Axis Drive Description; Axis Position Monitoring; Axis Position Calibration - Advent VISLINK Newswift 90 Technical Operation Handbook

VISLINK
4.11

AXIS DRIVE DESCRIPTION

Each drive axis (azimuth, elevation, and polarisation) is motorised using a 2-phase stepper
motor. The motor movement is monitored using a motor rotation detector. The azimuth and
elevation drive also have an electrically operated brake to hold the axis when power is not
applied to the motor.
The motor rotation detector (MRD) is used to monitor for motor stall and freewheel
conditions, and to measure the angular position of the antenna. The MRD's produces two
pulses per motor rotation. The stepper motors require 200 pulses for each half turn of
movement. If the system detects that the number of pulses per half turn is significantly larger
than expected, a motor stall is declared. If the number of pulses per half turn is significantly
smaller than expected, a motor freewheel is declared.
A stall indicates that an axis is attempting to move against a large torque. Either high wind or
snow load, or the system is driving against a physical hard stop. A freewheel is caused by
the motor being driven by a force other than by the stepper motor drive pulses.
4.12

AXIS POSITION MONITORING

The angular position of all axes is calculated by counting the motor turn pulses from the
MRD's and the number of pulses since the last motor rotation pulse. This angle is directly
used to display the azimuth and polarisation angle. The elevation axis has both an internal
axis angle, based on motor turn and pulse counting, and an RF axis angle based on the
output from the inclinometer.
Once deployed the elevation angle is based on the inclinometer output. This results in the RF
angle being true even when the base of the antenna is not level. If the inclinometer fails, the
RF angle is estimated by using the internal elevation axis angle based on motor turns and
pulse counting. In this case, an offset of the antenna from level will cause the estimated RF
angle to be in error.
The system has axis 'softlimits', which stop the antenna from moving beyond certain limits
based on the internal axis angles. For the elevation angle these are positioned at the nominal
RF angles of 10° and 90°. If the system is on a slope, these angles will be different, as the
softlimits apply to the internal axis angle rather than the RF angle.
As the system uses motor turn and pulse counting to calculate the angular position of the
antenna, any motor stall or freewheel can result in errors accumulating in the axis angle. Any
stall or freewheel detected causes that axis to be declared uncalibrated.
4.13

AXIS POSITION CALIBRATION

The system uses motor turn and pulse counting to calculate the axis angle from factory set
calibration markers, these are metal tabs placed at the stow positions for each axis. The
system calibrates each axis on deployment using these metal markers. These are detected
using Hall Effect devices that can sense the presence of the metal as it passes through the
detector.
On deployment each axis goes through an automatic calibration procedure. If an axis
becomes uncalibrated whilst deployed, the axis can be recalibrated using these markers. If
the inclinometer is working the elevation axis uses the inclinometer to recalibrate its internal
axis. If the inclinometer has failed, then the elevation axis can only be recalibrated by
completely stowing the antenna.
Newswift 90-180 Motorised Antenna 19