User`s guide

2 Choosing Your System Identification Strategy
model object. For example, the estimation method n4sid is unique to the
state-space model object
idss.
Every class has a special method for creating objects of that class, called the
constructor. U sing a constructor creates an instance of the corresponding
class or instantiates the object. The constructor name is the same as the class
name. For example,
idpoly is both the name of the class representing linear
black-box polynomial models and the n ame of the constructor for instantiating
the model object.
For a tutorial about estimating models at the command line, see “Tutorial –
Identifying Linear Models Using the Command Line” in System Identification
Toolbox Getting Started G uide.
When to Construct a Model Structure Independently
of Estim ation
You use model constructors to create a model object a t the command line by
specifying all required model properties ex plicitly.
You must construct the model object independently of estim ation whe n you
want to:
• Simulate a model
• Analyze a model
• Specify an initial guess for specific m odel parameter values before
estimation
In most cases, you can use the estim ation comm ands to both construct
and estimate the model—without having to construct the model object
independently. For e xample, the estimation command
pem lets you specify
both the model structure w ith unknown parameters and the estimation
algorithm. For information about how to both construct and estimate models
with a single command, see “Commands for M odel Estimation” on page 2-9.
In case of grey-box models, you must always construct the model object first
and then estimate the parameters of the ordinary differential or difference
equation. For more information, see Chapter 5, “ODE Parameter Estimation
(Grey-Box Modeling)”.
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