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vanbeek:thesis [Wednesday, 19 March 2008 : 15:56:32] (current)
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 +===== Exception Handling in Control Systems =====
 +//D.A. van Beek//
 +This thesis deals with the required concepts and
 +mechanisms for exception handling in control systems.
 +There is much confusion in the literature about
 +the meaning of exceptions and the relationship of exceptions and
 +errors. In this thesis, the most important terms relating to
 +errors and exceptions are accurately defined, while retaining a
 +high level of compatibility with the way these terms are used in
 +the literature.
 +A treatment is presented of the most important
 +concepts relating to the three stages of error handling: error
 +detection, error diagnosis and confinement,​ and error recovery
 +and fault repair. Only forward error recovery is covered in this
 +Several satisfactory and more or less similar
 +exception handling mechanisms exist for the handling of internal
 +exceptions. These mechanisms and a traditional mechanism are
 +briefly evaluated. The resume response from an exception handler
 +is rejected as being undesirable in both single and multi-process
 +A literature search has yielded several proposals
 +and existing mechanisms for exception handling in controlling
 +systems or, more generally, in a multi-process environment. No
 +publications,​ however, have been found which describe the essence
 +of the required functionality of such mechanisms. Therefore,
 +'​constraint of an operation'​ and '​constraint violation'​ have been
 +introduced as new concepts. The constraint of an operation is
 +that part of its precondition which is invariant over the
 +operation: it has to be valid throughout the execution of the
 +operation. A violation of an operation'​s constraint causes an
 +exception occurrence in the process executing the operation and
 +should result in the raising of an exception.
 +The concepts constraint and constraint violation
 +have been used to describe the required functionality of
 +mechanisms for the handling of exceptions in controlling systems.
 +Several existing and proposed mechanisms have been evaluated
 +using this functionality. The mechanisms have been evaluated as
 +either offering a functionality which is too restricted for
 +controlling systems, as offering an incorrect or undesirable
 +functionality,​ or as inadequate in other ways.
 +A new mechanism for the handling of constraint
 +violations has been introduced. The mechanism has been realized
 +by means of constraint monitors which are used to specify and
 +monitor constraints of operations independently of other
 +operations, which is an important requirement for the creation of
 +modular subprograms. A constraint monitor bound to a single
 +operation can also be used to specify a constraint which is
 +common to several operations.
 +A constraint monitor is bound to an operation,
 +and consists of a constraint and an exception. The violation of
 +the constraint which is monitored by a constraint monitor results
 +in the creation of a pending exception. The exception is not
 +immediately raised, since this can result in time-dependent
 +run-time errors due to violations of the internal invariants of a
 +process. Pending exceptions are raised at interaction points,
 +which are natural places for internal invariants to hold, but
 +they are not raised in exception handlers.
 +Several constraints can be violated at the same
 +time by concurrently executing processes. This can result in more
 +than one pending exception in a process. Some criteria for the
 +selection of a pending exception have been evaluated. The pending
 +exception that should be selected is the one belonging to the
 +constraint monitor which was enabled first, and thus at the
 +outermost level. The other pending exceptions can be discarded.
 +The only systems considered are systems that can
 +be modeled as discrete event systems.
 +The new mechanism is independent of a particular
 +programming language. The functionality of the mechanism
 +therefore deals with the common requirements of languages for the
 +control of industrial systems. Language-specific elements are not
 +Constraint monitors have been successfully
 +implemented in Process Calculus, which is a language for the
 +specification,​ simulation and control of industrial systems. The
 +simplicity and power of the new mechanism is illustrated with a
 +case concerning the control of a transport system.
 +//PhD thesis//, Eindhoven University of Technology, The Netherlands,​ 1993.
vanbeek/thesis.txt · Last modified: Wednesday, 19 March 2008 : 15:56:32 (external edit)