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1492 lines (1217 loc) · 51.5 KB
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"""statesp.py
State space representation and functions.
This file contains the StateSpace class, which is used to represent linear
systems in state space. This is the primary representation for the
python-control library.
"""
# 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.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. Neither the name of the California Institute of Technology nor
the names of its contributors may be used to endorse or promote
products derived from this software without specific prior
written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL CALTECH
OR THE CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
Author: Richard M. Murray
Date: 24 May 09
Revised: Kevin K. Chen, Dec 10
$Id$
"""
import math
import numpy as np
from numpy import any, array, asarray, concatenate, cos, delete, \
dot, empty, exp, eye, isinf, ones, pad, sin, zeros, squeeze
from numpy.random import rand, randn
from numpy.linalg import solve, eigvals, matrix_rank
from numpy.linalg.linalg import LinAlgError
import scipy as sp
from scipy.signal import lti, cont2discrete
from warnings import warn
from .lti import LTI, timebase, timebaseEqual, isdtime
from . import config
from copy import deepcopy
__all__ = ['StateSpace', 'ss', 'rss', 'drss', 'tf2ss', 'ssdata']
# Define module default parameter values
_statesp_defaults = {
'statesp.use_numpy_matrix': True,
}
def _ssmatrix(data, axis=1):
"""Convert argument to a (possibly empty) state space matrix.
Parameters
----------
data : array, list, or string
Input data defining the contents of the 2D array
axis : 0 or 1
If input data is 1D, which axis to use for return object. The default
is 1, corresponding to a row matrix.
Returns
-------
arr : 2D array, with shape (0, 0) if a is empty
"""
# Convert the data into an array or matrix, as configured
# If data is passed as a string, use (deprecated?) matrix constructor
if config.defaults['statesp.use_numpy_matrix'] or isinstance(data, str):
arr = np.matrix(data, dtype=float)
else:
arr = np.array(data, dtype=float)
ndim = arr.ndim
shape = arr.shape
# Change the shape of the array into a 2D array
if (ndim > 2):
raise ValueError("state-space matrix must be 2-dimensional")
elif (ndim == 2 and shape == (1, 0)) or \
(ndim == 1 and shape == (0, )):
# Passed an empty matrix or empty vector; change shape to (0, 0)
shape = (0, 0)
elif ndim == 1:
# Passed a row or column vector
shape = (1, shape[0]) if axis == 1 else (shape[0], 1)
elif ndim == 0:
# Passed a constant; turn into a matrix
shape = (1, 1)
# Create the actual object used to store the result
return arr.reshape(shape)
class StateSpace(LTI):
"""StateSpace(A, B, C, D[, dt])
A class for representing state-space models
The StateSpace class is used to represent state-space realizations of linear
time-invariant (LTI) systems:
dx/dt = A x + B u
y = C x + D u
where u is the input, y is the output, and x is the state.
The main data members are the A, B, C, and D matrices. The class also
keeps track of the number of states (i.e., the size of A). The data
format used to store state space matrices is set using the value of
`config.defaults['use_numpy_matrix']`. If True (default), the state space
elements are stored as `numpy.matrix` objects; otherwise they are
`numpy.ndarray` objects. The :func:`~control.use_numpy_matrix` function
can be used to set the storage type.
Discrete-time state space system are implemented by using the 'dt'
instance variable and setting it to the sampling period. If 'dt' is not
None, then it must match whenever two state space systems are combined.
Setting dt = 0 specifies a continuous system, while leaving dt = None
means the system timebase is not specified. If 'dt' is set to True, the
system will be treated as a discrete time system with unspecified sampling
time.
"""
# Allow ndarray * StateSpace to give StateSpace._rmul_() priority
__array_priority__ = 11 # override ndarray and matrix types
def __init__(self, *args, **kw):
"""
StateSpace(A, B, C, D[, dt])
Construct a state space object.
The default constructor is StateSpace(A, B, C, D), where A, B, C, D
are matrices or equivalent objects. To create a discrete time system,
use StateSpace(A, B, C, D, dt) where 'dt' is the sampling time (or
True for unspecified sampling time). To call the copy constructor,
call StateSpace(sys), where sys is a StateSpace object.
"""
if len(args) == 4:
# The user provided A, B, C, and D matrices.
(A, B, C, D) = args
dt = None
elif len(args) == 5:
# Discrete time system
(A, B, C, D, dt) = args
elif len(args) == 1:
# Use the copy constructor.
if not isinstance(args[0], StateSpace):
raise TypeError("The one-argument constructor can only take in a StateSpace "
"object. Received %s." % type(args[0]))
A = args[0].A
B = args[0].B
C = args[0].C
D = args[0].D
try:
dt = args[0].dt
except NameError:
dt = None
else:
raise ValueError("Needs 1 or 4 arguments; received %i." % len(args))
# Process keyword arguments
remove_useless = kw.get('remove_useless', True)
# Convert all matrices to standard form
A = _ssmatrix(A)
B = _ssmatrix(B, axis=0)
C = _ssmatrix(C, axis=1)
if np.isscalar(D) and D == 0 and B.shape[1] > 0 and C.shape[0] > 0:
# If D is a scalar zero, broadcast it to the proper size
D = np.zeros((C.shape[0], B.shape[1]))
D = _ssmatrix(D)
# TODO: use super here?
LTI.__init__(self, inputs=D.shape[1], outputs=D.shape[0], dt=dt)
self.A = A
self.B = B
self.C = C
self.D = D
self.states = A.shape[1]
if 0 == self.states:
# static gain
# matrix's default "empty" shape is 1x0
A.shape = (0,0)
B.shape = (0,self.inputs)
C.shape = (self.outputs,0)
# Check that the matrix sizes are consistent.
if self.states != A.shape[0]:
raise ValueError("A must be square.")
if self.states != B.shape[0]:
raise ValueError("A and B must have the same number of rows.")
if self.states != C.shape[1]:
raise ValueError("A and C must have the same number of columns.")
if self.inputs != B.shape[1]:
raise ValueError("B and D must have the same number of columns.")
if self.outputs != C.shape[0]:
raise ValueError("C and D must have the same number of rows.")
# Check for states that don't do anything, and remove them.
if remove_useless: self._remove_useless_states()
def _remove_useless_states(self):
"""Check for states that don't do anything, and remove them.
Scan the A, B, and C matrices for rows or columns of zeros. If the
zeros are such that a particular state has no effect on the input-output
dynamics, then remove that state from the A, B, and C matrices.
"""
# Search for useless states and get indices of these states.
#
# Note: shape from np.where depends on whether we are storing state
# space objects as np.matrix or np.array. Code below will work
# correctly in either case.
ax1_A = np.where(~self.A.any(axis=1))[0]
ax1_B = np.where(~self.B.any(axis=1))[0]
ax0_A = np.where(~self.A.any(axis=0))[-1]
ax0_C = np.where(~self.C.any(axis=0))[-1]
useless_1 = np.intersect1d(ax1_A, ax1_B, assume_unique=True)
useless_2 = np.intersect1d(ax0_A, ax0_C, assume_unique=True)
useless = np.union1d(useless_1, useless_2)
# Remove the useless states.
self.A = delete(self.A, useless, 0)
self.A = delete(self.A, useless, 1)
self.B = delete(self.B, useless, 0)
self.C = delete(self.C, useless, 1)
self.states = self.A.shape[0]
self.inputs = self.B.shape[1]
self.outputs = self.C.shape[0]
def __str__(self):
"""String representation of the state space."""
str = "A = " + self.A.__str__() + "\n\n"
str += "B = " + self.B.__str__() + "\n\n"
str += "C = " + self.C.__str__() + "\n\n"
str += "D = " + self.D.__str__() + "\n"
# TODO: replace with standard calls to lti functions
if (type(self.dt) == bool and self.dt is True):
str += "\ndt unspecified\n"
elif (not (self.dt is None) and type(self.dt) != bool and self.dt > 0):
str += "\ndt = " + self.dt.__str__() + "\n"
return str
# represent as string, makes display work for IPython
__repr__ = __str__
# Negation of a system
def __neg__(self):
"""Negate a state space system."""
return StateSpace(self.A, self.B, -self.C, -self.D, self.dt)
# Addition of two state space systems (parallel interconnection)
def __add__(self, other):
"""Add two LTI systems (parallel connection)."""
# Check for a couple of special cases
if isinstance(other, (int, float, complex, np.number)):
# Just adding a scalar; put it in the D matrix
A, B, C = self.A, self.B, self.C
D = self.D + other
dt = self.dt
else:
other = _convertToStateSpace(other)
# Check to make sure the dimensions are OK
if ((self.inputs != other.inputs) or
(self.outputs != other.outputs)):
raise ValueError("Systems have different shapes.")
# Figure out the sampling time to use
if self.dt is None and other.dt is not None:
dt = other.dt # use dt from second argument
elif (other.dt is None and self.dt is not None) or \
(timebaseEqual(self, other)):
dt = self.dt # use dt from first argument
else:
raise ValueError("Systems have different sampling times")
# Concatenate the various arrays
A = concatenate((
concatenate((self.A, zeros((self.A.shape[0],
other.A.shape[-1]))),axis=1),
concatenate((zeros((other.A.shape[0], self.A.shape[-1])),
other.A),axis=1)
),axis=0)
B = concatenate((self.B, other.B), axis=0)
C = concatenate((self.C, other.C), axis=1)
D = self.D + other.D
return StateSpace(A, B, C, D, dt)
# Right addition - just switch the arguments
def __radd__(self, other):
"""Right add two LTI systems (parallel connection)."""
return self + other
# Subtraction of two state space systems (parallel interconnection)
def __sub__(self, other):
"""Subtract two LTI systems."""
return self + (-other)
def __rsub__(self, other):
"""Right subtract two LTI systems."""
return other + (-self)
# Multiplication of two state space systems (series interconnection)
def __mul__(self, other):
"""Multiply two LTI objects (serial connection)."""
# Check for a couple of special cases
if isinstance(other, (int, float, complex, np.number)):
# Just multiplying by a scalar; change the output
A, B = self.A, self.B
C = self.C * other
D = self.D * other
dt = self.dt
else:
other = _convertToStateSpace(other)
# Check to make sure the dimensions are OK
if self.inputs != other.outputs:
raise ValueError("C = A * B: A has %i column(s) (input(s)), \
but B has %i row(s)\n(output(s))." % (self.inputs, other.outputs))
# Figure out the sampling time to use
if (self.dt == None and other.dt != None):
dt = other.dt # use dt from second argument
elif (other.dt == None and self.dt != None) or \
(timebaseEqual(self, other)):
dt = self.dt # use dt from first argument
else:
raise ValueError("Systems have different sampling times")
# Concatenate the various arrays
A = concatenate(
(concatenate((other.A,
zeros((other.A.shape[0], self.A.shape[1]))),
axis=1),
concatenate((np.dot(self.B, other.C), self.A), axis=1)),
axis=0)
B = concatenate((other.B, np.dot(self.B, other.D)), axis=0)
C = concatenate((np.dot(self.D, other.C), self.C),axis=1)
D = np.dot(self.D, other.D)
return StateSpace(A, B, C, D, dt)
# Right multiplication of two state space systems (series interconnection)
# Just need to convert LH argument to a state space object
# TODO: __rmul__ only works for special cases (??)
def __rmul__(self, other):
"""Right multiply two LTI objects (serial connection)."""
# Check for a couple of special cases
if isinstance(other, (int, float, complex, np.number)):
# Just multiplying by a scalar; change the input
A, C = self.A, self.C
B = self.B * other
D = self.D * other
return StateSpace(A, B, C, D, self.dt)
# is lti, and convertible?
if isinstance(other, LTI):
return _convertToStateSpace(other) * self
# try to treat this as a matrix
try:
X = _ssmatrix(other)
C = np.dot(X, self.C)
D = np.dot(X, self.D)
return StateSpace(self.A, self.B, C, D, self.dt)
except Exception as e:
print(e)
pass
raise TypeError("can't interconnect systems")
# TODO: __div__ and __rdiv__ are not written yet.
def __div__(self, other):
"""Divide two LTI systems."""
raise NotImplementedError("StateSpace.__div__ is not implemented yet.")
def __rdiv__(self, other):
"""Right divide two LTI systems."""
raise NotImplementedError("StateSpace.__rdiv__ is not implemented yet.")
def evalfr(self, omega):
"""Evaluate a SS system's transfer function at a single frequency.
self._evalfr(omega) returns the value of the transfer function matrix
with input value s = i * omega.
"""
warn("StateSpace.evalfr(omega) will be deprecated in a future "
"release of python-control; use evalfr(sys, omega*1j) instead",
PendingDeprecationWarning)
return self._evalfr(omega)
def _evalfr(self, omega):
"""Evaluate a SS system's transfer function at a single frequency"""
# Figure out the point to evaluate the transfer function
if isdtime(self, strict=True):
dt = timebase(self)
s = exp(1.j * omega * dt)
if omega * dt > math.pi:
warn("_evalfr: frequency evaluation above Nyquist frequency")
else:
s = omega * 1.j
return self.horner(s)
def horner(self, s):
"""Evaluate the systems's transfer function for a complex variable
Returns a matrix of values evaluated at complex variable s.
"""
resp = np.dot(self.C, solve(s * eye(self.states) - self.A,
self.B)) + self.D
return array(resp)
# Method for generating the frequency response of the system
def freqresp(self, omega):
"""Evaluate the system's transfer func. at a list of freqs, omega.
mag, phase, omega = self.freqresp(omega)
Reports the frequency response of the system,
G(j*omega) = mag*exp(j*phase)
for continuous time. For discrete time systems, the response is
evaluated around the unit circle such that
G(exp(j*omega*dt)) = mag*exp(j*phase).
Parameters
----------
omega : array
A list of frequencies in radians/sec at which the system should be
evaluated. The list can be either a python list or a numpy array
and will be sorted before evaluation.
Returns
-------
mag : float
The magnitude (absolute value, not dB or log10) of the system
frequency response.
phase : float
The wrapped phase in radians of the system frequency response.
omega : array
The list of sorted frequencies at which the response was
evaluated.
"""
# In case omega is passed in as a list, rather than a proper array.
omega = np.asarray(omega)
numFreqs = len(omega)
Gfrf = np.empty((self.outputs, self.inputs, numFreqs),
dtype=np.complex128)
# Sort frequency and calculate complex frequencies on either imaginary
# axis (continuous time) or unit circle (discrete time).
omega.sort()
if isdtime(self, strict=True):
dt = timebase(self)
cmplx_freqs = exp(1.j * omega * dt)
if max(np.abs(omega)) * dt > math.pi:
warn("freqresp: frequency evaluation above Nyquist frequency")
else:
cmplx_freqs = omega * 1.j
# Do the frequency response evaluation. Use TB05AD from Slycot
# if it's available, otherwise use the built-in horners function.
try:
from slycot import tb05ad
n = np.shape(self.A)[0]
m = self.inputs
p = self.outputs
# The first call both evaluates C(sI-A)^-1 B and also returns
# Hessenberg transformed matrices at, bt, ct.
result = tb05ad(n, m, p, cmplx_freqs[0], self.A,
self.B, self.C, job='NG')
# When job='NG', result = (at, bt, ct, g_i, hinvb, info)
at = result[0]
bt = result[1]
ct = result[2]
# TB05AD frequency evaluation does not include direct feedthrough.
Gfrf[:, :, 0] = result[3] + self.D
# Now, iterate through the remaining frequencies using the
# transformed state matrices, at, bt, ct.
# Start at the second frequency, already have the first.
for kk, cmplx_freqs_kk in enumerate(cmplx_freqs[1:numFreqs]):
result = tb05ad(n, m, p, cmplx_freqs_kk, at,
bt, ct, job='NH')
# When job='NH', result = (g_i, hinvb, info)
# kk+1 because enumerate starts at kk = 0.
# but zero-th spot is already filled.
Gfrf[:, :, kk+1] = result[0] + self.D
except ImportError: # Slycot unavailable. Fall back to horner.
for kk, cmplx_freqs_kk in enumerate(cmplx_freqs):
Gfrf[:, :, kk] = self.horner(cmplx_freqs_kk)
# mag phase omega
return np.abs(Gfrf), np.angle(Gfrf), omega
# Compute poles and zeros
def pole(self):
"""Compute the poles of a state space system."""
return eigvals(self.A) if self.states else np.array([])
def zero(self):
"""Compute the zeros of a state space system."""
if not self.states:
return np.array([])
# Use AB08ND from Slycot if it's available, otherwise use
# scipy.lingalg.eigvals().
try:
from slycot import ab08nd
out = ab08nd(self.A.shape[0], self.B.shape[1], self.C.shape[0],
self.A, self.B, self.C, self.D)
nu = out[0]
if nu == 0:
return np.array([])
else:
return sp.linalg.eigvals(out[8][0:nu, 0:nu], out[9][0:nu, 0:nu])
except ImportError: # Slycot unavailable. Fall back to scipy.
if self.C.shape[0] != self.D.shape[1]:
raise NotImplementedError("StateSpace.zero only supports "
"systems with the same number of "
"inputs as outputs.")
# This implements the QZ algorithm for finding transmission zeros
# from
# https://dspace.mit.edu/bitstream/handle/1721.1/841/P-0802-06587335.pdf.
# The QZ algorithm solves the generalized eigenvalue problem: given
# `L = [A, B; C, D]` and `M = [I_nxn 0]`, find all finite lambda
# for which there exist nontrivial solutions of the equation
# `Lz - lamba Mz`.
#
# The generalized eigenvalue problem is only solvable if its
# arguments are square matrices.
L = concatenate((concatenate((self.A, self.B), axis=1),
concatenate((self.C, self.D), axis=1)), axis=0)
M = pad(eye(self.A.shape[0]), ((0, self.C.shape[0]),
(0, self.B.shape[1])), "constant")
return np.array([x for x in sp.linalg.eigvals(L, M, overwrite_a=True)
if not isinf(x)])
# Feedback around a state space system
def feedback(self, other=1, sign=-1):
"""Feedback interconnection between two LTI systems."""
other = _convertToStateSpace(other)
# Check to make sure the dimensions are OK
if (self.inputs != other.outputs) or (self.outputs != other.inputs):
raise ValueError("State space systems don't have compatible inputs/outputs for "
"feedback.")
# Figure out the sampling time to use
if self.dt is None and other.dt is not None:
dt = other.dt # use dt from second argument
elif other.dt is None and self.dt is not None or timebaseEqual(self, other):
dt = self.dt # use dt from first argument
else:
raise ValueError("Systems have different sampling times")
A1 = self.A
B1 = self.B
C1 = self.C
D1 = self.D
A2 = other.A
B2 = other.B
C2 = other.C
D2 = other.D
F = eye(self.inputs) - sign * np.dot(D2, D1)
if matrix_rank(F) != self.inputs:
raise ValueError("I - sign * D2 * D1 is singular to working precision.")
# Precompute F\D2 and F\C2 (E = inv(F))
# We can solve two linear systems in one pass, since the
# coefficients matrix F is the same. Thus, we perform the LU
# decomposition (cubic runtime complexity) of F only once!
# The remaining back substitutions are only quadratic in runtime.
E_D2_C2 = solve(F, concatenate((D2, C2), axis=1))
E_D2 = E_D2_C2[:, :other.inputs]
E_C2 = E_D2_C2[:, other.inputs:]
T1 = eye(self.outputs) + sign * np.dot(D1, E_D2)
T2 = eye(self.inputs) + sign * np.dot(E_D2, D1)
A = concatenate(
(concatenate(
(A1 + sign * np.dot(np.dot(B1, E_D2), C1),
sign * np.dot(B1, E_C2)), axis=1),
concatenate(
(np.dot(B2, np.dot(T1, C1)),
A2 + sign * np.dot(np.dot(B2, D1), E_C2)), axis=1)),
axis=0)
B = concatenate((np.dot(B1, T2), np.dot(np.dot(B2, D1), T2)), axis=0)
C = concatenate((np.dot(T1, C1), sign * np.dot(D1, E_C2)), axis=1)
D = np.dot(D1, T2)
return StateSpace(A, B, C, D, dt)
def lft(self, other, nu=-1, ny=-1):
"""Return the Linear Fractional Transformation.
A definition of the LFT operator can be found in Appendix A.7,
page 512 in the 2nd Edition, Multivariable Feedback Control by
Sigurd Skogestad.
An alternative definition can be found here:
https://www.mathworks.com/help/control/ref/lft.html
Parameters
----------
other : LTI
The lower LTI system
ny : int, optional
Dimension of (plant) measurement output.
nu : int, optional
Dimension of (plant) control input.
"""
other = _convertToStateSpace(other)
# maximal values for nu, ny
if ny == -1:
ny = min(other.inputs, self.outputs)
if nu == -1:
nu = min(other.outputs, self.inputs)
# dimension check
# TODO
# Figure out the sampling time to use
if (self.dt == None and other.dt != None):
dt = other.dt # use dt from second argument
elif (other.dt == None and self.dt != None) or \
timebaseEqual(self, other):
dt = self.dt # use dt from first argument
else:
raise ValueError("Systems have different time bases")
# submatrices
A = self.A
B1 = self.B[:, :self.inputs - nu]
B2 = self.B[:, self.inputs - nu:]
C1 = self.C[:self.outputs - ny, :]
C2 = self.C[self.outputs - ny:, :]
D11 = self.D[:self.outputs - ny, :self.inputs - nu]
D12 = self.D[:self.outputs - ny, self.inputs - nu:]
D21 = self.D[self.outputs - ny:, :self.inputs - nu]
D22 = self.D[self.outputs - ny:, self.inputs - nu:]
# submatrices
Abar = other.A
Bbar1 = other.B[:, :ny]
Bbar2 = other.B[:, ny:]
Cbar1 = other.C[:nu, :]
Cbar2 = other.C[nu:, :]
Dbar11 = other.D[:nu, :ny]
Dbar12 = other.D[:nu, ny:]
Dbar21 = other.D[nu:, :ny]
Dbar22 = other.D[nu:, ny:]
# well-posed check
F = np.block([[np.eye(ny), -D22], [-Dbar11, np.eye(nu)]])
if matrix_rank(F) != ny + nu:
raise ValueError("lft not well-posed to working precision.")
# solve for the resulting ss by solving for [y, u] using [x,
# xbar] and [w1, w2].
TH = np.linalg.solve(F, np.block(
[[C2, np.zeros((ny, other.states)), D21, np.zeros((ny, other.inputs - ny))],
[np.zeros((nu, self.states)), Cbar1, np.zeros((nu, self.inputs - nu)), Dbar12]]
))
T11 = TH[:ny, :self.states]
T12 = TH[:ny, self.states: self.states + other.states]
T21 = TH[ny:, :self.states]
T22 = TH[ny:, self.states: self.states + other.states]
H11 = TH[:ny, self.states + other.states: self.states + other.states + self.inputs - nu]
H12 = TH[:ny, self.states + other.states + self.inputs - nu:]
H21 = TH[ny:, self.states + other.states: self.states + other.states + self.inputs - nu]
H22 = TH[ny:, self.states + other.states + self.inputs - nu:]
Ares = np.block([
[A + B2.dot(T21), B2.dot(T22)],
[Bbar1.dot(T11), Abar + Bbar1.dot(T12)]
])
Bres = np.block([
[B1 + B2.dot(H21), B2.dot(H22)],
[Bbar1.dot(H11), Bbar2 + Bbar1.dot(H12)]
])
Cres = np.block([
[C1 + D12.dot(T21), D12.dot(T22)],
[Dbar21.dot(T11), Cbar2 + Dbar21.dot(T12)]
])
Dres = np.block([
[D11 + D12.dot(H21), D12.dot(H22)],
[Dbar21.dot(H11), Dbar22 + Dbar21.dot(H12)]
])
return StateSpace(Ares, Bres, Cres, Dres, dt)
def minreal(self, tol=0.0):
"""Calculate a minimal realization, removes unobservable and
uncontrollable states"""
if self.states:
try:
from slycot import tb01pd
B = empty((self.states, max(self.inputs, self.outputs)))
B[:,:self.inputs] = self.B
C = empty((max(self.outputs, self.inputs), self.states))
C[:self.outputs,:] = self.C
A, B, C, nr = tb01pd(self.states, self.inputs, self.outputs,
self.A, B, C, tol=tol)
return StateSpace(A[:nr,:nr], B[:nr,:self.inputs],
C[:self.outputs,:nr], self.D)
except ImportError:
raise TypeError("minreal requires slycot tb01pd")
else:
return StateSpace(self)
# TODO: add discrete time check
def returnScipySignalLTI(self):
"""Return a list of a list of scipy.signal.lti objects.
For instance,
>>> out = ssobject.returnScipySignalLTI()
>>> out[3][5]
is a signal.scipy.lti object corresponding to the transfer function from
the 6th input to the 4th output."""
# Preallocate the output.
out = [[[] for _ in range(self.inputs)] for _ in range(self.outputs)]
for i in range(self.outputs):
for j in range(self.inputs):
out[i][j] = lti(asarray(self.A), asarray(self.B[:, j]),
asarray(self.C[i, :]), self.D[i, j])
return out
def append(self, other):
"""Append a second model to the present model. The second
model is converted to state-space if necessary, inputs and
outputs are appended and their order is preserved"""
if not isinstance(other, StateSpace):
other = _convertToStateSpace(other)
if self.dt != other.dt:
raise ValueError("Systems must have the same time step")
n = self.states + other.states
m = self.inputs + other.inputs
p = self.outputs + other.outputs
A = zeros((n, n))
B = zeros((n, m))
C = zeros((p, n))
D = zeros((p, m))
A[:self.states, :self.states] = self.A
A[self.states:, self.states:] = other.A
B[:self.states, :self.inputs] = self.B
B[self.states:, self.inputs:] = other.B
C[:self.outputs, :self.states] = self.C
C[self.outputs:, self.states:] = other.C
D[:self.outputs, :self.inputs] = self.D
D[self.outputs:, self.inputs:] = other.D
return StateSpace(A, B, C, D, self.dt)
def __getitem__(self, indices):
"""Array style access"""
if len(indices) != 2:
raise IOError('must provide indices of length 2 for state space')
i = indices[0]
j = indices[1]
return StateSpace(self.A, self.B[:, j], self.C[i, :], self.D[i, j], self.dt)
def sample(self, Ts, method='zoh', alpha=None):
"""Convert a continuous time system to discrete time
Creates a discrete-time system from a continuous-time system by
sampling. Multiple methods of conversion are supported.
Parameters
----------
Ts : float
Sampling period
method : {"gbt", "bilinear", "euler", "backward_diff", "zoh"}
Which method to use:
* gbt: generalized bilinear transformation
* bilinear: Tustin's approximation ("gbt" with alpha=0.5)
* euler: Euler (or forward differencing) method ("gbt" with
alpha=0)
* backward_diff: Backwards differencing ("gbt" with alpha=1.0)
* zoh: zero-order hold (default)
alpha : float within [0, 1]
The generalized bilinear transformation weighting parameter, which
should only be specified with method="gbt", and is ignored
otherwise
Returns
-------
sysd : StateSpace
Discrete time system, with sampling rate Ts
Notes
-----
Uses the command 'cont2discrete' from scipy.signal
Examples
--------
>>> sys = StateSpace(0, 1, 1, 0)
>>> sysd = sys.sample(0.5, method='bilinear')
"""
if not self.isctime():
raise ValueError("System must be continuous time system")
sys = (self.A, self.B, self.C, self.D)
Ad, Bd, C, D, dt = cont2discrete(sys, Ts, method, alpha)
return StateSpace(Ad, Bd, C, D, dt)
def dcgain(self):
"""Return the zero-frequency gain
The zero-frequency gain of a continuous-time state-space
system is given by:
.. math: G(0) = - C A^{-1} B + D
and of a discrete-time state-space system by:
.. math: G(1) = C (I - A)^{-1} B + D
Returns
-------
gain : ndarray
An array of shape (outputs,inputs); the array will either
be the zero-frequency (or DC) gain, or, if the frequency
response is singular, the array will be filled with np.nan.
"""
try:
if self.isctime():
gain = np.asarray(self.D-self.C.dot(np.linalg.solve(self.A, self.B)))
else:
gain = self.horner(1)
except LinAlgError:
# eigenvalue at DC
gain = np.tile(np.nan, (self.outputs, self.inputs))
return np.squeeze(gain)
# TODO: add discrete time check
def _convertToStateSpace(sys, **kw):
"""Convert a system to state space form (if needed).
If sys is already a state space, then it is returned. If sys is a transfer
function object, then it is converted to a state space and returned. If sys
is a scalar, then the number of inputs and outputs can be specified
manually, as in:
>>> sys = _convertToStateSpace(3.) # Assumes inputs = outputs = 1
>>> sys = _convertToStateSpace(1., inputs=3, outputs=2)
In the latter example, A = B = C = 0 and D = [[1., 1., 1.]
[1., 1., 1.]].
"""
from .xferfcn import TransferFunction
import itertools
if isinstance(sys, StateSpace):
if len(kw):
raise TypeError("If sys is a StateSpace, _convertToStateSpace \
cannot take keywords.")
# Already a state space system; just return it
return sys
elif isinstance(sys, TransferFunction):
try:
from slycot import td04ad
if len(kw):
raise TypeError("If sys is a TransferFunction, "
"_convertToStateSpace cannot take keywords.")
# Change the numerator and denominator arrays so that the transfer
# function matrix has a common denominator.
# matrices are also sized/padded to fit td04ad
num, den, denorder = sys.minreal()._common_den()
# transfer function to state space conversion now should work!
ssout = td04ad('C', sys.inputs, sys.outputs,
denorder, den, num, tol=0)
states = ssout[0]
return StateSpace(ssout[1][:states, :states], ssout[2][:states, :sys.inputs],
ssout[3][:sys.outputs, :states], ssout[4], sys.dt)
except ImportError:
# 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?
# I think this will fail if num and den aren't 1-D after
# the squeeze
A, B, C, D = sp.signal.tf2ss(squeeze(sys.num), squeeze(sys.den))
return StateSpace(A, B, C, D, sys.dt)
elif isinstance(sys, (int, float, complex, np.number)):
if "inputs" in kw:
inputs = kw["inputs"]
else:
inputs = 1
if "outputs" in kw:
outputs = kw["outputs"]
else:
outputs = 1
# Generate a simple state space system of the desired dimension
# The following Doesn't work due to inconsistencies in ltisys:
# return StateSpace([[]], [[]], [[]], eye(outputs, inputs))
return StateSpace(0., zeros((1, inputs)), zeros((outputs, 1)),