CONTROL USING MAXIMUM AVAILABLE FEEDBACK

‘Maximum Available Feedback’ is the term coined by Bode for the highest possible loop gain over a given bandwidth, with specified stability margins, in a single loop feedback system. He derived a method to achieve this using asymptotic approximations. Bode, however, designed amplifiers for telephone systems, with different requirements from those for control systems, yet it is appropriate for a control system to have high loop gain, and the method is a good example of a design of a non-trivial controller. This paper shows how, using some novel analysis of the asymptotes, the method can be approached from a different perspective so as to make it suitable for designing control systems. To generate a stable single loop feedback system with maximum low frequency gain, three commodities are specified: gain margin and phase margin, to set the relative stability of the system, and the bandwidth over which the loop gain is maximum. Then, the maximum loop gain, expressed in decibels, is given by: