USING THE MEX FILE IN MATLAB:

For general information on S-functions, see the SIMULINK manual.
In addition to the standard input arguments (time, states, inputs, flag),
TE_MEX requires 4 parameters, as follows:

p1    idv(20)  -- a vector of length 20 containing the desired values of
      the disturbance flags (see paper by Downs and Vogel).

      The following are all used by the TE routines to save data between
      Matlab calls.  They are initialized by a call to this function with
      time<0 and flag=0.  You must create Matlab variables of the 
      appropriate size (a vector of zeros is convenient),
      and pass them to te_mex as input arguments.  Once you have
      initialized these variables, DO NOT modify them during
      the simulated plant operation.

p2    Vector of length 153
p3    Vector of length 586
p4    Vector of length 139



Here are some example calls from Matlab:

t=-1;
x=[];
u=[];
flag=0;
idv=zeros(20,1);
p2=zeros(153,1);
p3=zeros(586,1);
p4=zeros(139,1);
[sys,x0]=te_mex(t,x,u,flag,idv,p2,p3,p4);
      
u=[specify a vector, length 12];
flag=1;
t=0;
dxdt=te_mex(t,x0,u,flag,idv,p2,p3,p4);
flag=3;
y=te_mex(t,x0,u,flag,idv,p2,p3,p4);

The first call to te_mex (with t=-1) initializes the storage vectors, p2 to p4,
and returns the (default) initial state vector in x0. This is a good test
to verify that the code is working (especially for those of you who have 
compiled and installed it yourselves).  You can compare x0 to the values 
listed for YY(1) to YY(50) in the TEINIT subroutine supplied
by Downs and Vogel.  Doing this, for example, helped me to figure out how
to port the code from a Mac, where it was developed, to a VAX.  Our installation
requires the G_Float option (see Matlab/VAX manual), and the x0 vector was
incorrect unless this was set properly for the compilation 
(and the fmexg .com file was used for linking).

The second call to te_mex (with flag=1) calculates the time-derivatives of
the states at the initial conditions (x0) and for a set of inputs specified by
the "u" vector.  The derivatives are returned in the "dxdt" variable.

The third call (with flag=3) returns the values of the outputs (all 41 of them -
see paper by Downs and Vogel) for the given x0 and u.  As explained in the
paper, when t>0, measurement noise is added to the outputs automatically.  
Also, the composition analyzers are only sampled at certain times.  Thus,
proper specification of the time variable is important.

The typical use of TE_MEX.MEX is to simulate operation of the plant for
a given set of conditions.  

For example, the following sequence of Matlab statements
shows how to reproduce the simulations of Figure 3 of Downs and Vogel.
The SIMULINK function, RK45, is used to integrate the differential
equations for a period of 0.3 hours.  See the SIMULINK manual for details
on this function.

% Initialize variables:

idv=zeros(20,1);
p2=zeros(153,1);
p3=zeros(586,1);
p4=zeros(139,1);
[sys,x0]=te_mex(-1,[],[],0,idv,p2,p3,p4); 
u0=[63.53,  53.98, 24.644, 61.302, 22.21, ...
    40.064, 38.1,  46.534, 47.446, 41.106, ...
    18.114, 50];       % Base case inputs (XMV variables).
u0(10)=38;             % Specifies a step in XMV(10)

% Set up for integration:

tvec=[0; 0.3];         % Starting and ending time.
ut=[tvec [u0;u0] ];    % Specifies constant inputs.
options=[.001, .00001, .001, 0, 0, 0, 0];

% Integrate over entire 0.3 hour time period:

[tt,xt,yt]=rk45('te_mex',tvec,x0,options,ut,idv,p2,p3,p4);

% Plot results: 

clf
subplot(221)
plot(tt,yt(:,6)),title('Reactor Feed'),xlabel('Time (hours)')
subplot(222)
plot(tt,yt(:,7)),title('Reac Pressure'),xlabel('Time (hours)')
subplot(223)
plot(tt,yt(:,8)),title('Reac Level'),xlabel('Time (hours)')
subplot(224)
plot(tt,yt(:,9)),title('Reac Temp'),xlabel('Time (hours)')
pause

clf
subplot(221)
plot(tt,yt(:,11)),title('Prod Sep Temp'),xlabel('Time (hours)')
subplot(222)
plot(tt,yt(:,12)),title('Prod Sep Level'),xlabel('Time (hours)')
subplot(223)
plot(tt,yt(:,10)),title('Purge Rate'),xlabel('Time (hours)')
subplot(224)
plot(tt,yt(:,18)),title('Stripper Temp'),xlabel('Time (hours)')


I found that a specification of a maximum step size of about 0.001
was necessary to get reliable accuracy from the RK45 function
(see use of "options" variable, above).  Other settings
may give better performance.  I didn't experiment with it much.
Elapsed time for the integration was about 1 minute on a Mac IIfx.

The following additional plot commands show how the concentrations
from the analyzers vary with time.  You can observe the discrete
sampling behavior, e.g., the product concentrations vary every 0.25
hours, and the others vary every 0.1 hour.

clf
subplot(221)
plot(tt,yt(:,23)),title('A in Feed'),xlabel('Time (hours)')
subplot(222)
plot(tt,yt(:,30)),title('B in Purge'),xlabel('Time (hours)')
subplot(223)
plot(tt,yt(:,40)),title('G in Product'),xlabel('Time (hours)')
subplot(224)
plot(tt,yt(:,41)),title('H in Product'),xlabel('Time (hours)')


Many other uses of TE_MEX.MEX should be obvious to those familiar with
the analysis tools in SIMULINK and the various Matlab toolboxes.



USING THE SOURCE CODE TO COMPILE/LINK YOUR OWN .MEX FILE:

The linking procedure is machine-dependent.  
See your Matlab manual.  I have verified that the code 
works on a Macintosh and a VAX.  I would expect it to work on other 
machines, but one can never be sure.  You're on your own if you
attempt it (or modify the source code).

The following assumes Matlab 3.x, so is now outdated.

Some hints follow:

On the Macintosh, I used MPW 3.2 with Language Systems Fortran 3.0.  I
compiled TEPROB.F and TE_MEX.F using the following options:

-i '{MexDir}' -mc68020 -mc68881

The symbol {MexDir} was set as follows:

Set MexDir      "{Boot}MATLAB:MEX:"

You may need to modify this, depending on your directory structure.
The Link step is tricky.  I used the following command:

  Link -o "te_mex.mex" -d -f -ad 4 -rt 'MEX0'=0 -sg MEX -m mexmain ¶
        -t MEX0 -c MATL -srt -br on  ¶
       "{MexDir}MPW:cmex.c.o"  ¶
       "{MexDir}MPW:LS:fmex.c.o"  ¶
       'te_mex.f.o'  ¶
       'teprob.f.o'  ¶
       "{FLibs}FORTRANLib.o"  ¶
       "{FLibs}IntrinsicLibFPU.o"  ¶
       "{FLibs}Interface.o"  ¶
       "{FLibs}Runtime.o"

with the symbol {FLibs} set to "{Boot}MPW:Libraries:Libraries:FLibraries:".
This assumes that you have the compiled object files te_mex.f.o, and 
teprob.f.o, stored in the default directory, and it creates te_mex.mex in 
the same directory.  Modify as appropriate.  The key to making it work
is the "-br on" option, which allows linking of large code segments.  It
is apparently unavailable in MPW prior to version 3.2.