Centralized vs decentralized adaptive generalized predictive control of a biodiesel reactor
Asia-Pacific Journal of Chemical Engineering (Impact Factor: 0.79). 01/2012;
A second look at biodiesel reactor control using Recursive Least Squares (RLS)-based adaptive Generalized Predictive Control (GPC) strategy revealed the possibility of a simpler alternative to the previously published centralized RLS-based GPC controller (CRLS-GPC). New results show that the simpler decentralized RLS-based GPC controller (DRLS-GPC) was on par with the more sophisticated centralized version in terms of servo and regulatory control, process interactions handling, and the resultant controller moves. Moreover, the simplified control scheme remained superior to the conventional Proportional–Integral controller. Such attributes make the DRLS-GPC an attractive compromise between complexity and performance.
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ABSTRACT: In this work, the Recursive Least Squares (RLS) algorithm, which traditionally was used in the Generalized Predictive Controller (GPC) framework solely for model adaptation purposes, was extended to cater for auto-tuning of the controller. This new combination which eases the task of controller tuning, contains both model adaptation and auto-tuning capabilities within the same controller structure. Hereafter this scheme will be referred to as the Adaptive-Model Based Self-Tuning Generalized Predictive Control (AS-GPC). The Variable Forgetting Factor Recursive Least Squares (VFF-RLS) algorithm was selected to capture the dynamics of the process online for the purpose of model adaptation in the controller. Based on the evolution of the process dynamics given by the VFF-RLS algorithm in the form of First Order Plus Dead Time (FOPDT) model parameters, the move suppression weight for the AS-GPC was recalculated automatically at every time step based on existing Single Input Single Output (SISO) analytical tuning expressions originally used for offline tuning of constraint-free predictive controllers. Closed loop simulation on a validated transesterification reactor model, known for inherent nonlinearities, revealed the superiority of the proposed constrained control scheme in terms of servo and regulatory control as compared to the GPC with model adaptation only, the conventional GPC as well as the conventional PID controller. The tuning expressions used, although intended for constraint-free predictive controllers, yielded good results even in the constrained case.
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