Fail operational dual electromechanical servo actuator for aircraft with model monitoring

An electromechanical servo actuator system having dual redundant servo channels for positioning a positionable load in response to input command means, comprising

• first and second electromechanical servo loops responsive to said input command means having first and second electric servo motors respectively, differential gear means having two inputs coupled to receive said outputs of said first and second servo motors respectively and having an output for positioning said positionable load, first and second brake means respectively for clamping the respective inputs to said differential means and first and second tachometer generators coupled respectively to said first and second servo motors for providing first and second actuator rate signals respectively as rate feedback around said first and second servo loops respectively,
• electronic model means for electronically simulating the electromechanical characteristics of said servo loops in response to said input command means and for providing a model rate signal representative of simulated actuator rate, and
• monitor means for comparing said actuator rate signals with said model rate signal and for providing a failure signal, indicative of a failed channel, to actuate said brake means of said failed channel when the difference between at least one of said actuator rate signals and said model rate signal exceeds a predetermined threshold,
• thereby clamping said failed channel and permitting the operative servo channel to continue positioning said load through said differential gear means.

In a redundant dual servo actuator system of the type including first and second substantially identical servo actuators responsive respectively to first and second substantially identical control signal channels, each channel comprising a servo amplifier having an input responsive to a command signal and an output for controlling said actuator, a tachometer for providing an actuator rate feedback signal to said amplifier input and brake means for arresting movement of said actuator, differential means having input means coupled respectively to the outputs of said actuators and an output coupled to a positionable load and position feedback means responsive to said load position for supplying a position feedback signal to each of said amplifier inputs, monitoring means for said system comprising,

• electronic model means electronically duplicating the electromechanical chatacteristics of said servo actuator including means for producing a signal corresponding to the rate characteristics thereof,
• comparator means responsive to said actuator rate feedback signals and said rate characteristic signal of said model means for providing a signal indicative of a failed channel, and
• means for supplying said last mentioned signal to the brake means of the failed channel for arresting its movement and providing a load at its corresponding differential input whereby the operative servo actuator channel may continue to position said positionable load.