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|  MIL-N-81604C(AS)
Para 3.4.4.5b
(cont)
(8) Weight on Wheels - This discrete from the ANCU
shall be supplied to the PSU to limit the PSU
battery discharge to 5.5 seconds 1.5 seconds
in the-event of a primary power interruption
while the aircraft is on the ground. T h e
true
state of this discrete shall be ground, as
described in 3.4.3.2c. The PSU input circuit
shall require an input current no greater than
-100 ma. This discrete shall not be dependent
upon primary aircraft power (i.e. , the state
of the signal shall not be affected by a primary
power interrupt).
3.5 Detail Requirements
3.5.1 Inertial Measurinq Unit (IMU)
3.5.1.1 Function - The IMU shall contain gimbal-mounted synchros
which furnish unambiguous platform pitch, roll, and azimuth.
Velocity increment signals shall be derived from three accelerom-
eters orthogonally mounted on the stable element structure. The
stable element shall be decoupled from aircraft motion by means
of four gimbals. The gimbals shall be driven by torquer motors
which are, in turn, driven by electronic servos whose input signals
are derived from 2 two-degree-of-freedom gyros mounted on the
stable element. The gyros, and therefore the stable element,
shall be controlled by ANCU supplied torquing signals during
normal system operation. A nonvolatile, refillable memory shall
be contained within the IMU for the purpose of storing calibration
data peculiar to a particular platform. The calibration data shall
be used by the computer to correct the platform velocity outputs
by providing compensating gyro torquing command inputs. The IMU
shall be capable of providing attitude and heading reference infor-
mation in the event of an ANCU malfunction. The backup heading
reference shall be free gyro unless compensated by a sine latitude/
magnetic slave heading error signal generated by the CAU (3.4.2.2)
where this is a specific aircraft requirement.
3.5.1.2 Sealing - The IMU shall be adequately sealed to prevent
contamination of the unit. If the IMU requires external cooling
air, it shall be cooled by thermal conduction into a convective
heat exchanger chamber through which cooling air is forced, thus
maintaining the integrity of the gimbal assembly seal.
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