s-function 使用及应用举例.txt

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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. 
% 2 DS Update discrete states SYS = X(n+1) 
% 3 Y Return outputs in SYS. 
% 4 TNEXT Return next time hit for variable step sample 
% time in SYS. 
% 5 Reserved for future (root finding). 
% 9 [] Termination, perform any cleanup SYS=[]. 
% 
$参看后面相应函数 
% 
% The state vectors, X and X0 consists of continuous states followed 
% by discrete states. 
% 
% Optional parameters, P1,...,Pn can be provided to the S-function and 
% used during any FLAG operation. 
% 
% When SFUNC is called with FLAG = 0, the following information 
% should be returned: 
% 
% SYS(1) = Number of continuous states. 
% SYS(2) = Number of discrete states. 
% SYS(3) = Number of outputs. 
% SYS(4) = Number of inputs. 
% Any of the first four elements in SYS can be specified 
% as -1 indicating that they are dynamically sized. The 
% actual length for all other flags will be equal to the 
% length of the input, U. 
% SYS(5) = Reserved for root finding. Must be zero. 
% SYS(6) = Direct feedthrough flag (1=yes, 0=no). The s-function 
% has direct feedthrough if U is used during the FLAG=3 
% call. Setting this to 0 is akin to making a promise that 
% U will not be used during FLAG=3. If you break the promise 
% then unpredictable results will occur. 
% SYS(7) = Number of sample times. This is the number of rows in TS. 
% 
$需要说明的是sys的顺序不能乱 
% 
% X0 = Initial state conditions or [] if no states. 
% 
% STR = State ordering strings which is generally specified as []. 
% 
% TS = An m-by-2 matrix containing the sample time 
% (period, offset) information. Where m = number of sample 
% times. The ordering of the sample times must be: 
% 
% TS = [0 0, : Continuous sample time. 
% 0 1, : Continuous, but fixed in minor step 
% sample time. 
% PERIOD OFFSET, : Discrete sample time where 
% PERIOD > 0 & OFFSET 〈 PERIOD. 
% -2 0]; : Variable step discrete sample time 
% where FLAG=4 is used to get time of 
% next hit. 
% 
% There can be more than one sample time providing 
% they are ordered such that they are monotonically 
% increasing. Only the needed sample times should be 
% specified in TS. When specifying than one 
% sample time, you must check for sample hits explicitly by 
% seeing if 
% abs(round((T-OFFSET)/PERIOD) - (T-OFFSET)/PERIOD) 
% is within a specified tolerance, generally 1e-8. This 
% tolerance is dependent upon your model's sampling times 
% and simulation time. 
% 
% You can also specify that the sample time of the S-function 
% is inherited from the driving block. For functions which 
% change during minor steps, this is done by 
% specifying SYS(7) = 1 and TS = [-1 0]. For functions which 
% are held during minor steps, this is done by specifying 
% SYS(7) = 1 and TS = [-1 -1]. 

% Copyright (c) 1990-1998 by The MathWorks, Inc. All Rights Reserved. 
% $Revision: 1.12 $ 

% 
% The following outlines the general structure of an S-function. 
% 
switch flag, 
%%%%%%%%%%%%%%%%%% 
% Initialization % 
%%%%%%%%%%%%%%%%%% 
case 0,[sys,x0,str,ts]=mdlInitializeSizes; 
$大家是不是觉得此函数名太长，当然可以根据自己的爱好加以改变，不过后面的相应改。 
%%%%%%%%%%%%%%% 
% Derivatives % 
%%%%%%%%%%%%%%% 
case 1,sys=mdlDerivatives(t,x,u); 
%%%%%%%%%% 
% Update % 
%%%%%%%%%% 
case 2,sys=mdlUpdate(t,x,u); 
%%%%%%%%%%% 
% Outputs % 
%%%%%%%%%%% 
case 3,sys=mdlOutputs(t,x,u); 
%%%%%%%%%%%%%%%%%%%%%%% 
% GetTimeOfNextVarHit % 
%%%%%%%%%%%%%%%%%%%%%%% 
case 4,sys=mdlGetTimeOfNextVarHit(t,x,u); 
%%%%%%%%%%%%% 
% Terminate % 
%%%%%%%%%%%%% 
case 9,sys=mdlTerminate(t,x,u); 
%%%%%%%%%%%%%%%%%%%% 
% Unexpected flags % 
%%%%%%%%%%%%%%%%%%%% 
otherwise, error(['Unhandled flag = ',num2str(flag)]); 
end 
% end sfuntmpl 

% 
%============================================================================= 
% mdlInitializeSizes 
% Return the sizes, initial conditions, and sample times for the S-function. 
%============================================================================= 
% 
function [sys,x0,str,ts]=mdlInitializeSizes 
% 
% call simsizes for a sizes structure, fill it in and convert it to a 
% sizes array. 
% 
% Note that in this example, the values are hard coded. This is not a 
% recommended practice as the characteristics of the block are typically 
% defined by the S-function parameters. 
% 
$关于函数simsizes大家必须遵循，因为把是内部函数，不得随便改变，其作用是返回未初始化的size结构。 
sizes = simsizes; 
$number of continuous states 
sizes.NumContStates = 0; 
$number of discrete states 
sizes.NumDiscStates = 0; 
$number of outputs 
sizes.NumOutputs = 0; 
$ number of inputs 
sizes.NumInputs = 0; 
$Flag for direct feedthrough 
sizes.DirFeedthrough = 1; 
$number of sample times 
sizes.NumSampleTimes = 1; 
% at least one sample time is needed 
$ 
sys = simsizes(sizes); 
% 
% initialize the initial conditions 
% 
x0 = []; 
% 
% str is always an empty matrix 
% 
str = []; 
% 
% initialize the array of sample times 
% 
ts = [0 0]; 
% end mdlInitializeSizes 
% 
%============================================================================= 
% mdlDerivatives 
% Return the derivatives for the continuous states. 
%============================================================================= 
% 
function sys=mdlDerivatives(t,x,u) 
sys = []; % end mdlDerivatives 
% 
%============================================================================= 
% mdlUpdate 
% Handle discrete state updates, sample time hits, and major time step 
% requirements. 
%============================================================================= 
% 
function sys=mdlUpdate(t,x,u) 
sys = []; % end mdlUpdate 
% 
%============================================================================= 
% mdlOutputs 
% Return the block outputs. 
%============================================================================= 
% 
function sys=mdlOutputs(t,x,u) 
sys = []; % end mdlOutputs 
% 
%============================================================================= 
% mdlGetTimeOfNextVarHit 
% Return the time of the next hit for this block. Note that the result is 
% absolute time. Note that this function is only used when you specify a 
% variable discrete-time sample time [-2 0] in the sample time array in 
% mdlInitializeSizes. 
%============================================================================= 
% 
function sys=mdlGetTimeOfNextVarHit(t,x,u) 
sampleTime = 1; % Example, set the next hit to be one second later. 
sys = t + sampleTime; % end mdlGetTimeOfNextVarHit 
% 
%============================================================================= 
% mdlTerminate 
% Perform any end of simulation tasks. 
%============================================================================= 
% 
function sys=mdlTerminate(t,x,u) 
sys = []; % end mdlTerminate 

在下面我将写几个具体的实例。 

之二、连续系统例子： 

function [sys,x0,str,ts] = csfunc(t,x,u,flag)