Emerging technologies in electrical drives and power distribution systems in future aircrafts

Emerging technologies in electrical drives and power				Electro hydrostatic Actuation (EHA)
distribution systems in future aircrafts
				1. In contrast to EMA, EHA (Figure 2) uses fluidic
u.sureshkumar*				gearing between the electric motor and the surface
				actuator. Hydraulic fluid provides an intermediate
* professor in EEE				means of transmitting power to the surface. Here, a
department				variable-speed electric motor (typically DC) is used to
				drive a fixed-displacement hydraulic pump, which in
Mohamed				turn, powers a conventional hydraulic piston jack.
sathak enginnering college				Change in direction is achieved by the use of a
				bi-directional motor. A major advantage to this
Kilakkarai				approach is that the EHA operating mode can be
				managed like a conventional hydraulic actuator. This
E mail				approach is achieved using standard hydraulic bypass
				or damping valves (Figure 3); thus traditional
				active-standby, or active-active, actuator configurations
				can be readily adopted. This capability makes the EHA
				more suitable for primary flight control applications than
				the EMA. Although EHA technology reintroduces
Abstract:				hydraulic components and fluid, it is totally
				self-contained within
It is projected that in future aircraft, all power, except				the actuator assembly. Compared to
propulsion, will be distributed and processed electrically.				traditional hydraulic actuator systems, the
In other words, electrical power will be utilized for				inconvenience of hydraulic
driving aircraft subsystems currently powered by				disconnection from aircraft supplies and the
hydraulic, pneumatic or mechanical means including				complications of bleeding the system during
utility and flight control actuation, environmental control				reinstallation are not encountered during
system, lubrication and fuel pumps, and numerous				maintenance.
other utility functions. These concepts are embraced
by what is known as the “More Electric Aircraft
(MEA)” initiative. The MEA emphasizes the utilization
of electrical power as opposed to hydraulic, pneumatic,
and mechanical power for optimizing aircraft
performance and life cycle cost. It would eliminate the
need for gearboxes and transmissions since the
power transmission is through electrical rather than				Electrohydrostatic Actuators (EHA)
mechanical means, which reduces the weight of the
aircraft and increases the fuel efficiency. Detailed
analysis ofinteraction between an Electro Mechanical
Actuator (EMA) connected to the DC bus of the
power distribution system in a next generation
transport aircraft with the bus regulator is presented.				Large EHA
Development of reliable power-by-wire actuation
systems for both aeronautical and space applications				EHA Control Schematic
has been sought recently to eliminate hydraulic				Benefits of electrically powered Actuators:
systems from aircraft. Aircraftengineers have tested
electrohydrostatic actuators (EHAs), which combine				The potential benefits of electric actuation at a system
electrical and hydraulic power, hence theevolutionary				level have been well publicized.
"more electric aircraft" idea. Efforts are being made to				Electric actuation can offer:
replaceall the hydraulic systems with electrical				- Improved aircraft maintainability:
systems, which will lead to a new technology called				- Fewer hydraulic components are required,
“All Electric Aircraft”.				- Faster aircraft turnaround,
				- Fewer spares and tools are needed,
Introduction:				- Improved fault-diagnosis through built-in
Flight Control System				test (BIT).
A flight control system consists of the flight control				- Improved system availability and reliability:
surfaces, the respective cockpit controls, connecting				- Electrical distribution is more practical and offers
linkage, and necessary operating mechanism to				system flexibility with respect to reconfiguration Ñ a
cont4rol aircraft in flight.				capability previously difficult to achieve using hydraulics,
Classification				- Improved mean-time-between-failures (MTBFs)
Flight control systems (FCS) are classified as follows:				through removal (electromechanical actuation or EMA)
- Mechanical FCS				or on-demand usage (electrohydrostatic actuation
- Hydro mechanical FCS (powered flight control units				or EHA) of hydraulic components.
(PFCU))				- Improved flight safety Ñ in the MEA configuration,
- Fly-by-wire FCS				improved system safety is achieved through dissimilar
Mechanical FCS:				actuator power supplies and subsequent avoidance of
The mechanical FCS is the most basic designs. They				common mode failures.
were used in early aircraft and currently in small				- Reduced system weight Ñ weight saving, achieved
aeroplanes where the aerodynamic forces are not				through the replacement of entire hydraulic systems,
excessive. The FCS uses a collection of mechanical				including pumps, distribution networks (pipes and fluid),
parts such as rods, cables, pulleys and sometimes				and valve blocks, by electric systems.
chains to transmit the forces of the cockpit controls to				The main benefit is the reduction of aircraft operating
the control surfaces.				costs, for example, reduced fuel cost (as a result of
				reduced weight), and lower maintenance costs
Hydro mechanical FCS (powered flight control units				(quicker turnaround). However, before such benefits
(PFCU)):				can be realized, additional work is required to improve
The complexity and weight of a mechanical FCS				the technology and provide the appropriate application
increases considerably with size and performance of				platforms to introduce the technology into service.
the airplane. Hydraulic power overcomes these				Furthermore, the aircraft maintenance industry must
limitations				realign its infrastructure so that it can reap the benefits
A hydraulic FCS has 2 parts:				of electric technologies.
- The mechanical circuit
- The hydraulic circuit				Some additional benefits of both EMA and EHA
The mechanical circuit links the cockpit controls with				actuators are:
the hydraulic circuits. Like the mechanical FCS, it is				- Low quiescent power consumption during standby
made of rods, cables, pulleys, and sometimes chains.				operation,
The hydraulic circuit has hydraulic pumps, pipes, valves				- Rapid start-up response,
and actuators. The hydraulic pressure generated by				- Can be easily adapted for use with AC or DC
the pumps in the hydraulic circuit powers the actuators.				electric supplies,
The actuators convert hydraulic pressure into control				- Insensitive to supply frequency variation of AC
surface movements. The servo valves control the				electric supplies.
movement of the actuators. The above two-control
system has a major drawback that it contains heavy				EHA versus EMA?
mechanical circuitry, which increases the weight of the
system. To overcome this drawback a new				An alternative to EHAs, are ‘electromechanical
technology “MORE ELECTRIC TECHNOLOGY IN				actuators’ (EMAs), in which the motor torque is
AIRCRAFT” was developed. The aircraft in which				mechanically amplified and transmitted to the control
this technology was used was called “MORE				surface using a gear set, screw or other mechanical
ELECTRIC AIRCRAFT”(MEA).				transmission device, can be seen as an alternative.
				Indeed, as far as complexity, weight, reliability and
When describing the MEA, flight control actuation				maintenance requirement are concerned, EMAs are
systems can be considered to involve two main				potentially more attractive than EHAs, at least for low
technological areas: fly-by-wire (FBW) and				power applications. In particular, all hydraulic technology
power-by-wire (PBW). FBW technology comprises the				relevant problems are obviously eliminated from the
design, development and implementation of electronics				EHA configuration. However, in the three following
for flight control systems. Electronic control provides				areas EHAs are still preferable to EMAs:
flight control and actuator control functionality				?The jamming probability of an EMA used in a primary
implemented using either centralized or distributed				flight control application is difficult to predict and
architectures. Distributed control systems reduce the				substantiate from existing in-service experience.
processing load on centralized flight control computers,				Jamming probability of an EHA, can be directly
and offer more flexibility during system architecture				assessed from the current servo control experience,
development. A further benefit is the reduction in				and shown as ‘extremely improbable’ if
weight achieved by reducing harness size and				properly bypassed. In contrast, the jamming probability
component quantity. In recent years, technological				of mechanical systems incorporating hundreds of gear
advancement has centered on the FBW field, to the				teeth and screw mechanisms is questionable and
extent that FBW control systems are now the				present-day experience in secondary flight control
standard in today’s commercial and military aircraft.				applications may not be directly transferable to primary
Power-by-wire (PBW) actuation is the next major				flight controls, due to very different duty cycles in
breakthrough in aircraft control. Just as the fly-by-wire				particular
flight control system eliminated the need for				Wear of the mechanical transmissions components
mechanical interfaces, power-by-wire actuators				may result in control surface ‘free-play’ or other
eliminate the need for central hydraulic systems.				non-linearities, which may generate unacceptable limit
Control power comes directly from the aircraft				cycles
electrical system. This has several advantages. Central				?The introduction of an EHA in parallel with regular
hydraulic systems are complicated and difficult to				servo control in the basic more-electric architecture
maintain. Removing these systems would greatly				described above is easier than an EMA. EHAs can
reduce the amount of support equipment and				easily be made reversible in standby mode, they can
personnel required to maintain and operate current air				incorporate identical damping devices to those
and space vehicles. In addition, PBW actuators have				currently used for flutter protection, and they can be
the potential to be more efficient than their hydraulic				built with many components common with the
counterparts. A central hydraulic system must				adjacent servo control such as the piston, cylinder,
generate and sustain significant hydraulic pressure				associated position transducer or the accumulator. In
(3,000 to 6,000 pounds per square inch) at all times,				an obvious move to spread the technical as well as
regardless of demand. PBW actuators only use				financial risk, Airbus has called on the talents of several
electrical power when needed. Finally, PBW actuation				companies for the design, production and supply of the
systems can be made far more fault tolerant than				many actuators on this mammoth aircraft. Specifically,
those depending on a central hydraulic supply. Once a				the A380 aileron and elevator EHAs, as well as rudder
hydraulic line is compromised, it usually leads to the loss				EBHAs are purchased from Goodrich, while
of that entire hydraulic circuit. As a result, multiple				Messier-Bugatti will supply the associated EHA pumps.
hydraulic circuits are required to maintain some level of				Meanwhile, the spoiler EBHAs are from Liebherr, which
redundancy. With a PBW system, a failed actuator				supplies its own pumps. Phil Hudson, Goodrich VP
can simply be switched off, isolating the problem to a				engineering for actuation systems notes: “The
single surface.				electronic EHA concept can also be designed to serve
				more functions than simply motor control. It can serve
Types of PBW Actuators				as a smart actuator controller in its own right and be
				part of a distributed control system or to control a set
There are several different types of PBW actuators,				of multiple actuators. Another benefit is that this
including electrohydrostatic actuators (EHA) and				distributed technology puts intelligence local to the
electromechanical actuators (EMA). EHAs use a				actuation elements in a control system and can
reversible, electrically driven pumpmotor to directly				substantially reduce harness weight and improve fault
pump self-contained hydraulic fluid to a piston. This				detection and isolation.”
drives the ram in the same fashion as a standard				Maintenance benefits are also substantial.
hydraulic actuator (Figure 1(a)). An EMA has no internal				Power-by-wire EHA actuation units are line-removable
hydraulic fluid, instead using electric motors to directly				with only mechanical and electrical connections to the
drive the ram through a mechanical gearbox (Figure				aircraft, which eliminates the need to refill or bleed
1(b)). Compared to an EHA, the EMA has certain				systems of hydraulic fluids as is required with central
advantages. It is lighter, smaller, and less complex than				hydraulics. Since power-by-wire actuators are self
an equivalent EHA because of the absence of an				contained and remotely located at the surfaces, the
internal hydraulic system. Since there is no hydraulic				area exposed to damage is greatly reduced.
fluid in the load path, the EMA tends to be stiffer than				Additionally, power-by-wire actuators can be designed
an equivalent EHA. The EMA tends to be more				as position sensitive, which means that the actuators
efficient because there are no windage losses or				provide only the flow and pressure necessary to
pump inefficiencies. Finally, since there is no leak				move and hold the actuator in a desiredposition.
potential with an EMA, it is better suited to long term				Conventional central-hydraulic systems are configured
storage or space applications.				to produce continuous pressure. Flow is metered at
				each actuator, which can lead to a large consumption
				of power and generate unwanted heat. William Schley,
				R&D supervisor, Parker Aerospace, Controls
				Systems Division explains that EHAs only consume
Electromechanical Actuation (EMA)				power on demand. Specifically, they consume power
				in proportion to the power delivered to the load. In
An EMA uses mechanical gearing to couple an electric				contrast, a conventional EVSV-equipped hydraulic
motor to a flight control surface. This is achieved using				servoactuator consumes power in proportion to output
a rotary gearbox, and depending on the actuation				speed, allocating power to output load as needed, with
method required, can include some form of				the remainder of the power being dissipated through
rotary-to-linear conversion, such as a ball screw.				pressure drop (heat) across the main control valve.
Electric motors requiring a DC electrical supply are				Whilst hydraulic actuators become more efficient the
typically used, although the addition of a diode				more they are loaded, loads are typically low during
rectification stage will also allow them to operate from				most of a flight.” Another important advantage of
an AC electrical supply. Motor speed, direction, and				electric actuators is survivability. Ballistic or explosive
torque translate directly to speed, direction, and load in				damage to an electric power distribution system or
the actuator. Figure 1 shows an EMA currently being				actuator usually does not cause loss of function of
developed by TRW for a high-power flight control				that entire channel, particularly if the damage is
application. In its basic form, the EMA is susceptible to				peripheral. In a hydraulic system, depending on its
certain single-point failures that can lead to a				design, even a small leak can cause a major loss of
mechanical jam, and consequently presents				function and/or fire. Although some electric actuators
complications for flight certification on certain surfaces.				containhydraulic fluid, the system as a whole is still
Additional devices can be used to mitigate against this				usually more survivable. For now, these more
failure mode, but in doing so, complexity, cost, and				advanced failure management functions are being
weight are increased. For these reasons, the basic				provided by the EHA and its variants. EHA combines
EMA is not suited for primary flight control applications.				the best of electric actuation and conventional
However, spoiler systems and secondary actuation				hydraulics for a hybrid design approach, which is more
systems could accommodate EMA technology.				fault tolerant than most current EMAs. Moreover,
				EHAs are mechanically simple, and immune to gear
				train jams. The typical long-term storage capability for
				EHA is 10 years plus.



EMA system layout				Next-Generation---All-Electric Aircraft:

				The “All-Electric” aircraft is a concept that
				emerged in the 1970s and has engendered a large
Large EMA for High-Power flight controls				amount of research activity. An all-electric engine,
				which could replace current aero gas turbines, would
				drive all accessories electrically, via a distribution
				network, from motor/generators embedded in the
				engine spools. Extending the function of the motor
				generators to include service as active magnetic
Baseline Power System Architecture				bearings would facilitate deletion of the oil system. The
				all-electric concept thus offers a huge scope for both
The proposed power distribution system is built around				engine and airframe reconfiguration and operational
a 270V DC distribution bus. The typical baseline power				improvements, with studies indicating benefits of overall
system architecture for a next generation aircraft is				weight reduction, increased reliability, easier
shown in Fig. 1. It can be seen that the key				maintainability, reduced operating costs (including
components that control the power are the				reduced fuel burn), and enhanced safety.
bidirectional power converters (BDCs). A bus regulator
provides an interface between the starter/generator
and the distribution bus. Most of the loads, including the
actuators, are regulated using bidirectional power
converters, which control and condition the power
from the DC bus.				Conclusion:

				Beginning with the scenario of a single hydraulic power
				supply replaced by an electric one, it is possible to
				establish the relativity and scale for the changes
				required in the migration toward the “All-Electric”
				aircraft concept. On a small civil airliner, typically a
				minimum of five electric actuators would be needed to
				provide one lane of electrical control for the primary
				flight control surfaces. If all hydraulic systems were
				converted to electric, in excess of 20 electric
				actuators would be needed to provide complete
				control of all primary and secondary flight control
				surfaces. The consequential increase in electrical
				power demand has major implications for electrical
With the proliferation of bidirectional power converters				power generation and distribution systems. Thus, a
and advanced actuators in the power distribution				significant amount of work is still needed to address
system, it is important to develop methods to analyze				the consequences of distributing many electrical
the interaction between the different subsystems. Due				actuators around an aircraft, and the consequential
to the complexity of the baseline power system and				start-up, steady state, and peak demands required of
the large number of subsystems, a sample power				aircraft electrical power supplies.
distribution system, which captures the essential				It is clear that the migration to electric actuation
features of the baseline system but is not as				systems is affecting both civil and military markets. As
complicated, is introduced. The sample power system				described previously, the replacement of a single
is represented as a interconnection of a source and				hydraulic system by an electric substitute is a major
load subsystem.				step in the transition to all-electric technologies. It is quite
				evident that the demands being made on aircraft
Sample Power Distribution System				generators and distribution architectures will increase
The sample power distribution system is shown in Fig.				considerably to meet the needs of this migration. A
2. The source subsystem represented by subsystem 1				company named TRW has already developed
consists of an ideal three phase voltage source, a				products to meet the current demands envisioned by
three-phase boost rectifier to provide the regulated				PBW and has programs to ensure that it will meet any
270V DC required by the DC bus. The load				future demands required by the all-electric aircraft.
subsystem represented by Subsystem 2 is an				Finally, it is envisioned that once in service, electric
electromechanical actuator used to control the				actuator technology and electrical system
secondary flight control surfaces on the aircraft. The				architectures will improve the commercial viability and
other loads on the DC bus are modeled by a current				in-service reliability of the airframes to which they are
source, or a simple resistance.				fitted. These improvements will undoubtedly drive the
				adoption of greater levels of electric actuation on
				future aircraft.

The EMA model shown in Fig. 5 is shown to include a				References:
DC motor with constant field, a ball screw transmission
between the motor and the control surface, and a				- Weimer J. A, “Power management and distribution
model of the surface dynamics. The motor voltage is				for the More Electric Aircraft”, Proceedings of
controlled by a PWM bidirectional buck converter with				the30thIntersocietyEnergyConversion Engineering
an input filter. The EMA is controlled by a multi-loop				Conference, vol. 1, July 1995, pp. 273-277
controller, which includes a motor current, motor speed,
and the ball screw position feedback loops.All of the
other loads on the bus are modeled by a resistor or a
current source.				- Technology Review Journal — Millennium Issue •
				Fall/Winter 2000

				- ACTUATOR DEVELOPMENT OVERVIEW
				D.

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Emerging technologies in electrical drives and power distribution systems in future aircrafts

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