Towards Using Behavior Trees in Industrial Automation Controllers

TL;DR

This work addresses the rigidity of traditional PLC-based industrial automation software by proposing a BT-based supervisory framework that separates device-specific functions from high-level task coordination. It presents three integration strategies (PLCopen-based BTs, embedded BT libraries for IEC 61131, and event-based BTs for IEC 61499) and demonstrates them with a small application on a ctrlX CORE platform. The key contributions include a BT-integrated PLC design that improves modularity, rapid reconfiguration, and compatibility with PLCopen behavior models, plus practical realizations for IEC 61131 and IEC 61499 environments. The approach promises better skill-based programming, easier collaboration between hardware developers and application engineers, and potential AI-driven code generation for BT-driven control logic in CPPS contexts.

Abstract

The Industry 4.0 paradigm manifests the shift towards mass customization and cyber-physical production systems (CPPS) and sets new requirements for industrial automation software in terms of modularity, flexibility, and short development cycles of control programs. Though programmable logical controllers (PLCs) have been evolving into versatile and powerful edge devices, there is a lack of PLC software flexibility and integration between low-level programs and high-level task-oriented control frameworks. Behavior trees (BTs) is a novel framework, which enables rapid design of modular hierarchical control structures. It combines improved modularity with a simple and intuitive design of control logic. This paper proposes an approach for improving the industrial control software design by integrating BTs into PLC programs and separating hardware related functionalities from the coordination logic. Several strategies for integration of BTs into PLCs are shown. The first two integrate BTs with the IEC 61131 based PLCs and are based on the use of the PLCopen Common Behavior Model. The last one utilized event-based BTs and shows the integration with the IEC 61499 based controllers. An application example demonstrates the approach. The paper contributes in the following ways. First, we propose a new PLC software design, which improves modularity, supports better separation of concerns, and enables rapid development and reconfiguration of the control software. Second, we show and evaluate the integration of the BT framework into both IEC 61131 and IEC 61499 based PLCs, as well as the integration of the PLCopen function blocks with the external BT library. This leads to better integration of the low-level PLC code and the AI-based task-oriented frameworks. It also improves the skill-based programming approach for PLCs by using BTs for skills composition.

Figures (9)

• Figure 1: BT-based design of industrial control software.
• Figure 2: Graphical representation of BT.
• Figure 3: State diagrams of BT and PLCOpen ETrigA FB
• Figure 4: Integration of BTs and FBs via adapters.
• Figure 5: BT for controlling PLCOpen liner axis.