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Fix bandwidth function for DT systems #1212

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Fix bandwidth function for DT systems #1212

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8 changes: 7 additions & 1 deletion control/lti.py
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Original file line number Diff line number Diff line change
Expand Up @@ -211,8 +211,14 @@ def bandwidth(self, dbdrop=-3):
# solve for the bandwidth, use scipy.optimize.root_scalar() to
# solve using bisection
import scipy

if self.isdtime(strict=True):
cvt_w = lambda w: np.exp(1j * w * self.dt)
else:
cvt_w = lambda w: 1j * w

result = scipy.optimize.root_scalar(
lambda w: np.abs(self(w*1j)) - np.abs(dcgain)*10**(dbdrop/20),
lambda w: np.abs(self(cvt_w(w))) - np.abs(dcgain)*10**(dbdrop/20),
bracket=[omega[idx_dropped[0] - 1], omega[idx_dropped[0]]],
method='bisect')

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5 changes: 5 additions & 0 deletions control/tests/lti_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,11 @@ def test_bandwidth(self):
np.testing.assert_allclose(sys1.bandwidth(), 0.099762834511098)
np.testing.assert_allclose(bandwidth(sys1), 0.099762834511098)

# test a first-order discrete-time system, compared with matlab
sysd1 = tf([0.1, 0], [1, -0.9], 1)
np.testing.assert_allclose(sysd1.bandwidth(), 0.105207775532932)
np.testing.assert_allclose(bandwidth(sysd1), 0.105207775532932)

# test a second-order system, compared with matlab
wn2 = 1
zeta2 = 0.001
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