non-zero threshold for event function

I have written the following event function and I want to detect an event which is non-zero. Therefore, I am using the logical operator since event function will only work for value=0. In this code, y is 4x1 vector.

function [value,isterminal,direction] = myevent12d1(t,y)
    % Locate the time when height passes through zero in a decreasing direction
    % and stop integration.
    z=y(1)<y(3);
    value = z;     % detect height = 0
    isterminal = 1;   % stop the integration
    direction = -1;   % negative 

    

However, upon running the code, I am getting the following error message:

    Undefined function 'sign' for input arguments of type 'logical'.

Error in odezero (line 46)
    indzc = find((sign(vL) ~= sign(vR)) & (direction .* (vR - vL) >= 0));

Error in ode45 (line 352)
    [te,ye,ie,valt,stop] = ...

Error in onedof2dof (line 19)
            [t1,y1,te1,ye1,ie1]=ode45(@(t,y) sp121(t,y,k1,k2,m1,m2),[tstart tfinal],y0,options1);
            
            

Any help in this regard would be appreciated.

Answer 1

[Star Strider]

I am not certain what you are doing. If you want to evaluate ‘z’ as numeric rather than logical, just do an arithmetic operation on it, here that being uplus (unary plus):

y = 1:4;                                        % Create ‘y’
z1 = y(1)<y(3)                                  % Logical Result
z2 = +(y(1)<y(3))                               % Double Result

Note the parentheses, so the logical operation is done first, then the arithmetic operation.

Using the ‘z2’ approach will at least avoid the problem with logical variables.

EDIT —

Also:

z3 = y(3) - y(1);

Note that the documentation on ODE Event Location states: ‘Event functions take an expression that you specify, and detect an event when that expression is equal to zero.’ So a variable in your ODE evalutation does not itself have to be zero, it simply needs for the ‘value’ output to be zero to trigger the event.

[9times6]

Thanks a lot, Star Strider! It worked :)

[Star Strider]

As always, my pleasure!