INDUSTRIAL TRAINING REPORT

TWO MONTHS

INDUSTRIAL TRAINING REPORT

Held at

ENGINEERING TECHNOLOGY SOLUTIONS(ETS)

KURUKSHETRA (HRY.)

Submitted in partial fulfilment of the requirement for the 6^th Semester Curriculum Degree of  Bachelor of Technology in

“INSTITUTE  OF INSTRUMENTATION  ENGINEERING”

Of

Kurukshetra University, Kurukshetra

DEPARTMENT OF INSTRUMENTATION ENGINEERING

INSTITUTE OF INSTRUMENTATION ENGINEERING, KURUKSHETR UNIVERSITY , KURUKSHETRA

                                                                                      SUBMITTED BY:-

contents

·        certificate

·        acknowledgement

·        objectivE of training

COMPANY profile     

·        INTRODUCTION

·        OBJECTIVES

                     

programmable logic controllers                 

·        PLC BASICS                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

·        HISTORY 

·        OLD WAYS

·        IT IS NOT HARD TO IMAGINE

·        EARLIER                                                                                            

·        Need for PLC                                                                                      

·        Advantages of PLC                                                 

·        AUTOMATION                                                                                            

·      What is a PLC?

·       GUTES INSIDE  

·        PLC OPRATION

·   PLC Basic COMPONENTS                                                                            

·        About Inputs and Outputs of PLC

·        ANOLOG VERSUS DIGITAL INPUTS & OUTPUTS

·        HOW PLC REMOVING COMPLEXITY?

·        RELAY

·        REPLACING RELAY                                           

·        WHAT IS CONTROL PROGRAM?                                                     

·        PLC Architecture 

·        About Ladder Logic  

 ·CONTACTS                                                                                                                                               

·  INDUSTRIAL SENSOR                                                                                                                                                                                                                                

ACKNOWLEDGEMENT

      I have reached to this opportunity to thank all those who have been guide me on interval of my training & this report to completion. I am also grateful to Sh.,                                  Director of ETS.

I am grateful to all staff members of  ETS  for this remarkable and outstanding cooperation.

They help me and guide me for right path of training.

I am also very thankful to my friends who contributed a lot in solving the problems & devoted their valuable time in spite of their own studies.

I take this opportunity to bring on record the inspiration, help and indulgence.

OBJECTIVES OF TRAINING

·        The main objective of training is to get first hand technical and practical knowledge of PLC

The main purpose is that the students would become familiar with industrial environment so that they can take up future challenges when they turn up in an industry with good confidence.

·        The students will thus understand the flow of control and information in an industry.

·        This eight weeks training has been introduced in degree Engg. Courses for the enhancement of students’ administrative and professional knowledge about their respective fields.

·        After completion of their degree courses this training experience will help them to make their workplace stronger in the ever-competitive job fields.

 

                                 

   

COMPANY PROFILE

INTRODUCTION

            ETS has grown to be a company with the potential to become the strategic partners for world-wide customers for their software needs for Semiconductors & Embedded.
ETS has a passionate team consisting of professionals having good experience. The team has diversified experience and expertise in both product development and services model and value the significance of Customer Satisfaction and Meeting Deadlines.

With key emphasis on Quality, ETS follows through the Quality Processes during Product Development and Project Execution.

ETS with it strong determination and commitment for providing quality machinery and relentless customer care seeks an opportunity to serve your esteemed organization with its range of products.

         

    OBJECTIVES

                       

1)     Complete understanding of customer requirement in respect of their end products and necessary technology for it.

2)     Ensuring to meet customer specification, time schedule and any requirement to match existing equipment by strictly adopting line concept, selection of equipment designed manufacturing, inspection installation and commissioning norms and procedures.

3)     All equipment in a line to be designed and supplied to ensure their working in synchronous mode for achieving the desired end product specification.

4)     To ensure reliability, aesthetics, easy maintenance, equipment and operator safety, environmental protection and easy of human stress.

5)  Selection of manufacturing practices to ensure “right first time” output.

6)     To maintain and demonstrate the QUALITY ASSURANCE CONCEPT.

7)     Ultimately, improvement in human relation both internal and external through team work principal, better social communication and relation development efforts.

PROGRAMMABLE LOGIC CONTROLLER

                                                                                 

             

                      PLC Basic

History:-

The PLC was invented in response to the needs of the American automotive industry. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles and trucks was accomplished using relays, timers and dedicated closed-loop controllers. The process for updating such facilities for the yearly model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an electronic replacement for hard-wired relay systems.

The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates eighty-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MODular DIgital Controller.

One of the very first 084 models built is now on display at Modicon's headquarters in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired from nearly twenty years of uninterrupted service.

The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models such that they end with eighty-four.

           In the 1960s and 1970s, industry was beginning to see the need for automation. Industry saw the need to improve quality and increase productivity. Flexibility had also become a major concern. Industry needed to be able to change processes quickly to meet the needs of the consumer.

 

 

OLD WAYS

·        There was always a huge wiring panel to control the system.

·        Inside the panel there were masses of electromechanical relays.

·        These relays were all hardwired together to make the system work.

·      Hardwiring means that an electrician had to install wires between the connections of the relays.

·        An engineer would design the logic of the system and electricians would be given a blueprint of the logic and would have to wire the components together.

 

 

It is not hard to imagine

·        That the engineer made a few small errors in his/her design.

·        That the electrician may have made a few errors in wiring the system.

·        That there are few bad components in the system.

Earlier ways        

·        The only way to see everything was correct was to run the system.

·  Troubleshooting was done by running the actual system. This was a very time-consuming process.

·    No product could be manufactured while the wiring was being changed and system had to be disabled for wiring changes. This means that all the production personnel associated with that production line were without work until the system was repaired.

·        The control system was based on mechanical relays.

·        Mechanical devices are usually the weakest links in the systems. Mechanical devices have moving parts that can wear out. If one relay failed, the electrician might have to troubleshoot the whole system again. The system was down again until the problem was found and corrected.

·    Another problem with hardwired logic is that if a change must be made, the system must be shutdown and the panel rewired. If a company decided to change the sequence of operations (even a minor change), it was a major expense and loss of production time while the system was not producing parts.

Need for PLC

·        Due to the disadvantages of the hardwired control panels industry saw the need to replace them and introduce PLCs.

·  Increased competition to manufacturers to improve both quality and Productivity.

·         Flexibility, rapid changeover and reduced downtime became important.

·        Industry realized that a computer could be used for logic instead of hardwired relays. Computer could take the place of huge, costly, inflexible, hardwired control panels.

·   If changes in the system logic or sequence of operations were needed, the program of the computer could be changed instead of rewiring.

·        Imagine eliminating all the downtime associated with wiring changes.

·   Imagine being able to completely change how a system operated by simply changing the software in the computer.

·        High reliability 

Advantages of PLC

·        Small space requirements

·        Computing capabilities

·        Reduced costs

·        Ability to withstand harsh environments

·        Expandability   

 

   Automation

Automation is the use of computers to control industrial machinery and processes, replacing human operators. It is a step beyond mechanization, where human operators are provided with machinery to help them in their jobs. The most visible part of automation can be said to be industrial robotics. Some advantages are repeatability, tighter quality control, waste reduction, integration with business systems, increased productivity and reduction of labor. Some disadvantages are high initial costs and increased dependence on maintenance.

By the middle of the 20th century, automation had existed for many years on a small scale, using mechanical devices to automate the production of simply shaped items. However the concept only became truly practical with the addition of the computer, whose flexibility allowed it to drive almost any sort of task. Computers with the required combination of power, price, and size first started to appear in the 1960s, and since then have taken over the vast majority of assembly line tasks (some food production/inspection being a notable exception).

In most cases specialized hardened computers referred to as PLCs (Programmable Logic Controllers) are used to synchronize the flow of inputs from sensors and events with the flow of outputs to actuators and events. This leads to precisely controlled actions that permit a tight control of the process or machine.

Human-Machine Interfaces (HMI) are usually employed to communicate to PLCs. e.g.: To enter and monitor temperatures or pressures to be maintained.

Another form of automation that involves computers is called test automation, where computers are programmed to mimic what human testers do when manually testing software applications. This is accomplished by using test automation tools to produce special scripts (written as computer programs) that tell the computer exactly what to do in order to run the same manual tests.

                  

 

Need For Automation:-

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       Automation is required because of

·        Shorter throughput times.

·        Shorter set-up times.

·        Higher productivity.

·        Stock reduction.

·        Higher availability.

·        Better capacity utilization.

·        Shorter process change times.

·        Stability and accuracy of control.

·        Greater reliability & equipment life.

·        To add flexibility to process. 

                                                     

        

What is a PLC?

A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results.

PLCs are used in many "real world" applications. If there is industry present, chances are good that there is a plc present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a plc.

For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.

As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time.

This site gives you enough information to be able to write programs far more complicated than the simple one above. We will take a look at what is considered to be the "top 20" plc instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence.
That's right, more than 80%! Of course we'll learn more than just these instructions to help you solve almost all your potential plc applications.

 A programmable logic controller, PLC or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. Where older automated systems would use hundreds or thousands of relays and cam timers, a single PLC can be programmed as a replacement.

The PLC is a microprocessor based device with either modular or integral input/output circuitry that monitors the status of field connected "sensor" inputs and controls the attached output "actuators" (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created logic program stored in the microprocessor's battery-backed RAM memory. The functionality of the PLC has evolved over the years to include capabilities beyond typical relay control; sophisticated motion control, process control, Distributed Control System and complex networking have now been added to the PLC's list of functions.

The Guts Inside

The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. Do these counters, timers, etc. really exist? No, they don't "physically" exist but rather they are simulated and can be considered software counters, timers, etc. These internal relays are simulated through bit locations in registers. (More on that later)

What does each part do?

·         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-(contacts) 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 initializing data that was stored.

·         COUNTERS-These again do not physically exist. They 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 think of these as physically existing. Most times these counters can count up, down or up and down.

·         TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.

·         OUTPUT RELAYS-(coils) these are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or traces 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 manipulation. 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. Very convenient and necessary!!

PLC Operation

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.

                       

                       

Step 1-CHECK INPUT STATUS-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... It records this data into its memory to be used during the next step.

Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe 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.

Step 3-UPDATE OUTPUT STATUS-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.

After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.

PLC Basic Component

Power Supply:

·        The power supply is used to supply the power for the central processing unit.

·        Most of the PLCs operate on 115VAC. This means that input voltage to power supply is 115VAC.

·        On some PLCs power supply is a separate module and some have integrated on it.

Input section:

·        This section performs two vital tasks.

·        It takes input from the outside world and also protects CPU from the outside world.

·        Input devices are often called Field Devices.

Output section:

·        The output section of PLC provides connection to the real world output devices.

·     The output devices might be motor starters, lights, coils, valves etc. These are also called the Field Devices.

·        They can be used to output analog or digital signals.

 

 

 

 

 

About inputs and outputs of PLC

The input/output (I/O) system is the section of a PLC to which all of the field devices are connected. If the CPU can be thought of as the brains of a PLC, then the I/O system can be thought of as the arms and legs. The I/O system is what actually physically carries out the control commands from the program stored in the PLC’s memory.

I/O modules are devices with connection terminals to which the field devices are wired. Together, the rack and the I/O modules form the interface between the field devices and the PLC. When set up properly, each I/O module is both securely wired to its corresponding field devices. This creates the physical connection between the field equipment and the PLC.

All of the field devices connected to a PLC can be classified in one of two categories:

• Inputs

• Outputs

Inputs are devices that supply a signal/data to a PLC. Typical examples of inputs are push buttons, switches, and measurement devices. Basically, an input device tells the PLC, that something is happening out here…you need to check this out & see how it affects the control program.

Outputs are devices that await a signal/data from the PLC to perform their control functions. Lights, horns, motors, and valves are all good examples of output devices. These devices stay unaffected, until the PLC says; now you need to turn on or you’d better open up your valve a little more, etc.

EXAMPLE

An input device sends a signal to a PLC...

An output device receives a signal from a PLC.....

 

                                        

There are two basic types of input and output devices:

• Discrete

• Analog

Discrete devices are inputs and outputs that have only two states: on and off. As a result, they send/receive simple signals to/from a PLC. These signals consist of only 1s and 0s. A 1 means that the device is on and a 0 means that the device is off.

Analog devices are inputs and outputs that can have an infinite number of states. These devices cannot only be on and off, but they can also be barely on, almost totally on, not quite off, etc. These devices send/receive complex signals to/from a PLC. Their communications consist of a variety of signals, not just 1s and 0s.

EXAMPLE

The overhead light and switch we just discussed are both examples of discrete devices. The switch can only be either totally on or totally off at any given time. The same is true for the light. A thermometer and a control valve are examples of the other type of I/O devices/analog. A thermometer is an analog input device because it provides data that can have an infinite number of states. Temperature isn’t just hot or cold. It can have a variety of states, including warm, cool, moderate, etc. A control valve is an analog output for the same reason. It can be totally on or totally off, but it can also have an infinite number of settings between these two states.

 

A discreet can only be on or off, an analog device can be either on, off or anywhere in between.

                           

Because different input and output devices send different kinds of signals, they sometimes have a hard time communicating with the PLC. While PLCs are powerful devices, they can’t always speak the “language” of every device connected to them. That’s where the I/O modules we talked about earlier come in. The modules act as “translators” between the field devices and the PLC. They ensure that the PLC and the field devices all get the information they need in a language that they can understand.

Analog versus digital inputs and outputs

Digital signals behave as switches, yielding simply an On or Off signal (logical 1 or 0, respectively). These are interpreted as boolean values by the PLC. Pushbuttons, limit switches, and photo-eyes are examples of devices providing a digital signal. Analog signals behave as volume controls, yielding a range of values between On and Off. These are typically interpreted as integer values by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. Pressure transducers, scales and gas leak detectors can provide analog signals.

Digital signals generally use either voltage or current, where a specific range is denominated as On (logical 1) and another as Off (logical 0). A typical PLC might use 24VDC I/O (with 24V representing On and 0V

representing Off). Analog signals generally use voltage or current as well, but do not have discrete ranges for On or Off. They define a range of valid values, typically the range in which the I/O device operates reliably. Other methods of signal I/O include serial communications (typically RS-232 or RS-485), and proprietary networks like Allen-Bradley's Data Highway, Opto 22's OptoMux or open and standardised networks like Profibus.

PLCs have a limited number of connections built in for signals such as digital inputs, digital outputs, analog inputs and analog outputs. Typically, expansions are available if the base model does not have sufficient I/O.

The average amount of inputs installed in the world is three times that of outputs for both analog and digital. The need for this rises from the PLC's need to have redundant methods to monitor a instrument to appropriately control another.

Examples                    

As an example, say the facility needs to store water in a tank. The water is used as needed, but spilling is not permitted.

The PLC has two digital inputs from float switches, and a timer. The PLC controls two digital outputs to open and close the two inlet valves into the tank, and an error light. The valves are one after the other so that either valve can turn off the water. This means that the water can be turned off even if one valve breaks. The valves have repeaters, little sensor switches, so the logic controller can sense whether they are open or closed.

If both float switches are off (down) the PLC will open the valves to let more water in, and starts a timer. If both float switches are on, both valves turn off. When the timer is done, it turns off both valves anyway, to prevent spills, and if both switches are not on, and both valves closed, an error light turns on to indicate that a switch or valve is broken. A test button provides a way to restart the timer and retest the switches. The maintenance engineer will have a schedule to test such equipment.

Another example might use a load cell (the sensor of a scale) that weighs the tank and a rate valve. The logic controller would uses a PID feedback loop to control the rate valve. The load cell is connected to one of the PLC's analog inputs and the rate valve is connected to one of the PLC's analog outputs. This system fills the tank faster when there's less water in the

tank. If the water level drops rapidly, the rate valve can be opened wide. If water is only dripping out of the tank, the rate valve adjusts to slowly drip water back into the tank.

In this system, the tricky thing is adjusting the PID loop so the rate valve doesn't wear out from many continual small adjustments. Many PLCs have a "deadband", a range of outputs in which no change is commanded. In this application, the deadband would be adjusted so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.

A real system might combine both approaches, using float switches and simple. valves to prevent spills, and a rate sensor and rate valve to optimize refill rates

 

How PLC helps in removing complexity?

In a traditional industrial control system, all control devices are wired directly to each other according to how the system is supposed to operate. In a PLC system, however, the PLC replaces the wiring between the devices. Thus, instead of being wired directly to each other, all equipment is wired to the PLC. Then, the control program inside the PLC provides the “wiring” connection between the devices.

The control program is the computer program stored in the PLC’s memory that tells the PLC what’s supposed to be going on in the system. The use of a PLC to provide the wiring connections between system devices is called soft wiring.

Let’s say that a push button is supposed to control the operation of a motor. In a traditional control system, the push button would be wired directly to the motor. In a PLC system, however, both the push button and the motor would be wired to the PLC instead. Then, the PLCs control program would complete the electrical circuit between the two, allowing the button to control the motor.

The soft wiring advantage provided by programmable controllers is tremendous. In fact, it is one of the most important features of PLCs. Soft wiring makes changes in the control system easy and cheap. If you want a device in a PLC system to behave differently or to control a different process element, all you have to do is change the control program. In a traditional system, making this type of change would involve physically changing the wiring between the devices, a costly and time-consuming process.

Let’s say that two push buttons, PB1 and PB2, are connected to a PLC. Two pilot lights, PL1 and PL2, are also connected to the PLC the way these devices are connected now pressing push button PB1 turns on pilot light PL1 and pressing push button PB2 turns on pilot light PL2. Let’s say that you want to change this around so that PB1 controls PL2 and PB2 controls PL1. In a traditional system, you would have to rewire the circuit so that the wiring from the first push button goes to the second pilot light and vice versa. However, because these devices are connected to a PLC, making this change is as simple as making a small change in the control program.

Relays

Now that we understand how the PLC processes inputs, outputs, and the actual program we are almost ready to start writing a program. But first let’s see how a relay actually works. After all, the main purpose of a plc is to replace "real-world" relays.

We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic field is generated. This magnetic field sucks the contacts of the relay in, causing them to make a connection. These contacts can be considered to be a switch. They allow current to flow between 2 points thereby closing the circuit.

Let's consider the following example. Here we simply turn on a bell (Lunch time!) whenever a switch is closed. We have 3 real-world parts a switch, a relay and a bell. Whenever the switch closes we apply a current to a bell causing it to sound.

Notice in the picture that we have 2 separate circuits. The bottom (blue) indicates the DC part. The top (red) indicates the AC part.

Here we are using a dc relay to control an AC circuit. That's the fun of relays! When the switch is open no current can flow through the coil of the relay. As soon as the switch is closed, however, current runs through the coil causing a magnetic field to build up. This magnetic field causes the contacts of the relay to close. Now AC current flows through the bell and we hear it. Lunch time!

A typical industrial relay

Replacing Relays

Next, let’s use a plc in place of the relay. (Note that this might not be very cost effective for this application but it does demonstrate the basics we need.) The first thing that's necessary is to create what's called a ladder diagram after seeing a few of these it will become obvious why it is called a ladder diagram. We have to create one of these because, unfortunately, a plc doesn't understand a schematic diagram. It only recognizes code. Fortunately most PLCs have software which converts ladder diagrams into code. This shields us from actually learning the plc's code.

First step- We have to translate all of the items we're using into symbols the plc understands. The plc doesn't understand terms like switch, relay, bell, etc. It prefers input, output, coil, contact, etc. It doesn't care what the actual input or output device actually is. It only cares that it’s an input or an output.

First we replace the battery with a symbol. This symbol is common to all ladder diagrams. We draw what are called bus bars. These simply look like two vertical bars One on each side of the diagram. Think of the left one as being + voltage and the right one as being ground. Further think of the current (logic) flow as being from left to right.
Next we give the inputs a symbol. In this basic example we have one real world input. (I.e. the switch) We give the input that the switch will be connected to, to the symbol shown below. This symbol can also be used as the contact of a relay.

 
       (A contact symbol)

Next we give the outputs a symbol. In this example we use one output (i.e. the bell). We give the output that the bell will be physically connected to the symbol shown below. This symbol is used as the coil of a relay.

 
        (A coil symbol)

The AC supply is an external supply so we don't put it in our ladder. The plc only cares about which output it turns on and not what's physically connected to it.

Second step- We must tell the plc where everything is located. In other words we have to give all the devices an address. Where is the switch going to be physically connected to the plc? How about the bell? We start with a blank road map in the PLCs town and give each item an address .Could you find your friends if you didn't know their address? You know they live in the same town but which house? The plc town has a lot of houses (inputs and outputs) but we have to figure out who lives where (what device is connected where). We'll get further into the addressing scheme later. The plc manufacturers each do it a different way! For now let's say that our input will be called "0000". The output will be called "500".

Final step- We have to convert the schematic into a logical sequence of events. This is much easier than it sounds. The program we're going to write tells the plc what to do when certain events take place. In our example we have to tell the plc what to do when the operator turns on the switch. Obviously we want the bell to sound but the plc doesn't know that. It's a pretty stupid device, isn't it!

The picture above is the final converted diagram. Notice that we eliminated the real world relay from needing a symbol. It's actually "inferred" from the diagram. Huh? Don't worry; you'll see what we mean as we do more examples.

What is control program?

The control program is a software program in the PLC’s memory. It’s what puts the control in a programmable controller. The user or the system designer is usually the one who develops the control program. The control program is made up of things called instructions. Instructions are, in essence, little computer codes that make the inputs and outputs do what you want in order to get the

Result you need.

There are all different kinds of instructions and they can make a PLC do just about anything (add and subtract data, time and count events, compare information, etc.). All you have to do is program the instructions in the proper order and make sure that they are telling the right devices what to do and when to do.

          If you want the system to act differently, just change the instructions in the control program. Different PLCs offer different kinds of instructions. That’s part of what makes each type of PLC unique. However, all PLCs use two basic types of instructions:

• Contacts

• Coils

Contacts are instructions that refer to the input conditions to the control program, that is, to the information supplied by the input field devices. Each contact in the control program monitors a certain field device. The contact waits

For the input to do something in particular (e.g., turn on, turn off, etc.—this all depends on what type of contact it is).

A contact is a computer code that monitors the status of an input coil is a computer code that monitors the status of an output.

Coils are instructions that refer to the outputs of the control program that is, to what each particular output device is supposed to do in the system. Like a contact, each coil also monitors a certain field device. However, unlike a contact, which monitors the field device and then tells the PLC what to do, a coil monitors the PLC control program and then tells the field device what to do. Coil makes the field device to on or off depending upon the PLC instructions.

Let’s say that turning on the switch is supposed to turn on the light. In this situation, the PLCs control program would contain a contact that examines the input device the wall switch for an on condition and a coil that references the light. When the switch turns on, the contact will energize, meaning that it will tell the PLC that the condition it’s been looking for has happened. The PLC will relay this information to the coil instruction by energizing it. This will let the coil know that it needs to tell its referenced output the light to turn on.

PLC Architecture

RAM: -

      Volatile

          User can read or write to the memory 

Program memory: -

          Store program to control various processes

          Divided into different blocks:

                   a) Organization Block - Main program

                   b) Program Block - Subroutines or Calculations

                   c) Function Block - Special functions

                   d) Data Block - Temporary data

                   e) Sequential Block - PLC operations

Timers: -

          a) Pulse Timer

          b) Extended Pulse Timer

          c) On-Delay Timer

          d) Extended Delay Timer

          e) Off Delay Timer

Counters:

          a) Up Counter

          b) Down Counter

c)     Up/Down Counter

Flags:

·        Bits, Bytes and Words used for temporary data storage

                  

PII:

·        Process Image Input

·        Stores status of all the inputs

PIQ:

·        Process Image Output

·        Stores status of all the outputs

System Data Image:

·                                Stores information related to the system like CPU type, CPU make and system bits, bytes, etc.

ROM:

·   Read Only Memory stores Operating System for PLCs. Operating System is burned into ROM by the PLC manufacturer. It controls the functions such as the system software that the user uses to program the PLC.

·         ROM is a non-volatile memory that is when electricity is switched off, the data in the memory is retained.

Memory Sub module/EEPROM:

·    Electrically Erasable Programmable Read Only Memory. This is a non-volatile memory.

·        EEPROM is used to store the programs or ladder logics, which can be copied to the PLC.

ALU:

·             Arithmetic & Logical Unit.

·    It performs all the calculations and logic functions of the PLC. It loads information from PII, processes it in the accumulator and then transfers the results to the PIQ.

Serial Port:

·    This port is used for communication with the external devices. If the external device understands some other protocol then we have to use some converter so that both PLC and the external device are able to understand each other’s language.

PLC programming devices:

·        There are many devices used to program the PLCs. These devices do not need to be attached to the PLC once the ladder is written. The devices are just used to write the user program for the PLC.

·        They may also be used to troubleshoot the PLC.

About Ladder Logic

Ladder logic or the Ladder programming language is a method of drawing electrical logic schematics. It is now a graphical language very popular for programming programmable logic controllers. It was originally invented to describe logic made from relays.

A program in ladder logic, also called a ladder diagram, is similar to a schematic for a set of relay circuits. Ladder logic is useful because a wide variety of engineers and technicians can understand and use it without much training.

Ladder logic is widely used to program industrial Programmable logic controllers, where a series of complex logic checks are required before something is turned on. Ladder logic is useful for simple but critical control systems, or for reworking old hardwired relay circuits.

Most providers of programmable logic controllers also provide associated ladder logic programming systems. Typically, the ladder logic languages from two different providers will not be compatible; ladder logic is better thought of as a set of closely related programming languages rather than one language.

 Example of a simple ladder logic program

The language itself can be seen as a set of connection between logical checkers (relay contacts) and actuators (coils).

Ladder logic has "contacts" that "make" or "break" "circuits" to control "coils."

There are two types of contacts:

--[ ]-- a relay normally open (N/O) contact. This contact turns on if the relay's coil is on.

--[\]-- a relay normally closed (N/C) contact. This contact turns off if teh relay's coil is on.

These are switched by a circuit (or more archaically the "coil" of a relay). They in turn switch a circuit. In a ladder programmign language, the circuits are represented by a name or number.

The contacts represent electrical switches. They form logic that can be configured to produce any logical function such as AND, OR, XOR, NAND, NOR, INV

Here is an example of what one rung in a ladder logic program might look like. In real life, there would typically be hundreds or thousands of rungs.

 ---[ ]-----[ ]---

     X       Y

Represents X AND Y, because both X and Y must be on for the circuit to be on.

                                                       

----|---[ ]---|------

     |    X     |

     |            |

     |---[ ]---|

          Y         

Represents X OR Y, because the circuit will be on if either X or Y are on.

 ----|---[\]---|------

      |    X     |

      |            |

      |---[\]---|

           Y         

Represents X NAND Y, because the circuit will turn off only if both X and Y are turned on.

Ladder logic is then used to drive output coils, so that when the preceding logical functions have been evaluated - the output coil changes state.

--( )-- an N/O output coil

--(\)-- an N/C output coil

For Example

 ----[ ]---------|--[ ]--|------( )--

      X             |   Y   |       S

                      |         |

                      |--[ ]--|

                          Z

Realises the function S= X.(Y+Z)

Complex ladder logic is 'read' in the same way as a western book (left to right). The first point on the left, is the input signal (or high potential), as each of the lines (or rungs) are evaluated the output coil of a rung may feed into the next stage of the ladder as an input. In a complex system there will be many "rungs" on a ladder, which are numbered in order of evaluation.

 1 ----| |-----------|-| |-----|----( )--

         X              |  Y      |      S

                          |           |

                          |---| |---|

                              Z

 2 ----| |----| |-------------------( )--

         S      X                         T

T=X.S

This represents a slightly more complex system for rung 2. After the first line has been evaluated, the output coil (S) is fed into rung 2, which is then evaluated and the output coil T could be fed into an output device (buzzer, light etc..) or into rung 3 on the ladder.

This system allows very complex logic designs to be broken down and evaluated fairly easily.

For more practical examples see below (my creations released to wikipedia):

 |                                                        |

 |                                                        |

 |--][------------][-------------------O---|

    keysw1     keysw2           door motor

This circut shows the two key switches that security guards might use to activate motor on a bank vault door. When the normally open contacts of both switches close, electricity is able to flow to the motor which opens the door. This is a logical AND.

 |                                                        |

 |                                     +-------+     |

 |----------------------------+         +----|

 |                                     +-------+     | 

 |                             Remote receiver |

 |-----][------------------------------O---|

 |   remote unlock|       lock solenoid |

 |-----][------------+

     interior switch

This circuit shows the two things that can trigger the power door locks in my imaginary car. The remote receiver is always powered. The lock solenoid gets power when either set of contacts is closed. This is a logical OR.

Since electrical engineers already knew how to read ladder logic, PLC makers made their systems programmable in ladder logic. This would allow electrical engineers to read, debug, troubleshoot and write computer programs for the PLCs which replaced their cabinets full of relays.

Additional Functionality

Additional functionality can be added to a ladder logic implementation by the PLC manufacturer as a special block. When the special block is powered, it executes code on predetermined arguments. These arguments may be displayed within the special block

 |                                                        |

 |                                     +-------+     |

 |-----][---------------------+  A    +----|

 | remote unlock             +-------+     | 

 |                            Remote counter  |

 |                                                       |

 |                                     +-------+    |

 |-----][---------------------+  B    +---|

 | interior unlock            +-------+    | 

 |                             Interior counter |

 |                                                       | 

 |                           +--------+             |

 |--------------------+ A + B +---------|

                            + into C +            

                            +-------- +            

                        Adder              

In this example, the system will count the number of times that the interior and remote unlock buttons are pressed. This information will be stored in memory locations A and B. Memory location C will hold the total number of times that the door has been unlocked electronically.

Origin of Ladder Diagram : -

•              The Ladder Diagram (LD) programming language originated from the graphical representation used to design an electrical control system

−      Control decisions were made using relays.

•              After a while Relays were replaced by logic circuits

−      Logic gates used to make control decisions.

•              Finally CPUs were added to take over the function of the logic circuits

−      I/O Devices wired to buffer transistors.

−      Control decisions accomplished through programming.

•              Relay Logic representation (or LD) was developed to make program creation and maintenance easier

−      Computer based graphical representation of wiring diagrams that was easy to understand.

−      Reduced training and support cost.

−       

−       

What is a Rung?

•              A rung of ladder diagram code can contain both input and output instructions. −      Input instructions perform a comparison or test and set the rung state based on the outcome. −      Normally left justified on the rung. ·             Output instructions examine the rung state and execute some operation or function. −            In some cases output instructions can set the rung state. −            Normally right justified on the rung.

 

                                              

Contacts

•              Normally Open Contact -| |-

–         Enables the rung to the right of the instruction if the rung to the left is enabled and underlining bit is set (1).

•              Normally Closed Contact -|/|-

–         Enables the rung to the right of the instruction if the rung to the left is enabled and underlining bit is reset (0).

•              Positive transition contact -|P|-

–         Enables the right side of the rung for one scan when the rung on left side of the instruction is true.

•              Negative transition contact -|N|-

Enables the right side of the rung for one scan when the rung on left side of the instruction is false.

Non Retentive Coils

  Non-retentive values or instructions are reset to some default state (usually 0) after a power cycle

·        Coil -(   )-

−            Sets a bit when the rung is true (1) and reset the bit when the rung is false (0).

·        Negative coil -( / )-

−            Sets a bit when the rung is false (0) and resets the bit when the rung is True (1).

−            Not commonly supported because of potential for confusion.

·        Set (Latch) coil -(S)-

−            Sets a bit (1) when the rung is true and does nothing when the rung is false.

·        Reset (Unlatch) Coil -(R)-

Resets a bit (0) when the rung is true and does nothing when the rung is false.

Retentive Coils

The referenced bit is unchanged when processor power is cycled or retentive values or instructions maintain their last state during a power cycle.

·        Retentive coil -(M)-

−                  Sets a bit when the rung is true (1) and resets the bit when the rung is false (0)

·        Set Retentive (Latch) coil -(SM)-

−                  Sets a bit (1) when the rung is true and does nothing when the rung is false

·        Reset Retentive (Unlatch) Coil -(RM)-

−                  Resets a bit (0) when the rung is true and does nothing when the rung is false

Transition Sensing Coils

•              Positive transition-sensing coil -(P)-

−                  Sets the bit (1) when rung to the left of the instruction transitions from off (0) to on (1)

−                  The bit is left in this state.

•              Negative transition-sensing coil -(N)-

−                  Resets the bit (0) when rung to the left of the instruction   transitions from on (1) to off (0)

−                  The bit is left in this state.

Industrial Sensors

Objectives: -

•Need for sensors

•Examine types and uses of different industrial sensors

•Digital and Analog sensors

•Wiring of sensors

Need: -

          In the past operators were the brains of the equipment. He was the source of all information about the operation of a process. He could see, hear and feel the problems of operation. Industry is now using PLCs and computers to control their operations, as they are faster and accurate than the operator for these tasks. Industrial sensors are used to give the industrial controllers these capabilities.

          Simple sensors can be used by the PLC to check if the parts are present or absent, to size the parts, even to check if the product is empty or full and also for the safety of equipment and operator.

          Infect, sensors perform simple tasks more efficiently and accurately than people do. Sensors are much faster and make far fewer mistakes.

Contact type:-

          The device must contact a part to sense a part.

         

Example: Limit Switch

Non-Contact type:-

          Sensors that can detect the product without touching the product physically.

          Advantages:

          1. Their operation is generally electronic and not mechanical, so they are more reliable and less likely to fail.

          2. Much faster and can perform at high production rates.

          3. You cannot slow down or interfere with process

We’ll go in details with Non-Contact type sensors as they are in much use.

Digital Sensors:-

          A digital sensor has two states - ON or OFF. Most applications involve presence/absence and counting. A digital sensor meets this need perfectly and inexpensively.

Digital output sensors are either on or off. They generally have transistor outputs. If the sensor senses an object the output will turn on. The transistor turns on and allows current   to flow. The output from sensor is usually connected to a PLC input module.

          Sensors are available with either normally closed or normally

Open output contacts. NO contact sensors are off until they sense an object. NC contact sensors are on until they sense an object. When they sense an object the output turns off.

Example:    Photo sensors (Dark-on, Light-on)

Analog Sensors:-

          They are more complex but can provide much more         information about a process. They are also called linear output sensors.

An analog sensor senses the parameter and sends a current        to the PLC. The output from the analog sensor can be any value in the range from low to high.

A 4-20mA current loop system can be used for applications where the sensor needs to be mounted a long distance from the control device. A 4-20mA loop is good to about 800m.

Example:    Temperature, Pressure sensor

# Digital sensors are more widely used because of their simplicity and ease of use.

Optical Sensors:-

          These sensors use light to sense objects. In the past they somewhat unreliable because they used common light and were affected by ambient lighting. Today they are very reliable because of the way they now operate.

          All optical sensors function almost in the same manner.  There is a light source (emitter) and a photo detector to sense the presence or absence of light. Light-emitting diodes (LEDs) are typically used for the light source. An LED is a semiconductor diode that emits light. LEDs are a PN-type semiconductor. Forward biased electrons from N-type material enter the P-type material where they combine with excess holes. When an electron and a hole combine, energy is released. These energy packets are called photons. Photons then escape as light energy. The type of material used for the semi-conductor determines the wavelength of the emitted light.

Why LEDS are chosen:-

• Small, sturdy, very efficient and can be turned on and off at the

• Extremely high speeds.

• Operate in a narrow wavelength and are very reliable.

• Not sensitive to temperature, shock or vibration

• Have almost endless life

Operation:-

          The LEDs in sensors are used in a pulse mode. The emitter is pulsed (turned off and on repeatedly). The “on-time” is extremely small as compared to the “off-time”.

LEDs are pulsed for two reasons:

          1. So that the sensor is unaffected by ambient light.

          2. To increase the life of the LED. This can also be called modulation.

Pulsed light is sensed by the photo detector. Photo emitter and photo receiver are both “tuned” to the frequency of the modulation. The photo detector sorts out all ambient light and looks for the pulsed light. Light sources chosen are typically invisible to human eye. Wavelengths are chosen so that sensors are not affected by other lightening in the plant. The use of different wavelengths allows some sensors, called color mark sensors, to differentiate between colors. The pulse method and the wavelength chosen make optical sensors very reliable. Optical sensors are available in either light or dark sensing. This is also called light on or dark-on. Infect many sensors can be switched between light and dark