Programmable logic controller

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The PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices. This is one of the most commonly used industrial automation technique in the world. This is very useful for factory automation, process control and manufacturing systems. The main technique used in this programming is called ladder logic which allows sequences of logical actions to be set up, inter-linked and timed.

It can also be defined as a microcomputer which is embedded in or attached to perform operations like switching, timing and machine or process control tasks.

In the past humans were used to control systems. Then electricity came into use with the help of electrical relays. These relays allow power to be switched on and off without a mechanical switch. But the development of the low cost computer has brought the most recent revolution , the programmable logic controller(PLC). The advent of the plc's began in the 1970's and is the most common choice for manufacturing controls.

Almost any production line, machine function, or process can be greatly enhanced using this type of control system. There are many advantages using the plc's. However, one of the biggest benefit in using a PLC is the ability to change and replicate the operation or process while collecting and communicating vital information.

Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to best suit your application.

History of PLCs

The first Programmable Logic Controllers were designed and developed by Modicon as a relay re-placer for GM and Landis.

  • These PLC'S(controllers) eliminated the need for rewiring and adding additional hardware for each new configuration of logic.
  • The new system introduced by the PLC drastically increased the functionality of the controls while reducing the cabinet space that housed the logic.
  • The first PLC, model 084, was invented by Dick Morley in 1969
  • The first commercial successful PLC, the 184, was introduced in 1973 and was designed by Michael Greenberg.


Flexibility: many machines can be run using one single programmable logic controller.

Space-efficient: there is a lot of space to generate more and more contacts, coils, timers, sequencers, counters and so on. There can be many number of timers in a single PLC.

Correcting errors: the correction of errors in a PLC is extremely short and cost effective. In the past there were wired relay types and any changes required time for rewiring panels. But in a PLC any change in circuit design is as simple as retyping the logic.

Low cost: these prices are cost efficient and affordable varying from few hundreds to few thousands.

What Is Inside A PLC?

The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:

  • Execute the Control Instructions contained in the User's Programs. This program is stored in "nonvolatile" memory, meaning that the program will not be lost if power is removed
  • Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.
  • Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.

Programmable Logic Controllers or PLC are the hub of many manufacturing processes. . These microprocessor based units are used in processes as simple as boxing machines or bagging equipment to controlling and tracking sophisticated manufacturing processes. They are in virtually all new manufacturing, processing and packing equipment in one form or another.

The microprocessor or processor module is the brain of a PLC system. It consists of the microprocessor, memory integrated circuits, and circuits necessary to store and retrieve information from memory. It also includes communications ports to other peripherals, other PLC's or programming terminals. Today's processors vary widely in their capabilities to control real world devices. Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more. One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process. The number of inputs and outputs PLCs can control are limited by the overall capacity of the PLC system

Code generation in PLC:

Importing I/O list: this tool allows us to work on I/O list and allows to do all changes and then export it to our PLC in few seconds.

Generation of ladder logic automatically: this software allows us to generate hundreds of motors and valves in few seconds in manual mode. It also creates the main structure of the program.

Equipment logic generation: this software allows us to generate motors and valves libraries which can then be imported to our PLC.

Operation of plc:

There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.

Four Steps In The PLC Operations
1.) Input Scan

* Detects the state of all input devices that are connected to the PLC

2.) Program Scan

* Executes the user created program logic

3.) Output Scan

* Energizes or de-energize all output devices that are connected to the PLC.

4.) Housekeeping

* This step includes communications with programming terminals,

internal diagnostics, etc...



A mechanism that causes a device to be turned on or off, adjusted or moved. The motor mechanism that moves the conveyor belt is called an actuator. In this Allen Bradley model we have five actuators. The first actuator is the upper conveyor belt motor, second actuator is the lower conveyor belt motor, actuator three is used to hit the ring in to ring chute, actuator four releases rings into assembly area, actuator five rejects the unassembled component.


It can be defined as a device such as a photoelectric cell that receives and responds to a signal or stimulus or a device that measures or detects a real-world condition, such as motion, heat or light and converts the condition into an analog or digital representation. An optical sensor detects the intensity or brightness of light, or the intensity of red, green and blue for colour systems there are eight types of sensors in this process. Sensor one detects the peg near to and in front of the solenoid. Sensor two also detects about the component in front of the solenoid. Sensor three detects the component at the very bottom of the ring chute beyond the rotary solenoid. Sensor four is as a black push button used on the system. Sensor five has a red push button used to shut down. Sensor six used to detect the presence of passing complete assemblies. Sensors seven is used as a reflective IR sensor. Sensor eight is used detect the presence and reject the incomplete assemblies. Sensor nine is use to sense the complete assembly.


A Programmable Logic Controller, PLC, or Programmable Controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result


Micrologix 1000 brings high speed, powerful instructions and flexible communications to applications that demand compact, cost-effective solutions.The Micrologix 1000 programmable controller is available in 32-point digital I/O versions. Analog versions are also available with 20 digital I/O points, with 4 analog inputs (two voltages and two current) and 1 analog output (configurable for either voltage or current).

Sensor number

Sensor type, location and function.

PLC Input


Download-looking reflective IR sensor at the upper sort area.

Detects the presence of a peg near to and in front of the solenoid at the top of the ring chute.

I: 0/4


Sideways-looking reflective IR sensor at the upper sort area. Detects a component in front of the solenoid at the top of the ring chute.

I: 0/1


Reflective IR sensor at the assembly area.

Detects the presence of component at the very bottom of the ring chute beyond the rotary solenoid.

I: 0/0


Black pushbutton.

Used to commence assembling say.

I: 0/18


Red pushbutton.

Used to terminate assembling, say.

I: 0/19


Capacitive sensor, near the lower sort area.

Detects the presence of passing complete

assemblies near the reject solenoid at the motor end of the lower conveyor.

I: 0/6


Reflective IR sensor at capacitive sensor near lower sort area

I: 0/3


Reflective IR sensor at the lower sort area.

Detects the presence of components and assemblies in front of the reject sorlenoid at the motor end of the lower conveyor.

I: 0/2


Through-beam IR sensor, just after the assembly area.

Detects components on the lower conveyor leaving the assembly area.

I: 0/5

Actuator No.

Actuator type, location and desired function

PLC Output


Upper conveyor motor.

Drives the upper toothed chain conveyor

O: 0/3


Lower conveyor motor.

Drives the lower plain belt conveyor.

O: 0/4


Solenoid at upper sort area.

Knocks rings into the ring chute.



Rotary solenoid at the bottom of the ring chute before the assembly area.

Releases rings into the assembly area.

O: 0/1


Solenoid at the reject area.

Rejects unassembled components before the complet assembly collection tray.

O: 0/2

The model of plc used in this process is ALLEN-BRADLEY MICROLOGIX 1000 PLC and the programming software is RSLOGIX 500.

Relay: Switch operated by an electromagnet is a relay.

Counter: Digital counters in the form of relay contact when a preassigned count value is reached.

Timer: A timer consists of an internal clock, a count value register, and an accumulator is used for timing purpose.

Ladder diagram: A ladder diagram is a means of graphical representing the logic required in relay logic system.

Requirements of the plc diagram:

  • Ladder diagram
  • Structure text programming
  • Functional block programming
  • Instruction list
  • Sequential functional chart.

PLC Ladder Diagram instructions:

  • Relay,
  • Timer and counter,
  • Program control,
  • Arithmetic,
  • Data manipulation,
  • Data transfer, and
  • Others, such as sequencers.

Functional block programming:

Functional block is a well packaged element of software that can be re-used in different parts of an application or even in different projects. Functional blocks are the basic building blocks of a control system and can have algorithms written in any of the IEC languages.

Instructions list:

A low level language having a structure similar to an assembly language. It is simple and easy to understand and ideally for small hand held programming devices. Each line of the code is divided in to four they are label, operator, operand and comment.

Sequential function chart:

A graphics language used for depicting sequential behaviour. A SFC is indicated as a rectangular boxes connected by a vertical lines. Each step represents the state of system being controlled. Horizontal bar indicates the condition. Each step have a number of actions. Action qualifier causes the action to behave in certain ways and the indicator variable is optional.

Requirements & Constraints:


  • Conveyor belts should be should be kept moving during the operation.
  • The actuator 3 should knock only the ring when sensor 1 senses it.
  • The ring should not enter the peg chute or vice versa.
  • Excess rings should be collected in the surplus rings box.
  • The peg chute and the ring chute should be clean for the ring and peg to slide in that.
  • Unassembled components should be knocked down by the actuator 5.
  • Constraints
  • Actuator shouldn't knock the ring when there are more than 5 rings in the chute if there are more rings 5 rings then the movement will be blocked.
  • The area between the actuator 4 and the sensor 3 should be clear for the next peg to occupy the space.
  • The actuator 3 cannot knock if there are more than 5 rings in the ring chute.

Only if the peg moves past the ring the assembly will be over


  • Actuator shouldn't knock the ring when there are more than 5 rings in the chute if there are more rings 5 rings then the movement will be blocked.
  • The area between the actuator 4 and the sensor 3 should be clear for the next peg to occupy the space.
  • The actuator 3 cannot knock if there are more than 5 rings in the ring chute.
  • Only if the peg moves past the ring the assembly will be over.

Explanation of program:

In this program there are 23 steps called rungs.These rungs help us to achieve the objective of our assignment.


B3:1/1 is the starting flag which is black push button in on state and I:0/19 which is red push button in off state. If this is the condition the the flag B3:1/1 is latched and runs the next instruction otherwise flag is unlatched.


If B3:1/1 is latched then it enters to the next flag B3:1/2 and then actuator 3 and actuator 4 having addresses O:0/3 AND O:0/4 are enabled respectively such that the upper and lower conveyor motors will be running.


Here we check that I:0/19 is pressed or not. If it is pressed then B3:1/2 flag is latched if it is unlatched then it will be the flag B3:1/2.


Here the starting flag is B3:2/1 where sensors 2(I:0/1) and sensor 1(I:0/4) are used. Sensor 2 detects a component in front of the solenoid at the top of the ring chute and sensor 1 detects the peg. Here sensor 1 is off and sensor 2 is on.


Since sensor 2 is on, actuator 3(O:0/0) detects the rings coming from the conveyer belt and pushes them into ring chute. Here we start a timer on delay(T4:0) to 0.01 sec and preset to 50 which enables O:0/0.


If B3:2/2 is on and timer 0(T4:0) decrementing is on then we unlatch B3:2/2 and latch B3:2/3.


If B3:2/3 is on then we have to start timer 2(T4:1) to 0.01 sec and preset to 20.


When B3:2/2 is on then timer 2 is decremented and B3:2/4 is latched and B3:2/3 is unlatched.


When B3:2/4 is on then a counter(C5:0) is on and preset to 5.


When B3:2/4 is on, then counter (C5:0) is started and then we unlatch B3:2/4 and B3:2/1 is latched.


Here B3:3/1 is starting flag and we use sensor 3(I:0/0) which detects the component at the bottom of the ring chute. Here we also check the number of rings in accumulator and if B3:3/3 is on, we have to latch B3:3/2 and B3:3/1 is unlatched.


If B3:3/2 is on, then we have to set timer 3(T4:2) to 0.01 sec and preset to 50 and we have to enable O:0/1 which is actuator 4 used to release the rings into the assembly area.


When B3:3/2 is on then the timer 3(T4:2) is decremented and we have o latch B3:3/3 and B3:3/2 is unlatched.


When B3:3/3 is on, then counter(C5:0) is decremented.


If B3:3/3 is on and input 0(I:0/0) is not true then latch B3:3/4 AND UNLATCH B3:3/3.


When B3:3/4 is on we have to enable the sensor 5(I:0/19) and we have to latch B3:3/1 and B3:3/4 is unlatched.


When B3:4/1 is on then we have to enable sensor 9(I:0/5) which detects components on lower conveyer belt leaving assembly area and we have to latch B3:4/2 and B3:4/1 is unlatched.


When B3:4/2 is on we have to enable sensor 8(I:0/2) which detects the presence of components and assembles in front of the reject solenoid at motor end and we have to latch B3:4/3 an B3:4/2 is unlatched.


If B3:4/3 is on then sensor 8(I:0/2) is disabled an we have to latch B3:4/1 and unlatch B3:4/3.


When B3:4/1 is on, the sensor 8(I:0/2) is enabled and we have to latch B3:4/4 and unlatch B3:4/1.


When B3:4/4 is on then we have to enable actuator 5(O:0/0) which is a solenoid at the reject area. It rejects unassembled components before the complete assembly collection tray.


If B3:4/4 is on then we have to sensor 8(I:0/2) is ff and we have to latch B3:4/1 and B3:4/4 is unlatched.