njit-etd2003-081 - New Jersey Institute of Technology

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The bulk of the theory and applications of SID evolved from control systems
research. In control language, the dynamic system being identified (or modeled) is
called the plant. The plant can take on a wide variety of forms. In general, a plant is an
object or collection of objects, producing an output by modifying an input. The input to
the plant could be temperature, pressure, voltage, current, position, velocity,
acceleration, and so on. The input can also contain more than one variable (e.g., several
signals from transducers measuring various conditions of an engine). The output from
the plant can be in the form of any of the inputs (temperature, pressure, position, etc.)
and it also can contain several signals.
Single-input, single-output (siso) plants are addressed here since they are the
most prevalent. The siso models can be combined to create multi-input, multi-output
models if necessary.
Figure 3.13 depicts an example of an electrical plant. The plant can be
accurately described mathematically by linear constant/coefficient, differential
equations. A plant described by these equations is said to be Lumped-Linear-Time-
Invariant (LLTI). Even plants that are mildly non-linear, slowly time varying, and
possibly distributed in nature (e.g., those requiring partial differential equations for a full
description) can often be adequately modeled using these LLTI assumptions.
3.13.1 Estimating Transfer Functions of Non -linear Systems
Accurate transfer function estimation of linear, noise-free, dynamic systems is typically
an easy task. Often, however, the system being analyzed is noisy or not perfectly linear.
All real-world systems suffer from these deficiencies to a degree, but biological systems