Swing low

19 January 2005

There is considerable choice when it comes to specifying anti-sway systems, as Tim Watson explains

Load sway or swing on cranes is a pendulum effect where the load moves independently of the crane. This does not always matter while the load is in the air (although a big swing may affect the load radius and hence the stability of certain cranes). However, it becomes crucially important when the crane operator is trying to land the load accurately.

An experienced crane operator can counter sway by driving into the swing to dampen it. The extent to which this can be done will, however, depend not only on the skill of the operator, but also the response of the crane control system.

Besides, however successful the operator is in overcoming the swing of the load, this will always take time and add to theoretical cycle times.

So how does sway occur? A load on the end of a long hoist rope acts rather like a pendulum. With a pendulum, once the bob or weight is offset from the vertical it will swing back to a point the same distance on the other side of the vertical. In a frictionless environment, it would keep swinging backwards and forwards forever, but in the real world the friction of the air and the internal friction of the rope dampens the motion which will gradually die away. The weight will then come to rest vertically below the suspension point.

The length of time that the weight takes to swing through one cycle from its initial position and back again is called the ‘pendulum period’. This depends on the length of the pendulum, and is independent of the mass of the load. In the case of a crane, it is generally the fulcrum or point of suspension of the load that moves rather than the load.

If we imagine an overhead crane moving a load down the length of a workshop, when the hoist unit moves and accelerates the load on the hoist rope the load will lag the point of suspension or fulcrum and start to swing as a pendulum. The load will then move down the workshop in a series of jerky movements, first lagging the fulcrum – which is moving at a constant speed – and then overtaking it.

When the load is decelerated and the fulcrum stopped above the desired landing point, the load will continue to move forward to end of its swing and continue to swing backwards and forwards until friction causes it to come to rest.

There are a number of systems that overcome, to varying degrees, the problem of load sway on cranes.

Those that have been around the longest are mechanical solutions using either a special hoist reeving technique and/or by a load block retaining scheme. These aim to prevent sway from occurring in the first place rather than controlling it once it has occurred. Effective mechanical solutions are expensive both to purchase and maintain.

Container or rubber tired gantry cranes commonly use hydraulic sway damping systems (such as those produced by the Rima Group in Italy). In these systems, hydraulic circuits are used to dissipate the sway energy of the load by acting on inclined auxiliary ropes acing on the container lifting spreaders. The control of the forces on these ropes is generally achieved by hydraulic cylinders or winches.

These hydraulic sway damping systems are passive systems which damp out existing sway, but do not prevent it from starting. They have the additional disadvantage that when a travel motion is stopped, the crane operator has to wait a short time before the sway motion stops and he can lower the load.

As with mechanical systems, hydraulic systems have a significant maintenance requirement and hence cost.

With the advent of variable frequency drives and PLC control, it has recently become easier to use microprocessors to beat the load sway problem, and there are a number of systems on the market.

All ‘electronic’ anti-sway systems work by controlling the acceleration and deceleration of the crane motion drives without the additional ropes or mechanisms of mechanical and hydraulic systems.

A set of mathematical rules or algorithms modify the speed of a crane motion so that any sway is prevented from starting or is quickly damped out. These algorithms are used either in a closed loop system or an open loop system.

In a closed loop or feed back system, information about the position and velocity of the load is measured and compared with the desired values, and adjustments made.

Open loop systems, on the other hand, rely on making adjustments to the speed of the crane motions based on a mathematical model which approximates the response of the crane and its controls. These systems cannot deal with sway due to wind or impact on the load, and are therefore generally used on indoor overhead cranes.

KCI Konecranes in Finland has produced an anti-sway system for RTG cranes which, it claims, combines the best of the electronic and hydraulic systems in that it uses four ac motor driven winches controlling four auxiliary ropes.

The auxiliary ropes are reeved from the main hoist rope drum down to the head block, and then back up to the auxiliary winches on the trolley. These ropes are inclined in both trolley and gantry travel directions, making sway prevention effective in both directions.

Each winch is controlled independently by the cranes PLC system using algorithms to adjust the forces on the auxiliary ropes so that the sway of the load is prevented.

Swedish Engineering giant ABB produces two crane anti-sway systems. One is a closed loop system for use on container cranes which, by measuring the parameters of the movement of the load, can dampen not just the pendulum swing from moving the load, but also sway from external influences like wind load. The anti sway system control is incorporated in the main PLC control of the crane.

The other is an open loop system which can prevent and dampen load movement induced sway, but cannot compensate for external influences as there is no feedback of actual load position. It is therefore intended for indoor use such as overhead cranes in factories. The anti-sway system modifies the crane operators command to the crane motion drives so that acceleration or deceleration will not induce load swing.

This is achieved using a mathematical model which predicts the behaviour of the load on the hoist rope as a pendulum and calculates the appropriate acceleration at any instant to avoid sway. The parameters of the pendulum at any time are estimated by the system using the hoist drum position and the load characteristics.

This neatly illustrates the point that, in choosing an electronic anti-sway system, it is very much a case of ‘horses for courses’. The closed loop system can be justified outdoors (such as in container handling or ship unloading applications), where the load can be acted upon by external forces such as the wind or impact with other loads, and cycle times must be kept to a minimum.

In an indoor location, such as a factory, where external influences are less significant and cycle times are perhaps not so critical, an open loop system with its lower initial and maintenance costs is more appropriate.