Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to stabilized mount for
a platform, particularly a stabilized mount for a plats
form of an antenna to track a satellite. The mount can
be used on a fixed surface but is intended principally
for use on a moving vehicle, such as a ship at sea or a
large land vehicle.
In tracking a signal from, for example, a
satellite, it is essential that accurate tracking be
carried out as quite small deviations by the antenna can
result in a loss of the signal. The signal reception is
independent of simple translations of the antenna base
because the satellite is so far away that small motions
of the antenna do not noticeably change the azimuth or
elevation of the satellite as detected by the antenna.
Lo however signal strength falls off very suddenly as the
axis of the dish deviates from correct alignment. The
axis of a dish is a line normal to the surface of the
dish at the centre of the dish. For example angular
errors of greater -than 0.5 can cause a loss of operating
capability. The power signal at 0.5 error will be about
1.5 volts less than it is with correct alignment and
reception of pilot signals from the antenna will be lost
when the angular error is about 2. Up to that point the
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power signal will be positive.
It is therefore important -to provide an antenna
with a motion compensation system which will enable the
antenna to function correctly despite movements of the
carrying vehicle affecting the alignment of the antenna
with the signal. A stabilization system to be effective
must prevent the platform from rotating relative to fixed
space even though the carrying vehicle is doing so. A
stabilization system cannot, of course, isolate the plats
form from accelerations of the vehicle along the axis of
the antenna.
Known platform stabilization systems have been
either the passive or active. A typical passive stabile-
ration system balances the platform on gimbals with low
friction bearings. with careful balancing sideways acre-
aeration of a platform support will not cause the plats
form to rotate. The only rotational force on -the
platform will be small amounts of torque transmitted by
the gimbal bearings. But the inertia of the platform
will tend to make it stay aligned in absolute space. A
gyroscope is an example of a passive system. A gyroscope
relies on the low friction of its gimbal bearings to pro-
vent the stabilized platform from rotating. The spinning
flywheel found in a gyroscope serves to increase the
effective inertia of the platform.
An active system differs in having instruments
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for sensing motion of the platform and a control system
that determines how quickly to rotate the platform rota-
live to its supports to reduce the motion to a minimum.
All active control systems send out a control signal that
corresponds directly to a rate of rotation of the plats
form relative to its support structure. If the incitory-
mint sensing motion of the platform detects a roll rate
of a ship to be o, ED per second a signal will be sent to
the control system to drive the stabilized platform at a
rate of minus 0.5 per second relative to the ship so
that the net rotation of the platform will be 0.
With passive systems friction ensures that a
perfect system is not possible. That is friction cannot
be eliminated completely in the gimbal bearings and
therefore deviation of the platform from the required
position is inevitable. A problem with active systems is
their relative complication and the fact that errors are
still inevitable both in sensing the motion and sending
the appropriate compensating motion to the platform.
The present invention offers a combination of
active and passive systems. In the present invention the
platform is balanced on low friction gimbals to isolate
it from rotations of its support structure. However
instruments are used to measure the motion of the plats
25 form and a control system is used to counteract animation that is observed. However it should be emphasized
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that in the present invention the active control system
does not control the motion of the platform relative to
the support structure. In normal operation the platform
is allowed to swing freely in its gimbals and the active
control system merely gives the platform boosts of torque
only when the platform starts to swing away from correct
orientation
Thus, in a first aspect the present invention
provides a stabilized mount for a platform comprising a
lo support; a gimbal ring mounted on the support by a
diametrically opposed first gimbal; diametrically
opposed second gimbal to mount the platform on the gimbal
ring; the improvement whereby a first shaft extends
through one of the first gimbals; a first motor attached
to the first shaft; a second shaft extending through one
of the second gimbals; a second motor attached to the
second shaft; sensing means to sense movement of the
platform whereby any movement of the platform induced by
movement of the gimbal ring and transmitted to the plats
form through gimbal friction may be compensated for by
torque applied by a motor.
An embodiment of the invention is illustrated,
merely by way of example, in the accompanying drawings in
which
Figure l is a side view of a stabilization
system according to the present invention;
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Figure 2 is an elevation, partly in section, of
the stabilized platform; and
Figure 3 illustrates a control system useful
with the stabilization system shown in Figures 1 and 2.
Figures 1 and 2 show a stabilized mount
comprising rotatable circular base 2 having gimbal sup-
port arms 4 and 6 rigidly rotated on its upper surface.
The circular base 2 is located by a pin 8 extending down-
warmly to engage a deck 10 of a ship There is a gimbal
ring 12 mounted between the support arms 4 and 6 on horn-
zontal, diametrically opposed shafts 14 and 16 that
extend outwardly from the gimbal ring 12 to engage the
support arms 4 and 6. There is a further pair of
diametrically opposed shafts 18 and 20 rigidly fixed in a
platform 22, a portion of which is shown in Figure 2 -
but located in KIWI friction bearings 24 and 26 in the
gimbal ring 12. There is a first electric motor 28
attached to the shaft 14 and a second electric motor 30
mounted on plate 32 and attached to the shaft 20. The
shaft 20 extending from motor 30 rotates freely in the
gimbal ring 12 but is rigidly connected to the platform
22. Shaft 14 extending from motor 28 is rigidly con-
netted to the gimbal ring 12 but rotates freely in the
platform support 4 in low friction bearings. All
bearings used have low starting torque and both electric
motors 28 and 30 are selected as low friction electric
motors.
There is an angular rate sensor 34 - see Figure
1 - able to detect movement of the platform 22 in the two
directions permitted by the gimbal ring structure.
The platform 2 is mounted on rollers 36 and is
provided with teeth 38 on its periphery. An electric
motor 40 is mounted on the platform 2 and has a gear 42 on
its drive shaft to permit rotation of the platform 2 by
driving motor 40. The system is protected by a cover 44
often referred to as a rhodium to protect i-t from the
weather. A particularly important function of the rhodium
is to protect the antenna from gusts of wind.
A conventional antenna 46 is mounted on the
platform 22 and its angular inclination is preset either
automatically or by hand. An optional automatic sky-
scanning function of the active system is possible.
Operation of the device is shown in the control
system illustrated as a block diagram in Figure 3.
Figure 3 shows a control system for only one of the two
motor actuators 28 and 30. The system is duplicated for
the other motor 28 or 30.
Before discussing Figure 3 some general come
mints are appropriate. The stabilized mount illustrated
in Figure 2 has three axes of rotation. There is an axis
of rotation around pin 8 which, with the aid of electric
motor 40, provides a coarse azimuthal orientation of the
assembly. There are two axes in the gimbal for precise
control ox the direction of the antenna. For an antenna
system on an anchored vessel, for example an oil rig,
where the heading of the vessel is not changing -the
coarse azimuthal orientation can be preset and the base
clamped to the stand. However on ships that are liable
to change their heading, electric motor 40 is used and is
desirably connected to a signal provided by the ship's
compass.
Finer control of the mount is provided actively
and passively. The platform 22 is free to rotate on the
gimbal ring 12 and the active control gives the platform
boosts of torque only when the platform starts to swing
away from correct orientation for the antenna. The
control system never "knows" how fast -the antenna is
rotating relative to the ship only how fast it is
rotating in the absolute frame of reference. This is in
contrast to the prior art systems discussed above where,
of course, active systems are constantly controlling
movement of the platform relative to the ship.
The antenna 46 carried by the platform 22 is
carefully balanced on its gimbals so that lateral acre-
aeration of the ship's deck will not induce -torque on it.
To achieve this, the centre of gravity of the rotating
member, that is the antenna, the low noise amplifier
associated with the antenna and the stabilization equip-
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mint, must be exactly in the centre of -the gimbal
mounting and thus in line with both axes of rotation of
the gimbal. Thus counterweight 48 is used and, further-
more, as indicated the counterweight must be able to
receive small masses to trim the balance of the antenna.
With the antenna free swinging on its gimbal
the only force that will cause it to rotate will be
torque transferred through the gimbal bearings as Eric-
lion. If the bearings were friction less there would, of
course, be no need for the active control system. The
antenna would remain pointing in the same direction while
the vessel rolled beneath i-t simply because no forces
would be transmitted through the gimbal ring. However,
the active system is necessary to compensate for Eric-
lion. In this regard it should be emphasized that it is important to use bearings with as low friction as
possible The rotation rate will be slow so that
bearings with high starting friction, that is large break
away torque must be avoided. Further the bearings must
be low maintenance and suitable for use in salt spray.
however bearing tolerances are not critical and the load
is small.
Further concerning friction the motors 28 and
30 should be close coupled to the rotating shafts 14 and
20 and they never rotate through a full turn and never go
faster than about 2 per second. The normal condition of
the motors is to have zero current through them and in
this state they are free to rotate and therefore do not
add significantly to the friction of the motor gimbals.
When additional torque is needed to compensate for the
effects of friction in the gimbals a controlled amount of
current is put through the motor by a control algorithm
described below. It should be noted that the active
controller according to the present invention never has
direct control of the motion of the antenna. It only
gives the antenna boosts in one direction or the other.
Thus the control system illustrated in Figure 3
is a control system concerned only with the absolute
motion of the antenna and does not control the speed that
the antenna moves relative to the ship. The antenna is
isolated from rotation of the ship by low friction
bearings so that its inertia keeps it properly oriented.
The control system uses inertia sensors to monitor the
absolute motion of the antenna and applies rotational
torque to boost the antenna back onto track only if the
antenna starts to drift from its proper orientation.
Figure 3 illustrates one half of -that control
system, used to control the "roll" angle of the antenna
Referring to Figure 3, the torque due to the roll of the
ship, the torque applied by the motor and the dynamics of
the antenna and the gimbals determine the roll rate of
the antenna. This roll rate is measured by the angular
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rate sensor mounted on the antenna. That angular rate
sensor may be a gyro-stabilized inertial platform or a
solid state device. The device produces an electrical
signal proportional to the roll rate of the antenna and
that signal is filtered to an analog signal filter and
the information fed to an A/d converter. From there a
signal is passed through the microprocessor part of the
control system, which is that part between the A/d con-
venter and the d/A converter. Also fed into the
lo microprocessor is information derived from the power
signal r which is proportional to the strength of the
received signal and which is fed through an do converter
to provide a calculation of the desired roll angle of -the
antenna using a scanning algorithm. This part of the
control system will be discussed below.
The microprocessor sends out a control signal
to the current controlling device and the current
con-trolling device then sends the necessary current to an
electric motor 28 or 30, depending on which motor is
controlled by the particular circuit, which sends a boost
of torque to the attached shaft.
It is desirable that the control system be
supplemented with a signal tracking control system that
will drive the antenna towards an alignment where the
power signal is strongest. Such a system will refer to
the power signal as its primary controlling variable and
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will move the antenna through an arc to scan for an
orientation where the signal is strongest.
