Remote Terminal Unit – RTU or Station Computer, is one of the common solutions used in many automation systems. It is used for data acquisition, processing and transfer technological information from the object to higher system levels.
The specifics of the different types of automation object reflects here in the different field-busses and protocols used in the various industries.
So, on this level, in the Energetics the IEC 60870-5-103, IEC 61850 and Modbus, in the railway-objects – CAN_R and/or Profibus, in the industry automation – DeviceNet and Profibus, field-busses and protocols are used.
The structure of the RTU could be
The centralized structure is based on common parallel system BUS like VME BUS or PCI BUS, and includes:
- system- CPU modules with PowerPC or ARM processors
- various passive and intelligent peripheral modules incl.
– digital I/O’s
– analog I/O’s
– communication controllers etc.
- various interface modules for object
For this case all device signals are delivered to a given RTU.
The decentralized structure consists of:
- main CPU module
- a different number of single board computers – bay controllers
- a different number and type of extension peripheral modules, as also
- communication controllers
Here the device signals are fetched locally, direct on the technological devices and are delivered to the main CPU serial or by local LAN’s.
The RTU is intent to:
- acquire and process information inside the object
- supervise, control and manage technological processes inside object
- acquisition and concentrating of process data coming from neighboring objects, which are not accessible from the belonging Control Center – Data Concentrator – DC
- select, prepare and simultaneously deliver to different Control Centers, the necessary for each of them process data
- archive, log and visualize all the object information
- provide the local operator the complete technological, operational and diagnostic information about the object
For this purpose the RTU supports the following function groups:
- communication functions
- acquisition- and primary data processing functions – technological functions
- control functions
The RTU supports two main communication levels – process and object.
The process level is about the data transfer from/to Intelligent Electronic Devices – IED, and including:
- protective relays, measuring devices, Bay controllers, energy meters, in energetics
- Central Relay Interlocking – CRI, in railway transport
- robots’ controllers, Computerized Numerical Control – CNC machines, in industry automation
The object level is used for the data transfer between RTU and Front_End Servers of the system. Furthermore the RTU works as data concentrator center, collecting data from other RTUs, that don’t have no direct connection with the Control Station.
Communication functions could be performed on different type of environments, interfaces and protocols:
- RS 232, RS 485
- SPI etc.
- IEC 60870 – 5 – 101
- IEC 60870 – 5 – 104
- IEC 60870 – 5 – 103
- IEC 60870 – 5 – 102
- Modbus etc.
RTU could work in parallel as Server/Slave to more than one subscribers – Clients/Masters.
RTU could select, extract or deliver required data to each subscriber.
All communication processes inside all levels and channels occur in parallel and independently of each other.
In case of information loss caused by connection injury, the RTU preserves all events occurring in the meantime, and resends them on connection recovery.
For more detailed information about the communication process, please visit “Technologies – Communication”.
The technological functions of the RTU are:
- to collect and process in real time the information coming in parallel from digital-, analog- and counter I/O’s, and/or serial from Intelligent Electronic Devices – IED
- to support in parallel and independent communication through various channels and protocols along the whole system hierarchy
- to discover status changes in the objects
- to register the occurrence of status changes with an accuracy about 10 msec.
- filter (hardware or software) various disturbances, generated by the industrial environment
- to provide the primary processing and plausibility check on the incoming data
- to process commands delivered by operators or system structures, found in higher levels, to check their validity and there activation conditions, and to send them for execution
- to communicate with a local Control Station – option
- to check, in case of 2 bit signals, the transition time and to create a “timeout surpassed” message by exceeding the limit, as also to mark the signal as invalid – “not topical”
- to provide operators request to extracting certain signals from the process, and substitute them manually by the operator’s manipulation. Such manipulations are recorded in the system archives.
- any telemeasures could be brought in the RTU directly by means of analog I/O’s or serial by communication channels or field networks
- on a selected set of process signals and different logical equations, defined by the user, to build diverse “logical” signals
- RTU software could be adopted to various technological or communication specifics of many IEDs, including relay protections, measuring devices, robot controllers, processors of CNC machines etc.
Allows the control of objects by different system levels:
- on-site or manually
- locally, by a local Control Station
- remotely from Remote Control Centers located on different System levels
The control of a certain object at certain time is given to exactly one Control Center.
The control could be done through single commands as well as a of command sequences.
The commands are executed on two stages. The first stage is to:
- verify the validity of the command
- check if all requirements for its execution are present
- check the condition of the command outputs
- check presence of other active command outputs
In case of successful verification, the chosen command output is selected.
On the second stage, the system powers up the chosen command output of the RTU:
- Normally, the command outputs are de-energized. They could be energized only during the command execution.
- The duration of a command, i.e. the time the command chain is energized, is a parameter defined in the configuration.
- The activation and executions of the commands is provided in compliance with the interlocking conditions, which are composed of a number of physical and/or logical signals, or logical equations on their basis, defined by the user.
- The correctness of the command execution is verified through the feedback status information, coming from the controled object.
The bay controller is designed to control and surveil small objects as well as to build an RTU with decentralized or mixed structure.
It consists of one single board computer with resources for additional 4 peripheral modules. The single board computer consists of:
- PowerPC or ARM CPU
- Ethernet controller
- 3 x RS 232/485 UART channels
- 32 serial I/O’s
- 2 x I2C interfaces etc.
Additionally on could extended with up to 4 boards, each with 40 I/O’s. The Bay controller functions like an RTU, but its information capacity is limited.