Running counters using S-functions

Hello,

I am designing an autopilot waypoint tracking system. I have a list of waypoints. The autopilot is given the first as a reference waypoint and when the aircraft gets a distance D close to the waypoint, I want the reference to change to the next waypoint. I did this by having a flag based on whether the aircraft is D distance close to the waypoint or not (1 if it is and 0 if it is not). The flag increments a counter which keeps track of which waypoint is currently the reference.

I learned (the hard way) that variables passed to workspace during the simulation are not available until after the simulation and the only way to have a running counter would be through S-functions.

The question I have is: How can I (can I even?) build a simple S-function that increments a fixed counter based on a flag input and then outputs the value of the fixed counter (which will be used to determine the current waypoint)? I've been using the S-function builder but I'm not sure how to reference a static/global/base-workspace variable (I'm not too familiar with C either).

Any help would be tremendously appreciated.

~ IH

Answer 1

[Kaustubha Govind]

If this is all you want your S-function to do, then you can simply implement your own subsystem (using a unit delay block to store the counter's state) or use an Embedded MATLAB Fcn block containing code to the effect of:

function y = fcn(u)
%#codegen

persistent counter;

if isempty(counter)
    counter = 0; %initialize
end

if (u == 1) %implement this condition as per your requirement
    counter = counter + 1;
end

y = counter;

Is there a reason you chose to use a C-MEX S-function instead? (By the way, I would recommend Level-2 MATLAB file S-functions or Embedded MATLAB Functions if you are not comfortable with C/C++ and need to perform simple enough operations)

[Ismail Hameduddin]

Great, thanks! I managed to make it work (for the most part) using a variation of the above. I didn't know of the existence/validity of the persistent command in Simulink.