2009 BASIC STUDIED OF PLC & SCADA
RAJMAL MENARIYA
RAJMAL MENARIYA 11/14/2009
PREFACE: It gives me me an immense pleasure to submit this report as a part pa rt of pr pract actic ical al tr trai aini ning ng of 30 da days. ys. Pr Pract actic ical al tr trai aini ning ng is th the e mo most st important part of the engineering studies. During the course of the training a trainee learns to correlate both the practical problem to the possible possible theore theoretical tical knowl knowledge edge or so solu luti tion on.. Th This is tr trai aini ning ng re reco cord rd is prepared on the basis of my own experience gained during my practical training. traini ng. On the basis of information information collected and guidance provided I had prepared a comprehensive training report training report.. This report contains contains the history, introduction, quality policy and description of PLC & SCADA.
RAJMAL MENARIYA (Electronics & Comm. Engineer)
Contents PROGRAMMABLE
LOGIC CONTROLLERS
1) Int Introdu roducti ction on 2) PL PLC C Histo History ry 3) Hard Wired Relay Comparison 4) Co Comp mpone onent nts s 5) Bas Basic ic PLC Operat Operation: ion: 6)Terminology: a) Sensor
a) b) c)
d) e)
f)
Actuator Discrete Input Analog Inputs Discrete Outputs Analog Outputs CPU
7)Programming 7) Programming of PLC a) Ladder Logic b) Stat St ate emen entt list Progr gra amming c) Function Bl Block Diagrams Programming
8) Adv Advant antages ages of of PLCs PLCs SCADA
1) Int Introdu roducti ction on 2) Characteristics of SCADA RTU 3)Characteristics of SCADA master 4) HMI/S HMI/SCADA CADA – SIMPLICI SIMPLICITY TY 5) Benefi Benefits ts HMI HMI/SC /SCADA ADA 6) SCA SCADA DA REC RECOMM OMMEND ENDED ED
Conclusion
PROGRAMMABLE LOGIC CONTROLLERS 1) In Intr trod oduc ucti tion on:: A Programmable controller is a solid state user programmable control system with functions to control logic, sequencing, timing, arithmetic data manipula manipulation tion and counting capabilities. capabilitie s. It can be viewed as an industrial computer that has a central processor unit, memory, input output interface and a programming device. device. The central processing unit provides the intelligence of the controller. It accepts data, status information from various sensing devices like limit switches, proximity switches, executes the user control program stored in the memory and gives appropriate output commands to devices such as solenoid valves, switches etc. Input output interface is the t he communication link between field devices and the controllers. Through these interfaces the processor can sense and measure physical quantities regarding a machine or process, such as, proximity, position, motion, level, temperature, pressure, etc. Based on status sensed, the CPU issues command to output devices such as valves, motors, alarms, etc. The programmer unit provides the man machine interface. It is used to enter the application program, which often uses a simple user-friendly logic. Programmable logic controller controllers s (PLCs) have become the most predominant control elements for the discrete discrete event control of a mechatronics system. Simplification Simplification of engineering and precise control of manufacturing process can result in significant cost savings. The most cost-effective way which can pay big dividends in the long run is flexible automation; a planned approach towards integrated control systems. It requires a conscious effort on the part of plant managers and engineers to identify areas where automation can result in better deploymen deploymentt and/or utilization of human resources and savings in man-hours or down time. Controls automation need not be high ended and extremely sophisticated; sophisticated; it is the phased, step-by-step effort to automate, employing control systems tailored to one’s specific requirements that achieves the most attractive results. This is where programmab programmable le logic controls have been a breakthrough in the field of automation and control techniques. techniques. This report looks at the role PLCs play in these techniques. A constant demand for better and more efficient manufacturing and process machinery has led to the t he requirement for higher quality and reliability in control techniques. With the availability of intelligent, compact solid state electronic devices, it has been possible to provide control systems that can reduce maintenance, down time and improve productivity to a great extend. By installing an efficient and user friendly electronics systems for manufacturing machinery or processors, one can obtain a precise and reliable means for producing quality products. One of the latest techniques in solid state controls that offers flexible and efficient operation to the user is programmable 6 controllers. The basic idea behind these programmable controllers was to provide means to eliminate high cost associated with inflexibl inflexible, e, conventional relay controlled systems. Programmable Programmabl e controllers offer a system with computer flexibility that is suited to withstand the harsh industrial environment, environment, has simplicity of operation/readability, operation/readability, can reduce machine down time and provide expandability for future and is able to be maintained by plant technicians.
Programmable
Logic History
2) PL PLC C His Histo tory ry PLCs were first introduced in the 1960’s. The primary reason for designin designing g such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. The MODICON 084 brought the world's first PLC into commercial production. When production requirements requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices devices they also have a limited lifetime because of the multitude of moving parts. This also required strict adhesion to maintenan main tenance ce sche schedule dules. s. Troub Troublesh leshooti ooting ng was also quite tedi tedious ous whe when n so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or the outstands, of individual relays. The size could be mind boggling not to mention the complicated initial wiring of so many individual devices. These relays would be individually wired together in a manner that would yield the desired outcome. The problems for maintenance and installation were horrendous. These new controllers also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones which have no moving parts. Communications abilities began to appear in approximatel Communications approximately y 1973. The first such 9 system was Modicon's Modicon's Modbus. The PLC could now talk talk to other PLCs and they could befar away from the actual machine they were controlling. They could also now be used tosend and receive varying voltages to allow them to use analog signals, meaning that they were now applicable applicabl e to many more control systems in the world. Unfortunately, the lack of standardizatio standardization n coupled with continually changing technology has made PLC communications communicatio ns a nightmare of incompatible protocols and physical networks. The 1980’s saw an attempt to standardize communications communications with General Motor's manufacturing automation automation protocol (MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming programming terminals or handheld programmers. The 1990’s saw a gradual reduction in the introduction of new protocols, and the modernization modernizatio n of the physical layers of some of the more popular protocols that survived the 1980's. PLCs can now be programmable in function block diagrams, instruction lists, C and structured text all at the t he same time. PC's are also being used to replace PLCs in some applications. applications. The original company who commissioned commissioned the MODICON 084 has now switched to a PC based control system.
3) Hard Wired Wired Relay Relay Compari Comparison son At the outset of industrial revolution, especially during sixties and seventies, relays were used to operate automated machines, and these were interconnected using
wires inside the control panel. In some cases a control panel covered an entire wall. To discover an error in the system much time was needed, especially with more complex 10 process control systems. On top of everything, a lifetime of relay contacts was limited, so some relays had to be replaced. If replacement was required, machine had to be stopped and production as well. Also, it could happen that there was not enough room for necessary changes. A control panel was used only for one particular process, and it wasn’t easy to adapt to the requirements of a new system. As far as maintenance maintenance,, electricians had to be very skillful in finding errors. In short, conventiona conventionall control panels proved to be very inflexible. Typical example of conventional control panel is given in the following picture.
Figure: Typical Small Scale Control Panel In Figure 1 you can see a large number of electrical wires, relays, timers and other elements of automation typical for that period. period. The pictured control panel panel is not one of the more complicated ones, so you can imagine what complex ones looked like. The most frequently mentioned disadvantages of a classic control panel are: 1. Large amount of work required connecting wires 2. Difficulty with changes or replacemen replacements ts 3. Difficulty in finding f inding errors; requiring skillful/experienced skillful/experienced work force 4. When a problem occurs, hold-up time is indefinite, usually long 11With invention of programmable programmabl e controllers, much has changed in how a process control system is designed. Many advantages appeared. Typical example of control panel with a PLC controller is given in the following picture.
Advantages of control panel that is based on a PLC controller can be presented in few basic points: 1. Compared to a conventiona conventionall process control system, number of wires needed for connections is reduced by approximatel approximately y 80% 2. Diagnostic functions of a PLC controller allow for fast and easy error detection. 3. Change in operating sequence or application of a PLC controller to a different operating process can easily be accomplished by replacing a program through a console or using PC software (not requiring changes in wiring, unless addition of some input or output device is required). 4. Needs fewer spare parts 5. It is much cheaper compared to a conventional conventional system, especially in cases where a large number of Input/Output instruments are needed and when operational functions are complex 6. Reliability of a PLC is greater than that of an electro-mechanical electro-mechanical relay or a timer, because of less moving parts 7. They are compact and occupy less space 8. Use of PLC results in appreciable savings in Hardware and wiring cost
Programmable Logic Controller Components 4) Co Comp mpon onen ents ts The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. They don't physically exist but rather they are simulated and can be considered software counters, timers, etc. Each component of a PLC has a specific function: · Input Relays (contacts) - These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. · Internal Utility Relays - These do not receive signals from the outside world nor
do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializi initializing ng data that was stored. · Counters - These are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can t hink of these as physically existing. · Timers - These come in many varieties and increments. increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1 milliseco millisecond nd through 1 second. · Output Relays (coils) - These are connected to the outside world. They physically physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen. · Data Storage - Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipul manipulation. ation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed.
Figure: PLC Component A counter is a simple device intended to do one simple thing - count. Using them can sometimes be a challenge however however because every manufacturer seems to use them a different way. There are several different types of counters. There are up-counters called CTU CNT, or CTR that only count up, such as 1, 2, and 3. There are also down counters called CTD that only count down, such as 9, 8, 7, etc. In addition to these two, there are up-down counters, typically called UDC (up-down counter). These count up and/or down (1,2,3,4,3,2,3,4,5,...).
A timer is an instruction that waits a set amount of time before doing something. As
usual in industry, different types of timers are available with different manufacturers. The most common type of timer is an On-Delay Timer. This type of timer simply delays turning on its respective output. In other words, after our sensor (input) turns on we wait “x” number of seconds before activating a solenoid valve (output). This is the most common timer. It is oft en called TON (timer on-delay), TIM (timer) or TMR (timer). Another type of timer is an Off-Delay Timer. This type of timer is the opposite of the ondelay timer listed above. This timer delays turning off its respective output. After a sensor (input) sees a target we turn on a solenoid (output). When the sensor no longer sees the target we hold the solenoid on for x number of seconds before turning it off. It is called a TOF (timer off-delay) and is less common than the on-delay type listed above. Very few manufacturers include this type of timer, although it can be quite useful. The last type of timer is a Retentive or Accumulating timer. This type of timer needs 2 inputs. One input starts the timing event (i.e. the clock starts ticking) and the other resets it. The on/off delay timers above would be reset if the input sensor wasn't on/off for the complete timer duration. This timer however holds or retains the current elapsed time when the sensor turns off in mid-stream. For example, we want to know how long a sensor is on f or during a 1 hour period. If we use one of the above timers they will keep resetting when the sensor turns off/on. This timer however, will give us a total or accumulated time. It is often called an RTO (retentive timer) or TMRA (accumulating (accumulatin g timer).
5)
Basic PLC Operation:
1) Chec Check k Inp Input ut st stat atus us 2) Ex Exci cite te Pr Prog ogra ram m 3) Upd pdat ate e Outp Output ut A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values. The first type of scanning, as shown in the diagram below, is not as common as the type that will be discussed second. PLC SCAN DIAGRAM
The first step is to check the input status. This step is therefore generally referred to as the “Check Input Status” stage. First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third? This goes on and on through the entire program. It records this data into its memory to be used during the next st ep. Next the PLC executes your program one instruction at a time, called the “Execute Program” stage. For example, if your program said that if the first input was on
then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step. Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. A new style of scanning has been implemented in the more recent years, called “rung scanning”. This type basically scans each ladder rung individually in the entire ladder logic program, updating the outputs on that rung after scanning through the inputs. This changes the type of programming that will be used as well. If an output is in a rung above the inputs it depends on, you will not get the output updated until the next scan, as the program will keep scanning down until the last rung, then start over. This style is very advantageous in certain situations. If you want your outputs updated at the soonest possible moment, this is the style of scanning that you want to use.
Figure: PLC Operation PLCs consist of input modules or points, a Central Processing Unit (CPU), and output modules or points. An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and executes control instructions based on program instructions in memory. Output modules convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired instructions. These instructions determine what the PLC will do for a specific input. An operator interface device allows process information to be displayed and new control parameters to be entered.
6)
Terminology:
The language of PLCs consists of a commonly used set of terms; many of which are unique to PLCs. In order to understand the ideas and concepts of PLCs, an understanding of these terms is necessary.
a) Sens nso or : A sensor is a device that converts a physical condition into an electrical signal for use by the PLC. Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is connected to the PLC input. An electrical signal is sent from
the pushbutton to the PLC indicating the condition (open/closed) (open/closed) of the pushbutton contacts.
b) Ac Actu tuat ator or : Actuators convert an electrical signal from the PLC into a physical condition. Actuators are connected to the PLC output .A motor starter is one example of an actuator that is connected to the PLC output. Depending on the output PLC signal the motor starter will either start or stop the motor.
c) Discrete Input: A discrete input, also referred to as a digital input, is an input that is either in an ON or OFF condition. Pushbuttons, toggle switches, switches, limit switches, proximity switches, and contact closures are examples of discrete sensors which are connected to the PLCs discrete or digital inputs. In the ON condition a discrete input may be referred to as a logic 1 or a logic high. In the OFF condition a discrete input may be referred to as a logic 0 or a logic low. signal. d) Analog Inputs : An analog input is an input signal that has a continuous signal. Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. In the following example, a level transmitter monitors the level of liquid in a tank. Depending Dependin g on the level transmitter, the signal to t o the PLC can either increase or decrease as the level increases or decreases.
e) Discrete Outputs: A discrete output is an output that is either in an ON or OFF condition. Solenoids, Solenoids, contactor coils, and lamps are examples of actuator devices connected to discrete outputs. Discrete outputs may also be referred to as digital outputs.
f) Ana nallog Out Outpu puts ts : An analog output is an output signal that has a continuous signal. The output may be as simple as a 0-10 VDC level that drives an analog meter. Examples of analog meter outputs are speed, weight, and temperature. The output signal may also be used on more complex applications applications such as a current-to pneumatic transducer that controls an air-operated flow-control valve.
g) CPU : The central processor unit (CPU) is a microprocessor system that contains the system memory and is the PLC decision making unit. The CPU monitors the inputs and makes decisions based on instructions held in the program memory. The CPU performs relay, counting, timing, data comparison, and sequential operations.
7)
Programming of PLC
A program consists of one or more instructions that accomplish a task. Programming a PLC is simply constructing a set of instructions. There are several ways to look at a program such as ladder logic, statement lists, or function block diagrams.
a) Ladder Lad der Logi Logic c Definition Ladder logic is one form of drawing electrical logic schematics, and is a graphical language language very popular for programming Programmable Logic Controllers. Ladder logic was originally invented invented to describe logic made from relays. The name is based on the observation that programs in this language resemble ladders, with two vertical "rails" and a series of horizontal "rungs" between them. Figure 5 below is a very basic example of ladder logic used in a programmable logic logic controls program.
Comparison to Relay Logic The program used in a controls schematic, called a ladder diagram, is similar to a schematic for a set of relay circuits. An argument that aided the initial adoption of ladder logic was that a wide variety of engineers and technicians would be able to understand and use it without much additional training, because of the resemblance to f amiliar hardware systems. This argument has become less relevant lately given that most ladder logic programmers have a software background in more conventional programming languages, language s, and in practice implementations of ladder logic have characteristics such as sequential execution that make the analogy to hardware somewhat somewhat imperfect. Electricians and data cabling or control technicians still argue that this is the best graphical interface as they generally do not have any computer science or digital systems background, and are therefore taught with this interface in sequence with relay logic.
Relay logic is the precursor to ladder logic, and is a method of controlling industrial electronic circuits by using relays and contacts. Figure 6 above shows an average mechanical mechanical relay used in older relay logic systems. The schematic diagrams for relay logic circuits are often called line diagrams, because the inputs and outputs are essentially essentiall y drawn in a series of lines, with the lines representing actual wires run in the circuit. A relay logic circuit is an electrical network consisting of lines, in which each input/output group must have electrical continuity with all components in t hat group of devices to enable the output device. The Relay logic diagrams represent the physical interconnection interconnecti on of devices, while the relay logic circuit forms an electrical schematic diagram for the control of input and output devices. This is why electricians and control technicians can easily understand and interpret relay logic and ladder logic diagrams. Figure 7 below shows a basic relay logic circuit. Notice how it differs from the ladder logic circuit in Figure 5 in that the “virtual” inputs and outputs in the ladder logic circuit have replaced the actual relays and coils in the relay logic circuit.
Figure 7 is a small, basic relay logic circuit. You can see how in relay logic circuits the pushbuttons are represented with graphical drawings of a normally closed pushbutton for the stop button, and a normally open pushbutton for the start button. The coil that is marked “M” is a motor coil, and is a physical piece of equipment in the same location as the motor, which is represented by a circle with the letter M in the middle. The over current or overload device is represented by a normally closed coil symbol with “O.L.” over it. There would only be seven wires to connect in this circuit, so this would not be very difficult to wire, but when more inputs and outputs are added, the difficulty grows exponentially. Figure 8 shows an expanded relay circuit of Figure 7 in that a double pole single throw pushbutton is added into the diagram to be used as a “jog function”. As the diagram shows, a jog switch is used to run the output (motor). Only one component is added, but three wires need to be installed in the circuit for the component to be utilized in the intended manner.
Figure 9 below adds four more components to the system. Two of them are just coils from the motor apparatus that are used as inputs and the other two are a red and green light to be utilized as output/motor status indicators for the user
This circuit adds 6 additional wires to the original circuit in Figure 7. If both of the additions from Figure 8 and 9 were added to the original circuit, this would add 5 components and 9 additional wires. This illustrates how using a programmable logic logic controller is advantageous advantageo us in that adding any number of relays takes much less effort. It doesn’t seem like a large amount of work to connect just 9 additional wires, wires, but in a real world situation, the motor in question may be on top of a grain silo, and the start/stop station may be a few hundred feet away in a control booth. Pulling all these control wires would take hours instead of a few minutes sitting in front of a programming terminal. Programmable Programmabl e logic controllers coupled with ladder logic can make some of the most labor intensive tasks become easy, enjoyable projects.
Ladder logic is the most widely used program for programmable logic controllers where sequential control of a process or manufacturing operation operation is required. r equired. Ladder logic is useful for simple but critical control systems, or for reworking old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. Figure 10 above shows a much more complicated ladder ladder logic diagram than the one shown in Figure 5. It is relatable to the relay circuits in Figures 7, 8, and 9 as well in that some of the outputs are motors and status lights. In addition there are holding/latching holding/latching contacts included, but they are not a piece of hardware. In fact, they are just the address of the respective output being referenced, which will be discussed discussed in greater detail later. This is still st ill not a very large program. Ladder logic programs can easily grow to more than 500 “rungs” to finish some functions.
Ladder Logic Programming Introduction Ladder logic or ladder diagrams are the most common programming language used to program a PLC. Ladder logic was one of the first programming approaches used in PLCs because it borrowed heavily from the relay diagrams that plant electricians already knew. The symbols used in relay ladder logic consist of a power rail to the left, a second power rail to the right, and individual circuits that connect the left power rail to the right. The logic of each circuit (or rung) is solved from left to right. A common mistake made by most people is trying to think of the diagram as having to have current across the rung for the output to function. This has given many people trouble because of the fact that some inputs are “not” inputs, which will be true when there isn’t current
through this sensor. These concepts will be discussed more later. The symbols of these diagrams look like a ladder - with two side rails and circuits that resemble rungs on a ladder.
Figure 11 shows a simplified ladder logic circuit with one input and one output. The logic of the rung above is such: · If Input1 I nput1 is ON (or true) - power (logic) completes completes the circuit from the left rail to the right rail - and Output1 turns ON (or true). · If Input1 I nput1 is OFF (or false) - then the circuit is not completed and logic does not flow to the right - and Output 1 is OFF (or false). There are many logic symbols available in Ladder Logic - including timers, counters, math, and data moves such that any logical condition or control loop can be represented in ladder logic. With just a handful of basic symbols such as a normally open contact, normally closed contact, normally open coil, normally closed coil, timer and counter most logical conditions can be represented. Normally Open Contact
This can be used to represent any input to t he control logic such as a switch or sensor, a contact from an output, or an internal output. When solved the referenced input input is examined for a true (logical 1) condition. If it is true, the contact will close and allow logic to flow from left to right. If the status is FALSE (logical 0), the contact is open and logic will NOT flow from left to right.
Normally Open Coil
This can be used to represent any discrete output from the control logic. When "solved" if the logic to the left of the coil is TRUE, the t he referenced output is TRUE (logical 1).
Normally Closed Contact
When solved the referenced input is examined for an OFF condition. If the status is OFF (logical 0) power (logic) will flow from left to right. If the status is ON, power will not flow.
Normally Closed Coil
When "solved" if the coil is a logical 0, power will be turned on t o the device. If the device is logical 1, power will be OFF.
Basic AND & OR Gates The AND is a basic fundamental logic condition that is easy to directly represent in Ladder Logic. Figure 12 shows a simplified AND “gate” on a ladder rung.
In order for Light1 to turn TRUE, Switch1 must be TRUE, AND Switch2 must be TRUE. If Switch1 is FALSE, logic (not ( not power) power) flows from the left rail, but stops at Switch1. Light1 will be TRUE regardless of the state of Switch2. If Switch1 is TRUE, logic makes it to Switch2. If Switch2 is TRUE, power cannot flow any further to the right, and Light1 is FALSE. If Switch1 is TRUE, AND Switch2 is TRUE - logic flows to Light1 solving its state to TRUE. The OR is a logical condition condition that is easy to t o represent in Ladder Logic. Figure 13 shows a simple OR gate. Notice the differences in logic between the OR and AND gates.
If Switch1 is TRUE, logic flows to Light1 turning it to TRUE. If Switch2 is TRUE, logic flows through the Switch2 contact, and up the rail to Light1 turning it t o TRUE. If Switch1 AND Switch 2 are TRUE Light1 is TRUE. The only way Light1 is FALSE is if Switch1 AND Switch2 are FALSE. In other words, Light1 is TRUE if Switch1 OR Switch2 is TRUE.
Basic Timers & Counters Many times programs will call for action to be taken in a control program based on more than the states of discrete inputs and outputs. Sometimes, processes processes will need to turn on after a delay, or count the number of times a switch is hit. To do these simple tasks, Timers & Counters are utilized.
A timer is simply a control block that takes t akes an input and changes an output based on time. There are two basic types of timers. There are other advanced timers, but they won’t be discussed in this report. An On-Delay Timer takes an input, waits a specific amount of time, allows logic to flow after the delay. An Off-Delay Timer allows logic to flow to an output and keeps that output true until the set amount of time has passed, then turns it false, hence off-delay. Figure 14 above shows an On-Delay Timer with a 10 second delay before it passes the logic through it.
A counter simply counts the number of events that occur on an input. There are two basic types of counters called up counters and down counters. As its name implies, whenever a triggering event occurs, an up counter increments the counter, while a down counter decrements the counter whenever a triggering event occurs. Figure 15 shows the typical graphical representation of an Up Counter. Building a PLC/Ladder Logic Program Building a small ladder logic program to run on a PLC network is quite easy. For the beginner, it is easier to see the ladder diagram in the form of relay logic. Figure 16 below shows a basic start/stop station for a motor in relay logic. Figure 16: Ladder Diagram in Relay Logic.
Just as in Figure 16 above, relay logic shows all components in the system. This is because relay logic is the same as the wiring diagrams that the electricians use, so all the wiring needs to be shown for the logic to work. Because of this, some components may not need to be included in the plc ladder logic diagram.
Figure 17 above shows the same circuit as in Figure 16 with the overload removed. The overload is needed in relay logic because you have to have an overload device on any circuit; therefore it needs to be in t he wiring diagram. This way, if you push too much current to the motor, the overload device will interrupt interrupt the circuit. Overloads are included internally in most any device anymore, but you will still see this in diagrams. There is still an overload device in a plc ladder logic circuit, but ladder logic shows only those components that have an input or output address, so you do not see it. In Figure 17, you can see that the start and stop buttons along with the motor relay will all be turned to inputs in the plc diagram and the motor, signified by a circle with an ‘M’ in the middle will be an output. The motor relay will not be a physical entity in the plc ladder diagram as it is in this relay logic. It will simply be an input that uses the same I/O address as the motor output. The stop button input can be located on either side of the start button/relay gate, as long as it is still in series with it.
Figure 18 above shows the addition of a ‘Jog Function” to the relay circuit. The jog function is generally added to any circuit for troubleshooting purposes only. Most jog functions are set up so that the t he only time the motor will run with the help of the jog function is when the ‘Jog Button’ is pushed. In Figure 18 above, you can see this with the relay logic. As the circuit looks right now, when the Start Button is pressed, the motor will start, energizing the relay, and going across the Jog Button’s normally closed contacts. The motor will stay running this way until the Stop Button is pressed. If instead the Jog Button is pressed, the current will travel across the normally open Jog contacts that are now closed. The motor will stay running until the Jog Button is no longer
pressed.
Figure 19 above shows that same circuit with Status Indicators added. These are used in control rooms to inform users of the status of their motors or other moving parts. Green is the generally accepted color for a motor going, while red is stopped. The green light is energized when the normally open contact is energized by the moving motor, closing it. The red light is energized whenever a normally closed relay is closed, so it will turn off whenever the motor starts to run. From Figure 16 to Figure 19, one can see that with every component added, many wires need to be connected as well. Depending on how far away these components are away from each other, this can be very difficult and time consuming.
Figure 20 above was converted from the relay logic in Figure 19 to the PLC ladder logic seen here. If the PLC logic here was used in Figures 16-Figures 19, adding the various component would’ve taken much less time than physically wiring each component. PLC ladder logic can differ from relay logic in that different components are used as well. In the relay diagrams, a single button double pole switch was used so that it could perform two different functions. In PLC ladder logic, just a single pole button is needed, because the computer can be asked to look for a on or off state. For the status lights, instead of running wires to the motor relays the PLC diagram just looks for a true or false state of the motor output.
b)
Statement list Programming : A statement list (STL) provides another view of a set of instructions. The operation, operation, what is to be done, is shown on the left. The operand, the item to be operated on by the operation, is shown on the right. A comparison between the statementlist shown below, and the ladder logic shown on the previous page, reveals a similar structure. The set of instructions in this statement list perform the same task as the ladder diagram.
c)
Function Block Diagrams Programming: Function Block Diagrams (FBD) pr (FBD) provi ovide de ano anothe therr vie view w of a set of ins instru tructi ctions ons.. Eac Each h fun functi ction on ha has s a na name me to designate its specific task. Functions are indicated by a rectangle. Inputs are shown on
the left-hand side of the rectangle and outputs are shown on the right-hand side. The function block diagram shown below performs the same function as shown by the ladder diagram and statement list.
System scale 1.A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. 2.Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.
8) Adv Advant antages ages of of PLCs PLCs Following are just a few of the advantages of PLCs: • Smaller physical size than hard-wire solutions. solutions. • Easier and faster to make changes. • PLCs have integrated diagnostics diagnostics and override functions. • Diagnostics are centrally available. • Applications can be immediately documented. • Applications can be duplicated faster and less expensively.
SCADA
1) Introduction: SCADA is not a specific technology, but a type of application- “ SCADA stands for Supervisory Control And Data Acquisition- any application that gets data about a system in order to control that system is a SCADA application”..It is a purely software package that is positioned on top of hardware to which it is interfaced , in general via Programmable Logic Controllers (PLCs), or other commercial hardware modules.
A SCADA application has two elements: 1.The process/system/machinery you want to monitor a control — this can be a power plant, a water system, a network, a system of traffic lights, or anything else. 2. A network of intelligent devices that interfaces with the first system through sensors and control outputs. This network, which whi ch is the SCADA system, gives you the ability to measure and control specific elements of the first system.
These functions are performed by four kinds of SCADA components:
1.
Sensors (either digital or analog) and control
relays that
directly interface with the managed system.
2.
Remote telemetry units (RTUs). These are small
computerized units deployed in the field at specific sites and locations. RTUs serve as local collection points for gathering reports from sensors and delivering commands to control relays.
3.
3 SCADA master units. These are larger computer
consoles that serve as the central processor for the SCADA system. Master units provide a human interface to the system and automatically regulate the managed system in response to sensor inputs.
4.
4. The communications network that connects the
SCADA master unit to the RTUs in the field.
Data Acquisition: Real-life SCADA system needs to monitor hundreds or thousands of sensors. Some sensors measure inputs into the system (for example, water flowing into a reservoir), and some sensors measure outputs (like valve pressure as water is released from the reservoir). Some of those sensors measure simple events that can be detected by a straightforward on/off switch, called a discrete input (or digital input). For example, in our simple model of the widget fabricator, the switch that turns on the light would be a discrete input. In real life, discrete inputs are used to measure simple states, like whether equipment is on or off, or tripwire alarms, like a power failure at a critical facility. Some sensors measure more complex situations where exact measurement is important. These are analog sensors, which can detect continuous changes in a voltage or current input. Analog sensors are used to track fluid levels in tanks, voltage levels in batteries, temperature and other factors that can be measured in a continuous range of input. For most analog factors, there is a normal range defined by a bottom and top level. For example, you may want the temperature in a server room to stay between 60 and 85 degrees Fahrenheit. If the temperature goes above or below this range, it will trigger a threshold alarm. In more advanced systems, there are four threshold alarms for analog sensors, defining Major Under, Minor Under, Minor Over and Major Over alarms.
Data Communication: Real SCADA systems don’t communicate with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on closed proprietary protocols, but today the trend is to open, standard protocols and protocol mediation. Sensors and control relays are very simple electric devices that can’t generate or interpret protocol communication on their own. Therefore the remote telemetry unit (RTU) is needed to provide an interface between the sensors and the SCADA network. The RTU encodes sensor inputs into protocol format and forwards them to the SCADA master; in turn, the RTU receives control commands in protocol format from the master and transmits electrical signals to the appropriate control relays.
Data Presentation: SCADA system reports to human operators over a specialized computer that is variously called a master station, an HMI (HumanMachine Interface) or an HCI HC I (Human- Computer Interface). The SCADA master station has several different functions. The master continuously monitors all sensors and alerts the operator when there is an “alarm” — that is, when a control factor is operating outside what is defined as its normal operation. The master presents a comprehensive view of the entire managed system, and presents more detail in response to user requests. The master also performs data processing on information i nformation gathered from sensors — it maintains report re port logs and summarizes historical trends.
Control: SCADA systems automatically regulate all kinds of industrial processes. For example, if too much pressure is building up in a gas pipeline, the SCADA system can automatically open a release valve. Electricity production can be adjusted to meet demands on the power grid. Even these real-world examples are simplified; a fullscale SCADA system can adjust the managed system in response to multiple inputs.
2) Characteristics of SCADA RTU SCADA RTUs need to communicate with all your on-site equipment and survive under the harsh conditions of an industrial Environment.
Sufficient capacity
Rugged construction and ability to withstand extremes of temperature and humidity .
Secure, redundant power supply.RTU should support battery
to support the equipment at at site but not more capacity than actually will use. At every site,
power and, ideally, two power inputs.
Redundant communication ports. Network connectivity is as important to SCADA operations as a power supply. A secondary serial port or internal modem will keep your RTU online even if the LAN fails. Plus, RTUs with multiple communication ports easily support a LAN migration strategy.
Nonvolatile memory (NVRAM) for storing software and/or firmware. NVRAM retains data even when power is lost. New firmware can be easily downloaded to NVRAM storage,
Intelligent control. As I noted above, sophisticated SCADA
remotes can control local systems by themselves according to programmed re sponses to sensor inputs. This isn’t necessary for every application, but it does come in handy for some users.
Real-time clock for accurate date/time stamping of reports.
Watchdog timer to ensure that the RTU restarts after a power failure.
3) Characteristics of SCADA master SCADA should display information information in the most most useful ways to human operators and intelligently regulated your managed systems.
Flexible, programmable response to sensor inputs. Look for a system that provides easy tools for programming soft alarms (reports of complex events that track combinations of sensor inputs i nputs and date/time statements) and soft controls (programmed control responses to sensor inputs).
24/7, automatic pager and email notification There’s no need to pay personnel to watch a board 24 hours a day. If equipment needs human attention, the SCADA master can automatically page or email directly to repair technicians.
Detailed information display
Nuisance alarm filtering.Nuisance alarms desensitize your staff to
reports in plain English, with a complete description of what activity is happening and how you can manage it.
alarm reports, and they start to believe that all alarms are nonessential alarms. Eventually they stop responding even to critical alarms. Look for a SCADA master that includes tools to filter out nuisance alarms.
Expansion capability. A SCADA system is a long
Redundant, goo diverse backup. The best SCADA systems
term investment that will last for as long as 10 to 15 years. So you need to make sure it will support your future growth for up to 15 years.
support multiple backup masters, in separate locations.. If the primary SCADA master fails, a second master on the network automatically takes over, with no interruption of monitoring and control functions.
Support for multiple protocols and equipment types. Early SCADA systems were built on closed, proprietary protocols. Singlevendor solutions aren’t a great idea — vendors sometimes drop support for their products or even just go out of business. Support for multiple open protocols safeguards your SCADA system against unplanned obsolescence.
4) HMI/SCADA – SIMPLICITY Simplicity HMI supports redundancy at several levels to minimize the effect of any failure. These include backup Simplici Simplicity ty HMI Servers and network redundancy.
Options
Action Calendar: Calendar based event control HMI for CNC: Connectivity to Fanuc CNCs Marquee: Display alarm or event messages to marquee devices Pager: Send alarms to paging devices Recipes: Manage and download set points to multiple controllers SPC: Statistical Process Control integrated with alarm management System Sentry: Monitor and alarm off key computer parameters such as memory or disk space Web View: Thin Client for displaying graphic screens Terminal Services: Thin Client for full capabili capabilities ties including remote development and maintenance VCR: Visual CIMPLICITY Replay provides powerful analysis by replaying logged data back through your screens
5) Benefits HMI/SCADA : Powerful Monitoring and Control Over Your Production f or New and Experienced Users. Ease of Use for Connectivity to Other Software, Systems and Devices. Robust Connectivity True Client / Server Architecture for Easy Scalability. Powerful Thin Client Technologies. Sophisticated ted Alarming & Trending. Sophistica
6) SCADA RECOMMENDED LAN / WAN Support. Import from multiple PLC types Support for low bandwidth operation. Secure & Flexible. Low CPU and Memory requirements. Drivers work on RS232, 422, 485, TCP/IP. Unlimited number of tags (tags support 80 char). Graphics(Transparent parent color support, Advanced animations animations without coding, Import Graphics(Trans graphics Windows Windows Bitmap Bitmap -Auto Cad, Cad, Fax Image )
Conclusion This report has discussed the role that programmable logic controllers have in the
efficient design and control of mechanica mechanicall processes. Also discussed was the understanding understandi ng SCADA and the programming programming involved with it. Finally, Finally, the report has discussed relay logic and the evolution that ladder logic made from it. 1. Programmable Logic History: This section discussed the history and advancement of controls technology, with a comparison of programmable logic controllers controllers and hard-wired relays. 2. PLC components: This section defined what programmable logic logic is and described all hardware associated with it. 3. PLC Programming: This section covered various technique of PLC programming. 4 SCADA: This section contain basic introduction of SCADA system.
Some PLC: 984 – 785 E: 1. 984 – 785 785 E can support support 16 RI RI / O drops with with each each drop havin having g 1K in / 1K out out (I / O bits ). It can also support support 31 RI / O drops with each each drop having a maximum of 512 in / 512 out ( I / O bits ).
2. 984 – 785 E runs on 115 or 230 V Ac (47 to 63 Hz) and / or 24 V D.C. 3. 984 – 785 E has a memory fixture that contains both RAM and NAVRAM is attached to the CPU mother board and is not accessible to users.
4. 984 – 785 E has two Modbus ports and one Modbus Plus port.
QUANTUM 43412A 1.
The Quantum 140 CPU 434 12A is equipped with two nine-pin RS-232C connectors that support Modicon’s proprietary Modbus communica communication tion protocol.
2.
The 140 CPU 434 12A supports up to six network modules (i.e., Modbus Plus,Ethernet, and Multi-Axis Motion option modules) using the option module interface technique. However, only two Modbus Plus modules can have f ull functionality, functionali ty, including Quantum DIO support .
Electric power generation, transmission and Distribution . Manufacturing Industries. Mass transit & Water Management Systems.
Traffic signals. Cement and Petrochemical industries. Automobile Industries.
