LAPLACE TRANSFORMS. Prepared by Mrs. Azduwin Binti Khasri. LAPLACE TRANSFORMS. A mathematical tool. Important analytical method for solving LINEAR ORDINARY DIFFERENTIAL EQUATIONS. Application to nonlinear ODEs? Must linearize first. Transform ODEs to algebraic equation.
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Prepared by Mrs. AzduwinBintiKhasri
control concepts and techniques.
system design, Stability analysis
The Laplace transform of a function, f(t), is defined as
F(s)= symbol for the Laplace transform,
L =Laplace transform operator
f(t) =some function of time, t.
s= Complex Independent Variable
Note: The L operator transforms a time domain function f(t) into an s domain function, F(s).
INVERSE LAPLACE TRANSFORM, L1
By definition, the inverse Laplace transform operator, L1, converts an sdomain function back to the corresponding time domain function:
Important Properties:
Both L and L1 are linear operators. Thus, satisfies the superposition principle;
Because the step function is a special case of a “constant”, it follows from (34) that
initial condition at t = 0
Similarly, for higher order derivatives:
 n is an arbitrary positive integer

Special Case: All Initial Conditions are Zero
Suppose Then
In process control problems, we usually assume zero initial conditions. Reason: This corresponds to the nominal steady state when “deviation variables” are used.
Using integrating by parts or Euler Identity.
5. Exponential Functions
Consider where b > 0. Then,
6. Rectangular Pulse Function
It is defined by:
Time, t
The Laplace transform of the rectangular pulse is given by
7.Impulse Function (or Dirac Delta Function)
The impulse function is obtained by taking the limit of the
rectangular pulse as its width, tw, goes to zero but holding
the area under the pulse constant at one. (i.e., let )
SOLUTION OF ODEs BY LAPLACE TRANSFORMS
EXAMPLE 1
Solve the ODE,
First, take L of both sides of (326),
Rearrange,
Take L1,
From Table 3.1,
Basic idea: Expand a complex expression for Y(s) into simpler terms, each of which appears in the Laplace Transform table. Then you can take the L1 of both sides of the equation to obtain y(t).
Example:
Perform a partial fraction expansion (PFE)
where coefficients and have to be determined.
Multiply both sides by (s + 1)(s+4)
PARTIAL FRACTION EXPANSIONSMETHOD 2
Specify 2 value of s, and solve simultaneously.
PARTIAL FRACTION EXPANSIONSMETHOD 3HEAVISIDE EXPANSION
To find : Multiply both sides by s + 1 and let s = 1
To find : Multiply both sides by s + 4 and let s = 4
Consider a general expression,
Here D(s) is an nth order polynomial with the roots all being real numbers which are distinct so there are no repeated roots.
The PFE is:
Note:D(s) is called the “characteristic polynomial”.
EXAMPLE 2
Solve the ODE;
HEAVISIDE EXPANSION
HEAVISIDE EXPANSION
EXAMPLE 3
By using HEAVISIDE EXPANSION, solve the ODE;
IMPORTANT PROPERTIES OF LAPLACE TRANSFORMS
It can be used to find the steadystate value of a closed loop system (providing that a steadystate value exists.)
Statement of FVT:
providing that the limit exists (is finite) for all where Re (s) denotes the real part of complex variable, s.
2. Initial Value Theorem
3. Transform of an Intergal
4. Time Delay
Time delays occur due to fluid flow, time required to do an analysis (e.g., gas chromatograph). The delayed signal can be represented as
Also,