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29.1 System Data Structure

The OCST stores all dynamic systems in a single data structure format that can represent continuous systems, discrete-systems, and mixed (hybrid) systems in state-space form, and can also represent purely continuous/discrete systems in either transfer function or pole-zero form. In order to provide more flexibility in treatment of discrete/hybrid systems, the OCST also keeps a record of which system outputs are sampled.

Octave structures are accessed with a syntax much like that used by the C programming language. For consistency in use of the data structure used in the OCST, it is recommended that the system structure access m-files be used (see section System Construction and Interface Functions). Some elements of the data structure are absent depending on the internal system representation(s) used. More than one system representation can be used for SISO systems; the OCST m-files ensure that all representations used are consistent with one another.

Function File: sysrepdemo

Tutorial for the use of the system data structure functions.


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29.1.1 Variables common to all OCST system formats

The data structure elements (and variable types) common to all system representations are listed below; examples of the initialization and use of the system data structures are given in subsequent sections and in the online demo DEMOcontrol.

n
nz

The respective number of continuous and discrete states in the system (scalar)

inname
outname

list of name(s) of the system input, output signal(s). (list of strings)

sys

System status vector. (vector)

This vector indicates both what representation was used to initialize the system data structure (called the primary system type) and which other representations are currently up-to-date with the primary system type (see section Data structure access functions).

The value of the first element of the vector indicates the primary system type.

0

for tf form (initialized with tf2sys or fir2sys)

1

for zp form (initialized with zp2sys)

2

for ss form (initialized with ss2sys)

The next three elements are boolean flags that indicate whether tf, zp, or ss, respectively, are "up to date" (whether it is safe to use the variables associated with these representations). These flags are changed when calls are made to the sysupdate command.

tsam

Discrete time sampling period (nonnegative scalar). tsam is set to 0 for continuous time systems.

yd

Discrete-time output list (vector)

indicates which outputs are discrete time (i.e., produced by D/A converters) and which are continuous time. yd(ii) = 0 if output ii is continuous, = 1 if discrete.

The remaining variables of the system data structure are only present if the corresponding entry of the sys vector is true (=1).


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29.1.2 tf format variables

num

numerator coefficients (vector)

den

denominator coefficients (vector)


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29.1.3 zp format variables

zer

system zeros (vector)

pol

system poles (vector)

k

leading coefficient (scalar)


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29.1.4 ss format variables

a
b
c
d

The usual state-space matrices. If a system has both continuous and discrete states, they are sorted so that continuous states come first, then discrete states

Note some functions (e.g., bode, hinfsyn) will not accept systems with both discrete and continuous states/outputs

stname

names of system states (list of strings)


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