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fix discrete time simulation time step issue #332 #356

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Merged
merged 1 commit into from
Jan 2, 2020
Merged

fix discrete time simulation time step issue #332 #356

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39 changes: 39 additions & 0 deletions control/tests/timeresp_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -471,6 +471,45 @@ def test_time_vector(self):
squeeze=False)
self.assertTrue(isinstance(context.exception, ValueError))

def test_discrete_time_steps(self):
"""Make sure rounding errors in sample time are handled properly"""
# See https://github.com/python-control/python-control/issues/332)
#
# These tests play around with the input time vector to make sure that
# small rounding errors don't generate spurious errors.

# Discrete time system to use for simulation
# self.siso_dtf2 = TransferFunction([1], [1, 1, 0.25], 0.2)

# Set up a time range and simulate
T = np.arange(0, 100, 0.2)
tout1, yout1 = step_response(self.siso_dtf2, T)

# Simulate every other time step
T = np.arange(0, 100, 0.4)
tout2, yout2 = step_response(self.siso_dtf2, T)
np.testing.assert_array_almost_equal(tout1[::2], tout2)
np.testing.assert_array_almost_equal(yout1[::2], yout2)

# Add a small error into some of the time steps
T = np.arange(0, 100, 0.2)
T[1:-2:2] -= 1e-12 # tweak second value and a few others
tout3, yout3 = step_response(self.siso_dtf2, T)
np.testing.assert_array_almost_equal(tout1, tout3)
np.testing.assert_array_almost_equal(yout1, yout3)

# Add a small error into some of the time steps (w/ skipping)
T = np.arange(0, 100, 0.4)
T[1:-2:2] -= 1e-12 # tweak second value and a few others
tout4, yout4 = step_response(self.siso_dtf2, T)
np.testing.assert_array_almost_equal(tout2, tout4)
np.testing.assert_array_almost_equal(yout2, yout4)

# Make sure larger errors *do* generate an error
T = np.arange(0, 100, 0.2)
T[1:-2:2] -= 1e-3 # change second value and a few others
self.assertRaises(ValueError, step_response, self.siso_dtf2, T)

def test_time_series_data_convention(self):
"""Make sure time series data matches documentation conventions"""
# SISO continuous time
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11 changes: 7 additions & 4 deletions control/timeresp.py
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Expand Up @@ -365,7 +365,8 @@ def forced_response(sys, T=None, U=0., X0=0., transpose=False,
# Make sure that the time increment is a multiple of sampling time

# First make sure that time increment is bigger than sampling time
if dt < sys.dt:
# (with allowance for small precision errors)
if dt < sys.dt and not np.isclose(dt, sys.dt):
raise ValueError("Time steps ``T`` must match sampling time")

# Now check to make sure it is a multiple (with check against
Expand All @@ -374,19 +375,21 @@ def forced_response(sys, T=None, U=0., X0=0., transpose=False,
np.isclose(dt % sys.dt, sys.dt)):
raise ValueError("Time steps ``T`` must be multiples of "
"sampling time")
sys_dt = sys.dt

else:
sys.dt = dt # For unspecified sampling time, use time incr
sys_dt = dt # For unspecified sampling time, use time incr

# Discrete time simulation using signal processing toolbox
dsys = (A, B, C, D, sys.dt)
dsys = (A, B, C, D, sys_dt)

# Use signal processing toolbox for the discrete time simulation
# Transpose the input to match toolbox convention
tout, yout, xout = sp.signal.dlsim(dsys, np.transpose(U), T, X0)

if not interpolate:
# If dt is different from sys.dt, resample the output
inc = int(round(dt / sys.dt))
inc = int(round(dt / sys_dt))
tout = T # Return exact list of time steps
yout = yout[::inc, :]
xout = xout[::inc, :]
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