Glossary

Glossary

Published from _docs/07-glossary.md — edit it there, not here. Changes made in the wiki are overwritten on the next sync.

Automation is full of abbreviations. Here is what the ones used in these pages mean, in plain language. Nothing here is specific to this project unless it says so.

The big idea

Virtual commissioning — testing a machine's control program against a simulation of the machine, before the real one exists or while it is still being built. The point is to find the mistakes early, when fixing one costs an afternoon instead of a week on site.

Digital twin — a model of a real machine that behaves like the real machine: parts move, sensors switch, cylinders take time to travel. In this project the twin is built in Unity and is what you see on screen.

MIL · SIL · HIL — the three ways to run a twin, and the difference is only what drives it:

	Stands for	What drives the machine
MIL	Model in the Loop	Scripts inside the model itself. An animation — good for looking at the machine, proves nothing about a control program
SIL	Software in the Loop	A real control program, running on emulated controller software on your PC
HIL	Hardware in the Loop	The same control program, running on real controller hardware

Control systems

PLC — Programmable Logic Controller. The industrial computer that runs a machine: it reads the sensors, decides what to do, and switches the motors and valves. Every real production machine has one.

Control program — the software running on the PLC. In this project it is the thing being tested.

Control platform — a whole controls ecosystem: the controller, its programming software and its fieldbus. Beckhoff/TwinCAT and Siemens/TIA are two. This project keeps one module per platform.

PackML — a widely used standard for how a machine's states are named and how it moves between them (stopped, starting, executing, aborting, resetting …). The operator panel buttons in this machine follow it.

ST — Structured Text, the text programming language most PLC programs on this machine are written in. It looks a little like Pascal.

FB — Function Block. A reusable block of PLC code with its own memory, roughly what other languages call a class instance. One cylinder in the machine is served by one function block.

GVL — Global Variable List. A named list of variables a PLC program shares across its parts.

HMI — Human Machine Interface. The screen an operator uses to run the machine.

Signals and buses

I/O — Inputs and Outputs. The machine's senses and muscles: an input is a sensor telling the PLC something, an output is the PLC switching something on.

Fieldbus — the network that carries I/O between the PLC and the terminals out on the machine. The important part of this project is that the control program talks to a fieldbus, exactly as it would on the real machine, and never talks to the simulation directly.

EtherCAT — a fast fieldbus, made by Beckhoff, used by the TwinCAT setup here.

PROFINET — a fast fieldbus, widely used with Siemens controllers.

ADS — Automation Device Specification. Beckhoff's way for two programs on a PC to exchange data. Unity uses it to talk to the simulation project.

Process image — the block of memory holding the current value of every input and output, updated once per PLC cycle.

Cycle time — how often the PLC runs its program from top to bottom. 1 ms here. Everything the machine does happens in those steps.

Bit mapping — which individual bit of a byte carries which signal. A camera might report "ready" on bit 0 and "busy" on bit 1 of the same byte, so the mapping is the only thing that says what a value means. The machine pages state it as a table per device.

Products this project uses

Open Commissioning (OC) — the open-source framework this twin is built on: a Unity package of machine components, a TwinCAT library, and the OC Assistant tool that keeps the two in step. By SpiraTec AG. See github.com/OpenCommissioning.

OC Assistant — the tool that synchronises the Unity scene and the simulation project: the hierarchy, the components, their metadata and the interface GVLs. It also scans a fieldbus and creates the emulation unit for it.

TwinCAT — Beckhoff's control software for a PC. Here it does two jobs: it hosts the simulation project, and it can also run the control program being tested.

TIA Portal — Siemens' engineering software for their controllers.

PLCSIM Advanced — Siemens' emulator: a Siemens PLC running as software on a PC.

Unity — the real-time 3D engine the twin is built in.

CAD — Computer Aided Design. The 3D drawings of the real machine, which is where this twin's shapes come from.

Words used in this project

FG — Funktionsgruppe, functional group. One self-contained part of the machine, such as the conveyor or the press station. This machine has seven: FG_System, FG_Transport, and FG_01 to FG_05.

Station — one working position on the conveyor, where a pallet is stopped so a group can work on it.

Index unit — the transport station that stops a pallet, lifts it to hold the part still, asks a functional group to run, and only releases the pallet when the group reports it has finished.

Lift unit — the transport station that carries a pallet between the two conveyor levels. Its carriage is driven by a servo axis and has a short belt of its own to draw a pallet aboard and push it off.

Referencing (also homing) — finding a servo axis' zero by moving it onto a sensor. Until an axis is referenced its position is only a number; the lift units reference on their lower limit sensor at the first reset after power-up.

Pallet — the carrier that travels around the conveyor with the workpiece on it. This one has two slots.

Device — one addressable thing in the machine: a cylinder, a sensor, a drive, a lamp.

Device type — the kind of device, such as Cylinder or SensorBinary. One page describes each type; every device in the machine is an instance of one.

Simulation unit (also emulation unit) — the TwinCAT project holding one function block per twin device, and the emulated fieldbus. It sits between Unity and the control program, in every setup, whichever vendor's controller is being tested.

Twin PLC path — the address the twin uses for a device, such as MAIN.FG_01.P_Camera. A control platform usually spells the same device differently, so the two are never mixed on one page.

Behaviour contract — a written description of what the machine must do: the sequences, the interlocks, the fault codes and the recovery. It belongs to the machine, not to any one control platform, so every platform running this line has to honour the same one.

ContextNode — the engineering description carried by a part of the twin, as key/value entries in the Unity scene. Some keys are synced from the project (oc.plcPath, oc.deviceType); the rest are authored by a human and say what the part is for. Everything generated downstream comes from them.

Knowledge base — the generated machine description in context/: what exists, where, and of what type. Generated from the twin, so it cannot drift; never edited by hand.

Handoff — context/.machine.json, the machine's structure as schema-versioned JSON. It is what a control platform builds against, and each module keeps its own committed copy so it builds without this repository present.

Module — one control platform's realisation of this machine: a separate, optional repository cloned into this one's root, holding that platform's projects and its own documentation.

Prefab — Unity's word for a reusable object saved once and placed many times. The whole machine is one prefab, which is why every scene shows the same machine.

Scene — one Unity file you open and press Play on. There is one per control platform, plus the MIL one.

† — on a generated machine page, marks something that belongs to one control platform's example rather than to the machine itself. Today only the operator panel is marked.

PSA OPCV — the name of the original machine this twin was modelled from, carried over from the CAD model it was built out of.