[SciPy-user] Impulse/Step response troubles

[Ryan Krauss]

I have seen the problem with repeated roots.  I thought I reported it.

Your second case with a ramp response could be better handled with
signal.lsim using your original transfer function ([K],[1.0,8.0,K])
and u = t.

On Mon, Mar 31, 2008 at 11:50 AM, Kumar Appaiah <akumar at iitm.ac.in> wrote:
> Reply below quote.
>
>
>  On Mon, Mar 31, 2008 at 08:52:57AM +0530, Kumar Appaiah wrote:
>  > The question is to determine (plot) the step and ramp responses of
>  > K / (s^2 + 8s + K) for K = 7, 16 and 80
>  >
>  > So, the following code is used, and I'll keep changing a line:
>  > <code>
>  > from scipy import *
>  > from pylab import *
>  >
>  > K = 16.0 # should be redone with 7.0, 16.0 and 80.0
>  >
>  > r = signal.impulse(([K],[1.0,8.0,K]), T=r_[0:5:0.001])
>  > plot(r[0], r[1], linewidth=2)
>  > show()
>  > </code>
>  >
>  > The above code plots the impulse response of the given function. Now,
>  > on to the action:
>  >
>  > 1. Running the above code with K = 7.0 and K = 16.0 gives expected
>  > results. However, running the code with K = 16.0 doesn't work; if K =
>  > 16.0, the response should be 16 * t * exp(-4t) * u(t), which it
>  > isn't. Changing K to 16 + or - .00000000001 fixes it. There surely is
>  > some problem when the value of K is such that a double root is hit.
>
>  Well, I've zeroed in on the issue. Running the code of
>  signal.lti.impulse, we get here:
>
>  s,v = linalg.eig(sys.A)
>  vi = linalg.inv(v)
>
>  Now, v is a matrix which has the eigen vectors, which are EQUAL in
>  this case:
>
>  print sys.A
>  [[-0.9701425  -0.9701425 ]
>   [ 0.24253563  0.24253563]]
>
>  which is singular. However, the code goes on to invert this happily,
>  resulting in a bad matrix and horrendous values. I would be surprised
>  if nobody has encountered this yet (didn't find this on the Tickets).
>
>  What would be the best way to handle repeated roots in the transfer
>  function's denominator?
>
>  Thanks.
>
>
>
>  Kumar
>  --
>  Kumar Appaiah,
>  458, Jamuna Hostel,
>  Indian Institute of Technology Madras,
>  Chennai - 600 036
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