IEEE Electrification Magazine - December 2017 - 41

By eliminating one
hydraulic system,
aircraft weight was
reduced while
maintaining the
necessary actuation
redundancy
for safety.

A simplified block diagram of this
architecture is shown in Figure 2.
This block diagram shows the separation of the low power (brain) from
the high power (brawn). This conventional system has a hydraulic pump
driven by the aircraft engine. The
pump is designed with a mechanical
compensator to produce a (nearly)
constant pressure, analogous to how
a field-regulated generator maintains a constant voltage. Hydraulic
system loads (flow demand) cause
pressure fluctuations just as electrical system loads (current demand)
cause fluctuations of voltage. For servo-controlled actuation
in a conventional system, the high-power hydraulics are
controlled with a low-power electrohydraulic servo valve
(EHSV). EHSVs use a small (low-power) torque motor to
control porting of the fluid to and from the head end and
rod end of the actuator (which controls extension and
retraction of the actuator). The torque motor typically controls the first stage of a multistage hydraulic valve, which
amplifies the output stage so that large pressures and flows
can be controlled with low-power electrical devices.
Airbus eliminated one of the hydraulic systems by utilizing electrical power from the generators as the backup
source. This altered the basic architecture from two
hydraulic systems to one hydraulic (primary) and one
electric (backup) for the flight controls. Figure 3 shows
the high-level block diagram architecture of the electric

FlightControl
Computer

Command

Actuator
Controller

Electric (Low)
Power

Torque
Motor

EngineDriven
Pump

Spool
Valve

backup hydraulic actuator (EBHA).
The EBHA has a conventional hydraulic actuator powered by a hydraulic
source and controlled by an EHSV.
In addition, the EBHA has an integrated motor driven pump as a
backup hydraulic source. The EBHA
uses a manifold that switches the
hydraulic source from the primary
engine-driven pump to the backup
electric motor-driven pump. Because
the EBHA has more complexity than
a conventional hydraulic actuator, it
does weigh more. Even with the
weight of the EBHA being twice
that of the adjacent, conventional hydraulic actuator, a
significant weight savings was realized with the elimination of the entire third hydraulic system (van den
Bossche 2006).
Flight-control actuation redundancy does not necessarily mean that an individual actuator has two sources
to operate (a backup source in the event of failure of the
primary source) but rather that the control surface has
redundancy. A flight-control surface can have two independent actuators, each with only one power source,
and achieve redundancy as long as each actuator on the
surface is powered by a separate source. Separate power
sources prevent a failure in any one system from disabling the control surface. Airbus used electrohydrostatic actuators (EHAs) as another form of redundancy,
using electrical power from the engine as its primary

Feedback

Hydraulic
Actuator

Flight
Surface

EHSV
Hydraulic (High)
Power

Figure 2. The simplified block diagram architecture of common hydraulic flight-control actuation.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

41



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