Automatic control-system calibration

First of all, we should distinguish the difference between open-loop and closed-loop transfer functions: the transfer function represents output/input, so we should aim at what the output is and what the input is at any time, so as not to be messy. Therefore, the open-loop transfer function is the output of the feedback line/the input (deviation) of the first link, that is, B/E. The closed-loop transfer function is the output/input of the whole system. Therefore, the cutoff frequency of the two is also very different: the open-loop cutoff frequency is the frequency at which the amplitude-frequency characteristic curve crosses the 0dB line in the open-loop amplitude-frequency characteristic. Closed-loop cut-off frequency, also known as bandwidth frequency, refers to the corresponding frequency when the closed-loop amplitude-frequency characteristic drops to 3dB below the decibel value when the frequency is zero (0.707 times the zero-frequency amplitude), which is recorded as ω B.. So the bandwidth of the system refers to the closed-loop cutoff frequency. Generally speaking, it is the maximum frequency that can be tracked.

When that phase angle margin is not enough, the margin can be increase, and at the same time, because the link is rising, the slope of the middle frequency band will be raise, and the cut-off frequency will also be increased, so that not only the stability margin is improved, but also the rapidity is improved, but the high frequency band is also raised, which leads to the slow attenuation of the high frequency and weakens the anti-high frequency disturbance ability.