Computer algorithm for on-line scheduling , Mechanical Engineering

Computer Algorithm For On-Line Scheduling For Automated Manufacturing Systems 

Introduction

Now days  market  is  characterized  via  the  production  of  minute  batches  of  specialized products to satisfy exact demands of exact customers. Individual of the key ingredients of these categories of flexible production systems are Flexible Manufacturing Systems or FMS. A key feature of these systems is to manufacture quality products along with short response times. To fluctuating demands, short response times can always be attained via huge spare capacities in the production process. The high investment that is essential to install highly automated manufacturing systems conversely renders this solution not acceptable. Hence capable scheduling policies have become necessary that would be capable to support constraints as like: the due dates of the jobs and high load of the system.

Within a complex manufacturing system, resources are to be assigned optimally which should exceed the capacities of humans. The implementation of the same tools on a computer that is only utilized for graphical representation of the real state of decision process, as this is frequently offered via the vendors of computer based factory control systems, does not eliminate this bottleneck. Scheduling must be done online, that is depending upon the real condition in the production process, via suitable algorithms and only is eventually and controlled modified through the dispatchers.

The resource assigned problem in manufacturing systems of the job shop category is termed to be NP hard. This implies that the computational effort to determine the optimal solution grows exponentially along with the number of machines and the number of operations considered, and  a  true  optimization becomes  not feasible online even  for  extremely  minute systems. The standard solution to the scheduling problem practically is to produce sub optimal schedules by using priority rules are computationally extremely simple and can be implemented simply. For a test problem, effects of a detailed study upon the presentation of all general priority rules are detailed in the upcoming sections.

Observations of the scheduling errors caused via the unidentified priority rules motivated the introduction of an original rule, the WLS that is weighted loss of stalk rule. The coming discussion illustrates also this new rule and compares it along with conventional rules. To conquer the deficiencies of priority rules generally, the predictive strategies for multi machine problems are examined.

There approach is based upon the concept of predictive control as this has emerged in the context of standard continuous control problems. The fundamental concept of predictive control is: suppose that recent state of a system and a model of its dynamics are identified, and a preferred trajectory of several variables (outputs) is given. After that, at a specified instant of time, the result of all possible control inputs on the future evaluation of the system can be estimated, and input sequence that yields the suitable fit to the preferred trajectory can be found. Since both changes and disturbances of the desired trajectory may arise, this process is iterated, and only the initial or the first few control inputs are utilized.

Within the control of standard continuous time discrete dynamical systems along with continuous variables, the optimal input's computation that minimizes a specified cost function over a limited or  infinite horizon is  relatively easily in  several  cases, for instants: for quadratic cost functions. In our cases, because of the discrete nature of the problem, an analytical solution of the optimization problem is impossible and the computational effort rises exponentially along with the length of horizon that is considered. The key factor for the presentation and the applicability of predictive scheduling algorithms is an enough restriction of the search problem to dangerous decisions and or promising candidate control sequences.

Two various strategies for predictive control algorithms for FMS scheduling have been examined.  One strategy aims at escaping of an exact, frequently happening scheduling error produced via easy priority rules through future evolution's partial simulation of the systems.

The other utilizes a limited bound and branch search method to examine a promising part of the complete decision tree for the subsequently decisions. The products of application of both the techniques of the test problems for FMS scheduling under various load levels, time pressure levels, and queue lengths have been shown and the performance is compared along with those priority rules that were found to be Pareto Optimal for the similar problem.

The FMS scheduling policies are associated to the idea of decentralized-hierarchical production scheduling. Local scheduling should be complemented via a global assignment of tasks to the sub-systems along with earliest possible starting times and local due dates. After the local decisions are completed, the overall system should be coordinated since the local completion times in several cases find out the earliest possible starting times in another subsystem. This coordination procedure is disturbed more severely whether the important fraction of the jobs is ended along with huge delays than if approximately all jobs are ended along with an approximately equivalent delay. Consequently, the width of the tardiness distribution is very significant and not merely the average tardiness. Thus, concurrently three measures for tardiness are considered as: mean, RMS that is Root Mean Square, and tardiness. The subsequent discussion details several aspects of on line scheduling along with the assist of a test problem.

 

 

Posted Date: 3/5/2013 7:15:37 AM | Location : United States







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