32 - Reliability in instrumentation and control

Abstract

Reliability is generally defined as the probability that a component or assembly operates without failure for a prescribed period under specified conditions. To ensure a meaningful result, the conditions of operation, both physical and electrical, must be specified in detail as the standard of performance is required. The chapter also discusses reliability and mean time between failures (MTBF). It is convenient to look for value; the one usually adopted is MTBF. This applies to a maintained system in which each failure is repaired and the equipment is then restored to service. For systems, which cannot be maintained, such as satellite control systems, the value of interest is the mean time to failure (MTTF). Another measure of reliability that is often quoted is failure rate—that is, the number of failures per unit time. This is particularly useful in relation to individual components because it is the most convenient figure to use when estimating the reliability of a complete system in terms of the performance of its components. Equipment manufacturers generally quote MTBF as an indication of reliability as the value does not depend upon the operating period, which is often different for each user. However, reliability is the factor of prime concern to the user, and the relation between this and MTBF is thus important. The simplest case occurs when the failure rate, and thus, MTBF is constant with time.