Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology / Logistic Engineering
M.J. Terstegge Deadlocks in AGV-systemen.
Literature survey, Report 98.3.LT.5081, Transport Technology, Logistic Engineering.
On terrains where Automated Guided Vehicles (AGVS) operate, claim areas are often used. These are areas in which only one AGV is allowed at a time, ment to prevent AGVs from colliding with each other. The use of these claim areas may lead to deadlock. This is the phenomenon that occurs when each AGV in a set of AGVs is waiting for a claim area that is held by another AGV in the set. This chain of waiting vehicles cannot be broken without extemal intervention, and causes a great loss of time.
In literature a lot of methods are known to solve deadlock or keep deadlocks from occuring. These methods can be divided into three categories. Deadlock detection is a strategy in which no effort is made to prevent deadlocks from happening. Instead, the detection algorithm tries to detect deadlocks after they have appeared. After a deadlock is detected, a deadlock recovery mechanism brings the system back into a deadlock free state. This method works well in systems where deadlocks don't occur often, and where the costs of these are acceptable. Deadlock detection algorithms are also used as a support tool for deadlock avoidance.
Deadlock prevention and deadlock avoidance both aim to keep the system deadlock free, but this is achieved in different ways. Deadlock prevention is a category of methods that make sure that one of the necessary conditions for a deadlock to occur can never hold. Deadlock avoidance consists of a group of algorithms that, using the knowledge that is available about the routes to be covered, avoid the forming of a deadlock in a dynamic manner. The well-known banker's algorithm is an example of this. This is one of the best methods to keep AGV systems free from deadlocks.
An important tool that can be used for examining, deadlocks is the resource-allocation graph. This graph shows schematically to which AGVs occupied claim areas are granted, and which AGVs are waiting for certain claim areas. Many of the algorithms for deadlock detection, prevention and avoidance make use of this graph.
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