Analytical Multiloop Control for Multivariable Systems with Time Delays

In this paper, a new design method with performance improvements of multiloop controllers for multivariable systems is proposed. Precise expression is developed to show the relationship between the dynamic- and steady-state characteristics of the multiloop control system and its parameters. First, an equivalent transfer function (ETF) is introduced to decompose the multivariable system, based on which the multiloop controller parameters are calculated. According to the ETF matrix property, an analytical expression for the PI controller for multivariable systems is derived in terms of substituting the ETF matrix for the inverse open-loop transfer function. In the proposed controller design method, no approximation of the inverse of the process model is needed, implying that this method can be applied to some multivariable systems with high dimensions. The simulation results obtained from several examples demonstrate the effectiveness of the proposed method.

Decoupling Method for PI Controllers via Setpoint Modification Applied to HVAC Systems

The method described in this paper addresses the problem of interaction in a control system that consists of multiple legacy PI(D) controllers. In these systems, it is often not possible to intercept the signals between the controllers and plant and replacement with multivariable alternatives such as model predictive control (MPC) may not be viable from a cost perspective. To address these issues, we propose a decoupling method that involves first changing the tuning of the controllers and then dynamically adjusting the setpoints. The method enables decoupling of a multiloop control system via setpoint adjustment which can be carried out without having to re-engineer any of the legacy controllers. The paper focuses on the application area of energy systems in buildings where costs are constrained and PID multiloop configurations are prevalent. Simulation test results are presented from two systems to demonstrate the improvement in control achieved with the method.

A New Phase Advanced Multiloop Control System For Dynamic Voltage Restorer

A dynamic voltage restorer is a power quality (custom power) device used to correct the voltage disturbances by injecting voltage as well as power into the system. The compensation capability of a dynamic voltage restorer (DVR) depends primarily on the maximum voltage injection ability and the amount of stored energy available within the restorer. In this paper a new phase advance compensation (PAC) strategy for the DVR is proposed in order to enhance the voltage restoration property of the device. Also a method of determining the exact amount of voltage injection required to systematically correct a specific voltage drop with minimum active power injection is proposed. Using the proposed method it can be shown that a particular disturbance can be corrected with less amount of storage energy compared to that of existing in-phase boosting method. Analytical expressions for both the magnitude and angle of the injected voltage are also derived. A closed-loop controller that consists of an inner current loop and an outer voltage loop is also incorporated into the DVR system.

Comparison of Steady State and Dynamic Interaction Measurements in Multiloop Control Systems

The applicability of the steady-state Relative Gain Array (RGA) to measure dynamic process interactions in a multiloop control system was investigated. Several transfer function matrices were chosen, and the gains, time constants, and dead times of their elements were varied to represent the systems with dominant dynamic interactions. It was shown that the steady-state RGA method predicted the controller pairing accurately if the pairing elements recommended by RGA had the bigger gains and the same or smaller time constants compared to other elements in the corresponding rows. When these conditions were not met, the RGA would give a wrong result, and dynamic interaction measurements, such as the Average Dynamic Gain Array (ADGA) and the Inverse Nyquist Array (lNA), should be used instead to determine the best controller pairing in a multiloop control system. Keywords: Control pairing, dynamic process interaction, multiloop control systems, Relative Gain Array (RGA), and steady state.