Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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STABILIZED PLATFORM SYSTEMS FOR PAYLOADS
FIELD OF THE INVENTION
This invention relates to stabilized platform systems for isolating a payload
from
angular motions of a supporting structure.
BACKGROUND OF THE INVENTION
As imaging devices such as motion picture and video cameras are more
frequently
being mounted on unstable structures to achieve a desired point of view, image
stabilizing devices are becoming more necessary. With the long focal length
video
lenses in use today, even a tripod on a concrete stadium floor can impart
enough
undesirable motion to spoil the shot. Scaffolds, cranes and moving vehicles
all impart
significant levels of motion which can limit the use of long focal length
imaging
devices. This problem can be overcome by using a stabilized platform system
such as
described in U.S. Patent No. 3,638,502 (Leavitt et al.) issued February 1,
1972 and
U.S. Patent No. 4,989,466 (Goodman) issued February 5, 1991. However, the
platform systems described in these patents have many disadvantages, for
example
complexity, size and weight.
U.S. Patent No. 5,897,223 (Tritchew et al.) issued April 27, 1999 describes an
improved stabilized platform system for isolating a payload from angular
motion and
translational vibration of a supporting structure. The platform system has an
inner
gimbal for carrying the payload, a sprung shell containing and carrying the
inner
gimbal in a manner permitting the inner gimbal a limited amount of angular
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movement relative thereto amount pitch, roll and yaw axes, an outer gimbal
containing the sprung shell and inner gimbal, and a passive vibration isolator
connected between the sprung shell and the outer gimbal and having two
symmetrical arrays of dampened coil springs located on opposite sides of the
sprung
shell. The angular position measured between the inner and outer gimbals is
used
as an error signal to drive the outer gimbal to follow the inner gimbal,
thereby
allowing large ranges of steering motion.
While the platform system described by Tritchew et al has many advantages
and improvements over the previously mentioned systems of Leavitt et al and
Goodman, the universal joint and supporting structure still occupies the
central area
of the inner gimbal. For use with single sensors such as large video and film
cameras, the Tritchew et al platform system would require the use of large
counterweights to balance the sensor about the central pivot. The size and
weight
of such a platform system, relative to such a sensor, would therefore be
significant.
Conventional gimballing methods for freeing up the central area require the
use of large gimbal rings around the payload connected together through
bearing
axes orthogonally. Such large rings can limit system performance due to
structural
resonances and inertial effects. Such gimbal rings also add weight and
restrict
payload volume.
Another problem with such prior art platform systems is that it is difficult
to adapt them to standard film or video camera packages currently used by the
motion picture and broadcast industry. Instead, specific custom camera
packages are
normally engineered to operate with known Gimbal systems. While a certain
degree
of interchangeability can be designed into these systems, the camera packages
still
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have custom designed features or characteristics.
Many non-stabilized camera steering heads have been developed to utilize
standard camera packages. Such steering heads tend to have large open
structures
which are prone to low frequency structural resonances which contribute to
undesirable motion of the camera. Some attempts have been made to stabilize
such
steering heads. However, torquing through these large open structures severely
limits the attainable system bandwidth.
It is therefore an object of the present invention to provide a stabilized
platform system which at least substantially overcomes the problems described
above.
SUMMARY OF THE INVENTION
According to the present invention, a stabilized platform system for isolating
a payload from angular motions of a supporting structure has a base assembly
securable to the supporting structure, and a payload stabilizing assembly
carried by
the base assembly and mounted for angular movement relative thereto about two
or
more separate axes. At least one of the axes is non-orthogonal with respect to
another of the axes and is mounted for limited angular movement relative to
the
base assembly. The axes have extensions which meet at a common point, which is
preferably within the periphery of the payload.
The payload stabilizing assembly may include a first angular adjustment arm
with one end pivotally mounted on the base assembly for limited angular
movement
relative thereto about a first of said three axes, a second angular adjustment
arm
having one end pivotally mounted on another end of the first angular
adjustment
arm for limited angular movement relative thereto about a second of said three
axes,
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and a payload carrier pivotally mounted on another end of the second angular
adjustment arm for limited angular movement relative thereto about the third
of said
three axes.
The platform system may also include an array of at least three magnetic
torque motors, each motor having an electrically energizable coil portion
carried by
the base assembly and a magnetic structure portion carried by the payload
stabilizing
assembly, each magnetic torque motor having an active axis along with a
payload
stabilizing assembly positioning for can be applied but having freedom of
movement
about the other two axes, and a controller for controlling energization of the
motors to
apply controlled moments to the payload stabilizing assembly about any axis of
rotation.
The stabilized platform system may have at least one capacitive sensor having
a first portion carried by the base assembly and the second portion carried by
the
payload stabilizing assembly with an air gap between said first and second
portions,
said capacitive angle sensor being responsive to relative movement between the
first
and second portions to provide a signal indicative of the angular position of
the
payload stabilizing assembly relative to the base assembly.
The payload stabilizing assembly may carry at least one angular rate sensor
operable to provide the signal of angular movement of the payload stabilizing
assembly about a pre-determined axis.
The angular rate sensor may be a fibre optic gyro.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the present invention will now be described, by way of
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example, with reference to the accompanying drawings, of which:
Fig. 1 is an exploded perspective view of the stabilized platform system,
Fig. 2 is a perspective view of a stabilized platform system in accordance
with
one embodiment of the invention with some parts being omitted so as to show
other parts more clearly,
Fig. 3 is a perspective view of the universal joint arrangement used in the
platform system shown in Figs. 1 and 2,
Fig. 4 is an exploded perspective view of the universal joint arrangement
shown in Fig. 3,
Fig. 5 is a perspective view of one of the magnetic torque motors used in the
platform system,
Fig. 6 is a planned view of the torque motors and capacitive sensor array used
in the platform system,
Fig. 7 is a perspective view of the torque motors and capacitive angle sensor
array shown in Fig. 6 and
Fig. 8 is a block diagram of the control system for the platform system.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to the drawings, Fig. 1 shows a stabilized platform system with an
outer gimbal in the form of a base assembly 12 securable to a supporting
structure
(not shown) such as a camera boom, an inner gimbal in the form of a payload
stabilizing assembly 14 and a two-part casing 16.
The base assembly 12 has an octagonal base member 18 which is securable by
bolts (not shown) to the supporting structure. The base member 18 carries the
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electrically energizable coil portions 20 of four torque motors substantially
equally
spaced around its periphery and extending in upwardly and outwardly inclined
directions. Torque motors of this kind are described in more detail in
previously
mentioned U.S. Patent 5,897,223. The base member 18 also carries diametrically
opposite capacitive sensor arrays 22 of a pair of capacitive angle sensors
located
between adjacent pairs of motor coil portions 20 and also extending from the
periphery of the base member 18 in upwardly and outwardly inclined directions.
Capacitive angle sensors of this kind are also described in U.S. Patent
5,897,223.
The centre of the base member 18 has a circular stop portion 24 which limits
motion of the payload stabilizing assembly 14, as will be described in more
detail
later. The base member 18 further carries a mounting arm 26 for payload
stabilizing
assembly 14 which extends upwardly and outwardly from the periphery of the
base
member 18 and is located between adjacent pairs of motor coil portions 20 so
that
a pair of motor coil portions 20 with a capacitive sensor array 22
therebetween is
located on each side of the mounting arm 26.
Referring also now to Figs. 2 and 3, the payload stabilizing assembly 14 has
a mounting arm 28 securable by bolts (not shown) to the mounting arm 26 of the
base assembly 12. A first angular adjustment arm 30 has one end pivotally
mounted
by means of a bearing 32 on the mounting arm 28 so that the adjustment arm 30
is
capable of a limited amount of angular movement relative to the mounting arm
28
about an axis A which is upwardly and inwardly inclined in a manner which will
be described in more detail later. A second angular adjustment arm 34 has one
end
pivotally mounted by means of a bearing 36 on the other end of the first
adjustment
arm 30 so that the adjustment arm 34 is capable of a limited amount of angular
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movement relative to the first adjustment arm 30 about an axis B of which is
also
upwardly and inwardly inclined in a manner which will be described in more
detail
later.
A payload carrier 40 is pivotally mounted by means of a bearing 42 on the
other end of the second adjustment arm 34 so that the payload carrier 40 is
capable
of a limited amount of angular movement relative to the second adjustment arm
34
about a vertical axis C. When extended, axes A, B and C meet at a point D
which
is some distance above the payload stabilizing assembly and which, when a
payload
is mounted thereon, is within the periphery of the payload (not shown). As
shown
in Fig. 1, the orthogonal role, pitch and yaw axes x, y and z of the
stabilizing
assembly 14 meet at and pass through the point D at which extensions of axes
A,
B and C meet.
Referring again to Figs. 1 and 2, a mounting plate 44 is secured in any
suitable
manner to the bottom of the payload carrier 40 and carries the motor and
sensor
components which complement the components provided on the base assembly 12.
Thus, the mounting plate 44 carries the magnetic structure portions 46 of the
four
torque motors substantially equally spaced around its periphery and angled to
cooperate with the electrically energizable coil portions 20 mounted on the
base
assembly 12. Likewise, the mounting plate 44 also carries the capacitive
excitation
plates 48 of the two capacitive angle sensors located between adjacent pairs
of
magnetic structure portions 46 and angled to cooperate with the capacitive
sensor
arrays 22 on the base assembly 48. The payload carrier 40 also carries an
angular
rate sensor, such as a fibre optic gyro (FOG) 50, which is used in the same
manner
as described in U.S. Patent 5,897,223.
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The stabilizing assembly 14 also includes a payload interface plate 52 which
is secured by bolts (not shown) to the top of the payload carrier 40. An
annular
structural member 54 is secured by bolts (not shown) to the top of the
mounting
arm 28 of the stabilizing assembly 14 and also to the upper ends of mounting
plates
21, 23 on the base member 18 and on which motor coil portions 20 and the
capacitive sensor arrays 22 are mounted, as well as to the upper end of
mounting
arm 26.
Fig. 4 shows a construction of the adjustment arm bearings 32, 36, 42. Each
bearing has a bearing shaft 60, two bearing members 62, a bearing cap 64 at
the
lower end, a retaining ring 66 at the upper end and a shaft retaining pin 68.
Fig. 5 shown one of the magnetic torque motors in more detail, namely the
electrically energizable coil portion 20 which is carried by the base assembly
12 and
the magnetic structure portion 46 which is carried by the stabilizing assembly
14.
Again, reference is made to U.S. Patent No. 5,897,223 for a more detailed
description.
Fig. 6 is a plan view of the four magnetic torque motors 20, 46 and the two
capacitive angle sensors 22, 48 and their relation to the point of convergence
D of
the pivot axes A, B, and C shown in Figs. 1 and 3.
Fig. 7 is a perspective view of the same components from the same perspective
as Figs. 1 to 3.
Fig. 8 is a block diagram of the control system of the previously described
embodiment of the invention. The control system is based on a single
microprocessor and is generally similar to the control systems described in
U.S.
Patent 5,897,223.
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The primary control algorithms of this microprocessor are shown as separate
blocks in the figure. The angular rate sensor (or FOG) array 50 attached to
the
payload carrier 40 detects rates of rotation thereof relative to inertial
coordinates.
In the absence of external steering commands 70 (i.e. zero demanded rates) the
processor's inner gimbal control algorithm computes and causes the torque
motor
array 20 to apply small correction moments to the inner gimbal using the
principal
of negative feedback to maintain the angular orientation of the payload
stabilizing
assembly in space. Capacitive angle sensors 22 sense the angular displacement
between the base assembly 12 and the payload stabilizing assembly 14 about
three
orthogonal axes.
The processor's outer gimbal control algorithm resolves the three angular
displacements into components aligned with the axes of the outer follow-up
devices
servo axes. These displacements are then used to produce steering commands to
drive a follow-up steering device to null each of the three angular
displacements of
the capacitive sensors 22, i.e. to continually centre these sensors, in effect
causing the
follow-up steering device to follow the orientation of the payload stabilizing
assembly 14. Position feedback from the follow-up steering device may be used
as
part of the outer gimbal control algorithm when such information is available.
In the presence of external steering signals 70, these signals are resolved
into
three angular velocity vector components aligned with the angular rate sensing
(or
FOG) axes X, Y and Z, using the angles indicated by the capacitive angle
sensor
array and the position feedback from the follow-up steering device (if
available) to
determine the current orientation of the payload stabilizing assembly 14.
Three
negative feedback control loops then drive the payload stabilizing assembly 14
to
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follow the external rate steering signals. The outer gimbal control algorithm
causes
the follow-up steering device to follow the moving payload stabilizing
assembly 14
as before.
While the orientation of the payload stabilizing assembly 14 is maintained
stationary in space, the earth rotates at the rate of 15 degrees per hour,
causing the
image of the horizon in the camera to apparently rotate at some component of
this
rate. Pitch and roll inclinometers mounted on the payload stabilizing assembly
14
may be used to generate automatic rate steering signals to steer the payload
stabilizing assembly 14 to maintain a level horizon in the camera image.
An alternative steering mode (follow mode) may us the three angular
displacements measured by the capacitive sensors 22 to generate the three
steering
commands 70 to steer the payload stabilizing assembly 14 to null each of these
displacements of the capacitive sensors 22, i.e. to continually centre these
sensors, in
effect causing the payload stabilizing assembly 14 to follow the orientation
of the
supporting structure. In such a mode, the stabilized platform functions as a
low pass
filter between the payload and the supporting structure. Such a steering mode
may
be used with a tripod and a manually steered head.
It will be appreciated that one adjustment arm may be omitted so that there
are only two rotational axes. Alternatively, a further adjustment arm may be
provided so that there are four rotational axes.
Other embodiments of the invention will be readily apparent to a person
skilled in the art, the scope of the invention being defined in the appended
claims.
06141500.9pa
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