Manufacturing

• Conveyor belt (Conveyor):
  • special stock element introduced in Stella, redesigned in Modelica
  • functionality
    • operates at clock ticks t_n = t₀ + n Δt
    • stores input value
    • outputs the value after a given transit time t_tr
  • example
    • inflow =  5 (per time unit)
    • t₀ = 0.1, Δt = 1, t_tr = 3
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  • results
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  • details
    • discrete simulation at sample times t_n = 0.1 + n
    • needs discrete stock component StockD
    • input flow is 5 for 2.5 ≤ t ≤ 13 (implemented with TimeSwitchedConverter)
    • initial waiting time < Δt is rounded to Δt
• Manufacturing machine (Oven):
  • another discrete stock from Stella
  • baking tray model: fill tray, bake, unload
  • parameters initialLoad, capacity, cookingTime
  • processing phases
    • loading: get input, until capacity is reached
    • production: wait until cookingtime ends
    • unloading: output complete content, immediately start with reloading
  • example with initialLoad = 0, capacity = 3, cookingTime = 2
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  • results
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• Model of a simple assembly line:
  • 
  • delivery of 2 units per time unit of raw material
  • stored in inputStore with initial stock = 7
  • path2 transports 2 parts per time unit
  • machine1 has a capacity of 4 and needs 2 time units
  • conveyor12 has a transport time of 2 time units
  • intermediateStore for buffering, initially empty
  • path5 transports 2 parts per time unit
  • machine2 has a capacity of 2 and needs 1 time unit
  • outputStore for final products, initially empty
• Simulation results of model AssemblyLine:
  • expectation: throughput of 2 parts per time unit
  • inventory of the stores
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  • problem
    • inputStore grows continually
    • uneven growth of outputStore
    • average throughput: 1.5 parts per time unit
    • all stations allow for a throughput of 2 parts per time unit
  • cause
    • behavior at machine1
    • 
    • path2 transports 2 parts per time unit
    • can't deliver during production time of machine1
    • loading of machine1 needs 4 parts, but only 2 parts can be delivered in one step
  • solution:
    • path2 must be extended to transport 4 parts per time unit
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    • simulation shows necessary sizes of stores under optimal conditions