(〇) Foreword

What is the
    S function : The S function is the English abbreviation of the system function, which refers to
how to write a S function using a design language (non-graphical way) to describe the S function :
    you can use Matlab language, you can also use other programming languages into C , C ++, etc.
    I only introduce the commonly used MATLAB comes with "language" and S function template to write S function

(1) Write S function

①Enter the command in the command line window: >> edit sfuntmpl.m
to open the template file
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②The template file sfuntmpl.m contains a main function and six sub-functions

[1] Let's look at the main function
  first : the first sentence of the main function-the leading statement is:
function [sys, x0, str, ts, simStateCompliance] = sfuntmpl (t, x, u, flag)
     where fname is the function of the S function Name;
     input parameters t, x, y, flag are simulation time, state vector, input vector and call flag
     output parameter sys represents return parameters; x0 is the initial state value; str is set to empty array; ts is a Column matrix, one column is the sampling period of each state variable, and the other column is the offset of the corresponding sampling time.
     [This is how the S function is set in the M file]

   【2】再看看**子函数**：

  The prefix of the subfunction is mdl, and the flag controls the calling situation
. There are four common situations:
   Flag = 0: call the initialization subfunction mdlInitializeSizes;
   Flag = 1: call the continuous state update subfunction mdlDerivatives (t, x, u);
   Flag = 2: Call discrete state update subfunction mdlUpdate (t, x, u);
   Flag = 3: Call output subfunction mdlOutputs (t, x, u);

(2) Application of S function:

Take the simplest chestnut: y = kx + b

(1) Define the S function

① Initialize the main function

function [sys,x0,str,ts,simStateCompliance] = move(t,x,u,flag)
switch flag,
    case 0,
        [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes;
    case 1,
        sys=mdlDerivatives(t,x,u);
    case 2,
        sys=mdlUpdate(t,x,u);
    case 3,
        sys=mdlOutputs(t,x,u);
    case 4,
        sys=mdlGetTimeOfNextVarHit(t,x,u);
    case 9,
        sys=mdlTerminate(t,x,u);
    otherwise
        DAStudio.error('Simulink:blocks:unhandledFlag', num2str(flag));
end

② Initialization subfunction

function [sys,x0,str,ts,simStateCompliance]=mdlInitializeSizes
sizes = simsizes;
sizes.NumContStates  = 1;
sizes.NumDiscStates  = 0;
sizes.NumOutputs     = 1;
sizes.NumInputs      = 1;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;   % at least one sample time is needed
sys = simsizes(sizes);
x0  = [0];
str = [];
ts  = [0 0];
simStateCompliance = 'UnknownSimState';
function sys=mdlDerivatives(t,x,u)
sys = u;
function sys=mdlUpdate(t,x,u)
sys=[];
function sys=mdlOutputs(t,x,u)
sys=x;
function sys=mdlGetTimeOfNextVarHit(t,x,u)
sampleTime = 1;    %  Example, set the next hit to be one second later.
sys = t + sampleTime;
function sys=mdlTerminate(t,x,u)
sys = [];

(2) Use S function in Simulink model

    First of all, we need to build this model with a module.
    Novices may not find the device. Here I will briefly say: in the Simulink standard module library, there is Scope under sinks, Sine Wave under Sources, and S-Function. Just go up
    here and talk about how to modify the number of input terminals of the Scope: double-click to open the Scope interface, click on the upper left to set, modify the Input parameter, just remember to modify Apply .
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The model is built as follows: The

results are as follows:

    Sampling time: For the Simulink model, a step in the solver determines the minimum sampling time interval of the entire model.

(3) Special attention *-ensure correct operation

When I applied the S function for the first time, I was able to encounter many problems. I summarized several common problems and then I need to pay attention to them. I hope everyone can write the right one at a time.

    ①Note that the S function name must be the same as the .m file you write, otherwise you will get an error ②Note Insert picture description here
    that you must put the S function .m file and the model .slx file in a directory to ensure that you can find it when you click Edit And open the corresponding S function M file

    ③ Note that you must modify the current working directory of matlab to the directory where you saved these two files
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    so that you can basically guarantee the operation. If you have other problems, please query or comment yourself .