Hydraulic Actuator Sleeve Mechanism for Fail-Fixed Position Holding
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Solution Overview
Problem
Conventional hydraulic actuators in helicopters tend to revert to a fail-safe position (fully extended or retracted) during electrical failures, which is not desirable as it disrupts flight control parameters, making it challenging to maintain the last commanded position.
Innovation Solution
A fail-fixed hydraulic actuator system with a motor-driven sleeve and position sensors that maintains alignment between the piston head and sleeve aperture, utilizing a low pressure cavity to create a pressure differential and ensure the piston remains in the last commanded position by exposing it to either the retract or extend cavity upon failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hydraulic actuators are designed to automatically position in a fail-safe position during electrical failure, then reliability is improved, but flight control parameters are disrupted and the actuator cannot maintain the last commanded position
Solution Approach 1:
A sleeve with a sleeve aperture is introduced as an intermediary component between the piston head and the hydraulic system. The sleeve aperture selectively communicates with either the retract cavity or extend cavity based on piston position, enabling the system to maintain the last commanded position during failure without requiring active electrical control.
Solution Approach 2:
The invention uses hydraulic pressure differentials created through the sleeve aperture communication to passively maintain piston position. When the piston moves away from the centered position, the sleeve aperture communicates with one cavity, creating a pressure differential that generates a restoring force to return the piston to its last commanded position without electrical power.
2Ease of operation
If the actuator uses a motor-driven sleeve to maintain alignment between piston head and sleeve aperture, then the actuator can maintain the last commanded position, but device complexity increases
Solution Approach 1:
The sleeve mechanism is designed to automatically respond to piston position changes without requiring external control signals. The sleeve aperture's communication with the hydraulic cavities is passively determined by the piston's position relative to the sleeve, creating a self-regulating system that maintains position without continuous motor actuation or electrical control.
Solution Approach 2:
The sleeve is designed with dynamic characteristics that allow it to respond to pressure differentials and piston movement. The sleeve can move or rotate to align the aperture with the appropriate cavity based on the piston position, creating a dynamic, adaptive system that automatically maintains the last commanded position during failure conditions.
3Reliability
If the low pressure cavity is hydraulically connected to create pressure differential, then the piston head is forced to align with sleeve aperture, but force balance is disrupted during normal operation
Solution Approach 1:
The low pressure cavity is designed to only partially affect the hydraulic system during normal operation. The sleeve aperture selectively connects the low pressure cavity to either the retract or extend cavity only when the piston moves beyond the centered position, allowing the system to maintain force balance during normal operation while providing position-maintaining pressure differentials during failure conditions.
Solution Approach 2:
The system is designed to preemptively prevent position drift during failure by creating pressure differentials that counteract any movement away from the last commanded position. The sleeve aperture configuration ensures that if the piston moves, the low pressure cavity will communicate with the appropriate high pressure cavity to generate a restoring force that opposes the displacement and maintains position.
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
The system effectively keeps the actuator in the last commanded position during power failures, maintaining force balance and resisting external loads without the need for continuous power, ensuring stable flight control.
Implementation Method 1
the low pressure cavity is hydraulically connected to one of the retract cavity or the extend cavity to cause a pressure differential with the other of the extend cavity and the retract cavity and cause movement of the piston head to align with the sleeve aperture
Data Source
AI summary
Fail-fixed hydraulic actuator systems for aircraft include a hydraulic actuator having a piston in a housing. The piston separates the housing into a retract cavity and an extend cavity. A sleeve is moveably arranged within the housing and includes a sleeve aperture that is aligned with a piston head during normal operation. A driving mechanism is configured to drive movement of the sleeve to maintain alignment between the sleeve aperture and the piston head. A low pressure cavity is defined between an interior surface of the housing and the sleeve and, when the piston head is offset from the sleeve aperture, the low pressure cavity is hydraulically connected to one of the retract cavity or the extend cavity to cause a pressure differential with the other of the extend cavity and the retract cavity and cause movement of the piston head to align with the sleeve aperture.