Field Instrumentation
Field Instrumentation refers to the sensors and actuators that are directly interfaced to the plant or equipment. They generate the analog and digital signals that will be monitored by the Remote Station. Signals are also conditioned to make sure they are compatible with the inputs/outputs of the RTU or PLC at the Remote Station.
Field Instrumentation refers to the devices that are connected to the equipment or machines being controlled and monitored by the SCADA system. These are sensors for monitoring certain parameters; and actuators for controlling certain modules of the system.These instruments convert physical parameters (i.e., fluid flow, velocity, fluid level, etc.) to electrical signals (i.e., voltage or current) readable by the Remote Station equipment. Outputs can either be in analog (continuous range) or in digital (discrete values). Some of the industry standard analog outputs of these sensors are 0 to 5 volts, 0 to 10 volts, 4 to 20 mA and 0 to 20 mA. The voltage outputs are used when the sensors are installed near the controllers (RTU or PLC). The current outputs are used when the sensors are located far from the controllers.
Digital outputs are used to differentiate the discrete status of the equipment. Usually, <1> is used to mean EQUIPMENT ON and <0> for EQUIPMENT OFF status. This may also mean <1> for FULL or <0> for EMPTY.
Actuators are used to turn on or turn off certain equipment. Likewise, digital and analog inputs are used for control. For example, digital inputs can be used to turn on and off modules on equipment. While analog inputs are used to control the speed of a motor or the position of a motorized valve.
Digital outputs are used to differentiate the discrete status of the equipment. Usually, <1> is used to mean EQUIPMENT ON and <0> for EQUIPMENT OFF status. This may also mean <1> for FULL or <0> for EMPTY.
Actuators are used to turn on or turn off certain equipment. Likewise, digital and analog inputs are used for control. For example, digital inputs can be used to turn on and off modules on equipment. While analog inputs are used to control the speed of a motor or the position of a motorized valve.
The Remote Station
The Remote Station is installed at the remote plant or equipment being monitored and controlled by the central host computer. This can be a Remote Terminal Unit (RTU) or a Programmable Logic Controller (PLC). Field instrumentation connected to the plant or equipment being monitored and controlled are interfaced to the Remote Station to allow process manipulation at a remote site. It is also used to gather data from the equipment and transfer them to the central SCADA system. The Remote Station may either be an RTU (Remote Terminal Unit) or a PLC (Programmable Logic Controller). It may also be a single board or modular unit.
The RTU (Remote Terminal Unit) is a ruggedized computer with very good radio interfacing. It is used in situations where communications are more difficult. One disadvantage of the RTU is its poor programmability. However, modern RTUs are now offering good programmability comparable to PLCs. The PLC (Programmable Logic Controller) is a small industrial computer usually found in factories. Its main use is to replace the relay logic of a plant or process. Today, the PLC is being used in SCADA systems to due its very good programmability. Earlier PLC’s have no serial communication ports for interfacing to radio for transferring of data. Nowadays, PLC's have extensive communication features and a wide support for popular radio units being used for SCADA system. In the near future we are seeing the merging of the RTUs and the PLC’s. Micrologic is offering an inexpensive RTU for SCADA system wherein the PLC may be an overkill solution. It is a microcontroller-based RTU and can be interfaced to radio modems for transmitting of data to the CMS.
| Remote Terminal Unit (RTU) |
| Programmable Logic Controller (PLC) |
The Communications Network
The Communications Network is the medium for transferring information from one location to another. This can be via telephone line, radio or cable. The Communication Network refers to the communication equipment needed to transfer data to and from different sites. The medium used can either be cable, telephone or radio. The use of cable is usually implemented in a factory. This is not practical for systems covering large geographical areas because of the high cost of the cables, conduits and the extensive labor in installing them. The use of telephone lines (i.e., leased or dial-up) is a cheaper solution for systems with large coverage. The leased line is used for systems requiring on-line connection with the remote stations. This is expensive since one telephone line will be needed per site. Besides leased lines are more expensive than ordinary telephone line. Dial-up lines can be used on systems requiring updates at regular intervals (e.g., hourly updates). Here ordinary telephone lines can be used. The host can dial a particular number of a remote site to get the readings and send commands. Remote sites are usually not accessible by telephone lines. The use of radio offers an economical solution. Radio modems are used to connect the remote sites to the host. An on-line operation can also be implemented on the radio system. For locations wherein a direct radio link cannot be established, a radio repeater is used to link these sites.
Central Monitoring Station
The Central Monitoring Station (CMS) refers to the location of the master or host computer. Several workstation may be configured on the CMS, if necessary. It uses a Man Machine Interface (MMI) program to monitor various types data needed for the operation. The following is a sample configuration of a SCADA system for water distribution.
The Central Monitoring Station (CMS) is the master unit of the SCADA system. It is in charge of collecting information gathered by the remote stations and of generating necessary action for any event detected. The CMS can have a single computer configuration or it can be networked to workstations to allow sharing of information from the SCADA system.
A Man-Machine Interface (MMI) program will be running on the CMS computer. A mimic diagram of the whole plant or process can be displayed onscreen for easier identification with the real system. Each I/O point of the remote units can be displayed with corresponding graphical representation and the present I/O reading. The flow reading can be displayed on a graphical representation of a flowmeter. A reservoir can be displayed with the corresponding fluid contents depending on the actual tank level.
Set-up parameters such as trip values, limits, etc. are entered on this program and downloaded to the corresponding remote units for updating of their operating parameters.
The MMI program can also create a separate window for alarms. The alarm window can display the alarm tag name, description, value, trip point value, time, date and other pertinent information. All alarms will be saved on a separate file for later review.
A trending of required points can be programmed on the system. Trending graphs can be viewed or printed at a later time. Generation of management reports can also be scheduled on for a specific time of day, on a periodic basis, upon operator request, or event initiated alarms.
Access to the program is permitted only to qualified operators. Each user is given a password and a privilege level to access only particular areas of the program.. All actions taken by the users are logged on a file for later review.
A Man-Machine Interface (MMI) program will be running on the CMS computer. A mimic diagram of the whole plant or process can be displayed onscreen for easier identification with the real system. Each I/O point of the remote units can be displayed with corresponding graphical representation and the present I/O reading. The flow reading can be displayed on a graphical representation of a flowmeter. A reservoir can be displayed with the corresponding fluid contents depending on the actual tank level.
Set-up parameters such as trip values, limits, etc. are entered on this program and downloaded to the corresponding remote units for updating of their operating parameters.
The MMI program can also create a separate window for alarms. The alarm window can display the alarm tag name, description, value, trip point value, time, date and other pertinent information. All alarms will be saved on a separate file for later review.
A trending of required points can be programmed on the system. Trending graphs can be viewed or printed at a later time. Generation of management reports can also be scheduled on for a specific time of day, on a periodic basis, upon operator request, or event initiated alarms.
Access to the program is permitted only to qualified operators. Each user is given a password and a privilege level to access only particular areas of the program.. All actions taken by the users are logged on a file for later review.
| Central Monitoring Station (CMS) |
