Process analysis / Model configuration

Closed control loop synthesis

Another important task of project design work is undoubtedly the configuration and commissioning of the designed closed control loop and the binary control system. On the basis of the automation of process technology, the most important task in this respect is the commissioning of the closed control loop. This article therefore explains the main aspects involved in the solution of this problem.

Process analysis / Model configuration

To be able to solve an automation task (configuration and commissioning of closed control loops), it is essential to have the most comprehensive information possible with regard to the static and dynamic characteristics of the control system (processes) to be automated. The attainable quality of the solution is largely dependent on the qualitative and quantitative knowledge available concerning the technical process to be automated, in order to be able to define in detail appropriate algorithms for its control and the required hardware and software tools for its realisation.
The analysis of the behaviour and the characteristics of technical systems (controlled systems) is known as process analysis or model configuration; the result of which is known as a process model or, in short, model. Models of this type not only assist in the design of automation systems, but also are of fundamental importance for other areas of technology, natural sciences, economics, etc.

Behavioural models

In the area of technology, so-called behavioural models play a major part and are intended to reflect accordingly the system behaviour with regard to cause, effect and correlation. Behavioural models are mainly used to predict events. They are used with the intention of being able to determine future system behaviour with sufficient accuracy, i.e. to determine the reactions of the system to causes (input signals), which are not yet of importance at the model configuration point. A "good" model reflects the behaviour of the original concept as adequately as possible with the use of simple means. It should merely reflect the behaviour of the original, which is of relevance to the solution of a particular task. Being a substitute of the original, the objective of the model must be in agreement with the functional behaviour of its original. The performance (quality) of models must therefore be sufficiently tested with regard to application prior to its practical utilisation, e.g. for the design of automation systems.
Since system behaviour is largely dependent on the signals acting upon the system (correcting and disturbance variables), a process model generally consists of a system model and a signal model. To design a controlling device, you need to know whether a system is operated primarily by means of step-type, periodic or accidental signals. A signal analysis therefore generally also forms part of the process analysis; in some cases, the signal analysis is the sole aim of the process analysis.
Models are therefore used in the design of open and closed control systems for the:

>> selection and definition of appropriate measured variables and correcting variables;
>> detection and evaluation of interference signals;
>> description of static and dynamic behaviour of controlled systems;
>> detection of functional links between process variables;
>> simulative calculation of design variants;
>> selection of control algorithms and dimensioning of the characteristic values of the controlling device.

Model configuration strategies

The model can be configured along theoretical and/or experimental lines (Picture 1). In the case of theoretical model configuration, the physical/chemical processes occurring in an actual technical system are analysed and mathematically formulated with the help of the familiar laws of mechanics, thermodynamics, etc. With experimental model configuration, the input and output signals of the actual technical systems are measured and evaluated, whereby artificially modulated or naturally occurring input signals such as stepchange signals may be used. To a certain degree, the objects to be modelled are therefore analysed externally.

Picture 1: Fundamentals of model configuration

At this point, it should be emphasised that a model obtained along this experimental line seldom permits any information about the actual physical/chemical processes in the process. The model merely describes the interaction between input and output variables and is therefore also known as an I/O model. It is therefore quite common for the two methods of model configuration to be combined by determining the model configuration by means of theoretical system analysis and the model parameters (characteristic system values) by means of experimenting.