ID: 96
Corresponding Author: Stan JAROSLAWSKI
Experiment: General Interest
Sub-system: General Interest
Topic: Systems reliability and quality assurance

Impact of Reliability Specification on Electrical System Design
Stan Jaroslawski

Abstract:

Advantages of addressing Reliability issues very early in any electrical system design is emphasised. An example of an impact of the Reliability specification on the design of a power subsystem is described.  The power subsystem is part of  High Resolution Limb Sounder (HIRDLS) instrument which is to be flown in space as part of Chem1 mission. The main component of the subsystem is a power supply designated Power Converter Unit (PCU). The PCU has to meet HIRDLS instrument system a very tight Reliability requirement of 0.99. The PCU must also meet HIRDLS instrument power requirement (220Watts total), be compliant with spacecraft requirements, and NASA specifications.

Summary:

Reliability issue is a very important factor in the design of any electrical system and equally applies to products destined for consumer market and to scientific apparatus. Instrumentation for LHC will comprise vast electronic front end and data readout systems that will need to work with very high operational efficiencies. Prediction of failures will allow preventive maintenance to be done during shutdowns and will be instrumental in maintaining operational functionality of experiments. Reliability analysis will also very valuable in selection of electrical system configurations in critical areas. Reliability of systems is routinely studied in space satellite projects and the authors hope that the experience gained from building space instruments could be applied to LHC electronic systems in critical areas.
In engineering, and in mathematical statistics, reliability has a real meaning but is very dependent on the quality of manufacturing and assembly processes. In space programmes this is addressed by Quality Assurance (QA) based on extensive Product Assurance (PA) plans. PA plans comprise a wide range of manufacturing procedures that ensure a very high quality of workmanship. PA plans also define the strategy for procurement components, components’ quality, and components’ screening levels. The plans also lay down rules for levels of testing of the end products. The design to the reliability specification of the power supply described in this paper was baselined on such premises.
This paper gives an example of the actual completed design of a power supply and emphasises the impact that the reliability specification. The power supply was designed for the instrument called High Resolution Limb Sounder (HIRDLS) which is to be flown in the space mission designated CHEM1. Design of the power supply was based on DC-DC converters. The power supply converts d.c. power provided by the spacecraft at 29Volts (primary power) into power rails at +5Volts, +/-15Volts and +30Volts rails (secondary power). Secondary power is supplied independently to seven HIRDLS subsystems. The power supply’s operational life in space is  five years and its design theoretical reliability is specified at 0.99 . The reliability specification was achieved after investigating a number of the on board DC-DC converter configurations; a single set of converters parallel redundancy, and standby redundancy. Reliability was analysed in each case and the best result was achieved by a configuration based on the standby redundancy. In the analyses the overall mean time between failures of all EEE parts was calculated and applied to formulae derived form Poisson density function f(t) =  lambda e - lambda t. (All converter configurations are to be shown in separate diagrams).
In conclusion the paper hopes to leave a message that electronics systems that are buried in the depth of complicated LHC detectors are just inaccessible as are instruments flying in space. One could argue that the LHC front-end electronics would be exposed to even more hazardous conditions than instrument flown in space (ionising radiation, for instance). It is therefore prudent to include in the design of exciting electronics a measure of reliability analysis in order to extend its life.