Transfer function from ARX-model

Im trying to get a transfer function from an arx model. I have tried to follow an MATLAB-example that i found on the homepage. I think my ARX model is pretty good but i dont really know how to get a good op-point for the linearization

Im using the example file twotankdata with 1 input and 1 output.

Help would be greatly appreciated!

This is how far im currently at:

 load twotankdata.mat

ze = iddata(y, u, 0.2);
m3 = nlarx(ze,[2 2 3],wavenet('num',8));% na=nb=2 and nk=3
figure(2);step(m3,60)

stepinput = iddata([],[zeros(35,1);ones(1500,1)],m3.Ts); 
%Dont know what happends here!

x0 = findop(m3,'steady',1,NaN);
sys2 = linearize(m3,1,x0)

figure(4);step(sys2,60)

Answer 1

[Rajiv Singh]

Does the example on http://www.mathworks.com/help/ident/ref/idnlarxlinearize.html

help?

[Viktor]

Thank you for you answer.

Thats the example i have been following. I dont understand whats happening under point 4:

Compute the operating point corresponding to T = 20.
stepinput = iddata([],[zeros(10,1);ones(200,1)],...
                   nlsys.Ts);
% Compute operating point.
[x,u] = findop(nlsys,'snapshot',20,stepinput);

[Arkadiy Turevskiy]

As stated here:

[X,U] = findop(SYS,'snapshot',T,UIN,X0) computes the operating point at a simulation snapshot of time T using the specified input and initial state values