#### 29.2.2 State space system interface functions

— Function File: ss (a, b, c, d, tsam, n, nz, stname, inname, outname, outlist)

> Create system structure from state-space data. May be continous, discrete, or mixed (sampled data)

> Inputs

> a
> b
> c
> d
>   usual state space matrices.

>   default: d = zero matrix

> tsam
>   sampling rate. Default: tsam = 0 (continuous system)
> n
> nz
>   number of continuous, discrete states in the system

>   If tsam is 0, n = `rows`(a), nz = 0.

>   If tsam is greater than zero, n = 0, nz = `rows`(a)

>   see below for system partitioning

> stname
>   cell array of strings of state signal names

>   default (stname=[] on input): `x_n` for continuous states, `xd_n` for discrete states

> inname
>   cell array of strings of input signal names

>   default (inname = [] on input): `u_n`

> outname
>   cell array of strings of input signal names

>   default (outname = [] on input): `y_n`

> outlist
>   list of indices of outputs y that are sampled

>   If tsam is 0, outlist = [].

>   If tsam is greater than 0, outlist = 1:`rows`(c).

> Unlike states, discrete/continous outputs may appear in any order.

> `sys2ss` returns a vector yd where yd(outlist) = 1; all other entries of yd are 0.

> Outputs outsys = system data structure

> System partitioning

> Suppose for simplicity that outlist specified that the first several outputs were continuous and the remaining outputs were discrete. Then the system is partitioned as

>               x = [ xc ]  (n x 1)
>                   [ xd ]  (nz x 1 discrete states)
>               a = [ acc acd ]  b = [ bc ]
>                   [ adc add ]      [ bd ]
>               c = [ ccc ccd ]  d = [ dc ]
>                   [ cdc cdd ]      [ dd ]

>                   (cdc = c(outlist,1:n), etc.)

> with dynamic equations: d/dt xc(t) = acc*xc(t) + acd*xd(k*tsam) + bc*u(t)

> xd((k+1)*tsam) = adc*xc(k*tsam) + add*xd(k*tsam) + bd*u(k*tsam)

> yc(t) = ccc*xc(t) + ccd*xd(k*tsam) + dc*u(t)

> yd(k*tsam) = cdc*xc(k*tsam) + cdd*xd(k*tsam) + dd*u(k*tsam)

> Signal partitions

>                       | continuous      | discrete               |
>               ----------------------------------------------------
>               states  | stname(1:n,:)   | stname((n+1):(n+nz),:) |
>               ----------------------------------------------------
>               outputs | outname(cout,:) | outname(outlist,:)     |
>               ----------------------------------------------------

> where cout is the list of in 1:`rows`(p) that are not contained in outlist. (Discrete/continuous outputs may be entered in any order desired by the user.)

> Example

>               octave:1> a = [1 2 3; 4 5 6; 7 8 10];
>               octave:2> b = [0 0 ; 0 1 ; 1 0];
>               octave:3> c = eye (3);
>               octave:4> sys = ss (a, b, c, [], 0, 3, 0, {"volts", "amps", "joules"});
>               octave:5> sysout(sys);
>               Input(s)
>                       1: u_1
>                       2: u_2

>               Output(s):
>                       1: y_1
>                       2: y_2
>                       3: y_3

>               state-space form:
>               3 continuous states, 0 discrete states
>               State(s):
>                       1: volts
>                       2: amps
>                       3: joules

>               A matrix: 3 x 3
>                  1   2   3
>                  4   5   6
>                  7   8  10
>               B matrix: 3 x 2
>                 0  0
>                 0  1
>                 1  0
>               C matrix: 3 x 3
>                 1  0  0
>                 0  1  0
>                 0  0  1
>               D matrix: 3 x 3
>                 0  0
>                 0  0
>                 0  0

> Notice that the D matrix is constructed by default to the correct dimensions. Default input and output signals names were assigned since none were given.

— Function File: [a, b, c, d, tsam, n, nz, stname, inname, outname, yd] = sys2ss (sys)

> Extract state space representation from system data structure.

> Input

> sys
>   System data structure.

> Outputs

> a
> b
> c
> d
>   State space matrices for sys.
> tsam
>   Sampling time of sys (0 if continuous).
> n
> nz
>   Number of continuous, discrete states (discrete states come last in state vector x).
> stname
> inname
> outname
>   Signal names (lists of strings); names of states, inputs, and outputs, respectively.
> yd
>   Binary vector; yd(ii) is 1 if output y(ii) is discrete (sampled); otherwise yd(ii) is 0.
> A warning massage is printed if the system is a mixed continuous and discrete system.

> Example

>               octave:1> sys=tf2sys([1 2],[3 4 5]);
>               octave:2> [a,b,c,d] = sys2ss(sys)
>               a =
>                  0.00000   1.00000
>                 -1.66667  -1.33333
>               b =
>                 0
>                 1
>               c = 0.66667  0.33333
>               d = 0