User`s guide

1 Overview of S-Functions

1-4

When to Use an S-Function

The most common use of S-functions i s to create custom Simulink blocks. You

can use S -functions for a variety of applications, including:

• Adding new g eneral purpose blocks to Simuli nk

• Incorporating existing C code into a simulation

• Describing a system as a mathematical set of equations

• Using graphical animations (see the inverted pendulum demo,

penddemo)

Anadvantageof usingS-functions isthatyou canbuild ageneral purposeblock

that you can use many times in a model, varying parameters with each

instance of the block.

How S-Functions Work

Each block w ithin a Simulink model has the following general charact erist ics:

a vector of inputs, u, a vector of outputs, y, and a vector of states, x,asshown

by this ill ustration:

The state vector may consist of continuous states, discrete states, o r a

combination of both. The mathematical relationships between the inputs,

outputs, and the states are expressed b y the following equations:

In M-file S-fun ctions, Simulink p artitions the state vector into two parts: the

continuous states and the discrete states. The continuous states occupy the

firstpart of the state vector,and the discrete states occupythe secondpart. For

(states)

(input) (output)

(output)

(derivative)

(update)

where xx

txu,,()=

k1+

txu,,()=