Future “more electric aircraft” (MEA) will require electric actuation systems for control surfaces and engine controls. Electric motors, drive electronics, and mechanisms are essential elements of aircraft actuation in MEAs that incorporate Electro-Magnetic Actuators (EMAs). High-temperature environments experienced in aircraft applications place demands on actuator components, materials, and insulation systems that dictate the use of new technologies and materials.

Figure 1: HTEMA actuator package (left) and interior view.

High Temperature Electromagnetic Actuator (HTEMA) options for high-temperature aircraft environments include appropriate motor types, drive and control electronics, mechanisms, materials, and construction methods. These options are evaluated to identify candidates that meet the challenges of tomorrow’s MEA actuators.

Efforts to design, build, and test a prototype demonstrating a high-temperature, high-reliability class of all-electromagnetic actuator design with minimal/ no heat load on aircraft cooling systems are ongoing. Size and weight goals are consistent with the aircraft engine limits. The key technical aspects being addressed include operation in the high temperature and vibration environment, actuator force, power, weight, size, efficiency, speed, stroke, mechanical robustness and reliability, lifecycle cost, failure modes and effects, and maintenance predictions.

Engine nozzle actuation is presently accomplished using fuel as a hydraulic working fluid (fueldraulic actuation) for the current-generation engine actuators. While the nozzle actuation has a low duty cycle, the heat generated is continuous and presents a significant load on the aircraft/engine thermal management system.

An electrically driven actuator can reduce the heat load of the aircraft. The present engine fueldraulic actuators and aircraft fuel systems are highly interrelated, leading to a challenging thermal management problem that has an impact on aircraft performance. Elimination of this fuel heat load reduces the fuel system’s thermal issues. High-temperature, electromagnetic actuator component solutions that meet the technology challenges and performance requirements for an actuator system operating engine nozzle actuation have been identified.

Convergent Nozzle Actuation

Figure 2: HTEMA representation in Simulink.

The design and development of HTEMA technology for a Convergent Nozzle Actuation System (CNAS) actuator has been completed. The CNAS is the first critical application of the HTEMA technology, providing the power to position the nozzle as required for the pilot-selected engine Power Level Angle (PLA). The CNAS actuators are mounted to the aft end of the engine. Current fueldraulic actuators for the CNAS are limited by the actuator O-ring material. In the CNAS application, fluid temperature limits approach 325 °F and seal temperature limits approach 400 °F. The current engine actuators are able to operate in a temperature environment approaching 325 °F continually and up to 560 °F for transients of 10 seconds by using the hydraulic fluid (fuel) as a means of cooling.

The CNAS actuators have a linear stroke of 4", a combined stall load (for 4 actuators) of 42,000 lbf, and weigh about 52 lbm (actuation system hardware including routing). Envelope goals provided by the AFRL are 11 × 2.5 × 6.5". The power type is 270 VDC.

Key technologies evaluated included motor, insulation, bearings, electronics, gearing, cooling, and signals and sensors for a notional actuator. The best candidates for detailed design and optimization were identified.

The notional CNAS HTEMA employs five actuators, each capable of 10,500 lbs (46.7 kN) of force, producing a total of 42,000 lbs (186.8 kN) with one actuator missing. Given the requirement for an electromagnetic solution, a direct drive actuator was considered first. Aerospace machines typically develop a pressure of 3-5 psi2 in the air gap due to magnetic fields. This dictates a gap area of at least 2.1 × 103 square inches is needed to generate the required force. Based on the 5 psi curve, an actuator with an 18" gap diameter has roughly a 2' outer diameter and 3' length. An approximate actuator weight would be greater than 1,000 pounds, which is clearly unacceptable. Given the envelope requirement of 11 × 2.5 × 6.5", an 11"-long machine would have over a 60" diameter, an unacceptable result.

Therefore, the CNAS application requires mechanical advantage to decrease the required electromagnetic actuator force and size, while increasing speed. The Navy investigated electromagnetic actuator replacements for hydraulic cylinders and found planetary roller screws to be a good candidate for rotor-to-linear conversion and mechanical advantage. Typical COTS roller screw ratings indicate a screw diameter in the 30- to 48-mm (~1.2 to 1.9") range is appropriate.

Inclusion of a single-pass, 5 to 1 planetary gear between the planetary roller screw and motor improves this situation. COTS planetary gears show a significant performance advantage for a small weight (<2.8 lbs). Issues with dry lubrication, life, and seals must be addressed in a custom detailed design for this high-temperature application. However, this information is sufficient for trade studies. Assuming a ~3:1 length-to-diameter ratio, the motor weight is less than 12 pounds.

Harmonic drives were considered for higher gear ratios in smaller packages. However, concerns about the ability to back-drive the unit in some failure modes prevented inclusion in the baseline design. Magnetic gearing is also an option but concerns about permanent magnet (PM) demagnetization during flux reversals at high temperature were considered high risk.

Care must be exercised in the lead screw and gear ratio selection if the actuator must be back-driven with the power off. There is a critical angle in lead screws where the unit will act as a friction lock at and below the critical value. Similarly, the gearing will multiply any drag and cogging torques present in the motor, which can also inhibit the ability to back-drive an actuator. It is important to keep this in mind during detailed trades.

Baseline Actuator Concept

The system design includes the following key components and technologies: roller screw, planetary gearing, high-temperature coatings, high-temperature motor materials and insulation system, high-temperature power electronics, and sensors.

Several motor types have been considered with high efficiency and minimal weight and volume. These include surface mount PM, Halbach Array PM, buried magnet PM, and hybrid stepper. Others that do not require PMs include wound rotor DC (brush and brushless), variable or switched reluctance (VR or SR) machines, and squirrel cage induction motors. Trades have identified brushless DC PM and SR machines are of primary interest. Both motors can use sensorless commutation methods. A VR resolver can be incorporated in the design if required for control feedback.

VR motors are robust with simple windings, facilitating application of high-temperature insulation. Because VR motors have no permanent magnets, they do not generate a back Electro Motive Force (EMF) voltage when unpowered, which may be advantageous in some failure modes.

The PM motor design includes a band to contain the surface-mount PMs. VR machines do not require such containment requirements, but have quite small mechanical gaps that must be maintained over temperature and life. Both machines can be back-driven, with VR machines having an advantage of no back EMF generated when unpowered. This may be an advantage in failure modes and effects analyses (FMEA). Given the motor is used in an aircraft application, high-saturation flux density lamination material is highly desirable. Brushless DC PM motors provide excellent efficiency with high bandwidth for servo applications. Brushless commutation is appropriate for life, reliability, and maintenance issues. High motor pole count is a big factor in minimizing weight. Maximum slew rate, speed, switching speed, and/or geometry will be the limiting factor, but the higher the number of poles, the better.

Mechanical gears are a mature technology with a long and rich history. When properly designed, applied, and maintained, they can provide long, failure-free performance. In addition, cost and manpower limitations are pushing hardware toward more robust, no-maintenance technologies.

Planetary gears were selected in the baseline design for their power-dense, high-torque transmission capacity and form factor. A COTS gearhead was identified with 5:1 gear ratio in a single pass. Modifications to the COTS gearhead with a 98- 95% efficiency for a single pass and 150,000 to 200,000 hours of life at room temperature to meet the high-temperature environment are anticipated. Flex spline or harmonic drives can also be used if failure modes and effects analyses show their inability to be backdriven is not an issue.

A simulation study of HTEMA was performed based on flow-down requirements. The simulation tool used was Simulink together with its SimPowerSystems (SPS) toolbox from MathWorks. Instead of using SPS library models, custom models were created for the permanent magnet synchronous motor, the motor drive, and some of the mechanical elements. The main reason for creating custom models is that the existing SPS models do not provide the flexibility needed for detailed study of the HTEMA.

As shown in Figure 2, a nested controller structure is used to provide the servo performance. The outermost control loop is the position controller, which generates the motor speed reference used by the speed controller. The speed controller calculates the motor current reference, and the innermost current controller regulates the motor current to follow this reference.

Key elements of the HTEMA have been investigated, and the baseline design meets the space, weight, and performance requirements. Development continues to mature the HTEMA system for the CNAS application, perform detailed design, build and test a prototype unit, and validate analytical models. This work will lead to efforts that transition the HTEMA to military flight-certified hardware supporting future engine upgrades and commercial aviation applications.

This article was written by Gerald Foshage, Richard Young, Yuntao Xu, Edward Wagner, and Dennis Mahoney of RCT Systems (Linthicum Heights, MD); and Alireza R. Behbahani of the Air Force Research Laboratory. For more information, Click Here .

Motion Control & Automation Technology Magazine

This article first appeared in the June, 2014 issue of Motion Control & Automation Technology Magazine.

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