Optimize production line design with simulation modeling

Simulation can be a powerful tool throughout a project and enables the project team to visualize many aspects of a production line during the design phase.

To plan a new production line or modify an existing one, many questions must be answered including:

1. What will this line produce?
2. How fast will it run?
3. What line efficiency can I expect?
4. How will this equipment fit within my space?
5. If conveyors are used, what are the constraints? (What is the conveyor’s capacity? What happens to the line during a backup condition and how long will it take to recover? Where should buffers be placed to be most effective? Can existing conveyors or unit operations developed for other applications be used?)

Technology designed to visualize a line and bring it to life prior to development is more accessible than ever. 3-D PDFs or videos can be viewed electronically and many software programs now integrate directly with virtual reality (VR) headsets, which allow stakeholders to step into a dimensionally accurate interactive line layout.

This visualization helps everyone understand design parameters more effectively and reach consensus on the final design. Augmented reality (AR) smartphone apps and headsets allow a 3-D model to be projected into an existing space, providing another view of a line. Static or dynamic modeling shows interferences and obstructions and helps avoid these problems at the beginning of a project (Figure 1).

Another way visualization can be used in the design phase is highlighting dynamics of product movement under different constraints. Simulation enables the user to set up many parameters of line operation – machine and conveyor speeds, conveyor lengths, device locations, control behavior – and see the system performs under these different settings. Animated simulations often uncover potential line design issues that are difficult or impossible to see when looking at a line layout or a spreadsheet.

How a simulation saved time

A recent example that underscores the importance of simulation involved a machine that filled four cartons at once, and then pushed all four cartons out of the machine at the same time. Although the average machine speed was 100 cartons/minute, the actual instantaneous output of the machine was either 0 or 200 cartons/minute. If the conveyor at the discharge of the machine wasn’t running twice as fast as the average speed, the cartons would back up into the machine as they exited, preventing the machine from loading new empty cartons.

On paper, the speeds looked correct, but the machine was being blocked. The simulation model identified this problem during the design phase so it could be corrected before installation.

Physics-based modeling is a valuable tool when attempting to understand a product’s real-time interactions on a line. Designers can see how products will move on the conveyor and tweak the conveyor design to maintain control of the product. A simulation can demonstrate how products will clump together or jam in a chute or accumulation table. In the example below (Figure 2), it shows how a buffer for frozen dough balls would be utilized.

Previously, this would be done with educated guesswork and perhaps a computer-aided design (CAD) layout. However, the dynamics of dough rolling around on the conveyor belt is hard to predict or visualize accurately. Physics modeling is also very demanding on computer hardware. Targeted models can be created and any lessons learned from the smaller models can be applied to larger models.

Avoid costs with the right equipment

Even with the best-designed line, machine downtimes are inevitable. The impact of random variable downtimes can be very difficult to predict. Manufacturers may be hesitant to build in buffers and accumulation, believing they hide problems or encourage unmotivated operators. Some buffers minimally impact performance due to the machine’s arrangement, resulting in unnecessary capital expense. A simulation can model various scenarios and consider normal running conditions to determine the optimal number, location, and capacity of buffers to improve line performance and avoid unnecessary expense (Figure 3).

Another important factor simulation can help with is how to control the line. Early in the design process, when no programmable logic controller (PLC) exists, the model allows the design team to consider the controls. Placement of photo eyes and other sensors can be tested and optimized before equipment is purchased.

Reduce start-up time with testing and training

Perhaps the most critical time to use simulation is when the PLC program is ready to be tested. Some modeling software can be connected to a PLC. The model signals to the PLC from simulated sensors and responds to the PLC signals to its simulated motors. Controls engineers can debug controls with a realistic, responsive system, rather than tracing through code manually or attempting to use the human-machine interface (HMI) to visualize performance. Sensor placement can be fine-tuned in the model to within a few inches of optimal real-world placement.

The HMI program can be tested alongside the PLC using the model, and since the model is being controlled by the PLC, buttons pressed in the HMI will mimic real-time production scenarios. Line commissioning start-up times, therefore, are greatly reduced using the simulation model (Figure 4).

The process of connecting a simulation model to a PLC benefits training. A new PLC or HMI programmer can identify mistakes, test new ideas, and build confidence in a low-risk environment prior to live production. Line operators can practice running the line and become acquainted with new PLC programs prior to installation.

Discover difficulties earlier

There are other indirect benefits to the simulation process. Drawing upon background knowledge of line dynamics, a modeling programmer may ask questions early in the design process that normally would not be addressed until much further in development. Schedule adherence is another benefit. Too often, the line has been designed and installed, but time and project constraints result in start-up and commissioning activities beginning prior to the PLC program’s completion. If the model is tested before it goes to the factory floor, it helps validate the program that is written and debugged and is performing as expected.

While simulation has many potential benefits, there are some limitations. The model’s output is only as good as the inputs or assumptions going into it. The simulation won’t predict certain factors such as poor operator habits, bad materials, or condensation build-up. It’s important to revisit and adjust the model to ensure it is reflecting realistic behavior and constraints.

Simulation modeling, used properly, can identify and remove risks, maximize value, and help produce successful outcomes.

Christy Starner is director of simulation and modeling, Dennis Group. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

MORE ANSWERS

Keywords: Simulation modeling, project management

Simulation modeling enables the project team to visualize many aspects of a production line during the design phase.

Another simulation modeling benefit is it allows users to see possible outcomes that are impossible to see on a spreadsheet.

Simulation modeling helps remove risks and produce successful outcomes.

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