Constraining Nonnegative states in a NOT EXPLICITLY NAMED ODE FUNCTION

Hi Everybody!

I've written a function which contains a system model:

function [xdot] = BR_1D_model(t,x,valve)
xdot = A*x + B*u

I want to constrain some of the states in the model to be non-negative values. Specifically, one of the states is vertical position, so I want to constrain it to [0 inf). I can successfully do this in the workspace by using ODESET.

options = odeset('NonNegative',[3 5]);
[t, x] = ode45(@BR_1D_model,tspan,initial,[],valve);

However, I'm also using this model in an s-function, and I can't figure out how to use the odeset 'NonNegative' attribute, because I'm not directly calling the ode45 solver in the s-function.

function [sys,x0,str,ts]=BR_1D_sfcn(t,x,u,flag,Pressure,Mass,Pos,Vel,Thrust)

switch flag
    case 0          %initialize
        str=[] ;
        ts = [0 0] ;
        s = simsizes ;
        s.NumContStates = 5 ;
        s.NumDiscStates = 0 ;
        s.NumOutputs = 5 ;
        s.NumInputs = 1 ;
        s.DirFeedthrough = 0 ;
        s.NumSampleTimes = 1 ;
        sys = simsizes(s) ;
        x0 = [Pressure,Mass,Pos,Vel,Thrust]' ;
    case 1          %derivatives
        valve = u ;
        sys = BR_1D_model(t,x,valve) ;
    case 3          %output          
        sys = x;

What do you guys think? Any thoughts?