Sub‐optimal control of a multivariable non‐linear chemical reactor using quasilinearization

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Abstract
The performance of a multivariable non‐linear stirred tank chemical reactor has been used to compare the practicability of an optimal controller and two types of suboptimal controller. The optimal controller, which by its nature required lengthy calculations, was suitable for open‐loop control only and therefore sensitive to modelling errors. By contrast a suboptimal controller, which used the method of quasilinearization to solve the two point boundary value problem, needed only a small amount of computation time and was capable of online implementation. This controller was much less sensitive to modelling errors, and also had the advantage of decoupling the control variables. An extension of this controller incorporated a feedback feature which compensated further for modelling errors and, in addition, unmeasured disturbances. This controller, which is expected to be of interest to the process industries facing inaccurate‐model problems, produced responses close to the true optimal response even when the actual model used was in gross error.