Problem using the bvp4c solver. First derivate seems to be constant on the graphic

Hello there, I have implemented a code to simulate the deflection of a beam. It works fine but physically there seems to be a problem with my non-linear solution. I wonder if the bvp4c solver is considering my first derivate correctly. It only appears on the non-linear solution so the graphics from the linear(in blue) and non linear(in red) should be somehow different, but the red looks as if they were multiplied by a constant.

It may be a syntax problem but I have tried to find it for quite a while and can´t. My question is very simple: is the "Non linear solver" correctly considering that y(2) is my dy/dx? The code is below:

function 2modificado= 2modificado(qo,L)

global qmax Lg EI;
qmax = qo;
Lg = L;
Em=200*10^9

%This part below is not relevant for the problem

      for i=1:3
      if i==1
          b=0.055;
          h=0.1;
          Im=b*h^3/12
      end
      if i==2
          Im=2.5175*10^(-6)
      end
      if i==3
          re=0.065;
          ri=0.060;
          Im=pi/4*((re^4)-(ri^4))
      end
  EI=(Em*Im)*(1000^2)

x0 = [0 0];
%Längenintervall
xspan = [0 L];

% This part below is:

%Call Solver -> Linear
solinit = bvpinit(linspace(0,L,10),[0 0]);
solL=bvp4c(@bvp4odeL, @bvp4bcL, solinit);
xint=linspace(0,L)
Sxint=deval(solL,xint)
%Plot result
figure
plot(xint,Sxint(1,:))
if i==1  title('y(x) in a Retangular Profile') 
end
if i==2  title('y(x) in a Iprofile(IPE 100)')  
end
if i==3  title('y(x) in a Round hollow section') 
end
set(gca,'yDir','reverse')
xlabel('[mm]');
ylabel('[mm]');
hold on;
%Plot result
plot(xint,Sxint(1,:));

%Call Solver Non-linear
solinitNL = bvpinit(linspace(0,L,10),[0 0]);
solNL=bvp4c(@bvp4odeNL, @bvp4bcNL, solinitNL);
xintNL=linspace(0,L)
SxintNL=deval(solNL,xint)
plot(xintNL,SxintNL(1,:),'-r')
set(gca,'yDir','reverse')
end   
return

%---------------------------------------------------------------
function dydx = bvp4odeL(x,y)
global qmax Lg EI;
dydx = [y(2) (qmax*x*(x-Lg)/(2*EI))]

function res=bvp4bcL(ya,yb)
res=[ya(1) yb(1)]

function dydx = bvp4odeNL(x,y)
global qmax Lg EI;
dydx = [y(2) (qmax*x*(x-Lg)/(2*EI))*(1+y(2)^2)^1.5]

function res=bvp4bcNL(ya,yb)
res=[ya(1) yb(1)]

Answer 1

[Sean de Wolski]

One thing sticks out: dydx should be a column vector

dydx = [y(2); (qmax*x*(x-Lg)/(2*EI))*(1+y(2)^2)^1.5]

Otherwise, try putting a function in there that you know for sure should make it highly non-linear.

[Daniel Clemente]

Thanks Sean! I did not notice it.