MATLAB Examples

Explicit MPC Control of an Aircraft with Unstable Poles

This example shows how to control an unstable aircraft with saturating actuators using Explicit MPC.

Reference:

[1] P. Kapasouris, M. Athans and G. Stein, "Design of feedback control systems for unstable plants with saturating actuators", Proc. IFAC Symp. on Nonlinear Control System Design, Pergamon Press, pp.302--307, 1990

[2] A. Bemporad, A. Casavola, and E. Mosca, "Nonlinear control of constrained linear systems via predictive reference management", IEEE® Trans. Automatic Control, vol. AC-42, no. 3, pp. 340-349, 1997.

See also MPCAIRCRAFT.

Contents

Define Aircraft Model

The linear open-loop dynamic model is as follows:

A = [-0.0151 -60.5651 0 -32.174;
     -0.0001 -1.3411 0.9929 0;
      0.00018 43.2541 -0.86939 0;
      0      0       1      0];
B = [-2.516 -13.136;
     -0.1689 -0.2514;
     -17.251 -1.5766;
      0        0];
C = [0 1 0 0;
     0 0 0 1];
D = [0 0;
     0 0];
plant = ss(A,B,C,D);
x0 = zeros(4,1);

The manipulated variables are the elevator and flaperon angles, the attack and pitch angles are measured outputs to be regulated.

The open-loop response of the system is unstable.

pole(plant)
ans =

  -7.6636 + 0.0000i
   5.4530 + 0.0000i
  -0.0075 + 0.0556i
  -0.0075 - 0.0556i

Design MPC Controller

To obtain an Explicit MPC controller, you must first design a traditional MPC (also referred as Implicit MPC) that is able to achieves your control objectives.

% *MV Constraints*

Both manipulated variables are constrained between +/- 25 degrees. Since the plant inputs and outputs are of different orders of magnitude, you also use scale factors to facilitate MPC tuning. Typical choices of scale factor are the upper/lower limit or the operating range.

MV = struct('Min',{-25,-25},'Max',{25,25},'ScaleFactor',{50,50});

OV Constraints

Both plant outputs have constraints to limit undershoots at the first prediction horizon. You also specify scale factors for outputs.

OV = struct('Min',{[-0.5;-Inf],[-100;-Inf]},'Max',{[0.5;Inf],[100;Inf]},'ScaleFactor',{1,200});

Weights

The control task is to get zero offset for piecewise-constant references, while avoiding instability due to input saturation. Because both MV and OV variables are already scaled in MPC controller, MPC weights are dimensionless and applied to the scaled MV and OV values. In this example, you penalize the two outputs equally with the same OV weights.

Weights = struct('MV',[0 0],'MVRate',[0.1 0.1],'OV',[10 10]);

Construct the traditional MPC controller

Create an MPC controller with plant model, sample time and horizons.

Ts = 0.05;          % Sampling time
p = 10;             % Prediction horizon
m = 2;              % Control horizon
mpcobj = mpc(plant,Ts,p,m,Weights,MV,OV);

Generate Explicit MPC Controller

Explicit MPC executes the equivalent explicit piecewise affine version of the MPC control law defined by the traditional MPC. To generate an Explicit MPC from a traditional MPC, you must specify range for each controller state, reference signal, manipulated variable and measured disturbance so that the multi-parametric quadratic programming problem is solved in the parameter space defined by these ranges.

Obtain a range structure for initialization

Use generateExplicitRange command to obtain a range structure where you can specify range for each parameter afterwards.

range = generateExplicitRange(mpcobj);
-->Converting model to discrete time.
-->Assuming output disturbance added to measured output channel #1 is integrated white noise.
-->Assuming output disturbance added to measured output channel #2 is integrated white noise.
-->The "Model.Noise" property of the "mpc" object is empty. Assuming white noise on each measured output channel.

Specify ranges for controller states

MPC controller states include states from plant model, disturbance model and noise model in that order. Setting the range of a state variable is sometimes difficult when the state does not correspond to a physical parameter. In that case, multiple runs of open-loop plant simulation with typical reference and disturbance signals are recommended in order to collect data that reflect the ranges of states.

range.State.Min(:) = -10000;
range.State.Max(:) = 10000;

Specify ranges for reference signals

Usually you know the practical range of the reference signals being used at the nominal operating point in the plant. The ranges used to generate Explicit MPC must be at least as large as the practical range.

range.Reference.Min = [-1;-11];
range.Reference.Max = [1;11];

Specify ranges for manipulated variables

If manipulated variables are constrained, the ranges used to generate Explicit MPC must be at least as large as these limits.

range.ManipulatedVariable.Min = [MV(1).Min; MV(2).Min] - 1;
range.ManipulatedVariable.Max = [MV(1).Max; MV(2).Max] + 1;

Construct the Explicit MPC controller

Use generateExplicitMPC command to obtain the Explicit MPC controller with the parameter ranges previously specified.

mpcobjExplicit = generateExplicitMPC(mpcobj, range);
display(mpcobjExplicit);

Regions found / unexplored:      483/       0

 
Explicit MPC Controller
---------------------------------------------
Controller sample time:    0.05 (seconds)
Polyhedral regions:        483
Number of parameters:      10
Is solution simplified:    No
State Estimation:          Default Kalman gain
---------------------------------------------
Type 'mpcobjExplicit.MPC' for the original implicit MPC design.
Type 'mpcobjExplicit.Range' for the valid range of parameters.
Type 'mpcobjExplicit.OptimizationOptions' for the options used in multi-parametric QP computation.
Type 'mpcobjExplicit.PiecewiseAffineSolution' for regions and gain in each solution.

Use simplify command with the "exact" method to join pairs of regions whose corresponding gains are the same and whose union is a convex set. This practice can reduce memory footprint of the Explicit MPC controller without sacrifice any performance.

mpcobjExplicitSimplified = simplify(mpcobjExplicit, 'exact');
display(mpcobjExplicitSimplified);

Regions to analyze:      471/     471

 
Explicit MPC Controller
---------------------------------------------
Controller sample time:    0.05 (seconds)
Polyhedral regions:        471
Number of parameters:      10
Is solution simplified:    Yes
State Estimation:          Default Kalman gain
---------------------------------------------
Type 'mpcobjExplicitSimplified.MPC' for the original implicit MPC design.
Type 'mpcobjExplicitSimplified.Range' for the valid range of parameters.
Type 'mpcobjExplicitSimplified.OptimizationOptions' for the options used in multi-parametric QP computation.
Type 'mpcobjExplicitSimplified.PiecewiseAffineSolution' for regions and gain in each solution.

The number of piecewise affine region has been reduced.

Plot Piecewise Affine Partition

You can review any 2-D section of the piecewise affine partition defined by the Explicit MPC control law.

Obtain a plot parameter structure for initialization

Use generatePlotParameters command to obtain a parameter structure where you can specify which 2-D section to plot afterwards.

params = generatePlotParameters(mpcobjExplicitSimplified);

Specify parameters for a 2-D plot

In this example, you plot the pitch angle (the 4th state variable) vs. its reference (the 2nd reference signal). All the other parameters must be fixed at a value within its range.

Fix other state variables

params.State.Index = [1 2 3 5 6];
params.State.Value = [0 0 0 0 0];

Fix other reference signals

params.Reference.Index = 1;
params.Reference.Value = 0;

Fix manipulated variables

params.ManipulatedVariable.Index = [1 2];
params.ManipulatedVariable.Value = [0 0];

Plot the 2-D section

Use plotSection command to plot the 2-D section defined previously.

plotSection(mpcobjExplicitSimplified, params);
axis([-10 10 -10 10]);
grid;
xlabel('Pitch angle (x_4)');
ylabel('Reference on pitch angle (r_2)');

Simulate Using Simulink®

To run this example, Simulink® is required.

if ~mpcchecktoolboxinstalled('simulink')
    disp('Simulink(R) is required to run this example.')
    return
end

Simulate closed-loop control of the linear plant model in Simulink, using the Explicit MPC Controller block. Controller "mpcobjExplicitSimplified" is specified in the block dialog.

mdl = 'empc_aircraft';
open_system(mdl)
sim(mdl)

The closed-loop response is identical to the traditional MPC controller designed in the "mpcaircraft" example.

bdclose(mdl)