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Novel schemes for highly reliable aerospace electromechanical primary actuation systems


OBJECTIVE: Define and demonstrate a novel design scheme for high-reliability, fault tolerant electromechanical actuation for critical aerospace applications. DESCRIPTION: Many emerging and future USAF and USN aircraft programs, including efforts related to Next Generation Air Dominance, drive to demanding actuator packaging requirements that today"s electrohydrostatic or hydraulic actuators cannot easily meet. Lighter, smaller, more reliable actuation technology can help enable game-changing new aircraft capabilities. Electromechanical actuation offers great potential advantages in improved maintainability, ease of distribution, actuator packaging and installation, system-level power-to-weight ratios, stiffness, installation flexibility, and application-customized designs [Ref 1]. These theoretical advantages over hydraulic and pneumatic systems have been recognized for at least forty years [Ref 2]. At the same time, hydraulic actuation dominates critical applications in aerospace today, particularly for the movement of aircraft primary flight control surfaces. New aircraft, including the Lockheed Martin F-35, Boeing 787, and Airbus A380, still use hydraulics in primary flight control applications. Airworthiness standards for large aircraft expect system design to drive to a very low probability of catastrophic failure. As an example, the Federal Aviation Administration"s advisory circular AC25-1309 mandates system design with a probability of catastrophic failure of less than one in ten to the ninth operating hours [Ref 3]. Conventionally, hydraulic actuation achieves the requisite reliability at a system level by use of multiple independent hydraulic circuits driving a single control surface with very low probability of failure of the final hydraulic actuator output link or links. By contrast, existing electromechanical actuators typically employ a rotary-to-rotary or rotary-to-linear output elements that depend on mechanical rolling element bearings, which, in serial arrangement, do not achieve the requisite reliability ratios. Single-point failures that can lead to a mechanical jam are generally not certifiable in a primary control application. While parallel devices or complex arrangements may mitigate these failures, system complexity, cost, and weight are increased to the point that hydraulics are generally favored [Ref 1]. Alternate electromechanical actuator designs and arrangements are needed that can achieve failure rates less than one incidence per ten to the ninth hours, to enable their use in flight critical applications as well as in critical space applications. Novel approaches to electromechanical actuator design are sought, which robustly and comprehensively consider component reliability levels, and apply creative architectural solutions to achieve fault tolerance and high reliability. Fault tolerance is judged at a system level by a rigorous fault hazards analysis (FHA) and fault-tree buildup. An exemplary buildup for a state-of-the-art fault-tolerant mechanical actuator design is provided in Ref 4. Note that this dual-lane fault-tolerant electric drive architecture is still only capable of achieving a probability of system failure of 8.68 10^-6 failures per hour, far short of the 1 10^-9 failures per hour goal for unaided primary flight control. The limiting factors are identified to be the mechanical elements gearbox and actuator mechanism. From a system perspective, simply creating more reliable mechanical components will not achieve the requisite failure rate goals; a novel electromechanical system architecture is required. Electromechanical design approaches should be widely scalable but readily demonstrable at a small scale in a laboratory bench setting. If component performances are more than an order of magnitude greater of those found in standard catalogues of electronic or non-electronic component reliability, a full justification must be provided. Rigorous consideration should be given to the complexity and reliability of any required sensing, fault-detection, and toggling equipment incorporated in a design. Preferred design approaches are those readily scaled across different actuator sizes and across different aircraft applications. Approaches are preferred that could be scaled to encompass a family of actuators that could eliminate the hydraulic flight control actuation systems of a large, man-rated aircraft. Design concepts should also identify electrical bus requirements, including preferred numbers of electrical circuits, efficiency, and thermal considerations, whether the actuator will put electric power back on the bus, and if it will need to be conditioned. Designs are desired that will accommodate power-reconditioning and regeneration from actuator back-driving, to lower the overall heat rejection requirements. Additionally, thorough consideration of integration into an aircraft electrical bus is encouraged. In particular, minimizing peak power usage and power regenerated by the actuation system is considered important in order to reduce the load on the aircraft's electrical system. Preferred actuator designs should be operable on 270 VDC or +/- 270 VDC buses, although lower DC bus voltages are considered acceptable for bench tests. Actuators are desired that provide at least 3hp of peak power, although lower powers are considered acceptable for intermediate testing activity. Larger designs are preferred, especially actuator architectures that are modular or can be easily scaled across different applications requiring higher stroke, higher frequency response, higher output force, or frequent control reversal. High stiffness designs are preferred. The scaling limitations of any architectural approach must be clearly identified. PHASE I: Design a concept for a fault-tolerant, ultra-high reliability electromechanical actuator. Develop an analysis of predicted performance, and define key component technological milestones. Establish performance goals in terms of power-to-weight ratio, and contrast with existing systems. Perform a fault- tree analysis demonstrating a design approach capable of achieving the requisite reliability level. Perform initial hardware risk reduction or mockup of actuator output portion mechanical arrangement, possibly using 3D printed parts. Phase I deliverables will include a description of the conceptual actuator design, performance assessment against existing approaches, a thorough reliability analysis, and a risk reduction and demonstration plan. PHASE II: Develop, demonstrate, and validate the architectural approach to high actuator reliability. Construct and demonstrate the operation of a laboratory prototype actuator that has all of the requisite architectural features needed to achieve high reliability actuation. Exercise the relevant fault modes, and show robust operation. Perform additional analyses to project eventual performance capabilities of the architectural approach. PHASE III: High reliability, high power-to-weight ratio fault tolerance electromechanical actuators have applicability in many future USAF and USN aircraft programs, including especially Next Generation Air Dominance, which have demanding packaging requirements that current electrohydrostatic actuators cannot easily meet. Unmanned aircraft programs, including demonstration programs executed by DARPA, may have particular benefit from such actuators, as the inclusion of hydraulic systems adds great expense, complexity, and weight to an aircraft. The military transition path would be inclusion of an actuator into a future aircraft program of a record. Aerospace-grade electromechanical actuators can also find application in the commercial aerospace industry. It is an oft-stated goal to move towards more-electric aircraft, however electromechanical actuators have largely been relegated to non- flight-critical applications. A commercial transition path would be development of a flight-grade actuator and inclusion into a future aircraft program of record. As an example, Airbus explicitly states that a move towards electric actuation is in their long-term goals, as they desire to reduce conversion losses and increase overall systems efficiency [Ref 5]. REFERENCES: 1. Stephen L. Botten, Chris R. Whitley, and Andrew D. King,"Flight Control Actuation Technology for Next-Generation All-Electric Aircraft", 2000 2. Roskam, J., Rice, M., Eysink, H. A comparison of hydraulic, pneumatic, and electromechanical actuators for electromechanical general aviation flight control, SAE PAPER 790623, 1979 3. Federal Aviation Administration Advisory Circular AC25-1309 4. J.W. Bennett, B.C. Mecrow, D.J. Atkinson, and G.J. Atkinson,"Safety-critical design of electromechanical actuation systems in commercial aircraft", IET Electric Power Applications, 2010. 5. All-Electric Aircraft, Clean Sky Initiative FAA Advisory Circular, AC 25.1309-1A - System Design and Analysis
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