Chris Atkinson - How to Make an S-Function

8/4/2019 Chris Atkinson - How to Make an S-Function

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How To Make an S-Function

Chris Atkinson

Aero 400 - Flight Simulation


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S-Functions

Used to create a block that runs C++ code in Simulink

Reasons you need to write an S-Function

Your Simulink model is too complex

You need to read in from or write to a file You need to access hardware (AD or DA cards)

You need to access external programs (3DLinx)


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Simulink model for the x-axis of one

wheel in the landing gear model


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Equivalent Code in C++

xWPos[i]+=vWheel[0]*dt;

xPos=xWPos[i]+offset[i]*cos(angle+rotation[i]);

if(xAlreadySliding[i] || yAlreadySliding[i])Fmax=Fkenitic;

else

Fmax=Fstatic;

FWheel[0]=-kx[i]*(xPos-xWheelPos[i])-bx[i]*vWheel[0];

xSliding=false;

if(brake


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Introduction to C++

Variablesdouble y;

const int SIZE = 10;

bool z = true;

Arraysdouble a[SIZE], b[SIZE][SIZE];

a[2] = 3.0;

b[2][3] = y/2.0;

Strings (Character arrays)

char s[100] = Chris;(stored as C h r i s \0 You need one extra character for the \0)

typedef char String[100];

String s = Chris;


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Ifif(x>3 && x


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Whilewhile(i>2)

i-=10.0;

Functions// Prototype (You need the semicolon)

double CalculateSomething(int x, double y);

double CalculateSomething(int x, double y)

{

double answer;if(x>3)

answer = sin(y); // #include

else

answer = double(x);

return answer;

}


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Starting an S-Function

Open a pre-existing S-Function in Visual C++ and save a

copy with a new name like Test.cpp

Change this line to have the same name as the file name#define S_FUNCTION_NAME Test

Now you have the general format so all you have to do is

change the code to make it do what you want it to do.


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Callback Functions

Functions that you write and Simulink calls to run your code

Initialize sizes

Sets the number and sizes of the output and input ports

static void mdlInitializeSizes(SimStruct *S)

Initialize sample times

Sets the number and type of the sample timesstatic void mdlInitializeSampleTimes(SimStruct *S)

Initialize conditions

Initializes anything you need it to (variables, read in file, etc.)static void mdlInitializeConditions(SimStruct *S)


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Start model

Runs when you start the model running

static void mdlStart(SimStruct *S)

OutputsRuns each time step to calculate the outputs of the block

static void mdlOutputs(SimStruct *S, int_T tid)

Terminate

Runs when you stop the model

static void mdlTerminate(SimStruct *S)


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Simulink FunctionsFunctions that you can call to get information or set up stuff

Initializing sizes

Set the number of input ports and set the with of each port

if (!ssSetNumInputPorts(S, 2))

return;

ssSetInputPortWidth(S, 0, 1);ssSetInputPortWidth(S, 1, 1);

Set the number of output ports and set the with of each port

if (!ssSetNumOutputPorts(S,1))

return;

ssSetOutputPortWidth(S, 0, 1);


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Initializing sample times

Set the number and type of sample times

ssSetNumSampleTimes(S, 1);

ssSetSampleTime(S, 0, CONTINUOUS_SAMPLE_TIME);

ssSetOffsetTime(S, 0, FIXED_IN_MINOR_STEP_OFFSET);

Inputs

Read in numbers from the inputs to the block

InputRealPtrsType inPtrs1 =

ssGetInputPortRealSignalPtrs(S,0);

InputRealPtrsType inPtrs2 =

ssGetInputPortRealSignalPtrs(S,1);

double x = *inPtrs1[0];

double y = *inPtrs2[0];


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Outputs

Write the results to the outputs of the block

real_T *out = ssGetOutputPortRealSignal(S,0);

out[0]=x+y;

Other

Get the step size

double dt=ssGetStepSize(S);

Get the simulation timedouble time=ssGetT(S);


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Parameters (users can type parameters into a text box in Simulink)

Setup the number of parameters

ssSetNumSFcnParams(S, 0);

if(ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S))

return; // Parameter mismatch will bereported by Simulink

Read in the parameter as a string

mxGetString(ssGetSFcnParam(S,0),fname,N_STRING);

Printing to the Simulink window

printf(Hello!\n);printf(The altitude is %f ft.\n, altitude);


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Reading in From a File

#include

void ReadFile(double data[MAX_NUMBER], const string filename)

{

const int N_IGNORE=1000;

ifstream input;

input.open(filename);

number=0;

input.eatwhite();

while(!input.eof() && number>data[number++]; // Reads in data

input.ignore(N_IGNORE,'\n'); // Ignores any comments

input.eatwhite(); // Ignores any white space

}input.close();

}


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Transformation Matrix// Calculates the transformation matrix from body to earth// coordinates from the given Euler angles.

void Transformation(double answer[3][3], double psi,

double theta, double phi)

{

answer[0][0]=cos(psi)*cos(theta);answer[0][1]=sin(psi)*cos(theta);

answer[0][2]=-sin(theta);

answer[1][0]=cos(psi)*sin(theta)*sin(phi)-sin(psi)*cos(phi);

answer[1][1]=sin(psi)*sin(theta)*sin(phi)+cos(psi)*cos(phi);

answer[1][2]=cos(theta)*sin(phi);answer[2][0]=cos(psi)*sin(theta)*cos(phi)+sin(psi)*sin(phi);

answer[2][1]=sin(psi)*sin(theta)*cos(phi)-cos(psi)*sin(phi);

answer[2][2]=cos(theta)*cos(phi);

}


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Matrix Multiplication

// Multiplies a 3x3 matrix by a 3 element vector.void Multiply(double answer[3], const double A[3][3],

const double B[3])

{

for(int i=0; i


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Example: Converting gravity to body

coordinatesdouble earthToBody[3][3], bodyToEarth[3][3];

double fgEarth[3], fgBody[3];

// Transformation from Body to Earth coordinates

Transformation(bodyToEarth,psi,theta,phi);

Transpose(earthToBody,bodyToEarth);

fgEarth[0]=0.0;

fgEarth[1]=0.0;

fgEarth[2]=g*mass;

Multiply(fgBody, earthToBody, fgEarth);


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Compiling Your S-Function

You cant compile an S-Function with Visual C++directly, because it has to access Matlab libraries.

Compile Through Matlab using the Visual C++ compiler Setup mex

>> mex -setup

It will ask you if you want it to search for compilers

It should find Visual C++ if it is installed

Just type in the number for Visual C++

Compile

>> mex Test.cpp

>>

It will probably give you a list of compiler errors

If not, it will make Test.dll


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Putting Your S-Function into a Model

Date post:	07-Apr-2018
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Chris Atkinson - How to Make an S-Function

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