Question

I’m experimenting with ode45 in Matlab. I’ve learned how to pass parameters to the ode function but I still have a question. Let’s suppose that I want to compute the trajectory (speed profile) of a Car and I have a function, e.g. getAcceleration, that gives me the acceleration of the car but also the right gear: [acceleration, gear] = getAcceleration(speed,modelStructure) where modelStructure represents the model of the car.

The ode function would be:

function [dy] = car(t,y,modelStructure)

dy           = zeros(2,1);
dy(1)        = y(2);
[dy(2),gear] = getAcceleration(y(1),modelStructure);

Then I call the Ode45 integrator in this way:

tInit = 0;
tEnd  = 5,
[t,y] = ode45(@car,[tInit tEnd], [speedInitial,accelerationInitial],options,modelStructure);

The problem is: how do I get the vector storing gears? Should I have something like [t,y,gear]=ode45(....) or should gear be within the y vector?

---

I’ve been working on my code and using the events function I’m now able to get the car ‘gears’ changes (as events).
Now I have a new problem related to the same code.
Imagine that when I evaluate de ‘dy’ vector I’m able to get a further value Z which let me to have a massive speed up calling the acceleration computation (getAcceleration):

function [dy] = car(t,y,modelStructure)

dy           = zeros(2,1);
dy(1)        = y(2);
[dy(2),Z(t)] = getAcceleration(y(1),modelStructure,Z(t-1)); 

and suppose that I’m also able to compute Z at the initial condition. The problem is that I’m not able to compute the Z derivative.

Is there a way to pass Z value throw the stepping without integrating it?

Thanks guys.

Answer 1

First off: why are the initial values to the differential equation the initial speed (speedInitial) and the initial acceleration (accelerationInitial)? That means that the differential equation car will be computing the acceleration (y(1)) and the jerk (y(2)), the time-derivative of the acceleration, at each time t. That seems incorrect…I would say the initial values should be the initial position (positionInitial) and the initial speed (speedInitial). But, I don’t know your model, I could be wrong.

Now, getting the gear in the solution directlty: you can’t, not without hacking ode45. This is also logical; you also cannot get dy at all times directly, can you? That’s just not how ode45 is set up.

There’s two ways out I see here:

Global variable

DISCLAIMER: don’t use this method. It’s only here to show what most people would do as a first attempt.

You can store gear in a global variable. It’s probably the least amount of coding, but also the least convenient outcome:

global ts gear ii

ii    = 1;
tInit = 0;
tEnd  = 5,
[t,y] = ode45(...
    @(t,y) car(t,y,modelStructure), ...
    [tInit tEnd], ...
    [speedInitial, accelerationInitial], options);

...

function [dy] = car(t,y,modelStructure)
global ts gear ii

dy    = zeros(2,1);
dy(1) = y(2);
[dy(2),gear(ii)] = getAcceleration(y(1),modelStructure);

ts(ii) = t;
ii = ii + 1;

But, due to the nature of ode45, this will get you an array of times ts and associated gear which contains intermediate points and/or points that got rejected by ode45. So, you’ll have to filter for those afterwards:

ts( ~ismember(ts, t) ) = [];

I’ll say it again: this is NOT the method I’d recommend. Only use global variables when testing or doing some quick-n-dirty stuff, but always very quickly shift towards other solutions. Also, the global variables grow at each (sub-)iteration of ode45, which is an unacceptable performance penalty.

It’s better to use the next method:

Post-solve call

This is also not too hard for your case, and the way I’d recommend you to go. First, modify the differential equation as below, and solve as normal:

tInit = 0;
tEnd  = 5,
[t,y] = ode45(...
    @(t,y) car(t,y,modelStructure), ...
    [tInit tEnd], ...
    [speedInitial, accelerationInitial], options);

...

function [dy, gear] = car(t,y,modelStructure)    

dy    = [0;0];
dy(1) = y(2);
[dy(2),gear] = getAcceleration(y(1),modelStructure);

and then after ode45 completes, do this:

gear = zeros(size(t));
for ii = 1:numel(t)
    [~, gear(ii)] = car(t(ii), y(ii,:).', modelStructure); 
end

That will get you all the gears the car would have at times t.

The only drawback that I can see here is that you’ll have many more function evaluations of car than ode45 would use by itself. But this is only a real problem if each evaluation of car takes in the order of seconds or longer, which I suspect is not the case in your setup.