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BugFix: tf2ss now handles static gains #129

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Merged
merged 1 commit into from
Feb 15, 2017
Merged

BugFix: tf2ss now handles static gains #129

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33 changes: 23 additions & 10 deletions control/statesp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@

# Python 3 compatibility (needs to go here)
from __future__ import print_function
from __future__ import division # for _convertToStateSpace

"""Copyright (c) 2010 by California Institute of Technology
All rights reserved.
Expand Down Expand Up @@ -647,6 +648,7 @@ def _convertToStateSpace(sys, **kw):
"""

from .xferfcn import TransferFunction
import itertools
if isinstance(sys, StateSpace):
if len(kw):
raise TypeError("If sys is a StateSpace, _convertToStateSpace \
Expand Down Expand Up @@ -679,16 +681,27 @@ def _convertToStateSpace(sys, **kw):
ssout[3][:sys.outputs, :states],
ssout[4], sys.dt)
except ImportError:
# If slycot is not available, use signal.lti (SISO only)
if (sys.inputs != 1 or sys.outputs != 1):
raise TypeError("No support for MIMO without slycot")

# TODO: do we want to squeeze first and check dimenations?
# I think this will fail if num and den aren't 1-D after
# the squeeze
lti_sys = lti(squeeze(sys.num), squeeze(sys.den))
return StateSpace(lti_sys.A, lti_sys.B, lti_sys.C, lti_sys.D,
sys.dt)
# No Slycot. Scipy tf->ss can't handle MIMO, but static
# MIMO is an easy special case we can check for here
maxn = max(max(len(n) for n in nrow)
for nrow in sys.num)
maxd = max(max(len(d) for d in drow)
for drow in sys.den)
if 1==maxn and 1==maxd:
D = empty((sys.outputs,sys.inputs),dtype=float)
for i,j in itertools.product(range(sys.outputs),range(sys.inputs)):
D[i,j] = sys.num[i][j][0] / sys.den[i][j][0]
return StateSpace([], [], [], D, sys.dt)
else:
if (sys.inputs != 1 or sys.outputs != 1):
raise TypeError("No support for MIMO without slycot")

# TODO: do we want to squeeze first and check dimenations?
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@slivingston slivingston Feb 14, 2017 •
edited
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Should "dimenations" be "dimensions"?

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I can't take the credit for this one, the comment was there already ;). Not sure it's applicable anymore: the check that sys.inputs and sys.outputs are both 1 implies that squeeze will bring num and den to 1-D?

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Yes, I agree. The comment is obsolete after the addition in commit c90a399 of the check that sys.inputs = sys.outputs = 1.

I am planning to review and perform maintenance on documentation and code comments, so I can delete this comment at that time.

# I think this will fail if num and den aren't 1-D after
# the squeeze
lti_sys = lti(squeeze(sys.num), squeeze(sys.den))
return StateSpace(lti_sys.A, lti_sys.B, lti_sys.C, lti_sys.D,
sys.dt)

elif isinstance(sys, (int, float, complex)):
if "inputs" in kw:
Expand Down
22 changes: 22 additions & 0 deletions control/tests/convert_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -184,6 +184,28 @@ def testConvertMIMO(self):
if (not slycot_check()):
self.assertRaises(TypeError, control.tf2ss, tfcn)

def testTf2ssStaticSiso(self):
"""Regression: tf2ss for SISO static gain"""
import control
gsiso = control.tf2ss(control.tf(23, 46))
self.assertEqual(0, gsiso.states)
self.assertEqual(1, gsiso.inputs)
self.assertEqual(1, gsiso.outputs)
# in all cases ratios are exactly representable, so assert_array_equal is fine
np.testing.assert_array_equal([[0.5]], gsiso.D)

def testTf2ssStaticMimo(self):
"""Regression: tf2ss for MIMO static gain"""
import control
# 2x3 TFM
gmimo = control.tf2ss(control.tf([[ [23], [3], [5] ], [ [-1], [0.125], [101.3] ]],
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How did you select these coefficients?

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It's an arbitrary selection of numbers over a (small) range of magnitudes and signs; in all cases the ratios are exactly representable with low precision (see d just below), so I could use np.assert_array_equal().

[[ [46], [0.1], [80] ], [ [2], [-0.1], [1] ]]))
self.assertEqual(0, gmimo.states)
self.assertEqual(3, gmimo.inputs)
self.assertEqual(2, gmimo.outputs)
d = np.matrix([[0.5, 30, 0.0625], [-0.5, -1.25, 101.3]])
np.testing.assert_array_equal(d, gmimo.D)


def suite():
return unittest.TestLoader().loadTestsFromTestCase(TestConvert)
Expand Down