Anti-sway control of a crane under operator's command

Details Anti-sway control of a crane under operator's command Anti-sway control of a crane under operator's command

2001-03-05

2003-07-08

Brahan, Thomas J. (Department: 3652)

Traversing hoists

Having means to prevent or dampen load oscillations

Antisway

C212S270000

Reexamination Certificate

active

06588610

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to systems and methods for controlling cable suspended, payload transfer systems. More particularly, this invention relates to anti-sway control systems and methods for a payload undergoing both horizontal trolley and vertical hoisting motions.
BACKGROUND OF THE INVENTION
Gantry-style cranes are used extensively for the transfer of containers in port operation. Typically, a crane has two inputs in the form of velocity commands. These two velocity commands independently control horizontal trolley and vertical hoisting motions of a payload. Undesirable swaying of a payload at the end of the transfer is one difficulty in accomplishing a transfer movement. Loading or unloading operations cannot be accomplished when a payload is swaying. Presently, only an experienced operator can efficiently bring the container to a swing-free stop. Other operators must wait for the sway to stop. Typically, the time spent waiting for the sway to stop, or the various maneuvers to fine position the load, can take up to one-third of the total transfer time.
Various prior art patents teach sway reduction systems. These patents relate to different aspects of payload transfer with reduced sway. For example, several patents describe operation in autonomous mode where system uses the starting and ending positions of the payload to generate the necessary control signals to achieve the payload transfer. Other non-autonomous systems attempt to minimize the amount of payload sway while following the operator's commands for horizontal trolley and vertical hoisting motions.
Autonomous systems are suitable for structured environments where positions of a payload are well identified, In a typical port environment, a container's position depends on the relative positioning of the ship relative to the crane. Therefore, the position of the container is rarely precisely known. In such an environment, a non-autonomous mode of operation is preferred. The present invention relates to such non-autonomous systems.
Several references disclose non-autonomous modes of operation. Many of these references use a fixed-length pendulum model as the basis for their sway reduction method and/or system. Consequently, these strategies do not eliminate sway when the cable length changes during horizontal motion. Several other references handle the effect of changing vertical cable length by using approximations. The present invention uses the full dynamical equation of a crane system without approximation in order to avoid error and to eliminate sway. In particular, the present invention uses cancellation acceleration for sway control. The computation of a cancellation signal is exact as it is based on the full dynamical equation of the crane model. This is particularly significant during simultaneous trolley and hoist motions. For the ease of discussion, the angle of sway of the load and the velocity of sway of the load are shown as &thgr; and {dot over (&thgr;)}, respectively, and the acceleration of the trolley is referred to as {umlaut over (&khgr;)}. All control systems use the horizontal acceleration of the trolley as the control for sway. Hence, horizontal acceleration is also termed the control.
There are two general approaches for sway minimization. In first approach, the trolley acceleration is given in the form {umlaut over (x)}=r+k
1
&thgr;+k
2
{dot over (&thgr;)} or something similar. Here, the value r is a time function that depends on the desired motion of the trolley. The use of this approach introduces additional damping into the system to control sway. The resultant system can be made to have any desirable damping ratio and natural frequency using the appropriate values of k
1
and k
2
.
Several references adopt this first approach. These references differ in the profile of the motion dependent time function, r, and the specific procedure by which values of the damping ratios, k
1
and k
2
, are determined. In the U.S. Pat. No. 5,443,566 to Rushmer, sway angle and sway angle velocity are estimated using a fixed-length cable model of the crane. Estimates of the sway angle, &thgr;, and the sway angle velocity, {dot over (&thgr;)}, are used together with the input velocity demand from the operator, {dot over (x)}
d
, to compute the control signal {umlaut over (x)}=k
1
({dot over (x)}
d
−{dot over (x)})+k
2
&thgr;+k
3
{dot over (&thgr;)}. In U.S. Pat. No. 5,490,601 to Heissat et al., the control signal is {umlaut over (x)}=k
1
&thgr;+k
2
{dot over (&thgr;)}+k
3
(x
d
−x). Sets of k
1
, k
2
, and k
3
are determined experimentally at various lengths of the cable. The exact values of k
1
, k
2
, and k
3
for a particular cable length are interpolated from these experimental sets using gain scheduling, or some form of fuzzy or neural network control. In U.S. Pat. No. 5,878,896 to Eudier et al., the speed demand send to the trolley is of the form {dot over (x)}
d
=k
1
&thgr;+k
2
{dot over (&thgr;)}+k
3
(x
d
−x) where x
d
is the desired position of the trolley. The values of k
1
, k
2
, and k
3
are determined experimentally.
This first approach can effectively damp out sway. The approach is based on standard mechanism of feedback and is therefore robust against model inaccuracies. The main disadvantage of this approach is its lack of intuitive control by the operator. As the trolley acceleration depends on &thgr;, {dot over (&thgr;)} and the operator's desired velocity, the motion of the trolley can be unpredictable and counter-intuitive to the operator. As a result, several manuevers may be needed to bring the system to a proper stop. As such, this first approach is suitable for an unmanned crane in a structured environment where payload position is well identified.
A second approach is based on the principle of sway cancellation. This is the mechanism used by most human operators for sway damping. The basic idea of this approach for a fixed-length pendulum is described in
Feedback Control Systems
, McGraw-Hill, New York, 1958, by O. J. Smith. In a fixed-length pendulum, the sway motion is a nearly sinusoidal time function with a frequency &ohgr;, defined by &ohgr;={square root over (g/l)}. Suppose that a short pulse of horizontal acceleration is applied at time t=0, this pulse will induce a sway oscillation of frequency &ohgr;. It is possible to cancel this oscillation using a second short pulse of the same magnitude and duration applied at time t=&pgr;/&ohgr;. After the application of the second pulse, the system will have no sway for the time thereafter. This method, known as double-pulse control or cancellation control, gives the shortest possible settling time for a constant length cable. While this method is readily applicable to a fixed-length pendulum, extensions to pendulums with varying cable length extension are not easy.
Several references teach the general approach of cancellation control. In U.S. Pat. No. 4,756,432 to Kawashima et al., it appears that double-pulse control is used in both the acceleration and deceleration phases of the trolley motion. For a specified final trolley location, the timing and magnitude of these pulses are computed based on a fixed-length pendulum. One double-pulse is used in each of the acceleration and deceleration phases. In between these two phases, the trolley travels at constant velocity and does not sway. In order for this method to work, the operator must provide the final position of the trolley to accurately determine the timing and magnitude of the pulses. This system works reasonably well when the cable length is constant during horizontal motion.
In U.S. Pat. No. 5,219,420 to Kiiski et al., it appears that the sway angle is measured and a best fit sinusoidal time function is made of the sway motion. With this estimated sinusoidal function, a cancellation pulse is generated to eliminate sway. The method assumes the presence of only one sinusoidal frequency. As such, the method is not effective for system

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