Why open loop systems are stable ?

[karthik_reddy00]

answers req :

1. why open loop systems are stable ?

2. what is the application of modulation in instrumentation/control engineering

 

[Miguel Gaspar]

answers req :

1. Not always

2. to transmit an analog signal for example

 

[mago]

answers req :

In a brainstorming session, i could tell you the following:
For the first question, open loop systems could be instable too, for instance, suppose that you have a transfer function in your system that has poles on the right side of the complex plane. It will be instable, unless a proper feedback is used.
For the second question, in instrumentantio, for instance, you have to deal with noise, and if your system works on low frequency, there is a kind of nose called 1/f noise that becomes insignificantly at higher frequencies, so it is better to modulate your signal, to escape from this zone of noise. Besides that if you study an important equipament, known as lock-in, you will see that is syncronous detection is something very important.
I hope i could help you a little
Regards

 

[demodb]

Re: answers req :

Hi Mago, you're talking about poles in the right complex plane and of making this stable when the right feedback is applied. I would like to know how you would shift the right half pole to the left half plane.

 

[raybo]

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OPEN LOOP are allways unstable UNSTABLE that is why we need some feedback to control this open loop instability.

 

[neils_arm_strong]

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open loop systems are not stable always.Feed back improves stability.

 

[karthik_reddy00]

Re: answers req :

feedback leads to instability


feedback can only detect & correct errors

 

[mago]

answers req :

Hi demodb,

You can shift the poles to the left, by applying a feedback circuit, for instance, apply a PI, PD, PID, or simiply adjusting the gain (in some cases). You might want to analyze your transfer function by appliying a tool like root-locus plot, or nyquist plot, where you can see where are your poles, and what you have to do to estabilize your system. There are many techniques of control that you can do. I recommend studying a good control book, like Ogata Control Engineering, Kuo Automatic control systems (this last gives you a general view and it´s a good reference, but has few exercises) there are many others, just take a look at this forum in other sections.

 

[vicky]

Re: answers req :

HI DEAR,

FEED BACK IMPROVES THE STABILITY CRITERIA


USE MODULATION IN COMMUNICATION FOR GETTING THE BEST RESULTS OF OUR TRANSMITTED SIGNAL

 

[safwatonline]

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sorry , but i got confused here , in open loop system , what do u mean by unstable, as what understand by unstable "oscillation" is due to +ve feedback which is obviously must be closed loop.
regards,
a.safwat

 

[easternbean]

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feedback itself dont fully decide the stability, and often negative feedback can stable the output but maybe not the system,positive feedback often introcuces
oscillation but at some applicatin it is used to eliminate the input error!
if you want to investigate the stability ,first derive the transfer functions ,and then use the routh cirteria or lyyapov stability criteria!

 

[safwatonline]

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again i dont get why the system will be unstable in open loop, i know the adv. of feedback but i think stability is not one of them

 

[engrvip]

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can someone tell how does time response of right half pole looks like???and how it comes into system.

 

[noghabi]

answers req :

often negetive feedback leads to stability and vice versa.

 

[kavita_mishra]

Re: time response of the system

can someone tell how does time response of right half pole looks like???and how it comes into system.

The time response of the system can be determined by applying inverse laplace transform to the transfer function of the control system. It can look like an exponential curve..the time response of system having transfer function [1/(s-1)] is exponential in nature and it is unstable..for a stable system, time response can look like the transient response image on this page
https://ecestudyaid.blogspot.in/2012/05/some-important-topics-in-control-system.html
how the time response will look depends entirely on the sytem..For more system responses you can refer Ch-5 of Control engg by IJ nagrath