1.1INTRODUCTION TO AUTOMATION
Automation is the use of control systems such as computers tocontrol industrial machinery and process, reducing the need forhuman intervention. In the scope of industrialization, automationis a step beyond mechanization. Whereas mechanizationprovided human operators with machinery to assist them withphysical requirements of work, automation greatly reduces theneed for human sensory and mental requirements as well.
Processes and systems can also be automated.
1. It increases productivity and reduce cost.
2. It gives emphasis on flexibility and convertibility of
manufacturing process. Hence gives manufacturers the ability
to easily switch from manufacturing products.
3. Automation is now often applied primarily to increase quality
in the manufacturing process, where automation can increase
4. Increase the consistency of output.
5. Replacing humans in tasks done in dangerous environments.
1.2Advantages of Automation:
1. Replacing human operators in tasks that involve hard physicalor monotonous work.
2. Performing tasks that are beyond human capabilities of size,weight, endurance etc.
3. Economy improvement: Automation may improve in economyof enterprises, society or most of humanity.
1.3Disadvantages of Automation:
1. Technology limits: Current technology is unable to automate all the desired tasks.
2. Unpredictable development costs: The research anddevelopment cost of automating a process may exceed the costsaved by the automation itself.
3. High initial cost: The automation of a new product or plantrequires a huge initial investment in comparison with the unitcost of the product.
1.4.1 Automated video surveillance:
Automated video surveillance monitors people and vehiclesin real time within a busy environment. Existing automatedsurveillance systems are based on the environment they areprimarily designed to observe, i.e., indoor, outdoor orairborne, the amount of sensors that the automated systemcan handle and the mobility of sensor, i.e., stationary cameravs. mobile camera. The purpose of a surveillance system is torecord properties and trajectories of objects in a given area,generate warnings or notify designated authority in case of occurrence of particular events.Industrial automation deals with the optimization of energy efficient drive systems by precise measurement and control technologies.Automated manufacturing refers to the application ofautomation to produce things in the factory way.Automated manufacturing refers to the application of automation to produce things in the factory way.
1.4.2 Automated manufacturing:
Automated manufacturing refers to the application of automation to produce things in the factory way. Most of the advantages of the automation technology has its influence inthe manufacture processes.The main advantages of automated manufacturing arehigher consistency and quality, reduced lead times,simplified production, reduced handling, improved workflow, and increased worker morale when a good implementation of the automation is made.
1.4.3 Home automation:
Home automation designates an emerging practice ofincreased automation of household appliances andfeatures in residential dwellings, particularly throughelectronic means that allow for things impracticable, overlyexpensive or simply not possible recent past decades.
1.4.4 Industrial automation:
Industrial automation deals with the optimization of energyefficientdrive systems by precise measurement and controltechnologies. Nowadays energy efficiency in industrialprocesses are becoming more and more relevant.Semiconductor companies like Infineon Technologies areoffering 8-bit microcontroller applications for examplefound in motor controls, general purpose pumps, fans, and ebikesto reduce energy consumption and thus increaseefficiency.
1.5Limitations to automation:
Current technology is unable to automate all the desired tasks.As a process becomes increasingly automated, there is less andless labour to be saved or quality improvement to be gained.This is an example of both diminishing returns and the logisticfunction.
Similar to the above, as more and more processes becomeautomated, there are fewer remaining non-automatedprocesses. This is an example of exhaustion of opportunities.?
PROGRAMMABLE LOGIC CONTROLLERS
Programmable logic controllers, also called programmable controllers orPLCs, are solid-state members of the computer family, using integratedcircuits instead of electromechanical devices to implement control functions.
They are capable of storing instructions, such as sequencing, timing,counting, arithmetic, data manipulation, and communication, tocontrolindustrial machines and processes. Programmable controllers have many definitions. However, PLCs can bethought of in simple terms as industrial computers with specially designedarchitecture in both their central units (the PLC itself) and their interfacingcircuitry to field devices (input/output connections to the real world).
2.1.1 The First Programmable Controller
The product implementation to satisfy Hydramatic’s specifications wasunderway in 1968; and by 1969, the programmable controller had its firstproduct offsprings. These early controllers met the original specifications andopened the door to the development of a new control technology.
The first PLCs offered relay functionality, thus replacing the originalhardwired relay logic, which used electrically operated devices to mechanicallyswitch electrical circuits. They met the requirements of modularity,expandability, programmability, and ease of use in an industrial environment.These controllers were easily installed, used less space, and were reusable.
The controller programming, although a little tedious, had a recognizableplant standard: the ladder diagram format. In a short period, programmable controller use started to spread to otherindustries. By 1971, PLCs were being used to provide relay replacement asthe first steps toward control automation in other industries, such as food andbeverage, metals, manufacturing, and pulp and paper.
2.1.2 Today’s Programmable ControllersMany technological advances in the programmable controller industrycontinue today. These advances not only affect programmable controllerdesign, but also the philosophical approach to control system architecture.
Changes include both hardware (physical components) and software (controlprogram) upgrades. The following list describes some recent PLChardware enhancements:-
• Faster scan times are being achieved using new, advanced microprocessorand electronic technology.
• Small, low-cost PLCs (see Figure 1-2), which can replace four to tenrelays,
• High-density input/output (I/O) systems (see Figure 1-3) providespace-efficient interfaces at low cost.
• Intelligent, microprocessor-based I/O interfaces have expanded distributedprocessing. Typical interfaces include PID (proportional integral-derivative), network, CANbus, fieldbus, ASCII communication,positioning, host computer, and language modules (e.g., BASIC,Pascal).
• Mechanical design improvements have included rugged input/outputenclosures and input/output systems that have made the terminal anintegral unit.
• Special interfaces have allowed certain devices to be connecteddirectly to the controller. Typical interfaces include thermocouples,strain gauges, and fast-response inputs.
• Peripheral equipment has improved operator interface techniques,and system documentation is now a standard part of the system. small plc.
Fig PLC System high density I/O (64 point modules)
2.2 PLC Varsus Relay Control
For years, the question many engineers, plant managers, and original equipment manufacturers (OEMs) asked was, “Should I be using programmable controller?” At one time, much of a systems engineer’s time spent trying to determine the cost-effectiveness of a PLC over relay control. Even today, many control system designers still think that they are faced with this decision. One thing, however, is certain—today’s demand for high quality and productivity can hardly be fulfilled economically without electronic control equipment. With rapid technology developments and increasing competition, the cost of programmable controls has been driven down to the point where a PLC-versus-relay cost study is no longer necessary or valid. Programmable controller applications can now be evaluated on their own merits.
When deciding whether to use a PLC-based system or a hardwired relay system, the designer must ask several questions. Some of these questions are:
• Is there a need for flexibility in control logic changes?
• Is there a need for high reliability?
• Are space requirements important?
• Are increased capability and output required?
• Are there data collection requirements?
• Will there be frequent control logic changes?
• Will there be a need for rapid modification?
• Must similar control logic be used on different machines?
• Is there a need for future growth?
• What are the overall costs?
The merits of PLC systems make them especially suitable for applications in which the requirements listed above are particularly important for the economic viability of the machine or process operation. A case which speaks for itself, the system shown in Figure 1-10, shows why programmable controller are easily favored over relays. The implementation of this system using electromechanical standard and timing relays would have made this control panel a maze of large bundles of wires and interconnections
Fig .The uncluttered control panel of an insulted PLC System
If system requirements call for flexibility or future growth, a programmable controller bring returns that outweigh any initial cost advantage of a relay control system. Even in a case where no flexibility or future expansion is required, a large system can benefit tremendously from the trouble shooting and maintenance aids provided by a PLC. The extremely short cycle (scan)time of a PLC allows the productivity of machines that were previously under electromechanical control to increase considerably. Also, although relay control may cost less initially, this advantage is lost if production down time due to failures is high.
2.3 Typical Areas Of PLC Applications
Since its inception, the PLC has been successfully applied in virtually every segment of industry, including steel mills, paper plants, food-processing plants, chemical plants, and power plants. PLCs perform a great variety of control tasks, from repetitive ON/OFF control of simple machines to sophisticated manufacturing and process control. Table 1-1 lists a few ofthe major industries that use programmable controllers, as well as some their typical applications.
2.3.1 Chemical/Petrochemical Manufacturing/Machining
Batch process Assembly machines
Finished product handling Boring
Materials handling Cranes
Mixing Energy demand
Off-shore drilling Grinding
Pipeline control Injection/blow molding
Water/waste treatment Material conveyors
Cullet weighing Painting
Forming Test stands
Lehr control Tracer lathe
2.3.3 Metals Food/Beverage
Blast furnace control
Accumulating conveyors Continuous casting
Blending Rolling mills
Brewing Soaking pit
Filling Bulk material conveyors
Load forming Loading/unloading
Metal forming loading/unloading Ore processing
Palletizing Water/waste management
Warehouse storage/retrieval Burner control
Weighing Coal handling
Batch digesters Load shedding
Chip handling Sorting
Because the applications of programmable controllers are extensive, it is impossible to list them all in this book. However, Table 1-2 provides a small sample of how PLCs are being used in industry.
Internal Combustion Engine Monitoring. A PLC acquires data recorded from sensors located at the internal combustion engine. Measurements taken include water temperature, oil temperature, RPMs, torque, exhaust temperature, oil pressure, manifold pressure, and timing.
Carburetor Production Testing. PLCs provide on-line analysis of auto motive carburetors in a production assembly line. The systems significantly reduce the test time, while providing greater yield and better quality carburetors. Pressure, vacuum, and fuel and air flow are some of the variables tested.
Monitoring Automotive Production Machines. The system monitors total parts ,rejected parts, parts produced, machine cycle time, and machine efficiency. Statistical data is available to the operator anytime or after each shift
Power Steering Valve Assembly and Testing. The PLC system controls a machine to ensure proper balance of the valves and to maximize left and right turningratios.
2.3.8 Chemical And Petrochemical
Ammonia and Ethylene Processing. Programmable controllers monitor and control large compressors used during ammonia and ethylene manufacturing. The PLC monitors bearing temperatures, operation of clearance pockets, compressor speed, power consumption, vibration, discharge temperatures, pressure, and suction flow.
Dyes. PLCs monitor and control the dye processing used in the textile industry. They match and blend colors to predetermined values.
Chemical Batching. The PLC controls the batching ratio of two or more materials in a continuous process. The system determines the rate of discharge of each material and keeps inventory records. Several batch recipes can be logged and retreived automatically or on command from the operator.
Fan Control. PLCs control fans based on levels of toxic gases in a chemical production environment. This system effectively removes gases when a preset level of contamination is reached. The PLC controls the fan start/stop, cycling, and speeds, so that safety levels are maintained while energy consumption is minimized.
Gas Transmission and Distribution. Programmable controllers monitor and regulate pressures and flows of gas transmission and distribution systems. Data is gathered and measured in the field and transmitted to the PLC system.The PLC controls and monitors the total rig operation and alerts the operator of any possible malfunctions
Pipeline Pump Station Control. PLCs control mainline and booster pumps for crude oil distribution. They measure flow, suction, discharge, and tank low/high limits. Possible communication with SCADA (Supervisory Control and Data Acquistion) systems can provide total supervision of the pipeline.
Oil Fields. PLCs provide on-site gathering and processing of data pertinent to characteristics such as depth and density of drilling rigs. The PLC controls and monitors the total rig operation and alerts the operator of any possible malfunctions
Paper Mill Digester. PLCs control the process of making paper pulp from woodchips. The system calculates and controls the amount of chips based on density and digester volume. Then, the percent of required cooking liquors is calculated and these amounts are added to the sequence. The PLC ramps and holds the cooking temperature until the cooking is completed.
Paper Mill Production. The controller regulates the average basis weight and moisture variable for paper grade. The system manipulates the steam flow adjusts the stock valves to regulate weight, and monitors and controls total flow.
2.4 Advantages Of PLCs
In general, PLC architecture is modular and flexible, allowing hardware and software elements to expand as the application requirements change. In the event that an application outgrows the limitations of the programmable controller, the unit can be easily replaced with a unit having greater memory and I/O capacity, and the old hardware can be reused for a smaller application.
A PLC system provides many benefits to control solutions, from reliability and repeatability to programmability. The benefits achieved with programmable controllers will grow with the individual using them—the more you learn about PLCs, the more you will be able to solve other control problems.
Inherent Features Benefits
Solid-state components • High reliability
Programmable memory • Simplifies changes
• Flexible control
Small size • Minimal space requirements
Microprocessor-based • Communication capability
• Higher level of performance
• Higher quality products
• Multifunctional capability
Software timers/counters • Eliminate hardware
• Easily changed presets
Software control relays • Reduce hardware/wiring cost
• Reduce space requirements
Modular architecture • Installation flexibility
• Easily installed
• Reduces hardware cost
Variety of I/O interfaces • Controls a variety of devices
• Eliminates customized control
Remote I/O stations • Eliminate long wire/conduit runs
Diagnostic indicators • Reduce troubleshooting time
• Signal proper operation
Modular I/O interface • Neat appearance of control panel
• Easily maintained
.Quick I/O disconnects
• Useful management/maintenance
Without question, the “programmable” feature provides the single greatest benefit for the use and installation of programmable controllers. Eliminating hard wired control in favor of programmable control is the first step towards achieving a flexible control system. Once installed, the control plan can be manually or automatically altered to meet day-to-day control requirement without changing the field wiring. This easy alteration is possible since there are no physical connections between the field input devices and output devices (see Figure 1-18), as in hardwired systems.
2.5 Plc Instructions For Discrete Inputs
The most common class of input interfaces is digital (or discrete). Discrete input interfaces connect digital field input devices (those that send non-continuous,fixed-variable signals) to input modules and, consequently, to the programmable controller. The discrete, non-continuous characteristic of digital input interfaces limits them to sensing signals that have only two states(i.e., ON/OFF, OPEN/CLOSED, TRUE/FALSE, etc.). To an input interfacecircuit, discrete input devices are essentially switches that are either open orclosed, signifying either 1 (ON) or 0 (OFF). Table 6-2 shows some examples of discrete input field devices.
Field Input Devices
Motor starter contacts
Thumbwheel switches (TWS)
2.6 Discrete Outputs
Discrete output modules receive their necessary voltage and current from their enclosure’s back plane (see Chapter 4 for loading considerations). The field devices with which discrete output modules interface may differ in their voltage requirements; therefore, several types and magnitudes of voltage are provided to control them (e.g., 120 VAC, 12 VDC). Table 6-4 illustrate some typical output field devices, while Table 6-5 lists the standard output ratings found in discrete output applications.
12–48 volts AC/DC
120 volts AC/DC
230 volts AC/DC
INTRODUCTION TO PROGRAMMING LANGUAGE
As PLCs have developed and expanded, programming languages have developed with them. Programming languages allow the user to enter a control program into a PLC using an established syntax. Today’s advanced languages have new, more versatile instructions, which initiate control program actions. These new instructions provide more computing power for single operations performed by the instruction itself. For instance, PLCs can now transfer blocks of data from one memory location to another while, at the same time, performing a logic or arithmetic
operation on another block.As a result of these new, expanded instructions, control programs can now handle data more easily
In addition to new programming instructions, the development of powerfull I/O modules has also changed existing instructions. These changes include the ability to send data to and obtain data from modules by addressing the modules’ locations. For example, PLCs can now read and write data to and from analog modules. All of these advances, in conjunction with projected industry needs, have created a demand for more powerful instructions that allow easier, more compact, function-oriented PLC programs.
3.1 Types Of Plc Languages
The three types of programming languages used in PLCs are:
Some PLC manufacturers use Boolean language, also called Booleanmnemonics, to program a controller. The Boolean language uses Boolean algebra syntax to enter and explain the control logic. That is it uses the AND, OR, and NOT logic functions to implement the control circuits in the control program. Figure 9-3 shows a basic Boolean program. The Boolean language is primarily just a way of entering the control program into a PLC, rather than an actual instruction-oriented language.
When displayed on the programming monitor, the Boolean language is usually viewed as a ladder circuit instead of as the Boolean commands that define the instruction. We will discuss Boolean programming, along with it sinstruction set, at the end of this chapter. a simple circuit represented in boolean. Note that boolean charts provide a flowchart-like representation of the events that take place in each stage of the control program. These charts use three components—steps, transitions, and actions—to represent events. The IEC 1131standard’s SFCs also use these components; however, the instructions in side the actions can be programmed using one or more possible languages including ladder diagram.For example, PLCs can now read and write data to and from analog modules. All of these advances, in conjunction with projectedindustry needs, have created a demand for more powerful instructions thatallow easier, more compact, function-oriented PLC programs.As PLCs have developed and expanded, programming languages havedeveloped with them. Programming languages allow the user to enter acontrol program into a PLC using an established syntax. Today’s advancedlanguages have new, more versatile instructions, which initiate controlprogram actions.Using this method, a boolean software manufacturer can providedifferent PLCs that use the same “language.” Figure 9-5 illustrates a typicaltranslation that occurs when using boolean.
Fig . Hard Wired Logic Circuit And Its Boolean Representation.
Grafcet(Graphe Fonctionnel de Command eÉtape Transition) is a symbolic,graphic language, which originated in France, that represents the controlprogram as steps or stages in the machine or process. In fact, the Englishtranslation of Grafcet means “step transition function charts.”
Figure. Hardwired logic circuit and its Grafcet representation
1illustrates a simple circuit represented in Grafcet. Note that Grafcet charts provide a flow chart like representation of the events that take place in each stage of the control program. The charts use three components—steps, transitions, and actions—to represent events. The IEC 1131standard’s SFCs also use these components; however, the instructions inside the actions can be programmed using one or more possible languages, including ladder diagram.
Few programmable controllers may be directly programmed using Grafcet.However, several Grafcet software manufacturers provide off-line Grafcet programming using a personal computer. Once programmed in the PC, the Grafcet instructions can be transferred to a PLC via a translator or driver that translates the Grafcet program into a ladder diagram or Boolean language eprogram. Using this method, a Grafcet software manufacturer can provide different PLCs that use the same “language.” Figure 9-5 illustrates a typical translation that occurs when using Grafcet.
3.1.3 Ladder Relay Instructions
Ladder relay instructions are the most basic instructions in the ladder diagram instruction set. These instructions represent the ON/OFF status ofconnected inputs and outputs. Ladder relay instructions use two types of
Contacts represent the input conditions thatmust be evaluated in a given rung to determine the control of the output.
Coils represent a rung’s outputs. lists common ladder relay instructions.
In a program, each contact and coil has a referenced address number, whichidentifies what is being evaluated and what is being controlled. The addressnumber references the I/O table location of the connected input/output or theinternal or storage bit output
Table 2.1Ladder relay instructions symbols: contacts and coils
A contact, regardless of whether it represents i/p/o/p connection or an internal output, can be used throughout thecontrol program whenever the condition it represents must be evaluated.
The format of the rung contacts in a PLC program depends on the desiredcontrol logic. Contacts may be placed in whatever series, parallel, or series/parallel configuration is required to control a given output. When logiccontinuity exists in at least one left-to-right contact path, the rung conditionis TRUE; that is, the rung controls the given output. The rung condition isFALSE if no path has continuity.
184.108.40.206 Examine-On/Normally Open
An examine-ON instruction, referred to as a normally open (NO) contactinstruction, tests for an ON condition in a reference address. This referenceaddress can be an input table bit corresponding to an inputdevice, an outputbit in the internal bit storage section of the data table, or an output table bitcorresponding to an output device.
During the execution of an examine-ON instruction in the control program,the processor examines the reference address of the instruction for an ONcondition. If the reference address is logic 0 (OFF), the processor will notchange the state of the normally open contact; thus, it does not providecontinuity to the rung (see Figure 9-17a). However, if the reference addressis logic 1 (ON), the processor will close the normally open condition toprovide power flow in the rung.
220.127.116.11 Examine-Off/Normally Closed
An examine-OFF instruction, also called a normally closed (NC) contactinstruction, tests for an OFF condition in the reference address. Like anexamine-ON instruction, the address can reference the input table, the outputtable, or the internal bit storage section of the output table.During the execution of an examine-OFF instruction, the processor examinesthe reference address for an OFF condition. If the reference address has a logic0 status (OFF), the instruction will continue to provide power (continuity)through the normally closed contacts (see Figure 9-18a). If the referenceaddress has a logic 1 status (ON), the instruction will open the normally closedcontact, thus breaking continuity to the rung (see Figure 9-18b). An examine-OFF instruction can be associated with a logic NOT function, so that if thereference address is NOT ON, logic continuity will be provided.
18.104.22.168 Output Coil
An output coil instruction controls either a real output (connected to the PLCvia output interfaces) or an internal output (control relay). This instructionuses an output coil address bit in the internal storage area as its reference address. The —( )— symbol may also represent an output coil instruction.
22.214.171.124 Latch Output Coil
A latch coil instruction causes an output to remain energized even if thestatus of the contacts that caused the output to energize changes. If any rungpath has logic continuity, this instruction turns the output ON and keeps it
ON, even if logic continuity or system power is lost. The latched output willremain ON until it is unlatched by an unlatch output instruction. An unlatchinstruction is the only automatic (programmed) way to reset a latched output.Although most PLCs allow latching of internal and external outputs, somecontrollers will latch internal outputs only. A latch output coil instruction mayalso be referred to as a set coil instruction, which can be unlatched by a resetcoil instruction. They activate or deactivate adevice after a time interval has expired or a count has reached a preset value Timer and counter instructions are generally considered internal outputs
126.96.36.199 Unlatch Output Coil
An unlatch coil instruction resets a latched output with the same referenceaddress. When any rung path has logic continuity, this instruction turns OFFthe latched reference address coil, or rather unlatches it to an OFF condition.
Figure 9-24 illustrates the use of latch and unlatch coils.
Fig2.3 Latch and unlatch coil instructions
188.8.131.52 Timers And Counters
PLC timers and counters are internal instructions that provide the samefunctions as hardware timers and counters. They activate or deactivate adevice after a time interval has expired or a count has reached a preset value.
Timer and counter instructions are generally considered internal outputs.
Timer instructions may have one or more time bases (TB) which they useto time an event. The time base is the resolution, or accuracy, of the timer.
Therefore, if the timer has a time base of 1 second, then the timer must countten times before it activates its output. This number of counts is referred to asticks. The most common time bases are 0.01 sec, 0.1 sec, and 1 sec.
Counter instructions are used to count events, such as parts passing on aconveyor, the number of times a solenoid is turned ON, etc. Counters, alongwith timers, must have two values, a preset value and an accumulated value.
These values are stored in register or word locations in the data table. Thepreset value is the target number of ticks or counting numbers that must beachieved before the timer or counter turns its output ON. The accumulatedvalue is the current number of ticks (timer) or counts (counter) that haveelapsed during the timer or counter operation. The preset value is stored in a preset register, while the accumulated value is kept in an accumulatedregister. Both of these registers are defined during the programming of theinstruction. Either the basic ladder format or the block instruction formatcan be used to implement timers and counters.
3.2 Timer Instructions
Figure 2.4. (a) Block format and (b) ladder format timer instructions.
PLCs provide several types of timer instructions. However, PLC manufacturersmay provide different definitions for each type of timer functionoffered.
Table. timer instructino.
The function of the various timer instructions is essentially the same, different only in the type of output provided. Figure 2.4 illustrates the two formatsused for timers. A block format timer has one or two inputs, depending on theprogrammable controller. These inputs are called the control line and the enable/reset line. If the control line is TRUE (i.e., it has continuity) and enable line is also TRUE, the block function will start
If the control lineis ON, the timer will start timing.
Figure 2.4. (a) Block format and (b) ladder format timer instructions.
3.2.1On-Delay Energize Timer
An ON-delay energize timer (TON) output instruction either provides timedelayaction or measures the duration for which some event occurs. Oncethe rung has continuity, the timer begins counting time-based intervals (ticks)and counts down until the accumulated time equals the preset time. Whenthese two values are equal, the timer energizes the output and closes the timedoutcontact associated with the output (see Figure 9-44). The timed contactcan be used throughout the program as either a normally open or normallyclosed contact. If logic continuity is lost before the timer times out, the timerresets the accumulated register to zero.
3.2.2 On-Delay De-Energize Timer
An ON-delay de-energize timer (TON) instruction operates in a mannersimilar to an ON-delay energize timer instruction, except that the timer’soutput is already ON. This instruction de-energizes the output once the runghas continuity and the time interval has elapsed (accumulated register value= preset register value). PLC manufacturers provide either ON-delay energizeor ON-delay de-energize timers, since it is easy to program one from theother. Figure 9-45 illustrates a timing diagram for both types of ON-delaytimer instructions.
3.2.3 Off-Delay Energize Timer
An OFF-delay energize timer (TOF) output instruction provides timedelayedaction. If the control line rung does not have continuity, the timerbegins counting time-based intervals until the accumulated time value equals theprogrammed preset value. When these values are equal, the timer energized output and closes the timed-out contact associated with the output (seeFigure 9-46). The timed contact can be used throughout the program as eithera normally open or normally closed contact. If logic continuity occurs beforethe timer times out, the accumulated value resets to zero
3.2.4 Off-Delay De-Energize Timer
An OFF-delay de-energize timer (TOF) instruction is similar to its OFF-delayenergize counterpart; however, this timer’s output is ON and will be deenergizedonce the rung loses continuity and the time interval has elapsed(accumulated register value = preset register value). Like ON-delay timers,PLC manufacturers usually provide either OFF-delay energize or de-energizetimers.
3.2.5 Retentive On-Delay Timer
A retentive ON-delay timer (RTO) output instruction is used if the timer’saccumulated value must be retained even if logic continuity or system poweris lost. If any rung path has logic continuity, the timer begins counting timebasedintervals until the accumulated time equals the preset value. Theaccumulated register retains this accumulated value, even if power or logiccontinuity is lost before the timer has timed out. When the accumulated timeequals the preset time, the timer energizes the output and turns ON (closes)the timed-out contact associated with the output. Again, these timer contactscan be used throughout the program as normally open or normally closedcontacts. A retentive timer reset instruction resets a retentive timer’saccumulated value.
3.2.6Retentive Timer Reset
A retentive timer reset (RTR) output instruction is the only way to automatically reset the accumulated value of a retentive timer. If any rung path has logic continuity, then this instruction resets the accumulated value of its referenced retentive timer to zero. Note that the retentive timer reset addresswill be the same as the retentive timer output instruction it is resetting. A retentive ON-delay timer (RTO) output instruction is used if the timer’saccumulated value must be retained even if logic continuity or system poweris lost. If any rung path has logic continuity,
3.3 Counter Instructions
There are two basic types of counters: those that can count up and those thatcan count down. Depending on the controller, the format of these countersmay vary. Some PLCs use the ladder format (output coil), while others usefunctional block format.
Table 2.4 counter instructions.
3.3.1 Up Counter
An up counter (CTU) output instruction adds a count, in increments of one,every time its referenced event occurs. In a control application, this counterturns a device ON or OFF after reaching a certain count (i.e., the preset value in the preset register). Also, this counter can keep track of the number of parts(e.g., filled bottles, machined parts, etc.) that pass a certain point. An upcounter increases its accumulated value (the count value in its accumulatedregister) each time the up-count event makes an OFF-to-ON transition. Whenthe accumulated value reaches the preset value, the counter turns ON the output,finishes the count, and closes the contact associated with the referencedoutput. After the counter reaches the preset value, it either resets its accumulatedregister to zero or continues its count for each OFF-to-ON transition,depending on the controller. In the latter case, a reset instruction is used toclear the accumulated value.
3.3.2 Counter Reset
A counter reset (CTR) output instruction resets up counter and down counteraccumulated values to zero. When programmed, a counter reset coil has thesame reference address as the corresponding up/down counter coils. If thecounter reset rung condition is TRUE, the reset instruction will clear thereferenced address. The reset line in a block format counter instruction setsthe accumulated count to zero (accumulated register = 0). Figure 9-49illustrates a typical block-formatted counter rung with up, down, and resetcounter instructions. The counter will count up when contact 10 closes, countdown when contact 11 closes, and reset register 1003 to 0 when contact 12closes. If the count is equal to 15 as a result of either an up or down count,output 100 will be ON. If the contents of register 1003 are greater than 15,output 101 will be ON. Output 102 will be ON if the accumulated count valueis less than 15.
3.3.3 Down Counter
A down counter (CTD) output instruction decreases the count value in itsaccumulated register by one every time a certain event occurs. In practicaluse, a down counter is used in conjunction with an up counter to form an up/down counter, given that both counters have the same reference registers.
In an up/down counter, the down counter provides a way to correct data thatis input by the up counter. For example, while an up counter counts thenumber of filled bottles that pass a certain point, a down counter with thesame reference address can subtract one from the accumulated count valueevery time it senses an empty or improperly filled bottle. Depending on theprogrammable controller, the down counter will either stop counting downat zero or at a specified maximum negative value. In a block formatinstruction, a down count occurs every time the down input of the countertransitions from OFF to ON.
Fig. counter function block up, down and reset instruction
An end (END) instruction signifies the last rung of a master control relay orzone control last state instruction. This instruction is usually unconditional (i.e., programmed without any conditions to energize). An end instruction reference address may or may not reference a MCR or ZCL. If a reference is included, the END instruction will end that particular MCR or ZCL. If the instruction does not include a reference address, it will terminate the latestMCR or ZCL instruction.
Arithmetic instructions in a PLC include the basic four operations of addition, subtraction, multiplication, and division. In addition to these four math functions, large PLCs may also include square root operations. Table9-7 lists these typical arithmetic instructions and their symbols
Table. arithmetic instructions.
3.6 Data Manipulation Instructions
Data manipulation instructions are enhancements of the basic ladderdiagram instruction set. Whereas relay-type instructions are limited to thecontrol of internal and external outputs based on the status of specific bitaddresses, data manipulation instructions allow multibit operations. Datamanipulation instructions handle operations that tsake place within one, two,or more registers. Table 9-8 presents some data manipulation instructions
Table. data manipulation instruction
(SUPERVISORY CONTROL AND DATA ACQUISITION)
SCADA stands for Supervisory Control And Data Acquisition. Asthe name indicates, it is not a full control system, but rather focuses on the supervisory level. As such, it is a purely software package that is positioned on top of hardware to which it isinterfaced, in general via Programmable Logic Controllers (PLCs),or other commercial hardware modules.SCADA systems are used to monitor and control a plantor equipment in industries such as telecommunications, waterand waste control, energy, oil and gas refining and transportation.These systems encompass the transfer of data between a SCADAcentral host computer and a number of Remote Terminal Units(RTUs) and/or Programmable Logic Controllers (PLCs), and thecentral host and the operator terminals. A SCADA system gathersinformation (such as where a leak on a pipeline has occurred),transfers the information back to a central site, then alerts thehome station that a leak has occurred, carrying out necessaryanalysis and control, such as determining if the leak is critical, anddisplaying the information in a logical and organized fashion.SCADA systems consist of:
1. One or more field data interface devices, usually RTUs, or PLCswhich interface to field sensing devices and local controlswitchboxes and valve actuators
2. A communications system used to transfer data between fielddata interface devices and control units and the computers inthe SCADA central host. The system can be radio, telephone,cable, satellite, etc., or any combination of these.
3. A central host computer server or servers (sometimes called aSCADA Center, master station, or Master Terminal Unit (MTU)
Fig : Typical SCADA System
Generally SCADA system is a centralized system which monitorsand controls entire area. It is purely software package that ispositioned on top of hardware. A supervisory system gathers dataon the process and sends the commands control to the process.For example, in the thermal power plant the water flow can be setto specific value or it can be changed according to therequirement. The SCADA system allows operators to change theset point for the flow, and enable alarm conditions incase of lossof flow and high temperature and the condition is displayed andrecorded. The SCADA system monitors the overall performance ofthe loop. The SCADA system is a centralized system tocommunicate with both wire and wireless technology to Clintdevices. The SCADA system controls can run completely all kindsof industrial process.
EX: If too much pressure in building up in a gas pipe line theSCADA system can automatically open a release valve.
The generally SCADA system can be classified into two parts:
• Clint layer
• Data server layer
The Clint layer which caters for the man machine interaction. Thedata server layer which handles most of the process dataactivities. The SCADA station refers to the servers and it iscomposed of a single PC. The data servers communicate withdevices in the field through process controllers like PLCs or RTUs.The PLCs are connected to the data servers either directly or vianetworks or buses. The SCADA system utilizes a WAN and LANnetworks, the WAN and LAN consists of internet protocols usedfor communication between the master station and devices. Thephysical equipments like sensors connected to the PLCs or RTUs.The RTUs convert the sensor signals to digital data and sends
digital data to master.
Most of the servers are used for multitasking and real timedatabase. The servers are responsible for data gathering andhandling. The SCADA system consists of a software program toprovide trending, diagnostic data, and manage information suchas scheduled maintenance procedure, logistic information,detailed schematics for a particular sensor or machine and expertsystem troubleshooting guides. This means the operator can sea aschematic representation of the plant being controlled.
EX: alarm checking, calculations, logging and archiving; pollingcontrollers on a set of parameter, those are typically connected tothe server. . The SCADA system monitors the overall performance ofthe loop. The SCADA system is a centralized system tocommunicate with both wire and wireless technology to Clintdevices. The SCADA system controls can run completely all kindsof industrial process.
4.2 Human machine interface:
The SCADA system uses human machine interface. Theinformation is displayed and monitored to be processed by thehuman. HMI provides the access of multiple control units whichcan be PLCs and RTUs. The HMI provides the graphicalpresentation of the system. For example, it provides the graphicalpicture of the pump connected to the tank. The user can see theflow of the water and pressure of the water. The important part ofthe HMI is an alarm system which is activated according to thepredefined values.
Fig : Human machine interface
We all know that the second year project is the one on which our future after engineering
depends.So we have searched for a project that is industry oriented and compatible with the
modern trend.The microcontroller based train system has various advantage compared to
normal train.Metro train can be relatively inexpensive.This makes them special effective for
photovoltaic system using high efficiency panel.
The project is a compact,user friendly,well organized which has a special purpose computer
that is a microcontroller.The aim pof this report is briefy cover whatever during our training
program.It has enhanced our knowledge and confiedence to understand and work on electronic
Reference of my training report is taken from
1. “Programmable Controller” by L.A BRYAN