Shipyard Giants
September 3, 2002 By: Seungnam Kim, Richard Langley, Donghyun Kim GPS WorldHigh-Precision Crane Guidance
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Giant cranes moving in busy container yards require precise positioning to operate efficiently and safely.
To accomplish this, the University of New Brunswick has developed ultra high-precision GPS RTK software that works in conjunction with dual-frequency GPS receivers and wireless data modems installed on the cranes. The software monitors a crane's deviations from its tracks and feeds the data to the crane's auto-steering system.
Metal banging, engines roaring, machines whizzing, and sirens wailing - these are the sounds that fill the air at Korea International Terminals' Kwangyang Port, a busy container terminal. Constantly moving above this hubbub are gigantic cranes - rail-mounted quayside cranes for loading/unloading containers from ships and rubber-tired gantry cranes (RTGCs) for stacking/unstacking containers in the container yard.
 A rubber-tired gantry crane (RTGC) moving along its track. |
The movement of these cranes is carefully choreographed by the crane control system - a key component of the port's management system. The control system was developed to improve container-handling productivity and operational safety. It comprises the anti-sway system, which helps operators accurately position a crane's "spreader" to grab containers; the position detection system, used to identify and cross-check the positions of stacked/ unstacked containers; and the auto-steering system, which keeps the wheels of an RTGC moving along a track - either a painted line or an electrical guide wire - and prevents it from hitting containers or other cranes in the tightly packed yard.
For that purpose the auto-steering system must consistently identify the line mark and calculate the corresponding deviations of the RTGC's front and rear wheels. The most efficient and reliable way to accomplish this is by using real-time kinematic (RTK) GPS technology.
 An RTGC unloading a truck at Korea International Terminals' Kwangyang Port |
RTGC Auto-steering Technologies
In an automated system, a programmable logic controller (PLC) is usually the central part of a process control system which comprises a group of electronic devices and equipment. With execution of a program stored in program memory, the PLC continuously monitors the status of the system through signals from input devices. Based on the logic implemented in the program, the PLC determines which actions the output devices need to execute.
In an RTGC auto-steering system, the calculated deviations of the front and rear wheels are fed into the PLC so that it can adjust the speed of the left and right wheels to keep the crane on track. (Operators turn the crane's wheels only to make ninety degree changes to its direction of movement and only when the crane is stationary at special low-friction turning pads.)
 Two GPS antennas and a wireless LAN antenna (far left) on an RTGC |
Conventional Approaches. Several technologies for identifying the line mark - such as the induction-loop, transponders, and charge-coupled device (CCD) cameras - have been adopted for RTGC auto-steering systems. Although these technologies have been employed successfully, there is a growing concern that they may not provide the greatest possible system reliability and economic efficiency. Induction-loop and transponder systems have a limited effective range of about 10 centimeters. If a crane exceeds this range for some reason, there is no way to get it back on track easily. Furthermore, these systems require frequent maintenance. CCD systems are highly dependent on environmental factors (such as surface reflection and line mark condition) which cannot be overcome completely by the system's hardware and software. Also, CCD systems suffer the same limited range problem as the induction-loop and transponder systems and both the CCD hardware subsystem and the line marks require continuous maintenance to guarantee the performance of the auto-steering system.
GPS-Based Approach. An auto-steering control system which is independent of environmental factors requires a technology not based on physical line marks in the container yard. This can be accomplished by an electronic map with virtual lines and a GPS receiver to precisely locate an RTGC on the map. The control system can then compare the crane's position as reported by the GPS receiver with the virtual lines and steer the crane accordingly.
 RTGC tracking a line mark. The length of an RTGC's typical run is about 500 meters and its frame spans about 23 meters. |
GPS Auto-steering
The GPS-based auto-steering system recently installed at Kwangyang Port consists of three major components: GPS hardware (dual-frequency receivers and antennas), RTK processors (industrial panel PCs and RTK software), and a 2.4 GHz wireless local area network (LAN) base unit, access point, and station adapters. It includes one GPS hardware unit for the base station and two remote units on each crane. It also includes one RTK processor unit for the base station and one for each crane. The base unit of the wireless LAN is installed at the base station while each crane has an individual station adapter. To improve the performance of wireless LAN communication, if necessary, additonal access points can be installed in the container yard.
A fully operational and safe RTGC auto-steering system requires GPS RTK software with high levels of accuracy, integrity, continuity, availability, and computational efficiency.
Accuracy. For this application, the horizontal accuracy requirement of GPS positioning solutions is 1.5 centimeters at a 95 percent confidence level. This enables the integration of GPS with the auto-steering control system and is almost the highest real-time accuracy level currently attainable from GPS. While this accuracy level is generally achievable in short-baseline and static applications, it is very challenging to achieve it in kinematic mode due to the dynamics of a moving platform and the problem of multipath (produced by the crane itself, and any lighting towers or other cranes in the vicinity). Therefore, in order to attain the required accuracy we had to devise a robust quality control scheme.
 GPS receiver (right) at base station |
Integrity and Continuity. For the system to be safe, the GPS RTK software must include a self-diagnosis routine able to detect failures when the positioning accuracy degrades beyond what could be expected from the GPS observations the system is using. For that purpose, two parameters - integrity and continuity - should be considered rigorously. The risk associated with equipment latency or design failure is specified by an integrity parameter while the risk associated with unscheduled function interruptions is specified by a continuity parameter.
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