A process-reliable PLC integration requires the deterministic definition of the trigger signal, synchronous exchange of product identifiers, and structured feedback of inspection data. Modern vision systems use real-time-capable fieldbuses such as PROFINET, EtherCAT or EtherNet/IP for process data, as well as OPC UA for higher-level IT and MES systems.
The deterministic inspection cycle in detail
The reliable operation of an inline quality inspection is based on a strictly timed communication chain between the machine control (PLC) and the camera system:
- The part reaches the defined inspection position in the tool or on the conveyor.
- The PLC transmits the part type identifier (recipe number) to the vision system in advance.
- A time-critical hardware trigger initiates image capture with microsecond precision.
- The vision system evaluates the image and maintains the busy status.
- The result data (good/bad, error codes, measurement values) is passed to the PLC and acknowledged.
Comparison of industrial communication protocols
| Interface | Real-time capability | Typical application (THOP practice) |
|---|---|---|
| Digital I/O (24V) | Very high (hardware) | Simple triggering, ready signals, binary good/bad output. |
| PROFINET (RT/IRT) | High to very high | Standard in Siemens environments; transmission of recipes, error codes. |
| EtherCAT | Extremely high (< 1 ms) | Motion control applications, on-the-fly image capture synchronised to servo axes. |
| EtherNet/IP | High | Standard in Rockwell / Allen-Bradley automation environments. |
| OPC UA | Medium (non-real-time) | Vendor-independent export to MES, ERP systems or quality databases. |
Hardware triggering versus software bus communication
For image capture, industrial image processing follows the principle: the trigger for image acquisition belongs on a direct digital hardware input of the camera or vision controller (e.g. via a fast photoelectric sensor or a direct 24V signal from the PLC output module). A software trigger via fieldbus protocols is subject to jitter (runtime variation) on the network. For moving parts, this can cause positional shifts in the image and thus false rejects. Data such as product IDs, batch numbers or detailed measurement results, on the other hand, are transmitted in parallel and flexibly via the fieldbus.
Every THOPinspect system has galvanically isolated, ultra-fast hardware digital inputs for the camera trigger, as well as standardised software blocks (Siemens TIA Portal, Beckhoff TwinCAT), which significantly shortens integration time into your existing system.