Exercise for the course Python for MATLAB users

Original exercise by Claus F├╝hrer, modified by Olivier Verdier

In [1]:
%matplotlib inline
Using matplotlib backend: MacOSX
Populating the interactive namespace from numpy and matplotlib

System matrix

Consider the matrix $$ A=\begin{bmatrix} 0 & I \\ K & D \end{bmatrix} $$ where $0$ and $I$ are the $2 \times 2$ zero and identity matrices and $K$ and $D$ are $2 \times 2$ matrices of the following form: $$ K=\begin{bmatrix} -k & 0.5 \\ 0.5 & -k \end{bmatrix} \qquad D=\begin{bmatrix} -d & 1.0 \\ 1.0 & -d \end{bmatrix} $$ with $k$ and $d$ being real parameters.

Write a function stiffness which constructs the matrix $K$ above.

In [2]:
def make_sym(k,a):
    """Make a matrix of the form K or D"""
    M = -k*identity(2)
    M[0,1] = a
    M[1,0] = a
    return M
In [3]:
def stiffness(k):
    return make_sym(k,.5)
In [4]:
assert(allclose(stiffness(1.), array([[-1.,.5],[.5,-1.]])))

Write a function damping which constructs the matrix $D$ above.

In [5]:
def damping(d):
    return make_sym(d,1.)
    #return zeros([2,2]) # implement this!
In [6]:
assert(allclose(damping(1.), array([[-1.,1.],[1.,-1.]])))

Write a function system_matrix which takes $k$ and $d$ as input and which generates the matrix $A$.

Hint: use the function concatenate. Check its documentation by running:

In [7]:

Using concatenate:

In [8]:
def system_matrix(d,k):
    left = concatenate([zeros((2,2)), stiffness(k)])
    right = concatenate([identity(2), damping(d)])
    return concatenate([left, right], axis=1)

Using inplace replacements:

In [9]:
def system_matrix(d,k):
    M = zeros((4,4))
    M[2:,:2] = stiffness(k)
    M[2:,2:] = damping(d)
    M[:2,2:] = identity(2)
    return M

Use also identity (or eye), zeros (or zeros_like).

In [10]:
A = system_matrix(10.,20.)
assert(allclose(A[:2,:2], zeros([2,2])))
assert(allclose(A[:2,2:4], identity(2)))
assert(allclose(A[2:4,:2], stiffness(20.)))
assert(allclose(A[2:4,2:4], damping(10.)))
[[  0.    0.    1.    0. ]
 [  0.    0.    0.    1. ]
 [-20.    0.5 -10.    1. ]
 [  0.5 -20.    1.  -10. ]]

For samples of the values $d \in [0,100]$ and the fixed value $k=1000$, plot the four eigenvalues on the complex plane.

In [11]:
for d in linspace(0,100,200):
    M = system_matrix(d, 1000)
    eigs = eigvals(M)

Bonus question: there is a bifurcation in the diagram above. Can you find the bifurcation point programmatically?

First, collect the data:

In [12]:
def find_bifurcation(k):
    eigs = []
    ds = linspace(0,100,200)
    for d in ds:
        M = system_matrix(d, k)
    # make an array:
    aeigs = array(eigs)
    # let's plot the imaginary parts:
    # compute the part where the imaginary part is close to zero:
    mask = aeigs.imag < 1e-7
    # we need all eigenvalues to be real:
    mask_ = all(mask, axis=1)
    # at which coefficient to we switch from False to True?
    i = argmax(mask_)
    # which d was that:
    return ds[i]


Frequency Response Plot

In technical applications there occurs often linear systems of the form $$ \dot x(t) = A x(t) + B u(t) $$ where $u$ is an given input signal. $x$ is called the state. From the state some quantities $y(t)$ can be measured, this is decribed by the equation $$ y(t)=C x(t). $$ We assume here that the input signal is an harmonic oscillation $u(t)=\sin(\omega t)$ with a given frequency $\omega$ and amplitude one. Then, $y(t)$ is again a harmonic oscillation with the same frequency, but another amplitude. The amplitude depends on the frequency.

The relationship between the in- and out-amplitude is given by the formula $$ \mathrm{amplitude}(\omega)=\\|(G(i\omega))\\|\quad\text{where} \quad G(i\omega)=C \cdot (i\omega I -A)^{-1} \cdot B $$ and $i$ is the imaginary unit.

In [13]:
In [14]:
In [15]:
def amplitude(A, B, C, omega):
    M = 1j*omega*identity(len(A)) - A
    return norm(dot(C, dot(inv(M), B)))

Plot the amplitude versus omega, for $\omega \in [0, 160]$, with $A$ being the system matrix above with $d=20$ and $k=500$, and $$ C=\begin{bmatrix} 1 & 0 & 0 & 0 \end{bmatrix} \qquad B=\begin{bmatrix}0 \\ 0 \\ 0\\ 1 \end{bmatrix} . $$

In [16]:
C = zeros((1,4))
B = zeros((4,1))
C[0,0] = 1
B[-1,0] = 1
A = system_matrix(20, 500)
def get_amplitudes(omegas):
    amplitudes = []
    for omega in omegas:
        amplitudes.append(amplitude(A, B, C, omega))
    return amplitudes
omegas_ = linspace(0,160,200)
plot(omegas_, get_amplitudes(omegas_), lw=.5)
grid(lw=.5, ls='-', alpha=.2)

Find out the relationship between $A$'s eigenvalues and the peak(s) in the figure.