Loading and unloading of road trailer trucks transporting containers is an important part of the operations in nearly all container terminals. As a function, the unloading of trailer trucks does not differ much from a situation where the container is picked up in the container storage area, as in both cases an unladen spreader is lowered on top of the container, after which the container is locked to the spreader by means of twistlocks.
In automatic container crane systems, the container can be automatically lifted from a trailer truck for example by means of measuring systems based on laser scanners. Systems have also been developed for loading the containers on a trailer automatically. An absolutely essential requirement for these systems is, however, that the desired position of the container can be accurately measured. However, accurate measurement requires reference points that can be identified. In most terminals, unloading and/or loading of trailer trucks is, nevertheless, a manual procedure due to security regulations.
The loading of trailer trucks is considerably more challenging than the unloading. The trailer in road trailer trucks transporting containers is usually provided with twistlocks, by means of which it is ensured that the container stays in place during the transportation. Lowering the container with sufficient accuracy in such a way as to align corner castings of the lowered container sufficiently accurately with the twistlocks requires accuracy. The visibility of the crane operator is disturbed by the container hanging from the spreader and blocking the twistlocks in the truck at the final stage of the lowering process.
Handling of the trailers is facilitated by a variety of systems. For example, there are systems where the driver of a truck is assisted in stopping relative to the crane by means of a measuring system based on a laser scanner in such a way that the main movements of the crane would not have to be operated. By the main movements is meant the movement of the entire gantry or trolley.
An example of state of the art solutions representing laser scanner technology is patent publication CN201161875Y disclosing a system for determining locations of a spreader in a container crane and of a trailer truck to be loaded or unloaded, and particularly of twistlocks on a bed thereof (or of corner castings in a container) relative to the crane. Information produced by laser scanners in the system also allows one to conclude the type of a truck to be loaded and/or of a container to be lifted out from a truck.
Another example of solutions utilizing the laser scanner technology is disclosed in patent publication CN1884034A relating to a system for determining locations of a trailer truck loaded or unloaded by a container crane and of the containers on a bed thereof relative to the crane. Information produced by laser scanners in the system also allows one to conclude the type of a truck to be loaded and/or of a container to be lifted out from a truck.
A third example of solutions representing this technology is patent publication JP2005239343A relating to a system for determining locations of trailer trucks to be loaded or unloaded relative to a crane handling containers. The system uses laser scanners disposed on the frame of the crane.
A container can be automatically lifted from a trailer truck for example by means of the measuring systems based on laser scanners.
There are also systems based on cameras, wherein the aim is to recognize the desired position of the container of twistlocks in the trailer utilizing machine vision. One such solution is disclosed in patent publication DE3606363A1 relating to a system for determining the location of a trailer truck to be loaded or unloaded relative to a crane handling containers. The system uses cameras disposed on the frame of the crane at a height of a bed in the trailer truck. A problem of the camera-based machine vision systems is moderate reliability due to environmental conditions. In particular, wrong interpretations are caused by lighting that changes. When directing a spreader of a crane based on information from a measuring system, reliability is extremely important. False measurement data may cause material damages.
Yet another example of known state of the art is patent publication EP1337454A1, wherein a system for determining the location of a trailer truck to be loaded or unloaded relative to a crane handling containers is disclosed. The system uses sensors disposed on the frame of the crane at a height of a bed in the trailer truck.
However, not all trailers have measurable reference points, such as twistlocks; instead, cargo is fastened to the trailer e.g. by tie down straps or chains. In addition, the twistlocks in some trailers are not exposed until points for the fastening of a container are subjected to the weight of the container.