Fail-safe Electromechanical Actuator Redundant Load Paths
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Solution Overview
Problem
Existing electromechanical actuators for aircraft control surfaces lack redundant load paths, making them vulnerable to motor failures and mechanical jams, and require complex brake mechanisms for holding positions, which adds to system complexity.
Innovation Solution
The design incorporates two redundant load paths, each with a motor and a no-back device, allowing the actuator rod to be passively locked at a commanded stroke position without the need for a powered brake, ensuring continued operation even if one path is compromised.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a brake mechanism is used to hold the EMA at a commanded stroke position, then the EMA can maintain position, but the control system complexity increases due to additional control circuitry and active power requirements
Solution Approach 1:
The patent removes the brake mechanism entirely from the system. Instead of using a brake to hold position, the design relies on the inherent friction and mechanical interference between the ballscrew shaft and ballnut, combined with the motor's electromagnetic holding capability, to maintain the commanded stroke position without requiring separate braking components or control circuitry.
Solution Approach 2:
The system uses its own motor and mechanical components (ballscrew shaft, ballnut, and inherent friction) to achieve position holding without requiring external brake mechanisms. The motor can be commanded to maintain position through software control, and the mechanical friction between moving parts provides passive holding capability.
2Device complexity
If only one load path is provided through the ballscrew shaft, then the structure is simpler, but the system lacks redundancy and is vulnerable to motor failure or mechanical jams
Solution Approach 1:
The patent divides the single load path into two separate, redundant load paths. The first load path uses the traditional ballscrew shaft and ballnut mechanism. The second load path uses a separate drive shaft with gear teeth that engages with a gear on the actuator rod. This segmentation provides independent redundancy while maintaining relatively simple individual path structures.
Solution Approach 2:
The patent changes the mechanical configuration parameter from a single drive mechanism to dual drive mechanisms with different operational characteristics. The first path uses direct ballscrew conversion, while the second path uses gear engagement, providing different mechanical advantages and failure modes that enhance overall system reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances safety by providing a fail-safe mechanism that maintains the actuator position without active power, reducing system complexity and ensuring continuous operation in case of mechanical failures or jams.
Implementation Method 1
a screw rotatable about a screw axis and a nut mounted on the screw in mating fashion... A first load path is defined by a first motor operable to rotate the screw about the screw axis relative to the nut to cause the nut to travel linearly along the screw
Implementation Method 2
a spline member rotatable about a spline member axis, wherein the spline member includes a splined surface engaging the external toothed surface of the nut... A second load path is defined by a second motor operable to rotate the spline member about the spline member axis to cause the nut to rotate about the screw axis relative to the screw such that the nut travels linearly along the screw
Implementation Method 3
The first load path includes a no-back device between the first motor and the screw for holding the actuator rod at a commanded stroke position without the need for a powered brake mechanism. Likewise, the second load path may include a no-back device between the second motor and the spline member for the same purpose
Data Source
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AI summary
An electromechanical actuator ("EMA") is provided with redundant load paths for driving the actuator stroke. The EMA includes a rotatable screw, a nut mated with the screw and having an external toothed surface, and a rotatable spline member having a splined surface engaging the external toothed surface of the nut. An actuator rod is coupled to the nut for linear movement with the nut. A first motor is operable to rotate the screw relative to the nut to cause the nut to travel linearly along the screw, and a second motor is operable to rotate the spline member and nut such that the nut travels linearly along the screw. Consequently, the actuator rod moves linearly by operation of the first motor alone, by operation of the second motor alone, and by simultaneous operation of the first and second motors. A no-back device may be incorporated into each load path.