Design PID Controller Using FRD Model Obtained From "frestimate" Command

This example shows how to design a PI controller with frequency response estimated from a plant built in Simulink. This is an alternative PID design workflow when the linearized plant model is invalid for PID design (for example, when the plant model has zero gain).

Opening the Model

Open the engine control model and take a few moments to explore it.

mdl = 'scdenginectrlpidblock';

The PID loop includes a PI controller in parallel form that manipulates the throttle angle to control the engine speed. The PI controller has default gains that makes the closed loop system oscillate. We want to design the controller using the PID Tuner that is launched from the PID block dialog.

open_system([mdl '/Engine Speed (rpm)'])

PID Tuner Obtaining a Plant Model with Zero Gain From Linearization

In this example, the plant seen by the PID block is from throttle angle to engine speed. Linearization input and output points are already defined at the PID block output and the engine speed measurement respectively. Linearization at the initial operating point gives a plant model with zero gain.

% Hide scope
close_system([mdl '/Engine Speed (rpm)'])
% Obtain the linearization input and output points
io = getlinio(mdl);
% Linearize the plant at initial operating point
linsys = linearize(mdl,io)
linsys =
  D = 
                Throttle Ang
   EngineSpeed             0
Static gain.

The reason for obtaining zero gain is that there is a triggered subsystem "Compression" in the linearization path and the analytical block-by-block linearization does not support events-based subsystems. Since the PID Tuner uses the same approach to obtain a linear plant model, the PID Tuner also obtains a plant model with zero gain and reject it during the launching process.

To launch the PID Tuner, open the PID block dialog and click Tune button. An information dialog shows up and indicates that the plant model linearized at initial operating point has zero gain and cannot be used to design a PID controller.

The alternative way to obtain a linear plant model is to directly estimate the frequency response data from the Simulink model, create an FRD system in MATLAB Workspace, and import it back to the PID Tuner to continue PID design.

Obtaining Estimated Frequency Response Data Using Sinestream Signals

Sinestream input signal is the most reliable input signal for estimating an accurate frequency response of a Simulink model using frestimate command. More information on how to use frestimate can be found in the example "Frequency Response Estimation Using Simulation-Based Techniques" in Simulink Control Design examples.

In this example, we create a sine stream that sweeps frequency from 0.1 to 10 rad/sec. Its amplitude is set to be 1e-3. You can inspect the estimation results using the bode plot.

% Construct sine signal
in = frest.Sinestream('Frequency',logspace(-1,1,50),'Amplitude',1e-3);
% Estimate frequency response
sys = frestimate(mdl,io,in); % this command may take a few minutes to finish
% Display Bode plot

Designing PI with the FRD System in PID Tuner

SYS is a FRD system that represents the plant frequency response at the initial operating point. To use it in the PID Tuner, we need to import it after the Tuner is launched. Click Plant and select Import.

Click the 2nd radio button, select "sys" from the list, and click "OK" to import the FRD system into the PID Tuner. The automated design returns a stabilizing controller. Click Add Plot and select Open-Loop Bode plot. The plot shows reasonable gain and phase margin. Click Show Parameters to see the gain and phase margin values. Time domain response plots are not available for FRD plant models.

Click Update Block to update the PID block P and I gains to the PID.

Simulating Closed-Loop Performance in Simulink Model

Simulation in Simulink shows that the new PI controller provides good performance when controlling the nonlinear model.

Close the model.

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