# An introduction to the moment method for optimal control problems for polynomial ODEs

Author: - 04 February 2019

The aim of this note is to explain how one can solve an optimal control problem with the moment method. While the theory is collected in [1], we will explain how one is able to use the Globtipoly toolbox, which is a powerful toolbox able to solve many other problems. This toolbox is available in http://homepages.laas.fr/henrion/software/gloptipoly3/. A more detailed documentation is provided in this link, if you are insterested in other applications than the optimal control problem.

The use of this toolbox needs a proper installation either of Sedumi or the Mosek solvers, which are toolboxes instrumental to solve numerically SDP (Semidefinite Program) problems, i.e. optimization problems expressed with LMI (Linear Matrix Inequalities) constraints.

This note does not aim at explaining all the details of the toolbox Globtipoly, since there exists already the paper [2] which introduces really well the problem. It does not aim neither at introducing in details the moment framework developed by Jean Bernard Lasserre and Didier Henrion, which is much more general than the optimal control problem framework. In this note, we just give a simple example to explain how this method can be used in the context of optimal control (especially for ODEs). Thanks to the recent paper [3], we also think that this framework might also work for optimal control of PDEs.

## FRAMEWORK

We focus on the following ordinary differential equations:

where $x$ belongs to a subset $X$ of $\mathbb{R}^n$ and $u$ to a subset $U$ of $\mathbb{R}^m$. We assume that $f$ is a polynomial of $x$ and $u$. Moreover, we assume that $X$ and $U$ can be written with finitely many polynomial inequalities. The Gloptipoly toolbox allows to solve the following kind of optimal control problem:

where $X_T$ is also a subset of $\mathbb{R}^n$ defined with finitely many polynomial inequalities and $l$ is also a polynomial in the variables $x$ and $u$.

## A SIMPLER EXAMPLE AS A MOTIVATION

We focus on a specific example to make the reading of thenote easier. We have

$u\in\mathbb{R}$ and:

It is a simple chain of integrators, really well-known in automatic control theory. As described in [1], the related system can be equivalently rewritten as follows:

where $g$ is any test function in $C^1(X)$, $\mu_1(x)$ is a measure supported on a compact set $K_1$ representing the constraints on $x$ and $u$, $\mu_2(x)=$ is supported on a given compact set $K_2$ corresponding to performance requirements. For example $K_2 = 0$ indicates that state $x$ must reach the origin.

The approximation provided by Gloptipoly consists in reducing the class of test function $g$ to be polynomials, with maximal degree $d$. This maximal degree $d$ is the parameter of approximation of the program. As proved in [1], the higher the degree $d$ is, the better is the result of the approximation.

Our aim is to find the minimal time so that the trajectories of the system go to $0$. Obviously, this result is already known, but we provide this simple example to make clear the use of the Globtipoly toolbox

## IMPLEMENTATION

Before going to the optimal control problem, some (easy) technical lines of command are necessary for Gloptipoly to work.

First, you have to add to the path the toolbox Globtipoly

untar('http://homepages.laas.fr/henrion/software/gloptipoly3/gloptipoly3.tar.gz')


Second, you have also to add to the path the toolbox Sedumi.

unzip('https://github.com/sqlp/sedumi/archive/master.zip')


Then, you are ready to define the optimization problem.

x0 = [1; 1]; u0 = 0; %% initial conditions
d = 6; %% maximum degree of test function.


We compute the analytic minimum time. We want to know whether we are able to compute the good one with the toolbox Gloptipoly.

if x0(1) >= -(x0(2)^2-2)/2
tmin = 1+x0(1)+x0(2)+x0(2)^2/2;
elseif x0(1) >= -x0(2)^2/2*sign(x0(2))
tmin = 2*sqrt(x0(1)+x0(2)^2/2)+x0(2);
else
tmin = 2*sqrt(-x0(1)+x0(2)^2/2)-x0(2);
end


We define measures for constraints. In other words, we also define the vector field under consideration.

The command mpol in Globtipoly allows to define the variables under consideration. One can then define polynomials with respect to these variables.

The command meas allows to define a measure depending on several variables. Hence, when writing meas([x_1;u_1]), one is defining a measure depending on the variables x_1 and u_1.

mpol x1 2
mpol u1
m1 = meas([x1;u1]);


We now define a measure associated to $x_2$, which will be related to some performance requirement. Indeed, we will impose that this variable is dissipative.

mpol x2 2
m2 = meas(x2);


We then define the vector field.

f = [x1(2);u1];


We now define the test functions as polynomials, with maximal degree $d$. The command mmon in Globtipoly allows to define monomials up to an order $d$.

g1 = mmon(x1,d);
g2 = mmon(x2,d);


It is also necessary to assign the initial conditions, for the states as well for the control.

assign([x1;u1],[x0;u0]);
g0 = double(g1);


We define $K_1$ and $K_2$. The set $K_1$ is defined as: $K_1= \lbrace x_1,u_1 \in \mathbb{R}^2\mid x_1(2)\geq -1,: u_1^2\leq 1\rbrace$ and $K_2$ is defined as: $K_2 =\lbrace x_2\in\mathbb{R}\mid x_2^2\leq 0\rbrace$.

We have also to define the cost function of the problem under consideration. The one we are considering is quite simple and just reduces to being

In terms of functions, this means that you are minizing $x_1(t)$. This implies in particular that you are looking for the minimal time such that the trajectory $(x_1(t),x_2(t))$ will converge to $(0,0)$.

In other words, we are asking that the mass the measure $\mu_1$ is minimal.

Finally, we define also the differential equation with the polynomials truncated up to the order $d$. The command mom corresponds to the integral of polynomials against the suitable measure. For instance, for mom(g_2), one integrates the the polynomial g_2 against the measure $\mu_2$.

P = msdp(min(mass(m1)),... %%Definition of the cost function
u1^2 <= 1,... %% Definition of $K_1$
x1(2) >= -1,... %% Definition of $K_&$
x2'*x2 <= 0,... %% Definition of $K_2 mom(g2) - g0 == mom(diff(g1,x1)*f)); %% Definition of the dynamics.  GloptiPoly 3.8 of 15 December 2014 Define moment SDP problem Valid objective function Number of support constraints = 3 Number of moment constraints = 28 Measure 1 Degree = 6 Variables = 3 Moments = 84 Measure 2 Degree = 6 Variables = 2 Moments = 28 Relaxation order = 3 Total number of moments = 112 Generate moment and support constraints Generate moment SDP problem  The command msdp allows to define the problem and all the constraints. As written in [1], this means that you are transforming the problem on functions into a SDP (Semi-definite programming) problem. Once you do that, one has to solve this SDP problem. The command msol allows to do that. [status,obj] = msol(P);  GloptiPoly 3.8 of 15 December 2014 Solve moment SDP problem ***************************************************** Calling SeDuMi SeDuMi 1.32 by AdvOL, 2005-2008 and Jos F. Sturm, 1998-2003. Alg = 2: xz-corrector, Adaptive Step-Differentiation, theta = 0.250, beta = 0.500 Put 28 free variables in a quadratic cone eqs m = 112, order n = 59, dim = 766, blocks = 7 nnz(A) = 597 + 0, nnz(ADA) = 10192, nnz(L) = 5152 it : b*y gap delta rate t/tP* t/tD* feas cg cg prec 0 : 6.97E-01 0.000 1 : -4.85E+00 2.61E-01 0.000 0.3749 0.9000 0.9000 -0.46 1 1 7.9E+00 2 : -1.58E+01 1.08E-01 0.000 0.4124 0.9000 0.9000 -0.63 1 1 5.7E+00 3 : -1.60E+01 4.52E-02 0.000 0.4189 0.9000 0.9000 0.74 1 1 2.0E+00 4 : -7.27E+00 1.65E-02 0.000 0.3663 0.9000 0.9000 1.88 1 1 4.7E-01 5 : -4.16E+00 5.56E-03 0.000 0.3361 0.9000 0.9000 1.60 1 1 1.3E-01 6 : -3.52E+00 2.17E-03 0.000 0.3912 0.9000 0.9000 1.07 1 1 5.0E-02 7 : -3.37E+00 1.06E-03 0.000 0.4892 0.9000 0.9000 0.88 1 1 2.6E-02 8 : -3.24E+00 4.94E-04 0.000 0.4642 0.9000 0.9000 0.70 1 1 1.5E-02 9 : -3.17E+00 2.14E-04 0.000 0.4334 0.9000 0.9000 0.58 1 1 7.8E-03 10 : -3.12E+00 9.06E-05 0.000 0.4235 0.9000 0.9000 0.46 1 1 4.3E-03 11 : -3.08E+00 4.45E-05 0.000 0.4907 0.9000 0.9000 0.48 1 1 2.6E-03 12 : -3.05E+00 2.39E-05 0.000 0.5378 0.9000 0.9000 0.34 1 1 1.9E-03 13 : -3.02E+00 1.07E-05 0.000 0.4486 0.9000 0.9000 0.46 1 1 1.1E-03 14 : -2.98E+00 5.94E-06 0.000 0.5540 0.9000 0.9000 0.23 1 1 8.3E-04 15 : -2.95E+00 2.34E-06 0.000 0.3932 0.9000 0.9000 0.35 2 1 4.4E-04 16 : -2.91E+00 1.06E-06 0.000 0.4528 0.9000 0.9000 0.22 2 2 3.1E-04 17 : -2.88E+00 2.15E-07 0.000 0.2032 0.9000 0.7324 0.34 2 2 1.8E-04 18 : -2.84E+00 9.48E-08 0.000 0.4410 0.7706 0.9000 0.22 2 2 1.2E-04 19 : -2.82E+00 4.23E-08 0.000 0.4464 0.9000 0.9000 0.34 3 2 7.3E-05 20 : -2.78E+00 1.96E-08 0.000 0.4627 0.9000 0.9000 0.20 5 5 5.2E-05 21 : -2.76E+00 8.49E-09 0.000 0.4334 0.9000 0.9000 0.34 7 7 3.0E-05 22 : -2.73E+00 3.76E-09 0.000 0.4427 0.9000 0.9000 0.19 8 8 2.1E-05 23 : -2.70E+00 1.56E-09 0.000 0.4156 0.9000 0.9000 0.33 12 12 1.2E-05 24 : -2.67E+00 6.45E-10 0.000 0.4129 0.9000 0.9000 0.20 14 14 7.9E-06 25 : -2.66E+00 2.71E-10 0.000 0.4202 0.9000 0.9000 0.35 41 41 4.5E-06 Run into numerical problems. iter seconds digits c*x b*y 25 0.4 Inf -2.6569125651e+00 -2.6557155494e+00 \vertAx-b\vert = 9.3e-06, [Ay-c]_+ = 1.9E-06, \vertx\vert= 5.2e+03, \verty\vert= 4.9e+04 Detailed timing (sec) Pre IPM Post 6.598E-03 2.390E-01 7.343E-04 Max-norms: \vert\vertb\vert\vert=1, \vert\vertc\vert\vert = 1, Cholesky \vertadd\vert=1, \vertskip\vert = 7, \vert\vertL.L\vert\vert = 3155. ***************************************************** Check feasibility (eps = 1.0000e-03): Marginally feasible SDP: residual = -5.0513e-07 Check Euclidean norm of solution (max = 1.0000e+06): Norm = 4.9305e+04 Check ranks of moments matrices (rel gap svd = 1.0000e-03): Measure 1 Rank shift = 1 Moment matrix of order 1 has size 4 and rank 4 Moment matrix of order 2 has size 10 and rank 10 Moment matrix of order 3 has size 20 and rank 20 Measure 2 Rank shift = 1 Moment matrix of order 1 has size 3 and rank 1 Rank conditions cannot ensure global optimality Try to extract solutions (rel tol basis detection = 1.0000e-06): Measure 1 Incomplete basis - no solution extracted Global optimality cannot be ensured Measure 2 Maximum relative error = 0.0000e+00 1 solution extracted Inconsistent number of solutions amongst respective measures No globally optimal solution could be extracted Global optimality cannot be ensured  You display the minimal time and the lower bound of the cost functional. disp(['Minimum time = ' num2str(tmin)]); disp(['LMI ' int2str(d) ' lower bound = ' num2str(obj)])  Minimum time = 3.5 LMI 6 lower bound = 2.6557  For the initial condition x0 = [1 1] the exact minimum time is equal to 3.5. In table 1 of [2], a table is provided where you can see that when increasing$d\$ the minimum time becomes closer and closer to the analytic one. We copy it in the note.

## Conclusion

As noticed before, this note aimed at providing a quick course on how using the toolbox Gloptipoly toolbox to solve optimal control problem. The document [2] provides more examples, even for other kinds of optimization problem. Hence, we refer the interested reader to this document if he is interested in using this toolbox for other problems. Let us note moreover that this toolbox has been used to solve nonlinear hyperbolic PDEs, in the paper [3].

## References

[1] J. B. Lasserre, D. Henrion, C. Prieur, and E. Trélat, Nonlinear optimal control via occupation measures and LMI-relaxations, SIAM J. Control Opt., vol. 47, 4, pp. 1643-1666, 2008.

[2] D. Henrion, J.B. Lasserre and J. Löfberg. GloptiPoly 3: moments, optimization and semidefinite programming. Optimization Methods & Software, 24(4-5), 761-779, 2009.

[3] S. Marx, T. Weisser, D. Henrion and J.B. Lasserre. A moment approach for entropy solutions to nonlinear hyperbolic PDEs arXiv preprint arXiv:1807.02306