Parallel redundant actuator systems

A parallel redundant actuator system comprising:

• an output member;
• a plurality of signal lanes each having an input, an output and a position feedback loop;
• means for applying to the input of each said lane a respective demand signal representing a desired position of said output member and a feedback signal produced by said position feedback loop representing the actual position of said output member, to produce at the output of that lane a signal representative of the difference between said demand and feedback signals;
• a summing arrangement having a plurality of inputs which are respectively connected to the outputs of said signal lanes to produce at an output of said summing arrangement an output signal representative of the sum of said output signals of said lanes;
• and an actuator connected between said summing arrangement and said output member and responsive to said output signal of said summing arrangement to control the position of said output member;
• said actuator system exhibiting a non-linear functional relationship between the input signal of each lane and the output signal of said summing arrangement over a range of values of input signal for which the lane is capable of operation whose slope over a range of values of input signals for which changes occur due to failure of another lane is greater than if said relationship were linear over said range of values of input signal for which the lane is capable of operation.