Magnetic Flowmeters

Magnetic Flowmeters

A magnetic flowmeter is a very common type of velocity style flowmeter that can be found in use in most industries, including mining, pulp and paper and water treatment and electrical generations, amongst others.

Theory

Magnetic flowmeters or magmeters have been available since the mid 1950’s and are ideally suited for measuring liquids which are clean or contain particles (slurries).  Magmeters are sized from (1/10 inch to 96 inch and can measure flow from 0.01 gallons per minute to 500,000 gallons per minute.

Magmeters are obstruction less, contain no moving parts to wear out and provide practically no pressure drop.

The basic construction consists of a flow tube complete with a non-conductive, non-magnetic liner.  Coils to produce a magnetic field surround the flowtube.  Two electrodes protrude into the flowtube to pickup the very small induced voltage.  Electronic components are used to excite the coils and amplify the voltage developed at the electrodes.  The wetted parts (contact the process), which consists of the liner and electrodes are selected for compatibility with the process with respect to corrosion and/or contamination. (Figure 1)

Magnetic Flowmeters

A magnetic flowmeter is a very common type of velocity style flowmeter that can be found in use in most industries, including mining, pulp and paper and water treatment and electrical generations, amongst others.

Theory

Magnetic flowmeters or magmeters have been available since the mid 1950’s and are ideally suited for measuring liquids which are clean or contain particles (slurries).  Magmeters are sized from (1/10 inch to 96 inch and can measure flow from 0.01 gallons per minute to 500,000 gallons per minute.

Magmeters are obstruction less, contain no moving parts to wear out and provide practically no pressure drop.

The basic construction consists of a flow tube complete with a non-conductive, non-magnetic liner.  Coils to produce a magnetic field surround the flowtube.  Two electrodes protrude into the flowtube to pickup the very small induced voltage.  Electronic components are used to excite the coils and amplify the voltage developed at the electrodes.  The wetted parts (contact the process), which consists of the liner and electrodes are selected for compatibility with the process with respect to corrosion and/or contamination. (Figure 1)

Figure 1: Mag-Flo Meter

The small voltage developed at the electrodes is proportional and linear with respect to flow through the pipe.  Flow measurement with a magnetic flowmeter is independent of temperature, pressure, density or viscosity of the liquid.  Accuracy over a wide range (typically 10 to 1) has evolved from 1% of full scale reading to 1% of rate as standard.

Applications of magnetic flowmeters vary from corrosive chemicals to sanitary requirements such as milk and other food products.

Faraday’s Law

The operation of a magnetic flowmeter is based on Faraday’s Law which states that a voltage developed in a conductor is proportional to the strength of the magnetic field (B),  the length of the conductor (L) and the velocity (V) of the conductor as it moves through the magnetic field.  This relationship can be shown in the following formula.

E = B x L x V

In the conventional magnetic flowmeter (figure 2) the magnetic field is created by a  set of coils mounted outside of the non-magnetic, non-conductive flow tube.  These coils are excited by either an AC voltage or a DC pulse depending upon the type of meter used (more detail provided later in this unit).   In the magnetic flow meter the actual fluid becomes the moving conductor (must be conductive) therefore the path length (L) is set by the distance between the pick-up electrodes.  These probes are mounted on opposite sides of the pipe and protrude through the pipe and liner making the distance between them almost equal to the flow tube diameter.  The velocity of the conductive fluid through the pipe becomes the ‘V’ term in Faraday’s equation.

Since the magnetic field (B) and flow tube diameter (L) are known and the induced voltage (E) can be measured by electronic circuitry, the velocity (V) of the fluid can be calculated.  Magnetic flowmeters are consequently a type of velocity measuring flow meter.   Using the continuity equation and knowing the cross-sectional area of the flow tube,  this velocity can then be used to calculate the flow rate through the magnetic flowmeter.

Continuity Flow Equation:  Q = V x A

Where:

Q – volumetric flow rate

V – average velocity of liquid

A – cross-sectional area of the pipe

It should be noted that most manufacturers have established the maximum flow  velocity of 10 m/sec (32.8 ft/sec) and minimum velocity of 1 m/sec (3.28 ft/sec) for optimum meter operation.  Abrasive slurries velocities are limited to 2 m/sec (6.5 ft/sec) to effectively increase service life of the liner and electrodes.  Sludge and grease bearing liquids should be measured at velocities greater than 2 m/sec (6.5 ft/sec) in order to reduce coating of the liner and electrodes.

Parts of a Magnetic Flowmeter

The major components of a magnetic flowmeter are the:

• Flow tube and liner

• Magnetic coils

• Electrodes

• Electronic (transmitter) housing

Although the process fluid may not be considered an actual part of the magnetic flow meter it is still the first parameter to consider when designing a flow system using a magmeter.

The process fluid has to be conductive as has been shown from Faraday’s Law – just how conductive depends upon the requirements specified by the manufacturer.  The standard minimum is typically 1 micromhos/cm (mhos/cm) to 5 mhos/cm.

Note that the standard (imperial) unit of conductivity is the ‘mho’ which is just the word ohm written backwards.

Conductivity is thus defined as the ability to conduct current.

The SI unit of conductivity is microsiemens/cm (S/cm), which is numerically equal to mho/cm.

Typical conductivities are:

• Sodium hydroxide 80000 S/cm

• Sulphuric acid 107000 S/cm

• Water 10 S/cm

• Ethyl alcohol 0.44 S/cm

• Fuel oil 0.000001 S/cm

As conductivity levels drop below 5 S/cm electrical noise becomes a problem.  As velocity increases the amount of noise tends to increase, thus low conductivity liquid applications should have magmeters sized to a maximum flow of about 3 ft/sec (0.914 m/sec).

Magnetic flowmeters require the process fluid to have a minimum conductivity with no maximum.   Since gases have a low conductivity and thus are not measured, magmeters are generally referred to as liquid flowmeters.

The magmeter flow tube has a liner to electrically isolate the electrodes from the meter body and pipe.   The liner can be made from a variety of materials, the most common are Teflon, polyurethane, and rubber.  Since the liner is in contract with the process fluid (a wetted part) it must be selected to not deteriorate in the given application.  Polyurethane and rubber are mainly used in abrasive services whereas Teflon is used in sanitary, highly corrosive, and high temperature applications.

The coils of the magmeter create the magnetic field.  The two general types of coils are the AC excited and the DC excited which will be discussed later in this unit.

The electrodes are physically mounted  in the flow tube such that they are at right angles to both the magnetic field and the axis of the pipe.  Like the liner, the electrodes are also a wetted part and therefore must be made of a material that is compatible with the process.  Some common materials are stainless steel, Hastelloy, Monel.

One of the common problems associated with a magmeter is electrode coating since the electrodes are in contact with the process.   An insulating type coating on the electrodes can create a significant shift in the magmeter zero and span.   Several different methods have and are being used to prevent or reduce electrode coating.   The electrodes can be manually brushed,  however the pipeline has to be shutdown during this procedure.   A momentary  AC voltage can be applied to the electrodes to ‘burn-off’ the insulating coating however there is a loss of flow signal during this time.  Electrodes can be rounded and designed to  protrude slightly into the process to create a turbulence which scrubs the tip and thus are classified as a self-cleaning electrodes.  Some electrodes can be removed from the magmeter for cleaning and some of these without stopping the flow.  These electrodes must be carefully re-installed as the distance between the probes is one of the determining factors in determine the quantity of  induced voltage.   Ultrasonic generators can also be to clean the electrodes without interrupting the process.  The more modern approached to electrode insulating coating is to ignore it through the use of a pulsed DC excitation systems which will be explained late.

The transmitter component of the magmeter converts the small voltage induced at the electrodes to a useable output signal.   Traditionally the output of the transmitter is an analog  4 -20 mA signal although more modern magmeter transmitters are microprocessor based and have a digital, frequency output.   Microprocessor based transmitters can automatically measure bi-directional flow;  can be configured for totalizing or scaling,  can display different units of flow, alarm points can be set,  bi-directional communication established, and many more features.

Magnetic Field Excitation

There are two common methods used to create the magnetic field in the magmeter

 AC excitation

 Pulsed DC excitation

AC excitation was the first to be used however the advantages of the pulsed DC type have resulted in its more predominate use.

An AC voltage with its sinusoidal nature is always changing.  When an AC voltage excites the coils of a magmeter two signals are induced at the electrodes – one created by the velocity of the fluid  (Faraday’s Law), the other by the changing magnetic field itself.  The second signal is called noise and must be reduced or eliminated for an accurate flow measurement.

The amount of noise can be determined by stopping the liquid flow.  Once stopped, the only signal the transmitter would see is the noise due to the changing magnetic field.   This ‘noise’ signal could then be zeroed-out in the transmitter.

The non-practicality of periodically stopping the flow to re-zero the meter,  the large size of the AC coils compared to DC coils,  and the electrode coating problems of AC magmeters have lead to the decline of the AC magmeter.

It should be noted however that an advantage of the AC excited meter is its fast speed of response as the coils are continuously energized although with advances in pulsed DC technology even this may not be an advantage for long.

A pulsed DC excited magmeter purposely has the coils de-energized for a period of time and energized for a period of time.   During the ‘on-time’ the voltage supplied to the coils is constant and therefore the voltage signal induced at the electrodes and seen by the transmitter is due only to the fluid velocity.  There is no noise due to changing excitation current.

It should be noted that it is also possible for noise to be generated from sources external to the magmeter.  These may be due to other magnetic fields from inductive loads (motors) or voltage build-up at the electrodes dues to stray electrical current on the pipe or even electrochemical reactions within the process liquid itself.  These could create a voltage noise at the electrodes that is not a result of the flow rate.  These external noise signals can be automatically (electronically in the transmitter) zeroed-out in a pulsed DC magmeter once every cycle when the coils are de-energized or turned off (figure 3).

This is not possible in the AC excited magmeter because the coils are never really turned off.

The advantages of a pulsed DC magmeter include lower power consumption and automatic re-zeroing due to its ‘turn-off’ time.  The coils of an DC meter are smaller and therefore have less weight than an comparable AC magmeter.   A limitation of a pulsed DC meter is its speed of response again due to its off time.  During this off-time there is no flow reading and for very fast changing flow conditions this may result in a less accurate average flow rate reading.  The more modern pulsed DC magmeters have an adjustable on/off time that allows flexibility for a given flow situation and thereby reduces flow errors.

Installation

The importance of installation of a magmeter cannot be overemphasized.   The direction of flow, the orientation of the meter for given process, bolting, and grounding has a significant impact on the operation of the flowmeter.

A magmeter can measure flow in either direction – the only difference is in the polarity of the voltage induced at the electrodes.    Unless the transmitter is designed to detect this change, reverse flow will give the meter a ‘zero flow’ indication.  For this reason magmeters have a flow direction stamped on the housing.

Magnetic flowmeters are designed to measure liquids in full pipes thus the preferred meter orientation is vertical with the flow upward through the meter.  Sloping upward or horizontal piping is acceptable however piping must be designed to ensure the flow tube is always full (see figure 4)

The minimum straight run pipe lengths for a typical magmeter is significantly less than for orifice plates – one resource states 20D upstream and 5D downstream for an orifice plate and 10D upstream and 3D downstream for a magmeter.   The manufacturer’s information should be consulted for actual values for a given application.

Grounding and Ground Rings

Proper process grounding is one of the most important installation details, as it ensures that the flowtube and process fluid are at the same potential and only the induced flow signal is therefore measured.

This is necessary to provide a stable reference for the measuring electronics, which is usually an ungrounded differential amplifier. In addition, stray electrical currents can be induced in metal pipelines by capacitive or inductive coupling from other electrical sources.  These electrical currents can create voltage zero shifts at the magmeter electrodes.  Proper bonding of the magmeter to either side of the pipeline connection allows these stray currents to pass by the meter rather than go through it.

By connecting the magmeter flowtube, the fluid, and the reference for the amplifier to a stable, noise free reference point, the user is ensured of getting the best performance from their magnetic flowmeter

Where necessary magnetic flow meters are provided with 2 grounding rings of a conductive material recommended for the intended service. These grounding rings although designed primarily to provide proper grounding on installations with insulated piping, also serve to protect and shield the flowtube liner edge from process abrasion.

Most manufacturers include grounding ring kits if installing on insulated or plastic pipe and a detailed explanation of how to properly ground their magmeter (see figure 5). However inexperienced construction personnel have been known to “forget” to install these components, so be aware.

One of the most common faults of initial magmeter failure is improper tightening of the flange bolts.   The liner of the magmeter can be easily damaged beyond repair by over-torquing the flange bolts.   Manufacturer’s literature should be consulted for the proper torque requirements for their particular meter.

Application

Magnetic flowmeters measure the volumetric flow rate of conductive liquids.  Since they are velocity flowmeters their signal output is linear and directly proportional to the flow rate.

Meters range in size from small (0.1 inches, 2 mm) to large (96 inches, 243 cm) and with flow rates from 0.01 USGPM (37 mL/min) to 500, 000 USGPM (1.89 ML/min).   Generally, magmeters have a relatively wide rangeability (10:1) and inaccuracies of 1% of full scale to 1% of rate.

Magmeters are virtually obstruction less and therefore have no pressure drop.  They are widely used in the pulp and paper industry and in mining applications.  They are not used in the oil industry as hydrocarbons are not conductive.

Advantages and Limitations

Although the features of the magmeter have been discussed throughout this document they are outlined here.

Advantages

• No moving parts

• Obstruction less, no pressure drop

• Low power requirements, especially for pulsed DC excited meters

• Wide application for corrosive and slurry process because of liner

• Can measure low and high flow rates, good rangeability

• Can measure bi-directional flow

• Relatively high accuracy

Limitations

• Can only measure conductive liquids, no hydrocarbons or gases

• Relatively heavy, especially AC excited meters

• Caution must be used in installing the meter (grounding, torquing, full pipe)

• Relatively expensive

• Electrodes can become coated

• Velocity measurement is limited to a maximum of 30 ft/sec (10m/sec) in practice.

Calibration

A magnetic flowmeter system has two major components – the magmeter body that is installed in the pipeline and the transmitter, which may be mounted remotely from the body.

Each of these has its own calibration techniques and can be done independently however better accuracy is achieved if they are done together as a system.

Before a flowmeter leaves the factory it is calibrated with its own unique K Factor.

Passing a volume of liquid through the meter performs this true hydraulic calibration.  This specific volume is measured using a weigh tank or a master meter such as a turbine meter.  This accurate volume is then compared to the frequency count (number of pulses).  From several tests the number of pulses per unit volume (K Factor) is determined.  Each magnetic flowmeter is then stamped with its own unique K Factor.  If these calibrations are performed in a certified facility then a calibration certificate traceable to NIST (National Institute of Standards and Technology) can be obtained.

As ‘real’ flows are difficult to attain in the field, only the transmitter part of the magmeter is field calibrated.  A field calibrator (usually manufacturer specific) is used to simulate the input electrical output from the electrodes and to test and troubleshoot the electronic circuitry of the transmitter.

Conclusion

With the many advantages of a magnetic flowmeter, it is a widely used flow meter in many industries other than petroleum.  This unit described the basic parts of a magmeter:  the flow tube and liner, the electrodes, the coils, and the transmitter.  The basic principle of operation of the magmeter shows an application of Faraday’s Law.   Some of the operational considerations of a magmeter were mentioned such as the coating of the electrodes and ways to reduce it.   The importance of proper installation practices was emphasized.  The methods of generating the magnetic field through AC and pulsed DC exciting were described.

Data relating to possible applications of the magmeter along with its advantages and limitations were outlined.  A brief explanation of the calibration involved with magmeters concluded the unit.

Satuan Pengukuran (Unit of Measure.IND)

Satuan SI (Sistem Internasional )

Sistem SI pada pengukuran ini dikembangakan sejak tahun 1960 dan diakui sebagai versi yang terakhir dan kemudian dikenal sebagai sistem metrik (Metrick).

Sistem pengukuran ini pada awalnya dikembangkan di Negeri Perancis pada abad tujuh belas, dan SI atau “System Interntional” adalah sistem ukuran yang sekarang  dipakai pada industri dan telah diakui di seluruh dunia (Internasional).

Sistem SI mempunyai tujuh satuan dasar pengukuran dan satuan lain yang berasal dari ke tujuh satuan ini. Daftar berikut ini mendefinisikan tujuh satuan dasar beserta lambang (simbol)-nya.

Kuantitas Nama Unit Simbol

Panjang (length) meter m

Massa (mass) kilogram kg

Waktu (time) second s

Arus listrik

(electrical current) ampere A

suhu (temperature) Kelvin K

banyaknya zat

(amount of substance) mole mol

intensitas cahaya Candela cd

Anda harus pahami bahwa satuan ukuran kilogram adalah satuan dasar yang mempunyai awalan “kilo”. Ini karena satuan gram dianggap terlalu kecil jika digunakan untuk kebanyakan ukuran massa.

Semua satuan sistem SI yang lain diperoleh dengan menurunkan dari satuan dasar atau dari kombinasi satuan dasar yang lain. Misalnya kita perhatikan satuan SI untuk volume, yaitu liter. Satuan liter sama dengan  1/1000 meter kubik (m3). Satu  m3  sesuai untuk volume dengan ukuran ketiga sisinya satu meter.

Marilah kita memeriksa satuan pengukuran SI yang lain, Pascal atau digunakan untuk tekanan. Tekanan didefinisikan sebagai satuan gaya yang digunakan dibagi dengan luas tertentu. Gaya didefinisikan sebagai massa kali percepatan, atau sedangkan percepatan bisa berupa  gaya grafitasi.

Gaya = massa  percepatan

= kg  m/s2

= kgm/s2

= a Newton (N)

Oleh karena itu satu Pascal adalah: Gaya dibagi satuan luas (force per unit area)

Pa = N/ m2

Pa = kgm/s2 m2

Pa = kg/m s2

Hal ini penting untuk dimengerti berbagai kata awalan yang berhubungan dengan penggunaan satuan ukuran sistem SI. Awalan-awalan ini adalah perkalian dasar dan perkalian lanjutan dari satuan yang dideskripsikan sebagai berikut:

Prefix Symbol Factor to Multiply the Unit By Exponential Notation

exa E 1 000 000 000 000 000 000 1018

peta P 1 000 000 000 000 000 1015

tera T 1 000 000 000 000 1012

giga G 1 000 000 000 109

mega M 1 000 000 106

kilo k 1 000 103

hecto h 100 102

deca da 10 10

deci d 0.1 10-1

centi c 0.01 10-2

milli m 0.001 10-3

micro  0.000 001 10-6

nano  0.000 000 001 10-9

pico  0.000 000 000 001 10-12

femto f 0.000 000 000 000 001 10-15

atto a 0.000 000 000 000 000 001 10-18

Perkalian dari “kilo” dan “mega” dipergunakan sebagai perkalian biasa untuk ukuran dalam sistem SI.

Perkalian pada cabang “centi”, “milli” dan “micro” dipergunakan sebagai perkalian cabang biasa untuk pengukuran pada sistem SI.

Maksud pada tujuan unit ini kita akan mendiskusikan satuan dasar SI yaitu waktu (time), suhu, (temperature), massa (mass), arus listrik (current) dan panjang (length), yang berkaitan dengan bidang instrumentasi.

Panjang (Length)

Satuan dasar untuk panjang (length) dalam sistem SI didefinisikan sebagai meter (m). Perkalian dan perkalian lanjutan yang sering digunakan adalah: kilometer (km), centimeter (cm) dan millimeter (mm).

Massa (Mass)

Satuan dasar untuk massa (mass) dalam sistem SI adalah kilogram (kg). Massa  didefinisi sebagai banyaknya bahan yang berada di dalam satu benda. Massa bukan berat benda, karena berat bergantung pada gaya gravitasi (gravity) dan gaya grafitsi ini akan berkurang jika berat benda menjadi lebih jauh dari pusat bumi.

Walaupun satuan gram merupakan satuan dasar yang benar untuk massa, tapi satuan gram untuk massa sangatlah kecil dan karena itu sekarang satuan kilogram diakui sebagai satuan dasar. Perkalian dan perkalian lanjutan yang terkenal baik untuk massa adalah: milligram (mg), dan ton (t). Satuan ton metrik sesuai dengan 1000 kg.

Waktu (Time)

Satuan dasar untuk waktu (time) dalam sistem SI didefinisikan sebagai detik (s).

Perkalian lanjutan yang  umum adalah millisecond (ms) dan nanosecond (s). Perkalian yang umum sama dengan sistem Inggris/Imperial, adalah menit (min), jam (hr), hari (da), dan lain-lain.Perkalian ini telah diterima di seluruh dunia dan untuk dipertahankan sebagai sistem SI.

Suhu (Temperature)

Satuan dasar untuk suhu (temperature) di dalam sistem SI didefinisikan sebagai Kelvin (K). Antara derajat Celcius (C) dan Kelvin mempunyai hubungan yaitu  0 K sesuai -273 C. Temperatur Kelvin merupakan temperatur absolut karena tidak mungkin ada suhu lebih dingin dari pada 0 K.

Untuk tujuan (kecuali ketika dipakai untuk penggunaan ilmu panas) banyak kita memakai suhu Celsius sebagai pembagian derajat dalam sistem.

Arus Listrik (Current)

Satuan dasar untuk arus listrik di dalam sistem SI didefinisikan sebagai ampere  (A). Perkalian lanjutan yang yang biasa digunakan adalah milliamp (mA) dan microamp (A). Perkalian dasar untuk ampere tidak umum dipergunakan dalam bidang Instrumentasi.

Sistem Inggris/Imperial

Sejak berabad-abad yang lalu Sistem Inggris (Imperial) mulai digunakan di negeri Inggris. Ada banyak variasi satuan dasar pengukuran. Lain halnya dengan sistem SI, dimana perkalian biasa dan perkalian lanjutan menggunakan satuan perkalian sepuluh yang dinyatakan dalam bentuk eksponen (misalnya: m103 atau kilometer). Pada sistem imperial, perkalian biasa dan perkalian lanjutan mempunyai banyak factor dari satuan dasar, dan menggunakan nama-nama yang  berbeda, serta tidak ada modifikasi pada awalannya.

Daftar berikut ini menyajikan definisi satuan Sistem Inggris (Imperial) yang umum digunakan untuk mengukur panjang (length), berat (weight), waktu (time), suhu (temperature), dan arus listrik (current).

Kuantitas Nama dasar satuan Simbol

Panjang (length) kaki foot ft

Berat (weight) pound lb

Waktu (time) second sec

Suhu (temperature) Rankin R

Arus listrik (current) Ampere Amp

Ini harus disadari pentingnya penggunaan sistem Inggris (Imperial), di sini tekanan juga didefinisikan sebagai gaya dibagi dengan luas penampang, atau pounds per square inch (pon dibagi dengan inci kuadrat/psi).

Satu pound gaya atau pound of force (lbf) hanya sesuai dengan satu pon berat yaitu kalau ada gravitasi (gaya berat) yang konstan, yaitu 32.2 ft/ sec2  atau 32.2 ft/detik2 dalam sistem Inggris/Imperial.

Unit satuan volume, dalam sistem Inggris adalah variasi dari satuan panjang kubus (ft3 ) galons (gallon/gal) dan ounces (ons / oz ). Tidak ada hubungan secara langsung antara gallon dan kubik feet.

Ada perbedaan antara satu gallon Amerika (US gal) dan satu gallon Inggris (Imp gal) karena satu gallon Amerika lebih kecil dari satu gallon Inggris. Antara sistem Amerika US dan sistem  Inggris bisa disesuaikan dengan faktor (1 US gal = 0.8327 Imp gal) atau ( 1 Imp. gal = 1.2009 US gal). digunakan untuk mengkonversi dari sistem satuan US menjadi versi Imperial atau sebaliknya.

Daftar untuk perkalian biasa dan perkalian lanjutan dalam satuan dengan sistem Inggris menjadi sangat luas. Perkalian biasa dan perkalian lanjutan untuk tiap satuan yang telah disebutkan di atas, akan  diuraikan pada bab berikut ini.

Panjang (Length)

Satuan dasar untuk panjang dalam sistem Inggris didefinisikan sebagai foot  (ft). Perkalian lanjutan umumnya adalah inch (inch=in) yang mana ada 1/12 inch = 1 kaki (atau foot). Perkalian yang umum untuk kaki (foot) yaitu yard (yar=yd) atau  kaki kubik (feet), dan satu mile (mil=mi) atau 5280 kaki (feet).

Berat (Weight)

Satuan dasar untuk berat (weight) dalam sistem Inggris didefinisikan sebagai (pound = lb). Perkalian lanjutan yang paling umuml adalah ounce (ons=oz), atau sebesar 1/16 onns = 1 pound (pon). Perkalian umum yang biasa untuk berat (weight) yaitu ton  atau dua ribu (=2000) pound. Satun ton buiasanya tidak disingkat.

Waktu (Time)

Satuan dasar untuk waktu (time) dalam sistem Inggris didefinisikan sebagai detik (second =s), satuan ini dipergunakan oleh kedua sistem, yaitu sistem Inggris dan SI. Perkalian lanjutan yang umum adalah millisecond (ms) dan nanosecond (ns). Perkalian yang umum adalah sama dengan sistem SI, seperti menit (min), jam (hr), hari (da), dan lain-lain. Perkalian ini telah diterima di seluruh dunia untuk dipertahankan pada kedua sistem.

Suhu (Temperature)

Satuan dasar untuk suhu (temperature) pada sistem Inggris didefinisikan sebagai  Rankin. Antara derajat Fahrenheit (F) dan Rankin mempunyai hubungan yaitu 0°R = -460 F. Derajat suhu mutlak (temperature absolut) yaitu Rankin, karena tidak mungkin ada suhu lebih dingin daripada 0°R.

Untuk banyak maksud (kecuali dipakai untuk menggunakan ilmu panas) kita bisa menggunakan skala suhu Faherheit sebagai pembagian derajat dalam sistem Inggris.

Arus Listrik (Current)

Satuan dasar untuk arus listrik pada sistem Inggris didefinisikan sebagai ampere (A). satuan ini dipergunakan pada kedua sistem, yaitu sistem Inggris dan SI. Perkalian lanjutan yang umum adalah milliamp (mA) dan microamp (A). Perkalian dasar untuk ampere tidak umum dipergunakan pada bidang Instrumentasi.

Satuan Konversi

Daftar factor-faktor konversi yang lengkap digunakan untuk konversi antara dua sistem pengukuran dapat diperoleh dari macam-macam text book dan sumber lainnya. Salah satu sumber yang bagus yang memuat factor konversi adalah tabel Foxboro. Anda dapat melihat foto copy dari table Foxboro yang telah dilampirkan pada bab ini.

Agar lebih mudah dalam melakukan konversi satuan, pertahankanlah kontinyuitas satuan tersebut. Berikut ini diberikan contoh dalam mengkonversi satuan.

Untuk mengubah dari   273 kilogram (kg) ke dalam pound (lb).

Faktor konversinya adalah  1 kg sama dengan  2.205 lb

lb = kg  2.205 lb/ kg

lb = 273 kg  2.20 lb/ kg

lb = 601.86 lb

Sangat penting bahwa setiap satuan pada sistem harus tetap digunakan, dan diteruskan di dalam perhitungannya, dan kemudian dihapus untuk mendapatkan hasil akhir. Dengan cara ini, jika dalam perhitungan menghasilkan satuan yang baik berarti tidak membuat kesalahan matematika, dan jawaban anda menjadi benar.

Untuk menkonversi antara kedua sistem, kita sering menggunakan mesin hitung. Akan tetapi yang benar-benar kita butuhkan adalah kecakapan ilmu pasti dan pendekatan-pendekatan sewaktu mengkonversi dengan menggunakan factor-faktor konversi yang benar.

Contoh  2

Konversikan dari 32 °C ke dalam °F.

Sekali lagi kita harus menggunakan factor konversi yang baik, yaitu  F = 9/5 C + 32.

Oleh karena itu:

F = 9/5C + 32

F = 9/5,(32) + 32

F =  288/5 + 32

F = 57,6 + 32

F = 89,6

Satuan SI (Sistem Internasional )

Sistem SI pada pengukuran ini dikembangakan sejak tahun 1960 dan diakui sebagai versi yang terakhir dan kemudian dikenal sebagai sistem metrik (Metrick).

Sistem pengukuran ini pada awalnya dikembangkan di Negeri Perancis pada abad tujuh belas, dan SI atau “System Interntional” adalah sistem ukuran yang sekarang  dipakai pada industri dan telah diakui di seluruh dunia (Internasional).

Sistem SI mempunyai tujuh satuan dasar pengukuran dan satuan lain yang berasal dari ke tujuh satuan ini. Daftar berikut ini mendefinisikan tujuh satuan dasar beserta lambang (simbol)-nya.

Kuantitas Nama Unit Simbol

Panjang (length) meter m

Massa (mass) kilogram kg

Waktu (time) second s

Arus listrik

(electrical current) ampere A

suhu (temperature) Kelvin K

banyaknya zat

(amount of substance) mole mol

intensitas cahaya Candela cd

Anda harus pahami bahwa satuan ukuran kilogram adalah satuan dasar yang mempunyai awalan “kilo”. Ini karena satuan gram dianggap terlalu kecil jika digunakan untuk kebanyakan ukuran massa.

Semua satuan sistem SI yang lain diperoleh dengan menurunkan dari satuan dasar atau dari kombinasi satuan dasar yang lain. Misalnya kita perhatikan satuan SI untuk volume, yaitu liter. Satuan liter sama dengan  1/1000 meter kubik (m3). Satu  m3  sesuai untuk volume dengan ukuran ketiga sisinya satu meter.

Marilah kita memeriksa satuan pengukuran SI yang lain, Pascal atau digunakan untuk tekanan. Tekanan didefinisikan sebagai satuan gaya yang digunakan dibagi dengan luas tertentu. Gaya didefinisikan sebagai massa kali percepatan, atau sedangkan percepatan bisa berupa  gaya grafitasi.

Gaya   = massa x percepatan

= kg x m/s2

= kgm/s2

= a Newton (N)

Oleh karena itu satu Pascal adalah: Gaya dibagi satuan luas (force per unit area)

Pa = N/ m2

Pa = kgm/s2 m2

Pa = kg/m s2

Hal ini penting untuk dimengerti berbagai kata awalan yang berhubungan dengan penggunaan satuan ukuran sistem SI. Awalan-awalan ini adalah perkalian dasar dan perkalian lanjutan dari satuan yang dideskripsikan sebagai berikut:

Prefix Symbol Factor to Multiply the Unit By Exponential Notation

exa E 1 000 000 000 000 000 000 1018

peta P 1 000 000 000 000 000 1015

tera T 1 000 000 000 000 1012

giga G 1 000 000 000 109

mega M 1 000 000 106

kilo k 1 000 103

hecto h 100 102

deca da 10 10

deci d 0.1 10-1

centi c 0.01 10-2

milli m 0.001 10-3

micro u 0.000 001 10-6

nano n 0.000 000 001 10-9

pico p 0.000 000 000 001 10-12

femto f 0.000 000 000 000 001 10-15

atto a 0.000 000 000 000 000 001 10-18

Perkalian dari “kilo” dan “mega” dipergunakan sebagai perkalian biasa untuk ukuran dalam sistem SI.

Perkalian pada cabang “centi”, “milli” dan “micro” dipergunakan sebagai perkalian cabang biasa untuk pengukuran pada sistem SI.

Maksud pada tujuan unit ini kita akan mendiskusikan satuan dasar SI yaitu waktu (time), suhu, (temperature), massa (mass), arus listrik (current) dan panjang (length), yang berkaitan dengan bidang instrumentasi.

Panjang (Length)

Satuan dasar untuk panjang (length) dalam sistem SI didefinisikan sebagai meter (m). Perkalian dan perkalian lanjutan yang sering digunakan adalah: kilometer (km), centimeter (cm) dan millimeter (mm).

Massa (Mass)

Satuan dasar untuk massa (mass) dalam sistem SI adalah kilogram (kg). Massa  didefinisi sebagai banyaknya bahan yang berada di dalam satu benda. Massa bukan berat benda, karena berat bergantung pada gaya gravitasi (gravity) dan gaya grafitsi ini akan berkurang jika berat benda menjadi lebih jauh dari pusat bumi.

Walaupun satuan gram merupakan satuan dasar yang benar untuk massa, tapi satuan gram untuk massa sangatlah kecil dan karena itu sekarang satuan kilogram diakui sebagai satuan dasar. Perkalian dan perkalian lanjutan yang terkenal baik untuk massa adalah: milligram (mg), dan ton (t). Satuan ton metrik sesuai dengan 1000 kg.

Waktu (Time)

Satuan dasar untuk waktu (time) dalam sistem SI didefinisikan sebagai detik (s).

Perkalian lanjutan yang  umum adalah millisecond (ms) dan nanosecond (s). Perkalian yang umum sama dengan sistem Inggris/Imperial, adalah menit (min), jam (hr), hari (da), dan lain-lain.Perkalian ini telah diterima di seluruh dunia dan untuk dipertahankan sebagai sistem SI.

Suhu (Temperature)

Satuan dasar untuk suhu (temperature) di dalam sistem SI didefinisikan sebagai Kelvin (K). Antara derajat Celcius (oC) dan Kelvin mempunyai hubungan yaitu  0 K sesuai -273 oC. Temperatur Kelvin merupakan temperatur absolut karena tidak mungkin ada suhu lebih dingin dari pada 0 K.

Untuk tujuan (kecuali ketika dipakai untuk penggunaan ilmu panas) banyak kita memakai suhu Celsius sebagai pembagian derajat dalam sistem.

Arus Listrik (Current)

Satuan dasar untuk arus listrik di dalam sistem SI didefinisikan sebagai ampere  (A). Perkalian lanjutan yang yang biasa digunakan adalah milliamp (mA) dan microamp (oA). Perkalian dasar untuk ampere tidak umum dipergunakan dalam bidang Instrumentasi.

Sistem Inggris/Imperial

Sejak berabad-abad yang lalu Sistem Inggris (Imperial) mulai digunakan di negeri Inggris. Ada banyak variasi satuan dasar pengukuran. Lain halnya dengan sistem SI, dimana perkalian biasa dan perkalian lanjutan menggunakan satuan perkalian sepuluh yang dinyatakan dalam bentuk eksponen (misalnya: mx1000 atau kilometer). Pada sistem imperial, perkalian biasa dan perkalian lanjutan mempunyai banyak factor dari satuan dasar, dan menggunakan nama-nama yang  berbeda, serta tidak ada modifikasi pada awalannya.

Daftar berikut ini menyajikan definisi satuan Sistem Inggris (Imperial) yang umum digunakan untuk mengukur panjang (length), berat (weight), waktu (time), suhu (temperature), dan arus listrik (current).

Kuantitas Nama dasar satuan Simbol

Panjang (length) kaki foot ft

Berat (weight) pound lb

Waktu (time) second sec

Suhu (temperature) Rankin R

Arus listrik (current) Ampere Amp

Ini harus disadari pentingnya penggunaan sistem Inggris (Imperial), di sini tekanan juga didefinisikan sebagai gaya dibagi dengan luas penampang, atau pounds per square inch (pon dibagi dengan inci kuadrat/psi).

Satu pound gaya atau pound of force (lbf) hanya sesuai dengan satu pon berat yaitu kalau ada gravitasi (gaya berat) yang konstan, yaitu 32.2 ft/ sec2  atau 32.2 ft/detik2 dalam sistem Inggris/Imperial.

Unit satuan volume, dalam sistem Inggris adalah variasi dari satuan panjang kubus (ft3 ) galons (gallon/gal) dan ounces (ons / oz ). Tidak ada hubungan secara langsung antara gallon dan kubik feet.

Ada perbedaan antara satu gallon Amerika (US gal) dan satu gallon Inggris (Imp gal) karena satu gallon Amerika lebih kecil dari satu gallon Inggris. Antara sistem Amerika US dan sistem  Inggris bisa disesuaikan dengan faktor (1 US gal = 0.8327 Imp gal) atau ( 1 Imp. gal = 1.2009 US gal). digunakan untuk mengkonversi dari sistem satuan US menjadi versi Imperial atau sebaliknya.

Daftar untuk perkalian biasa dan perkalian lanjutan dalam satuan dengan sistem Inggris menjadi sangat luas. Perkalian biasa dan perkalian lanjutan untuk tiap satuan yang telah disebutkan di atas, akan  diuraikan pada bab berikut ini.

Panjang (Length)

Satuan dasar untuk panjang dalam sistem Inggris didefinisikan sebagai foot  (ft). Perkalian lanjutan umumnya adalah inch (inch=in) yang mana ada 1/12 inch = 1 kaki (atau foot). Perkalian yang umum untuk kaki (foot) yaitu yard (yar=yd) atau  kaki kubik (feet), dan satu mile (mil=mi) atau 5280 kaki (feet).

Berat (Weight)

Satuan dasar untuk berat (weight) dalam sistem Inggris didefinisikan sebagai (pound = lb). Perkalian lanjutan yang paling umuml adalah ounce (ons=oz), atau sebesar 1/16 onns = 1 pound (pon). Perkalian umum yang biasa untuk berat (weight) yaitu ton  atau dua ribu (=2000) pound. Satun ton buiasanya tidak disingkat.

Waktu (Time)

Satuan dasar untuk waktu (time) dalam sistem Inggris didefinisikan sebagai detik (second =s), satuan ini dipergunakan oleh kedua sistem, yaitu sistem Inggris dan SI. Perkalian lanjutan yang umum adalah millisecond (ms) dan nanosecond (ns). Perkalian yang umum adalah sama dengan sistem SI, seperti menit (min), jam (hr), hari (da), dan lain-lain. Perkalian ini telah diterima di seluruh dunia untuk dipertahankan pada kedua sistem.

Suhu (Temperature)

Satuan dasar untuk suhu (temperature) pada sistem Inggris didefinisikan sebagai  Rankin. Antara derajat Fahrenheit (oF) dan Rankin mempunyai hubungan yaitu 0°R = -460 oF. Derajat suhu mutlak (temperature absolut) yaitu Rankin, karena tidak mungkin ada suhu lebih dingin daripada 0°R.

Untuk banyak maksud (kecuali dipakai untuk menggunakan ilmu panas) kita bisa menggunakan skala suhu Faherheit sebagai pembagian derajat dalam sistem Inggris.

Arus Listrik (Current)

Satuan dasar untuk arus listrik pada sistem Inggris didefinisikan sebagai ampere (A). satuan ini dipergunakan pada kedua sistem, yaitu sistem Inggris dan SI. Perkalian lanjutan yang umum adalah milliamp (mA) dan microamp (uA). Perkalian dasar untuk ampere tidak umum dipergunakan pada bidang Instrumentasi.

Satuan Konversi

Daftar factor-faktor konversi yang lengkap digunakan untuk konversi antara dua sistem pengukuran dapat diperoleh dari macam-macam text book dan sumber lainnya. Salah satu sumber yang bagus yang memuat factor konversi adalah tabel Foxboro. Anda dapat melihat foto copy dari table Foxboro yang telah dilampirkan pada bab ini.

Agar lebih mudah dalam melakukan konversi satuan, pertahankanlah kontinyuitas satuan tersebut. Berikut ini diberikan contoh dalam mengkonversi satuan.

Untuk mengubah dari   273 kilogram (kg) ke dalam pound (lb).

Faktor konversinya adalah  1 kg sama dengan  2.205 lb

lb = kg x 2.205 lb/ kg

lb = 273 kg x 2.20 lb/ kg

lb = 601.86 lb

Sangat penting bahwa setiap satuan pada sistem harus tetap digunakan, dan diteruskan di dalam perhitungannya, dan kemudian dihapus untuk mendapatkan hasil akhir. Dengan cara ini, jika dalam perhitungan menghasilkan satuan yang baik berarti tidak membuat kesalahan matematika, dan jawaban anda menjadi benar.

Untuk menkonversi antara kedua sistem, kita sering menggunakan mesin hitung. Akan tetapi yang benar-benar kita butuhkan adalah kecakapan ilmu pasti dan pendekatan-pendekatan sewaktu mengkonversi dengan menggunakan factor-faktor konversi yang benar.

Contoh  2

Konversikan dari 32 °C ke dalam °F.

Sekali lagi kita harus menggunakan factor konversi yang baik, yaitu  oF = 9/5 oC + 32.

Oleh karena itu:

oF = 9/5 oC + 32

oF = 9/5,(32) + 32

oF =  288/5 + 32

oF = 57,6 + 32

oF = 89,6

Download lampiran : Satuan Ukuran – Presentasi

Sepintas Tentang Modbus

Modbus adalah salah satu protokol untuk komunikasi serial yang di publikasikan oleh Modicon pada tahun 1979 untuk di gunakan pada PLC Modicon (PLC pertama di dunia yang di kembangkan oleh Schneider). Protokol ini menjadi standard komunikasi dalam industri dan menjadi yang paling banyak dipakai untuk komunikasi antar peralatan elektronik pada industri. Alasan utama mengapa Modbus Protokol banyak di gunakan adalah:
1. DI publikasikan secara terbuka tanpa royalty fee untuk penggunaannya.
2. Relatif mudah untuk di aplikasikan pada industrial network.
3. Modbus mempunyai struktur bit tanpa memiliki banyak larangan bagi vendor lain untuk mengaksesnya.

Modbus digunakan untuk komunikasi antara devices yang terkoneksi pada jaringan yang sama, misalnya untuk sebuah system yang mengukur temperature, tekanan dsb kemudian mengkomunikasikan hasilnya pada komputer. Modbus biasanya digunakan untuk supervisory pada RTU (remote terminal unit( pada System SCADA.

2 Media Protokol
Media Modbus Protokol terbagi atas tiga yaitu Serial port dan Ethernet port dan versi tambahan.

Kebanyakan Modbus devices berkomunikasi dengan serial RS485 Port.
Pada koneksi serial, ada dua varian yaitu Modbus RTU dan Modbus ASCII, Modbus RTU menggunakan representasi bit pada pengiriman datanya sedan Modbus ASCII menggunakan format data ASCII dalam pengiriman datanya. Dode dengan varian ASCII tidak bisa berkomunikasi dengan varian RTU dan sebaliknya.
Pada versi tambahan, ada jenis Modbus + (MB+). Modbus ini memiliki kecepatan transfer data 1MBps tetapi merupakan proprietary dari Modicon . Tidak seperti Modbus, MB+ harus menggunakan media transfer spesial untuk komunikasinya.

3 Cara Komunikasi
Pada tiap modbus devices memiliki address yang unik dari 0 s/d 247. Pada komunikasi serial, hanya node yang di set master yang bisa menginisiasi command kepada slave dengan address tertentu( Slave ID). Tiap command mengandung data address dari slave yang dituju, hanya slave dengan address yang dituju yang akan merespond command tersebut.

4 Function Code
Function Code adalah bit dalam word yang dikirim ke node slave untuk menyatakan slave akan di tulis atau dibaca pada tabel data yang mana.

Function Code Action Table Name
01 (01 hex) Read Discrete Output Coils
05 (05 hex) Write single Discrete Output Coil
15 (0F hex) Write multiple Discrete Output Coils
02 (02 hex) Read Discrete Input Contacts
04 (04 hex) Read Analog Input Registers
03 (03 hex) Read Analog Output Holding Registers
06 (06 hex) Write single Analog Output Holding Register
16 (10 hex) Write multiple Analog Output Holding Registers

PLC Programming Language

Ladder Diagram
Ladder Diagram (LD or simply Ladder) is probably the most widely used controller programming language. Invented to replace hardwired relay-based control systems, Ladder programming is used in probably 95% of all applications. Visually, this language resembles a series of control circuits, with a series of inputs needing to be “made” or “true” in order to activate one or more outputs.

Ladder has experienced such widespread adoption that almost every programmer in any country or industry can read and write this language. Because it resembles the familiar electric circuit format, even a non-programmer with an electrical background can follow the program for purposes of troubleshooting a problem.

It’s also easy to start writing a program in Ladder. With just a basic outline of input and output signals, one can start churning out code. Most other IEC languages require more preparation, such as flowcharting of all potential process flows. Finally, most Ladder implementations allow a program to be organized into folders or subprograms that can be downloaded to the PLC, allowing easy program segmentation.

Ladder programming is ideal for simple material handling applications, for example, where a sensor detects the presence of a box, other sensors check for obstructions, and then an output fires an actuator to push the box to another conveyor. Digital inputs check for various conditions, and the program analyzes the inputs and fires digital outputs in response. There may be timers in the program, or some basic comparisons, or math, but there are no complex functions involved.

As the complexity of PLC functionality has grown, however, Ladder has been challenged to meet these advances and still maintain the paradigm of easy visualization and understanding. Functions, such as PID loops, trigonometry, and data analysis, now required in many control applications can be difficult to implement. Another challenge is that as program size grows, the ladder can become very difficult to read and interpret unless it’s extensively documented. Finally, implementing full processes in Ladder can be daunting – picture a ladder rung with an output used in several phases of a process with many input conditions attempting to control exactly when that output needs to turn on.

 

Function Block Diagram
Although Ladder may be the most widespread language, a survey conducted by Control Engineering magazine several months ago highlighted growth in the use of programming languages other than Ladder. Function Block Diagram (FBD) programming is an example. Even though the adoption rate for this language has recently slowed relative to other languages such as Structured Text, FBD is probably the second most widely used language.

In many ways, this graphical language resembles a wiring diagram even more so than Ladder code. With FBD, the blocks are “wired” together into a sequence that’s easy to follow. It uses the same instructions as Ladder, but visually is more understandable to a viewer who is not versed in relay logic. The major advantage is that programs written in FBD tend to be easy to follow – just follow the path!

FBD is ideal for simpler programs consisting of digital inputs, such as photoelectric sensors, and outputs such as valve manifolds, and could be used in any application where Ladder works well.

However, FBD is not ideal for large programs using special I/O and functions. The large amount of screen space required can quickly make a program unwieldy if it reaches any substantial size. Also, writing a program in FBD requires more upfront preparation to understand the program and how it will flow before any code is written, since it can be difficult to make corrections later.

 

Sequential Function Chart
Sequential Function Chart (SFC) programming resembles the computer flowcharts that many engineers remember drawing up in their college days. An initial step “action box” (the starting point of a flowchart) is followed by a series of transitions and additional action steps. The SFC concept is simple: an action box, with code inside written in any language of the programmer’s choice, is active until the transition step below it activates. The current action box is turned off, and the next one in the sequence is active. The transition step also has code to check that the necessary conditions are met to allow the program to advance to the next step. The language is very friendly to maintenance engineers because its visual nature and natural code segmentation makes it easy to troubleshoot.

On the downside, this style of programming is not suitable for every application, as the structure it forces on a program could add unneeded complexity. A large amount of time must be spent up front preparing and planning before any programming is attempted, or the charts can become unwieldy and difficult to follow. The overhead required for this type of program causes it to execute slower than the other languages.

A final consideration is the inability to convert to other languages. IL, FBD, ST and Ladder programs can easily be interconverted, allowing a piece of code to be displayed in the way most comfortable to the user. SFC, however, cannot be converted.

 

Instruction List
Instruction List programming (IL) consists of many lines of code, with each line representing exactly one operation. Thus, it is very step-by-step in layout and format, which makes the entry of a series of simple mathematical functions easy.

IL is a low-level language and, as such, will execute much faster than a graphical language, such as Ladder. IL is also much more compact and will consume less space in PLC memory. The simple one-line text entry method supported by this language also allows for very fast program entry – no mouse required, no tabs to click! In legacy systems, programs written in IL are easier to display and edit on a handheld programming unit, with no software or laptop required.

Despite IL’s advantages, it seems that maintenance and service engineers do not prefer it. This may be because it is less visual than Ladder, which may make it more difficult to interpret what the program is doing and what errors it is experiencing.

IL can make entering complex functions, such as PID loops and complex mathematical computations, a struggle. IL does not lend itself well to any form of structured programming, such as state programming or step ladder, further limiting its usefulness for implementing large programs. It is also arguable that the advantages of speed and compactness are less relevant, given the processing speeds of modern PLCs and the large amounts of memory available.

 

 

 

 

Structured Text

With its IF…THEN loops, CASE selectors, and lines ending in semicolons, Structured Text (ST) closely resembles a high-level computer programming language such as Pascal and C. The aforementioned Control Engineering survey indicated that of all the IEC61131-defined programming languages, ST has seen the greatest increase in adoption.

Among IEC languages, ST perhaps best embraces the growing complexity of PLC programming, such as the process control functions involved in plastics or chemical manufacturing. Trigonometry, calculus, and data analysis can be implemented far more easily than in Ladder or IL. Decision loops and pointers (variables used to do indirect addressing) allow for a more compact program implementation than can be achieved in Ladder. The flexible ST editor that is common in most programming packages makes it easy to insert comments throughout a program, and to use indents and line spacing to emphasize related sections of code. This makes the task of structuring a complex program easier.

ST text-based, non-graphical nature, which is similar to IL, also runs much faster than Ladder. An additional ST benefit is that it comes closer than most of the other languages in achieving the transferability goals of the IEC61131 standard, emancipating a programmer from the hardware platform.

A final benefit is that many students currently graduating from engineering studies have a better background in computer languages than in the basics of electrical wiring, and therefore can more easily become proficient in ST than Ladder programming.

A disadvantage is that, for many previously experienced programmers or maintenance and service personnel, the ST environment is unfamiliar. Writing the code and structure to make it maintenance-friendly can reduce some of its compactness advantages.

As a result, control engineers tend to use ST “behind the scenes.” For example, IEC 61131 allows a programmer to build his or her own functions in one language, then insert them as sub-programs in another language. With this option, programmers often encapsulate an ST program inside an instruction, which is then embedded in a Ladder program.

Reference :

Using PLC Programming language

As the previous article mention at IEC61131-3.  PLC have 5 common programming language. Sometimes we have to consider when have to use the one of the programming language. But offcourse the last choice is which language do you familiar with.

However all the language have the benefit as following:

• Ease of maintenance by the final user: SFC;
• Universal acceptance of language: LD;
• Acceptance in Europe: IL or ST;
• Speed of execution by the PLC: IL or ST;
• Applications mainly using digital I/O and basic processing: LD or FBD;
• Ease of changing code later: LD;
• Ease of use by newer engineers: ST;
• Ease of implementing complex mathematical operations: ST; and
• Applications with repeating processes or processes requiring interlocks and concurrent operations:

One of the consideration also is the type of your PLC. Sometimes one PLC cannot afford all of the languages. Its regarding the capability of the PLC.  Also depend to the costumer. Sometimes the costumer which have maintenance background more like the FBD or the Ladder diagram. But the person with coding backround will be like IL or ST structure Language.

What is PLC?

The first to heard the “PLC” word was very strange.  Maybe for the beginner at school it will be like a strange topic to be discuss. But do you know if the PLC is the most common brain in the industrial automation? It’s going to be main brain without it the industry will cannot produce million of products in the short time.

PLC is Programmable Logic Controller. It has began in 1970s to change the ordinary mechanical system and contact relays that had been using in industrial control System. Since the usage of relays and mechanical switch was very complex so the choice to use PLC is become very gratify
PLC have a lot of gain to use in control and automation system as PLC :

• Cost effective for controlling complex systems.
• Flexible and can be reapplied to control other systems quickly and easily.
• Computational abilities allow more sophisticated control.
• Trouble shooting aids make programming easier and reduce downtime.
• Reliable components make these likely to operate for years before failure.

The Programmable ability has made the PLC is very flexible and cost effective.  PLC have 5 of programming language. The programming language refer to IEC 61131-3.

IEC 61131-3 is the third part of the open international standard IEC 61131, and was first published in December 1993 by the IEC. The current (second) edition was published in 2003.
Part 3 of IEC 61131 deals with programming languages and defines two graphical and two textual PLC programming language standards:

• Ladder diagram (LD), graphical
• Function block diagram (FBD), graphical
• Structured text (ST), textual
• Instruction list (IL), textual
• Sequential function chart (SFC), has elements to organize programs for sequential and parallel control processing.

We will talk about the programming language in the others topic. C u.

Reference:
Automating Manufacturing Systems with PLCs by:Hugh Jack
http://en.wikipedia.org/wiki/IEC_61131-3

Let’s think automation!

Hi all, this is my first post. I have inspired to this  blog about automation as my profession   is  in Instrument fiels and the main subject at automation system. With this blog I hope I can share and learn more about instrumentation and automation with the others professional in the entire world. I’m sorry if my English is not so good a  I’m Indonesian but I will try my best to better speak in English.

Hope you interest to join with me to think automation!