Open-Loop Process Identification

This section will examine the principles and key results for modelling an open-loop process modelled using the general prediction error model given by Eq. (6.4). The foundation for such modelling is the prediction error method, which uses the fact that most models in system identification are used for predicting future values of the process. 

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Theorem 6.3 Open-Loop Process Identification (properties of the prediction error method). The prediction error method produces parameter estimates that are unbiased if the prediction error is a white noise signal. 

Final Considerations in Open-Loop Process Identification... 

An automated pilot-scale 1-litre experimental polymer reactor system with facilities for on-line measurement of flow rate, temperature and density has been set up by Chien and Penlidis (1994a, b). These authors describe a set of open-loop process identification experiments and closed-loop control experiments performed on this system where monomer conversion is controlled in the presence of reactive impurities using the initiator flow rate as the manipulated variable. 

Chien, D. C. H. A. Penlidis (1994o), Effect of impurities on continuous solution methyl methacrylate polymerization reactors. I. Open-loop process identification results , Polymer Reaction Engineering 2, 163-213. 

The primary objective is to perform system identification, that is, obtain a plant model, especially that of the process, in order to design a controller. Two different situations can be considered open-loop system identification and closed-loop system identification. 

The identification of plant models has traditionally been done in the open-loop mode. The desire to minimize the production of the off-spec product during an open-loop identification test and to avoid the unstable open-loop dynamics of certain systems has increased the need to develop methodologies suitable for the system identification. Open-loop identification techniques are not directly applicable to closed-loop data due to correlation between process input (i.e., controller output) and unmeasured disturbances. Based on Prediction Error Method (PEM), several closed-loop identification methods have been presented Direct, Indirect, Joint Input-Output, and Two-Step Methods. 

The PBL reactor considered in the present study is a typical batch process and the open-loop test is inadequate to identify the process. We employed a closed-loop subspace identification method. This method identifies the linear state-space model using high order ARX model. To apply the linear system identification method to the PBL reactor, we first divide a single batch into several sections according to the injection time of initiators, changes of the reactant temperature and changes of the setpoint profile, etc. Each section is assumed to be linear. The initial state values for each section should be computed in advance. The linear state models obtained for each section were evaluated through numerical simulations. 

Show that, for open-loop identification irrespective of the true plant model, an output-error model will provide an unbiased estimate of the process parameters. Provide a useful implication of this result... 

Model identification is the process of quantifying process dynamics. The techniques available fall into one of two approaches - open loop and closed loop testing. Open loop tests are performed with either no controller in place or, if existing, with the controller in manual mode. A disturbance is injected into the process by directly changing the MV. Closed loop tests may be used if a controller exists and already provides some level of stable control. Under these circumstances the MV is changed indirectly by making a change to the SP of the controller. 

The concept of extension the linear Intemal Model Control to a nonlinear form allowed to obtain a more accuracy model what also improved the control performance, which is expressed by reducing the overshoot and settling time for a wide range of work points. In this study the parametric model of the process was created by means of identification based of an open-loop step response by different rudder angle thereby the different ship velocity components. The proposed controller can be successfully used to reach a new set-point of course and also after the necessary modifications also in vessel maneuvering along a desired path. 

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