Create a piecewise-linear nonlinearity estimator object
NL = idPiecewiseLinear
NL = idPiecewiseLinear(Name,Value)
creates a default
piecewise-linear nonlinearity estimator object with 10 break points for estimating
Hammerstein-Wiener models. The value of the nonlinearity at the break points are set to
NL = idPiecewiseLinear
. The initial value of the nonlinearity is determined from the
estimation data range during estimation using
nlhw. Use dot notation to customize the object properties, if
creates a piecewise-linear nonlinearity estimator object with properties specified by
one or more
NL = idPiecewiseLinear(
Name,Value pair arguments. The properties that you do
not specify retain their default value.
idPiecewiseLinear is an object that stores the piecewise-linear
nonlinearity estimator for estimating Hammerstein-Wiener models.
idPiecewiseLinear to define a nonlinear function , where y and x are scalars, and
θ represents the parameters specifying the number of break points
and the value of nonlinearity at the break points.
The nonlinearity function, F, is a piecewise-linear
(affine) function of x. There are
(xk,yk), k =
1,...,n, such that yk =
F(xk). F is
linearly interpolated between the breakpoints.
F is also linear to the left and right
of the extreme breakpoints. The slope of these extensions is a function
of xi and yi breakpoints.
The breakpoints are ordered by ascending
which is important when you set a specific breakpoint to a different
There are minor difference between the breakpoint values you set and the values stored in the object because the toolbox has a different internal representation of breakpoints.
For example, in the following plot, the breakpoints are xk = [-2,1,4] and the corresponding nonlinearity values are yk = [4,3,5].
F(x) is computed by
NL is the
idPiecewiseLinear object. When
evaluate, the break points have to be initialized
idPiecewiseLinear object properties, see Properties.
NL = idPiecewiseLinear;
Specify the number of break points.
NL.NumberOfUnits = 5;
Load estimation data.
load twotankdata; z = iddata(y,u,0.2,'Name','Two tank system'); z1 = z(1:1000);
idPiecewiseLinear object, and specify the breakpoints.
InputNL = idPiecewiseLinear('BreakPoints',[-2,1,4]);
BreakPoints is specified as a vector, the specified vector is interpreted as the x-values of the break points. The y-values of the break points are set to 0, and are determined during model estimation.
Estimate model with no output nonlinearity.
sys = nlhw(z1,[2 3 0],InputNL,);
comma-separated pairs of
the argument name and
Value is the corresponding value.
Name must appear inside quotes. You can specify several name and value
pair arguments in any order as
Name,Value arguments to specify additional properties of
idPiecewiseLinear nonlinearity. For example,
idPiecewiseLinear('NumberofUnits',5) creates a piecewise-linear
nonlinearity estimator object with 5 breakpoints.
idPiecewiseLinear object properties include:
Number of breakpoints, specified as an integer.
Break points, xk, and the corresponding nonlinearity values at the breakpoints, yk, specified as one of the following:
When the nonlinearity object is created, the breakpoints are ordered by
Option to fix or free the values in the mapping object, specified as a
logical scalar. When you set an element of
NL— Piecewise-linear nonlinearity estimator object
Piecewise-linear nonlinearity estimator object, returned as an
idnlhwmapping object names is not recommended.
Not recommended starting in R2021b
Starting in R2021b, the mapping objects (also known as nonlinearities) used in the nonlinear components of the
idnlhw objects have been renamed. The following table lists the name changes.
|Pre-R2021b Name||R2021b Name|
Scripts with the old names still run normally, although they will produce a warning. Consider using the new names for continuing compatibility with newly developed features and algorithms. There are no plans to exclude the use of these object names at this time