In the Control System
Toolbox™ software, you can use
frd models to store, manipulate,
and analyze frequency response data. An
frd model stores a
vector of frequency points with the corresponding complex frequency response data
you obtain either through simulations or experimentally.
For example, suppose you measure frequency response data for the SISO system you want to model. You can measure such data by driving the system with a sine wave at a set of frequencies ω1, ω2, ,...,ωn, as shown:
The measurement yields the complex frequency response G at each input frequency:
You can do most frequency-domain analysis tasks on
models, but you cannot perform time-domain simulations with them. For information on
frequency response analysis of linear systems, see Chapter 8 of .
Use the following commands to create FRD models.
This example shows how to create a single-input, single-output (SISO)
frequency-response model using
A frequency-response model stores a vector of frequency points with corresponding complex frequency response data you obtain either through simulations or experimentally. Thus, if you measure the frequency response of your system at a set of test frequencies, you can use the data to create a frequency response model:
Load the frequency response data in
This command loads the data into the MATLAB® workspace as the column vectors
resp. The variables
resp contain 256 test frequencies and the
corresponding complex-valued frequency response points, respectively.
To inspect these variables, enter:
whos freq resp
Create a frequency response model.
sys = frd(resp,freq);
sys is an
frd model object, which is a data
container for representing frequency response data.
You can use
frd models with many frequency-domain analysis
commands. For example, visualize the frequency response data using
By default, the
frd command assumes that the frequencies
are in radians/second. To specify different frequency units, use the
frd model object. For example:
sys = frd(resp,freq,'TimeUnit','min','FrequencyUnit','rad/TimeUnit')
sets the frequency units to radians/minute.