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Laplace Transforms Use
EGR 386
8/31/2011
Common Laplace Transforms
Process for using Laplace Transforms
to solve differential equations
• Determine governing differential equation as function
of time ‘t’
• Use Laplace transform table to convert each term to
algebraic equation as function of ‘s’
• Apply initial conditions
• Express as “ Ouput(s)=…” or “Y(s)=“…
• Use algebra to have each term of Y(s) able to be
found on Laplace transform table
• Convert each term of Y(s) usingLaplace transform
table , obtain y(t)
Laplace Transform yields complete solution
Laplace Transform yields complete
solution (continued)
x(t)x(t)x(t)x(t)
Transfer Function
)(
)(
)(
)()(
sU
sY
sInput
sOutputsG ==≡
Initial Value Theorem (IVT)
{ })()0( ssFs
Limtf
∞→==
Final Value Theorem (FVT)
{ })(0
)( ssFs
Limtf
→=∞=
FVT is typically used to find the steady state value of f(t)FVT is typically used to find the steady state value of f(t)
FVT is more often used than the IVT
Final value may not exist*
*Note: There is no final value for oscillatory functions,
such as)sin()( tCtf ω=
Partial Fraction Expansion
PFE continued
Substituting for R & P we have easily converted Laplace Transforms
So taking the inverse Laplace Transforms we get h(t)
PFE w/ TF with ‘s’ in the numerator
Transfer Function
Terminology : Poles and Zeros
• Poles [at a ‘pole’ G(s) →∞ ]
• values ‘s’ of D(s)=0 ,
))()((
))((
)(
)()(
esdscs
bsas
sD
sNsG
−−−
−−==
• values ‘s’ of D(s)=0 ,
• roots of the denominator: s= c, d, e
• Zeros [at a ‘zero’ G(s) →0 ]
• values ‘s’ of N(s)=0 ,
• roots of the numerator : s= a, b
Poles and Zeros
• Later we will see that poles and zeros of a
transfer function will characterize
performance of system, and will show what
effects feedback may haveeffects feedback may have
Examples