Similar, but not the same
This technical article shed light on terminology used in the field of SCADA and industrial automation. The terms SCADA, distributed control system (DCS), programmable logic controller (PLC), remote terminal unit (RTU) and smart instrument are very important when we’re talking about concept of a industrial telemetry system.
- Distributed control system (DCS)
- Programmable logic controller (PLC)
- Remote terminal unit (RTU) and
- Smart instrument
A SCADA (or supervisory control and data acquisition) system means a system consisting of a number of remote terminal units (or RTUs) collecting field data connected back to a master station via a communications system.
The master station displays the acquired data and also allows the operator to perform remote control tasks.
The accurate and timely data (normally real-time) allows for optimization of the operation of the plant and process. A further benefit is more efficient, reliable and most importantly, safer operations. This all results in a lower cost of operation compared to earlier non-automated systems.
There is a fair degree of confusion between the definition of SCADA systems and process control system. SCADA has the connotation of remote or distant operation.
A successful SCADA installation depends on utilizing proven and reliable technology, with adequate and comprehensive training of all personnel in the operation of the system.
There is a history of unsuccessful SCADA systems – contributing factors to these systems includes inadequate integration of the various components of the system,
unnecessary complexity in the system, unreliable hardware and unproven software.
It should be noted in passing that many operators judge a SCADA system not only by the smooth performance of the RTUs, communication links and the master station (all falling under the umbrella of SCADA system) but also the field devices (both transducers and control devices).
The field devices however fall outside the scope of SCADA in this manual and will not be discussed further. A diagram of a typical SCADA system is given below.
On a more complex SCADA system there are essentially five levels or hierarchies:
- Field level instrumentation and control devices
- Marshalling terminals and RTUs
- Communications system
- The master station(s)
- The commercial data processing department computer system
The RTU provides an interface to the field analog and digital signals situated at each remote site.
The communications system provides the pathway for communications between the master station and the remote sites. This communication system can be radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data.
The master station (and submasters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, submaster sites gather information from remote sites and act as a relay back to the control master station.
In addition there has been a growing trend to use smart instruments as a key component in all these systems. Of course, in the real world, the designer will mix and match the four approaches to produce an effective system matching his/her application.
Considerations of SCADA system
Typical considerations when putting a SCADA system together are:
- Overall control requirements
- Sequence logic
- Analog loop control
- Ratio and number of analog to digital points
- Speed of control and data acquisition
- Master/operator control stations
- Type of displays required
- Historical archiving requirements
- System consideration
- Speed of communications/update time/system scan rates
- System redundancy
- Expansion capability
- Application software and modeling
Benefits of a SCADA system
Obviously, a SCADA system’s initial cost has to be justified. A few typical reasons for implementing a SCADA system are:
- Improved operation of the plant or process resulting in savings due to optimization of the system
- Increased productivity of the personnel
- Improved safety of the system due to better information and improved control
- Protection of the plant equipment
- Safeguarding the environment from a failure of the system
- Improved energy savings due to optimization of the plant
- Improved and quicker receipt of data so that clients can be invoiced more quickly and accurately
- Government regulations for safety and metering of gas (for royalties & tax etc)
Definition – In a DCS, the data acquisition and control functions are performed by a number of distributed microprocessor-based units situated near to the devices being controlled or the instrument from which data is being gathered.
The data highway is normally capable of fairly high speeds (typically 1 Mbps up to 10 Mbps).
Since the late 1970s, PLCs have replaced hardwired relays with a combination of ladder–logic software and solid state electronic input and output modules.
They are often used in the implementation of a SCADA RTU as they offer a standard hardware solution, which is very economically priced.
How Are Field Devices Wired to PLCs (VIDEO)
An RTU (sometimes referred to as a remote telemetry unit) as the title implies, is a standalone data acquisition and control unit, generally microprocessor based, which monitors and controls equipment at some remote location from the central station.
It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station. There is also a facility to be configured locally by some RTU programming unit.
Although traditionally the RTU communicates back to some central station, it is also possible to communicate on a peer-to-peer basis with other RTUs. The RTU can also act as a relay station (sometimes referred to as a store and forward station) to another RTU, which may not be accessible from the central station.
Small sized RTUs generally have less than 10 to 20 analog and digital signals, medium sized RTUs have 100 digital and 30 to 40 analog inputs. RTUs, having a capacity greater than this can be classified as large.
A typical RTU configuration is shown in Figure 5:
Typical RTU hardware modules include:
- Control processor and associated memory
- Analog inputs
- Analog outputs
- Counter inputs
- Digital inputs
- Digital outputs
- Communication interface(s)
- Power supply
- RTU rack and enclosure
Typical requirements for an RTU system:
In the writing of a specification, the following issues should be considered:
Individual RTU expandability (typically up to 200 analog and digital points)
- Off the shelf modules
- Maximum number of RTU sites in a system shall be expandable to 255
- Modular system – no particular order or position in installation (of modules in a rack)
- Robust operation – failure of one module will not affect the performance of other modules
- Minimization of power consumption (CMOS can be an advantage)
- Heat generation minimized
- Rugged and of robust physical construction
- Maximization of noise immunity (due to harsh environment)
- Temperature of –10 to 65°C (operational conditions)
- Relative humidity up to 90%
- Clear indication of diagnostics
- Visible status LEDs
- Local fault diagnosis possible
- Remote fault diagnostics option
- Status of each I/O module and channel (program running/failed/communications OK/failed)
- Modules all connected to one common bus
- Physical interconnection of modules to the bus shall be robust and suitable for use in harsh environments
- Ease of installation of field wiring
- Ease of module replacement
- Removable screw terminals for disconnection and reconnection of wiring
The RTU is normally installed in a remote location with fairly harsh environmental conditions.
Typically it is specified for the following conditions:
- Ambient temperature range of 0 to +60°C (but specifications of –30°C to 60°C are not uncommon)
- Storage temperature range of –20°C to +70°C
- Relative humidity of 0 to 95% non condensing
- Surge withstand capability to withstand power surges typically 2.5 kV, 1 MHz for 2 seconds with 150 ohm source impedance
- Static discharge test where 1.5 cm sparks are discharged at a distance of 30 cm from the unit
- Other requirements include dust, vibration, rain, salt and fog protection.
Software (and firmware)
- Compatibility checks of software configuration of hardware against actual hardware available
- Log kept of all errors that occur in the system both from external events and internal faults
- Remote access of all error logs and status registers
- Software operates continuously despite powering down or up of the system due to loss of power supply or other faults
- Hardware filtering provided on all analog input channels
- Application program resides in non volatile RAM
- Configuration and diagnostic tools for:
- System setup
- Hardware and software setup
- Application code development/management/operation
- Error logs
- Remote and local operation
Another device that should be mentioned for completeness is the smart instrument which both PLCs and DCS systems can interface to.
Although this term is sometimes misused, it typically means an intelligent (microprocessor based) digital measuring sensor (such as a flow meter) with digital data
communications provided to some diagnostic panel or computer based system.
Reference // Practical SCADA for Industry by David Bailey, Edwin Wright