Kyung Soo Park

Optimal Number of Minimal Repairs before Replacement

This paper presents a model for determining the optimal number of minimal repairs before replacement. The basic concept paralles the periodic replacement model with minimal repair at failure introduced by Barlow & Hunter, the only difference being the replacement is signaled by the number of previous minimal repairs performed on the unit. In the case of Weibull distribution, which is widely used for failures, the optimal solution is simple and more cost effective compared to Barlow & Hunters Policy II.

Fuzzy Apportionment of System Reliability

The concept of reliability apportionment is general and has even been applied to the allocation of man-machine reliability. Typically, however, the process of optimally apportioning individual component reliability to meet some desired system reliability level subject, perhaps, to constraints on cost, volume, weight, etc. has always been imprecise and vague at best. In real problems, the resource constraints are no more sacred than the objective system reliability; they are frequently flexible. In view of the inherent vagueness of the reliability objective as well as constraints in a typical ill-structured reliability apportionment problem, this paper formulates the nonlinear optimization problem in the fuzzy-set theoretic perspective. To illustrate the philosophy, a simple reliability apportionment example with a budgetary constraint for a 2-component series structure is analyzed. Then the concept is generalized into a more realistic problem with multiple components and constraints. This tutorial paper is an easy introduction for the newcomer to fuzzy-system theory.

Optimal continuous-wear limit replacement under periodic inspections

An item breaks down when it wears continuously beyond a certain threshold. The item is preventively replaced as the wear at periodic inspections exceeds a certain wear limit; on failure, it is replaced immediately. The optimal wear limit for preventive replacement that minimizes the long-run total average-cost rate is derived. A numerical example demonstrates its computability. >

Optimal number of minor failures before replacement

This paper extends a replacement model based on failure number to include a repair-cost limit for general failure distributions. The stochastic behaviour of failures and repair costs are modelled by a non-homogeneous Poisson process. An efficient marginal analysis algorithm is presented for finding the optimal failure number for replacement that minimizes the average cost rate.

Optimal wear-limit replacement with wear-dependent failures

The optimal wear-limit for preventive replacement for an item with wear-dependent failure rate is derived by minimizing the long-run total mean cost rate. The generic term wear connotes any type of degradation that accumulates through use and is observed continuously in time. The optimal strategy has the same form as the age replacement policy. >

Condition-based predictive maintenance by multiple logistic function

For complex systems, the system failures exhibit an exponential nature, negating the benefit of traditional, time-based preventive maintenance. Under this circumstance, the condition-based predictive maintenance (CbPM) is more appropriate where the system condition can be monitored from the surface of running machinery, and maintenance is performed only when needed as the failure prognosis dictates. The author presents a cost optimal prognostic criterion for CbPM using a multiple logistic function of risk variables to be monitored, which fluctuate randomly according to a certain probability distribution. A numerical example demonstrates the procedure and utility of the method. >

Reality and human performance in a virtual world

Abstract Two experiments were designed to understand the performance characteristics of human interaction with a virtual environment. In the first experiment, the effects of control feel (isometric vs. isotonic 3D input device) on tracking performance were measured in a virtual world. The results showed that the isometric device gives better control performance than the isotonic device. In the second experiment, the hypothesis that increased visual depth cues can improve 3D control performance was tested. The presence of binocular depth cues improved tracking performance. However, tracking performance was not significantly improved under perspective depth cues.

Comparison of group replacement policies under minimal repair

Three replacement policies for a group of identical units are compared. ( 1 ) Units are periodically replaced all together. Between the periodic group replacements, minimal repairs are performed at failures. ( 2 ) The group replacement interval is divided into repair and waiting intervals. Minimal repairs are performed at failures during the repair interval, but no repair is made in the waiting interval, and the unit remains failed until the group replacement time comes. ( 3 ) Each unit undergoes minimal repair at failure during the repair interval. Beyond the interval, no repair is made until a number of failures. The expected cost rate expressions under each policy are derived. It is shown that the third policy is better economically than the other two policies. Numerical examples are given to demonstrate the results.

Solving Nontrivial Recursive Equations on a Microcomputer

A multiple-line function, subprogrammed in Basic language on a microcomputer, has the ability to reference itself recursively. This feature is particularly useful in computing nontrivial recursive equations.

Optimal Use of Product Warranties

Under a warranty service contract, the manufacturer of a product agrees to repair and maintain the product over a certain stated period of time for a fee payment by the customer. When the warranty period is over, the user is responsible for any future repairs. The problems facing the user in this situation are the determination of the economic life of the product and the selection of the best warranty policy, given several optional warranties. This paper develops a simple stochastic model of product failures and the associated repair costs to obtain the economic life (of a warranted product) that minimizes the average total cost rate. The result is useful in evaluating warranty alternatives.