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Merging and Slug Building   

Merge conveyors can organize items flowing from multiple infeed conveyor lines. The most common purpose of merging is to combine items from multiple conveyor lines into a single line, which then sends the items downstream for further processing. This section will describe how to use FlexSim conveyor objects to create various kinds of merges. It will also discuss accumulation methods that could possibly be used when merging items.

Slug Building

The conveyor industry uses the term slug to refer to a queue of accumulated items on a conveyor that will eventually be released downstream as a single group. Building slugs of items can maintain high rates of throughput while keeping equipment speeds as slow as possible.

You can design a conveyor that builds slugs and waits to release them based on a specific set of criteria such as:

  • What percentage of the conveyor must be filled before a slug is ready for release
  • The number of items that must be on a conveyor before a slug is ready for release
  • The amount of time that must elapse in building the slug before a slug is ready for release

For example, in the following image, each conveyor in this system is set to build a slug of at least 5 items before release:

To design conveyors that build slugs before releasing them, you will need to create a Conveyor Type that is specifically designed to build slugs. In the settings for your Conveyor Type, check the Slug Builder check box to enable slug building. Then, in the Ready Criteria, check the boxes next to the conditions you would like the conveyor to use when determining a slug is complete and ready for release. See Conveyor Type Settings - Slug Building Settings for more instructions on these settings.

The next section will provide a step-by-step set of instructions for building the conveyor system used in the preceding image.

Sawtooth Merging Using the Merge Controller

The Merge Controller uses its Lane Clear Table and Release Strategy in determining how to prioritize merging lanes. Release strategies can be:

  • Round Robin - Conveyor lanes will merge in the order the lanes are listed in its Lane Clear Table. Only after the first lane has merged will the second lane merge, etc.
  • Round Robin if Available - Conveyor lanes will merge as soon as the lane becomes available for merging. If two lanes are available at the same time, it will then use the ordering in the Lane Clear Table to determine which lane should take priority.

Also, since the Release Strategy is just a code field, you can manually write your own release strategy if the pre-defined options do not fit your needs.

To see the Merge Controller in action, we'll return to the Slug Building example from the previous section. In the following image, each conveyor will not be available to merge until it has built a slug of at least 5 items. The Merge Controller in this case will use the Round Robin if Available strategy and allow lanes to merge whenever the slug is built:

To create this kind of merging system:

  1. Add the conveyors and add any additional Fixed Resources such as Sources and Sinks. (See Adding Conveyors to a Simulation Model.)
  2. Connect the conveyors together and connect to the conveyors to the Fixed Resources. (See Connecting Conveyors.)
  3. On the View menu, click Toolbox to open the Conveyor System Tool. (See Accessing Type Properties for other methods of opening the tool.)
  4. In the Conveyor System list, right-click Conveyor Types and select Add Conveyor Type from the menu. This will help you to create a type of conveyor that builds slugs before making the slug lane available to merge. (See Conveyor Type Settings for more information.)
  5. In this example, we're going to set the conveyor to build a slug of at least 5 items before becoming available to merge. On the Behavior tab, check the Slug Builder check box. This will enable the Ready Criteria options. Check the Item Count check box and change the number next to it to 5. Now it will build a slug of at least 5 items before making the lane available for release.

  1. If you would like, you could change the name of this new Conveyor Type by editing the top field. In this example, we'll change the name to SlugLanes.
  2. Click OK to apply these settings and close the Properties dialog box. The SlugLanes conveyor type should now appear in the list of Conveyor Types in the Conveyor System Tool.
  3. Double-click each slug-building conveyor to open its Properties dialog box. From the Conveyor Type menu, select the SlugLanes conveyor type. The slug-building conveyor will now inherit all of those settings and will build slugs of at least 5 items.

  1. Add a Merge Controller anywhere to the simulation model.
  2. Add a Decision Point on the conveyor in the location where the merging items will be sent.
  3. Using an 'A' connector, connect the Merge Controller to the Decision Point and to each slug-building conveyor.
  4. Double-click the Merge Controller to open the Properties dialog box. On the Merge Controller tab, on the Release Strategy menu, select Round Robin If Available.
  5. If needed, you can reorganize the order in which the Merge Controller prioritizes the lanes using the Lane Clear Table. In this table, the conveyors will be prioritized from left to right. To reorganize the prioritization, right-click a column and select Move Column(s) Left or Move Column(s) Right.

  1. Click OK to apply the settings and close the Properties dialog box.
  2. To test the simulation, click Reset and Run on the simulation control bar. As the simulation runs, the lanes should build slugs of at least five items and they should be released one at a time.

You can use these basic methods to do other kinds of complex merging with Merge Controllers. You could possibly familiarize yourself with the available Behaviors by reading the chapter on the Conveyor System Tool and Photo Eye and Decision Point Behaviors.

Merging Using Decision Points and Photo Eyes

You can use a Decision Point or Photo Eye to build complex merging logic into your conveyor system. You can build nearly any kind of system by positioning and connecting Decision Point and Photo Eyes and adding some Behaviors to them. A few of the Decision Point or Photo Eye Behaviors that could be useful for merging are:

  • Send Item - Can send items to different destinations (such as different conveyor lines) based on criteria you specify
  • Stop/Resume - Can stop an item, delay an item, and can set conditions that can make a conveyor motor stop or restart
  • Area Restriction - Can create an area that only allows a fixed number of items to flow through a restricted area on the conveyor at a time

See the chapter on Decision Point and Photo Eye Behaviors for a more in-depth discussion of each Behavior and its available settings.

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FlexSim 2016.1