slow ilapalce calculation for systems with higher polynomial orders

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S H 2019-2-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/443439-slow-ilapalce-calculation-for-systems-with-higher-polynomial-orders

评论： S H 2019-2-6

Is it possible to calculate the output o(t) of a system h(s) for the input x(s) faster than following codes? Why the following codes run extremely slow? What is wrong?

syms s t

time=linspace(0,4/1e09,401);

h=((5034923382327397*s^3)/627710173538668076383578942320766641610235544446403451289600 + (6766076405664713*s^2)/71362384635297994052914298472474756819137331200 + (210759975314673*s)/64903710731685345356631204115251200 + 376686340223973/281474976710656000000000000)/((5034923382327397*s^3)/12554203470773361527671578846415332832204710888928069025792 + (2284590225627523*s^2)/356811923176489970264571492362373784095686656 + (7512718920571563*s)/649037107316853453566312041152512 + 765850114395651/37778931862957161709568);

x=(100e-03/s)*(1-exp(-s/(2*1e09)))/(1+exp(-s/(2*1e09)));

y=ilaplace(x*h,s,t);

o=subs(y,t,time);

plot(time,o)

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Walter Roberson 2019-2-6

your expression for h is either incorrect or meaningless. Numbers with that many digits will be approximated with floating point which will introduce false behaviour with roots and poles in different places than you expect .

Chances are high that you should be using symbolic constants not floating point .

sym('26271892563873553613')

S H 2019-2-6

Thank you Walter. Your suggestion improved my codes but still ilaplace doesn't work. I will open a new thred by resubmitting my revised question.

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Answer 1

Walter Roberson 2019-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/443439-slow-ilapalce-calculation-for-systems-with-higher-polynomial-orders#answer_359714

subs(vpa(y,16),t,time)

No point in calculating with large numbers to indefinite precision only to throw away that precision since plot() only has single precision resolution according to some tests I have done.