Estimate the open-loop response of the plant in the watertank model. First, open the model.

model = 'watertank';
open_system(model);

Define a linearization I/O set that specifies the plant, and find a steady-state operating point for estimation.

io(1)=linio('watertank/PID Controller',1,'input');
io(2)=linio('watertank/Water-Tank System',1,'openoutput');

watertank_spec = operspec(model);
opOpts = findopOptions('DisplayReport','off');
op = findop(model,watertank_spec,opOpts);

Then, create an input signal for estimation, and estimate the frequency response of the specified portion of the model. Use the simout output argument to store the estimation data.

input = frest.Sinestream('Frequency',logspace(-3,2,10));
[sysest,simout] = frestimate(model,op,io,input);

Open the Simulation Results Viewer.

frest.simView(simout,input,sysest)

The viewer shows you the steady-state time response and the FFT of that response for all frequencies within the range you select on the Bode Diagram section of the viewer. These plots can help you identify when the response deviates from the expected response. For more information about using the Simulation Results Viewer, see Analyze Estimated Frequency Response.

If you have a linear model of the system you are estimating, you can use the model as a baseline response for comparison in the viewer. For instance, you can compare a model obtained by exact linearization to the estimated frequency response. Use the linearization I/O set and the operating point to compute an exact linearization of the watertank plant.

syslin = linearize(model,io,op);

Open the Simulation Results Viewer again, this time providing syslin as an input argument.

frest.simView(simout,input,sysest,syslin)

The Bode Diagram section of the viewer includes a line showing the exact response syslin. This view can be useful to identify particular frequencies where the estimated response deviates from the linearization.