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math guide - 34.83

e.g.,

dy

y

the equation given

 

----- =

- – 1

 

 

dx

x

 

 

 

y

the substitution chosen

 

u =

x-

 

Put the substitution in and solve the differential equation,

 

dy

u – 1

 

 

----- =

 

 

dx

 

 

 

 

du

u – 1

u + x----- =

 

 

dx

 

 

du

–1

 

----- = -----

 

 

dx

x

 

 

du

1

 

–-----

= --

 

 

dx

x

 

 

u = ln ( x)

+ C

Substitute the results back into the original substitution equation to get rid of ’u’,

y

= ln ( x) + C

x--

y = – x ln ( x) – Cx

34.6.3.2- Second-order Differential Equations

These equations have at least one second-order derivative.

In engineering we will encounter a number of forms,

-homogeneous

-nonhomogeneous

34.6.3.2.1 - Linear Homogeneous

• These equations will have a standard form,

math guide - 34.84

d

 

2

y + A

d

y + By = 0

----

 

 

----

dt

 

 

dt

 

• An example of a solution is,

 

d

2

d

 

 

 

 

 

e.g.,

 

----

y + 6

 

----

 

 

 

 

 

dt

y + 3y = 0

 

 

 

 

dt

 

 

 

 

 

 

Guess,

 

 

 

 

 

 

 

 

 

 

y = eBt

 

 

 

 

 

 

 

 

d

y =

Be

Bt

 

 

 

 

----

 

 

 

 

 

 

dt

 

 

 

 

 

 

 

 

 

 

d

2

y =

B

2

e

Bt

 

 

 

----

 

 

 

 

dt

substitute and solve for B,

B2eBt + 6BeBt + 3eBt = 0

B2 + 6B + 3 = 0

B = – 3 + 2.449j, – 3 – 2.449j substitute and solve for B,

y = e( – 3 + 2.449j) t

y = e–3te2.449jt

y = e–3t( cos ( 2.449t) + j sin ( 2.449t) )

Note: if both the roots are the same,

y= C1eBt + C2teBt

34.6.3.2.2- Nonhomogeneous Linear Equations

• These equations have the general form,

math guide - 34.85

d 2

y + A

d

 

y + By = Cx

----

----

 

dt

 

dt

 

• to solve these equations we need to find the homogeneous and particular solutions and then add the two solutions.

y = yh + yp

 

 

to find yh solve,

 

 

d

2

d

y + B = 0

----

y + A

----

dt

 

dt

 

to find yp guess at a value of y and then test for validity, A good table of guesses is,

Cx form

Guess

 

 

A

C

Ax + B

Cx + D

eAx

CeAx CxeAx

B sin ( Ax) or B cos ( Ax)

C sin ( Ax) + D cos ( Ax)

 

or Cx sin ( Ax) + xD cos ( Ax)

 

 

• Consider the example below,

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