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07-28-2010
08:50 AM

07-28-2010
08:50 AM

mathcad errors; inverse laplace

hi there.

i got some problems in mathcad(a new user) regarding laplace (err. inverse laplace) transforms

browsing the net, i got some exercise for mathcad: http://www.unf.edu/~mzhan/chapter6.pdf

problem arises when i tried to get the inverse laplace of 8a*s^2/(s^2+a^2)^3..

i didn't get any answer other than mathcad(14) making what i typed to red....

also, when i didn't even get the right inverse laplace of (-(2 a (a^2 - 3 s^2))/(a^2 + s^2)^3))..

-----------------correct answer is t^2*sin(a t)..

the thing is, inverse laplace-ing the laplace of f(t) don't give me f(t)...

mathematica did solved both my problems correctly..

mathcad seem to fail in that aspect?

9 REPLIES 9

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07-28-2010
10:39 AM

07-28-2010
10:39 AM

Re: mathcad errors; inverse laplace

Could you please post a worksheet.

Also, which version of Mathcad are you running, including service releases?

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07-28-2010
11:30 AM

07-28-2010
11:30 AM

Re: mathcad errors; inverse laplace

There are two possible answers:

1. You don't instruct your Mathcad version correctly

2. Your version of Mathcad is short and lazy in Laplace !

Don't take my word, but it seemed to me that the Mathcad versions above 11 [that aren't Maple ] , Laplace was Carson-Laplace. That's probably why it was so much reported as "bug" in Mathcad-MuPad ! That stuff is few [few = many] years back, and no recollection of Engineering work based on Carson-Laplace. You could check by comparing your result vs a conversion table. As you mentioned Mathematica, version 4.0 is "Laplace".

jmG

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07-28-2010
02:10 PM

07-28-2010
02:10 PM

Re: mathcad errors; inverse laplace

I know I used to do Laplace / Inverse Laplace in school all the time back in MathCad 11 and it worked great. After seeing this post, I tried it today in MathCad 14 M030 and I agree with the original poster, I'm not too pleased with the output...

I tried some additional symbolic operators (simplify, collect, etc.) to see if I could "encourage" an output in the form I was looking for....with no luck.

So now I'm off to read about the what changes were made in MathCad that are causing it to be "lazy" (as termed by Jean in the previous post) like this and not produce the result that I would expect (after using MathCad 11 for so many years to do this perfectly).

August

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07-28-2010
02:39 PM

07-28-2010
02:39 PM

Re: mathcad errors; inverse laplace

So now I'm off to read about the what changes were made in MathCad that are causing it to be "lazy" (as termed by Jean in the previous post) like this and not produce the result that I would expect (after using MathCad 11 for so many years to do this perfectly).

Going from MC13 to MC14 the symbolic engine was chnaged from Maple to MuPad. The MuPad engine has very poor integral tables and has problems with integrals, Fourier transforms, and Laplace transforms. Sometimes it can't solve them, sometimes it gets them wrong. Some of this has been fixed, but obviously not all of it.

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07-28-2010
10:22 PM

07-28-2010
10:22 PM

Re: mathcad errors; inverse laplace

August,

Mathcad 14 solution reveals the system it is reading is under determined, You could try the partial fraction expansion. If it does not work, the question is: MuPad can it do or should it do ? The answer is: it should but if it can't , then zap.

Jean

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07-29-2010
06:13 AM

07-29-2010
06:13 AM

Re: mathcad errors; inverse laplace

I tried it on 15.0.0.436 (trial) and ended up with the same result as August's..

-license says Feature Version 14.1, Version 15.0 Beta

Attached is side-by-side compasion with mathematica...

@Jean

What version of MathCad are you using?

So is it better to downgrade and just use Jean's version?

Thanks a lot.

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07-29-2010
08:10 AM

07-29-2010
08:10 AM

Re: mathcad errors; inverse laplace

@Jean What version of MathCad are you using?

So is it better to downgrade and just use Jean's version?

Any version prior to 14 will give that result, because they all use the Maple engine. If you downgrade get either MC13 or MC11. Do not get MC12! If you downgrade you may not be able to get Mathcad to run on either Vista or Windows 7.

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07-29-2010
12:42 PM

07-29-2010
12:42 PM

Re: mathcad errors; inverse laplace

Nowel Lubi wrote:

I tried it on 15.0.0.436 (trial) and ended up with the same result as August's..

-license says Feature Version 14.1, Version 15.0 Beta

Attached is side-by-side compasion with mathematica...

@Jean

What version of MathCad are you using?

So is it better to downgrade and just use Jean's version?

Thanks a lot.

I am Win XP Mathcad 11.2a Maple symbolic.. That MuPad is not a symbolic engine or a very limited one is one thing, but it shouldn't give any result when not capable. If you need the inverseLaplace, you need it correct or **No symbolic result was found** . Being so crappie, excludes Mathcad from the symolic DE Laplace solver, it excludes Mathcad from the Process Control & Instrumentation most essential tools, it surely excludes Mathcad doing more normal applications. Try more in the attached ... you could try running some of the standard Laplace table . I have a good collection to test, but what would be the point if PTC does not participate and hides in the sand ! And what would be the point if MuPad can't be fixed. MuPad was a single man creation [from a report I was reading in 2000 and already classified of low utility as it was at that time]. Maple is the product of the best past and recent mathematicians, a product based on "advanced algebra" and they have either discovered or re-used rules that produce result. The most notorious capability of Maple is the "RootOf". How many examples of RootOf I have passed in the former Mathcad collab = 100's. Not a single acknowledge except Alvaro, not a single acknowledge from PTC since 2006.

What's the use of Win 7, Vista ? I use Excel to draw lines, and little more gadgets, Paint [a lotlotlot], NotePad [a lot too].

What's the use of Win 7 32 decimals ln(1.23456789) ? If you have Win 7 and Mathematica, ask Mathematica what is your "Machine precision" , if it replies 16, then no numerical maths will take or exploit 32 decimals,

Jean

____________________________________

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07-30-2010
07:39 AM

07-30-2010
07:39 AM

Re: mathcad errors; inverse laplace

thanks a lot jean.

thank you guys (and gals), also, for those insights and replies..

Good day!