Simulating Conveyor Systems
The term digital twin often gets used to reference the visual side of a facility. Is it a virtual factory? Can I go through it and see what it will look like? The more valuable twin allows engineers to evaluate a design. A design that they can see in action to determine if the production system will perform as envisioned. To do this accurately, the simulation engines must model equipment behavior accurately. Today’s post will look at modeling conveyors in detail to make sure the models reflect reality.
Even in the subset of conveyors, the number of variations and conveyor features can be quite extensive. This post will focus on pulling gaps, photoeyes, conveyor merges, and skewed rollers. These models are all in 3D and “to-scale”. While some simulation products take a flowchart approach to modeling processes, conveyor systems require a level of detail that requires accurate 3D representations to ensure the quality of the answer and the easy modification.
Often boxes on a conveyor are back-to-back. Sensors such as photoeyes need space between the boxes to effectively count or sort the boxes. To do this, designers will introduce short belts of increasing speed to create space between consecutive boxes. When simulations have variable size boxes and varying speed, these are difficult calculations without a simulation. The video below shows two gap belts pulling spaces between boxes on a conveyor line.
Photoeyes detect the beginning and ends of boxes by providing a beam that is interrupted by a passing box. The length of the interruption allows the control system to know when conveyors are backed up or the size of an object that is passing. The control system can execute different sections of code based on the length of these “gaps”. Photoeyes can also be place above the surface of the conveyor. In the video below, notice how the photoeye changes color to red when boxes pass except for the purple box. The purple box is flat enough that is passes underneath the photoeye. This is a good example of how the third dimension (Z-Axis) can be important in conveyor modeling.
Merging together streams of boxes on to a common conveyor trunk line is one of the most complex scenarios in modeling conveyor systems. In order to be efficient, the lines feeding the trunk line will accumulate “slugs” of parts and they claim the merge point to move a slug through the merge as quickly as possible. The logic to allocate and release the merge so slugs can take turns can be complex. The placement of photoeyes to claim and release the merge is an excellent application of simulation. The video below shows a section of a large conveyor system. The conveyor merge is in the lower left corner of the video.
Sometimes the simplest thing can be difficult to model. Certain pieces of equipment like a labeler often want the boxes justified to the edge of a conveyor to operate correctly. When the boxes are different sizes conveyor manufacturers have methods to accomplish the positioning. One method is to skew rollers slightly to push all the items to one side or another. In the video below, a 5-degree skew is placed on the rollers to push the items to the edge of the conveyor. Without this feature being built into the commercial software (FlexSim in this case), this would be very difficult to model.
These are just four examples of conveyor modeling issues that arise when simulating complex systems. Other unique situations include Power & Free conveyor, Bulk conveyors, Bidirectional conveyors and Side Transfers. What unique situations have you run across? Were you able to simulate them accurately or did you have to make some critical assumptions? Let us know in the comment section.
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