Abstract
The development of supervisory controllers for cyber-physical systems is a laborious and error-prone process. Supervisor synthesis enables control designers to automatically synthesize a correct-by-construction supervisor from a model of the plant combined with a model of the control requirements. From the supervisor model, controller code can be generated which is suitable for the implementation on a programmable logic controller (PLC). Supervisors for industrial systems that operate in close proximity to humans have to adhere to strict safety standards. To achieve these standards, safety PLCs (SPLCs) are used. For SPLC implementation, the supervisor has to be split into a regular part and a safety part. In previous work, a method is proposed to automatically split a supervisor model for this purpose. The method assumes that the provided plant model is a collection of finite automata. In this paper, the extension to extended finite automata is described. Additionally, guidelines are provided for modeling the plant and the requirements to achieve a favorable splitting. A case study on a rotating bridge is elaborated which has been used to validate the method. The case study spans all development steps, including the implementation of the resulting supervisor to control the real bridge.
Highlights
Due to the large number of sensors and actuators present in modern cyber-physical systems and the growing functionality that has to be provided by the control system, the development of the associated supervisory controllers is a laborious and error-prone process
Supervisory controllers are often implemented on programmable logic controllers (PLCs)
A PLC is connected to sensors and actuators in the system
Summary
Due to the large number of sensors and actuators present in modern cyber-physical systems and the growing functionality that has to be provided by the control system, the development of the associated supervisory controllers is a laborious and error-prone process. Supervisor synthesis, introduced in Ramadge and Wonham (1987), enables control designers to automatically construct a supervisor from a discrete-event model of the uncontrolled system (the plant) combined with a model of the control requirements From this supervisor a supervisory controller can be derived which can be used for controller code generation. The use of supervisor synthesis significantly reduces the development time and increases the quality of the supervisory controller, as — by construction — it adheres to the requirements and it is nonblocking. SPLCs differ from regular PLCs (RPLCs — PLCs without safety functionality), as they have additional dedicated inputs and outputs for safety certified sensors and actuators, and they require separate safety controller code. The previously mentioned code generation tools do not support distributed implementation of a single supervisor on the regular part and the safety part of the SPLC. This is automatically derived from the requirement model
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