Optimal maintenance strategy for product considering degradation level with preventive maintenance cycles

Minjae Park(박민재) 한국신뢰성학회 2019 한국신뢰성학회 학술대회논문집 Vol.2019 No.10

In this study, we develop the maintenance policy for a used product to determine a periodic preventive maintenance cycle from the dealer’s point of view. Because replacing failed products may not be possible with used products, we introduce the concept of full refund instead of replacement. We set a repair time threshold and give a full refund, not continuing to provide a repair service for failed used product when a repair time exceeds its threshold. Otherwise, the user has only minimal repairs to the failed product, and if the dealer does not provide a full refund during the warranty period, the periodic preventive maintenance cycle is finished at the moment when the warranty expires. Given a certain cost models charged to the dealer, we develop formulas to evaluate the optimal maintenance cycle and the total maintenance cost under warranty. Assuming the process for used product failures and a Weibull distribution for the repair times, we describe the proposed policy numerically and carry out sensitivity analysis to investigate the impacts of several parameters on the preventive maintenance cycle and corresponding expected cost rate. The real application is implemented using the proposed approach and numerical examples are discussed to demonstrate the appropriateness of the methodologies derived in this paper.

Cost Analysis on Warranty Policies Using Freund's Bivariate Exponential Distribution

Park, Minjae,Kim, Jae-Young The Korean Society for Quality Management 2014 품질경영학회지 Vol.42 No.1

Purpose: In this paper, the minimal repair-replacement warranty policy is used to carry out a warranty cost analysis with warranty servicing times and failure times that are statistically correlated to bivariate distributions. Methods: Based on the developed approach by Park and Pham (2012a), we investigate the property of the Freund's bivariate exponential distribution and obtain the number of warranty services using the field data to conduct the warranty cost analysis. Results: Maximum likelihood estimates are presented to estimate the parameters and the warranty model is investigated using a Freund's bivariate exponential distribution. A numerical example is discussed to deal with the applicability of the developed approach in the paper. Conclusion: A novel approach of analyzing the warranty cost is proposed for a product in which failure times and warranty servicing times are used simultaneously to investigate the eligibility of a warranty claim.

Cost models for age replacement policies and block replacement policies under warranty

Park, Minjae,Pham, Hoang Elsevier 2016 Applied mathematical modelling Vol.40 No.9

<P><B>Abstract</B></P> <P>In this paper, we elaborate on cost models by examining the renewable and non-renewable warranty policies subject to minimal repair within the warranty period and the post-warranty period. Among various maintenance policies, the block replacement policy and the age replacement policy have been investigated and compared under the broader warranty perspective. The cost model is developed from the perspective of the customer. This analytical model should provide manufacturers with a better understanding of customer behavior. We consider failure time and repair time simultaneously instead of the traditional two dimensions such as age and usage because it may be difficult to obtain usage information. For the customer's satisfaction, the repair time threshold, which has several types based on the properties of a product, is fixed. However, if the repair time exceeds the repair time threshold, the decision is to discontinue providing the repair service and to provide a replacement service. We obtain and compare the differences between maintenance policies using the expected cost rate, both under the non-renewable warranty policy and the renewable warranty policy. The suggested model provides better guidance for finding an optimal maintenance policy. Numerical examples are discussed to demonstrate the applicability of the methodology derived in the paper.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This paper deals with the optimal maintenance policy under renewable and non-renewable warranty policies. </LI> <LI> The maintenance policies, block replacement policy and age replacement policy have been investigated under the warranty. </LI> <LI> The cost model is developed from the aspect of the customers. </LI> <LI> The analytical model should provide the manufacturers with better understanding with customer's behavior. </LI> </UL> </P>

Investigation of the synergy effect of urea and ammonium carbonate in thermal decomposition to produce ammonia

( Minjae Park ),( Horim Kim ),( Young Su Lee ),( Hyeri Jo ),( Xin Zhao ),( Jae Young Kim ) 한국폐기물자원순환학회 2022 ISSE 초록집 Vol.2022 No.-

Nitrogen oxides (NO_X) are air pollutants that cause respiratory and cardiovascular diseases which are mainly produced by the combustion process of power plants, incinerators, and automobiles. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are well-known methodologies of reducing nitrogen oxides to nitrogen by ammonia. Anhydrous ammonia (NH₃, l), ammonia solution (NH₃, aq), and urea solution (NH₂CONH₂, aq) are conventional reductants of SCR and SNCR. Anhydrous ammonia and ammonia solution are highly corrosive resulting high risk of storage and transportation. Compared to the previous two reductants, urea solution is much safer and therefore generally applied to SCR of various systems. To convert urea into ammonia, urea solution requires a pyrolysis process with a temperature of 400℃. Still, the conversion rate of urea solution is low because of by-products produced by complicated reactions. Recently, ammonium carbonate, which decomposes completely into ammonia at 58℃, is being studied as a reductant of SCR and SNCR. However, ammonium carbonate is much more expensive than urea. In this study, to take advantage of both urea and ammonium carbonate, the synergy effect of urea and ammonium carbonate was investigated by comparing the concentration of produced ammonia. Three types of solutions, 4 wt. % urea solution, 2 wt. % ammonium carbonate solution, and a mixture of 4 wt.% urea and 2 wt.% ammonium carbonate solution, were thermally decomposed in a tube furnace. The pyrolysis was performed at a temperature of 200, 300, 350, 400, and 500°C with a residence time of 2 seconds. The produced gas was dissolved into 250 ml of distilled water inside the impinger using air as carrier gas. Ammonia concentration was measured with the Nessler method and UV-Visible Spectrophotometer. This study investigated the synergy effect of urea and ammonium carbonate and how the synergy effect varies depending on the pyrolysis conditions.

Any Meeting: Mobile Application System for investigating students’ engagement

Minjae Park,Jundong Cho 한국HCI학회 2014 한국HCI학회 학술대회 Vol.2014 No.12

In-class team discussion is considered as pedagogically motivated learning method. It reinforces students’ class participation. Through our mobile application system, Any Meeting, we suggest instructional strategies that elevate participation and interactive communication. This mobile application provides an opportunity for students to have a team discussion by simple manipulation during class. Through the team discussion, students enhance their understanding of class materials and engagement. Any Meeting not only generates in-class discussion among students but also supports communication between students and the instructor. Students use this application to communicate with their instructor instead of confronting the instructor face to face. Students ask questions, get feedback, and make unclear clear without interfering flow of the class. Using Any Meeting, students stay free from the anxiety of impeding the progress of classwork for clarifying confusing information. They raise hands, not physically but through their mobile devices, and request for the class discussion. The instructor, on the other hand, catches a sense of how well students comprehend the class materials, as he checks the number of hands raised up on his mobile device. Any Meeting supports both students and the instructor to communicate despite the size of the class. Even in large-scale lectures, students and the instructor interact with each other through Q&A system offered in the application. From user study, we have demonstrated that students adequately use Any Meeting in augmenting their understanding of class materials. The study shows positive relationship between our system and user satisfaction.

Aspects of the occurrence of Planthopper Species in Rice Production Regions in Asia

Minjae Kong,Won-Young Kim,Bue-Yong Park,Ho-Jun Rim,Jum-Rae Cho,Hong-Hyun Park 한국응용곤충학회 2022 한국응용곤충학회 학술대회논문집 Vol.2022 No.10

Warranty cost modeling using the parametric method

Minjae Park 한국신뢰성학회 2011 신뢰성응용연구 Vol.11 No.1

In the paper, we consider two-dimensional warranty policy with failure times and repair times. The failure times are considered within the warranty period and the repair times are considered within the repair time limit. Under the renewable warranty policy and non-renewable warranty policy, we consider the number of warranty services in the censored area by warranty period and repair time limit to conduct warranty cost analysis. We investigate the field data to check their dependency and implement our proposed approaches to conduct warranty cost analysis using the parametric methods. Numerical examples are discussed to demonstrate the applicability of the methodologies and results based on the proposed approach in the paper.

A Design of a 92.4% Efficiency Triple Mode Control DC–DC Buck Converter With Low Power Retention Mode and Adaptive Zero Current Detector for IoT/Wearable Applications

Park, Young-Jun,Park, Ju-Hyun,Kim, Hong-Jin,Ryu, Hocheol,Kim, SangYun,Pu, YoungGun,Hwang, Keum Cheol,Yang, Youngoo,Lee, Minjae,Lee, Kang-Yoon IEEE 2017 IEEE TRANSACTIONS ON POWER ELECTRONICS - Vol.32 No.9

<P>This paper presents a retention/pulse frequency modulation (PFM)/pulse width modulation (PWM) mode dc-dc buck converter with adaptive zero current detector (AZCD) and spread spectrum clock generation (SSCG) for IoT/Wearable systems. The proposed dc-dc buck converter is capable of handling loads from 10 mu A to 20 mA with high efficiency by applying triple mode (retention mode, PFM mode, and PWM mode), gate split technique, and AZCD. Retention mode is proposed to extend wide load range at ultralight load. Gate split technique adjusts the conduction loss and switching loss. AZCD reduces the operation duty of the high-speed comparator and avoid reverse current below light load. In IoT applications, each node communicates with other nodes through a Bluetooth low energy transceiver, which consumes very low current and is highly sensitive to supply noise. Therefore, the proposed dc-dc buck converter adopts the SSCG technique to reduce electromagnetic interference by up to 18 dB. This chip is implemented using 0.13 mu m CMOS technology with an active area of 820 x 800 mu m(2). The maximum power efficiency of the proposed dc-dc buck converter is 92.4% at a switching frequency of 2.5 MHz when the load current range is 10 to 20 mA. The input voltage range and the regulated output voltage are 2.2-3.3 and 1.7 V, respectively. In addition, the proposed dc-dc buck converter achieves over 74.2% efficiency in retention mode when the load current range is from 10 to 500 mu A.</P>

Two-dimensional policy for a warranted system with periodic preventive maintenance

( Minjae Park ) 한국품질경영학회 2022 한국품질경영학회 학술대회 Vol.2022 No.1

This paper considers a two-dimensional warranty policy for a repairable product with increasing failure rate, during which a fixed number of periodic preventive maintenances are provided by the manufacturer or the seller. The preventive maintenance is the common planned action to delay the wearout of the product, while it is still in the operating state. The product is warranted by taking into account both age and usage and the warranty is expired when the product reaches a specified age or specified usage, whichever comes first. In this paper, we develop a cost model to evaluate the expected total warranty cost from the manufacturer’s perspective under a certain cost structure and determine the optimal warranty period under the two-dimensional warranty policy under study. As a practical application of the proposed optimal warranty policy, we present the optimal maintenance strategy under a certain type of lemon law, by which the manufacturer is required to either refund or replace the purchased product if the product failures can’t meet certain conditions of repair thresholds set by the law. In addition, we discuss the optimal premium prices for the warranty policy from the manufacturer’s perspective.

Replacement model under warranty with age-dependent minimal repair

Park, Minjae The Korean Reliability Society 2017 International Journal of Reliability and Applicati Vol.18 No.1

In this paper, we consider a renewable repair-replacement warranty strategy with age-dependent minimal repair service and propose an optimal maintenance model during post-warranty period. Such model implements the repair time limit under warranty and follows with a certain form of system maintenance strategy when the warranty expires. The expected cost rate is investigated per unit time during the life period of the system as for the standard for optimality. Based on the cost design defined for each failure of the system, the expected cost rate is derived during the life period of the system, considering that a renewable minimal repair-replacement warranty strategy with the repair time limit is provided to the customer under warranty. When the warranty is finished, the maintenance of the system is the customer's responsibility. The life period of the system is defined and the expected cost rate is developed from the viewpoint of the customer's perspective. We obtain the optimal maintenance strategy during the maintenance period by minimizing such a cost rate after a warranty expires. Numerical examples using field data are shown to exemplify the application of the methodologies proposed in this paper.