In general, there are two ways of improving the performance characteristics of automatic control system, i. e. inserting series compensating networks and adding minor loops.
In the first case of using series compensating networks, the resultant frequency responses of systems can easily be obtained by usual method.
But in the second case of using minor loops, the determination of the frequency response is not so easy as the first case.
Therefore, we present two new methods of determining frequency responses as to the second case. One is a very simple approximate method and the other is more accurate but less simple method.
Both of them mainly consist of the following:
(1) Inverse transfer function of the system with minor loops is written as an algebraic sum of terms, each of which is constructed from the transfer functions of system elements in a quotient form.
(2) The gain of this inverse transfer function at some frequency interval can be approximately considered as the gain of the most predominating term at that interval.
(3) The sum of two transfer functions can easily be obtained using “Nichols” charts, on Bode Diagrams.
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