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Model 1278K Mobile VSAT Antenna System 12 Operation and Maintenance Manual
3.5 AZIMUTH POSITION FEEDBACK
The azimuth position feedback is produced by a 10 turn, 1K-ohm potentiometer driven by the output shaft
of the worm gear box. (Fig 3.1) Since the AvL cable drive has no backlash, the position feedback is as
accurate (1%) as the resolution (.3°) and accuracy of the potentiometer and the backlash between the
potentiometer and the azimuth capstan. The potentiometer is sealed and rated for IP 65 environment -
wind, rain, dust, etc. In addition, a optical encoder mounted to output shaft on motor and provides
redundant feedback on azimuth position. Since the AvL cable drive has no backlash, the position feedback
is as accurate as the resolution and accuracy of the potentiometer, encoder and the backlash between the
potentiometer and the output shaft of the worm gear box.
The potentiometer is mounted on a bracket that straddles the az gear box. The azimuth potentiometer is
slotted and driven with a cross pin in the az capstan. The potentiometer is mechanically centered at the
travel position (5 turns from either end) when az platform is in the az stow position. This is accomplished
by loosening the 1/2” nut, which secures the potentiometer to the bracket. The controller may also
electrically zero the potentiometer. The limits are controlled by using inputs from the potentiometer and
optical encoder together and are set in and controlled by the TracStar Systems controller.
The AvL has the mechanical capability of ±400º or ±200º from stow position.
3.6 AZIMUTH STOW SWITCH
The TracStar System controller uses the az potentiometer voltage
to determine limits. Therefore, the AvL positioner is equipped with
only an azimuth stow position switch. Limit switch
mounting holes are provided if needed for other brand controllers.
The azimuth stow switch is mounted on the azimuth platform
accessible from under az/el cover. (Fig 3.4) The switch is fixed
relative to the mounting surface and is actuated by magnet located
on the azimuth bearing clamp ring.
Since the controller drives to 0° az after sensing the stow switch,
the az stow position can be varied by either changing the zero
position of the az potentiometer or zero voltage of the controller.
Rotation Direction Convention : + is clockwise (CW) viewed from above mount. - is counter clockwise
(CCW) viewed from above mount.
NOMINAL AZIMUTH LIMITS
Model 1278K Mobile VSAT (With 400° AZ ) +210° / -190°
Notes:
1) The limits should be set after the potentiometer has been centered as described above. If the
potentiometer is moved from this position, the limits should be reset.
2) The az angle displayed by the controller is affected by several parameters in the controller. (These
include az Scale Factor, az Reference Position).
3) While setting the limits, watch the az capstan to make sure it does not run into the spring block at the
end of the cables.
4) The angles at the limits in the chart shown above are for nominal values of these parameters, and the
values actually displayed by the controller at the limits may vary.
Figure 3.4
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