Comparison of Replacement Strategies for Items that Fail

Replacement strategies applicable to items which fail with monotonically increasing failure rate are considered. In particular, the important characteristics of n-stage and 1-stage strategies are compared. This study, though somewhat tutorial in nature, reveals interesting properties which are useful in selecting an appropriate strategy. In general, n-stage strategy has economic advantage over the corresponding 1-stage strategy. The advantage is obtained by transferring failures from stages where failure costs are high to stages where these costs are low. In many cases, a quick examination of the system will reveal whether n-stage strategy will be advantageous or not. Subsequently, based on a detailed study an appropriate n-stage strategy can be designed. The more stages the greater would be the economy, up to where practical considerations and increasing transfer costs will limit the number of stages.     

A dimensionality reducing model for distributed filtering

The necessity of filtering noisy data generated by multidimensional processes arises in many diverse settings. The direct application of the Kalman-Bucy results is hindered by dimensionality difficulties inherent in multidimensional problems. This paper shows that for linear steady-state problems significant dimensionality reductions can be accomplished, thus making routine the solution of many interesting problems.     

Stability of moving surfaces in fluid systems with heat and mass transport — I: Stability in the absence of surface tension effects

A mechanism by which transport processes can produce instability of moving surfaces in fluid systems in considered. Stability of a moving boundary at which fog forms or vaporizes is analyzed using techniques of existing work on stability of moving solid surfaces. It is found that the vaporization case is always stable. But instability is predicted when fog forms at a boundary moving toward an initially supersaturated region. Stability of moving reaction fronts and of moving boundaries in ionic systems is discussed.     

Computer-aided system for the database storage structure design

An overview is given of a computer aided system for the design of a schema for a CODASYL DBMS. The system helps the designer to trade off between the conflicting objectives; short retrieval time for a user query, low database updating cost, small storage requirements, and low total cost of the system. Different relative weights can be assigned to each of the users query and update transactions, and the design objectives can be assigned different priorities. The model evaluates the performance of the database for a specified set of input parameters and finds the optimal location mode of each database record type. The designer can interactively change any of the design parameters, priority and weights while performing the analysis. The system has been tested on the design of a department store database.     

Improved implementation of search technique to find spanning trees

The authors have developed a set of algorithms to find the spanning trees, the minimal paths and minimal cutsets of a graph, starting from the incidence matrix of the graph [1,3]. All the above algorithms employ a unique tracing process based on search techniques. The above algorithms have a number of salient features. The arithmetic and logic operations are very simple, which makes it possible to design small desk top calculators capable of handling reasonably large and complex graphs. The major constraint of these equipments is the memory capacity vis à vis their capability of handling larger graphs. The authors designed a microprocessor based system [2] to find spanning trees. The end results were available in the form of code numbers of branches appearing in a spanning tree, which had to be noted down, every time a tree was generated. In the new system the end results are in a more compact form, i.e. the vectors (see definition), one vector for one tree. The user can easily note down the vectors and decode them later to obtain the branches of a tree. In the new system the user can reallocate the available working memory space to suit the problem. The memory requirement in the new approach is also less.     

The influence of heavy doping effects on the reverse recovery storage time of a diode

During the reverse recovery process in a modern Si p-n junction diode, the value of J_EO/J_BO (the ratio of emitter to base dark saturation currents) increases and the recombination of carriers in the emitter becomes important due to heavy doping effects. A theory is developed to take these effects into account. The emitter and the base components of the current during the reverse recovery phase are found to vary with time. However, their sum remains equal to the constant reverse current J_R, which flows in the external circuit. The ratio of the total quantity of charge present in the base to that present in the emitter is found to increase rapidly with time. Values of the storage time t_s for different values of J_EO/J_BO are calculated. In a typical case, the storage time is reduced by a factor 5 in a diode with J_EO/J_BO = 2. In such cases, the values of lifetime τ_B calculated using measured t_s values and the Kingston's formula, become inaccurate. Theoretical expression for the total charge Q_BS left in the base at t = t_s in a base dominated diode is derived.An earlier semi-empirical formula known as Kuno's formula is derived theoretically. It is found that the formula is valid both for the base dominated diode as well as in a diode with large contribution of the emitter but only when J_R/J_F is small. According to this formula t_s vs 1n(1 + J_F/J_R) plot is approximately a straight line with slope approximately equal to τ_B in both cases. For large values of J_R/J_F when t_s values are small, the correct formula shows that the plot is highly curved. An analysis of this part of the curve yields a value of J_EO/J_BO.     

The effect of built-in drift field and emitter recombinations on FCVD of a p-n junction diode

This paper discusses the Forward Current induced open circuit Voltage Decay (FCVD) of a p-n junction diode including the effects of recombinations in the emitter as well as the built-in drift fields in the base and in the emitter. The analysis is based on the quasi-static approximation (QSA) of the carrier profiles in the emitter. It is shown that the emitter effects on FCVD is completely determined by J_EO, the dark saturation current in the emitter. The value of J_EO in general, depends on the heavy doping effects in the emitter, the drift field in the emitter, emitter thickness and surface recombination velocity at the emitter surface. It is shown that for a diode with retarding drift field in the base, emitter recombinations play a very significant role in FCVD. The decay time constant for large values of time in this case is given by $\text{τ}{}_{\mathrm{<mtext>eff</mtext>}}\text{= τ}{}_{\mathrm{B}}\text{/[1 + ?}{}_{\mathrm{B}}{}^2\text{? (a ? ?}{}_{\mathrm{B}}\text{)}{}^2\text{],}\text{where}\text{a = J}{}_{\mathrm{EO}}\text{/J}{}_{\mathrm{BO}}\text{, ?}{}_{\mathrm{B}}$ is the drift field parameter in the base. The higher value of a, the faster is the voltage decay. For accelerating fields in the base, the time constant for large values of time is independent of emitter recombinations and is given by $\text{τ}{}_{\mathrm{<mtext>eff</mtext>}}\text{= τ}{}_{\mathrm{B}}\text{/(1 + ?}{}_{\mathrm{B}}{}^2\text{)}$. However, the decay rate for small values of time is strongly affected by emitter recombinations for both types of the field; the higher the emitter recombinations, the faster is the initial rate of the voltage decay. For extremely strong drift fields in the base, QSA in the emitter is not valid. The coupled continuity equations are solved with the conditions $\text{?}{}_{\mathrm{B}}{}^2\text{? τ}{}_{\mathrm{B}}\text{/τ}{}_{\mathrm{E}}$ and an analytic expression for FCVD is derived. It is seen that FCVD for strong base fields is determined solely by emitter lifetime τ_E except for small values of time of the order of a few τ_E.