LHC-GCS Process Tuning: selection and use of PID and Smith predictor for the regulations of the LHC experiments' gas systems

The LHC experiment’s Gas Control System (LHC GCS) has to provide LHC experiments with homogeneous control systems (supervision and process control layers) for their 23 gas systems. The LHC GCS process control layer is based on Programmable Logic Controllers (PLCs), Field-Buses and on a library, UNICOS (UNified Industrial COntrol System). Its supervision layer is based on a commercial SCADA system and on the JCOP and UNICOS PVSS frameworks. A typical LHC experiment’s gas system is composed of up to ten modules, dedicated to specific functions (e.g. mixing, purification, circulation). Most of modules require control loops for the regulation of pressures, temperatures and flows or ratios of gases. The control loops of the 23 gas systems can be implemented using the same tools, but need specific tuning according to their respective size, volume, pipe lengths and required accuracy. Most of the control loops can be implemented by means a standard PID (Proportional, Integral and Derivative) controller. When this is not appropriate the Smith Predictor can be used as an alternative. This paper will describe the limitations of a standard PID approach as well as the results of the Smith Predictor implementation when a PID controller is insufficient. It will also explain the feasibility, identification, testing and the conclusions of both approaches.