dual DOF control suggested different controller settings for outer and inner-loop controllers. But, in the current study, both the controllers are proposed to be the same which drastically reduces the complexity of the design. To offer good robustness in the closed-loop response, the controller is synthesized with a user-defined maximum sensitivity. Case studies on CSTRs for both the nominal and disturbed process models are conducted and the same is compared with recently developed control laws. Lastly, a performance comparison on ISE, ITAE, and IAE is provided.

MSP designing with optimal fractional PI–PD controller for IPTD processes

An effective tuning methodology of modified Smith predictor (MSP) based fractional controller designing for purely integrating time delayed (IPTD) processes is reported here. IPTD processes with pole at the origin are truly difficult to control; exhibit large oscillations once get disturbed from their steady state. Proposed MSP design consists of fractional PI (proportional-integral) and fractional PD (proportional-derivative) controllers together with P (proportional) controller. Fractional controllers are competent to provide improved closed loop responses due to flexibility of additional tuning parameters. Fractional tuning parameters of PI and PD controllers are derived through optimization algorithms where integral absolute error (IAE) is considered as cost function. Efficacy of the proposed methodology is validated for IPTD processes having wide range of time delay. Stability and robustness issues are explored under process model uncertainties with small gain theorem. Performance of the proposed MSP-FO(PI–PD) controller is validated through simulation study relating five IPTD process models. Overall satisfactory closed loop responses are observed for each case during transient as well as steady state operational phases.

Fractional-order PI plus D controller for second-order integrating plants: Stabilization and tuning method

In this paper, a fractional-order PI plus D (PI^λ-D) structure is proposed for second-order integrating plants. A feedback controller is designed to locate the integrating pole(s) to improve the stability region of the controlling system. Then, the explicit formulae are derived to construct the complex root boundary (CRB) for a given plant model with fractional integrator. By stabilizing approach, it is feasible to put the constraint to calculate the optimal controller parameters. Its effectiveness is presented through numerical examples and a hardware experiment on physical integrating system, namely the altitude control of a quadrotor. The results reveal the improved performances with robustness by the new approach.

Simple internal model control based modified Smith predictor for integrating time delayed processes with real-time verification

Internal model control (IMC) tuned simple modified Smith predictor structure for integrating time delayed processes (IPTD) is reported here. Pole position at origin implies its non-self-regulating behaviour. Processes like distillation column, liquid supply to large storage tank, superheated steam flow to turbine etc. are usually found IPTD processes. Reported modified Smith predictor (MSP) design with multiple controllers is adequate to exhibit anticipated closed loop performance for IPTD processes. Tuning complexity of the reported multi-controller based structure is mitigated by the sole tuning parameter (closed loop time constant) obtained from IMC design. Proposed scheme shows considerable performance improvement during set point tracing by zero overshoot. Additionally, smooth as well as reasonably fast load recovery is ensured. Eminence of the reported scheme is established in terms of performance indices along with stability margins in assessment through recently reported modified Smith predictor techniques. Real-time evaluation of the proposed design is demonstrated on an indigenous set up of level control loop considered as IPTD process.

Novel control strategy for non-minimum-phase unstable second order systems: generalised predictor based approach

A control structure based on generalized predictor is proposed to control non-minimum phase unstable second order processes with time delay. The scheme contains a predictor structure and a direct synthesis method based primary controller for servo tracking. The predictor structure consists of two filters acting on input and current output which are designed to provide noise attenuation and disturbance rejection. A set-point filter minimises the overshoot caused by the introduction of additional zeros of the controller in the overall closed loop transfer function so as to smooth the tracking performance. Different second order unstable time delay systems are considered and Integral Absolute Error (IAE) and Total Variation (TV) measures are used for comparing the performances quantitatively. The method is implemented experimentally on an inverted pendulum. The proposed predictive strategy is found to provide enhanced control performances in comparison to the existing literature methods.

Simplified tuning of IMC based modified smith predictor for UFOPDT processes

A simplified tuning guideline for internal model control (IMC) based modified Smith predictor technique is reported here for unstable lag dominated first-order processes with dead time (UFOPDT). Pole location in right half section of s-plane signifies the unstable behaviour of UFOPDT processes. Mostly, chemical processes like isothermal chemical reactor, bioreactor, dimerization reactor, fluid catalytic cracker etc. are found to be lag dominated and unstable along with considerable time delay. Smith predictor technique based control methodology is considered to be a well-accepted approach for such cases. However, conventional Smith predictor technique is not capable enough for controlling UFOPDT processes. Whereas modified Smith predictor is found to be quite competent in such cases as its design involves more than one controller. Modified Smith predictor structure is capable to provide desirable closed loop response during set point tracking along with the load recovery phases. To mitigate the tuning complexity of multiple controllers involved in modified Smith predictor designing, suggested IMC structure employs single tuning parameter λ i.e. closed loop time constant for all three controllers concerned. Noticeable performance enhancement is reported by the proposed scheme as no overshoot is observed during set point tracking. Moreover, smooth and efficient load rejection behaviour is also obtained. Supremacy of the proposed tuning is established through closed loop performance comparison with others’ reported modified Smith predictor based tuning relations for chemical reactor and bioreactor in terms of performance indices as well as stability margins.

Parametric and Nonparametric PI Controller Tuning Method for Integrating Processes Based on Magnitude Optimum

Integrating systems are frequently encountered in the oil industry (oil–water–gas separators, distillation columns), power plants, paper-production plants, polymerisation processes, and in storage tanks. Due to the non-self-regulating character of the processes, any disturbance can cause a drift of the process output signal. Therefore, efficient closed-loop control of such processes is required. There are many PI and PID controller tuning methods for integrating processes. However, it is hard to find one requiring only a simple tuning procedure on the process, while the tuning method is based either on time-domain measurements or on a process transfer function of arbitrary order, which are the advantages of the magnitude optimum multiple integration (MOMI) tuning method. In this paper, we propose the extension of the MOMI tuning method to integrating processes. Besides the mentioned advantages, the extension provides efficient closed-loop control, while PI controller parameters calculation is still based on simple algebraic expressions, making it suitable for less-demanding hardware, like simpler programmable logic controllers (PLC). Additionally, the proposed method incorporates reference weighting factor b that allows users to emphasize either the disturbance-rejection or reference-following response. The proposed extension of the MOMI method (time-domain approach) was also tested on a charge-amplifier drift-compensation system, a laboratory hydraulic plant, on an industrial autoclave, and on a solid-oxide fuel-cell temperature control. All closed-loop responses were relatively stable and fast, all in accordance with the magnitude optimum criteria.

Design of Noise Filters for Integrating Time Delay Processes

In this research, a second order noise filter is designed for reducing the noise effect on PID controlled Integrating plus time delay (IPTD), Integrating plus first order plus time delay (IFOPTD), Double integrating plus time delay (DIPTD) processes. The second order noise filter time constant is designed based on an iterative approach, which is a function of loop gain crossover frequency ( {\omega _{gc}}) and a design parameter \,\left( {{\beta }} \right). Based on the data obtained from an integrating process and the controller, the controller parameters are calculated using Optimal H2 -IMC based technique. The design parameter ( {{\beta }}) is selected as a compromise among robustness, performance and reduction of measurement noise. Simulation studies are carried out on different IPTD, IFOPTD and DIPTD processes to illustrate the amount of reduction of noise effect on process output and controller output. The effect is also calculated based on Integral Absolute Error (IAE), Integral Square Error (ISE) and Total Variance (TV) criteria by considering perfect and perturbations in the process parameters.

Analytical design of an industrial two-term controller for optimal regulatory control of open-loop unstable processes under operational constraints

This paper proposes a novel optimization-based approach for the design of an industrial two-term proportional-integral (PI) controller for the optimal regulatory control of unstable processes subjected to three common operational constraints related to the process variable, manipulated variable and its rate of change. To derive analytical design relations, the constrained optimal control problem in the time domain was transformed into an unconstrained optimization problem in a new parameter space via an effective parameterization. The resulting optimal PI controller has been verified to yield optimal performance and stability of an open-loop unstable first-order process under operational constraints. The proposed analytical design method explicitly takes into account the operational constraints in the controller design stage and also provides useful insights into the optimal controller design. Practical procedures for designing optimal PI parameters and a feasible constraint set exclusive of complex optimization steps are also proposed. The proposed controller was compared with several other PI controllers to illustrate its performance. The robustness of the proposed controller against plant-model mismatch has also been investigated.

A generalized smith predictor for unstable time-delay SISO systems

In this work, a generalization of the Smith Predictor (SP) is proposed to control linear time-invariant (LTI) time-delay single-input single-output (SISO) systems. Similarly to the SP, the combination of any stabilizing output-feedback controller for the delay-free system with the proposed predictor leads to a stabilizing controller for the delayed system. Furthermore, the tracking performance and the steady-state disturbance rejection capabilities of the equivalent delay-free loop are preserved. In order to place this contribution in context, some modifications of the SP are revisited and recast under the same structure. The features of the proposed scheme are illustrated through simulations, showing a comparison with respect to the corresponding delay-free loop, which is here considered to be the ideal scenario. In order to emphasize the feasibility of this approach, a successful experimental implementation in a laboratory platform is also reported.