Rabu, 15 Agustus 2007

basic instrument

Many of people almost don't know what instrument is, and with this article we'll share about instrument. It's not musical instrument but industry instrument. Instrument is important part in the industry, cause all industry both small or big company must be use instrument.
So, what is instrument?; what is instrumentation?; why do industry need instrument?
For the beginning, in this session we'll discuss about basic instrument, which talk about basic instrumentation knowledge including principle and definition also.
Note : Our body is instrument system also, but not valid. For example, we can say that some water hot or warm but also we can measure that in the right unit data ( C or F ). Exactly that the reason why we need instrument.

Ok, instrumentation is defined as " the art and science of measurement and control ". Instrumentation can be used to refer to the field and engineers work, or it can refer to the available methods of measurement and control and the instrument with facilitate this. Control system depends on set point and measurement result, and measurement is the first step and the most important action from the control system.
And instruments are the devices which are used to measure attributes of physical system or chemical system. The variable measured can include practically any measurable related to the physical science.
Measurement is the act or the result of quantitative comparation between a predetermined standard and unknown magnitude. The measurement is expressed as a number of the units of the standard ( a real number times unit ), such as distance being indicated by a number of miles or koilmeters.
There are 2 classification of methods of measurement :
1. Mechanic type, or self operated type ( like mercury in the glass thermometer ).
2. Power type, required the source of auxiliary power ( such as compressed air, electrical power, and hydraulic supply.
There are 2 basic methods of measurement :
1. Direct comparasion,commonnly used for measured length.
2. Indirect Comparasion, the comparasion is done with standard through use of calibrated system.In the industry commonly have 5 factor selection of measuring instrument. They are how accurate is the measurement to be made, when are final data required, the cost criterion, in what form data should
be displayed, whether quanttity to be measured has constant value or is it a time variant.
For the measuring system, they should be have primary sensing device, tranducer, intermediate modifying stage, and terminating stage. They all are used for indirect measuring system.

We have some question now, what are the function of instrument?. Before we answer that question we must know the principle of instrument, that is the control system.
Based that words we can say that the function of instrument are :
1. Indication, the value of quantity can be read by movement of needle on a calibrated scale provided in instrument.
2.Signalling, digital or analog ( 4-20 mA ).
3.Transmitting, transmitters are provided in the instrument which provided a signal corresponding to the value of the quantity being measured.
4. Registering, in such instruments instrument merely indiacates, by number or some others symbol of discrete increments, the value of quantitive.
5. To perform various manipulation. Multiplication, differentation, ratio control, etc.
Same as electronic system, control system like instrument has error also.Commonly that error is called source error. In this part we ecspecially talk about 3 source error, they are :
1. Noise, its defined as any signal that doesn't convey useful information.
2.Response time, any measuring has some got response time, i.e., it can't immeditially the input signal applied to it. This is another factor which contributes of the neasurenment.
3. Transmission. In the transmission of information from primary sensing element to indicator the following type error may arise.
After talk about principle of instrument, now we'll talk about variable and process variable. Variable is that suantity or characteristic which is the object of measurement in an instrumentation / control system. Variable generally are classified in two ways : physical characteristic and measurement signal.
And process variable is the current status of the process under control. An example of this would be the temperature of the furnance. The current temperature is called the process variable, while the desired temperature is known as the set point. Specilally, a process variable may be defined as a physical or chemical quantity.
Process variable are measured and controlled for a number reasons, including the following :
Product Quality, Material Saving, Cost Accounting,Production inspecting and testingPlant safety and comfort.

process variable

Now, we'll continue the last topic that is about process variable. For attention, article in this blog is talk about instrumentation in pulp and paper industry. But the principles of instrumentation in all industry must be same. The chapter write what we call as process variable in main thing, cause in next chance we'll discuss it one by one.

1. Pressure
Pressure ( symbol= p ) is the force per unit area applied on a surface in a direction perpendicular to that surface. Mathematically : p = F/A, where F is the normal force and A is the area..
Pressure is a scalar, and has SI units of Pascal, 1 Pa = 1N/m2
2. Flow meters
Measuring the flow of liquids is a critical need in many industrial plants. In some operatons, the ability to conduct accurate flow measurement is so important that it can make the difference between making a profit or taking a loss. With most liquid measurement instruments, the flow rate is determined inferentially by measuring the liquids velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross sectional area is known and remains constant, the average velocity is an indicaton of the flow rate. The basic relationship for determining the liquid's flow rate in such cases is : Q = VxA
Q = liquid flow through the pipe
A = average velocity of the flow
V = cross-sectional area of the pipe
3. Temperature
Temperature, a measure of the quantity of heat in the object, usually measured on thermometer. Temperature can be measured via a diverse array of the sensors. All of them infer temperature by sensing come change in physical characteristic. Six types with which the engineer is likely to come into contact are thermocouples, RTDs and thermistor, infrared radiator, bimetallic devices, liquid expansion devices, and change of state devices.
4. Level
These are two ways of measuring level :
Direct methods, which use the varying of the liquid as a means of obtaining the measurements. Idirect methods, which use a variable that change with the liquid level, to actuate the measuring mechanism.
5. Density
For a fluid, density is define as the mass per unit volume ( grams per cubic centimeter ).
Specific gravity scale, is the ratio of the density of the fluid to density water.
6. Radiation
Radiation in physics is the process of emitting energy in the form of waves or particles. Various types of radiation maybe distinguished, depending on the properties of the emitted energy/matter, the type of emission source, properties and purposes of the emission, etc. Whwn used by general public, the word radiation commonly refers ro ionizing radiation.
7. Electric Current
Electric current is by definition the flow of the electric charge. The SI unit of electric current is ampere ( A ), which is equal to a flow of one coulomb of charge per second.
8. Voltage
Volatage is the difference of electrical potential between two points of an electrical network,expressed i volts. It is measure of the capacity of an electric field to cause an electric current in an electric conductor.
9. Inductance
Inductance ( or electric inductance ) is a measure of the amount o magnetic fluk produced for a given electric current. The SI unit of inductance is the Henry ( H ), the symbol L is used for inductance has rhe following relationship : L is the inductance in henrys,
i is current i amperes
10. Capacitance
Capacitance is a measure of the amount of electric charge stored ( or separated ) for a given electric potential. In a capacitors, there are two conducting electrodes which are insulated from one another. The charge on the electrodes is +Q and -Q, and V represent the potential difference between the electrodes. The SI unit of capacitance is the farad; 1 farad = 1 coulomb per volt.
11. Frequency
Frequency is the measurement of the number of times that a repeated event occurs per unit o the time. It is also defined as the rate of change of phase of a sinusoidal waveform.

Kamis, 09 Agustus 2007


Components of the control loop communicate with each other via standard signal, either pneumatic or electronic. Process information such as pressure, flow, temperature and level are converted to standard signals through instrumentation. The standard electrical signals are sent to the control system ( DCS ) through signal cable.

The signal cable are going through the following path :
1. Sensor to converter / transmitter ( single pair cable or special cable )
2. Converter / Transmitter to junction box single pair instrument cable )
3. Junction box to DCS Marshalling cabinet ( multicore cable )

As an example, consider a standard level control loop on 5 meters high tank of water consisting of an electronic differential pressure transmitter, a controller and a linear control valve using 4-20 mA signals. Assume a level is at steady state with the tanks at a level of 2 meters and a valve opening at 42%. Signal around the loop would be calculated as follows :

Transmiter output = ( 2 mtr /5 mtr ) x (20 mA-4mA) + 4 mA
= 10,4 mA
Controller output = ( 42%/100%0 x (20 mA-4 mA) + 4 mA
= 10,72 mA

Note : this example uses a standard output signal 4-20 mA, cause every company has a different standard. Maybe 0-20 mA, 0-10 V, or in pneumatic uses 3-15 psi.

DCS and Related Part

Distributed Control System ( DCS ) has a bundle of a part with related together in a rack room, electrical room, and a control room.

2. Basic DCS I/O electronic

2.1 Electronic Review

In electronic system, it is necesarry to be familiar with some basic terms. There are current, voltage, resistance, and impedance.

Current is a flow of electronic in a wire. this flow is usually considered to be from negative end to positive end. Sometime, the opposite of electron flow is used to define current. In order for electron to flow, a pressure or force must be applied to make them move. this is called voltage and it is measured in volts. The higher the voltage, the greater the force applied to the electronics. If a current is open for any reason, such as a switch or breaker wire, then the current will not flow. Voltage might still be present, the effective resistance of the circuit is so high that the power supply can not push any electron through it, so current stops.

Every device in use in a electric circuit has a resistance to the force of voltage.This will also tend to restrict the flow of electron through the device. The relationship between voltage, current and resistance is V = IxR. Impedance is a type of resistance found in AC circuit. Since our DCS using field I/O is DC, impedance is not discussed.

2.2 Fusing and Circuit Protection

Fusing is used to protect power supplies from short circuit and other electro-mechanical faults. Short circuits are condition in which the total circuit resistance is dropped to zero. This will cause the power supply to generate an infinite currrent, resulting in severe damage to the output components of the power supply.

A fuse is a special piece of wire,enclosed in a glass tube, designed to pass a fixed amount of current. if the current exceeds the designed amounts,the wire will melt, opening the circuit and preventing any voltage or current from passing.

2.3 Relay

Relays are electro-mechanical devices that are used to switch current off and on only. They are consists of an electronic coil and one or more sets of mechanical switches called contacts. The contancts are held inplace by spring. the type of contact can be either normally open or normally closed, this depending upon the state they are in when the relay is sitting on a shelf and not connected to anything.

The relay operates on the basis that a voltage, when applied to the electric coil, will cause the coil to develop a magnetic field ( this is called energized ). The magnetic field overcomes the force of the spring and will cause the contacts to change state. In this way, it operates as a switch and can open and close circuits under direction from a processor. When the coil is de-energized, the spring holding the contacs will cause the switches to return to their normal state.

3. DCS Part

In general, DCS has 3 important part with connecting together to carry on/back data to the field. They are :
1. Controller/control processing unit
2. Cross Connection cabinet
3. Operator Station

3.1 Controller

Inside this cabinet, it has a controller as a brain of process control. Controller in this case called AC450( Advant Controller ) includes :
a. CPU with memory ( system software and application software )
b. Submodules carrier SC510 or SC520
c. Voltage Regulator SR511
d. Backup Power Supply SB510
e. Supervisory module TC520
f. LAN module CS513
g. Program Card Inteface MB510

3.2 Cross Connection Cabinet

I/O modules are set up on cross connection cabinet which include end of external wiring from field and also internal wiring connected to each module terminal. The I/O modules consist of theree basic components :

1. Connection Unit
Different types of connecton unit used are S100 and S800. In my pulp mill uses S800 and in power mill uses S100. S800 Connection Unit has :

1. TU830 for AI810, AO810, and DI810
2. TU831 for DO820

2. Board Modules

- S800 Board Modules
- AI810 for analog input signal
- AO810 for analog output signal
- DI810 for digital input signal
- DO820 for digital output signal

3. Cable
Connection from controller C1520 to S800 I/O CI810VI has 2 kid of connection :
- TK515 cable, used to connect from CI520 to TC512
- Fiber Optic, used to connect from CI520 to TC514

3.3 Operator Station

The operator station called human machine interface is an interface between the oprator and controller. All process information from controllers is presented on the operator station. The controller via operator station also carries out the operator command. Operator station is connected to the MB300 (Mater bus 300) via RTA board (back of operator station ). The stream of information controller using CS513 on the controller, use AUI cable (TK575) through tranceiver.

4. Software Application

The ABB DCS software application or process application consist of 2 parts :
1. Database (DB)
Every single connection unit like S800 will have its own database in the controller. The data base for I/Os corresponds to their type of I/Os are :
- AI database element for the analog input I/O
AI = analog input ( signal fro field )
- AO database element for the analog output I/O
AO = analog output ( signal from DCS )
- DI database element for the digital input I/O
DI = digital input ( contact at field )
- DO database element for the digital output I/O
DO - digital output ( power 24vdc to field )
2. PC program
The PC program are the process control application programmed in ABB Master Programming Language (AMPL). It is a function block language with graohics representation which has been developed especially for process control application.The smallest unit of the language is called PC element. A fully functional control loop consist of several PC elemnets and the input and output of the element are connected to the input and outputs of other elements or from/to process I/O point.
3. List Of Drawing
List of drawing which could explain what is interlock, what's the properties included its database, alarm settins, list of alarm, and also software which is installed ON ABB system.

Senin, 06 Agustus 2007

basic automation

Automation or industrial automaton is the use of computers to control industrial machinery and processes, replacing human operators. It's step beyond mechanizaton, where human operators are provided with machinery to help them in their job. Some advantages of using automation in industrial are repeatibility, tighter quality control, wasted reduction, integrated with business system, increased productivity and of course reduction of labor. On contrary, some disadvantages are high initial costs and increased dependence and maintenance.
By the middle of the 20th century, automation had existed for many years on a small scale, using mechanical devices to automate the production of a simply shaped items. However the concepts only become truly practical with the addition of the computer, which flexibility allowed it to drive almost any sor of task. Computers with the required combination pf power, price, and size first started to appear in the 1960s., and since then have taken over the vast majority of assembly line tasks.

Another form of automation that involves computers in industrial scale is called control system, where computers are programmed to make easy operators in handling by interfacing with HMI ( human machine interface ).

DCS ( Distributed Control System )

DCS is a part of manufacturing system. It is generallty digitally, and normallly consist of field instrument,connected via wiring or buses, analog or digital and finally stream to HMI or control consoles.
DCS is used in industrial and applications to monitor and controlled equipment with remote human intervention. DCS is a very broad umbrella that describes solution across large victory of industries, including :
- electrical power distribution
- enviromental control system
- water management
- refining
- manufacturing ( pulp and paper, polymer, oil and gas,etc )

Control System Concept

A control system is a group of electronic or mechanical devices connected together in an effort to predict and control the behaviour of process. A process can be defined as a method of effecting changes in the properties of some physical objects.
A control system consist of the number of independent devices, each serving particular purpose within the system. Some of these devices are linked together to predict and control individual steps in the process.

1.1 Control Loop

a control loop is a group of devices the collective tasks of which is to perform the prediction and control of a very specific steps in the process. It's fundamentally designed to control only one physical aspect of the process. This physical aspect,since it's capable of change, is called the procrss variable. The control loop is designed so that the primary element or measurement is installed at some point to the process where any changed made to the process variable can be readily monitored are evaluated. If we link the primary element to the final control element through controller then we hava created a controlling loop.

1.1.1 Primary Element

Primary elements are the measuring components of any control loop. The entire purpose of these components is to observe the quality or quantity of a process variable and translate it to a signal that is meaningful to the operator and/or to the controller. A primary elements is either a tranduser or a transmitter. If it's a tranduser then it must be coupled to a transmitter to allow it to communicate within the control loop. Transmitter allow the sensed value to be sent over relatively long distance without serious degradation of the signal.

Trandusers are generally defined as a device, which will convert one type of energy into another. Such as the orifice plate which convert kinetic energy ( flow velocity ) into a potential energy ( differential pressure ). Similarly thermocouple converts temperature differential into electric voltage.

Transmitter are defined as trandusers that include a pneumatic or electronic amplifier capable of producting one of the standard transmission signal used in control loops.

1.1.2 Controllers

Controllers are the brains of any control loop. The task of this device is to receive the signal transmitted from the prymary element, compare it to the value decided by the operator, determine if any changes need to be made the process variable and implement those changes.
Any devices can be considered a controllers if is processes theree fundamental characteristic :

- Process Variable
This is necessary to allow the controller the asses the value of the process and determine the degree of change the process is experiencing.

- Set point
This is the value that is generated by an operator or some other device that informs to the controller as to the value that is desired for the processs being controlled. the controller will attempt to maintain the process variable at this set point.

- Output
This is a signal that is used to manipulate final element such as control valve or pump in an effort to changed the measured value of the process variable.

1.1.3 Final Control Element

The most common final control element is the control valve. This is a mechanical devices designed to modulate the process loss across itself through the use of variable sized opening. the valve is usually attached to an actuator that is pneumatically, hydraulically or electrically driven. The function of actuator is to provide linear or rotary motion to valve body, thereby causing the valve opening vary. Some valves have velve positioners installed. these devices are mini controllers in themselves, the purpose is to guarantee the position of valve in relation to the signal. valve positioners have linear response to the incoming signal as the most common characteristic. Most positioners, however, are capable of being field configured and calibrated to provide various signal to motion responses.


Add to Technorati Favorites