A vast amount of freight is shipped around the world in standard shipping containers. At each point of transfer from one transport means to another, for example in ports and harbours, there is a tremendous number of containers that must be unloaded, transferred to a temporary stack, and later loaded on to another ship or to another form of transport. To achieve the shortest unloading and loading times container handling equipment has to be partly and/or preferably completely automated in normal operation.
The technical demands of handling containers are great. The tare weight of containers is usually consistent, but the gross weight varies considerably. The width of shipping containers is standardised at 8 ft, but the height varies between from 8 and 9.5 ft. The most common standard lengths are 20 ft and 40 ft long. The 40 ft container is very common today and even longer containers up to 53 ft long are also in use.
A container may be handled by a crane, a crane moving on rails, a self-propelled container handling apparatus, or a lift or winch of any type all of which are referred to herein as a crane. Each crane has a lifting device usually incorporating a spreader of some kind that directly contacts a container, to grip it, lift it, lower it and release it. In this description the term spreader is used to denote a part of a lifting device that is in direct contact with a container. Spreaders are normally designed to handle more than one size of container, typically 20–40 ft or 20–40–45 ft long containers. Cranes and other automatic container handling apparatuses and systems are used to speed up the process of loading and unloading containers transferred from one point to another.
The point of transfer of a container between a crane and a truck can be a source of delay. Usually a crane is positioned over one or more parallel loading/unloading lanes on which a truck or other vehicle, hereafter called a truck chassis, stands to receive a load from or unload a container to the crane. The crane is usually arranged such that it has limited freedom to move the spreader in the direction parallel to the long axis of the loading/unloading lanes and it is important that the truck driver positions the truck chassis accurately under the crane. Inaccurate positioning of the truck chassis leads to delay in the process of transferring containers.
U.S. Pat. No. 5,142,658 describes a container chassis positioning system. The system uses a video camera to capture images of a truck chassis which are then compared by a computer to stored templates corresponding to known parts of known types of truck. The images are processed as gray scale images and matched using methods of statistical probability to determine a close match. A relative position of the truck and the container is determined and the exact stopping point is signaled to the driver of the truck. The signaling is carried out by means of a number of lamps in a system that light up progressively to indicate as the truck has drives up to the correct position.
However, processing of video images and gray scale images require. substantial computer processing power to process such images quickly. Moreover it can be difficult in practice to recognize a truck chassis using video imaging based on graphic images such as gray scale. The color intensity of chassis parts photographed against a background of asphalt varies according to degree of sunlight and shade, color of truck chassis, and color contrast between them. It can be difficult, especially in poor light or adverse weather conditions, to recognize a truck chassis using video based images of that type without resorting in addition to fixing lamps or highly reflective signs or markers to the chassis to be obtain a fast and certain recognition.