Latchable Electrohydraulic Servovalve Fail-Safe Positioning
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Solution Overview
Problem
Two-stage electrohydraulic servovalves do not maintain the spool in a predetermined fail-safe position after power restoration, lacking the ability to latch and hold the spool independently of motor power.
Innovation Solution
The design includes a spool with specific geometrical features and passages that allow it to shift between fail-safe positions based on motor current levels, using a latching mechanism to maintain the spool in a desired position even after power loss, utilizing a nozzle and motor connected to control fluid flows and a spring for biasing, ensuring the spool remains latched until pressure decay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a two-stage EHSV uses a torque motor to control spool position, then the spool can be positioned accurately during normal operation, but the spool cannot be maintained in a predetermined fail-safe position after power loss and restoration
Solution Approach 1:
The spool is segmented into multiple lands (first land, second land, third land) that can independently block different passages (latching passage, fourth passage) at different positions. This segmentation allows the spool to achieve multiple distinct fail-safe positions by blocking different combinations of passages, providing reliable position maintenance without requiring additional complex latching mechanisms.
Solution Approach 2:
The invention uses hydraulic pressure from the fluid supply to drive the spool to predetermined fail-safe positions. The first fail-safe position is achieved by hydraulic pressure blocking the latching passage, while the second fail-safe position is achieved by hydraulic pressure blocking the fourth passage. This hydraulic actuation eliminates the need for mechanical springs or external actuators, maintaining reliability while controlling device complexity.
2Adaptability or versatility
If the spool is designed with multiple lands and passages for latching, then the spool can be latched in multiple fail-safe positions, but the valve structure becomes more complex
Solution Approach 1:
The spool serves multiple functions simultaneously: it acts as a flow control element during normal operation, a latching element for fail-safe positioning, and a switching element for different operational modes. The same spool structure with its multiple lands provides both the flow control function and the latching function, eliminating the need for separate latching mechanisms and reducing overall device complexity while increasing adaptability.
Solution Approach 2:
The invention merges the latching function with the flow control function in a single spool-sleeve assembly. The lands on the spool that control fluid flow also serve as the latching surfaces that block passages to maintain fail-safe positions. This merging of functions allows the valve to achieve multiple fail-safe positions without adding separate latching components, thereby increasing versatility while managing structural complexity.
3Extent of automation
If the motor current is used to control spool position, then the spool responds to electrical commands, but the spool position is lost when power is interrupted
Solution Approach 1:
The invention prepares the system for power failure by providing predetermined fail-safe positions that the spool can automatically assume when power is interrupted. The hydraulic passages and lands are pre-configured so that loss of electrical power automatically results in the spool moving to a known fail-safe position, eliminating the need for active control during power loss and ensuring position stability without continuous electrical power.
Solution Approach 2:
The invention converts the harmful effect of power loss (which causes loss of control) into a beneficial outcome (automatic positioning at fail-safe positions). When power is interrupted, the hydraulic system automatically drives the spool to predetermined positions by blocking appropriate passages, transforming the power failure condition into a controlled state rather than an uncontrolled one, thereby improving position stability despite electrical control limitations.
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
Enables the electrohydraulic servovalve to be commanded to a fail-safe position independently of the torque motor null bias, providing a second fail-safe position that is maintained until pressure decay, allowing for reliable operation during power interruptions and restoration.
Implementation Method 1
a motor operably connected to the nozzle for controlling the position of the nozzle based on current provided to the motor
Implementation Method 2
a spool having first and second ends slidably mounted in a sleeve... A first passage extends from near the nozzle to the first end of the sleeve, a second passage extends from near the nozzle to the second end of the sleeve
Implementation Method 3
a latching passage is located near the first end of the sleeve... The first land blocks the latching passage when a current to the motor is below a predetermined level but opens the latching passage to the sleeve first end when the current exceeds the predetermined level
Data Source
AI summary
An electro-hydraulic servovalve (EHSV) (10) that includes a nozzle (16), a motor (12) operably connected to the nozzle (16) for controlling the position of the nozzle (16) based on current provided to the motor (12), a spool (18) having first and second ends (22,26) slidably mounted in a sleeve (34) having first and second ends (31,33) and a first land (21) near the first end (22), a first passage (40) extending from near the nozzle (16) to the first end (31) of the sleeve (34), a second passage (42) extending from near the nozzle (16) to the second end (33) of the sleeve (34), the spool (18) shifting to a first fail-safe position when the current is below a first predetermined level and latching in a second fail-safe position different than the first fail-safe position when the current exceeds a second predetermined level greater than the first predetermined level.