The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » Software » comp.soft-sys.matlab

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: Problem with inverse laplace
Replies: 5   Last Post: Mar 19, 2014 11:05 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]

Posts: 1
Registered: 12/6/12
Re: Problem with inverse laplace
Posted: Dec 6, 2012 3:07 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

that the roots of the polynomial are complex is quite obvious because this kind of equation represent the transient of an electric system (active and reactive parts), the problem is that matlab is perfectly able to compute ilaplace over a (polynomial of degree <=2) divided by a (polynomial of degree <=2) but when the function on the denominator grows one degree it return this kind "result"

for example:

syms s;
t = 10:0.01:50;

% Data
M1 = -3.5;
D1 = 1;
M2 = 4;
D2 = 0.75;
T = 6.283;
T_G1 = 0.01;
T_G2 = 0.05;
T_CARGA1 = 10;
T_CARGA2 = 20;
inc_P_L1 = 0.2;
inc_P_L2 = 0.0;

inc_w2= inc_P_L1*((-T)/((M1*s+D1)*((M2*s+D2)*s+T)+(M2*s+D2)*T));
inc_w2_i = ilaplace(inc_w2);

the result of this is:

-(25132*sum(exp(r4*t)/(11000*r4 - 168000*r4^2 + 15566), r4 in RootOf(s4^3 - (11*s4^2)/112 - (7783*s4)/28000 - 6283/8000, s4)))/5

Completely unusable, if I apply vpa() to this result I get a sum of exponentials, it might be ok but it is not, for this result to be usable, we have to apply euler's equation:
exp(x+iy)=exp(x)*(cos(y) + i*sin(y))
which is not possible because the result is a symbolic type. (at least I couldn't)

but if we downgrade the equation:

inc_w2= inc_P_L1*((-T)/((M1*s+D1)*((M2*s+D2)+T)+(M2*s+D2)*T));
inc_w2_i = ilaplace(inc_w2)

the result is now something quite usable :


a function that effectively represents the transient of a sinusoidal wave.

So after googling a lot I have not seen a proper answer to this issue.

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.