Device DrivePosition

DrivePosition

This page is generated from the Unity twin. Published from _docs/context/devices/driveposition.md — edit it there, not here. Changes made in the wiki are overwritten on the next sync.

A positioned axis: told where to be, it travels there at its own speed and reports where it actually is and whether it is moving.

Twin device type · twin FB FB_Sercos_VC · 2 instances in FG_Transport · Unity component OC.Components.DrivePosition

What it is

A servo axis — in this machine, the carriage of each lift unit, which it carries between the lower and upper conveyor levels. Unlike a cylinder it has no end positions of its own: any position along its travel is a valid target, and which positions matter is decided by the control program.

What the PLC sees

Symbol	Meaning
ControlData	Target position, in the twin's units (mm)
StatusData	Actual position, in the twin's units (mm)
Status.6	Moving
Status.7	Initialisation pulse — one second, after the device was restored from a metadata snapshot

No control bit is used. The target position is the whole of the command.

How the twin models it

Each cycle the axis moves towards the target at a constant Speed (per instance, units per second) — no acceleration ramp, no following error. With Speed at zero it jumps to the target at once. The twin is a follower: it does not close a position loop, plan a profile or know about referencing. Whatever position the simulation side sends, it goes to.

Watch out

The twin's zero is not the control program's zero. The twin starts at its own zero, which here is the lower conveyor level. The control program only knows a position once it has referenced the axis on a sensor, and its zero is wherever that sensor is — so the same carriage reads different numbers on the two sides. A position is meaningful only after referencing, and comparing it with the twin's StatusData needs the offset.

The component has no travel limits. Nothing in DrivePosition stops at the end of travel; the limit sensors beside it report, they do not restrain. Keeping the axis inside its travel is the control program's job — which is exactly what a twin is there to prove.

Status.7 is not a health bit. It is a one-second pulse after a metadata restore, and it must not be wired as "drive ready".

In this machine

Lift servo drive. Positions the carriage between the lower and upper conveyor levels.

Signal — Position-controlled. Referenced on B_AxisMin, which reads 0 mm; the lower conveyor level is at 10 mm and the upper at 364 mm.

There are 2 of them, in FG_Transport:

Device	Twin PLC path	Group
Y_Lift	MAIN.FG_Transport.Lift01_Y_Lift	FG_Transport
Y_Lift	MAIN.FG_Transport.Lift02_Y_Lift	FG_Transport

Control implementations

The twin is master for structure, and that is all this page states. Which control function block each instance becomes, its process-image members and its published HMI struct is the control platform's own knowledge base:

• Beckhoff
• Siemens — no knowledge base published yet