Actuating drive for positioning a rotary element

An actuating drive for positioning a rotary element to a position determined by a desired value signal, comprising the combination of

• (a) an angle pick-off, which provides an actual value signal indicative of the position of the rotary element,
• (b) a first controller, to which a difference of said actual value signal and said desired value signal is applied as error signal and which provides a first controller output signal, wherein said first controller comprises an adaptive transfer element with limitation controlled by said error signal of said first controller, said transfer element limiting said first controller output signal to a maximum value, said maximum value being reduced with increasing error signal down to a lowermost value,
• (c) a first torque motor,
• a rotor of which is directly connected with said rotary element, and
• to which said first controller output signal from said first controller is applied,
• (d) a second controller, to which the difference of said actual value signal and said desired value signal is applied as error signal and which provides a second controller output signal,
• (e) a second torque motor, to which the second controller output signal from said second controller is applied,
• (f) a step down system, an input member of which is connected to said second torque motor and an output member of which is in driving connection with said rotary element, and
• (g) means for linking control loops of said first and second controllers such that said second torque motor through said step down system provides for coarse positioning of said rotary element to be positioned, and said first torque motor provides for fine positioning after such coarse positioning.