Application of Smart Material- Hydraulic Actuators

material, specifically a piezoelectric material, with internal servohydraulic components. Large displacement outputs are produced, while the high force capacity of the stiff smart material is retained, for a net high- energy output.<br><br> The actuator is considered cpower-by-wire d because only electrical power is provided from the vehicle or system controller. A primary motivating application is in unmanned combat air vehicles (UCAVs). The particular actuation needs of these vehicles, in flight control and other utility functions, are described and distilled to a set of relevant device requirements.<br><br> Other potential applications, such as flight motion simulation, are also highlighted. The new actuation architecture offers specific advantages over centralized hydraulic systems and has capabilities not present in electromechanical actuators (EMAs). The main advantage over centralized hydraulic systems is the elimination of the need for hydraulic lines.<br><br> Compared to motor-driven ball screw type EMAs, the new actuators offer higher frequency response, and a larger peak-to-average output. A laboratory test facility designed to represent the loading experienced by a UCAV control surface is described. Key steps necessary to flight qualify the actuator are introduced.<br><br> Keywords: Piezoelectrics, smart materials, piezohydraulic, actuation, power by wire, EHA, pumps INTRODUCTION In aerospace and other applications, there is a long-term trend towards delivery of power to actuators over wires. This is in contrast to the more established practices using direct mechanical connections, or more commonly for aircraft, power transfer by pressurized hydraulic fluid. The electrical power transfer offers lower mass, better redundancy, and increased reliability.<br><br> One necessary aspect of power-by-wire systems is some means of converting electrical power into mechanical power, i.e. some means of actuation. There are several classes of power-by-wire actuation that may be appropriate for certain applications.<br><br> This paper concentrates on one, a type of hybrid electro-hydraulic actuation that uses solid-state materials to convert electrical to mechanical energy. Materials such as piezoelectrics, magnetostrictives and electrostrictives, sometimes called csmart materials, d have a long history in precision control applications. Because of their limited shape change capability, these materials are not normally used in actuators requiring large linear motion.<br><br> However, these stiff materials do offer relatively high energy density. A transducer having a mechanical output energy density of even 1 J/kg, could generate 1 kW/kg of power when operated at 1000 Hz; existing and new piezoelectric and magnetostrictive materials offer considerably higher energy densities. Delivering that power to a mechanical load in a meaningful way is a large challenge, one that the development described here is attempting to meet.<br><br> Because of the known requirements for aircraft and other applications, the interest is not necessarily in producing large motions using these stiff smart materials; the more fundamental goal is exploitation of the high energy densities of the materials. Over the last several decades there have been dozens of designs that achieve increased motion from smart material cores by various techniques. Among the common ones are mechanical amplification or transformation, for example those using levers and pivots, 1 and step-and-repeat types, for example the Inchworm".<br><br> 2 In general, these types of actuators have been quite effective in a narrow range of applications. More recently, researchers have recognized the potential of integrating smart materials and fluids, making the smart material driven pump a fundamental element to be exploited for linear actuation. 3-8 This newer approach holds promise for high power actuation with long stroke, and the possibility of competing directly with more established actuation types.<br><br> Smart material 3 hydraulic, more specifically piezoelectric-hydraulic or cpiezohydraulic d actuation, brings advantages and disadvantages compared to other types of actuation, including conventional servo-hydraulic and various electromagnetic types. The primary advantage compared to traditional hydraulics is the power-by-wire aspect, i.e. the elimination of hydraulic distribution lines.<br><br> Compared to electromagnetic actuation methods including motor-driven ball screws, the piezohydraulic actuation provides the same high force and a potentially more rapid response time. The new class of actuators has disadvantages compared to conventional hydraulics in the areas of heat distribution and tolerance for fluid loss. Compared to electromagnetic actuators, the new architecture, despite the small amount of fluid used, still requires both electrical and hydraulic integration.<br><br> Many of the features of piezohydraulic actuation are common to electrohydrostatic actuators (EHAs), such as those used on the Joint Strike Fighter aircraft. Where piezohydraulic actuation has a potential advantage over other EHAs is in the energy density of the smart material itself. The remainder of the paper discusses motivating applications for this type of actuator, particularly actuation of flight control surfaces in small air vehicles.<br><br> Available and new actuation options are considered. Then the piezoelectric- hydraulic actuation is described in more detail, and compared to conventional actuators. A flexible hardware-in-the- loop test system is described.<br><br> Finally, major integration issues for the new class of actuator are summarized. MOTIVATING APPLICATIONS The range of applications requiring actuation is broad and growing. Of particular interest in the present development are those applications for which high power density is required, where excessive weight or volume carries a high price.<br><br> Aerospace applications generally place a premium on mass, and these present a main impetus for development of actuation based on smart material cores. Figure 1: Overview of required power output and frequency response for aerospace actuator applications The range of applications within aerospace vehicles is also large, as Figure 1 shows. The maximum required power output, at the corner of a force-rate or torque-angular rate plot, is shown against the required frequency response.<br><br> The figure was derived from over 20 different actuator specifications. In general, larger vehicles require higher power actuation. Thus the actuators for space vehicles, such as those that move the elevons on the Space Shuttle, have capability well above 100 kW.<br><br> Tactical aircraft operate at high speeds and therefore require high power actuators with more bandwidth than those used for space launch. Missiles often demand high frequency response, making selective use of smaller actuators to influence and control high-speed flight. Unmanned combat aerial vehicles (UCAVs) are a subset of unmanned aerial vehicles (UAVs) that will place demands on actuators overlapping with requirements for tactical aircraft and missiles.<br><br> Among UAVs, UCAVs will generally be larger and faster, and will therefore require higher power, higher bandwidth actuation. In some ways, because UAV designs are evolving rapidly, they represent an open area for consideration of new types of actuators. There are still opportunities to exploit specific actuation advantages in new aircraft design.<br><br> The X-45A is one UCAV currently under development (Figure 2), with two test vehicles having flown in 2002. This vehicle, weighing approximately 8000 pounds without a payload, is 8 m long with a 10 m wing span. Some future versions of the X-45, as well as other UCAVs, may be larger, and may require higher actuator power output, with Figure 1 approximately enveloping the requirements for many of the concepts under consideration.<br><br> Figure 2: The X-45A requires actuation for flight control and other purposes For actuation of the primary flight control surfaces, the X-45A uses actuators with established heritage from a non- aircraft application. Requirements for these actuators include the following: " Dimensions " Stall force " Rate under load " Stroke " No load rate " Operating temperature " Weight (actuator and electronics) " Moderate to high voltage DC input The specific values for these quantities are known, and were used to motivate development of the smart material hydraulic actuation, though their publication is not yet possible. Additional information from the X-45B and other aircraft was also considered.<br><br> Many of the uses of actuators within these and other aircraft involve utility functions rather than flight control. Other applications under study include missile fin actuation, helicopter flaps or pitch links, control surfaces for naval vessels, and morphing aircraft. This last application area is in a research phase.<br><br> Morphing aircraft have been in use for decades, but some of the new concepts are considerably more comprehensive in the degree of shape change as designers attempt to support multiple missions with single aircraft. Among the desired features for this application are power by wire across shape-change interfaces, high power output in small volume, and non-standard device form factors. While power density is critical, these actuators may operate at a very low duty cycle, with morphing functions carried out only a small number of times during any given flight.<br><br> Figure 3: Large angle, lower bandwidth flight motion simulator under construction at Carco Electronics (left) and small angle high bandwidth simulator (right) Another specialized application is in flight motion simulators (FMS), particularly those used for testing missiles rather than those dedicated to aircraft flight training. In a typical state-of-the-art system (Figure 3, left), a missile seeker is bolted to the innermost stage of a large test system. Outer axes control target motion, and inner axes pitch, roll and yaw the seeker in part of a hardware-in-the-loop simulation.<br><br> There is interest in extending the bandwidth of these simulations beyond the low tens of Hertz typical of current systems. This would allow more complete testing of seekers and control systems with fewer expensive flight tests. The system on the right in the figure is a standalone high frequency stage that was designed to be mated to the innermost stage of the large simulator.<br><br> 9 One of the main challenges in the integration is the transfer of hydraulic power, with the fluid lines appearing prominently in the photo. All-electric systems are generally heavy. A hybrid power-by-wire system could potentially meet the simulator requirements.<br><br> But, in contrast to aircraft flight control actuation, this system would have to have a bandwidth well above 100 Hz. Considering these applications and others, it is possible to group the requirements for new actuation into multiple categories. As Table 1 shows, power density is ultimately the most direct measure of actuator performance in volume and mass constrained applications.<br><br> Power per unit volume is frequently most critical, when the form factor of an actuator is severely constrained. Requirements are often specified in terms of both peak and continuous power. Two actuation technologies may have equivalent continuous power outputs, but much different peak power outputs.<br><br> The table also indicates that power density is of greater importance than end-to-end efficiency for most applications, though high efficiency is certainly desired. There is also a crucial distinction between stroke, which can be realized by adding length to a device, and velocity, which is more critical and drives core device design. Table 1: Actuation requirements ranked in approximate order of importance within each column, with typical units Primary Performance Secondary Performance Other Power density (W/cc), peak and continuous Efficiency (W/W) Reliability, longevity, fail safety Power density (W/kg), peak and continuous Stroke (cm) Heat generation management Force (N) Stiffness (N/mm) Electromagnetic compatibility (EMC) Velocity (cm/s) Precision ( µ m) Power system compatibility So, Table 1 serves as a general guideline for what is considered important in development of a new actuation technology.<br><br> The next section turns to an overview of available technologies, with a bias towards aircraft applications, and suggests a possible role for new smart material based electro-hydrostatic actuators, or EHAs. ACTUATION OPTIONS The specifications for flight control on a UCAV suggest a number of actuation options. The relatively smaller size and moderate power output required in these vehicles means that more options are feasible.<br><br> The primary approaches (Figure 4) are normally classified as hydraulic, electric (or electromechanical), and electro-hydraulic (or electro- hydrostatic). MechanicalElectric/ Electromechanical (EMA) Hybrid/ Electro-hydrostatic (EHA) Flight Control Actuation Motor-driven screws Direct-drive linear motors Electric motor pump core Smart material core Hydraulic (centralized) Figure 4: Classification of actuation types for flight control and other aircraft applications The established option is a hydraulic system in which a central hydraulic power unit (HPU) uses power likely originating from the aircraft engines to pressurize hydraulic fluid. A typical system might run at 3000 psi (21 MPa).<br><br> From this central HPU, fluid is distributed to multiple actuators throughout the aircraft, including sets of redundant actuators. Hydraulic actuation has been a staple of aircraft flight control and utility control systems for decades. It produces the required power output, force, velocity and stroke, along with a desirable output impedance or apparent stiffness.<br><br> For this mature technology, reliability is good. This type of actuation often fails more gracefully than its EMA counterparts. Newer systems are being built and operated at higher pressures, up to 5000 psi (35 MPa), allowing smaller actuator cylinders.<br><br> But the distribution of high-pressure fluid throughout the aircraft is not without problems. The network of lines takes space and can be relatively complex to install, route and service. The central hydraulic unit is moderately heavy, as are the distribution lines.<br><br> Maintenance requires considerable ground equipment, and it can be relatively messy. For certain types of UAVs, the need to maintain a support infrastructure for hydraulics is unappealing. All-electric actuation systems have been developed over the last ten to twenty years, and motor technology has reached the point where electromechanical actuators (EMAs) can be practical for moderate-force aircraft applications.<br><br> In general, EMA architectures incorporate a motor that operates at relatively high speed. This motor is coupled to a screw shaft of some type through a transmission. Figure 5 shows a typical EMA with a motor in the center and the ball screw output on the right attached to a dummy flap load.<br><br> EMAs, like all other flight critical actuators, incorporate redundancy features, typically using two separate motors to drive through a transmission to an output shaft. The dual-motor arrangement provides excess torque or force. EMAs can also be driven from a centralized location, with power distributed mechanically, for example over the span of a wing.<br><br> One of the issues with EMAs is their potential for failing in a locked state. Another concern is the bandwidth limitation imposed by angular momentum in the motor. Figure 5: Example of a motor-driven electromechanical actuator (EMA) manufactured by Hamilton-Sundstrand Direct drive electromagnetic linear or rotary motors may be feasible for some aircraft applications, but these actuators are not in common use.<br><br> In a direct drive motor, there is no transmission stage. Thus, the motor output shaft output is attached more directly to the surface it is controlling. The relatively low speeds and long strokes or large angles associated with aircraft flight control do not necessarily favor this actuator type.<br><br> While in the more typical motor-driven screw arrangement, the high energy density motor is separated from the shaft, in the direct drive case there is no separation. For linear actuation in particular this can result in heavy units. Electrohydrostatic actuators, or EHAs, have seen increasing use over the last few years.<br><br> This actuator class combines the power-by-wire features of EMAs with the desirable high force and other load interface and graceful failure properties of hydraulic devices. As Figure 6 shows, the centralized hydraulic power unit and hydraulic distribution system are replaced by an electric current distribution system terminating in pressurization units or pumps local to each actuator. With such an actuator, using 5000 psi pressure versus 3000 psi becomes more attractive.<br><br> The same force can be achieved with three-fifths the actuator piston area, yet all the high pressure fluid is contained in a small volume, and there are no 5000 psi distribution lines. In general, EHAs use electric motors to pressurize and pump fluid. With local control, there is less excess pressurized fluid that is dumped as heat, and overall efficiency is improved.<br><br> Compared to centralized hydraulic systems, there is some reduction in fail safety, as leaks of small amounts of fluids have greater effects. The F-35/JSF is notable for its use of 5000 psi EHAs in primary flight control, while the A380 also uses EHAs to augment a more conventional, partially centralized system. Figure 6: Basic architectures of centralized and distributed hydraulic actuation EHAs are entering use increasingly, with new high profile applications.<br><br> The local pressurization and pumping assembly is the critical subsystem. It is there that high energy density smart materials may offer a more compact geometry. SMART MATERIAL EHAS AND PIEZOELECTRIC-HYDRAULIC ACTUATION This section describes the basis of operation of smart material EHAs in more detail.<br><br> It also compares features of this actuation class to those of the other types of actuation described above. The basic actuator architecture can be viewed as a sequence of power conversions (Figure 7). The role of the smart material is to act as prime mover within a pump, converting electric to fluid power.<br><br> Electric power conversion/drive Smart material transducer Fluid transmission Mechancial conversion Electrical power input Mechanical power output Figure 7: Basic power conversion path for a smart material - hydraulic actuator With an emphasis on power conversion efficiency, it is apparent that this actuator type is not similar to many of the other familiar uses for smart materials. The goal is not vibration control or precision positioning. It is not quasi-static positioning, or displacement amplification using levers or mechanisms.<br><br> It is not a low power application. Rather, it is energy conversion, more specifically AC to (near) DC power conversion via rectification. In its use of smart materials for high power output it is more akin to sonar systems.<br><br> Piezoelectric, magnetostrictive, and electrostrictive stiff smart materials are the normal candidates for the pump energy conversion. Figure 8 shows a photo of one piezoelectric-hydraulic actuator prototype, with a diagram summarizing the device architecture. The actuator assembly has a conventional output shaft that is driven by the fluid output from the pressurization and valve assembly (PVA).<br><br> The PVA incorporates a pump and a set of valves that provide directional control. As the diagram shows, the PVA includes a compression chamber and another valve set that interfaces with the piezoelectric to form what is essentially a source of pressurized fluid. More details on device architecture and subsystems are provided elsewhere.<br><br> 10,11 Smart material actuator (piezoelectric) Valve Set 2 Accumulator Compression chamber Output actuator Load Valve Set 1 High power amplifier Other drivers/amps Microcontroller/ DSP Power conversion Commands Status Elec. power Figure 8: Basic architecture of the smart material-hydraulic hybrid actuator So these actuators are a subset of EHAs with the following main differences. They have higher expected energy output per unit volume or mass because of smart material energy density.<br><br> There is potential of greater efficiency and rapid reversibility under slow and fast time-varying loads. Finally, there is possible additional performance to be gained via resonant drive or use of resonant amplification within the pump. Compared to EMAs, the new actuator class also offers several possible advantages.<br><br> Reversals and slews are faster because of reduced inertia. Better performance can be achieved in cases of light duty cycles and high peak-to- continuous power requirements. Performance under failure conditions is also better, with less likelihood to lock fully or open completely free.<br><br> Finally, the ability for dithering during actuation is retained. Figure 9: Recent piezoelectric pump unit used as part of a smart material EHA As this technology is being developed, the gains from higher energy density are the primary driver, but secondary features must be recognized to determine where the new actuators can be best used. For example, the local fluid transmission system is a site where energy can be stored to allow high peak load delivery.<br><br> While the device technology progresses (Figure 9), it is necessary to establish test methods consistent with those used to qualify established and competitive actuator technologies. FLEXIBLE TESTBED FOR ACTUTOR EVALUATION With a goal of testing smart material based EHAs in realistic flight environments, a necessary first step is the creation of a ground-based system, a laboratory test facility, that provides many of the features of flight control. At a minimum, the actuator must push against a load.<br><br> The simplest representation of a typical aircraft control surface is a spring (Figure 10). With such a load, the actuator 9s force-velocity characteristics can be established, and mechanical power output can be quantified. Figure 10: A stiff spring is one of the simplest loads used in basic actuator testing; force-velocity load lines can be measured as an actuator pushes against the spring load With applications such as the UCAV in mind, a hardware-in-the-loop (HWIL) test capability has been established to test smart material EHAs driving realistic loads.<br><br> Several options were considered for the HWIL simulator, with the following specifications driving the tester design " Programmable, time-varying load designed to mimic an aircraft flap or other component " -Stroke: 8 inches " -Stall load: 13,000 lb " -Maximum no-load rate: 7.5 in/sec " -Bandwidth: 80 Hz These specifications represent a compromise between information available for existing and future applications, with some additional capability to allow for future device growth or test of competitive EMAs or EHAs. The specifications are written similarly to those for an actuator, because at the heart of the simulator is an actuator. Figure 11 shows the basic layout of the testbed.<br><br> The assembly is located on a large stiff work plate. A servovalve- controlled hydraulic actuator is used in conjunction with sensors to mimic a load (Figure 12). With feedback the system can present, within a certain range, arbitrary stiffness and damping characteristics.<br><br> Although mass effects can also be synthesized actively, a fixed mass is positioned on a sliding rail to capture the main portion of the inertia presented to the actuator under test. Figure 11: Solid model of hardware in the loop (HWIL) simulator for testing new actuators Figure 12: Control diagram for HWIL tester By incorporation of the digital signal processor (DSP), it will be possible to present to the actuator the time-varying load that it would see during a set of flight maneuvers. A rudimentary capability demonstrating variable spring stiffness is shown in Figure 13.<br><br> The tests were done at the low end of load and position ranges and therefore sensor noise is visible in the plotted data. Figure 13: Force-displacement characteristics of HWIL load: stiffnesses of 1000, 3000, and 5000 lb/in result in different slopes Servohydraulic actuator Load cell Mass on slide Actuator under test The HWIL simulator provides a notable object lesson for development of the new type of actuation. The hydraulic power unit used to pressurize fluid for the simulator actuator is quite large and includes considerable means of heat management.<br><br> A smart material actuator capable of exercising the testbed to its full capabilities must have extremely high power density if it is to be kept small. INTEGRATION ISSUES Smart material EHAs using piezoelectrics and magnetostrictives are currently under development. Much remains to be proved in terms of basic performance of the technology including its effective power density.<br><br> Looking ahead, there await a series of other issues that must be addressed prior to successful integration in a flight demonstration system. Among these concerns are electronic drive, thermal management, smart material longevity and device redundancy. Electrical drive of the smart material will require a high efficiency system to maintain low volume, low weight and overall device efficiency.<br><br> Therefore a Class D amplifier has been baselined. The drive requirements for the PVA or pump are demanding, with the current requirements for the piezoelectric-based devices generally exceeding those of nearly all established piezo drive technology. Yet, the fidelity of the drive signal needed is nowhere near that required in audio or other applications.<br><br> Some ability to vary drive frequency, amplitude, and perhaps waveform shape is all that is required. Amplifiers built to date to support the development have shown greater than 80% efficiency, with high switching frequencies used. The piezoelectric drive development leverages advances in high power components for motor drive and other applications.<br><br> Yet, a flight weight unit that minimizes coupling to on-board power sources and other electronics is still challenging for the mainly capacitive piezoelectric load. Newer designs with efficiencies approaching 90% will be integrated with actuator test units in the near future. For practical use in a flight application, reliability must be increased, volume must be reduced by 50-75%, and mass must be reduced by 30-40%.<br><br> Another issue emerging as critical is thermal management. For many smart material applications, power transfer is low, and heat management is not a design driver. This is not the case for the smart material EHAs.<br><br> Piezoelectrics and magnetostrictives are both hysteretic in their response and heat is dumped into the material during each cycle. Further, the reactive component of the load in piezoelectrics is temperature dependent, increasing with increasing temperature. Fluid within the transmission stage heats up, and unlike traditional centralized hydraulic systems, there is not regular flow into and out of individual devices.<br><br> Heat management is also a consideration for the drive electronics, making their efficiency more critical. The present device designs incorporate temperature monitoring for possible use in control modulation, and maximize opportunities for convective and conductive heat transfer. This actuation technology is really no different from EMAs in the thermal management concern, and the ongoing development is making use of the same techniques used with EMAs.<br><br> A third issue requiring attention is the longevity of smart material actuators under high mechanical, electrical, and thermal loading. Piezoelectrics and other materials perform well over long periods in quasi-static positioning applications, or when the materials are used as sensors. Increasingly, such materials are finding successful use in valve control.<br><br> But the present application is almost unique in the persistent high frequency high drive operation. Since the goal is energy conversion, it is not desirable to de-rate the actuators significantly. Monolithic magnetostrictive elements have an advantage over stacked piezoelectric wafers.<br><br> Newer material compositions, including single crystal piezoelectric are unproven with respect to their longevity under high cyclic loads. A final issue that must be addressed for successful integration of the new actuation technology is redundancy. Flight critical aircraft actuation systems simply must incorporate some redundant features.<br><br> Often this takes the form of completely separate systems. In specifying EMAs, there is often the assumption of using dual torque-generating motors driving the same output shaft. Similar concepts are relevant to EHAs, and designs using smart materials are no different.<br><br> Series and parallel combinations of pumps or PVAs and output shafts have been investigated. A flight version of the piezoelectric-hydraulic actuation will certainly incorporate this feature. CONCLUSIONS This paper has attempted to serve two purposes.<br><br> First it has identified generic requirements for aircraft actuation and described available technologies for meeting these needs. Second, it has discussed a newer technology based on smart materials and described both how this technology may apply, highlighting some of the features and challenges in converting the new class of actuators from concepts and prototypes to flight-ready devices and systems. Unmanned aerial vehicles (UAVs) are considered one potential application with actuation needs that might be met by smart material based electrohydrostatic actuators (EHAs).<br><br> The moderate size of many of these aircraft makes it feasible to consider the new type of actuation, whereas primary flight control for large aircraft is probably not feasible. The new actuators share EHA advantages over traditional centralized hydraulics, offering a lower weight approach that is easier to integrate with other aircraft subsystems. Compared to electromechanical actuators (EMAs), a larger acceleration is possible, and failure modes are potentially less harmful.<br><br> Ultimately, exploitation of the inherently high energy density in smart materials is the key to viability of the new actuator class. Ongoing efforts are constructing and testing devices with an aim to produce smart material units having greater power densities than existing technologies. ACKNOWLEDGEMENTS The paper describes actuator development funded under the DARPA Compact Hybrid Actuator Program, with a contract administered by the Air Force Research Laboratory.<br><br> The authors thank the sponsors as well as Jason Lindler, Marc Regelbrugge, Brian Hurlbut, Richard Warner and Nathan Roth for their contributions. REFERENCES 1. Uchikawa, T., cMechanical Amplification Mechanism Combined with Piezoelectric Elements, d US Patent 4570095, 1986.<br><br> 2. 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Conf.<br><br> On Adaptive Structures Technologies , College Park, MD, Oct. 2001. 9.<br><br> Anderson, E.H., Evert, M.E., Flannery, P.S., Fowler, B.L., Glaese, R.M. and Janzen, P.E., cImage Stabilization Testbed, d SPIE Aerospace Sensing Conf., Orlando, FL, 2001, SPIE Paper 4366-24. 10.<br><br> Lindler, J., Anderson, E. and Regelbrugge, M., cDesign and Testing of Piezoelectric-Hydraulic Actuators, d Smart Structures and Materials 2003: Industrial and Commercial Applications of Smart Structures Technologies , E. H.<br><br> Anderson, ed., SPIE paper 5054-11. 11. Regelbrugge, M., Lindler, J.<br><br> and Anderson, E., cDesign Model for Piezohydraulic Actuators, d AIAA Adaptive Structures Conference , 2003, AIAA paper 2006-1640.

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