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Designing a Family of PID Controllers for Multiple Operating Points

This example shows how to design an array of PID controllers for a nonlinear plant in Simulink that operates over a wide range of operating points.

Opening the Plant Model

The plant is a continuous stirred tank reactor (CSTR) that operates over a wide range of operating points. A single PID controller can effectively use the coolant temperature to regulate the output concentration around a small operating range that the PID controller is designed for. But since the plant is a strongly nonlinear system, control performance degrades if operating point changes significantly. The closed-loop system can even become unstable.

Open the CSTR plant model.Open the CSTR plant model.

mdl = 'scdcstrctrlplant';
open_system(mdl)

For background, see Seborg, D.E. et al., "Process Dynamics and Control", 2nd Ed., 2004, Wiley, pp.34-36.

Introduction to Gain Scheduling

A common approach to solve the nonlinear control problem is using gain scheduling with linear controllers. Generally speaking designing a gain scheduling control system takes four steps:

(1) Obtain a plant model for each operating region. The usual practice
is to linearize the plant at several equilibrium operating points.
(2) Design a family of linear controllers such as PID for the plant
models obtained in the previous step.
(3) Implement a scheduling mechanism such that the controller
coefficients such as PID gains are changed based on the values of the
scheduling variables.  Smooth (bumpless) transfer between controllers
is required to minimize disturbance to plant operation.
(4) Assess control performance with simulation.

For more background reading on gain scheduling, see a survey paper from W. J. Rugh and J. S. Shamma: "Research on gain scheduling", Automatica, Issue 36, 2000, pp.1401-1425.

In this example, we focus on designing a family of PID controllers for the CSTR plant described in step 1 and 2.

Obtaining Linear Plant Models for Multiple Operating Points

The output concentration C is used to identify different operating regions. The CSTR plant can operate at any conversion rate between low conversion rate (C=9) and high conversion rate (C=2). In this example, divide the whole operating range into 8 regions represented by C = 2, 3, 4, 5, 6, 7, 8 and 9.

In the following loop, first compute equilibrium operating points with the findop command. Then linearize the plant at each operating point with the linearize command.

% Obtain default operating point
op = operspec(mdl);
% Set the value of output concentration C to be known
op.Outputs.Known = true;
% Specify operating regions
C = [2 3 4 5 6 7 8 9];
% Initialize an array of state space systems
Plants = rss(1,1,1,8);
for ct = 1:length(C)
    % Compute equilibrium operating point corresponding to the value of C
    op.Outputs.y = C(ct);
    opoint = findop(mdl,op,findopOptions('DisplayReport','off'));
    % Linearize plant at this operating point
    Plants(:,:,ct) = linearize(mdl, opoint);
end

Since the CSTR plant is nonlinear, we expect different characteristics among the linear models. For example, plant models with high and low conversion rates are stable, while the others are not.

isstable(Plants,'elem')'
ans =

     1     1     0     0     0     0     1     1

Designing PID Controllers for the Plant Models

To design multiple PID controllers in batch, we can use the pidtune command. The following command will generate an array of PID controllers in parallel form. The desired open loop crossover frequency is at 1 rad/sec and the phase margin is the default value of 60 degrees.

% Design controllers
Controllers = pidtune(Plants,'pidf',pidtuneOptions('Crossover',1));
% Display controller for C=4
Controllers(:,:,4)
ans =
 
             1            s    
  Kp + Ki * --- + Kd * --------
             s          Tf*s+1 

  with Kp = -12.5, Ki = -0.108, Kd = -14.4, Tf = 0.00875
 
Continuous-time PIDF controller in parallel form.

Plot the closed loop responses for step set-point tracking as below:

% Construct closed-loop systems
clsys = feedback(Plants*Controllers,1);
% Plot closed-loop responses
figure;
hold on
for ct = 1:length(C)
    % Select a system from the LTI array
    sys = clsys(:,:,ct);
    sys.Name = ['C=',num2str(C(ct))];
    sys.InputName = 'Reference';
    % Plot step response
    stepplot(sys,20);
end
legend('show','location','southeast')

All the closed loops are stable but the overshoots of the loops with unstable plants (C=4, 5, 6 and 7) are too large. It can be improved by increasing the open loop bandwidth to 10 rad/sec.

% Design controllers for unstable plant models
Controllers(:,:,3:6) = pidtune(Plants(:,:,3:6),'pidf',10);
% Display controller for C=4
Controllers(:,:,4)
ans =
 
             1            s    
  Kp + Ki * --- + Kd * --------
             s          Tf*s+1 

  with Kp = -163, Ki = -13.2, Kd = -138, Tf = 0.0471
 
Continuous-time PIDF controller in parallel form.

All the closed loop responses are satisfactory now.

% Construct closed-loop systems
clsys = feedback(Plants*Controllers,1);
% Plot closed-loop responses
figure;
hold on
for ct = 1:length(C)
    % Select a system from the LTI array
    sys = clsys(:,:,ct);
    set(sys,'Name',['C=',num2str(C(ct))],'InputName','Reference');
    % Plot step response
    stepplot(sys,20);
end
legend('show','location','southeast')

We designed an array of PID controllers and each of them should give reasonable performance around the local operating point. The next step is to implement the scheduling mechanism, which is beyond the scope of this example.

Close the model.

bdclose(mdl);
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