Simulink Control Design™ - MathWorks

Step Signals

Step inputs are quick to simulate. Like a random signal, a step signal can excite the

system at all frequencies up to the Nyquist frequency. For those reasons, a step input can

be useful as a first try when you do not have much knowledge about the system you are

trying to estimate. However, the amplitude of the excitation decreases rapidly with

increasing frequency. Therefore, step signals are best used to identify low-order plants

where the slowest poles are dominant. Step inputs are not recommended for estimation

across a wide range of frequencies.

To create a step input signal for estimation, use frest.createStep. This function

creates a MATLAB timeseries that represents a step input having the sample time, step

time, step size, and total signal length that you specify when you call

frest.createStep.

To use the step input signal you created in the MATLAB workspace:

• In the Linear Analysis Tool, in the Estimation tab, select it from the Existing Input
  Signals section of the Input Signal drop-down list.


• At the command line, use it as an input argument to frestimate.

When you use a step input signal for estimation, the frequencies returned in the

estimated frd model depend on the length and sampling time of the signal. They are the

frequencies obtained in the fast Fourier transform of the input signal (see the Algorithm

section of frestimate).

Arbitrary Signals

If you want to use a signal other than a sinestream, chirp, step, or random signal, you can

provide your own MATLAB timeseries object. For instance, you can create a timeseries

representing a ramp, sawtooth, or square wave input.

To use a timeseries object as the input signal for estimation, first create the timeseries

in the MATLAB workspace. Then:

• In the Linear Analysis Tool, in the Estimation tab, select it from the Existing Input
  Signals section of the Input Signal drop-down list.


• At the command line, use it as an input argument to frestimate.

When you use an arbitrary input signal for estimation, the frequencies returned in the

estimated frd model depend on the length and sampling time of the signal. They are the

5 Frequency Response Estimation