ARMY TM 5-6675-308-34
MARINE CORPS TM 08837A-34/2
FUNCTIONING OF EQUIPMENT
repairable at the direct support or general support
2-1. General. This chapter contains functional descrip-
levels. The IMU has many calibration parameters which
tions for the Position and Azimuth Determining System
are applied as correction factors in the computer pro-
(PADS) and its major units. Depth of coverage is based
gram. Many of these parameters can be recalibrated at
on the requirement to support direct and general sup-
general support using the PADS test set and a special
port maintenance. Functional descriptions for the fol-
diagnostic computer program. The diagnostic program
lowing major units of PADS are included:
also checks those terms which must be recalibrated at
Inertial Measurement Unit (IMU)
the depot level. A punched tape of the calibration data
Control and Display Unit (CDU)
is provided with each IMU.
Computer
g. Items which do not affect the integrity of the
IMU seal maybe replaced.
Power Supply (PS)
h. Porro prism replacement requires recalibration
A block diagram of the PADS is shown in figure FO-1.
of the IMU.
for operator communication and interaction with the
PADS. The CDU functional block diagram is shown in
the key survey sensors mounted on a stable platform.
The platform is isolated from IMU case rotations by a
follows:
set of four motor-driven gimbals. Gyroscopes mounted
on the stable element sense rotational rates. The gyro-
Serial data bus (SDB) logic
scope outputs are amplified to drive the gimbal motors
Display logic registers and drivers
so the stable platform is always level and pointed north.
Keyboard and associated logic
a. Three accelerometers mounted on the stable
element sense change in motion in the north, east, and
Discrete logic
vertical directions. This information is processed by the
computer to determine the present position of the
a. Serial Data Bus Logic. The SDB logic provides
system.
for serial data communication between the CDU and
b. The gyroscopes and accelerometers are very sen-
the computer. The signals associated with SDB opera-
sitive to temperature variations. Therefore, tempera-
tion are address envelope, data envelope, serial address/
tures inside the IMU are tightly controlled. Tempera-
data, and 250-kHz clock.
ture sensors provide control signals to the computer
(1) The CDU contains eight 16-bit storage regis-
and power supply. Electrical resistance heaters and
ters with associated drivers. Each of these registers may
thermal-electric coolers are powered by the PS to
be loaded via the SDB by first addressing the appropri-
maintain the desired IMU temperatures,
ate register with an address word and then transmitting
the appropriate data word.
the computer to determine the orientation of the stable
(2) Depending on the contents of the address
element with respect to the IMU case,
word, the CDU will either accept data or transmit data.
d. The various sensors provide both analog and
Received data is stored in one of the eight 16-bit
digital outputs. The IMU accepts several digital control
storage registers as designated by bits 5, 6, and 7 of the
signals.
address word. Two types of data words are transmitted
e. A porro prism, which is in a known mechanical
by the CDU as follows:
alignment to the stable element, is used to transfer
(a) The first type is the keyboard word which
azimuth out of PADS using a standard surveyor's theod-
indicates when a keyboard pushbutton switch is pressed
olite.
(keyboard data valid bit 2) and the pushbutton switch
f. Because the gimbal assembly is a precision me-
code (bits 3 thru 7). Bits O and 1 indicate that an
address or data word transmitted from the computer
chanical device which is protected by a dry nitrogen
was received with a parity error. After the computer
atmosphere, the IMU is hermetically sealed and is not
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