resolvers which measure the head movement in pitch and
e l e c t r i c a l f o r c e b a l a n c e t r a n s d u c e r s secured to a support
y a w are fed with a 400 Hz reference signal from the EPU
f r a m e by means of grommets and bolts, together with a
pneumatic connector, a printed circuit board assembly
containing the transducer control electronics and a 54-pin
p e r f o r m e d in the EPU, where the outputs give sine and
cosine values of the AADS yaw and pitch angles with
connected to the motherboard on the Chassis by a 54-pin
respect to the helicopter.
connector, and the Central Processor Unit circuit card is
c o n n e c t e d with a 90-pin connector.
b. Temperature measurements from the resistive ATS
are made in the EPU, where the constant current source
a n d differential amplifier circuits associated with the ATS
e. The Power Supply Unit consists of a machined
cast chassis which acts as a heat sink for all dissipating
components, and supports two printed circuit boards and
a 20-pin connector.
c . AADS monitoring is carried out by the EPU and
includes out-of-range tests and sine, cosine compatability
testing. AADS failures are displayed on the EPU front
panel by a ADS BITE indicator.
EPU Functional Description. Airflow pressure
angle and temperature data are demodulated and monitored
E P U P h y s i c a l D e s c r i p t i o n . The EPU (fig. 1-14)
contains seven Shop Replaceable Units, replaceable at
R a d a r Altitude input interface and the A to D converter.
AVIM level. The seven units are the Chassis, Front Panel
Assembly, Rear Cover, Pressure Transducer Unit (PTU),
Analog Interface Unit (AIU), Central Processor Unit (CPU)
microprocessor based computing system. Input power from
and Power Supply Unit (PSU). See figure 1-15.
t h e h e l i c o p t e r 28 V d c su p p l y is con v e r t e d a n d re g u l a t e d
into the system voltage requirements by the Power Supply
Unit. A block diagram of the EPU is shown in figure FO-1.
a . An exploded view of the EPU is shown in figure
a. P r e s s u r e
(PTU). T h e
a l u m i n i u m and the unit is secured in the helicopter by
contains two Electrical Force Balance (EFB) transducers
f o u r bolts through cast metal feet. Removal of the rear
t o m e a s u r e t h e S t a t i c ( Ps )
( Pt- Ps )
cover gives accesss to the PTU, AIU, CPU and PSU. These
pressures. A typical transducer is shown in figure 1-17.
SRUs plug into the motherboard, which is part of the
With their associated control electronics they are capable
front panel assembly, using two part keyed connectors.
of measuring very low pressures with great accuracy. The
When these four SRUs have been removed, the front panel
transducers are temperature-sensitive, and each transducer
assembly, containing the BITE panel and connectors as
well as the motherboard, may be removed.
programmable read only memory (PROM) located on the
P T U printed circuit card.
b . All electrical and pneumatic connections are made
(1) Operation of EFB Transducer. The low mass beam
via the front panel. Static and pitot lines are connected to
of each transducer (figure 1-18) is supported by four
standard pneumatic connectors and electrical connections
flexible leaf springs which form a frictionless pivot, whose
axis is aligned with and attached to a rotary pick-off. The
l o c a t e d on the front panel and contains the LRU BITE
far end of the beam is connected to both a precision
indicators, SELF TEST indicator lamp and switch and
bellows and a forcer coil which sits centrally in a stable