Similar, but not the same
This technical article sheds light on the terminology used in the field of SCADA and industrial automation. The terms SCADA, Distributed Control System (DCS), Programmable Logic Controller (PLC), Remote Terminal (RTU) and Intelligent Instrument are very important when it comes to industrial telemetry systems concept.
Five terms you MUST know: SCADA, DCS, PLC, RTU and Smart Instrument (pictured: controller panel)
Let’s see these five terms in light:
- Distributed Control System (DCS)
- Programmable logic controller (PLC)
- Remote terminal (RTU) and
- Smart instrument
A SCADA system (or supervisory control and data acquisition) refers to a system consisting of a certain number of remote terminals (or RTUs) collecting field data connected to a master station via a communication system.
The main station displays the acquired data and also allows the operator to perform remote control tasks.
Accurate and up-to-date data (normally in real time) helps optimize plant and process operation. Another advantage is efficiency, reliability and, more importantly, the safety of operations. This translates into a lower operating cost than previous non-automated systems.
There is a reasonable degree of confusion between the definition of SCADA systems and that of the process control systems. SCADA has the connotation of remote or remote operation.
The inevitable question is how far the ‘distance’ is – typically this means over such a distance that the distance between the controlling location and the controlled location is such that direct wire control is impractical (ie. that is, a communication link is an essential component of the system).
A successful SCADA installation depends on using proven and reliable technology, with adequate and comprehensive training of all personnel in the use of the system.
SCADA systems have always failed. Inadequate integration of the various system components has also contributed to these systems.
unnecessary system complexity, unreliable hardware, and unproven software. Today’s hardware reliability is less of a problem, but the increasing complexity of software brings new challenges.
Il convient de noter en passant que de nombreux opérateursjuger un système SCADA non seulement par le bon fonctionnement des RTU, des liaisons de communication et du poste principal (qui relèvent tous du système SCADA), mais également des dispositifs de terrain (transducteurs et dispositifs de contrôle).
Les appareils de terrain n’entrent cependant pas dans le champ d’application de SCADA dans ce manuel et ne seront pas traités plus en détail Un schéma d’un système SCADA typique est donné ci-dessous.
On a more complex SCADA system, there are basically five levels or hierarchies:
- Field instrumentation and control devices
- Marshalling terminals and RTUs
- Communication system
- The master position (s)
- The computer system of the IT department
The RTU provides an interface to the analogue and digital field signals located at each remote site.
The communications system provides the path for communications between the master station and 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 optimal data transfer.
The main station (and sub-masters) collect data from different RTUs and generally provide an operator interface for viewing information and controlling remote sites. In large telemetry systems, sub-master sites collect information from remote sites and act as a relay to the main command post.
SCADA technology has been around since the early 1960s and two other competing approaches are now possible: Distributed Control System (DCS) and Programmable Logic Controller (PLC).
In addition, there has been a growing trend to use smart instruments as a key component of all of these systems. Of course, in the real world, the designer will combine the four approaches to create an effective system corresponding to his application.
SCADA system considerations
Typical considerations when mounting a SCADA system are as follows:
- Global control requirements
- Sequence logic
- Analog loop control
- Ratio and number of analogue to digital points
- Speed of control and data acquisition
- Master / operator control stations
- Display type required
- Historical archiving conditions
- System consideration
- Reliability / availability
- Communication speed / update time / system scan rate
- System redundancy
- Expansion capacity
- Application software and modelling
Benefits of a SCADA system
Obviously, the initial cost of a SCADA system must be justified. Some typical reasons for implementing a SCADA system are as follows:
- Improved plant or process operation resulting in cost savings through system optimization
- Increase in staff productivity
- Improved system security through better information and better control
- Plant equipment protection
- Protecting the environment from system failure
- Improved energy savings thanks to the optimization of the installation
- Improved and fast data reception so customers can be billed faster and more accurately
- Government regulations on security and gas metering (fees, taxes, etc.)
2. Distributed control system (DCS)
Definition – In a DCS, data acquisition and control functions are performed by a number of distributed microprocessor-based units located near the controlled devices or the instrument from which the data is collected.
DCS systems have evolved into systems providing highly sophisticated analogue control capability (eg, loop). A tightly integrated set of operator interfaces (or human-machine interfaces) are provided to allow easy system configurations and operator control.
The data highway is normally capable of fairly high speeds (typically 1 Mbps to 10 Mbps).
Programmable logic controller (PLC)
Since the late 1970s, programmable logic controllers have replaced hard-wired relays with a combination of relay logic software and solid-state electronic input and output modules.
They are often used in the implementation of a SCADA RTU because they offer a standard hardware solution, which is very economical.
An RTU (sometimes called a remote telemetry unit) as the title suggests, is a stand-alone data acquisition and control unit, usually based on a microprocessor, which monitors and controls equipment at a location remote from the substation. central
Its main task is to control and acquire data from the processing equipment at the remote site and transfer it to a central station.
It usually also has the option of having its configuration and control programs dynamically downloaded from a central station. It is also possible to locally configure an RTU programming unit.
Although traditionally the RTU communicates with a central station, it is also possible to communicate peer-to-peer with other RTUs. The RTU can also act as a relay station (sometimes referred to as a recording station and retransmission) to another RTU, which may not be accessible from the central station.
Typical RTU hardware modules include:
- Control processor and associated memory
- Analogue inputs
- Analogue outputs
- Counter entries
- Digital inputs
- Digital outputs
- The communication interface (s)
- Power source
- RTU rack and enclosure
Typical requirements for an RTU system:
When writing a specification, the following issues should be considered:
Individual RTU expandability (typically up to 200 analogue and digital points)
- Modules on the shelf
- The maximum number of RTU sites in a system must be expandable to 255
- Modular system – no particular order or installation position (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)
- Minimized heat production
- Robust and robust physical construction
- Maximization of noise immunity (due to the hostile environment)
- Temperature from –10 to 65 ° C (operating conditions)
- Relative humidity up to 90%
- A clear indication of the diagnosis
- Visible status LEDs
- Local fault diagnosis possible
- Remote fault diagnosis option
- Status of each module and I / O channel (program running/failed / communication OK / failure)
- Modules all connected to a common bus
- The physical interconnection of the modules to the bus must be robust and suitable for use in harsh environments
- Ease of on-site wiring installation
- Ease of module replacement
- Removable screw terminals for disconnection and reconnection of wiring
The RTU is normally installed in a remote location under fairly harsh environmental conditions.
Typically, it is specified for the following conditions:
- Ambient temperature range 0 to + 60 ° C (but specifications of –30 ° C to 60 ° C are not uncommon)
- Storage temperature range from –20 ° C to + 70 ° C
- Relative humidity 0 to 95% non-condensing
- Surge resistor able to withstand surges, typically 2.5kV, 1MHz for 2 seconds with a 150-ohm source impedance
- Static discharge test where 1.5cm sparks are discharged at a distance of 30cm from the unit
- Other requirements include protection against dust, vibration, rain, salt and fog.
Software (and firmware)
- Hardware-software configuration compatibility checks against actually available hardware
- Log all errors that occur in the system both from external events and internal failures
- Remote access to all error logs and status registers
- The software runs continuously despite powering off or on the system due to power failure or other failures
- Hardware filtering provided on all analogue input channels
- Application program resides in non-volatile RAM
- Configuration and diagnostic tools for:
- System installation
- Hardware and software configuration
- Development / management / exploitation of application code
- Error logs
- Remote and local operation
Another device that should be mentioned for completeness is the intelligent instrument to which PLC and DCS systems can interface.
Although this term is sometimes misused, it generally refers to an intelligent digital measuring sensor (such as a flowmeter) (based on a microprocessor) with digital data.
communications provided to a diagnostic panel or computer system.