System identification

In previous chapters it was assumed that a model of the process exists and that the designer’s task is to use that model in the creation of a feedback controlled system. In fact, although a model structure may exist, it is frequently the case that its parameters are unknown, or at least are not known to the required precision. Hence a motor model may be derivable from basic electromagnetics and physics, but the moment of inertia of a particular motor may only be approximately known; a missile’s mass may decrease linearly with burn time of the rocket motor, but the rate and initial mass may only be guaranteed to a few per cent. This may or may not be a problem — after all, the reason for using feedback control in the first place is to reduce errors due to such factors as imprecisely known parameters — but accurate estimation of the parameters is sometimes important.