finding a system behaviour

Question

Why is step response so important? for finding properties of a system we use step response of a system why we don't use other basic signals such as impulse or ramp? thanks for your responses.

Answer 1

I think, the answers/comments as contained in the discussion referenced by Roger Rowland in his comment (above) are very helpful (primarily: theoretical aspects).

However, one rather practical aspect was not mentioned in linked thread: The step response of a standard second-order system is a good indication for the systems stability properties. It is a simple and reliable method for measuring the characteristic pole data in the time domain. The step response overshoot is a measure for the pole quality factor Qp and the observed damped oscillation (if any) gives the natural frequency of the system (app. equal to the pole frequency wp).

Moreover, the form of the step response contains infornmation about the systems phase margin.

Answer 2

Here are a number of reasons:

1. Theoretically speaking, it excites the system at all frequencies
2. It allows for simple, visual analysis of the system's behaviour given a change of input (or setpoint, in case of a closed-loop system)
3. It makes overshoot (i.e., maximum gain of Bode plot) clearly visible
4. It makes dominant time constants easy to establish
5. It makes eventual complex poles evident by appearance of oscillatory behaviours, and with them their natural frequency
6. In case of closed-loop systems, it displays steady-state errors (error between final value of output and given setpoint)
7. And obviously, it shows whether the system is stable

A trained person is able to extrapolate this information (and more) by simply looking at a transfer function, or by analyzing a Bode plot. But step response gives a clearer feeling because it's expressed in the time domain.