Evaluation of Software and Electronics Technologies for the Control of the E-elt Instruments: a Case Study

In the scope of the evaluation of architecture and technologies for the control system of the E-ELT (European-Extremely Large Telescope) instruments, a collaboration has been set up between the Instrumentation and Control Group of the INAF-OATs and the ESO Directorate of Engineering. The first result of this collaboration is the design and implementation of a prototype of a small but representative control system for an E-ELT instrument that has been setup at the INAFOATs premises. The electronics has been based on PLCs (Programmable Logical Controller) and Ethernet based fieldbuses from different vendors but using international standards like the IEC 61131-3 and PLCopen Motion Control. The baseline design for the control software follows the architecture of the VLT (Very Large Telescope) Instrumentation application framework but it has been implemented using the ACS (ALMA Common Software), an open source software framework developed for the ALMA project and based on CORBA middleware. The communication among the software components is based on two models: CORBA calls for command/reply using the client/server paradigm and CORBA notification channel for distributing the devices status using the publisher/subscriber paradigm. The communication with the PLCs is based on OPC UA, an international standard for the communication with industrial controllers. The results of this work will contribute to the definition of the architecture of the control system that will be provided to all consortia responsible for the actual implementation of the E-ELT instruments. This paper presents the prototype motivation, its architecture, design and implementation.