What is a Distributed Control System (DCS)?

By Larry O'Brien

Industry Trends

A typical  Distributed Control System (DCS) consists of  functionally and/or spatially distributed  digital controllers capable of executing from one to 256 or more regulatory control loops  in  one  controller “box.”  The sensor I/O  can  be  integral  with  the controller  or  located  remotely  via  a  field  network.    Controllers  currently available  have  extensive  computational  capabilities  and,  in  addition to continuous PID  control,  generally  can also perform  logic  and sequence control.  

A DCS may employ one or several workstations and can be configured at  the  workstation  or  by  an  off-line personal  computer.   A control network  with  transmission  over  twisted  pair,  coaxial,  or  fiber  optic cables handles  local  communication.    A  server  and/or  applications processor  may  be  included  in  the  system to provide system  services as  well  as  additional computational,  data storage,  and  reporting capability.

Distributed Control System (DCS) Hardware

With  the increasingly  amorphous  nature  of  evolving  DCS  architecture and the  ever-expanding  business model  of  DCS  suppliers,  ARC  has  made  an effort to focus its definition of DCS, rather than including all possible inter-related elements  of  plant  automation. Distributed Control System (DCS) Size Definitions

At  the  hardware  level,  DCS  begins  at  the sensor I/O  and extends all  the way through controllers, application processors, workstations, PCs, and the networking  equipment falling within  the  DCS  control  domain.   PLCs and PLC-type controllers sold as part of an integrated system offering, such as  the  Siemens  Simatic  PCS  7  and  Rockwell  Logix  Integrated  Architecture system, are included in our definition of DCS.  Also included are DCS controllers that perform PLC functions only and replace PLCs in an integrated system.  

Excluded  from  the  DCS  hardware  definition  are  field  instruments,  control valves,  analytical  devices,  all specialty  measurement  equipment(such  as paper  machine  gauges  and  condition  monitoring  devices), and  any  other devices  that  exist  outside  the  I/O  level  of  the  system.   Also  excluded are dedicated supervisory   workstations   that   execute applications   such   as advanced    control,    production   management, real-time optimization, simulation, or plant asset management.

ARC’s  primary  segmentation  of  the  DCS  marketplace  is  by  DCS  size.  Segmentation  of DCSs  into  small,  medium,  and  large  systems is based  on hardware content.   Hardware    specifically    includes    all    controllers, application  processors,  I/O modules,  workstations, and  communications networks.    PCs  used  for  system  configuration  and  real-time  control  are included  in  DCS scope,  as  are  computers  used  for supervisory  control, if sold as part of the system.  ARC also includes PLCs sold by DCS suppliers that are integrated with the DCS.  

Distributed Control System (DCS) Software

DCS  software  includes  embedded controller  software  as  well  as  some,  but not  all,  software  sold  bundled  with  a  system.    DCS  software  includes control, HMI, systems management software,     engineering  and configuration software, and plant information management (PIM) software such as bundled data historians in addition  to performing  HMI  functions,  DCS  HMI  software  can  also perform  functions  such  as  I/O  communication.    HMI  software  typically resides  in  the  DCS  workstation  or  PC.  Control  software  typically  resides embedded in    controllers and performs    basic    control    functions.  Programming   software, used   to   program   and   configure   the   various functions  of  the  control  system  at  startup, accounts  for  a  small  part  of  the total DCS software business.

The  DCS  definition  excludes  optional  supervisory software  that  performs production   management   (MES) and   advanced   control   functions and typically   resides  in a supervisory DCS server.  Supervisory   software includes   categories   such   as   advanced   process   control(APC), model-predictive control (MPC),  real-time  process  optimization  (RPO),  process simulation  and  optimization  (PSO),  and  plant  asset  management  (PAM) applications.  

Distributed Control System (DCS) Applications

From   an   application perspective, ARC includes all DCSs sold for continuous and batch control. We have included SCADA systems sold on a traditional DCS platform for oil & gas and water & wastewater applications.  However, dedicated SCADA  systems are not included.

ARC  includes  quality  control  systems  (QCSs)  for  paper,  plastic  sheet,  and similar  applications,  such  as those sold  by  ABB,  Honeywell,  and  Metso.  Custom and specialty measurement equipment for these applications, such as  profilers  and  vision  systems,  are  not  included.   Only  the  workstations, I/O, and related hardware, software, and services under the DCS definition are included in QCS definition.

ARC’s DCS definition excludes all  safety  systems. Safety  systems  and  emergency  shutdown (ESD) systems are governed by their own set of rules and standards that are distinct from the area of process control  and  automation.   Safety  system  hardware, software, and services are excluded from the scope of  this  study  regardless  of  their  application. 

ARC has also excluded systems serving dedicated automation applications (for  example turbine  control  systems and burner  management  systems) except  when  these  applications  are  implemented  using  DCS  equipment.  For   example,   a   PLC-like   turbine   controller   provided   by   a   turbine automation  specialist  would  be  excluded,  while  a  turbine  control  system implemented  on  an  Emerson  Ovation  or  ABB  Symphony  Plus  platform would be included.

Distributed Control System (DCS) Project Services

Project   services   include   all   services   provided   by   DCS   suppliers   for installing,  configuring,  and  commissioning  DCSs,  but  only services  for the hardware  and  software included  in  the  DCS  scope.  It  does  not  include services  for field  instruments  or  other non-DCS plant  equipment. 

The project  services segment includes only  those  services provided  by  direct  employees  of  DCS  suppliers  and  exclude  indirect contributors.    For  example,  project  services  performed  by  distributors  and authorized systems integrators    (SIs)    for    DCS suppliers  are  excluded  from  the  overall  tabulation.  Also  excluded   are  project   services  performed  by representatives of instrument companies.  ARC also excluded   DCS   project   services delivered by end users,     consultants,     EPCs,     process     equipment manufacturers, and independent SIs

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