basic of control system

[bhuvaneshnick]

i have two basic questions

1) why control systems are low pass filter?

2) input of control system is random in nature .But why we are testing the system mostly with step signals.How does the step signal alone completely tell about the system?

 

[steveB]

i have two basic questions

1) why control systems are low pass filter?

It seems to me that it is an oversimplification to say that control systems are low pass filters, but I think I understand what you are asking and it's not too far off the mark to say that it is a low pass filter.

Typically a control system rolls off at high frequency. There are many advantages to have the high frequency roll off. First, it is easier to stabilize a system by allowing the gain to go below one before the phase gets too close to 180 degrees. Also, high frequency noise is reduces by allowing the bandwidth of the control system to be the minimum required to meet specifications.

2) input of control system is random in nature .But why we are testing the system mostly with step signals.How does the step signal alone completely tell about the system?

It seems to me that it is not accurate to say the input is random in nature, but I think I understand what you are asking and it is not too far off the mark to say that the input is random. First, the input has random noise as one component of the input, and the exact input is not always know, so in some sense something unknown is psuedo-random in a way.

The reason why step signals are used is that it is an easy signal to generate and it has very high frequency content that reveals the character of the circuit over the full bandwidth. Impulses are also a useful signal for similar reasons.