Orlando Arrieta

Analytical robust tuning of PI controllers for first-order-plus-dead-time processes

This paper presents an analytically deducted procedure for tuning two-degree-of-freedom proportional integral (PI) controllers for first-order-plus-dead-time (FOPDT) controlled process. The equations incorporate a design parameter, which relates the feedback control systempsilas time constant, with the controlled process time constant. The design procedure considers the control-loop robustness by means of maximum sensitivity requirements, allowing the designer to deal with the performance-robustness trade-off.

Simple PID Tuning Rules with Guaranteed Ms Robustness Achievement

Abstract The design of the closed-loop control system must take into account the system performance to load-disturbance and set-point changes and its robustness to variation of the controlled process characteristics, preserving the well-known trade-off among all these variables. This paper face with this combined servo/regulation performance and robustness problem. The proposed method is formulated as an optimization problem for combined performance (not for independent operation modes), including also the robustness property as a constraint. The accomplishment of the claimed robustness is checked and then, the PID controller gives a good performance with also a precise and specific robustness degree. The proposed robust based PID control design is tested against other tuning methods.

Two-Degree-of-Freedom PI/PID tuning approach for smooth control on cascade control systems

A design approach for two-degree-of-freedom (2-DOF) PID controllers within a cascade control configuration that guarantees smooth control is presented in this paper. The rationale of operation associated to both, the inner and outer controllers, determines the need of good performance for disturbance attenuation (regulation) as well as set-point following (tracking). Therefore the use of 2-DOF controllers is introduced. However the use of 2-DOF controllers introduces additional parameters that need to be tuned appropriately. Specially for the case of PI/PID controllers there are not known clear auto-tuning guidelines for such situation. The approach undertaken in this paper provides the complete set of tuning parameters for the inner (2-DOF PI) controller and the outer (2-DOF PID) controller. The design equations are formulated in such a way that a non-oscillatory response is specified for both the inner and outer loop. A side advantage of providing the complete set of parameters is that it avoids the need for the usual identification experiment for the tuning of the outer controller.

Simple robust autotuning rules for 2-DoF PI controllers.

This paper addresses the problem of providing simple tuning rules for a Two-Degree-of-Freedom (2-DoF) PI controller (PI(2)) with robustness considerations. The introduction of robustness as a matter of primary concern is by now well established among the control community. Among the different ways of introducing a robustness constraint into the design stage, the purpose of this paper is to use the maximum sensitivity value as the design parameter. In order to deal with the well known performance/robustness tradeoff, an analysis is conducted first that allows the determination of the lowest closed-loop time constant that guarantees a desired robustness. From that point, an analytical design is conducted for the assignment of the load-disturbance dynamics followed by the tuning of the set-point weight factor in order to match, as much as possible, the set-point-to-output dynamics according to a first-order-plus-dead-time dynamics. Simple tuning rules are generated by considering specific values for the maximum sensitivity value. These tuning rules, provide all the controller parameters parameterized in terms of the open-loop normalized dead-time allowing the user to select a high/medium/low robust closed-loop control system. The proposed autotuning expressions are therefore compared with other well known tuning rules also conceived by using the same robustness measure, showing that the proposed approach is able to guarantee the same robustness level and improve the system time performance.

Servo/regulation tradeoff tuning of PID controllers with a robustness consideration

This paper analyzes optimal controller settings for controllers with PID structure. The analysis is conducted from the point of view of the operating mode (either servo or regulation mode) of the control loop and tuning mode of the controller. It is well known that, specially for optimization based settings, the performance of the control loop is defined in terms of the expected operating mode of the control loop. When the control system is not operating on the same operating mode as the controller was tuned the performance may exhibit very poor results. The performance degradation with respect of the optimal performance is defined and analyzed for settings based on ISE-like optimization criteria. Also the gain and phase margins that optimal tuning provides are analyzed. As a consequence, in order to get a minimal performance degradation (with respect to both operating modes) with also a minimal reduction of the gain margin, tradeoff tuning settings are proposed.

Considerations on Set-Point Weight choice for 2-DoF PID Controllers

Abstract Abstract This papers aim is to present an analysis of the influence of the 2-DoF controllers proportional set-point weight over the servo-control performance and to show that the removal of the existing constraint for its selection (0 ≤ β ≤ 1.0) will allow to improve its performance when a high robust regulatory control system is required. A concrete analysis is conducted by using 2-DoF PID tuning approaches that explicitly take the desired robustness level as a design parameter. It is seen that as the desired robustness increases the tuning methods suggest values β > 1.0. Performance looses are evaluated if we are to be constrained to the case β ≤ 1.0.

NORT: a non-oscillatory robust tuning approach for 2-DoF PI controllers

A tuning method for Two-Degree-of-Freedom (2-DoF) proportional integral (PI) controllers for First-Order-Plus-Dead-Time (FOPDT) models is presented. It allows the designer to deal with the closed-loop control system performancerobustness tradeoff specifying the required minimum robustness level selecting its maximum sensitivity in the 1.2 to 2.0 range. In addition, the performance is measured by forcing the closedloop transfer functions to perform as close as possible to a target non-oscillatory dynamics. Controller tuning equations are provided for FOPDT models with normalized dead-times from 0.1 to 2.0. Examples show the effectiveness of the proposed tuning method.

Considerations on PID controller operation: Application to a continuous stirred tank reactor

This paper analyzes optimal controller settings for controllers with One-Degree-of-Freedom (1-DOF) Proportional-Integral-Derivative (PID) structure. The analysis is conducted from the point of view of the operating mode (either servo or regulation mode) of the control loop and tuning mode of the controller. Performance of the optimal tuning settings (ISE-like) can be degraded when the operating mode is different from the select one for tuning. An index for measuring the overall Performance Degradation is proposed and from the minimum of this, tradeoff tuning settings are given. The proposed procedure is applied to control a Continuous Stirred Tank Reactor (CSTR), which is a non-linear system.

Disturbance compensation on uncertain systems: Feedforward control design for stable systems

Abstract This paper considers the design of feedforward controllers when model uncertainty is present. The main contribution is an alternative approach to the generation of the feedorward control action on the basis of the Internal Model Control formulation. This new structure allows for completely independent tuning of the feedback and feedforward controllers and provides an explicit expression for the achieved nominal performance degradation when the uncertain case is considered. The formulation of the feedforward controller as an Internal Model Controller allows existing design approaches to be applied and uncertainty effect taken into account by means of the corresponding analysis equation.

Stability margins characterization of a combined servo/regulation tuning for PID controllers

Robustness and performance analysis for a control system with a PID controller tuning from optimal settings are presented. Those analysis are conducted from the point of view of the stability margins characterization and of the operating mode (either servo or regulation mode) of the control loop and tuning mode of the controller, respectively. It is well known that, specially for optimization based settings, the performance of the control loop is defined in terms of the expected operating mode of the control loop. When the control system is not operating in the same operating mode as the controller was tuned, the performance may exhibit very poor results. The performance degradation with respect to the optimal performance is defined and analyzed for settings based on ISE-like optimization criteria. As a consequence, in order to get a minimal overall performance degradation with respect to both operating modes, tradeoff settings are proposed and the stability margins for the transition between both servo and regulation tunings are obtained.