There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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Rf Circuit Design Theory And Applications

Introduction Redundant Models Partition Policies Maintenance Policies for a Finite Interval Forward and Backward Times in Reliability Models Optimum Retrial Number of Reliability Models Optimum Checkpoint Intervals for Fault Detection Maintenance Models with Two Variables System Complexity and Entropy Models Management Models with Reliability Applications

Advanced reliability models and maintenance policies

Production issues at Volkswagen in 2016 lead to dramatic losses in sales of up to 400 million Euros per week. This example shows the huge financial impact of a working production facility for companies. Especially in the data-driven domains of Industry 4.0 and Industrial IoT with intelligent, connected machines, a conventional, static maintenance schedule seems to be old-fashioned. In this paper, we present a survey on the current state of the art in predictive maintenance for Industry 4.0. Based on a structured literate survey, we present a classification of predictive maintenance in the context of Industry 4.0 and discuss recent developments in this area.

A Survey on Predictive Maintenance for Industry 4.0.

The paper deals with a method for optimal threshold calculation during implementation of radio equipment condition-based maintenance. During radio equipment operation, sudden and gradual failures are possible. Such failures lead to deterioration of technical condition of radio equipment. Sudden failures are unpredictable and cannot be prevented. In case of gradual failure, its possible time moment of occurrence can be calculated during observation on diagnostic variables of equipment. Such variables are transmitter power, voltages at equipment units input and output, currents in the branches of equipment circuits. To prevent gradual failure, it is necessary to implement preventive maintenance. Such actions are performed based on the result of comparison of measured data with previously calculated preventive threshold. The problem of preventive threshold determination is solved for efficiency measure in the form of average specific operational costs in the case of known probability density function for parameters of diagnostic variables model. 

Method of Optimal Threshold Calculation in Case of Radio Equipment Maintenance

Modelling of Predictive Maintenance for a Periodically Inspected System

Electronic devices and circuits with negative differential resistance (NDR) are widely used in oscillators, memory devices, frequency multipliers, mixers, etc. Such devices and circuits usually have an N-, S-, or Λ-type current-voltage characteristics. In the known NDR devices and circuits, it is practically impossible to increase the negative resistance without changing the type or the dimensions of transistors. Moreover, some of them have three terminals assuming two power supplies. In this paper, a new NDR circuit that comprises a combination of a field effect transistor (FET) and a simple bipolar junction transistor (BJT) current mirror (CM) with multiple outputs is proposed. A distinctive feature of the proposed circuit is the ability to change the magnitude of the NDR by increasing the number of outputs in the CM. Mathematical expressions are derived to calculate the threshold currents and voltages of the N-type current-voltage characteristics for various types of FET. The calculated current and voltage thresholds are compared with the simulation results. The possible applications of the proposed NDR circuit for designing single-frequency oscillators and voltage-controlled oscillators (VCO) are considered. The designed NDR VCO has a very low level of phase noise and has one of the best values of a standard figure of merit (FOM) among recently published VCOs. The effectiveness of the proposed oscillators is confirmed by the simulation results and the implemented prototype.

https://www.mdpi.com/2079-9292/8/4/409/pdf

Electronic Circuit with Controllable Negative Differential Resistance and its Applications

Among the different maintenance techniques applied to wind turbine (WT) components, online condition monitoring is probably the most promising technique. The maintenance models based on online condition monitoring have been examined in many studies. However, no study has considered preventive maintenance models with incorporated probabilities of correct and incorrect decisions made during continuous condition monitoring. This article presents a mathematical model of preventive maintenance, with imperfect continuous condition monitoring of the WT components. For the first time, the article introduces generalized expressions for calculating the interval probabilities of false positive, true positive, false negative, and true negative when continuously monitoring the condition of a WT component. Mathematical equations that allow for calculating the expected cost of maintenance per unit of time and the average lifetime maintenance cost are derived for an arbitrary distribution of time to degradation failure. A numerical example of WT blades maintenance illustrates that preventive maintenance with online condition monitoring reduces the average lifetime maintenance cost by 11.8 times, as compared to corrective maintenance, and by at least 4.2 and 2.6 times, compared with predetermined preventive maintenance for low and high crack initiation rates, respectively.

https://www.mdpi.com/1996-1073/12/19/3801/pdf

Optimal Preventive Maintenance of Wind Turbine Components with Imperfect Continuous Condition Monitoring

This paper presents a mathematical model for assessing the availability of a non-redundant telecommunications system, which is continuously tested during operation. The types of faults that can occur in such systems are defined as permanent and intermittent faults. Mathematical equations are derived to calculate the mean times spent by the system in the operable and repair states using the properties of the regenerative process. The availability expression is derived under arbitrary fault distributions. Numerical example illustrates the impact of the faults rate on the system availability under exponential distribution of faults.

Availability assessment of a telecommunications system with permanent and intermittent faults

Modern avionics can account for around 30% of the total cost of the aircraft. Therefore, it is essential to reduce the operational cost of avionics during a lifetime. This article addresses the critical scientific problem of creating the appropriate maintenance models for digital avionics systems that significantly increase their operational effectiveness. In this research, we propose the lifecycle cost equations to select the best option for the maintenance of digital avionics. The proposed cost equations consider permanent failures, intermittent faults, and false-positives occurred during the flight. The lifecycle cost equations are determined for the warranty and the post-warranty interval of aircraft operation. We model several maintenance options for each period of service. The cost equations consider the characteristics of the permanent failures and intermittent faults, conditional probabilities of in-flight false-positive and true-positive as well as the cost of different maintenance operations, duration of the flight, and some other parameters. We have demonstrated that a three-level post-warranty maintenance variant with a detector of intermittent faults is the best because it minimizes the total expected maintenance cost several folds compared to other maintenance options.

Through-Life Maintenance Cost of Digital Avionics

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