SCADA RTU (Outstation) versus PLC
Acronym for Supervisory Control And Data Acquisition, a computer system for gathering and analyzing real time data. SCADA systems are used to monitor and control a plant or equipment in industries such as telecommunications, water and waste control, energy, oil and gas refining and transportation. A SCADA system gathers information, such as, where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion. SCADA systems can be relatively simple, such as one that monitors environmental conditions of a small office building, or incredibly complex, such as a system that monitors all the activity in a nuclear power plant or the activity of a municipal water system. SCADA systems were first used in the 1960s. A lot of major companies offer hardware and software for implementing a SCADA system, such as Yokogawa, Honeywell, Emerson Process Management, ABB, Harris, Siemens, Rockwell Automation, HunterWaterTech, Serck Controls, etc.
SCADA RTU (Outstation) versus PLC
SCADA Remote Terminal Unit (RTU) or "Outstation" as someone calls it, are dedicated blocks of microprocessor-based hardware comprising of digital inputs, digital outputs, analog inputs and analog outputs. In addition to this basic Input/Output setup, it also comprises of an inbuilt power supply, modem, GSM, Ethernet port, RS485 port and a battery backup. These RTUs are also designed as data-loggers as they have a huge amount of RAM inbuilt on their motherboards. Therefore, if the communication links breaks down, then the RTU logs the data collected a long period of time (it can save log-data collected over a period of around 6 to 12 months) which can be downloaded once the communication link is up and running. RTU can be programmed in its own proprietary language or it can also support PLC ladder logic. RTU have come a long way since their introduction back in 1960 and now they are so advanced that they are "web-enabled" and can update a webpage over a communication network. This feature is very useful for web-enabled SCADA software, which allows users to monitor and control the SCADA network from any corner of the world. Being a feature-rich equipment, the price of a RTU is usually high.
Now-a-days its seen that some companies have started using PLC as a "make-shift" RTU. Maybe the intension is just to save a little amount of money, but the disadvantages are too high and the "return-on-investment" factor is not so good on a PLC-based SCADA RTU hardware. First of all, a PLC is NOT designed to meet the hostile/harsh environment as a RTU is suppose to !!! secondly, a PLC has limited usable registers and the feature of "data logging" is not possible. PLCs are not designed for any long distance (modem-based) communications. PLCs do not have a battery-backup facility and many more disadvantages are noticed when such a system is implemented. Its not that PLC should be totally avoided to function as a RTU, in smaller areas like a small sugar-factory or a glass-factory (just for example) a PLC-based SCADA system can be implemented, where investing a fortune is not justified.
One can use any standard SCADA software to communicate to the RTU/Outstations/PLC situated at remote sites via 2-wire 4-wire (P&T) telephone lines, gsm network, direct (point-to-point), or Ethernet based communication link. The SCADA software are very easy to use and one has to first design the mimic screens of the process to be monitored and controlled. Then the remote RTU/Outstation/PLC must be configured in the SCADA software and then linking of "points" to the mimic "objects" to be done. For example, if the object on the mimic screen is a "pump" and if it is linked to a digital input point of a distant pump using a RTU. Then whenever the pump starts, the mimic shows an animation on the screen and the operator of the system "sees" that the remote pump is now running. Similarly, objects of switches on the mimic screen are linked the actual digital outputs of the RTU at a remote site. Therefore, when the operator flips the mimic "switch" on his computer, the actual switching takes place at the remote site via RTU.
A
functional block diagram of a typical Gas Chromatograph consists of a
sample line
of the gas to be analyzed from the process stream by a sample probe
installed in the
process line. The sample passes through a sample line to the sample
conditioning system where it is filtered or otherwise conditioned. After
conditioning, the sample flows to the analyzer for separation and
detection of the components of the gas.
The chromatographic separation of the sample gas into its components is
accomplished in the analyzer in the following manner. A precise volume
of sample gas is injected into one of the units analytical columns. The
column contains a stationary phase (packing) that is either an active
solid (adsorption partitioning) or an inert solid support that is coated
with a liquid phase (absorption partitioning). The gas sample is moved
through the column by means of a mobile phase (carrier gas). Selective
retardation of the components of the sample takes place in the column
that causes each component to move through the column at a different
rate. This action separates the sample into its constituent gases and
vapors.
A detector located at the outlet of the analytical column senses the
elution of components from the column and produces electrical outputs
proportional to the concentration of each component. Outputs from the
analyzer detectors are pre-amplified in the analyzer electronics, then
transmitted to the controller for further processing. The controller
output is normally displayed on a service recorder or a printer. The
controller output may also be displayed on an optional Personal Computer
(IBM or equivalent) via the MODBUS compatible communication interface.
Sampling System
A well designed, properly adjusted sampling system is essential to
optimum performance of any process gas chromatograph. The following
points should be considered in selecting and installing a sampling
system.
Sampling Point Location
Gas samples must be representative of the process stream and must be
taken from a location where stratification or separation of components
does not occur. The sampling point should be as close as feasible to the
analyzer.
Sample Volume & Flow Rate
An adequate response time for sample analysis requires that sample
volumes should generally be as small as possible, and the flow rate
between the sampling point and the analyzer should be as high as
possible, consistent with accuracy. To minimize time lag and to prevent
back diffusion, dryers and filters in the sampling line should be as
small as possible. When long sampling lines cannot be avoided, flow
velocity in the line can be increased by decreasing the downstream
pressure. Regulating the pressure upstream of a needle valve or orifice
and venting the sample to atmosphere normally provides adequate flow
control. The possibility of entrainment can be minimized by using a
bypass loop to remove only a small portion of the total sample for
conditioning and analysis.
Sample Conditioning
Sample systems should contain at least one filter to remove solid
particles from the sample stream. Most applications require fine-element
filters upstream of the analyzer.
Contamination Precautions
Several precautions are recommended to minimize the possibility of
contaminating samples. Except in special applications, filters should be
of either the ceramic or the porous metallic type to avoid the
absorption losses characteristic of fiber or paper filters. Pressure
regulators and flow controllers containing cork or felt filters or
having absorbent diaphragms should not be used. Sampling lines for
non-corrosive streams should be stainless steel tubing and must be clean
and free of grease. Lines must be pressure tight to prevent diffusion of
moisture or atmospheric gases into the sample. Pipe threads should be
dressed only with Teflon tape on pipe threads and never with pipe thread
compounds.
Valving
A block valve should be installed immediately downstream of the sample
takeoff point to permit shutdown of the system for maintenance. Block
valves should be either needle valves or cocks of the proper material
and packing, and should be rated for the process line pressure. Tight
seating of all connections is essential.
Calibration Gas
A calibration gas used for BTU analysis should be blended of gases
specified as Primary Standards. Primary Standard gases are blended using
weights that are traceable to the National Bureau of Standards (NBS).
The calibration gas should not have any component that could drop out at
the coldest temperature the gas will be subjected to.
Principle of operation
Spectroscopy is the study of the absorbance and emission of electromagnetic radiation (light) by matter. The collection of frequencies absorbed by a sample is its absorption spectrum. When light passes through or is reflected from a sample, the amount of light absorbed is the difference between the Incident radiation and the transmitted radiation. The amount of light that is absorbed is expressed either as transmittance or absorbance.
All spectrophotometers consist of four major sub-units
- a source that generates electromagnetic radiation
- a dispersion device that selects a particular wavelength from the broad band radiation of the source
- a sample area
- a detector to measure the intensity of radiation
Diode array
Diode arrays are assemblies of individual photodiodes in a linear array. Self-scanned arrays have the read-out electronics included on the chip with the array. When read out, all elements of the array must be read out in series. The array has 1024 elements. Light of all wavelengths falls on the diode-array and is measured simultaneously, that is, data acquisition is done in parallel. Speed is the best known advantage of diode-array spectroscopy. Data is acquired in parallel, the detectors are read-out by "electronic scanning", and microprocessors and computers are used to process data. Measurements are made at different wavelengths at the same time. Conventional spectrophotometers can make multi-wavelength measurements but there is a time differential between each measurement.
Basic functionality requirement of a control system can be designed around a few relays and electronic timers. For example, a sequencer, level controller or a direct-on-line starter can be wired using a few general purpose relays and timers.
Whenever, the number of relays increase or if the complexity of the functionality becomes serious then switching to a PLC system is more advisable as it offers more flexibility and is a cost effective solution. PLC manufacturers promote their basic PLC processors with various options for Digital Inputs (DI), Digital Outputs (DO), Analog Inputs (AI), Analog Outputs (AO), Modems etc. The number of DI, DO, AI & AO can be calculated based on your input and output signals. PLC can be programmed using the native "ladder logic" or other high-level programming tools supplied by the specific manufacturers of PLC such as Allen Bradley, GE-Fanuc, Mitsubishi, Siemens etc.
NEMA 1 Enclosures constructed for indoor use to provide
a degree of protection to personnel against incidental contact
with the enclosed equipment and to provide a degree of
protection against falling dirt.
NEMA 2 Same as NEMA 1 including protection against
dripping and light splashing of liquids.
NEMA 3 Enclosures constructed for either indoor or
outdoor use to provide a degree of protection to personnel
against incidental contact with the enclosed equipment; to
provide a degree of protection against falling dirt, rain,
sleet, snow, and windblown dust; and that will be undamaged by
the external formation of ice on the enclosure.
NEMA 3R Same as NEMA 3 excluding protection against
windblown dust.
NEMA 3S Enclosures constructed for either indoor or
outdoor use to provide a degree of protection to personnel
against incidental contact with the enclosed equipment; to
provide a degree of protection against falling dirt, rain,
sleet, snow, and windblown dust; and in which the external
mechanism(s) remain operable when ice laden.
NEMA 4 Enclosures constructed for either indoor or
outdoor use to provide a degree of protection to personnel
against incidental contact with the enclosed equipment; to
provide a degree of protection against falling dirt, rain,
sleet, snow, windblown dust, splashing water, and hose-directed
water; and that will be undamaged by the external formation of
ice on the enclosure.
NEMA 4X Same as NEMA 4 including protection against
corrosion.
NEMA 5 Enclosures constructed for indoor use to provide
a degree of protection to personnel against incidental contact
with the enclosed equipment; to provide a degree of protection
against falling dirt; against settling airborne dust, lint,
fibers, and flyings; and to provide a degree of protection
against dripping and light splashing of liquids.
NEMA 6 Enclosures constructed for either indoor or
outdoor use to provide a degree of protection to personnel
against incidental contact with the enclosed equipment; to
provide a degree of protection against falling dirt; against
hose-directed water and the entry of water during occasional
temporary submersion at a limited depth; and that will be
undamaged by the external formation of ice on the enclosure.
NEMA 6P Same as NEMA 6 including protection against the
entry of water during prolonged submersion at a limited depth.
NEMA 7 Enclosures are for indoor use in locations
classified as Class I, Groups A, B, C, or D and shall be capable
of withstanding the pressures resulting from an internal
explosion of specified gases, and contain such an explosion
sufficiently that an explosive gas-air mixture existing in the
atmosphere surrounding the enclosure will not be ignited.
Enclosed heat generating devices shall not cause external
surfaces to reach temperatures capable of igniting explosive
gas-air mixtures in the surrounding atmosphere. Enclosures shall
meet explosion, hydro-static, and temperature design tests.
NEMA 9 Enclosures are intended for indoor use in
locations classified as Class II, Groups E, F, or G, and shall
be capable of preventing the entrance of dust. Enclosed heat
generating devices shall not cause external surfaces to reach
temperatures capable of igniting or discoloring dust on the
enclosure or igniting dust-air mixtures in the surrounding
atmosphere. Enclosures shall meet dust penetration and
temperature design tests, and aging of gaskets (if used).
NEMA 12 Enclosures constructed (without knockouts) for
indoor use to provide a degree of protection to personnel
against incidental contact with the enclosed equipment; to
provide a degree of protection against falling dirt; against
circulating dust, lint, fibers, and flyings; and against
dripping and light splashing of liquids.
NEMA 12K Same as NEMA 12 including enclosures
constructed with knockouts.
NEMA 13 Enclosures constructed for indoor use to
provide a degree of protection to personnel against incidental
contact with the enclosed equipment; to provide a degree of
protection against falling dirt; against circulating dust, lint,
fibers, and flyings; and against the spraying, splashing, and
seepage of water, oil, and non-corrosive coolants.
Sensors or transducers play a very important role in our everyday life and unknowingly we use or come across various sensors and transducers when we use our car, cell-phone, blood-pressure monitor, bathroom scale, washing machine etc. (the list is long...)
Sensors (as the name suggests) as used to "sense" a particular non-electrical quantity such as temperature, pressure, humidity, light-intensity, speed, velocity, RPM, flow etc. whereas a transducer is a synonym of a sensor since it "transduces" a non-electrical quantity into an electrically measurable quantity.
Sensors & Transducers fall into the basic category of "active" and "passive". All sensors & transducers which gives a direct electrical voltage output are called as "active" sensors or transducers (eg. ceramic pickup for turntables, peizo-sensors). Most of the sensors & transducers fall under the "passive" segment as they basically convert a non-electrical physical quantity into a passive electrical component value such as change in "resistance", "capacitance" or "inductance" which can be further measure accurately by using it as an unbalanced arm of a Wheatstone Bridge setup. Usually, they are coupled (mechanically linked) to a potentiometer, or form a movable plate of an air-capacitor, or a movable core of a transformer. Like for example, linking a potentiometer to a bourdon-tube makes an excellent pressure transducer, or a sliding ferrite core of a Linear Variable Differential Transformer (LVDT) to a machine shaft to measure displacement.
As sensors are used to read the non-electrical quantities, similarly, meters or indicators are used to display the quantities or values in a format understandable by the human beings, such as 7-segment displays, analog moving coil meters or just simple bulbs in red, green or yellow colors. Meters are basically identified as "digital" and "analog". A digital meter is the one which is capable of displaying a measured quantity in numerical format such as "2.341" etc. whereas an analog meter uses pointer "needle" to point a reading, which the reader (user) has to decipher after removing the parallax. It is usually understood that analog meters are less accurate then digital meters, but its a myth, a nicely calibrated and trimmed analog meter is far more accurate then a lowly calibrated and basic digital meter.