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Vary Input and Output Bounds at Run Time

This example shows how to vary input and output saturation limits in real-time control. For both command-line and Simulink® simulations, you specify updated input and output constraints at each control interval. The MPC controller then keeps the input and output signals within their specified bounds.

For more information on updating linear constraints at run time, see Update Constraints at Run Time.

Create Plant Model and MPC Controller

Define a SISO discrete-time plant with sample time Ts.

Ts = 0.1;
plant = c2d(tf(1,[1 .8 3]),Ts);
[A,B,C,D] = ssdata(plant);

Create an MPC controller with specified prediction horizon, p, control horizon, c, and sample time, Ts. Use plant as the internal prediction model.

p = 10;
m = 4;
mpcobj = mpc(plant,Ts,p,m);
-->The "Weights.ManipulatedVariables" property of "mpc" object is empty. Assuming default 0.00000.
-->The "Weights.ManipulatedVariablesRate" property of "mpc" object is empty. Assuming default 0.10000.
-->The "Weights.OutputVariables" property of "mpc" object is empty. Assuming default 1.00000.

Specify controller tuning weights.

mpcobj.Weights.MV = 0;
mpcobj.Weights.MVrate = 0.5;
mpcobj.Weights.OV = 1;

For this example, the upper and lower bounds on the manipulated variable, and the upper bound on the output variable are varied at run time. To do so, you must first define initial dummy finite values for these constraints in the MPC controller object. Specify values for MV.Min, MV.Max, and OV.Max.

At run time, these constraints are changed using an mpcmoveopt object at the command line or corresponding input signals to the MPC Controller block.

mpcobj.MV.Min = 1;
mpcobj.MV.Max = 1;
mpcobj.OV.Max = 1;

Simulate Model Using Simulink

Open Simulink Model.

mdl = 'mpc_varbounds';
open_system(mdl)

In this model, the input and output constraint input ports of the MPC Controller block are enabled. The umin, umax, and ymax ports are connected to signals which change during the simulation. Since the minimum output bound is unconstrained, the ymin input port is disconnected.

Configure the output setpoint, ref, and simulation duration, Tsim.

ref = 1;
Tsim = 20;

Since the ymin port is disconnected, disable input/output not connected warnings.

set_param(mdl,'UnconnectedInputMsg','off')
set_param(mdl,'UnconnectedOutputMsg','off')

Run the simulation, and view the input and output responses in the I/O scope.

sim(mdl)
open_system([mdl '/I//O'])
-->Converting the "Model.Plant" property of "mpc" object to state-space.
-->Assuming output disturbance added to measured output channel #1 is integrated white noise.
-->The "Model.Noise" property of the "mpc" object is empty. Assuming white noise on each measured output channel.
The MPC block inport "ymin" is unconnected.  Limits specified in the controller object will be used.  This behavior differs from earlier releases.

Simulate Model at Command Line

Specify the initial state of the plant and controller.

x = zeros(size(B,1),1);
xmpc = mpcstate(mpcobj);

Store the closed-loop output, manipulated variable, and state trajectories of the MPC controller in arrays YY, UU, and XX, respectively.

YY = [];
UU = [];
XX = [];

Create an mpcmoveopt object for specifying the run-time bound values.

options = mpcmoveopt;

Run the simulation loop.

for t = 0:round(Tsim/Ts)
    % Store the plant state.
    XX = [XX; x];

    % Compute and store the plant output. There is no direct feedthrough
    % from the input to the output.
    y = C*x;
    YY = [YY; y'];

    % Get the reference signal value from the data output by the Simulink
    % simulation.
    ref = yout.Data(t+1,2);

    % Update the input and output bounds. For consistency, use the
    % constraint values output by the Simulink simulation.
    options.MVMin = uout.Data(t+1,2);
    options.MVMax = uout.Data(t+1,3);
    options.OutputMax = yout.Data(t+1,3);

    % Compute the MPC control action.
    u = mpcmove(mpcobj,xmpc,y,ref,[],options);

    % Update the plant state and store the input signal value.
    x = A*x + B*u;
    UU = [UU; u'];
end

Compare Simulation Results

Plot the input and output signals from both the Simulink and command-line simulations along with the changing input and output bounds.

figure
subplot(1,2,1)
plot(0:Ts:Tsim,[UU uout.Data(:,1) uout.Data(:,2) uout.Data(:,3)])
grid
title('Input')
legend('Command-line input','Simulink input','Lower bound',...
    'Upper bound','Location','Southeast')
subplot(1,2,2)
plot(0:Ts:Tsim,[YY yout.Data(:,1) yout.Data(:,3)])
grid
title('Output')
legend('Command-line output','Simulink output','Upper bound',...
    'Location','Southeast')

The results of the command-line and Simulink simulations are the same. The MPC controller keeps the input and output signals within the specified bounds as the constraints change throughout the simulation.

bdclose(mdl);
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