Page 51 - Mechatronics with Experiments

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JWST499-c01
JWST499-Cetinkunt
INTRODUCTION 37
For redundancy purposes, all hydraulic actuators (electro-hydraulic servo actuators
(EHSA) and motors) are powered by one of three independent hydraulic power supply
systems. A typical hydraulic power supply system is of constant pressure type in aerospace
applications. The constant pressure may be supplied by a fixed displacement pump with a
pressure limiting unload-valve or a output pressure regulated variable displacement pump.
The second option is more energy efficient in converting mechanical engine power to
desired hydraulic power.
These hydraulic power supply systems are named Blue, Green, and Yellow. In order
to meet reliability requirements for the supply systems, several independent power sources
are used. The main power sources for aircrafts are the engines. They not only create the
necessary thrust for flight, but also supply power to drive generators (integrated drive gen-
erator, IDG) and hydraulic pumps (engine driven pump (EDP)). An additional electrically
driven pump is connected to the yellow system. A backup for the blue system is the ram air
turbine (RAT), which is activated (roughly located below the cockpit) in case both engines
fail. The green and the yellow hydraulic systems are connected by a so-called power transfer
unit (PTU), comprising of two hydraulic machines (one fixed, one variable displacement),
both of which can operate as hydraulic pumps or motors. Thus, energy can be transferred
between both supply systems. The PTU might be used during normal operation in case
the power demand from one of the two hydraulic systems is too high (e.g., during deploy-
ment of slats and flaps). The “hydraulic power consumers” (actuators) are connected to the
hydraulic supply systems with different priorities controlled by a priority valve, with the
primary flight control actuators at highest priority (Figure 1.27).
Every actuator in flight control surfaces uses one of the constant supply pressure
lines as a hydraulic power source, and meters (controls) the flow to the rotary hydraulic

Control surface

Wing
rear spar

Mode selector valve
Servo valve

Hydraulic Hydraulic
system 1 system 2
(e.g. blue) (e.g. yellow)
FIGURE 1.27: Two redundant hydraulic actuators for a flight control surface. In addition to
redundant cylinder actuators, there are redundant flow control valves as well as redundant
hydraulic power source (blue and yellow supply). As shown in this example, redundancy for all
three main components of a hydraulic control system is designed into the hardware in the
design: redundant pumps, redundant valves, and redundant actuators (hydraulic cylinders or
motors).