When working on a preview setup-, or when only (production) process variation- and timing is relevant you will want to use Discrete Event Simulation (DES). DES is what a mechanical engineer would call - Kinematic - meaning ‘Simulating Motion without considering Force’. Arguably this way of simulating moving parts is more akin to creating a standard 3D animation: the transformations and material interaction are continuously calculated over a set of paths in the 3D world (called Splines).
Due to its continuous nature DES can also run much faster-than-real-time - allowing you to use it turn your material handling setup directly into a throughput simulation, or even extend it to help validate order planning.
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DES & Material Handling
In Chapter 2 of the Digital Twin Planning & Development guide (add link after move) we present our core planning model for Digital Twins. The following Figure 1 is a cross-section of that models' Material Handling axis mapped onto DES Temporal and Spatial fidelity; this allows us to visually discuss:
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For DES to function in a scene all DES Monobehavior components must be transform children of a DES Controller component. You can change Material Handling playback speed in this Controller to run it faster-than-real-time (for e.g. throughput simulation) or slower-than-real-time (for e.g. analyzing motion faster than 25hz). You link DES material handling to A | Visualization & Other Modalities and B | External Interaction in the scene.
II | Shots & Splines
In the scene your Material Actors (Agents) can find their way over paths (or Splines) - To correctly define these splines we use ‘shots’ or ‘poses’ of said actors in the scene.
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By default a spline offers a single degree-of-freedom (the actor can move up-and-down the spline) - but using different Motion Types and Spline Tensor Products much more complicated multi-degree motion (such as highly deterministic 2D Stacking) can be simulated. |
III | Cues & Junctions
Points-of-Interest on these paths, can be specified using Cues (which can also be used as motion sensors) - and an interconnected network of splines can be created using Junction Cues.
IV | Instructors
As Material Handling systems become more complicated and interconnected you will find yourself in need of sub-system control - these are managed by Instructors. Note that Instructors are rarely the material itself; rather they guide the behavior of Actors based on events they witness in the scene.
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Arguably you can follow the known subsystem subdivision you have in your plant (e.g. each conveyor has its own PLC, so each conveyor gets its own control mechanism); or you can base it on more ‘implicit’ systems of control (e.g. in the image above a traffic light is used to prevent unwanted ‘material interaction’ (e.g. collisions) - but note there’s no physical barrier for cars to run the light; participation in the sub-system is voluntary). |
V | Actors
Actors (or Agents) are the Simulation participants that actually move through your material handling scenario - they are the containers of data that allow you to evaluate your material flow.
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All DES Participants (Actors and Instructors) can execute Activities during the simulation. Most notably Timers and (for Actors) Motion and (un-) Parenting
VII | Activity Sequencing
Actors in a Material Handling Scenario can adhere a temporal and spatial step-by-step planning - this firstly means you must be able to sequence Activities using Motion Web Instruction Sequences.
VIII | Compound Actors
Sometimes you need more detailed control over the sequencing of (nested) motion (e.g. Imagine a robot arm that can either move its end-effector, or its base - but not both simultaneously) In such a scenario it can be prudent to split up a single Actor into multiple nested Actors, each in control of a specific motion - this is when Compound Actors come into play.
IX | (Assignment) Buffers
A single Actor often cannot execute 2 tasks at the same time - this means you must be able to reserve an actor, or queue your assignment with the Actor - this is when you use Assignment Buffers. Where Assignment Buffers are used for temporal reservation you will need Material buffers for spatial reservation
X | Process Simulation
After combining the above tools to create an accurate simulation of your material handling scenario you may want to test what would happen when you release actual work-orders into the system; what would be the lead time? and what will the utilization of the individual (resource) actors be? The Process Simulation toolset offers you a way to quickly convert your DES setup to a Throughput Simulation.
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