Why am I getting the wrong output with this MATLAB code

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James Mwage 2019-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/490309-why-am-i-getting-the-wrong-output-with-this-matlab-code

评论： Walter Roberson 2019-11-10

why am I gettng the wrong output in MATLAB with this code? The gain in the numerator is 2 in my input but comes out as 0.5 in the output

s=zpk('s');
g5= (2*(s+8))/((s+3)*(s+0.1)*(4*s+6));
%%output
g5 =
 
        0.5 (s+8)
  ---------------------
  (s+3) (s+1.5) (s+0.1)

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Answer 1

Walter Roberson 2019-11-10

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zpk and tf representation normalizes the first coefficient of the denominator to be 1.

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James Mwage 2019-11-10

Its the numerator that has the problem.

Walter Roberson 2019-11-10

Notice the denominator has 4*s+6 . Normalize that to 4*(s+1.5) in the denominator, and then divide the 2 in the numerator by the 4 in the denominator to get 0.5 in the numerator and 1 in the denominator, and you can see the s+1.5 term in the middle of the output. Therefore the expressions are equivalent. and there is no calculation problem.

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