Is this graph indicating stable system or unstable system?

Discussion in 'General Electronics Chat' started by iVenky, Apr 24, 2012.

  1. iVenky

    Thread Starter New Member

    Feb 15, 2012
    2
    1
    Actually I have plotted this for the function

    e(½*t) * sin(2*pi*t)

    This should mean that the system is unstable. But from the nyquist plot I am not getting it as stable. What I have read is this- If the amplitude( which is the radial length in the Nyquist plot) is greater than 1 then the system is unstable but what I get is less than 1.

    Here's image
    http://images.elektroda.net/76_1335200502.jpg
     
  2. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    Well, I'm no top mathematician, but for a start your graph show an imaginary axis, yet your equation does not contain an imaginary part.

    Please correct your formula and then wait for a "proper" maths expert ;)
     
  3. iVenky

    Thread Starter New Member

    Feb 15, 2012
    2
    1
    It's a nyquist plot. I guess you don't know about Nyquist criterion and Nyquist plot.
     
  4. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    I guess I don't then :)

    So why don't you help the OP?
     
  5. BSomer

    Member

    Dec 28, 2011
    433
    106
    I'd say because iVenky IS the OP. :)
     
  6. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    I'm sorry iVenky, my bad!

    But thanks to you I'm now aware of Nyquist plots and am studying them! :)
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    This appears to be an output response of a 'system' to an input stimulus rather than the system definition itself. Could you clarify how the response is obtained.
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    You probably haven't fully grasped the concept as yet. There are two matters of interest with the Nyquist Criteria.

    1. How many of the open loop function poles fall within the (positive / unstable) right half of the complex plane
    2. How many positive or negative encirclements of the (-1,0) point occur with the full Nyquist plot.
    You may need to do some further reading to be confident of how these matters impact the system closed loop stability.
     
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