ss2tf adds additional wrong zeros #1068
Description
I have a DC motor model in Statespace (tf_ss), cast this to a transfer function (tf_1), and compare the poles and zeros with the analytical correct transfer function (tf_2).
The poles are correctly mapped by ss2tf, but the ss2tf adds additional zeros.
This might be a bug?
import numpy as np
import control as ct
Li = 1e-7
k = 1e-3
Rm = 1
Lm = 1e-3
A = np.array([
[0, 1, 0],
[0, 0, k/Li],
[0, - k/Lm, - Rm/Lm]])
B = np.array([[0], [0], [1/Lm]])
C = np.array([[1, 0, 0]])
D = np.array([[0]])
s = ct.TransferFunction.s
tf_ss = ct.ss(A, B, C, D)
tf_1 = ct.ss2tf(A, B, C, D)
tf_2 = k/s / ( k**2 + s*Li*(s*Lm + Rm))
print(tf_ss.poles())
print(tf_1.poles())
print(tf_2.poles())
# output identical
#[ 0. +0.j -10.10205144+0.j -989.89794856+0.j]
#[-989.89794856+0.j -10.10205144+0.j 0. +0.j]
#[-989.89794856+0.j -10.10205144+0.j 0. +0.j]
print(tf_ss.zeros())
print(tf_1.zeros())
print(tf_2.zeros())
# output tf_1 adds zeros ?
#[]
#[120.+6.63177692e+09j 120.-6.63177692e+09j]
#[]
Python 3.12.7
Control Version: 0.10.1