Install guide

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