EHSV Fail-Fixed Valve Position Maintenance
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Solution Overview
Problem
Electro-hydraulic servo valves (EHSVs) often drive controlled devices to a shut-off position upon loss of position command, which is undesirable in applications requiring maintenance of the last commanded position.
Innovation Solution
A fail-fixed valve system that utilizes pressure differentials between control pressure lines to maintain the device's position within a tolerance band by moving from its spring-biased default position, ensuring the device remains at its last commanded position by equalizing pressure and applying a braking force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the EHSV uses a spring-biased mechanism to drive the device to a shut-off position upon loss of position command, then the device is reliably driven to a safe default position, but the device position cannot be maintained at the last commanded position
Solution Approach 1:
The patent applies dynamics by making the valve's default position adaptive rather than fixed. The valve body can shift position based on pressure differential conditions, allowing it to maintain the last commanded position under normal operation while automatically moving to equalize pressures and brake the device upon loss of control signal. This dynamic repositioning resolves the contradiction between maintaining position and providing a safe default state.
Solution Approach 2:
The patent changes the parameter of valve body position from a fixed spring-biased position to a variable position determined by pressure differential. By monitoring the pressure differential across the control lines and allowing the valve body to shift accordingly, the system can maintain the device at its last commanded position when pressures are balanced while automatically transitioning to a brake position when pressure imbalance indicates loss of control signal.
2Reliability
If the valve equalizes pressure across the device upon loss of command to maintain position, then the device stays at the last commanded position, but pressure imbalances from external loads cannot be compensated
Solution Approach 1:
The patent applies the counterweight principle by using the pressure differential itself as a counterbalancing force. When external loads create pressure imbalances, the valve body shifts position in response to this differential, allowing one side of the valve to experience higher pressure that compensates for the external load. This enables the system to maintain position stability while simultaneously compensating for external forces through the pressure-balanced design.
3Measurement precision
If the valve moves in response to pressure differential to maintain device position, then the device remains within tolerance band, but the valve requires additional movement range beyond spring-biased position
Solution Approach 1:
The patent applies dimensionality change by utilizing the valve body's ability to move in the axial direction in response to radial pressure differentials. Instead of using a complex multi-dimensional actuation system, the invention leverages the natural response of the valve body to pressure differential along one dimension (axial movement) to achieve precise position control within a tolerance band, thereby maintaining measurement precision without significantly increasing device complexity.
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
Effectively keeps the device at its last commanded position with minimal shift, even in the presence of external mechanical loads or pressure imbalances, ensuring stability and accuracy within a defined tolerance window.
Implementation Method 1
the valve moves as a result of the pressure differential between the first and second ends created as a result of the loss of the input position command to the EHSV
Implementation Method 2
The valve is biased towards the second control pressure line by a spring
Implementation Method 3
The pressure seal port is connected to the EHSV's first control pressure line (whose pressure is approaching drain pressure) by initial movement of the valve after loss of the input position command thereby creating a side-load on the valve spool. The side-load brakes any movement of the valve spool
Data Source
AI summary
A valve that keeps a electro-hydraulic servo valve (EHSV) controlled device such as a fuel metering valve or actuator at the last commanded position upon loss of input command in the EHSV is described. The valve fixes the position of the positioning device within a tolerance of the position the positioning device was at prior to the loss of the position command. Upon loss of the input position command, the valve moves from its spring-biased default position to a position that keeps the positioning device at a position within a tolerance band of its last commanded position as a result of the differential pressure created when the EHSV loses its input position command