matlab 怎样用function编写程序,[转载]matlab的s-function使用及举例

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)

%CSFUNC An example M-file S-function for defining a continuous

system.

% Example M-file S-function implementing continuous

equations:

% x' = Ax + Bu

% y = Cx + Du

%

% See sfuntmpl.m for a general S-function template.

%

% See also SFUNTMPL.

% Copyright (c) 1990-1998 by The MathWorks, Inc. All Rights

Reserved.

% $Revision: 1.5 $

A=[-0.09 -0.01; 1 0];

B=[ 1 -7; 0 -2];

C=[ 0 2; 1 -5];

D=[-3 0; 1 0];

switch flag,

%%%%%%%%%%%%%%%%%%

% Initialization %

%%%%%%%%%%%%%%%%%%

case

0,[sys,x0,str,ts]=mdlInitializeSizes(A,B,C,D);

%%%%%%%%%%%%%%%

% Derivatives %

%%%%%%%%%%%%%%%

case

1,sys=mdlDerivatives(t,x,u,A,B,C,D);

%%%%%%%%%%%

% Outputs %

%%%%%%%%%%%

case

3,sys=mdlOutputs(t,x,u,A,B,C,D);

%%%%%%%%%%%%%%%%%%%

% Unhandled flags %

%%%%%%%%%%%%%%%%%%%

case { 2, 4, 9 },sys =

[];

%%%%%%%%%%%%%%%%%%%%

% Unexpected flags %

%%%%%%%%%%%%%%%%%%%%

otherwise, error(['Unhandled

flag = ',num2str(flag)]);

end

% end csfunc

%

%=============================================================================

% mdlInitializeSizes

% Return the sizes, initial conditions, and sample times for the

S-function.

%=============================================================================

%

function [sys,x0,str,ts]=mdlInitializeSizes(A,B,C,D)

sizes = simsizes;

sizes.NumContStates = 2; sizes.NumDiscStates = 0; sizes.NumOutputs

= 2;

sizes.NumInputs = 2; sizes.DirFeedthrough = 1; sizes.NumSampleTimes

= 1;

sys = simsizes(sizes); x0 = zeros(2,1); str = []; ts = [0 0];

% end mdlInitializeSizes

%

%=============================================================================

% mdlDerivatives

% Return the derivatives for the continuous states.

%=============================================================================

%

function sys=mdlDerivatives(t,x,u,A,B,C,D)

sys = A*x + B*u; % end mdlDerivatives

%

%=============================================================================

% mdlOutputs

% Return the block outputs.

%=============================================================================

%

function sys=mdlOutputs(t,x,u,A,B,C,D)

sys = C*x + D*u; % end mdlOutputs

之三、有关离散系统

关于离散系统定义的书写在此举一例：

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

%DSFUNC An example M-file S-function for defining a discrete

system.

% Example M-file S-function implementing discrete equations:

% x(n+1) = Ax(n) + Bu(n)

% y(n) = Cx(n) + Du(n)

%

% See sfuntmpl.m for a general S-function template.

%

% See also SFUNTMPL. % Copyright (c) 1990-1998 by The MathWorks,

Inc. All Rights Reserved.

% $Revision: 1.13 $

% Generate a discrete linear system:

A=[-1.3839 -0.5097 1.0000 0]; B=[-2.5559 0 0 4.2382];

C=[ 0 2.0761 0 7.7891]; D=[ -0.8141 -2.9334 1.2426 0];

switch flag,

% Initialization

case 0,[sys,x0,str,ts] = mdlInitializeSizes(A,B,C,D);

% Update % %%%%%%%%%%

case 2, sys = mdlUpdate(t,x,u,A,B,C,D); %%%%%%%%%%

% Output % %%%%%%%%%%

case 3, sys = mdlOutputs(t,x,u,A,C,D); %%%%%%%%%%%%%

% Terminate % %%%%%%%%%%%%%

case 9, sys = []; % do nothing %%%%%%%%%%%%%%%%%%%%

% Unexpected flags % %%%%%%%%%%%%%%%%%%%%

otherwise, error(['unhandled flag = ',num2str(flag)]);

end %end dsfunc

%

%=======================================================================

% mdlInitializeSizes

% Return the sizes, initial conditions, and sample times for the

S-function.

%=======================================================================

%

function [sys,x0,str,ts] = mdlInitializeSizes(A,B,C,D)

sizes = simsizes;

sizes.NumContStates = 0; sizes.NumDiscStates = size(A,1);

sizes.NumOutputs = size(D,1);

sizes.NumInputs = size(D,2); sizes.DirFeedthrough = 1;

sizes.NumSampleTimes = 1;

sys = simsizes(sizes); x0 = ones(sizes.NumDiscStates,1); str = [];

ts = [1 0];

% end mdlInitializeSizes

%

%=======================================================================

% mdlUpdate

% Handle discrete state updates, sample time hits, and major time

step

% requirements.

%=======================================================================

% function sys = mdlUpdate(t,x,u,A,B,C,D)

sys = A*x+B*u; %end mdlUpdate

%

%=======================================================================

% mdlOutputs

% Return Return the output vector for the S-function

%=======================================================================

%

function sys = mdlOutputs(t,x,u,A,C,D)

sys = C*x+D*u; %end mdlUpdate