|
High Reliability Electro-Hydraulic Servo Valve (EHSV)
Navy SBIR 2012.1 - Topic N121-029 NAVAIR - Ms. Donna Moore - [email protected] Opens: December 12, 2011 - Closes: January 11, 2012 N121-029 TITLE: High Reliability Electro-Hydraulic Servo Valve (EHSV) TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Electronics, Space Platforms ACQUISITION PROGRAM: JSF-Prop OBJECTIVE: Develop an innovative electro-hydraulic servo valve (EHSV) to increase the reliability of fueldraulic systems on aircraft gas turbine engines. DESCRIPTION: Aircraft engine hydraulic systems which utilize fuel as the working fluid are called fueldraulics. Fueldraulics provide some clear benefits for aircraft engines over traditional hydraulics and over many other actuation technologies. A fueldraulic system can be lighter than a closed loop hydraulic system because the fueldraulic system does not require a dedicated pump and fluid reservoir. Instead, it utilizes the engine's fuel pump to provide motive power. A dedicated reservoir is eliminated because the working fluid is necessarily available whenever the engine is operating. EHSVs are electromechanical devices which are frequently used in fueldraulic systems to control actuator positioning. They are often utilized in gas turbine engine control systems for various control functions. For example, an EHSV may be utilized to precisely position a fuel metering valve to control engine power output. An EHSV may also be utilized to operate a piston actuator to position the variable guide vanes in an axial compressor or to set the nozzle area of an afterburning engine. EHSVs are a leading cause of fueldraulic system failures on aircraft gas turbine engines. A drawback of fueldraulics is that contaminants are continuously introduced into the system as thousands of gallons of fuel may be processed through the system during each flight. Though all fuel is filtered before it passes through a fueldraulic system, small contaminants pass through the filter yet may be large enough to affect the operation of fueldraulics, and of EHSVs in particular. The secondary stage of an EHSV includes a valve spool which moves within a sleeve. The clearance between these parts is on the order of the particle size which may pass through the upstream fuel filter, so the EHSV secondary stage is prone to stiction and seizure by these contaminants. EHSV stiction leads to increased positioning errors for the engine actuators and to erratic fuel flow control. These in turn can reduce aircraft controllability and may place the gas turbine at risk of compression system stalls and surges. Reducing the filter mesh size significantly to avoid this problem is not a viable solution because it would lead to unacceptably rapid filter clogging. Another common EHSV failure symptom is null bias shift. For failsafe purposes, the EHSV design incorporates a spring feature to drive the EHSV output flow in a safe direction if energizing current is lost. During normal operation, a specific level of electrical current is required to maintain the valve spool in the centered position. Null bias shift is a common EHSV failure symptom in which the actual null bias current requirement shifts away from the specified value. This failure mode can manifest itself as an actuator position error. Several features of popular EHSV designs contain failure modes which can cause null bias shift. Prevalent EHSV designs also contain the following failure modes within the torque motor section: breakdown or short circuit in the coil, faulty wires to the driver current source, imbalance in the air gaps. In the flapper nozzle section, failure modes include: nozzle hole blockage, flapper erosion, broken flapper. An innovative EHSV design solution is sought to reduce or eliminate these failure modes in fueldraulic systems. A successful solution will also have key characteristics comparable to typical fueldraulic EHSV designs on metrics of accuracy, response, hysteresis, package size, and electrical interface. Key environmental concerns include temperature and vibration extremes. Collaboration with an aircraft gas turbine manufacturer is strongly encouraged. PHASE I: Prove feasibility of an EHSV design concept which will be insensitive to contaminant particles at least 64 microns in size and reduce or eliminate the failure modes which may lead to null bias shifts. Design shall reduce the number of other EHSV failure modes within the design concept. PHASE II: Develop prototype EHSV hardware and characterize the transient response behavior, accuracy, and hysteresis through testing. Demonstrate performance relative to the Phase 1 objectives utilizing laboratory test equipment. PHASE III: Mature EHSV design to production-grade compatible with a current fueldraulic control system and test using procedures defined by MIL-STD-810C. Transition to appropriate platforms. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: EHSVs are frequently utilized in hydraulics applications whenever electronic control systems interface with hydraulics to provide position and flow control. Therefore, improved EHSV technology can benefit multiple industries including industrial motion control, flow control, and ground vehicle applications including steering control and active suspension control. REFERENCES: 2. SAE Aerospace Recommended Practices. (2008, February). Electrohydraulic Servovalves, SAE Standard ARP490F. http://standards.sae.org/arp490f/ 3. MIL-STD-810C. http://www.everyspec.com/MIL-STD/MIL-STD+(0800+-+0899)/MIL-STD-810C_13770/ KEYWORDS: EHSV; Reliability; actuator; fueldraulics; hydraulics; mechatronics
|