A feedback link which transmits a movement of a servo piston to a control sleeve of a regulator is constituted by a link lever formed of a rigid material and an expansion spring formed of a spring material. The method operates in a sequence of detection frames. The common bus carries the current signals from the voltage-to-current transformers to an output bus. At least part of the adjustment portions is located in a margin area produced in the external terminal area by a difference between the first size and the second size.