Sea Tel 9797A-27, Installation And Operation Manual

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9797A-27 C-Band TXRX Maintenance and Troubleshooting
7-3
1
Inspect inside of radome for signs that the dish or feed have been rubbing against the inside of
the fiberglass radome.
2
Rotate the pedestal through its full range of azimuth motion. The antenna should rotate freely
and easily with light finger pressure.
3
Rotate the pedestal through full range of elevation rotation. The antenna should rotate freely
and easily with light finger pressure.
4
Rotate the pedestal through full range of cross-level rotation. The antenna should rotate freely
and easily with light finger pressure.
5
Rotate the level cage through the full 90 degrees of rotation from CCW stop to CW stop. The
level cage antenna should rotate freely and easily with light finger pressure. Attached cables
should not cause the cage to spring back more that a few degrees from either stop when
released.
6
Inspect all drive belts for wear (black dust on/under the area of the belt).
7
Inspect AZ Drive chain. IF chain is beginning to show signs of rust or corrosion, apply a light
coat of light duty oil to the chain. Wipe excess oil off to leave a light coating on the chain. DO
NOT over-lubricate .
7.2.8. Check Balance
Check the balance of the antenna, re-balance as needed (refer to the Balancing the Antenna procedure
below).
7.2.9. Observe Antenna Initialization
Observe the Antenna Initialization as described below.
7.3. Troubleshooting
7.3.1. Theory Of Operation
The antenna system is mounted on a three axis stabilization assembly that provides free motion with 3
degrees of freedom. This assembly allows the inertia of the antenna system to hold the antenna pointed
motionless in inertial space while the ship rolls, pitches and yaws beneath the assembly. Three low
friction torque motors attached to each of the three free axes of the assembly provide the required force to
overcome the disturbing torque imposed on the antenna system by cable restraints, bearing friction and
small air currents within the radome. These motors are also used to re-position the antenna in azimuth
and elevation.
The Pedestal Control Unit (PCU) uses inputs from the level cage sensors to calculate the amount of
torque required in each axis to keep the antenna pointed /-0.2 degrees. The basic control loops for
Cross Level, Level and Azimuth are shown in the Control Loop Diagram, drawing 116287. The primary
sensor input for each loop is the rate sensor mounted in the Level Cage Assembly. This sensor reports all
motion of the antenna to the PCU. The PCU immediately responds by applying a torque in the opposite
direction to the disturbance to bring the antenna back to its desired position. Both the instantaneous
output of the rate sensor (Velocity Error) and the integrated output of the rate sensor (Position Error) are
used to achieve the high pointing accuracy specification.
The calculated torque commands are converted to a 5 volt differential analog signal by a Digital to Analog
converter (D/A) and sent to each of three Brush-Less Servo Amplifiers. These amplifiers provide the
proper drive polarities and commutation required to operate the Brush-Less DC Servo Motors in torque
mode. The Torque acting on the mass of the antenna cause it to move, restoring the rate sensors to their
original position, and closing the control loop.
Since the rate sensors only monitor motion and not absolute position, a second input is required in each
axis as a long term reference to keep the antenna from slowly drifting in position. The Level and Cross
Level reference is provided by a two axis tilt sensor in the level cage assembly. The Azimuth reference is