Automotive Applications of Discrete Event Simulation
The automotive industry has used discrete event simulation for many years to investigate the capabilities of the different manufacturing systems involved in building automobiles. Here we will look at the use of simulation in body shops, paint shops, trim/chassis/final, engine assembly, machining and stamping.
By Matt Rohrer and Bucky Strong
Simulation of body shop systems during the concept, design and build phases of a product life cycle allows a company to investigate the impact that new tooling, conveyor and material delivery will have on throughput.
There are two different strategies used to analyze the system characteristics of a body shop. The first is to model the body shop at the station level of detail. The second is to model the body shop at the line or subassembly level of detail.
The station-level simulation model is used to analyze the stand alone capability of a subassembly area. Station cycle times and downtimes are entered into a simulation model and the subassembly throughput is estimated. The subassembly throughput can be compared directly to the body shop target throughput. The subassembly throughput must be greater than the full body shop throughput or new design of the subassembly area would be needed.
If complex manual operations occur at a station, these operations can be added to the station level model. Modeling the walk, pickup and set down times can identify if an individual station can meet the required cycle time for the subassembly area.
While the station-level analysis of the subassembly areas is happening, a line level model can be developed. The throughput estimates from the individual subassembly models are entered into the line level model and the conveyor systems are modeled in detail. The interaction between the subassemblies and the conveyor systems can be used to identify sets of subassemblies or individual subassemblies that bottleneck the body shop. Sizing of the conveyor can be accomplished by increasing buffer between bottleneck subassembly areas and reducing buffer between non-bottleneck areas. This process continues through the design phase.
An alternative to this approach is to integrate all of the subassembly models into one large detailed model. Any time a subassembly changes or the conveyor layout changes, the model is updated to analyze the full body shop system.
The full body shop model is also used to investigate the impact that operational parameters have on the system once it is implemented. The operational parameters can include: preventative maintenance, reducing mean time to repair, batch sizing, and overtime. Changing operational parameters and identifying their impact on system performance will show the opportunities for increasing system performance and will aid in the prioritization of funding.
An additional analysis that occurs in the body shop is the delivery of materials to the line locations. This analysis investigates the impact of the number of fork trucks, fork truck assignments, the amount of stock at the line, the location of storage areas, the locations of docks, and the number of docks on delays caused by parts not present at the line. In the concept phase, this analysis can influence the location of subassemblies and identify strategic locations for stock to minimize the material delivery times.
When automotive paint shops are simulated, the focus is usually on the power-and-free conveyor systems that transport parts through the painting operations. Power-and-free systems are different from other conveyors because of carriers that fix themselves to the drive chain by means of a hooking mechanism called a "dog." When two different speed chains merge, carriers can accumulate and are picked up by the dogs on the new chain. There are two types of chains: production chains (low speed), which are while parts are being painted, and transport chains (high speed), to move parts between different production areas.
During the design phase of a paint system, the power-and-free layout is evaluated using simulation to determine if throughput goals can be met. Power-and-free systems can contain miles of production and transport chain in different connecting sub-systems used to transport different part types. Simulation is used to determine the number of carriers required, how chain merges and transfers will operate, and whether adequate accumulation is provided. In some areas where higher density empty carrier storage is required, carriers are rotated before accumulation, which is called "bias banking." The size of bias bank buffers can be evaluated using simulation.
Simulation is used while operating a paint system to determine the effects of adding different body styles and paint types to existing lines. As model years change and vehicle paint options proliferate, it becomes necessary to modify existing paint lines to handle the changing requirements. Simulation provides a "test bed" where production requirement changes can be evaluated prior to making any changes to the existing system. This reduces system downtime during changeover, and also gives operators and managers greater confidence that the changes can be accommodated.
Trim, chassis, and final assembly (TCF) systems are where all the parts are assembled to form a completed vehicle. Simulation helps determine how all the TCF assembly sub-systems will interact and what the production capacity of the facility will be. TCF plants include manual and automated assembly stations, with buffers in between stations to allow for the randomness in cycle times from one job to the next.
During the design phase for TCF, the layout is evaluated to determine the efficiency of flow. Parts are usually received just in time and delivered directly to the assembly line. Smooth, efficient flow of parts between dock doors and the line is important. Additionally, during the design phase a line balance is performed to determine the work requirements at each assembly station. Simulation is used to evaluate the line balance under dynamic conditions. In a TCF, as in most manual assembly applications, every job's cycle time will be slightly different. Simulation can represent the randomness seen in the real world, which helps determine the interactions between stations on the line. Buffering between stations is evaluated by simulation as well. Buffers provide enough jobs between stations so that assembly line workers are blocked or starved as little as possible.
Design phase simulation can also include a downtime analysis of the TCF facility. TCF simulation should include downtime of subassembly areas, like the door and engine lines, to determine actual system throughput and identify buffering requirements.
Operation of a TCF can be enhanced using a simulation. Vehicle mix changes and increasing production requirements can be tested using an "as-built" model of the facility. Simulation can be used to answer questions like:
- "What do we change to get two more jobs per hour?"
- "What happens if we change tire suppliers, with a different delivery schedule?"
- "Why is there always a backup prior to the engine line?"
Simulation of engine assembly areas allows a company to investigate the impact that new equipment, conveyor, and material delivery will have on throughput during the concept, design, and build phases of a product life cycle.
The analysis of engine assembly facilities generally focuses on the engine assembly line, the feeder lines and the delivery conveyor, and material delivery through the facility. Engine assembly line designs vary widely from completely non-synchronous systems to synchronous areas separated by non-synchronous sections.
The cycle time and downtime for each station and synchronous section are input into the engine assembly line simulation. The effects of starvation from the feeder lines are also added to the model. The system capabilities can then be estimated from the model, and bottlenecks can be identified. The conveyor between stations can stretch and shrink between stations and synchronous sections, and the number of pallets can be increased or decreased to try and increase throughput. Additionally, complexity may be transferred from one station to another to reduce the downtime at a station.
Feeder lines, like the piston assembly area, are also modeled in detail to investigate their impact on the engine assembly line throughput. A similar analysis to the body shop analysis is made when trying to identify if the feeder lines should be integrated into one large model. Computer performance, model detail, and model maintenance are considerations that each project team must evaluate when determining which method to select.
Engine assembly plants also model the material delivery within the facility. This analysis investigates the impact of the number of fork trucks, fork truck assignments, the amount of stock at the engine assembly line, the location of storage areas, the locations of docks, and the number of docks on delays caused by parts not present at the engine assembly line or feeder lines. In the concept phase, this analysis can influence the location of the feeder areas and identify strategic locations for stock to minimize material delivery times.
Simulation of machining areas allows a company to investigate the impact that new equipment, conveyor, and production schedule will have on throughput during the concept, design, and build phases of a product life cycle.
Machining models investigate the impact of synchronous machining areas separated by conveyor and the impact on system performance. These analyses are similar to engine assembly lines.
An additional consideration in machining areas is what batch sizes to produce, and what shift pattern is needed to minimize the interaction with the assembly areas. Traditionally, machining lines have longer downtimes and setup times than engine assembly areas. Assembly areas often try to reduce the batch sizes they build to satisfy downstream demand. Because of the conflicting needs from the two different areas, analysis of batch size and production schedule are important to minimize the interaction between the systems.
In stamping plants, simulation helps determine the number of stamping machines required, and how they should be configured. Stamping also has scheduling requirements where simulation based scheduling can answer sequencing questions.
When designing a stamping plant, the goals are to streamline the flow of material while minimizing the amount of capital equipment required. Simulation can report not only the production of a stamping plant, but also the utilization of each piece of equipment. Flow of parts between machines is performed by a combination of manually operated fork trucks and automated equipment. Simulation can help determine the right mix of automated to manual transportation, along with testing the capacity of the transportation equipment.
When operating a stamping facility, proper scheduling of equipment and human resources plays an important role in facility performance. Setups, tool and fixture requirements, preventative maintenance, and other factors influence stamping performance. Simulation based scheduling tools allow for detailed models that answer equipment sequencing questions. Scheduling tools help determine the sequence that each machine and operator should follow to meet delivery and production requirements.
*AutoSimulations is a founding member of Sim.TECH, the simulation software vendors' association. For a complete listing of members, or more information, visit http://www.sim-tech.org. Or contact its executive director, William Hakanson, at (412) 781-3255. AD&P
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