The steps to be followed in the identification of process dynamics are developed logically. These steps cover experiment design, the use of physical laws and a priori knowledge, model structure determination, parameter estimation and model validation. The presentation is directed toward guiding the control engineer in industry to the intelligent use of the copious, well-documented supply of theory and case studies. Emphasis is laid upon techniques which have proven practical value.
I ntroductionFollowing Box and Jenkinsl and Mehra,2 the steps in system identification are as shown schematically in Fig 1, which is self-explanatory. A first step is often to install measurement transducers and put data logging equipment on to the plant. Thereby valuable information is obtained on normal plant operation under steady-state and transient conditions. Often identification stops at this point in industrial applications. A typical example is in the plotting of engine performance maps for diesel and internal combustion engines. Such maps give a static relation between mean output torque, mean piston speed and brake specific fuel consumption. From a knowledge of the load-speed duty cycle, perhaps expressed in joint probabilities of various loads and speeds, the mean fuel consumption rate can be calculated, and hence gearbox design/control strategies can be engineered to minimise fuel consumption. This is a problem where a deterministic static model is adequate, and is characteristic of many industrial control problems.Not infrequently, the dynamics of the system cannot be totally neglected. For example, in start-up and shut-down of plant, the sequencing controls may have to take account of time delays in long pipelines and thermal inertias in heat processes. Therefore, as a further step in identification, simple test inputs may be applied, such as step, pulse or sinewave functions, to give information on time delays, gains and resonances. This brings in classical control theory, where information on frequency response characteristics may be used to design a single-loop compensator.The routine uses of identification in the process industries are largely as already outlined, and represent superficial application of the identification format indicated in Fig 1. The use of statistical techniques (correlation analysis, spectral analysis and maximum likelihood methods) in process identification,3 despite wide reporting in the literature, is not routine, although there are specialist applications, for example of spectral analysis in vibration and fatigue testing of jet engines and vehicle structures. However, the situation is changing dramatically. Three innovative forces are acting simultaneously to cause this:(1) The new-generation control engineers are well versed in modem control theory and the use of computers and are anxious to apply their knowledge.(2) Low-cost microprocessor technology is enabling data 4 to be collected more easily and in pre-processed form.4 In addition, the advent of effective time sharing systems with che...