Anti-sway systems

Load sway or swing, on cranes, is basically a pendulum effect where the load moves independently of the crane. This does not always matter whilst the load is in the air, although significant swing may affect the load radius and hence the stability of certain cranes, it becomes of great significance when the crane operator is trying to land the load accurately. A skilled, 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. However successful the operator is in overcoming the swing of the load this will always take time and add significantly to theoretical cycle times. For container handling operations where load positioning must be accurate to a few millimetres, some sources suggest that countering sway can occupy up to 30% of the average move time. In a high speed, high pressure environment such as a process environment, the time spent on countering sway can have a significant effect on the productivity of the port and ship turnaround times.

A swinging load travelling above a factory floor poses a risk to personnel and other equipment. Sway has a detrimental effect on the crane itself by increasing loads on the structure and mechanisms, with the attempts of the operator to counteract sway by ‘jogging’ the controls, causing accelerated wear to both the controls and the crane drive mechanisms. The tendency of a load suspended from an overhead crane to swing also has a detrimental effect on crane productivity, increasing theoretical cycle times by, it is claimed, 100%. It also requires that the load is kept a safe distance from walls, reducing the available capacity of storage areas.

Load sway does not seem to be such an issue with the slewing cranes used in construction such as tower cranes, telescopic jib mobile cranes and strut jib cranes. This is probably due to the fact that they generally carry out individual lifts with a wide variety of loads, rather than cycle duties with repetitive loads.

The physics of sway

A load on the end of a long hoist rope acts like a simple 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 will keep swinging backwards and forwards forever, but in the real world the friction of the air and the internal friction of the rope will dampen the motion which will gradually die away and weight 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, firstly lagging the fulcrum which is moving at a constant speed, and then overtaking due to the pendulum effect. Similarly, 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, which may take a significant time.

This sway can be avoided with well-timed movements of the fulcrum. At the initiation of movement this involves first accelerating the fulcrum to half speed and one-half pendulum period later, accelerating it to full speed. If this is achieved accurately, then the load will be hanging straight down below the fulcrum. This what a skilful crane driver will do instinctively, providing that the crane controls are sufficiently accurate to allow him to do this. The problem with this approach is that it requires a skilful operator with long experience of driving the crane in question and also tends to slow the movement of the crane, increasing cycle times.

Solutions

There are a number of systems on the market which overcome in 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 occurring in the first place, rather than controlling it once it has occurred. Whilst 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 until 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 the computing power of 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 in two ways, 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. On the other hand open loop systems 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.

A diagram showing the effect of an anti-sway system on the swing of a load after bringing the travel motion of a gantry crane to a stop is shown below.

KCI Konecranes in Finland have produced an anti-sway system for RTG cranes which they claim 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. KCI Konecranes claim that the advantage of this system over pure electronic systems is that the reeving of the four auxiliary ropes automatically prevents rotational sway of the load and the winches can be used for fine horizontal movement of the load during final load positioning.

The 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 system 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.

Each type has its own uses. In an outdoor situation, such as container handling, 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, the closed loop system with its feed back sensors and associated higher initial and maintenance costs can be justified. In an indoor location, such as a factory, where external influences are of less significance and cycle times are perhaps not so critical, an open loop system with its lower initial and maintenance costs is more appropriate.

The UK overhead crane manufacture Konecranes UK has recently launched its DynAPilot anti-swing control system for its range of indoor overhead cranes. They claim that load swing is dampened automatically using load height information, integrated with the driver's commands to calculate the optimal acceleration path. This allows bridge and trolley speeds to be increased, even close to building walls, consequently the maximum available space can be used as there is no need to operate the crane load with dedicated safety load swing areas surrounding the operating envelope (See diagram below). DynAPilot works with both manual and automatic modes, allowing operation of all motions simultaneously while dampening load swing and it is claimed will reduce “wear and tear and damage to the crane, its components, the load it is carrying and to the driver”.

Konecranes UK say that the new anti swing system can be fitted to most cranes, although it is easier to incorporate into new cranes. However, when older cranes are being upgraded by the addition of inverter controls it is a simple matter to add the DunAPilot system add on. The cost of adding this system varies from new to older cranes, and depends on the type of controls that are in current use or can be added as an upgrade. It is claimed that in industries where load damage is frequent, due to load swing or error then the payback period can be measured in months.

The System Developers

It appears that a lot of the development work on electronic anti sway systems is being carried out by specialists, rather than the crane manufacturers themselves.

One such small specialist company is CePlus GmbH of Magdeburg in Germany. CePlus specialise in the supply of crane control systems and including anti-sway systems. They provide a range of anti-sway systems, both open loop and closed loop, for overhead, gantry, container, ship to shore and slewing cranes. Their CeSAR maxx and CeSAR slew closed loop systems use a CCTV camera to measure the load movements and provide feedback to the crane’s control system.

Another anti-sway specialist is Innocrane OY of Hyvinkää in Finland. Set in 1992 Innocrane have developed their ICRAS system, which is available in both open and closed loop forms. Innocrane supplies complete anti-sway systems for fitting to existing cranes and their systems have been installed on P & H, Morris, Konecranes, Demag, Stahl, and Street Cranes among others. It has also licensed its designs to crane manufacturers as part of their OEM control systems such as Street Cranes (X-Y Zero), Konecranes (DynAPilot), SEOHO and Ansaldo.

Future Developments

State of the art electronic closed loop anti-sway systems are firmly established in the port crane market, where the requirement for high productivity coupled with the relatively low cost of the systems in relation to the overall cost of a port crane has driven the development and fitment of such systems. Lower cost open loop systems are starting to be offered on overhead cranes for indoor use, although the take up is slower and depends on minimising the additional costs attributable to the systems. Whilst the basic concept of both open and closed loop systems may well remain unchanged, both the software and hardware is improving all the time, bringing lower costs, improved reliability and better performance. An example of this is Moeller Electric’s Limited use of their Fuzzy Logic PLC to provide anti-sway control on an overhead crane. They claim that the use of “fuzzy logic” simplifies and speeds up the development of the mathematical model that drives the anti-sway system, enabling rapid tailoring of a system to individual cranes.