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After being repaired, a machine functions for an exponential time with rate \(\lambda\) and then fails. Upon failure, a repair process begins. The repair process proceeds sequentially through \(k\) distinct phases. First a phase 1 repair must be performed, then a phase 2 , and so on. The times to complete these phases are independent, with phase \(i\) taking an exponential time with rate \(\mu_{i}, i=1, \ldots, k\). (a) What proporti?n of time is the machine undergoing a phase \(i\) repair? (b) What proportion of time is the machine working?

Short Answer

Expert verified
The proportion of time the machine undergoes a phase i repair is: \[P_i = \frac{\mu_i}{\lambda + \sum_{i=1}^{k} \mu_i}\] The proportion of time the machine is working is: \[P_w = \frac{\lambda}{\lambda + \sum_{i=1}^{k} \mu_i}\]

Step by step solution

01

Determine overall rates

Let's first find out the overall rate for the whole system, which includes the machine's functioning time (rate λ) and the sum of repair processes' times (sum of rates μi). The overall rate is given by: \[\Omega = \lambda + \sum_{i=1}^{k} \mu_i\]
02

(a) Proportion of time spent on phase i repair

To find the proportion of time the machine undergoes a phase i repair, we can calculate it using the rate for that phase and the overall rate. The proportion of time spent on phase i repair is: \[P_i = \frac{\mu_i}{\Omega}\]
03

(b) Proportion of time the machine is working

Similarly, to find the proportion of time the machine is working, we need to divide the rate λ by the overall rate Ω. The proportion of time spent working is: \[P_w = \frac{\lambda}{\Omega}\] Now you can use these formulas to calculate the proportion of time spent on each repair phase and the proportion of time the machine is working, given the rate values λ and μi for the machine functioning and repair processes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Machine Repair Process
Understanding the machine repair process is crucial in managing the life cycle and maintenance of industrial machinery. In the realm of operation management and system analysis, this process is often described using mathematical models to predict the behavior of the machines over time. In the given exercise, we are introduced to a model where a machine operates for a certain amount of time and, upon failure, must undergo repairs that are segmented into sequential phases, each with its own exponential distribution characterized by a rate \(\mu_i\) for the \(i\)-th phase.

Within this context, the conception of a multi-phase repair process reflects real-world scenarios where repairs are not immediate but involve several steps - from diagnosis to full functional restoration. The reliability of the machine depends on the efficiency of each repair phase. Hence, evaluating the proportions of time consumed by each repair phase provides insight into maintenance scheduling and helps optimize machine uptime.
Exponential Distribution
Diving deeper into the concept of the exponential distribution, it is a continuous probability distribution often used to model the time between events in a Poisson process, which translates to the time until some specific event occurs, such as the failure of a machine component.

The key parameter of this distribution is the rate \(\lambda\), which represents how frequently these events occur. For our machine repair exercise, both the functioning time of the machine and the time taken for each repair phase are exponentially distributed. These are independent events, meaning the occurrence of one event does not affect the probability of the next. It's important to grasp that in an exponential distribution, the probability of an event occurring remains constant, which is why it's well-suited for representing the 'memoryless' property of some processes like radioactive decay or, in this case, mechanical failures.
Time Proportion Analysis
Time proportion analysis is a valuable tool in both theoretical study and practical application of operational efficiencies. In this framework, the time proportions for various phases of a process, such as functioning and repair in a machine repair model, are examined.

In our problem, the calculation of time proportions relies on determining how much time a machine spends in operation versus undergoing repair. This analysis is vital in crafting strategies for maintenance cycles and can directly influence the cost-effectiveness and reliability of equipment. By understanding the time proportions for each phase of the repair, stakeholders can forecast downtime and plan activities to minimize the impact of maintenance on production.
System Reliability
The final concept that ties this all together is system reliability, which quantifies the ability of a machine or system to perform its required functions under stated conditions for a specified time period. Reliability is often expressed as a probability, with higher values indicating a more reliable system. In the context of our exercise, system reliability hinges on the balance between operational time and repair phases, demanding an understanding of both the machine's functioning rate \(\lambda\) and the rates of repair \(\mu_i\) for each phase.

By utilizing the computed time proportions for working and repair phases, one can evaluate the system's reliability. Machine reliability analysis is a cornerstone of predictive maintenance strategies, which aim to conduct repairs just before failures are likely to occur, thus improving the system's overall efficiency and lifespan. These calculations help maintenance teams preempt and manage breakdowns, ensuring that the system performs efficiently with minimal disruptions.

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Most popular questions from this chapter

Consider two machines that are maintained by a single repairman. Machine \(i\) functions for an exponential time with rate \(\mu_{i}\) before breaking down, \(i=1,2\). The repair times (for either machine) are exponential with rate \(\mu .\) Can we analyze this as a birth and death process? If so, what are the parameters? If not, how can we analyze it?

A service center consists of two servers, each working at an exponential rate of two services per hour. If customers arrive at a Poisson rate of three per hour, then, assuming a system capacity of at most three customers, (a) what fraction of potential customers enter the system? (b) what would the value of part (a) be if there was only a single server, and his rate was twice as fast (that is, \(\mu=4\) )?

If \([X(t)\\}\) and \(\\{Y(t)\\}\) are independent continuous-time Markov chains, both of which are time reversible, show that the process \(\\{X(t), Y(t)\\}\) is also a time reversible Markov chain.

Consider a set of \(n\) machines and a single repair facility to service these machines. Suppose that when machine \(i, i=1, \ldots, n\), fails it requires an exponentially distributed amount of work with rate \(\mu_{i}\) to repair it. The repair facility divides its efforts equally among all failed machines in the sense that whenever there are \(k\) failed machines each one receives work at a rate of \(1 / k\) per unit time. If there are a total of \(r\) working machines, including machine \(i\), then \(i\) fails at an instantaneous rate \(\lambda_{i} / r\). (a) Define an appropriate state space so as to be able to analyze the preceding system as a continuous-time Markov chain. (b) Give the instantaneous transition rates (that is, give the \(\left.q_{i j}\right)\). (c) Write the time reversibility equations. (d) Find the limiting probabilities and show that the process is time reversible.If there are a total of \(r\) working machines, including machine \(i\), then \(i\) fails at an instantaneous rate \(\lambda_{i} / r\). (a) Define an appropriate state space so as to be able to analyze the preceding system as a continuous-time Markov chain. (b) Give the instantaneous transition rates (that is, give the \(\left.q_{i j}\right)\). (c) Write the time reversibility equations. (d) Find the limiting probabilities and show that the process is time reversible.

Potential custoiners arrive at a full-service, one-pump gas station at a Poisson rate of 20 cars per hour. However, customers will only enter the station for gas if there are no more than two cars (including the one currently being attended to) at the pump. Suppose the amount of time required to service a car is exponentially distributed with a mean of five minutes. (a) What fraction of the attendant's time will be spent servicing cars? (b) What fraction of potential customers are lost?

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