Using sympy, having an issue with an expression not being able to convert to a float

Question

I am using sympy to be able to perform Laplace/Inverse Laplace transforms for a controls class. Currently I am using the inverse Laplace but am having the error shown below my code. I believe the issue has something to do with the package and not my code but I am unsure.

from sympy.integrals.transforms import inverse_laplace_transform
from sympy import exp, Symbol
from sympy.plotting import plot
from sympy.abc import s, t

a = Symbol('a', positive = True)

G_d = 0.5*exp(-30*s)/(60*s + 1)
G_t = 1
G_f_a = -0.25
G_f_b = -0.25*exp(-10*s)*(95*s + 1)/(60*s + 1)
G_v = 1
G_p = 2*exp(-20*s)/(95*s + 1)
G_m = 1
G_c = 95 + 95/(10*s)

TF1 = (G_d + G_t*G_f_a*G_v*G_p)/(1 + G_c*G_v*G_p*G_m)/s
TF2 = (G_d + G_t*G_f_b*G_v*G_p)/(1 + G_c*G_v*G_p*G_m)/s
y1 = inverse_laplace_transform(TF1, s, t)
y2 = inverse_laplace_transform(TF2, s, t)

plot(y1, y2, (t, 0, 500))

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File "C:\Users\jj\OneDrive\Documents\Spring 2023\15-10.py", line 30, in plot(y1, y2, (t, 0, 500))

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 1873, in plot plots.show()

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 251, in show self._backend.show()

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 1549, in show self.process_series()

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 1546, in process_series self._process_series(series, ax, parent)

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 1367, in _process_series x, y = s.get_data()

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 605, in get_data points = self.get_points()

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\plot.py", line 779, in get_points f_start = f(self.start)

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\experimental_lambdify.py", line 188, in call return self.call(args)

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\experimental_lambdify.py", line 196, in call return self.call(args)

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\experimental_lambdify.py", line 184, in call raise e

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\experimental_lambdify.py", line 176, in call result = complex(self.lambda_func(args))

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\plotting\experimental_lambdify.py", line 272, in call return self.lambda_func(*args, **kwargs)

File "", line 1, in

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\core\expr.py", line 356, in complex return complex(float(re), float(im))

File "C:\Users\jj\anaconda3\envs\rdkit_env\Lib\site-packages\sympy\core\expr.py", line 351, in float raise TypeError("Cannot convert expression to float")

TypeError: Cannot convert expression to float

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When my variable G_c is set to 0, the code runs fine and I get the correct plots, however, with any value other than 0, I get the "cannot convert expression to float" error. The plot should come looking similar to the one shown below.Plot for G_c = 0, plot for G_c other than 0 should look similar

score 0 · Answer 1 · answered Apr 25 '23 at 14:51

SymPy's inverse_laplace_transform is unable to compute the inverse Laplace transform of this expression:

In [205]: TF1
Out[205]: 
        -20⋅s        -30⋅s 
   0.5⋅ℯ        0.5⋅ℯ      
 - ────────── + ────────── 
    95⋅s + 1     60⋅s + 1  
───────────────────────────
  ⎛  ⎛      19⎞  -20⋅s    ⎞
  ⎜2⋅⎜95 + ───⎟⋅ℯ         ⎟
  ⎜  ⎝     2⋅s⎠           ⎟
s⋅⎜─────────────────── + 1⎟
  ⎝      95⋅s + 1         ⎠

There might not be any analytic expression for the ILT here because of the non-removable exponentials in the denominator:

In [207]: TF1.factor()
Out[207]: 
       ⎛      10⋅s           10⋅s    ⎞  -10⋅s  
  -0.5⋅⎝60⋅s⋅ℯ     - 95⋅s + ℯ     - 1⎠⋅ℯ       
───────────────────────────────────────────────
           ⎛    2  20⋅s      20⋅s             ⎞
(60⋅s + 1)⋅⎝95⋅s ⋅ℯ     + s⋅ℯ     + 190⋅s + 19⎠

Using sympy, having an issue with an expression not being able to convert to a float

1 Answers1