Anti-collision systems have been developed in the tower crane industry over the last 20 years. The concepts are based on detecting a potential collision and then taking the necessary action to avoid it. Collision is one of the greatest risks to the safe operation of lifting equipment, behind only collapse, power line contact or a load fall. Different types of collision are between crane and environment (or obstacle); between load and environment; or on a multi-machine site between cranes or between load and crane.
Collisions of types one and two concern individual cranes. Relevant systems are usually referred to as Zoning or Work Area Limitation systems (ArbeitsBereichsBegrenzung in German).
These problems can be, and often are, dealt with directly by the crane manufacturers, particularly on modern cranes equipped with sufficient onboard computing capacity (PLCs).
For older crane designs that use traditional technology, or where manufacturers have not developed their own anti-collision solution, a retrofit or OEM type system from an external supplier can be used.
Type three collisions, on the other hand, concern the combined operation of several machines moving at the same time. In this situation two or more pieces of hardware moving under totally independent, man-operated, control must come to a stop at a safe distance from each other. To do this, factors to be considered are coherence, simultaneity and interactivity of operation of the parts of different machines that may collide.
Technically, substantial differences have to be taken into account:
• geometry – horizontal jib or luffing tower cranes, harbour cranes, mobiles, EOTs, gantries, portal types, and even concrete pumps, can find themselves facing each other in dangerous configurations
• the technology of the available sensing accessories and machine commands (relay or PLC driven) varies
• any age difference between the machines has important consequences
• the machines may be produced by different manufacturers so any action taken involves interaction between them
Compatability
Apart from the question of mixing more than one crane manufacturer in the same operation, the owners may be completely independent, either cooperating or simply on adjacent construction sites, or there might be different rental/ownership structures and consequential human communication problems.
Experience has shown that compatibility (in terms of coherent interactive operation of two unrelated pieces of equipment) and ultimate responsibility for the site’s operation as a whole are integral parts of the problem to solve.
Where different types of cranes are being used, with different owners, events need to be validated as they occur, using any manufacturer’s black-box functions, time-stamped recordings, selective retrieval of happenings, databases, etc. Construction of such databases as well as their accessibility, whether locally at a site’s management centre or over the internet, have become an integral part of anti-collision know-how.
A final point is that the risk only arises when there is a misjudgement, mistake or other failure on the part of the driver. Like ABS braking for road vehicles, anti-collision systems are not meant to be full scale security systems, rather just a driver’s support system that assists with collision avoidance.
To illustrate the above, Liebherr has developed its own work area limitation device (called ABB) for all of its PLC and some of its relay (contactor) driven tower cranes. For all other cranes SMIE systems are proposed for the zoning function, as well as for anti-collision needs between cranes worldwide.
Market
The market for anti-collision systems has grown in recent years. A mandatory regulation came into force in France in 1987 and demand has spread worldwide, growing particularly strongly in the last couple of years. Approaches have varied, largely due to cultural factors.
• As previously stated, a mandatory regulation in France requires an electronic device to assist the driver’s action, both for work area limitation and anti-collision. The market for these systems has therefore flourished
• In Singapore hard-set operational limits are imposed. Cranes are not allowed to overlap and physical limit-switches are used to ensure compliance. These systems are popular as they are a productive solution for the user
• Germany has only occasional requirement for zoning or work area limitation. Crane manufacturers have developed their own systems, for example, ABB from Liebherr and Wolff’s CC90
• In the UK, where demand is developing strongly, there is no special regulatory requirement but crane users choose to use anti-collision systems to increase safety and enhance liability protection.
• In Japan individual construction companies have developed several systems because they wish to present a better safety image.
A common factor, however, is that all over ther world ‘near misses’ are a too regular occurrence. This may explain the lack of resistance to invest, even in countries without mandatory regulations.
Another important characteristic of the market is the demand for rental solutions, due to the fact that construction project times have decreased – 18 months is now an unusually long project. This has affected both the type of products developed and the nature of the services surrounding them: compatibility (or inter-crane coherence); universality; portability; reliability; communication; ease of installation; and local support.
New regulations and increased safety awareness in other industries, where lifting is central to operations, mean that the know-how already acquired in the tower crane domain is being transferred to other applications where lifting equipment interacts.
Solutions
My approach to anti-collision has been as follows:
• to calculate in real-time the exact geometric configuration of the moving elements at risk, including their individual dynamic parameters, i.e. their speeds and braking capacities
• from this is derived the capacity of each moving part to come to a stop before colliding with any other moving part of the given configuration
• as long as no such risk is detected (hopefully, almost all of the time) nothing special happens
• where such a risk is detected, however, all dangerous movements are slowed down or cut and the driver may receive the corresponding information so that he will know the reason for his machine’s behaviour. Risk-free movements remain unobstructed
• when the risk has disappeared, all movements become unobstructed.
In other words the system acts as a background watchdog. While no risks are being detected the operator freely drives his machine, without the system interfering. As soon as a driver’s handling of one machine, in combination with that of a neighbouring machine, creates a risk of collision, the system takes over and brakes the machine to avoid collision.
Systems
More practically the basic composition is as follows:
• the positions and speeds of all moving elements are measured, in real-time, by sensors with sufficient accuracy and repeatability (potentiometers, encoders, impulse counting, laser, and soon, GPS/GNSS, etc.)
• a computer on each machine, programmed with the site’s configuration and the machine’s characteristics, particularly its braking capacity on a teach-in approach, calculates the risks and sends the braking orders to the machine itself
• a communication system, either by cable if possible, or by a wireless link ensures safe transfer of the necessary information between the individual computing units
• the machine command receives the issued orders, either by direct relay interfacing or by another information exchange process (serial or other)
• the driver receives the necessary information for his understanding of the proceedings
• Additionally, a dedicated central computer system is connected (by cable or radio) to the anti-collision systems network and records all pertinent events, for example, each crane’s operation start or end, each system’s operation start or end, system overrides and faults.
Results on the database can be accessed and queried, either locally or over the internet, to provide users, and any overseeing body, for example, management or safety authorities, with real-time information on the operational state of the whole site and historical events.
Due to market pressures, particularly the demand for rental services, the systems were designed with the following aspects as a high priority:
• compatibility: the system must facilitate interactive operation of cranes from all manufacturers
• universality: the system must be suitable for cranes with all types of geometry, command type and generation (technological or other)
• portability: the system must be moveable from one crane to another with minimum alteration
• communication: flawless communication that is easy to install and maintain depends on reliable wireless solutions
• service: quick and efficient installation, ease of support and repair by telephone, fax or internet.
In summary, anti-collision is a function that goes beyond the individual machine to cover the needs of a whole site. Responsibility usually has to be brought under one neutral external entity, other than the usual lifting machine suppliers (manufacturers, rental entities or even owners). Experience acquired over the years and around the world points in that direction.
Perspectives
Transferring the construction site experience to the more general industrial environment will need a number of overall adaptations other than those pertaining to the different cultures of the markets themselves. Installations are permanent rather than temporary and, as users are often ready to acquire the necessary know-how and assume overall responsibility there are different servicing needs and a different approach is needed. Specific configuration needs are much more common as is the need to consider one-of-a-kind machines, and to make maximum use of existing sensors.
There is increasing demand for recording a wide range of information useful for, among other things, operations control and management, safety control and management, maintenance (preventive or general), and machine management for the owners (rental company or user), for example, to determine a crane’s age and how much use it has had.
More interactivity, locally or over the internet, is necessary for site configuration adjustments, maintenance and for function upgrades. Among many other factors that will appear there will be more sensitive, or just more complicated, environments for wireless data communications and transmission between machines and with the management centre.
From a technical perspective examples of situations being taken into account include:
• full 3D handling of machine and load movements
• the various types of movements and their combinations such as slewing, trolleying, travelling, telescoping, luffing and turning hooks
• mobile machines introduced temporarily and included automatically or semi-automatically on a site
• prohibited zones (work area limitations) and mandatory or restricted work zones
• special operation zones activated by conditional situation logic
• control of tandem lifting
• graphical real-time display of the load environment or video images
• centralised, selective override control
• a variety of event recordings from any type of sensors or any type of machine
• improved central management tools to make use of recorded information.
