Discrete-Time Systems: Sampling Continuous Systems and Z-Transforms, Study notes of Electrical and Electronics Engineering

The process of finding the equivalent discrete-time system from a sampled continuous system, including the use of cayley-hamilton methods, z-transforms, and the fundamental matrix for a discrete-time system. Examples and practice problems are provided.

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Pre 2010

Uploaded on 08/09/2009

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12. Discrete-Time Systems
Topics
Be ableto
.find the equivalent discrete-time system
from a sampled, continuous system.
find the solution to a first-ordermatrix
difference equation using Cayley-
Hamilton methods.
.
.use Z-transforms to find solutions to
first-order matrix difference equations.
find the fundamental matrix for a
discrete-time system in terms of
Z-transforms.
.
Ref: Sections 5.1-5.6, 5.8, 5.9, 5.11. Fall 2004
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12. Discrete-Time Systems

Topics

Be ableto

. find the equivalent discrete-time system

from a sampled, continuous system.

find the solution to a first-order matrix

difference equation using Cayley- Hamilton methods.

. use Z-transforms to find solutions to

first-order matrix difference equations.

find the fundamental matrix for a

discrete-time system in terms of Z-transforms.

.

Ref: Sections 5.1-5.6, 5.8, 5.9, 5.11. Fall 2004

Sampling Continuous Systems

u(t)

i(t) =Ax(t) + Bu(t) y(t) =Cx(t)

yet)

Controller

yet) is sampled every T seconds ~rhich generates

u(t)

note: u(t) = u(kT) for kT < t < (k'+ l)T

Example

Find the equivalent discrete-time system for

0 1 0

i(t) = I x(t) + I u(t)

-1 -2 1

yet) = [ 1 0 ] x( t)
This is
Yi(s) 1

= vvhen

U(s) s2 + 2rco ~ (^) n S + (1)2 n

run- 1 and S = 1 (critically damp'ed).
  • F=e-T [

I+T T

Ans. -T^ 1-^ T]

[

-T-l +eT ]

G = e -T T

Solution
Find the eigenvalues,

\

Use the Cayley-Hamilton techniqlle,
F=eAT=a 0 I+a 1 A

G = A-I (eAT -I)B (ifAisnonsingular)

0 1 I -2 -

IAI=I =1*O,A-1=

-1 -2 I 1 0

\

Find x(k) if

Example

I.. 0

-I Ix(k) + Ilu(k), k>O

x(k+l)= 1

(^4) .J

[ ]

T l-k -2k and u(k) = 1 and x(O) = 1 -1/2.. Ans. Ak=(~t_! 2 l+k

Solution

0 1

Find the eigenvalues of A = -! -I"

A k =a I 0 + a 1 A and A-k = a 0 + a 1 A-

aO^0 I^ I^0 al^ I^ I^ aO
Ak=1 + 1

J

l

l

0 (Xo L - 4 (Xl -(Xl - 4 (Xl (Xo - (Xl
where 1 1 1 1 - 1

a = - a + ( - _)k - k (- ) ( - _)k 1 + ( - - )k

021 2 22 2

= k ( 1. ) ( - 1. )k( - 1. ) - I + ( - 1. )k

2 2 2 2

= - k ( - 1. )k + ( - 1. )k = ( - 1. i (1 - k) 2 2 2

al

For u(k) = 1, x(O) =[1 -1/2 ]T,

k-l

x(k) =A kX(O) + L Ak-i-lBu(i)

i=O

  1. k-l. 0

x(k) =A kl 1 + L Ak-z-l I, k> 1

    • i-O 1 2 -

Z- Transforms

x(k+ 1 ) ~ Delay I ~ x(k)

or by transforms

Z[x(k+ 1)] 1 Vz I ~ Z[x(k)]
Dej

00

Z[x(k)] = L xU) Z -j = X(z)

j=O

How is the Z-transformof x(k+ 1) related to

x(k)?

00

Z[x(k+ 1)] = L xU+ 1) z-j

j=O

Ans.Z[x(k+ 1)] =zX(z).

\

Inverse Z-Transfc)rms

Example

Find x(k) for X(z) = 1/(z + 1)

Solution

This is the power series expansion method

Example

Find the inverse Z-transform ofz/(z-l).
Find the inverse Z-transform of z-4/(z-l).