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The CMS RPC D etector C ontrol S ystem: First operational experiences. Giovanni Polese Lappeenranta University of Technology On behalf of CMS RPC Collaboration. CHEP'09 17th International Conference on Computing in High Energy and Nuclear Physics 21 - 27 March 2009 Prague, Czech Republic.

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Detector Control System:

First operational experiences

Giovanni Polese

Lappeenranta University of Technology

On behalf of CMS RPC Collaboration


17th International Conference on Computing in High Energy and Nuclear Physics21 - 27 March 2009 Prague, Czech Republic

  • The RPC system in CMS
  • RPC DCS mission, functionalities and challenges.
  • Hardware and software: status and performances
  • DCS experience during cosmic global runs
The CMS RPC System


Part of the CMS muon system, the RPC detector has fast time resolutions (few ns) and a good spatial resolution ( cm), thatassure robustness and redundancy to the muon trigger.

The RPC in CMS

6 layers of RPCs are embedded in the barrel iron yoke closely following the DT segmentation.

The forward region is instrumented with four layers of RPCs covering up to η= 2.1.

A total of 480 + 432 RPC chambers at startup.

Role of the RPC DCS
  • The Detector Control System (DCS) is aimed to assure a constant, safe a coherent operation of CMS.
  • Detect abnormal and harmful situations, and take protective and automatic actions to minimize consequential damages


  • The RPC DCS controls and configure the detector and its electronics, calibrate it, monitor all detector conditions (but not event data) and its performance by mean of several average parameters that determine the effective behavior of the detector.
  • From the detector point of view, the DCS is in charge to provide and monitor the V-A characteristic, the environmental and operational conditions and correlate them with gas flowandgas mixture composition
RPC DCS layout
  • Several subsystems located around all the detector and in the electronic room.
  • Different communication protocols
  • One common supervisor
RPC DCS software
  • RPC DCS software applications is distributed among several computers, exploiting the distribution capability of the commercial ETM SCADA (Supervisory Control And Data Acquisition) software used, PVSS, enhanced by the standard Joint Control Project (JCOP) framework components.
  • The front end electronics control is developed via XDAQ, the official CMS online software framework, and the PVSS communication is handled via PSX server.
  • The main functionalities:
  • Final state machine architecture,
  • the graphical user interface,
  • the alarm handler and
  • the ORACLE database interface, for storing the data in the CMS online database and the loading of the hardware configuration from the CMS configuration database.

RPC Power system

Every RPC chamber has been equipped with 2 independent HV channels (one per layer up to 12kV ) and 2 LV channels for Front end boards. In addition 4 LV channels per sector are needed for supply the very front end part of the trigger chain.




> 90 m




RPC power system performance

The system is composed by 912 HV + 1580 LV channels and controlled by 4 different computers to optimize the CPU load

  • Stability during all the operation phase.
  • Reliability, failure rate less then 4%
  • All repairs realized without delaying operation

Power System Software challenge

20k parameters @ 100 Mbytes/hour raw data rates

RPC power system: Timing performance

Based on the OPC server communication protocol, developed by CAEN. It is based on an event-driven approach, where the most significant parameters are handled with a 2 s refresh time.

Hot startup time of the entire system (OFF->ON) is about 470 s.

Mainly depending by detector operation mode requests

(i.e. ramping up settings)

Max load per OPC server in our conf. : 480 channels.

Max parameters set simultaneously per ch during operation : 3

RPC environmental control
  • The performance of the RPC detector are strongly related to the temperature and humidity, in particular the noise rate and the dark current of the chamber.
  • The environmental sensor network is composed by about 500 sensors to measure the gap temperature, 60 sensors to measure the temperature of the gas and 60 relative humidity sensors.
  • Readout provide by EASY CAEN ADC, they assure the robustness, reliability and precision required (~ 0.1 °C for T, 2.5% for RH ) and can operate in the radiation and magnetic field environment.

All < 22 °C, max accepted value 24 °C

Front end boards monitoring
  • The RPC system is composed with about 7000front end boards, located on the chamber and aimed for detector calibration.
  • 20k parameters to be monitored like thresholds, boards temperature and status.
  • The control and communication is developed via CCU ring through XDAQ and then sent via soap messages to PSX/PVSS.
  • Values are stored in condition db and transferred into offline db for offline analysis.
  • Average bandwidth measured during normal operation for the entire chain is ~2Kb/s



RPC Gas monitoring

The gas quality and the mixture composition are of primary importance in the operation of the RPC system.

The gas system is controlled by a CERN centralized system, LHC GCS project. It aims to acquire the data from the Programmable Logic Controllers (PLCs) and supervise it with a dedicated control system .

Main information, like gas flow, and gas mixture composition, are acquired via DIP protocol by the RPC DCS and correlate online with other operational parameters online, assuring a general overview of the detector status to optimize its behavior.

RPC Supervisor

All the auxiliary systems under the RPC DCS control are handled centrally by a unique RPC Supervisor application, aimed to summarize and correlated the status of the entire detector and publish it to the central DCS.

Based on the same FSM logic, the central DCS sends commands, and reads back alarms and messages directly to the RPC DCS, publishing to the CMS Run Control the RPC status condition.

To synchronize the operation with the RPC acquisition, a direct connection to the RPC Run Control is foreseen, allowing to operate the RPC in standalone mode, during the commissioning and calibration phase.

RPC DCS Control Architecture

The RPC DCS software architecture has been developed following a hierarchical design, creating different tree-like structures: geographical and hardware view.

The hierarchical tree structure allows only vertical data flow: commands move downwards, while alarms and state changes propagate upwards.


Finite State Machine
  • The FSM offers an easy, powerfuland safe way to get the full detector control:
    • limited number of state,
    • drive it to predefined configuration,
    • all operation mode translate in simple actions.

Commands are propagated through the RPC FSM tree down to the devices, where they are interpreted accordingly as hardware commands.

In addition the status of the system is described by alarm messages that define how well the system is working (OK, WARNING, ERROR, FATAL) and alert about possible changes of conditions

Graphical User Interface











DCS on duty: Global Runs
  • During the last 2 years CMS has been performing numerous “Global Runs”, i.e. periods of data taking with cosmics
  • The whole RPC system has been commissioned using this data. Several millions of cosmic have been collected during this period.
  • Final Power, environmental and Gas systems has been operating since Summer 2007 as well as the final DAQ software and DCS, implemented for the detector readout and control.
  • About 3 GBytes of not-event data stored during the last years GRs.
  • Storing rate 30 Mbytes/day with a prescale factor about 200.





  • The RPC DCS system:
    • offers a complete solution for all auxiliary systems involved in the detector operation,
    • It is reliable and stable, able to drive the detector behavior during all operative phases,
    • A useful tools for a prompt detector physics analysis and a powerful tools to prevent serious damages,
    • It has been successfully tested and used during the last years for the RPC commissioning and CMS global runs.