Convert transfer function filter parameters to state-space form

`[A,B,C,D] = tf2ss(b,a)`

Consider the system described by the transfer function

$$H(s)=\frac{\left[\begin{array}{c}2s+3\\ {s}^{2}+2s+1\end{array}\right]}{{s}^{2}+0.4s+1}.$$

Convert it to state-space form using `tf2ss`

.

b = [0 2 3; 1 2 1]; a = [1 0.4 1]; [A,B,C,D] = tf2ss(b,a)

`A = `*2×2*
-0.4000 -1.0000
1.0000 0

`B = `*2×1*
1
0

`C = `*2×2*
2.0000 3.0000
1.6000 0

`D = `*2×1*
0
1

A one-dimensional discrete-time oscillating system consists of a unit mass, $\mathit{m}$, attached to a wall by a spring of unit elastic constant. A sensor samples the acceleration, $\mathit{a}$, of the mass at ${\mathit{F}}_{\mathrm{s}}=5$ Hz.

Generate 50 time samples. Define the sampling interval $\Delta \mathit{t}=1/{\mathit{F}}_{\mathrm{s}}$.

Fs = 5; dt = 1/Fs; N = 50; t = dt*(0:N-1); u = [1 zeros(1,N-1)];

The transfer function of the system has an analytic expression:

$\mathit{H}\left(\mathit{z}\right)=\frac{1-{\mathit{z}}^{-1}\left(1+\mathrm{cos}\Delta \mathit{t}\right)+{\mathit{z}}^{-2}\mathrm{cos}\Delta \mathit{t}}{1-2{\mathit{z}}^{-1}\mathrm{cos}\Delta \mathit{t}+{\mathit{z}}^{-2}}$.

The system is excited with a unit impulse in the positive direction. Compute the time evolution of the system using the transfer function. Plot the response.

bf = [1 -(1+cos(dt)) cos(dt)]; af = [1 -2*cos(dt) 1]; yf = filter(bf,af,u); stem(t,yf,'o') xlabel('t')

Find the state-space representation of the system. Compute the time evolution starting from an all-zero initial state. Compare it to the transfer function prediction.

[A,B,C,D] = tf2ss(bf,af); x = [0;0]; for k = 1:N y(k) = C*x + D*u(k); x = A*x + B*u(k); end hold on stem(t,y,'*') hold off legend('tf','ss')

`b`

— Transfer function numerator coefficientsvector | matrix

Transfer function numerator coefficients, specified as a vector or matrix. If
`b`

is a matrix, then each row of `b`

corresponds to an output of the system.

For discrete-time systems,

`b`

contains the coefficients in descending powers of*z*.For continuous-time systems,

`b`

contains the coefficients in descending powers of*s*.

For discrete-time systems, `b`

must have a number of columns equal
to the length of `a`

. If the numbers differ, make them equal by padding
zeros. You can use the function `eqtflength`

to accomplish this.

`a`

— Transfer function denominator coefficientsvector

Transfer function denominator coefficients, specified as a vector.

For discrete-time systems,

`a`

contains the coefficients in descending powers of*z*.For continuous-time systems,

`a`

contains the coefficients in descending powers of*s*.

`A`

— State matrixmatrix

State matrix, returned as a matrix. If the system is described by
*n* state variables, then `A`

is
*n*-by-*n*.

**Data Types: **`single`

| `double`

`B`

— Input-to-state matrixmatrix

Input-to-state matrix, returned as a matrix. If the system is described by
*n* state variables, then `B`

is
*n*-by-1.

**Data Types: **`single`

| `double`

`C`

— State-to-output matrixmatrix

State-to-output matrix, returned as a matrix. If the system has *q*
outputs and is described by *n* state variables, then
`C`

is *q*-by-*n*.

**Data Types: **`single`

| `double`

`D`

— Feedthrough matrixmatrix

Feedthrough matrix, returned as a matrix. If the system has *q*
outputs, then `D`

is *q*-by-1.

**Data Types: **`single`

| `double`

`tf2ss`

converts the parameters of a transfer
function representation of a given system to those of an equivalent state-space
representation.

For discrete-time systems, the state-space matrices relate the state vector

*x*, the input*u*, and the output*y*:$$\begin{array}{c}x\left(k+1\right)=Ax\left(k\right)+Bu\left(k\right),\\ y\left(k\right)=Cx\left(k\right)+Du\left(k\right).\end{array}$$

The transfer function is the Z-transform of the system’s impulse response. It can be expressed in terms of the state-space matrices as

$$H\left(z\right)=C{\left(zI-A\right)}^{-1}B+D.$$

For continuous-time systems, the state-space matrices relate the state vector

*x*, the input*u*, and the output*y*:$$\begin{array}{l}\dot{x}=Ax+Bu,\\ y=Cx+Du.\end{array}$$

The transfer function is the Laplace transform of the system’s impulse response. It can be expressed in terms of the state-space matrices as

$$H(s)=\frac{B(s)}{A(s)}=\frac{{b}_{1}{s}^{n-1}+\cdots +{b}_{n-1}s+{b}_{n}}{{a}_{1}{s}^{m-1}+\cdots +{a}_{m-1}s+{a}_{m}}=C{\left(sI-A\right)}^{-1}B+D.$$

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