How write an event function using a 6x1 state vector.

Hello, I am trying to write an event function where my state vector, Y, is a 6x1 vector made up of position and velocity

Y = [r; v] = [r1; r2; r3; v1; v2; v3]

r and v are 3x1 vectors as seen above.

I would like to have my ODE45 simulation stop when norm(r) <= (a certain scalar magnitude), but I am unsure how to do this with the event function.

Answer 1

[Walter Roberson]

function [value,isterminal,direction] = myEventsFcn(t,Y)
   value = SCALAR - norm(Y(1:3));
   isterminal = 1;
   direction = 0;
end

This codes to stop when SCALAR - norm(Y(1:3)) is positive (actually, when it crosses 0). constant minus function is positive when function becomes less than the constant.