# 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

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
783
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
783
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.