Two-Stage Servo Valve Fail-Fixed Position Holding on Power Loss
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Two-stage electrohydraulic servo valves (EHSVs) face challenges in maintaining the last commanded position of a device during power interruptions, as existing solutions are costly, complex, and bulky, and do not adequately address the unique technical challenges posed by two-stage EHSVs.
Innovation Solution
A system incorporating a fail-fixed valve with a bleed orifice and dynamic seals, in conjunction with a two-stage EHSV, which transitions to a fail-fixed position upon power loss, using a torque motor to maintain the last commanded position of a hydraulically operated device without adding significant cost, complexity, size, or weight.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fail-fixed valve is added to maintain position during power interruption, then reliability is improved, but device complexity increases
Solution Approach 1:
The fail-fixed valve is integrated with the existing two-stage EHSV by utilizing the second stage spool and its existing fluid passages. The fail-fixed functionality is merged into the existing valve structure, eliminating the need for a completely separate valve system and reducing overall complexity.
Solution Approach 2:
The second stage spool is designed to perform multiple functions: during normal operation, it controls fluid flow to the actuator, and during power interruption, it automatically transitions to a fail-fixed position to maintain the last commanded position. This multi-functionality eliminates the need for dedicated separate components.
2Reliability
If a fail-fixed valve is added to prevent fluid flow during power loss, then reliability is improved, but size increases
Solution Approach 1:
The fail-fixed valve components are integrated within the existing EHSV housing and fluid passage structure. The second stage spool and its associated passages serve dual purposes, eliminating the need for additional external valve assemblies and reducing overall system volume.
3Reliability
If a fail-fixed valve is added to maintain position during power interruption, then reliability is improved, but cost increases
Solution Approach 1:
The fail-fixed functionality is achieved by modifying the existing two-stage EHSV design to utilize the second stage spool for position holding during power interruption. This integration eliminates the need for separate fail-fixed valve components, reducing part count, assembly complexity, and manufacturing cost.
Solution Approach 2:
The second stage spool is designed to perform multiple functions: normal fluid control during operation and automatic position holding during power interruption. This multi-functionality reduces the need for additional components and reduces overall system cost.
4Reliability
If a fail-fixed valve is added to prevent actuator drift, then reliability is improved, but weight increases
Solution Approach 1:
The fail-fixed valve components are integrated within the existing EHSV structure, utilizing the same housing, mounting features, and fluid passages. This integration eliminates the need for additional external components and minimizes the increase in overall system weight.
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 maintains the last commanded position of a device during power interruptions with minimal additional cost, complexity, size, or weight, using a simple and compact fail-fixed valve configuration that limits actuator drift and prevents fluid flow, ensuring the device remains locked within its commanded position.
Implementation Method 1
A bleed orifice limits fluid flow between a first and second region
Implementation Method 2
A first set of dynamic seals prevent fluid communication between the first and second regions when the fail-fixed valve element is in the fail-fixed position
Implementation Method 3
Many EHSVs are implemented as two-stage EHSVs, which use a first stage motor, such as a torque motor, to control the position of a second stage spool
Implementation Method 4
The position of a device, such as an actuator, may be hydraulically controlled by varying the pressures of fluids applied to one or more sections of the actuator
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A system uses a two-stage, four-way electrohydraulic servo valve that includes an additional control port that commands a fail-fixed valve to lock the position of a device in the last commanded position. The additional port is modulated by an existing land on the EHSV valve element, adding little to no complexity. Major technical benefits of the disclosed system are that it adds little to no cost, complexity, size, or weight the device being controlled. The disclosed configuration allows for the use of a relatively small and simple fail-fixed valve, and the control ports on the controlled device keep "drift" to a minimum, when transitioning between normal operating mode and fail-fixed operating mode.