Help on sign function argument

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headlight
headlight 2015-3-25
编辑： Andrew Newell 2015-3-25
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];
xdot=A*x;
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.)

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Andrew Newell
Andrew Newell 2015-3-25
Use
2*sign(x-0.01)-1
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headlight
headlight 2015-3-25
There are 2 cases. If x>0.01, x'' = -22zwx' - wx If x<0.01, x'' = -2zwx' - wx
Andrew Newell
Andrew Newell 2015-3-25
编辑：Andrew Newell 2015-3-25
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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