A method and layout of serial-parallel scheduling problem Gy

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A method and layout of serial-parallel scheduling
problemGyörgy Schuster,Tamás SándorH1084
Budapest, Tavaszmezo 15-17hal_at_k2.jozsef.kando.hu,
sandor.tamas_at_kvk.bmf.hu
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Some stations of production line have longer
cycle time than the production period of the
whole production. In these kind of cases
parallel processing methods are the only
solutions, which can be applied. Scheduling of
this method may have several problems, therefore
it is very important that the geometrical layout
and control meet all the requirements. In this
presentation we suggest new methods to solve the
problems of those kind of stations.
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Problems

• Dead-lock, which means that the whole system can
  get into a state where its activity is
  impossible. The reason for this is the station
  have run out of resources.
• The resources of the station cannot be assigned
  optimally. As a consequence more resources have
  to be applied.

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If environment and cycle time are well determined
a quasi satisfactory solution can be
found.Unfortunately these conditions are not
fulfilled in every case. Some designers don't
recognise the above mentioned problems.
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Target of investigation is flash down loading
(FDL) procedure.The reasons are

• this operation is widespread in case of
  production of electronics,
• its uncertainty a little bit higher than any
  phase of production of electronics devices.

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The reasons of uncertainty

• unpredictable environmental effects,
• unit must be reprogrammed partially or fully
  because of various error.

Consequently, programming time is not fully
determined!
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The basic problem

• Tc is the cycle time of production,
• TFDL is the time of FDL procedure.
• The problem is TFDLgt Tc
• Solution is more than one cells should be
  applied.

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Investigated station
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Dead-lock
Dead-lock
The most important rule is here Avoid this
situation!
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Algorythm
If there is no uncertainty we can apply the so
called "greedy" or "catch it if you can"
algorithm. "Greedy algorithm" A cell is empty
and there is a unit in front of it the unit will
be loaded into the cell to be programmed. "Greedy
" algorithm is simple that means it is easy to
implement on PLC.
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The problem is greedy method doesn't always match
the optimal algorithm.

• Problem
• the last unit breaks down.
• Consequences
• The critical position shifts forward.
• Solution
• Controller should realize this situatition (PLCs
  are able to do this.)
• Problem
• a unit - not the last one -breaks down.
• Consequences
• The process splits into more slightly-independent
  parts with more dead-lock possibilities.
• Solution
• Controller must adapt to this situation (At least
  embedded computer should be applied to find an
  optimal solution).

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Two belt solution
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• Advantages
• dead-lock situation is solved,
• speed of conveyor belt is the same as the
  production line speed,
• scheduling is simple "greedy" algorithm can be
  applied,
• there is no special cell.

• Disadvantage
• two conveyor belts have to be applied, the input
  and the output aren't in the same line.

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Shuttle solution
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• Questions
• What is the priority of carry out and fill in
  procedure?
• Carry out has higher priority than fill in.
• What is rule of choice between empty programming
  cells if there are more than one.
• Decision is based on geometrical features that
  can be FIFO or LIFO methods.
• Decision based on statistical features. We choose
  the cell whose average process time and standard
  deviation are smaller.

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• Advantages
• easy to find the optimum scheduling,
• dead-lock is impossible,
• statistical method helps to find an optimal
  scheduling,
• PLCs can be applied.

• Disadvantages
• there are some collisions,
• speed of shuttle is quite high (5-10 times).

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Authors would like to express their special
thanks to Ms. Gabriella Csomár and Mr. Róbert
Grusz who gave some important viewpoints and
ideas to solve the discussed problem and tested
the simulation.