Stabilized shipboard crane

The invention is directed to a stablilized cargo-handling system using means for stabilizing suspended cargo in all six degrees of freedom using six individually controlled cable in tension in a unique kinematic arrangement. Inertial and distance sensors, coupled with high-performance cable drives, provide the means to control the multi-cabled crane automatically. The distance sensors are used to track the target container or lighter during the pickup and setdown modes of operation; the inertial sensors are used to prevent pendulation during transfer of the cargo from the seagoing cargo ship to the vicinity of the receiving lighter. The complete stabilized shipboard crane system permits safe and efficient operations in relatively high sea states.

BACKGROUND OF THE INVENTION 
When off-loading containers and break-bulk cargo from large oceangoing 
ships (or a dock) onto a variety of lighters (and other small craft), 
off-loading must be accomplished in all types of weather and in the 
presence of high sea states. Problems arise both due to the motions of the 
vessels in inertial space and the motions of the vessels relative to each 
other. 
Existing shipboard cranes use a single cable with a cargo coupling 
mechanism (or spreader) suspended from an overhead structure. Ship motion 
may force the cargo (or spreader) into pendulous action, allowing it to 
swing unchecked. Difficulty may be encountered when trying to mate the 
spreader with a container that is on the oceangoing vessel itself. 
Furthermore, depositing a swinging container onto a moving lighter without 
damage to cargo and hazards to personnel is also difficult. 
SUMMARY OF THE INVENTION 
The invention is directed to a new, stabilized cargo-handling system using 
means for stabilizing suspended cargo in all six degrees of freedom using 
six individually controlled cables in tension in a unique kinematic 
arrangement. Inertial and distance sensors, coupled with high-performance 
cable drives, provide the means to control the multi-cabled crane 
automatically. The distance sensors are used to track the target container 
or lighter during the pickup and setdown modes of operation; the inertial 
sensors are used to prevent pendulation during transfer of the cargo from 
the seagoing cargo ship to the vicinity of the receiving lighter. The 
complete stabilized shipboard crane system permits safe and efficient 
operations in relatively high sea states. 
The control systems for the stabilized crane have been designed for two 
operating modes. Both are under the primary control of a human operator, 
using visual feedback from the spreader/container and the target lighter. 
Assuming that the operator has picked up the container from his own ship's 
deck, the operator then rotates his turntable to swing the container out 
over the water. At this time the control system is in the "TRANSPORT" 
mode. In this case, inertial sensors on the spreader feed back signals 
through appropriate networks to modify the operator's command to 
automatically eliminate pendulous swinging. 
The operator then commands the system to lower the container. When the 
spreader approaches within about 20 ft. of the target lighter, the 
distance sensors "acquire" the target and automatically switch the control 
system to the "SET-DOWN" mode. The distance sensors employ a light source 
and receiver mounted on a spreader, and a plurality of reflectors are 
strategically mounted on the target lighter. The signals from the distance 
sensors are fed back through appropriate networks to modify the operator's 
commands in accordance with the relative motion of the cargo and deck; the 
drives run the cables in the direction necessary to synchronize the motion 
of the container with the motion of the moving lighter so the container 
may contact the lighter deck with a negligible difference in instantaneous 
velocity. 
In the preferred embodiment of the invention, the cables are attached in 
pairs at three symmetrically spaced apart points on the cargo spreader. 
The cables are run in adjacent pairs to the overhead crane structure and 
through three pairs of symmetrically spaced pulleys or sheaves on a 
triangular frame supported on three booms extending from three masts. All 
six cables are led back to the ship's deck crane tower, which supports six 
identical computer-controlled cable-drive systems. By the proper 
combination of cable extensions and retractions, these drives can position 
the cargo in six degrees-of-freedom. 
The three masts are mounted on a turntable on the deck of the oceangoing 
vessel and support the booms through cables from their tops. This 
minimizes the mass of the booms by eliminating bending moments and 
minimizing the reaction forces they must sustain. 
For the pickup and set-down modes of operation, distance sensors are 
provided that lock onto targets on the container during pickup and lock 
onto targets on the lighter during set-down. In these modes, the crane is 
designed to slave the motion of the spreader (cargo) to match the motion 
of its target, thereby eliminating cargo damage and safety hazards. 
When the picked-up container is being transported from the oceangoing 
vessel to the vicinity of the target lighter, a set of accelerometers is 
provided on the spreader to feed back any dynamic motions and stabilize 
the spreader in inertial space. This mode is designed to minimize power 
consumption and the stress on the stabilized crane structures and cable 
drives. If the stabilization is perfect, there are no dynamic forces 
required by the spreader and its container payload, only the static forces 
due to their weight. The cable drives will be constantly extending and 
retracting the cables, but the only power consumed will be due to the 
system losses. 
Both sets of sensors are designed to develop signals to control the cable 
drives. However, those signals are referenced to a coordinate system 
incompatible with the cable drives. The distance sensor is referenced to 
the target vehicle, the acceleration sensors are referenced to inertial 
coordinates, and the cable drives are referenced to the crane structures. 
Therefore, an on-line computer is employed to convert the error signals 
continuously from their respective coordinate systems into command signals 
for the six cable lengths. 
It is therefore an object of this invention to provide an improved 
apparatus for stabilizing cargo as it is transferred from one vessel to 
another under conditions where there may be relative motion between the 
two vessels. Specifically, it is an object of this invention to employ a 
stabilizing crane apparatus utilizing six cables, each individually 
controlled and operating in response to the relative position between the 
cargo and the deck of the vessel on which the cargo is to be placed, to 
maintain the cargo in a stable position relative to that deck during at 
least the set down phase of the transfer operation. 
It is a further object of this invention to provide a stabilized crane 
apparatus for transferring cargo from a first deck at one location to a 
second deck at another location wherein there may be relative motion 
therebetween, such as heaving, pitching, rolling, yawing and moving 
longitudinally and laterally, said apparatus comprising a plurality of 
winches associated with the first deck; a support cable attached to each 
of said winches; an upper cable guide including means for guiding pairs of 
said support cables; means for suspending said upper cable guide generally 
above the second deck; spreader means for connecting to and supporting the 
cargo; means for securing different pairs of said cables to said spreader 
means; position sensing means for measuring the distance from and the 
attitude of said spreader means relative to the second deck; and circuit 
means responsive to the output of said position sensing means for 
providing control signals to said winches to control the length of said 
cables and thereby maintain a stable positional relationship between the 
cargo and the second deck. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, and particularly to FIGS. 1 and 2, a 
stabilized crane apparatus is shown for use in transferring cargo from a 
cargo vessel 10 to a lighter 15 or other smaller craft. The cargo vessel 
and the lighter are subject to relative motion in all six degrees of 
freedom due to the action of the sea. The amount of relative motion will, 
of course, depend upon the state of the sea and the hydrodynamics of the 
vessels. 
The deck of the first or cargo vessel 10 supports a basic crane apparatus 
20 which includes one or more masts 25 which extend upwardly perpendicular 
from the first deck, and one or more boom members 30 which extend 
outwardly from the mast to a triangular frame member 35, shown in FIG. 3, 
which serves as an upper cable guide, as will be explained. Cables 40 
extend from the top of the mast to the upper cable guide 35. These support 
cables 40 are usually of fixed length and together with the booms 30 keep 
the plane of the upper cable guide 35 fixed relative to the plane of the 
deck of the first vessel 10. 
The crane apparatus 20 shown in FIG. 3 includes three masts 25 which would 
typically be mounted upon a rotating platform 45 so that the operator 
could pick up a cargo container 50 from the hold of the cargo ship, lift 
it free of the ship, and then the platform 45 would be rotated so that the 
cargo could then be placed on the deck of the receiving ship or lighter 
15. 
The cargo container 50, typically 40' long, 8' wide and 8' deep, is 
attached to a spreader 55 by conventional attaching means (not shown). The 
spreader itself includes means (61, 62, 63) for attaching cables which 
will be used both to support and to stabilize and position the cargo. The 
spreader 55 also carries a lower cable guide 155, an inertial sensor 70 
and a position sensor 72. As will be explained, the inertial sensor 
measures the movement of the cargo relative to inertial space, and from 
the output of this sensor, control signals are generated that will reduce 
or eliminate the swinging motion that usually accompanies the transfer of 
the cargo from the first to the second ship. The position sensor 72 
measures the distance and the relative position or attitude of the 
spreader with respect to the dec of the second ship, and this device will 
provide the control signals that will maintain the cargo in a stable 
positional relationship with the deck of the second ship during the las 
phase of the transfer operation. 
Referring to FIG. 3, the inertial sensor 70 mounted on the spreader is a 
conventional device which measures acceleration in three planes. The 
distance and attitude sensor 72 includes a light source 74 and a camera 75 
mounted on the lower cable guide 155 which cooperates with a target 78 
fixed relative to the deck of the second ship 15. As shown in FIGS. 4 and 
5, the camera 75 views the target 78 by means of a prism 76. The light 
source likewise is directed toward the target 78 by means of a similar 
prism (not shown) 
The target 78 includes three reflector elements 79 each of which reflects 
the light from the light source 74. These reflector elements are 
preferably retroreflectors so to return the greatest amount of light 
possible. These reflector elements form an image plane of the light source 
which is in a plane oriented other than normal to a line between the light 
source and the plane formed by the image. The light source is not within 
the circle formed by these three reflector elements, nor is it within the 
plane itself A more complete description of this distance and attitude 
sensing device may be found in U.S. Pat. Nos. 4,678,329 and 4,684,247. 
A total of six cables (81-86) are used to position the cargo. These cables 
pass through bearing assemblies (91, 92, 93) in the upper cable guide. As 
shown in FIG. 3, two cables pass through each bearing assembly, each 
including a pair of pulleys, one for each cable. Each cable is attached to 
a corresponding winch (101-106) which has response characteristics and 
power requirements sufficient to move the cables quickly in response to 
the relative motion between the cargo and the deck of the second ship. 
Hydraulic winches may be preferable for this purpose. 
Cables 81 and 82 pass through the first bearing assembly 91 and are 
attached to attachment points 61 and 62 on the spreader 55. Cables 83 and 
84 pass through the second bearing assembly 92 and are attached to 
attachment points 62 and 63. Cables 85 and 86 pass through the third 
bearing assembly 93 and are attached to attachment points 63 and 61, 
respectively. 
These cables (81-86) under appropriate control by the winches (101-106), 
permit the placement of the spreader relative to the upper cable guide in 
six degrees of freedom, thereby allowing the control mechanism to 
compensate for heaving, pitching, rolling, yawing, and a limited amount of 
both lateral and longitudinal motion of the second deck with respect to 
the cargo. 
It will be noted from FIGS. 6 and 7 that the spacing between the attachment 
points 61-63 on the spreader are substantially within the boundaries 
established by the bearing assemblies 91-93 on the upper cable guide 35. 
This permits the movement of the spreader within limits in all six degrees 
of freedom, with those limits being established by the size of the cargo 
container and the expected sea states and relative motion between the two 
ships. 
Likewise, the design characteristics of the winches must be able to 
accommodate the speed of relative motion between the two ships, and have 
sufficient power to raise and lower the load, especially during the 
transfer operation since it is not expected that any substantial energy, 
other than friction losses, are needed to maintain the cargo in a stable 
position during the inertial phase of the transfer operation. 
For the cargo-handling system to track the motion of the lighter as it sets 
a container down onto its deck, it is assumed that the following sea 
conditions are typical: 
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Heave .+-. 32 in (.+-. 81 cm) 
Surge .+-. 12 in (.+-. 30 cm) 
Sway .+-. 20 in (.+-. 51 cm) 
Pitch .+-. 3 deg (.+-. 0.05 rad) 
Roll .+-. 6 deg (.+-. 0.10 rad) 
Yaw .+-. 1 deg (.+-. 0.02 rad) 
______________________________________ 
These motions of the container must be produced without any of the six 
cables going slack (zero tension). 
Because of the symmetry of the system and the omnidirectional pattern of 
the wave motion, the optimum configuration would have all cable tensions 
equal when the container is stationary. To accomplish this, the attachment 
points 62 and 63 are placed half the distance between attachment point 61 
and the center of gravity (CG), as illustrated in FIG. 6. 
It was also recognized that the upper cable bearings must have a greater 
span than the attachment points on the spreader in order to provide 
lateral and longitudinal forces on the container. This means that the 
cables would have to "fan out" from the sides of the spreader. Therefore, 
the cables would rub the sides of the container stack when retrieving 
low-down, in-stack containers. 
FIG. 8, shows the retrieving of containers situated in an area where the 
cables above the spreader and the upper cable guide are spread out. The 
cables pass through a lower cable guide 155 which is provided with means 
to allow the cables to pass freely therethrough for normal connection with 
the spreader 55. The cable guide 155 normally rests on the top of the 
spreader, and it is provided with projections 156 at each corner which, 
when the spreader enters the container-sized hole in the stack, engage the 
top of adjacent containers in the restricted area 160. The cable guide 
will remain at the top of the stack, as shown in FIG. 8, and will guide 
the cables within the container-sized hole as the spreader 55 is lowered 
and raised. 
As an example of one typical application, the cable drives must be designed 
to accommodate two modes of operation: (1) lifting at a rate of 79 ft/min 
(24 m/min), and (2) tracking the movements of the lighter. Assuming a 
worst case condition of a container weight of 25.739 lb [115 kN]), at a 
20-ft (6.1-m) drop, and a maximum acceleration of 0.1 g, the maximum 
tension is multiplied by 1.1 to obtain the maximum force required in each 
cable. In this worst case condition, where the cables can be as much as 50 
degrees off vertical, then the cable velocity, V, is 79 ft/min/cos 
50.degree. or 2.05 ft/sec or 25 in/sec (63.5 cm/sec). The horsepower 
required for lifting each cable drive is therefore 
(25739.times.1.1.times.2.05)/550 or 106 hp (79 kW) 
For this invention the following data for the cable drives was found to be 
adequate: 
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Peak Cable Power 140 hp (104 kW) 
Average Cable Power 89 hp (66 kW) 
Maximum Force .times. 
170 hp (127 kW) 
Maximum Velocity 
Peak System Power 481 hp (359 kW) 
Average System Power 307 hp (229 kW) 
______________________________________ 
Either hydraulic or electric motors may be used to power the winches 
101-106. 
The functional block diagram of the complete, closed-loop control system 
for the stabilized crane is illustrated in FIG. 9. Beginning at the left 
side, the system can be described as follows. 
The crane operator 120 is in command of the system, using visual feedback 
from the spreader/container 50, 55 and the target lighter 15. Assuming 
that he has picked up the container 50 from the deck of his own ship 10, 
the operator 120 rotates the turntable 45 to swing the container out over 
the water. The mode switch 110 is in the "TRANSPORT" mode. The 
accelerometers or inertial sensors 70 on the spreader 55 feed back signals 
through appropriate shaping networks to modify the operator's command 
signals in order to eliminate pendulous swinging automatically. 
The operator then commands the system to lower the container. His command 
passes through a coordinate converter, which translates it into the 
appropriate commands to each of the six cable drives. The drives respond 
by producing torques proportional to the magnitude of the commands, which 
rotate the cable drums to vary the cable tensions. However, there is a 
significant mount of compliance (stretch) in the cables, so the equivalent 
forces are not applied to the spreader/container until the drive motors 
have rotated enough to relax the compliance of the cables. Then the forces 
applied to the spreader/container lower it as commanded by the crane 
operator. 
When the spreader approaches within about 20 ft (6.1 m) of the target 
lighter, the sensors acquire the target and automatically switch the 
system to the "SET-DOWN" mode. Now the signals from the distance sensors 
are fed back through appropriate networks to modify the operator's command 
signals. The winches run the cables in the direction necessary to 
synchronize the motion of the container with the motion of the moving 
lighter. 
While the form of apparatus herein described constitutes a preferred 
embodiment of this invention, it is to be understood that the invention is 
not limited to this precise form of apparatus and that changes may be made 
therein without departing from the scope of the invention, which is 
defined in the appended claims.