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from sympy import Basic, Mul, Pow, degree, Symbol, expand, cancel, Expr, exp, roots
from sympy.core.evalf import EvalfMixin
from sympy.core.logic import fuzzy_and
from sympy.core.numbers import Integer
from sympy.core.sympify import sympify, _sympify
from sympy.polys import Poly, rootof
from sympy.series import limit
__all__ = ['TransferFunction', 'Series', 'Parallel', 'Feedback']
def _roots(poly, var):
""" like roots, but works on higher-order polynomials. """
r = roots(poly, var, multiple=True)
n = degree(poly)
if len(r) != n:
r = [rootof(poly, var, k) for k in range(n)]
return r
class TransferFunction(Basic, EvalfMixin):
"""
A class for representing LTI (Linear, time-invariant) systems that can be strictly described
by ratio of polynomials in the Laplace Transform complex variable. The arguments
are ``num``, ``den``, and ``var``, where ``num`` and ``den`` are numerator and
denominator polynomials of the ``TransferFunction`` respectively, and the third argument is
a complex variable of the Laplace transform used by these polynomials of the transfer function.
``num`` and ``den`` can be either polynomials or numbers, whereas ``var``
has to be a Symbol.
Parameters
==========
num : Expr, Number
The numerator polynomial of the transfer function.
den : Expr, Number
The denominator polynomial of the transfer function.
var : Symbol
Complex variable of the Laplace transform used by the
polynomials of the transfer function.
Raises
======
TypeError
When ``var`` is not a Symbol or when ``num`` or ``den`` is not
a number or a polynomial. Also, when ``num`` or ``den`` has
a time delay term.
ValueError
When ``den`` is zero.
Examples
========
>>> from sympy.abc import s, p, a
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction(s + a, s**2 + s + 1, s)
>>> tf1
TransferFunction(a + s, s**2 + s + 1, s)
>>> tf1.num
a + s
>>> tf1.den
s**2 + s + 1
>>> tf1.var
s
>>> tf1.args
(a + s, s**2 + s + 1, s)
Any complex variable can be used for ``var``.
>>> tf2 = TransferFunction(a*p**3 - a*p**2 + s*p, p + a**2, p)
>>> tf2
TransferFunction(a*p**3 - a*p**2 + p*s, a**2 + p, p)
>>> tf3 = TransferFunction((p + 3)*(p - 1), (p - 1)*(p + 5), p)
>>> tf3
TransferFunction((p - 1)*(p + 3), (p - 1)*(p + 5), p)
To negate a transfer function the ``-`` operator can be prepended:
>>> tf4 = TransferFunction(-a + s, p**2 + s, p)
>>> -tf4
TransferFunction(a - s, p**2 + s, p)
>>> tf5 = TransferFunction(s**4 - 2*s**3 + 5*s + 4, s + 4, s)
>>> -tf5
TransferFunction(-s**4 + 2*s**3 - 5*s - 4, s + 4, s)
You can use a Float or an Integer (or other constants) as numerator and denominator:
>>> tf6 = TransferFunction(1/2, 4, s)
>>> tf6.num
0.500000000000000
>>> tf6.den
4
>>> tf6.var
s
>>> tf6.args
(0.5, 4, s)
You can take the integer power of a transfer function using the ``**`` operator:
>>> tf7 = TransferFunction(s + a, s - a, s)
>>> tf7**3
TransferFunction((a + s)**3, (-a + s)**3, s)
>>> tf7**0
TransferFunction(1, 1, s)
>>> tf8 = TransferFunction(p + 4, p - 3, p)
>>> tf8**-1
TransferFunction(p - 3, p + 4, p)
Addition, subtraction, and multiplication of transfer functions can form
unevaluated ``Series`` or ``Parallel`` objects.
>>> tf9 = TransferFunction(s + 1, s**2 + s + 1, s)
>>> tf10 = TransferFunction(s - p, s + 3, s)
>>> tf11 = TransferFunction(4*s**2 + 2*s - 4, s - 1, s)
>>> tf12 = TransferFunction(1 - s, s**2 + 4, s)
>>> tf9 + tf10
Parallel(TransferFunction(s + 1, s**2 + s + 1, s), TransferFunction(-p + s, s + 3, s))
>>> tf10 - tf11
Parallel(TransferFunction(-p + s, s + 3, s), TransferFunction(-4*s**2 - 2*s + 4, s - 1, s))
>>> tf9 * tf10
Series(TransferFunction(s + 1, s**2 + s + 1, s), TransferFunction(-p + s, s + 3, s))
>>> tf10 - (tf9 + tf12)
Parallel(TransferFunction(-p + s, s + 3, s), TransferFunction(-s - 1, s**2 + s + 1, s), TransferFunction(s - 1, s**2 + 4, s))
>>> tf10 - (tf9 * tf12)
Parallel(TransferFunction(-p + s, s + 3, s), Series(TransferFunction(-1, 1, s), Series(TransferFunction(s + 1, s**2 + s + 1, s), TransferFunction(1 - s, s**2 + 4, s))))
>>> tf11 * tf10 * tf9
Series(TransferFunction(4*s**2 + 2*s - 4, s - 1, s), TransferFunction(-p + s, s + 3, s), TransferFunction(s + 1, s**2 + s + 1, s))
>>> tf9 * tf11 + tf10 * tf12
Parallel(Series(TransferFunction(s + 1, s**2 + s + 1, s), TransferFunction(4*s**2 + 2*s - 4, s - 1, s)), Series(TransferFunction(-p + s, s + 3, s), TransferFunction(1 - s, s**2 + 4, s)))
>>> (tf9 + tf12) * (tf10 + tf11)
Series(Parallel(TransferFunction(s + 1, s**2 + s + 1, s), TransferFunction(1 - s, s**2 + 4, s)), Parallel(TransferFunction(-p + s, s + 3, s), TransferFunction(4*s**2 + 2*s - 4, s - 1, s)))
These unevaluated ``Series`` or ``Parallel`` objects can convert into the
resultant transfer function using ``.doit()`` method or by ``.rewrite(TransferFunction)``.
>>> ((tf9 + tf10) * tf12).doit()
TransferFunction((1 - s)*((-p + s)*(s**2 + s + 1) + (s + 1)*(s + 3)), (s + 3)*(s**2 + 4)*(s**2 + s + 1), s)
>>> (tf9 * tf10 - tf11 * tf12).rewrite(TransferFunction)
TransferFunction(-(1 - s)*(s + 3)*(s**2 + s + 1)*(4*s**2 + 2*s - 4) + (-p + s)*(s - 1)*(s + 1)*(s**2 + 4), (s - 1)*(s + 3)*(s**2 + 4)*(s**2 + s + 1), s)
See Also
========
Feedback, Series, Parallel
"""
def __new__(cls, num, den, var):
num, den = _sympify(num), _sympify(den)
if not isinstance(var, Symbol):
raise TypeError("Variable input must be a Symbol.")
if den == 0:
raise ValueError("TransferFunction can't have a zero denominator.")
if (((isinstance(num, Expr) and num.has(Symbol) and not num.has(exp)) or num.is_number) and
((isinstance(den, Expr) and den.has(Symbol) and not den.has(exp)) or den.is_number)):
obj = super(TransferFunction, cls).__new__(cls, num, den, var)
obj._num = num
obj._den = den
obj._var = var
return obj
else:
raise TypeError("Unsupported type for numerator or denominator of TransferFunction.")
@property
def num(self):
"""
Returns the numerator polynomial of the transfer function.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction
>>> G1 = TransferFunction(s**2 + p*s + 3, s - 4, s)
>>> G1.num
p*s + s**2 + 3
>>> G2 = TransferFunction((p + 5)*(p - 3), (p - 3)*(p + 1), p)
>>> G2.num
(p - 3)*(p + 5)
"""
return self._num
@property
def den(self):
"""
Returns the denominator polynomial of the transfer function.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction
>>> G1 = TransferFunction(s + 4, p**3 - 2*p + 4, s)
>>> G1.den
p**3 - 2*p + 4
>>> G2 = TransferFunction(3, 4, s)
>>> G2.den
4
"""
return self._den
@property
def var(self):
"""
Returns the complex variable of the Laplace transform used by the polynomials of
the transfer function.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction
>>> G1 = TransferFunction(p**2 + 2*p + 4, p - 6, p)
>>> G1.var
p
>>> G2 = TransferFunction(0, s - 5, s)
>>> G2.var
s
"""
return self._var
def _eval_subs(self, old, new):
arg_num = self.num.subs(old, new)
arg_den = self.den.subs(old, new)
argnew = TransferFunction(arg_num, arg_den, self.var)
return self if old == self.var else argnew
def _eval_evalf(self, prec):
return TransferFunction(
self.num._eval_evalf(prec),
self.den._eval_evalf(prec),
self.var)
def _eval_simplify(self, **kwargs):
tf = cancel(Mul(self.num, 1/self.den, evaluate=False), expand=False).as_numer_denom()
num_, den_ = tf[0], tf[1]
return TransferFunction(num_, den_, self.var)
def expand(self):
"""
Returns the transfer function with numerator and denominator
in expanded form.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction
>>> G1 = TransferFunction((a - s)**2, (s**2 + a)**2, s)
>>> G1.expand()
TransferFunction(a**2 - 2*a*s + s**2, a**2 + 2*a*s**2 + s**4, s)
>>> G2 = TransferFunction((p + 3*b)*(p - b), (p - b)*(p + 2*b), p)
>>> G2.expand()
TransferFunction(-3*b**2 + 2*b*p + p**2, -2*b**2 + b*p + p**2, p)
"""
return TransferFunction(expand(self.num), expand(self.den), self.var)
def dc_gain(self):
"""
Computes the gain of the response as the frequency approaches zero.
The DC gain is infinite for systems with pure integrators.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction(s + 3, s**2 - 9, s)
>>> tf1.dc_gain()
-1/3
>>> tf2 = TransferFunction(p**2, p - 3 + p**3, p)
>>> tf2.dc_gain()
0
>>> tf3 = TransferFunction(a*p**2 - b, s + b, s)
>>> tf3.dc_gain()
(a*p**2 - b)/b
>>> tf4 = TransferFunction(1, s, s)
>>> tf4.dc_gain()
oo
"""
m = Mul(self.num, Pow(self.den, -1, evaluate=False), evaluate=False)
return limit(m, self.var, 0)
def poles(self):
"""
Returns the poles of a transfer function.
Examples
========
>>> from sympy.abc import s, p, a
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction((p + 3)*(p - 1), (p - 1)*(p + 5), p)
>>> tf1.poles()
[-5, 1]
>>> tf2 = TransferFunction((1 - s)**2, (s**2 + 1)**2, s)
>>> tf2.poles()
[I, I, -I, -I]
>>> tf3 = TransferFunction(s**2, a*s + p, s)
>>> tf3.poles()
[-p/a]
"""
return _roots(Poly(self.den, self.var), self.var)
def zeros(self):
"""
Returns the zeros of a transfer function.
Examples
========
>>> from sympy.abc import s, p, a
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction((p + 3)*(p - 1), (p - 1)*(p + 5), p)
>>> tf1.zeros()
[-3, 1]
>>> tf2 = TransferFunction((1 - s)**2, (s**2 + 1)**2, s)
>>> tf2.zeros()
[1, 1]
>>> tf3 = TransferFunction(s**2, a*s + p, s)
>>> tf3.zeros()
[0, 0]
"""
return _roots(Poly(self.num, self.var), self.var)
def is_stable(self):
"""
Returns True if the transfer function is asymptotically stable; else False.
This would not check the marginal or conditional stability of the system.
Examples
========
>>> from sympy.abc import s, p, a
>>> from sympy.core.symbol import symbols
>>> q, r = symbols('q, r', negative=True)
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction((1 - s)**2, (s + 1)**2, s)
>>> tf1.is_stable()
True
>>> tf2 = TransferFunction((1 - p)**2, (s**2 + 1)**2, s)
>>> tf2.is_stable()
False
>>> tf3 = TransferFunction(4, q*s - r, s)
>>> tf3.is_stable()
False
>>> tf4 = TransferFunction(p + 1, a*p - s**2, p)
>>> tf4.is_stable() is None # Not enough info about the symbols to determine stability
True
"""
return fuzzy_and(pole.as_real_imag()[0].is_negative for pole in self.poles())
def __add__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Parallel(self, other)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(other.args)
return Parallel(self, *arg_list)
else:
raise ValueError("TransferFunction cannot be added with {}.".
format(type(other)))
def __radd__(self, other):
return self + other
def __sub__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Parallel(self, -other)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = [-i for i in list(other.args)]
return Parallel(self, *arg_list)
else:
raise ValueError("{} cannot be subtracted from a TransferFunction."
.format(type(other)))
def __rsub__(self, other):
return -self + other
def __mul__(self, other):
if isinstance(other, (TransferFunction, Parallel)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Series(self, other)
elif isinstance(other, Series):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(other.args)
return Series(self, *arg_list)
else:
raise ValueError("TransferFunction cannot be multiplied with {}."
.format(type(other)))
__rmul__ = __mul__
def __truediv__(self, other):
if (isinstance(other, Parallel) and len(other.args) == 2 and isinstance(other.args[0], TransferFunction)
and isinstance(other.args[1], (Series, TransferFunction))):
if not self.var == other.var:
raise ValueError("Both TransferFunction and Parallel should use the"
" same complex variable of the Laplace transform.")
if other.args[1] == self:
# plant and controller with unit feedback.
return Feedback(self, other.args[0])
other_arg_list = list(other.args[1].args) if isinstance(other.args[1], Series) else other.args[1]
if other_arg_list == other.args[1]:
return Feedback(self, other_arg_list)
elif self in other_arg_list:
other_arg_list.remove(self)
else:
return Feedback(self, Series(*other_arg_list))
if len(other_arg_list) == 1:
return Feedback(self, *other_arg_list)
else:
return Feedback(self, Series(*other_arg_list))
else:
raise ValueError("TransferFunction cannot be divided by {}.".
format(type(other)))
__rtruediv__ = __truediv__
def __pow__(self, p):
p = sympify(p)
if not isinstance(p, Integer):
raise ValueError("Exponent must be an Integer.")
if p == 0:
return TransferFunction(1, 1, self.var)
elif p > 0:
num_, den_ = self.num**p, self.den**p
else:
p = abs(p)
num_, den_ = self.den**p, self.num**p
return TransferFunction(num_, den_, self.var)
def __neg__(self):
return TransferFunction(-self.num, self.den, self.var)
@property
def is_proper(self):
"""
Returns True if degree of the numerator polynomial is less than
or equal to degree of the denominator polynomial, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction(b*s**2 + p**2 - a*p + s, b - p**2, s)
>>> tf1.is_proper
False
>>> tf2 = TransferFunction(p**2 - 4*p, p**3 + 3*p + 2, p)
>>> tf2.is_proper
True
"""
return degree(self.num, self.var) <= degree(self.den, self.var)
@property
def is_strictly_proper(self):
"""
Returns True if degree of the numerator polynomial is strictly less
than degree of the denominator polynomial, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf1.is_strictly_proper
False
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> tf2.is_strictly_proper
True
"""
return degree(self.num, self.var) < degree(self.den, self.var)
@property
def is_biproper(self):
"""
Returns True if degree of the numerator polynomial is equal to
degree of the denominator polynomial, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf1.is_biproper
True
>>> tf2 = TransferFunction(p**2, p + a, p)
>>> tf2.is_biproper
False
"""
return degree(self.num, self.var) == degree(self.den, self.var)
class Series(Basic):
"""
A class for representing product of transfer functions or transfer functions in a
series configuration.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Series, Parallel
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> tf3 = TransferFunction(p**2, p + s, s)
>>> S1 = Series(tf1, tf2)
>>> S1
Series(TransferFunction(a*p**2 + b*s, -p + s, s), TransferFunction(s**3 - 2, s**4 + 5*s + 6, s))
>>> S1.var
s
>>> S2 = Series(tf2, Parallel(tf3, -tf1))
>>> S2
Series(TransferFunction(s**3 - 2, s**4 + 5*s + 6, s), Parallel(TransferFunction(p**2, p + s, s), TransferFunction(-a*p**2 - b*s, -p + s, s)))
>>> S2.var
s
>>> S3 = Series(Parallel(tf1, tf2), Parallel(tf2, tf3))
>>> S3
Series(Parallel(TransferFunction(a*p**2 + b*s, -p + s, s), TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)), Parallel(TransferFunction(s**3 - 2, s**4 + 5*s + 6, s), TransferFunction(p**2, p + s, s)))
>>> S3.var
s
You can get the resultant transfer function by using ``.doit()`` method:
>>> S3 = Series(tf1, tf2, -tf3)
>>> S3.doit()
TransferFunction(-p**2*(s**3 - 2)*(a*p**2 + b*s), (-p + s)*(p + s)*(s**4 + 5*s + 6), s)
>>> S4 = Series(tf2, Parallel(tf1, -tf3))
>>> S4.doit()
TransferFunction((s**3 - 2)*(-p**2*(-p + s) + (p + s)*(a*p**2 + b*s)), (-p + s)*(p + s)*(s**4 + 5*s + 6), s)
Notes
=====
All the transfer functions should use the same complex variable
``var`` of the Laplace transform.
See Also
========
Parallel, TransferFunction, Feedback
"""
def __new__(cls, *args, evaluate=False):
if not all(isinstance(arg, (TransferFunction, Parallel, Series)) for arg in args):
raise TypeError("Unsupported type of argument(s) for Series.")
obj = super(Series, cls).__new__(cls, *args)
obj._var = None
for arg in args:
if obj._var is None:
obj._var = arg.var
elif obj._var != arg.var:
raise ValueError("All transfer functions should use the same complex"
" variable of the Laplace transform.")
if evaluate:
return obj.doit()
return obj
@property
def var(self):
"""
Returns the complex variable used by all the transfer functions.
Examples
========
>>> from sympy.abc import p
>>> from sympy.physics.control.lti import TransferFunction, Series, Parallel
>>> G1 = TransferFunction(p**2 + 2*p + 4, p - 6, p)
>>> G2 = TransferFunction(p, 4 - p, p)
>>> G3 = TransferFunction(0, p**4 - 1, p)
>>> Series(G1, G2).var
p
>>> Series(-G3, Parallel(G1, G2)).var
p
"""
return self._var
def doit(self, **kwargs):
"""
Returns the resultant transfer function obtained after evaluating
the transfer functions in series configuration.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Series
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> Series(tf2, tf1).doit()
TransferFunction((s**3 - 2)*(a*p**2 + b*s), (-p + s)*(s**4 + 5*s + 6), s)
>>> Series(-tf1, -tf2).doit()
TransferFunction((2 - s**3)*(-a*p**2 - b*s), (-p + s)*(s**4 + 5*s + 6), s)
"""
res = None
for arg in self.args:
arg = arg.doit()
if res is None:
res = arg
else:
num_ = arg.num * res.num
den_ = arg.den * res.den
res = TransferFunction(num_, den_, self.var)
return res
def _eval_rewrite_as_TransferFunction(self, *args, **kwargs):
return self.doit()
def __add__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Parallel(self, other)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(other.args)
return Parallel(self, *arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
__radd__ = __add__
def __sub__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Parallel(self, -other)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = [-i for i in list(other.args)]
return Parallel(self, *arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
def __rsub__(self, other):
return -self + other
def __mul__(self, other):
if isinstance(other, (TransferFunction, Parallel)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(self.args)
return Series(*arg_list, other)
elif isinstance(other, Series):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
self_arg_list = list(self.args)
other_arg_list = list(other.args)
return Series(*self_arg_list, *other_arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
def __truediv__(self, other):
if (isinstance(other, Parallel) and len(other.args) == 2
and isinstance(other.args[0], TransferFunction) and isinstance(other.args[1], Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
self_arg_list = set(list(self.args))
other_arg_list = set(list(other.args[1].args))
res = list(self_arg_list ^ other_arg_list)
if len(res) == 0:
return Feedback(self, other.args[0])
elif len(res) == 1:
return Feedback(self, *res)
else:
return Feedback(self, Series(*res))
else:
raise ValueError("This transfer function expression is invalid.")
def __neg__(self):
return Series(TransferFunction(-1, 1, self.var), self)
@property
def is_proper(self):
"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is less than or equal to degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Series
>>> tf1 = TransferFunction(b*s**2 + p**2 - a*p + s, b - p**2, s)
>>> tf2 = TransferFunction(p**2 - 4*p, p**3 + 3*s + 2, s)
>>> tf3 = TransferFunction(s, s**2 + s + 1, s)
>>> S1 = Series(-tf2, tf1)
>>> S1.is_proper
False
>>> S2 = Series(tf1, tf2, tf3)
>>> S2.is_proper
True
"""
return self.doit().is_proper
@property
def is_strictly_proper(self):
"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is strictly less than degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Series
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**2 + 5*s + 6, s)
>>> tf3 = TransferFunction(1, s**2 + s + 1, s)
>>> S1 = Series(tf1, tf2)
>>> S1.is_strictly_proper
False
>>> S2 = Series(tf1, tf2, tf3)
>>> S2.is_strictly_proper
True
"""
return self.doit().is_strictly_proper
@property
def is_biproper(self):
r"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is equal to degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Series
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(p, s**2, s)
>>> tf3 = TransferFunction(s**2, 1, s)
>>> S1 = Series(tf1, -tf2)
>>> S1.is_biproper
False
>>> S2 = Series(tf2, tf3)
>>> S2.is_biproper
True
"""
return self.doit().is_biproper
class Parallel(Basic):
"""
A class for representing addition of transfer functions or transfer functions
in a parallel configuration.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Parallel, Series
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> tf3 = TransferFunction(p**2, p + s, s)
>>> P1 = Parallel(tf1, tf2)
>>> P1
Parallel(TransferFunction(a*p**2 + b*s, -p + s, s), TransferFunction(s**3 - 2, s**4 + 5*s + 6, s))
>>> P1.var
s
>>> P2 = Parallel(tf2, Series(tf3, -tf1))
>>> P2
Parallel(TransferFunction(s**3 - 2, s**4 + 5*s + 6, s), Series(TransferFunction(p**2, p + s, s), TransferFunction(-a*p**2 - b*s, -p + s, s)))
>>> P2.var
s
>>> P3 = Parallel(Series(tf1, tf2), Series(tf2, tf3))
>>> P3
Parallel(Series(TransferFunction(a*p**2 + b*s, -p + s, s), TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)), Series(TransferFunction(s**3 - 2, s**4 + 5*s + 6, s), TransferFunction(p**2, p + s, s)))
>>> P3.var
s
You can get the resultant transfer function by using ``.doit()`` method:
>>> Parallel(tf1, tf2, -tf3).doit()
TransferFunction(-p**2*(-p + s)*(s**4 + 5*s + 6) + (p + s)*((-p + s)*(s**3 - 2) + (a*p**2 + b*s)*(s**4 + 5*s + 6)), (-p + s)*(p + s)*(s**4 + 5*s + 6), s)
>>> Parallel(tf2, Series(tf1, -tf3)).doit()
TransferFunction(-p**2*(a*p**2 + b*s)*(s**4 + 5*s + 6) + (-p + s)*(p + s)*(s**3 - 2), (-p + s)*(p + s)*(s**4 + 5*s + 6), s)
Notes
=====
All the transfer functions should use the same complex variable
``var`` of the Laplace transform.
See Also
========
Series, TransferFunction, Feedback
"""
def __new__(cls, *args, evaluate=False):
if not all(isinstance(arg, (TransferFunction, Series, Parallel)) for arg in args):
raise TypeError("Unsupported type of argument(s) for Parallel.")
obj = super(Parallel, cls).__new__(cls, *args)
obj._var = None
for arg in args:
if obj._var is None:
obj._var = arg.var
elif obj._var != arg.var:
raise ValueError("All transfer functions should use the same complex"
" variable of the Laplace transform.")
if evaluate:
return obj.doit()
return obj
@property
def var(self):
"""
Returns the complex variable used by all the transfer functions.
Examples
========
>>> from sympy.abc import p
>>> from sympy.physics.control.lti import TransferFunction, Parallel, Series
>>> G1 = TransferFunction(p**2 + 2*p + 4, p - 6, p)
>>> G2 = TransferFunction(p, 4 - p, p)
>>> G3 = TransferFunction(0, p**4 - 1, p)
>>> Parallel(G1, G2).var
p
>>> Parallel(-G3, Series(G1, G2)).var
p
"""
return self._var
def doit(self, **kwargs):
"""
Returns the resultant transfer function obtained after evaluating
the transfer functions in parallel configuration.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Parallel
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> Parallel(tf2, tf1).doit()
TransferFunction((-p + s)*(s**3 - 2) + (a*p**2 + b*s)*(s**4 + 5*s + 6), (-p + s)*(s**4 + 5*s + 6), s)
>>> Parallel(-tf1, -tf2).doit()
TransferFunction((2 - s**3)*(-p + s) + (-a*p**2 - b*s)*(s**4 + 5*s + 6), (-p + s)*(s**4 + 5*s + 6), s)
"""
res = None
for arg in self.args:
arg = arg.doit()
if res is None:
res = arg
else:
num_ = res.num * arg.den + res.den * arg.num
den_ = res.den * arg.den
res = TransferFunction(num_, den_, self.var)
return res
def _eval_rewrite_as_TransferFunction(self, *args, **kwargs):
return self.doit()
def __add__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(self.args)
arg_list.append(other)
return Parallel(*arg_list)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
self_arg_list = list(self.args)
other_arg_list = list(other.args)
for elem in other_arg_list:
self_arg_list.append(elem)
return Parallel(*self_arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
def __sub__(self, other):
if isinstance(other, (TransferFunction, Series)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(self.args)
arg_list.append(-other)
return Parallel(*arg_list)
elif isinstance(other, Parallel):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
self_arg_list = list(self.args)
other_arg_list = list(other.args)
for elem in other_arg_list:
self_arg_list.append(-elem)
return Parallel(*self_arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
def __mul__(self, other):
if isinstance(other, (TransferFunction, Parallel)):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
return Series(self, other)
elif isinstance(other, Series):
if not self.var == other.var:
raise ValueError("All the transfer functions should use the same complex variable "
"of the Laplace transform.")
arg_list = list(other.args)
return Series(self, *arg_list)
else:
raise ValueError("This transfer function expression is invalid.")
def __neg__(self):
return Series(TransferFunction(-1, 1, self.var), self)
@property
def is_proper(self):
"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is less than or equal to degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Parallel
>>> tf1 = TransferFunction(b*s**2 + p**2 - a*p + s, b - p**2, s)
>>> tf2 = TransferFunction(p**2 - 4*p, p**3 + 3*s + 2, s)
>>> tf3 = TransferFunction(s, s**2 + s + 1, s)
>>> P1 = Parallel(-tf2, tf1)
>>> P1.is_proper
False
>>> P2 = Parallel(tf2, tf3)
>>> P2.is_proper
True
"""
return self.doit().is_proper
@property
def is_strictly_proper(self):
"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is strictly less than degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Parallel
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(s**3 - 2, s**4 + 5*s + 6, s)
>>> tf3 = TransferFunction(s, s**2 + s + 1, s)
>>> P1 = Parallel(tf1, tf2)
>>> P1.is_strictly_proper
False
>>> P2 = Parallel(tf2, tf3)
>>> P2.is_strictly_proper
True
"""
return self.doit().is_strictly_proper
@property
def is_biproper(self):
"""
Returns True if degree of the numerator polynomial of the resultant transfer
function is equal to degree of the denominator polynomial of
the same, else False.
Examples
========
>>> from sympy.abc import s, p, a, b
>>> from sympy.physics.control.lti import TransferFunction, Parallel
>>> tf1 = TransferFunction(a*p**2 + b*s, s - p, s)
>>> tf2 = TransferFunction(p**2, p + s, s)
>>> tf3 = TransferFunction(s, s**2 + s + 1, s)
>>> P1 = Parallel(tf1, -tf2)
>>> P1.is_biproper
True
>>> P2 = Parallel(tf2, tf3)
>>> P2.is_biproper
False
"""
return self.doit().is_biproper
class Feedback(Basic):
"""
A class for representing negative feedback interconnection between two
input/output systems. The first argument, ``num``, is called as the
primary plant or the numerator, and the second argument, ``den``, is
called as the feedback plant (which is often a feedback controller) or
the denominator. Both ``num`` and ``den`` can either be ``Series`` or
``TransferFunction`` objects.
Parameters
==========
num : Series, TransferFunction
The primary plant.
den : Series, TransferFunction
The feedback plant (often a feedback controller).
Raises
======
ValueError
When ``num`` is equal to ``den`` or when they are not using the
same complex variable of the Laplace transform.
TypeError
When either ``num`` or ``den`` is not a ``Series`` or a
``TransferFunction`` object.
Examples
========
>>> from sympy.abc import s
>>> from sympy.physics.control.lti import TransferFunction, Feedback
>>> plant = TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> controller = TransferFunction(5*s - 10, s + 7, s)
>>> F1 = Feedback(plant, controller)
>>> F1
Feedback(TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s), TransferFunction(5*s - 10, s + 7, s))
>>> F1.var
s
>>> F1.args
(TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s), TransferFunction(5*s - 10, s + 7, s))
You can get the primary and the feedback plant using ``.num`` and ``.den`` respectively.
>>> F1.num
TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> F1.den
TransferFunction(5*s - 10, s + 7, s)
You can get the resultant closed loop transfer function obtained by negative feedback
interconnection using ``.doit()`` method.
>>> F1.doit()
TransferFunction((s + 7)*(s**2 - 4*s + 2)*(3*s**2 + 7*s - 3), ((s + 7)*(s**2 - 4*s + 2) + (5*s - 10)*(3*s**2 + 7*s - 3))*(s**2 - 4*s + 2), s)
>>> G = TransferFunction(2*s**2 + 5*s + 1, s**2 + 2*s + 3, s)
>>> C = TransferFunction(5*s + 10, s + 10, s)
>>> F2 = Feedback(G*C, TransferFunction(1, 1, s))
>>> F2.doit()
TransferFunction((s + 10)*(5*s + 10)*(s**2 + 2*s + 3)*(2*s**2 + 5*s + 1), (s + 10)*((s + 10)*(s**2 + 2*s + 3) + (5*s + 10)*(2*s**2 + 5*s + 1))*(s**2 + 2*s + 3), s)
To negate a ``Feedback`` object, the ``-`` operator can be prepended:
>>> -F1
Feedback(TransferFunction(-3*s**2 - 7*s + 3, s**2 - 4*s + 2, s), TransferFunction(5*s - 10, s + 7, s))
>>> -F2
Feedback(Series(TransferFunction(-1, 1, s), Series(TransferFunction(2*s**2 + 5*s + 1, s**2 + 2*s + 3, s), TransferFunction(5*s + 10, s + 10, s))), TransferFunction(1, 1, s))
See Also
========
TransferFunction, Series, Parallel
"""
def __new__(cls, num, den):
if not (isinstance(num, (TransferFunction, Series))
and isinstance(den, (TransferFunction, Series))):
raise TypeError("Unsupported type for numerator or denominator of Feedback.")
if num == den:
raise ValueError("The numerator cannot be equal to the denominator.")
if not num.var == den.var:
raise ValueError("Both numerator and denominator should be using the"
" same complex variable.")
obj = super(Feedback, cls).__new__(cls, num, den)
obj._num = num
obj._den = den
obj._var = num.var
return obj
@property
def num(self):
"""
Returns the primary plant of the negative feedback closed loop.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction, Feedback
>>> plant = TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> controller = TransferFunction(5*s - 10, s + 7, s)
>>> F1 = Feedback(plant, controller)
>>> F1.num
TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> G = TransferFunction(2*s**2 + 5*s + 1, p**2 + 2*p + 3, p)
>>> C = TransferFunction(5*p + 10, p + 10, p)
>>> P = TransferFunction(1 - s, p + 2, p)
>>> F2 = Feedback(TransferFunction(1, 1, p), G*C*P)
>>> F2.num
TransferFunction(1, 1, p)
"""
return self._num
@property
def den(self):
"""
Returns the feedback plant (often a feedback controller) of the
negative feedback closed loop.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction, Feedback
>>> plant = TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> controller = TransferFunction(5*s - 10, s + 7, s)
>>> F1 = Feedback(plant, controller)
>>> F1.den
TransferFunction(5*s - 10, s + 7, s)
>>> G = TransferFunction(2*s**2 + 5*s + 1, p**2 + 2*p + 3, p)
>>> C = TransferFunction(5*p + 10, p + 10, p)
>>> P = TransferFunction(1 - s, p + 2, p)
>>> F2 = Feedback(TransferFunction(1, 1, p), G*C*P)
>>> F2.den
Series(TransferFunction(2*s**2 + 5*s + 1, p**2 + 2*p + 3, p), TransferFunction(5*p + 10, p + 10, p), TransferFunction(1 - s, p + 2, p))
"""
return self._den
@property
def var(self):
"""
Returns the complex variable of the Laplace transform used by all
the transfer functions involved in the negative feedback closed loop.
Examples
========
>>> from sympy.abc import s, p
>>> from sympy.physics.control.lti import TransferFunction, Feedback
>>> plant = TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> controller = TransferFunction(5*s - 10, s + 7, s)
>>> F1 = Feedback(plant, controller)
>>> F1.var
s
>>> G = TransferFunction(2*s**2 + 5*s + 1, p**2 + 2*p + 3, p)
>>> C = TransferFunction(5*p + 10, p + 10, p)
>>> P = TransferFunction(1 - s, p + 2, p)
>>> F2 = Feedback(TransferFunction(1, 1, p), G*C*P)
>>> F2.var
p
"""
return self._var
def doit(self, **kwargs):
"""
Returns the resultant closed loop transfer function obtained by the
negative feedback interconnection.
Examples
========
>>> from sympy.abc import s
>>> from sympy.physics.control.lti import TransferFunction, Feedback
>>> plant = TransferFunction(3*s**2 + 7*s - 3, s**2 - 4*s + 2, s)
>>> controller = TransferFunction(5*s - 10, s + 7, s)
>>> F1 = Feedback(plant, controller)
>>> F1.doit()
TransferFunction((s + 7)*(s**2 - 4*s + 2)*(3*s**2 + 7*s - 3), ((s + 7)*(s**2 - 4*s + 2) + (5*s - 10)*(3*s**2 + 7*s - 3))*(s**2 - 4*s + 2), s)
>>> G = TransferFunction(2*s**2 + 5*s + 1, s**2 + 2*s + 3, s)
>>> F2 = Feedback(G, TransferFunction(1, 1, s))
>>> F2.doit()
TransferFunction((s**2 + 2*s + 3)*(2*s**2 + 5*s + 1), (s**2 + 2*s + 3)*(3*s**2 + 7*s + 4), s)
"""
arg_list = list(self.num.args) if isinstance(self.num, Series) else [self.num]
# F_n and F_d are resultant TFs of num and den of Feedback.
F_n, tf = self.num.doit(), TransferFunction(1, 1, self.num.var)
F_d = Parallel(tf, Series(self.den, *arg_list)).doit()
return TransferFunction(F_n.num*F_d.den, F_n.den*F_d.num, F_n.var)
def _eval_rewrite_as_TransferFunction(self, num, den, **kwargs):
return self.doit()
def __neg__(self):
return Feedback(-self.num, self.den)
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