Control design pattern based on safety logical constraints for manufacturing systems: application to a palletizer

Abstract

This paper presents an original approach for safe controller design for manufacturing systems controlled by PLC (Programmable Logic Controller). In this work, manufacturing systems are considered as Discrete Event Systems (DES) with logical Inputs (sensors) and logical Outputs (actuators). The proposed approach, which separates the functional control part from the safety control part, is easy to implement and ensures that the designed controller is safe. The methodology is based on the use of safety constraints in order to get a permissive safe controller which can be validated off line by model-checking. This controller is then constrained by functional constraints. The approach is illustrated with a palletizer simulated process using the ITS PLC software from the Real Games Company (www.realgames.pt). The control algorithm is presented and allows resulting in a safe control using a standard control design pattern, may be simpler than a conventional approach based on a complete specification in GRAFCET (IEC 60848) that does not distinguish the functional aspect from the safety aspect. This approach presents interesting perspectives like the management of several operating modes linked to a Manufacturing Execution System (MES) or the manual modes through Human-Machine Interfaces (HMI) or Supervisory Control and Data Acquisition (SCADA) systems.