Help on sign function argument

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headlight on 25 Mar 2015
Edited: Andrew Newell on 25 Mar 2015
Here's my function:
function xdot=gotp(t,x)
z=0.1; w=1;
A = [0 1; (-22*z*w*sign()-2*z*w) -w];
Called by:
[t,x]=ode45('gotp',[0 10],[0 -0.025]);
I want the sign function to return 1 if x is above 0.010 and 0 if it is below 0.010. How can I do this? I've tried putting different integers into the sign function's parentheses, such as sign(10000) and sign(1), but it has no effect on the output. If I use the variable "x," I get the an error (Dimensions of matrices being concatenated are not consistent.)

Accepted Answer

Andrew Newell
Andrew Newell on 25 Mar 2015
Andrew Newell
Andrew Newell on 25 Mar 2015
O.k., the first case would translate into
x' = y
y' = -22zwy - wx
or, in matrix form,
[x'; y'] = [0 1; -w -22*z*w]*[x; y]
(you have the bottom two elements in the wrong order). Thus, in your vector X = [x; y] (capitalized to avoid confusion), the element you want to test is X(1), so in your code,
s = sign(x(1)-0.01);
A = [0 1; -w (-12-10*s)*z*w]
However, it might be more accurate to create a terminal event so the integration stops when x crosses 0.01. Then you can switch to the other function and continue. See ode45 for more information.

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