Fault tolerance defined

Furthermore, where multiple SISs are used, one should take into account common cause failures. In addition, all of the other requirements defined in lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (lEC 61511-1 Mod) should be satisfied, including the minimum fault tolerance requirements defined in Table 5. 

Table 6 of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Modi defines the basic level of fault tolerance for sensors, final elements, and non-PE logic solvers having the required SIL claim limit in the first column. The requirements in Table 6 are based on the requirements in lEC 61508-2 for PE devices with a SFF between 60 and 90 %. The requirements are based on the assumption that the dominant failure mode is to the safe state or that dangerous failures are detected. 

Human error is defined as an act outside the tolerance bounds. These are determined by the technical boundary conditions and may therefore be influenced— within limits— by the designer in the sense that the tolerance region becomes large (fault-tolerant design). This reduces the probability of human error. 

To deal with variations in quality requirements, we can define a set of quality factors and metrics for the factors. The choice of factors that are important and their level depends on the application. For example, security is an important factor for e-commerce applications and fault-tolerance is important for flight software. Typical... 

For example, if a pressure transmitter configuration of HIPPS has 2 oo3 architecture, then the safety function can be kept although one dangerous hardware failure occur. According to lEC standard, the hardware fault tolerance of the pressure transmitters is defined as 1. 

If the quantity q is estimated from other quantities following one oriented path in the structural model, its fault tolerance level is assessed by the minimal fault tolerance level of quantities used to estimate it. Indeed, the used quantify with the lowest level defines the number of needed failures that avoid the quantify to be estimated. Consequently, we obtain ... 

Our future works aims to extend the architecture research to the whole control system, limited here to the instrumentation. It consists in determining the number of control units and the instrumentation associated to each of them that satisfy the specified fault tolerance level. This problem can be solved thanks to the set of constraints imposed to the instrumentation that defines indirectly groups of sensors and actuators that can simultaneously fail due to the failure of the unit control to which they are connected. Another aim concerns the structural analysis that can be improved. It may consist in deterrnining automatically the needed... 

The rehabdity modeling of fault-tolerant aircraft systems using SyRelAn can be divided into two modeling levels, one mapping the system architecture, the other defining the redundancy management. Therefore the SyRelAn tool uses ReUabdity Block Diagrams for the definition of the nominal system architecture. To map the multi-state behavior of different components Concurrent Finite State Machines are implemented. 

Between these five discrete states 16 transitions 2) can be defined, depending on the set of possible states for each component and an initial state for the nominal system stale. The conditions of those state transitions are defined by a logical syntax, addressing the system component and the component state. Additionally, it is possible to address not just single component states but also logical combinations of different component states by setting combined conditions in order to provide a system with fault-tolerant capabilities. 

NOTE 2 The minimum hardware fault tolerance has been defined to alleviate potential shortcomings in SIF design that may result due to the number of assumptions made in the design of the SIF, along with uncertainty in the failure rate of components or subsystems used in various process applications. 

RAID subsystems can be optimized for performance, highest capacity, fault tolerance, or a combination of these attributes. Different RAID levels have been defined and standardized in accordance with these... 

Let us first examine the two words fault tolerance. One can define fault as an incorrect step, process, or data function, etc., meaning that it is the malfunction or deviation from an expected result or behavior. On the other hand, tolerance stands for endurance, in this case, continuance of operation even after a fault has occurred. So, the two words together stand for the ability of the system to function even after a... 

Means There are several means for dependability. Fault tolerance is one of them and was defined earlier. The other means are ... 

Fault tolerance As defined earlier, fault tolerant designs are aimed at development of systems that could function correctly in the presence of faults. This is primarily achieved by some kind of redundancy to detect or mask a fault. Masking/detections are followed by fault location, containment, and recovery. 

In addition to a formal specification, we need a technique to analyze the fault tolerance behavior of a component in a formal way. Approaches such as [19] verify formalized fault trees against formal implementation models. Furthe-more, several fault injection analyzes that rely on model checking like [3] and [9] have been presented. In this paper we focus on a fault injection based-technique [16], [10] that is called model-based safety analysis MBS A. The MBS A processes functional requirements and provides complete results as cut-sets and allows to define custom faulty behavior in the implementation model, which is specified using Matlab/Statefiow. Cut-sets are unique combinations of malfunctions occurrences that can cause a system failure. A cut-set is said to be minimal if no event can be removed from the set and the combination of malfunctions still leads to a failure[ll]. 

Photoelectric safety devices are safety components , as defined in the Supply of Machinery Safety Regulations 1992. A notified body, who will use BS EN 61496 as the baseline standard, must therefore check their conformity with the Regulations. The standard lays down general requirements for electrosensitive protective equipment, in Part 1, with Part 2 specifying particular requirements for photoelectric systems. For example, the standard specifies two types of device according to their fault tolerance. Type 2 and Type 4 ... 

The next revision of the standard is likely to define a Type 3 device in which the fault tolerance characteristics fall between those of Type 2 and Type 4 devices. 

Hardware elements used in the safety function have a defined hardware fault tolerance. 

If a safe state is entered, usually the driver should be informed. This part of the fault reaction can be defined by user information requirements - Fault-ReactionUserlnformationRequirement ). For user information requirements, the fault tolerant time, a description of actions by the driver or other persons involved, and validation criteria for these actions can be added. For user information requirements, it is required to specify at least one safe state, and a description of actions by the driver or other persons involved (see Tab. 4, 2M06RA). 

Specified as a library with functions to protect a data item and to check it at the other end of the communication. In short, it adds a counter and identifier to the data, computes a checksum and sends the data and checksum over the bus instead of the raw data. At the other end, the checksum is used to see if the data got corrupted and if not, the data is compared to an earlier value to see if it can be trusted. By addressing a number of fault models once and for all with a library, the AUTOSAR software developers know what they can use when they are faced with specific safety requirements. However, what is achieved by using such libraries is not always clear by just using the protection mechanism of AUTOSAR, the software developer is not guaranteed to obtain a fault-tolerant system, and it is critical to define the context in which the system is safe and level of tolerance that is guaranteed. 

It could not be considered, that chassis systems considered as hard real-time, power-train systems as firm real-time and interior system are soft real time systems. ISO 26262 defines the fault-tolerant-time-interval as the basic criterion to specify safety-related timing-specific requirements. This interval defines a period of time, within faults should be controlled by the system (see also Chap. 3 of this book). The consequences of a missing deadline can only be evaluated by the consequences of the fault-error propagation in the latter, consequences of the possible hazard due to resulting malfunctions. 

Fault-tolerant is the capability to provide continued correct system operation in the presence of a defined set of hardware and/or software faults. 

To what extent can fine grain aspects be defined and composed to implement reconfigutable fault tolerance mechanisms ... 

The answer to the problem of undetected faults in PLCs lies in the concept of Fault Coverage and Fault Tolerant Systems The answer to the problem of hidden defects in software is high quality embedded (i.e. operating system) software combined with strictly defined and constrained user programming facilities. 

Formula set 2 is used to calculate the PFDavg for any system with automatic diagnostics (or a portion thereof). Note that the test interval, Tia, is very short and should be carried out within the fault tolerant time of the process (process safety time, hence the need to define this period in the SRS) if the test itself is not going to affect the availability of the system. Typically Tia would be in the range 1 to 10 seconds. 

The SIL value achievable by any sub system is determined by both its PFDavg for random hardware failures and by the level of fault tolerance required by the standard for a particular type of device. The standard defines these as architectural constraints in Tables 2 and 3 in part 2. 

A fault is a method of determining cause or affixing blame. It is best safety practice to define faults in terms of failures. Although not every fault in a system causes a failure in that system, faults that either exist within or interact with a system may result in a system failure. When a system is determined to have faults that do not cause failures in the system, the system is said to be fault tolerant. ... 

It may be anticipated that some faults will be found in any film, and a specification will define to what extent faults will be tolerated in material intended for a particular purpose. Bearing in mind that the speed of the calendered film may be 100 m min -, many faults will not be visible—and it is necessary therefore either to have an automatic inspection system or a statistical plan for taking samples and making the measurements required. Normally, samples can be obtained only at the end of each production roll—which may mean that more than 1000 m have been made before a fault is detected. 

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