# Transient response of a control system.

1. May 21, 2010

### koolraj09

Hi guys.
I am a Mechanical Engineering Student but we've a course called Control systems. My question is why does any system(physical like fan,motor or electronic like amplifier) have a TRANSIENT RESPONSE and specially why do electronic components like amplifiers,diode or for that matter any electronic circuit have a transient response. A physical interpretation(in depth)would really help.

2. May 21, 2010

### ask_LXXXVI

Any physical system has an inherent inertia , so it takes a finite amount of time to reach the full response.

Being an electrical engineer I will illustrate using an example from electrical engineering.
for example , consider an series RC circuit of first order , the time constant is RC

Let the input voltage be a step voltage with magnitude V.
Let the response to the input voltage be Vc(t). i.e we take output as voltage across the capacitor.
Then , Vc(t) will be governed by the equation :-

As you see in above response the transient response is basically the exponential term . if you want it to be zero RC must equal zero. when RC = 0 the response would be simply Vc(t) = V for t > 0 . So the transient response is completely absent.

So you want a circuit with either R=0 or C=0 or both . This does not happen in practical circuits . Any capacitance has some leakage resistance .Any resistance has some capacitance.

Such 'deviations' from the ideal conditions we desire happen in other systems also be it mechanical system or electrical system or any other control system. This leads to the inertia

Last edited: May 21, 2010
3. May 21, 2010

### Okefenokee

As an ME you must know your quantities like momentum, force, and mass pretty well right? Have a look at electromechanical analogies. It's a nice way to bridge the two fields.

It's more than that though. You could use the analogies to build a circuit that acts as an analog computer that simulates some mechanical system.

4. May 22, 2010

### n.karthick

The reason is due to presence of energy storage elements. For example a voltage source connected to a resistor (assuming everything in and out to be ideal) will not have a transient response. Once capacitance and/or inductance come in the way, due to finite time needed for these energy storage elements to respond, we have a transient response.

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