A flexible manufacturing facility must produce 1680 parts of one type per 80-hour lead (Wortman and Wilson, 1984; Kleijnen and Standridge, 1988). The part flow through the facility is follows.
1. Parts arrive to the facility from a lathe at a constant rate of 21 per hour.
2. Parts require three operations in the following sequence: Op10, Op20, and Op30. 3. A part is washed before and after each operation.
Each operation is performed either by a fixed machine or a flexible machine. A fixed machine can perform one and only one of three operations but a flexible machine can perform any of the three operations. Parts are moved between machines and the wash station by a single automated guided vehicle (AGV). The AGV system transports parts with little or no human assistance. The vehicle picks up loads at designated pick-up points and transports them to designated drop-off points. Each pick-up and drop-off point is associated with a machine or work station. A central computer assigns material movement tasks to the AGV and monitors the vehicle position. The AGV moves in one direction on a fixed track around the center of the system.
Operation processing times are 14.0, 5.0, and 8.0 minutes respectively for Op10, Op20, and Op30. Washing time is 18 seconds.
AGV travel time is 20 seconds around the entire loop. The following table shows AGV travel time between each pair of workstations.
The objective is to find the minimum cost combination of fixed machines (Op10, Op20, or Op30 only) and flexible machines (all three operations) that meets the throughput requirement. Flexible machines cost more than fixed machines. Thus, all possible work should be done by fixed machines. Flexible machines are employed to avoid buying an excessive number of fixed machines. In addition, management is also interested in minimizing lead time for a part. Thus, management will consider a configuration of machines that includes flexible machines and increases cost in order to reduce lead time as long as the total number of machines does not exceed the total number of fixed machines required to do the work by more than one. The following operating rule is employed for each operation to select between fixed and flexible machines. A part will use a fixed machine if it is available. If not, it will use a flexible machine if one is available. If neither a fixed machine nor a flexible machine is available, the part will use the first machine of either type, fixed or flexible, that becomes available.
One possibility that should be considered is using the minimum number of fixed machines needed to process all parts in a timely fashion with no flexible machines utilized.
Assess the structural variability seen in this system. Generate a trace of all system activities. Use the trace to identify the structural variability.
Case Problem Issues
1. What performance measure are important in this problem?
2. How will the AGV system be modeled?
3. Describe how to model the choice between a fixed and a flexible machine for an operation in the simulation language you are using.
4. When does a part entity acquire and free a machine resource relative to acquiring and freeing the AGV resource?
5. How can the effect on system operations of part waiting for the AGV be determined.
6. How many fixed machines of each type are needed if no flexible machines are used? 7. Construct an alternative to the machine configuration in number
6 as follows. Replace one fixed machine of each type with a sufficient number of flexible machines. Determine the number of flexible machines in this case.
8. Why tell why all parts do not have the same time in the system