User`s guide

3 Linear Model Identification
Identifying Low-Order Transfer Functions (Process Models)
In this section...
“What Is a Process Model?” on page 3-22
“Data Supported by a Process Model” on page 3-23
“How to Estimate Process Models Using the GU I” on page 3-23
“Estimating Process Models at the Command Line” on page 3-29
“Options for Specifying the Process-Model S tructure” on page 3-35
“Options for M ultiple-Input Models” on page 3-36
“Options for the Disturbance Model Structure” on page 3-37
“Options for Frequency-Weighing Focus” on p ag e 3 -38
“Options for Initial States” on page 3-39
What Is a Process Model?
The structure of a continuous-time process model is a simple transfer function
that describes linear system dynamics in terms of one or m ore of the follow ing
elements:
• Static gain K
p
.
• OneormoretimeconstantsT
pk
. For complex poles, the time constant is
called
T
ω
—equal to the inverse of the natural frequency—and the damping
coefficient is
ζ
(zeta).
• Process zero T
z
.
• PossibletimedelayT
d
before the system output res ponds to the input
(dead time).
• Possible enforced integration.
Process models are popular for describing system dynamics in many
industries and apply to various production environments. The primary
advantages of these models are that they provide delay estimation, and the
model coefficients have a physical interpretation.
3-22