This paper presents the conditions necessary to sustain a limit cycle in hydraulic position control systems with displacement feedback. The limit cycle performance of the system is then examined when the temperature of the working fluid varies between 283K(10°C) to 343K(70°C). Results are presented for an electrohydraulic system driving a load comprising inertia, viscous friction, and coulomb friction.
a b s t r a c tSystem identification is the experimental approach to deriving process models, which can take many forms depending upon their intended use. In the work described in this paper, the ultimate aim is to use them in the design of controllers for regulating engineering processes. Modelling always involves approximations since all real systems are to some extent non-linear, time-varying, and distributed. Thus, it is highly improbable that any set of models will contain the 'true' system structure. A more realistic aim is therefore to identify a model that provides an acceptable approximation, in the context of the application in which it is used. In controller design, a first step is often to determine the model using step and frequency response data. This paper compares different modern software approaches that exploit step response data, where the aim is to determine either a first-or second-order-plus-dead-time (FOPDT or SOPDT) transfer function. They include an integral equation method, an algorithm available in the MATLAB Optimization Toolbox, and recently developed in-house software that uses a particle swarm optimisation (PSO) approach.
In water treatment processes, the individual unit operations are complex, highly non-linear and poorly understood. Whilst many models have been developed to improve process understanding, these are rarely in a form easily exploited by the control engineer. Attempts to improve the performance of water treatment works through the application of improved control and measurement have had variable success. This paper discusses investigations into the application of feedback control on the clarification process of a large-scale pilot plant using a streaming current detector (SCD). The application is aimed towards maximising the efficiency of the chemical coagulation process. To achieve this, a simple model of the interactions of process operating conditions on the SCD measurements must be made.
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