Example: Path planning problem

Binder nbviewer

This example was borrowed from [1, IX. Examples, A] whose dynamics comes from the model given in [2, Ch. 2.4]. This is a reachability problem for a continuous system.

Let us consider the 3-dimensional state space control system of the form

\[\dot{x} = f(x, u)\]

with $f: \mathbb{R}^3 × U ↦ \mathbb{R}^3$ given by

\[f(x,(u_1,u_2)) = \begin{bmatrix} u_1 \cos(α+x_3)\cos(α^{-1}) \\ u_1 \sin(α+x_3)\cos(α^{-1}) \\ u_1 \tan(u_2) \end{bmatrix}\]

and with $U = [−1, 1] \times [−1, 1]$ and $α = \arctan(\tan(u_2)/2)$. Here, $(x_1, x_2)$ is the position and $x_3$ is the orientation of the vehicle in the 2-dimensional plane. The control inputs $u_1$ and $u_2$ are the rear wheel velocity and the steering angle. The control objective is to drive the vehicle which is situated in a maze made of obstacles from an initial position to a target position.

In order to study the concrete system and its symbolic abstraction in a unified framework, we will solve the problem for the sampled system with a sampling time $\tau$.

The abstraction is based on a feedback refinment relation [1,V.2 Definition]. Basically, this is equivalent to an alternating simulation relationship with the additional constraint that the input of the concrete and symbolic system preserving the relation must be identical. This allows to easily determine the controller of the concrete system from the abstraction controller by simply adding a quantization step.

For the construction of the relations in the abstraction, it is necessary to over-approximate attainable sets of a particular cell. In this example, we consider the used of a growth bound function [1, VIII.2, VIII.5] which is one of the possible methods to over-approximate attainable sets of a particular cell based on the state reach by its center. Therefore, it is used to compute the relations in the abstraction based on the feedback refinement relation.

For this reachability problem, the abstraction controller is built by solving a fixed-point equation which consists in computing the the pre-image of the target set.

First, let us import StaticArrays.

using StaticArrays

At this point, we import the useful Dionysos sub-module for this problem: Abstraction.

using Dionysos
using Dionysos.Abstraction
AB = Dionysos.Abstraction;

Definition of the system

Definition of the dynamics function $f$ of the system:

function F_sys(x, u)
    α = atan(tan(u[2])/2)
    return SVector{3}(
        u[1]*cos(α + x[3])/cos(α),
        u[1]*sin(α + x[3])/cos(α),
        u[1]*tan(u[2]))
end;

Definition of the growth bound function of $f$:

ngrowthbound = 5;
function L_growthbound(u)
    β = abs(u[1]/cos(atan(tan(u[2])/2)))
    return SMatrix{3,3}(
        0.0, 0.0, 0.0,
        0.0, 0.0, 0.0,
        β, β, 0.0)
end;

Here it is considered that there is no system and measurement noise:

sysnoise = SVector(0.0, 0.0, 0.0);
measnoise = SVector(0.0, 0.0, 0.0);

Definition of the discretization time step parameters: tstep and nsys:

tstep = 0.3;
nsys = 5;

Finally, we build the control system:

contsys = AB.NewControlSystemGrowthRK4(tstep, F_sys, L_growthbound, sysnoise,
                                       measnoise, nsys, ngrowthbound);

Definition of the control problem

Definition of the state-space (limited to be rectangle):

_X_ = AB.HyperRectangle(SVector(0.0, 0.0, -pi - 0.4), SVector(4.0, 10.0, pi + 0.4));

Definition of the obstacles (limited to be rectangle):

X1_lb = [1.0, 2.2, 2.2, 3.4, 4.6, 5.8, 5.8, 7.0, 8.2, 8.4, 9.3, 8.4, 9.3, 8.4, 9.3];
X1_ub = [1.2, 2.4, 2.4, 3.6, 4.8, 6.0, 6.0, 7.2, 8.4, 9.3, 10.0, 9.3, 10.0, 9.3, 10.0];
X2_lb = [0.0, 0.0, 6.0, 0.0, 1.0, 0.0, 7.0, 1.0, 0.0, 8.2, 7.0, 5.8, 4.6, 3.4, 2.2];
X2_ub = [9.0, 5.0, 10.0, 9.0, 10.0, 6.0, 10.0, 10.0, 8.5, 8.6, 7.4, 6.2, 5.0, 3.8, 2.6];

Definition of the input-space (limited to be rectangle):

_U_ = AB.HyperRectangle(SVector(-1.0, -1.0), SVector(1.0, 1.0));

Definition of the initial state-space (here it consists in a single point):

_I_ = AB.HyperRectangle(SVector(0.4, 0.4, 0.0), SVector(0.4, 0.4, 0.0));

Definition of the target state-space (limited to be hyper-rectangle):

_T_ = AB.HyperRectangle(SVector(3.0, 0.5, -100.0), SVector(3.6, 0.8, 100.0));

Definition of the abstraction

Definition of the grid of the state-space on which the abstraction is based (origin x0 and state-space discretization h):

x0 = SVector(0.0, 0.0, 0.0);
h = SVector(0.2, 0.2, 0.2);
Xgrid = AB.GridFree(x0, h);

Construction of the struct DomainList containing the feasible cells of the state-space:

Xfull = AB.DomainList(Xgrid);
AB.add_set!(Xfull, _X_, AB.OUTER)
for (x1lb, x2lb, x1ub, x2ub) in zip(X1_lb, X2_lb, X1_ub, X2_ub)
    box = AB.HyperRectangle(SVector(x1lb, x2lb, _X_.lb[3]), SVector(x1ub, x2ub, _X_.ub[3]))
    if box ⊆ _X_ && isempty(box ∩ _I_) && isempty(box ∩ _T_)
        AB.remove_set!(Xfull, box, AB.OUTER)
    end
end

Definition of the grid of the input-space on which the abstraction is based (origin u0 and input-space discretization h):

u0 = SVector(0.0, 0.0);
h = SVector(0.3, 0.3);
Ugrid = AB.GridFree(u0, h);

Construction of the struct DomainList containing the quantized inputs:

Ufull = AB.DomainList(Ugrid);
AB.add_set!(Ufull, _U_, AB.OUTER)

Construction of the abstraction:

symmodel = AB.NewSymbolicModelListList(Xfull, Ufull);
@time AB.compute_symmodel_from_controlsystem!(symmodel, contsys)
compute_symmodel_from_controlsystem! started
compute_symmodel_from_controlsystem! terminated with success: 13081981 transitions created
  4.977670 seconds (168.94 k allocations: 339.345 MiB, 2.75% gc time, 2.95% compilation time)

Construction of the controller

Computation of the initial symbolic states:

Xinit = AB.DomainList(Xgrid);
AB.add_subset!(Xinit, Xfull, _I_, AB.OUTER)
initlist = [AB.get_state_by_xpos(symmodel, pos) for pos in AB.enum_pos(Xinit)];

Computation of the target symbolic states:

Xtarget = AB.DomainList(Xgrid)
AB.add_subset!(Xtarget, Xfull, _T_, AB.OUTER)
targetlist = [AB.get_state_by_xpos(symmodel, pos) for pos in AB.enum_pos(Xtarget)];

Construction of the controller:

contr = AB.NewControllerList();
@time AB.compute_controller_reach!(contr, symmodel.autom, initlist, targetlist)
compute_controller_reach! started

compute_controller_reach! terminated with success
  2.724211 seconds (54 allocations: 162.985 MiB)

Trajectory display

We choose the number of steps nsteps for the sampled system, i.e. the total elapsed time: nstep*tstep as well as the true initial state x0 which is contained in the initial state-space _I_ defined previously.

nstep = 100;
x0 = SVector(0.4, 0.4, 0.0);

Here we display the coordinate projection on the two first components of the state space along the trajectory.

To complete

References

  1. G. Reissig, A. Weber and M. Rungger, "Feedback Refinement Relations for the Synthesis of Symbolic Controllers," in IEEE Transactions on Automatic Control, vol. 62, no. 4, pp. 1781-1796.
  2. K. J. Aström and R. M. Murray, Feedback systems. Princeton University Press, Princeton, NJ, 2008.

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