# Zermelos problem (version 2)

## Problem description

Time-optimal crossing by boat of a river with a position dependent current stream.

Applied Optimal Control, Bryson & Ho, 1975. Example 1 on page 77.

Programmers: Gerard Van Willigenburg (Wageningen University) Willem De Koning (retired from Delft University of Technology)

```% Copyright (c) 2009-2009 by Tomlab Optimization Inc.
```

## Problem setup

```% Array with consecutive number of collocation points
narr = [20 40];

toms t tf % Free final time

for n=narr
```
```    p = tomPhase('p', t, 0, tf, n);
setPhase(p)

tomStates x1 x2
tomControls u1

% Initial & terminal states
xi = [0;  0];
xf = [31; 0];

% Initial guess
if n==narr(1)
x0 = {tf == 2; icollocate({x1 == xi(1); x2 == xi(2)})
collocate({u1 == 0})};
else
x0 = {tf == tfopt; icollocate({x1 == xopt1; x2 == xopt2})
collocate({u1 == uopt1})};
end

% Box constraints
cbox = {1 <= tf <= 10};

% Boundary constraints
cbnd = {initial({x1 == xi(1); x2 == xi(2)});
final({x1 == xf(1); x2 == xf(2)})};

% ODEs and path constraints
v = 9;
% No water motion in x1 direction
dx1 = v*cos(u1);
% Water motion in x2 direction: 5*sin(pi*x1/31)
dx2 = v*sin(u1)+5*sin(pi*x1/31);

ceq = collocate({
dot(x1) == dx1
dot(x2) == dx2});

% Objective
objective = tf;
```

## Solve the problem

```    options = struct;
options.name = 'Ferry trajectory optimization';
solution = ezsolve(objective, {cbox, cbnd, ceq}, x0, options);

tfopt = subs(tf,solution);
xopt1 = subs(x1,solution);
xopt2 = subs(x2,solution);
uopt1 = subs(u1,solution);
```
```Problem type appears to be: lpcon
===== * * * =================================================================== * * *
TOMLAB - Tomlab Optimization Inc. Development license  999001. Valid to 2010-02-05
=====================================================================================
Problem: ---  1: Ferry trajectory optimization  f_k       3.681324200251761000
sum(|constr|)      0.000024237038659655
f(x_k) + sum(|constr|)      3.681348437290420600
f(x_0)      2.000000000000000000

Solver: snopt.  EXIT=0.  INFORM=1.
SNOPT 7.2-5 NLP code
Optimality conditions satisfied

FuncEv    1 ConstrEv   91 ConJacEv   91 Iter   54 MinorIter  110
CPU time: 0.156250 sec. Elapsed time: 0.172000 sec.
```
```Problem type appears to be: lpcon
===== * * * =================================================================== * * *
TOMLAB - Tomlab Optimization Inc. Development license  999001. Valid to 2010-02-05
=====================================================================================
Problem: ---  1: Ferry trajectory optimization  f_k       3.681324335382778000
sum(|constr|)      0.000002528802798087
f(x_k) + sum(|constr|)      3.681326864185576300
f(x_0)      3.681324200251761000

Solver: snopt.  EXIT=0.  INFORM=1.
SNOPT 7.2-5 NLP code
Optimality conditions satisfied

FuncEv    1 ConstrEv   40 ConJacEv   40 Iter   32 MinorIter  114
CPU time: 0.171875 sec. Elapsed time: 0.172000 sec.
```
```end

% Get solution
t  = subs(collocate(t),solution);
x1 = subs(collocate(x1),solution);
x2 = subs(collocate(x2),solution);
u1 = subs(collocate(u1),solution);

%Bound u1 to [0,2pi]
u1 = rem(u1,2*pi); u1 = (u1<0)*2*pi+u1;

% Plot final solution
figure(1)
subplot(2,1,1)
plot(t,x1,'*-',t,x2,'*-');
legend('x1','x2');
title('Ferry states');

subplot(2,1,2)
plot(t,u1,'+-');
legend('u1');
title('Ferry control');
``` 