Event Recorder - Washington Metropolitan Area
The Washington Metropolitan Area is the metropolitan area centered on Washington D.C., the capital of the United States. The area includes all of the federal districts and part of the U.S. States of Maryland and Virginia, along with a small portion of West Virginia.
Final Customer: Washington Metro
Project: Event Recorder Reliability Prediction and FMECA Analysis
Description: Z Lab produced RAM documentation, as Reliability Prediction and FMECA Analysis for each single module of Event Recorder System.
The RAMS is a long-term characteristic of a system and is obtained by the application of data, concepts, methods, techniques and tools of engineering during the system lifecycle (EN 50126). It is defined as a quality and quantity indicator of the system degree, regarding to the system function and to the availability.RAMS is the acronym of Reliability Availability Maintainability, Safety.
Reliability is the probability that an item can perform a required function under given conditions for a given time interval n(t1 –t2).
Availability is the ability of a product to be in state to perform a required function under given conditions at a given instant of time or over a given time interval assuming that the required external resources are provided.
Maintainability is the probability that a given active maintenance action for an item under given conditions of use can be carried out within a stated time interval when the maintenance is performed under stated conditions and using procedures and resources.
Safety is defined as freedom from unaccettable risk of harm.
Reliability prediction is a method to calculate the constant failure rate during the system life time. The reliability predictions is conducted at various system levels and detail’s degrees. It is based, on a system decomposition as tree that is called WBS (Work Breakdown Structure), in order to identify the major components and assign to each of them a failure rate, in accordance with the standard NPRD-2011 (mechanical parts) and MIL-HDBK-217F Notice 2 or Siemens 29500 (electriconic parts). The basic failure rate of the system is calculated by summing up the failure rates of each component in each category multiplied by their quantity (based on probability theory). This is applied under the assumption that a failure of any component is assumed could lead to a system failure. This model assumes that the component failure rate under reference or operating conditions is constant. The failure rate of the electonic items can be calculated:
- at reference conditions (parts count method);
- at operating conditions (parts stress method).
In the part-count method, the failure rate is calculated by appropriate databases that provide the basic failure rate value relative to the component operating environment. The Part-Stress method required detailed information such as: type of technology, year of manufacture, junction temperature, stress factors, thermal expansion characteristics, number of thermal cycles, thermal amplitude of variation, application of the device, etc.. It is also possible evaluate the mission reliability prediction. This analysis can be done after the FMECA analysis : through the FMECA is possible to analyze the failure modes and the percentage of occurrence of each failure mode. In this way it is possible to identify the critical components of the system.
FMECA analysis is a tool used to examine all possible failures, their consequences and the critical components or functions in the system under analysis. The FMECA purpose is to improve and ensure the reliability of complex systems. It is composed of two separate analyzes: FMEA (Failure Modes and Effects Analysis) and CA (Criticality Analysis). The FMECA Analisys can have a functional approach or structural approach:
- functional approach: It is performed on the functions. This approach focuses on the ways in which the functional objectives are not complied
- structural approach: it is performed on the HW system components. This approach tends to provide more detail about the system failure modes and effects at component level
Furthermore, to provide a qualitative assessment of the potential consequences, the level of criticality of failure modes is assigned, according to their effect on the regularity and / or service "comfort" and safety; Evaluating these results, it is possible to suggest mitigation measures relating to the failure mode under analysis. FMECA analysis allows to identify components failures that could be critical in terms of reliability and / or safety, in relation to a particular mission profile. FMECA is the basis of design choices in order to eliminate critical fault, or at least, to reduce the criticality (through corrective actions). FMD-97 is the standard used for FMECA Analysis (failure mode description) and “Yellow book” for the Reliability Failure Mode Classification and Mishap Severity Categories.