How do I extract extra parameters from the ode45 ordinary differential equation solver function?
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I have the following function which represents an ever increasing mass on a spring according to Hooke's Law:
function dydt = HookeWithRisingMass(t, y, m0, k, Cload)
%HookeWithRisingMass defines Hooke's Law but with a mass increasing at a constant rate over time.
%INPUTS:
%t = [start, finish], time interval
%y = [disp; velocity], initial conditions of the system; displacement from equilibrium and starting velocity
%k = the spring constant
%m0 = initial mass
%Cload = constant of mass increase
%OUTPUTS:
%dydt = [time, [diplacement, velocity]]
%See also
mass = m0 + Cload*t; %mass increases linearly with time
disp = y(1); %Initial displacement
dxdt = y(2); % set dx/dt = velocity
dvdt = -(k/mass).*disp; % set dv/dt = acceleration
dydt = [dxdt; dvdt]; % return the derivatives
It works correctly when called by e.g. [timeout,out] = ode45(@HookeWithRisingMass, tin, y, [], m0, k, Cload).
However I really need to get a time series of values of the mass vs. time for use elsewhere. At the moment I can do this by adding the following to the HookeWithRisingMass function definition:
paff = 'C:\matlabtemp\';
fileName = num2str(t);
fileName(regexp(fileName, '\D')) = 'D';
fileName = ['mass' fileName];
save([paff fileName], 'm', 't');
I can then sequentially load the saved files and store the values of m, t in an array. This is a slow method and gets very unwieldy when the maximum value of time, t, is large. Is there a method of extracting t, m, pairs and storing them in an array that is available in the workspace of the script/function that makes the ode45 call?
采纳的回答
One way is to use OutputFcn. I have attached an example function that will save mass to the workspace when the calculation is done. Please have myOutputFcn.m saved in the same directly with HookeWithRisingMass.m, then the following script would get you mass.
m0 = 1;
k = 1;
Cload = 1;
y0 = [1,1];
tin = [0,1];
odeFcn = @(t,y) HookeWithRisingMass(t,y, m0, k, Cload);
options = odeset('OutputFcn',@(t,y,flag) myOutputFcn(t,y,flag, m0, Cload));
[timeout,out] = ode45(odeFcn, tin, y0, options);
For details on this option, please see "OutputFcn" section of odeset. The following is the content of myOutputFcn.m.
function status = myOutputFcn(t,y,flag, m0, Cload)
% OutputFcn sample
persistent mass
switch flag
case 'init'
mass = m0 + Cload*t; %mass increases linearly with time
case ''
mass = [mass, m0 + Cload*t];
case 'done' % when it's done
assignin('base','mass',mass); % get the data to the workspace.
end
status = 0;
5 个评论
Robert
2016-10-6
Robert
2016-10-6
Vincent DUFOUR
2017-3-14
编辑：Vincent DUFOUR
2017-3-14
I am sorry to re-open this subject, but once you have done it, how do you get to the variable you extracted (I mean in the workspace) ?
Thanks in advance
Simon Silge
2020-5-5
That is done by the assignin('base','mass',mass)
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