For more than 100 years electric overhead cranes have been a fundamental part of industrial material handling. While crane structures have benefited from several material technology and design method advances, the operator control side has remained almost unchanged. Modern drive technology has made it possible to limit crane acceleration and to offer more speed variation, but control of the crane and the suspended load is still generally the responsibility of a human operator.
Controlling load sway is a difficult task for anyone. It takes a long time to learn to drive a crane efficiently. Skilled operators have a model in their mind of their crane’s load sway – they know, off by heart a few seconds in advance, how to catch load swing caused by moving the crane. When the need for exact load positioning is added to the damping requirement, the task becomes even more complicated. Even the most skilled and experienced operators are susceptible to fatigue from such work and efficient, continuous operating periods are limited to a few hours.
The trend in most industries is to combine the work of the crane operator with that of a shop floor worker and also to control cranes by radio remote control from ground level rather than a cabin. A major benefit is that an operator at floor level tends to have the best visibility of the load. A disadvantage is that crane operating efficiency is in most cases reduced. A worker who operates a crane as a sideline is not as skilled and is not likely to have the same feel for the machine as a dedicated cabin-based operator.
Benefits of load sway control
In addition to smaller load sway angles, benefits of fitting sway control are reduced maintenance costs and shorter cycle times. Smoother operation with less position jogging means less wear and tear on the crane’s mechanical components.
In cabin controlled cranes operators prefer to avoid shaking the crane because it causes discomfort. Without sway control operators allow the load to swing while the crane is moving and then use sharp, momentary control actions to control the load movement when stopping. Swing control systems drive the crane, reduce wear on the rope and drive systems, and put less load on the crane structure. Where an operator uses sharp control motions to catch the swing, the sway control system predicts the load swing and uses accurately timed and sized control motions to damp it.
If the crane is operated by remote control rather than from a cabin, the operator only observes the load movement. That allows the operator to use an aggressive control strategy without feeling uncomfortable. Brakes, motors, gears and hoist components can quickly wear out. This highlights the ability of automated sway control to use optimised actions. For example, instead of an operator inputting four, one second positioning actions, an anti sway system can put in one perfectly timed action.
Load sway control improves cycle times because of faster handling. The best crane operators can drive a crane using practically the fastest possible strategy, as can a computer based load sway control system. The difference is that the computer calculates the control every time to the same accuracy, while operator performance varies. Operating a crane can be repetitive work which soon causes operator fatigue and reduced efficiency. In the same way that electronics have improved conditions for car drivers, automated sway control liberates the crane operator from continuous, ‘mechanical’ load sway damping actions and allows them to concentrate on a higher level of control. An operator can move closer to a destination at full speed, reduce the load spacing time and use both the travel and hoist motions simultaneously, without special concentration. Kimmo Hytönen, managing director of Innocrane, a manufacturer of sway controls, says his customers report cycle times reduced by up to 44% with a sway control system.
Another benefit is that less time is needed to train operators. With automated sway control the operator moves the crane by giving speed references directly to the load. The system then solves the inverse dynamics problem and adjusts the travel speed so that the load follows operator controls. The operator can forget the crane running overhead and concentrate on moving and positioning the load. With a sway control system fitted most workers can use a crane efficiently after only a few hours of training.
Load sway can be harmful to workers as well as to machinery and the handled load. A swinging 1t load has a huge amount of kinetic energy. Loads such as metal coils or concrete elements are easily damaged during warehousing, causing significant loss of profit. But the greatest expense in industrial countries is injuries to personnel.
It might be that most of the time cranes operate satisfactorily, without automated load sway control, but the investment is justified because of that one time in a thousand when it is needed. An analogy is a car with an ABS braking system – most of the time it is not needed, but when it is, perhaps just once a year, the cost of fitting the ABS option is repaid many times over.
Load sway control principles
There are several basic mechanical and electrical methods of controlling load sway. Mechanical systems use either rope or mast designs which do not let the load swing at all. These can be called rigid mechanical systems. Another method is to let the load swing in the main hoist ropes and use a separate system to damp the sway. This is usually a hydraulic system combined with anti sway ropes.
Electrical systems also use two main methods, open loop (or feed forward) and closed loop control. In an open loop design the crane speed control system uses a mathematical model of how the load sway was affected by a previous control motion. This information is used to calculate compensatory speed control actions so the crane reaches its final speed without load swing. The performance of this method depends on the mathematical model and the control strategy used. Methods vary between manufacturers and several have been patented. Since open loop systems only operate satisfactorily as long as the model is accurate, the crane drive dynamic is very important. If crane speed does not follow the reference given by the controller, the output of the mathematical model and the actual load sway are not equal. This can cause the system to lose accuracy, efficiency and reliability.
Electrical systems with load sway angle measurement and closed loop control have advantages over open loop systems. Since the load sway angle is measured, instead of calculated by mathematical model, the system can guarantee its efficiency. The amount of end sway is limited only by the accuracy of the swing angle measurement and the controllability of the travel drive. These systems have been used successfully in demanding applications. Closed loop control is almost standard in harbour container crane applications.
While closed loop systems have advantages, their use also has limitations. Closed loop control operates well when the feedback law is well designed and tuned. The problem with many cranes, however, is that crane behaviour can vary significantly with different loads. That makes it important to tune the system for each load and lifting attachment. Complexity is added to the system and can make an open loop design more attractive.
According to information available it looks likely that the open loop swing control will be the solution for normal industrial overhead cranes and that closed loop control will be used in outside cranes and applications where guaranteed positioning accuracy is needed.
Advantages and disadvantages
Each different type of sway control system has its pros and cons. Rigid mast types have a heavy mast or telescopic structure which is also expensive and needs a lot of maintenance. It usually costs much more than a standard hoist system. In some cases though, such as automated warehousing of steel coils, the investment can be justified. In automated cranes rigid mast or rope systems are very good solutions. Problems are price, complexity and maintenance.
Anti sway rope systems are used successfully for container handling with low hoist heights. Other applications for the well proven design are for handling steel plate and coils. Disadvantages are high price, accelerated rope wear and stress on the crane structure. This design also puts large peak loads on the rope system, crane structure, motor and drives and even the building.
Systems with swing angle feedback are widely used in ship-to-shore container cranes as they can damp load sway caused by wind. There are also applications in automated cranes. Disadvantages are price, the need for complicated sensor systems and reduced flexibility. It has to be tuned for each load and task, and poor tuning can cause more trouble than it solves.
Simple model-based systems without load sway angle measurement can be used in most overhead cranes. They can be easily adapted to different types of load and crane task, and only conventional sensors are needed.
Cranes with mechanical swing damping or rigid suspension load sway control are generally as easy or easier to operate than a crane with electrical sway control. On the other hand, if there is an anti sway rope system but the lifting tool is swinging under the rigid rope arrangement, the result can be worse than an electrical sway control system.
Most cranes are used manually. The load sway control system needs to be easy to use, reliable and have repetitive behaviour. If the control system operates a different way each time, the user cannot learn to use it efficiently. This is a significant advantage of methods using open loop control based on a mathematical model – the same control action always results in the same crane movement.
Designs using swing angle feedback can be difficult to position manually. This is because the system continuously compensates the load sway, and a swinging load is a chaotic system by nature, meaning that the system never responds exactly the same way twice.
Open loop load sway control generally operates very reliably. The main disadvantage is that the system does not see the real load sway, and therefore cannot guarantee the amount of sway. This type of control is ideal for applications where the crane works under the supervision of an operator. If that cannot be done it is possible to install, for example, optical sensors to monitor the angle of the hoist rope. Open loop control is used in demanding applications such as steel plants for ladle and slab handling, metal refineries and concrete element factories.
Electrical load sway control with swing angle feedback is mainly used in very special applications where the high price of swing angle measurement and system tuning are acceptable. Special attention must be paid to the dynamics of lifting tools and the hoist rope arrangement to avoid controller instability. Multiple hook systems also need special attention since the feedback control can easily intensify tensional sway.
Ensuring a profitable investment
The sway control system cannot compensate for poor travel drives or an inadequate crane structure. A modern industrial crane with inverter drives on the travelling motions is a good platform for all swing control systems. Older cranes retrofitted with modern drive systems can also work well. The travel drives must have the capacity to give fast enough acceleration and deceleration ramps.
Speed controlled drive systems are usually used to limit acceleration of the crane bridge and trolley. When the acceleration rates are slower the resultant load sway is smaller. A disadvantage of this is that a slow rate of acceleration extends the crane cycle time and therefore reduces operating efficiency. With ICRAS and other sway control systems the crane can be accelerated using the fastest possible acceleration ramp and the sway is handled by modifying the acceleration speed pattern instead of just limiting the rate.
The hoist rope system must not be used to limit load sway. Normal “straight fall” rope systems with traditional hook blocks work well. If there is a multiple hook system or the loads vary, the supplier of the closed loop swing control system must be able to tune the system to operate in all work situations. If not, an open loop system should be used.
With swing control systems it is relatively easy to reach ±30mm load positioning accuracy with a rope length of 10m or less. Most of the inaccuracy comes from alignment of the crane on its rails and the hoist/rope system. If higher positioning accuracy is needed special attention must be paid to crane rails and drive systems.
Compatibility of the overall control strategy with the swing control system should be determined at an early stage of the project. If the crane is automated the swing control should first be operated in manual mode and thoroughly tested. The automation should then be built on top of that to avoid problems with positioning and tuning. An automated crane’s user interface should be designed for ease of use. Time taken to select the next task or operating function should not be more than one second.
Overhead cranes will remain a basic material handling tool in factories because of their high load carrying capacity, low cost and simple structure. Computer and control technology can turn this old workhorse into a more flexible 21st century material handling tool. In addition, by increasing the number of potential crane applications load sway control can push up crane sales. Kimmo Hytönen is managing director of Innocrane Oy, Finland
