Industrial Communication Guide

2.4. CONTROL COMPUTER

MES/ERP level

The function of the PLC (Programmable Logic Controller) is to con- trol a process or sub-process. For this reason, it is positioned as close to the process as possible. If the sensors/actuators are po- sitioned adjacent to the PLC, they are connected directly to the PLC. Sensors/actuators with a long distance to the controller are generally connected to the PLC via a so-called bus system. In larg- er systems with several sub-processes, a separate PLC is used for each sub-process, and these are networked with one another. Looking at the automation pyramid, the control level lies between the field level and the MES/ERP level. For a control computer this results in the following tasks:

Control level

Field level

A

S

A A

Structure of a PLC Depending on its type, a PLC has a different number of inputs and outputs as well as a processing unit. The classic PLC is cycle-based, in other words a sequence is constantly repeated internally. The system operates according to the input-function-output model: • The control computer receives signals from the sensors via its inputs (input). • The signals received are processed according to a predefined logic (PLC program) by the control computer (function). • Based on the logic, the control computer generates the appro- priate signals for the actuators (output). • Processing then starts from the beginning again (the control computer once again receives signals from the sensors).

• Control of the process or sub-process using the sensors and actuators at the field level.

• Alignment with the MES/ERP level to coordinate resources. In this way, it is possible to determine which machine is currently handling which order and when it is expected to be available again, for example.

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