S-FUNCTIONS的书写之一
s-function也就是system-function的缩写。说得简单,s-function就是用MATLAB所提供的模型不能完全满足用户,而提供给用户自己编写程序来满足自己要求模型的接口。要了解 s-function,必须了解以下知识:
(1)direct feedthrough
(2)dynamically sized inputs
(3)setting sample times and offsets
[由于我也不知道怎么把上面三句话精确的翻译成中文,因此在此不加翻译,大家自己理解,也许更好]由于上面三部分的重要性,在此详细进行分析。
一.direct feedthrough
direct feedthrough意思是说系统的输出或可变采样时间是否受到输入的控制。大家清楚有的系统是受到输入控制如:
y=k*u (u是输入,k是放大因子,y是输出)
而有的系统输出是不受到输入影响,如:
输出:y=x
dx=u
x表示状态
二.dynamically sized inputs
主要是给出:输入连续状态数目(size.NumContStates),离散状态数目(size.NumDiscStates)
,输出数目(size.NumOutputs),输入数目(size.NumInputs),Direct Feedthrough(size.Dir Feedthrough)。
三.setting sample times and offsets
setting smaple times and offsets主要设置采样时间 MATLAB为了用户方便,已经书写了S-FUNCTIONS模板函数sfuntmpl.m。为了更好的写S-FUNCTIONS,大家来看一下,该函数sfuntmpl.m内容如下:(我通过在该内容加注释来说明,以$开头)
function [sys,x0,str,ts] = sfuntmpl(t,x,u,flag)
$输出变量就此四个,大家必须注意它的顺序。$输入变量可以为t,x,u,flag,p1,...,pn等,但是前面的四个变量不能变,特此说明。
%SFUNTMPL General M-file S-function template
% With M-file S-functions, you can define you own ordinary differential
% equations (ODEs), discrete system equations, and/or just about
% any type of algorithm to be used within a Simulink block diagram.
%
$上面是其功能
% The general form of an M-File S-function syntax is:
% [SYS,X0,STR,TS] = SFUNC(T,X,U,FLAG,P1,...,Pn)
%
$调用格式
% What is returned by SFUNC at a given point in time, T, depends on the
% value of the FLAG, the current state vector, X, and the current
% input vector, U.
%
% FLAG RESULT DESCRIPTION
% ----- ------ --------------------------------------------
% 0 [SIZES,X0,STR,TS] Initialization, return system sizes in SYS,
% initial state in X0, state ordering strings
% in STR, and sample times in TS.
$具体怎样实现,大家参看后面的函数mdlInitializeSizes
% 1 DX Return continuous state derivatives in SYS.