Conveyor Control Using Programmable Logic Controller

International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014 25 ISSN 2278-7763

Copyright © 2014 SciResPub. IJOART

Conveyor Control Using Programmable Logic Controller

*Chitra.S,* Lecturer, Department of Electronics and

Communication Engg, Dr.TTIT, K.G.F,

Email:[email protected]

**Vijaya Raghavan. **Assistant professor, Department of Mining

Engg, Dr.TTIT, K.G.F, Email:raghavan_pp@rediffmail.

com

Abstract

A programmable Logic Controller (PLC) is a specialized computer used for the control and operation of manufacturing process and machinery. It uses a programmable memory to store instructions and execute functions including on/off control, timing, counting, sequencing, arithmetic, and data handling. Programmable Logic Controllers (PLC) is used in almost every aspect of industry to expand and enhance production. Where older automated systems would use hundreds or thousands of electromechanical relays, a single PLC can be programmed as an efficient replacement. The functionality of the PLCs has evolved over the years to include capabilities beyond typical relay control. Sophisticated motion control, process control, distributive control systems, and complex networking have now been added to the PLC’s Functions. Therefore, PLCs provide many advantages over conventional relay type of control, including increased reliability, more flexibility, lower cost, communication capability, faster response time and convenience to troubleshoot. The paper is based on systematic conveyor controller programming by programmable logic controller using omron software which is a world smallest plc, offers variety of expansion options and has user-friendly software. 1. Introduction The PLC is a microcontroller based device with input/output circuitry that monitors the status of field connected sensor (inputs) and controls the attached (output) actuators (motor -starters, Solenoids, Speed drives, Valves etc.) according to a user created logic program stored in the memory. The main difference from other computers is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such

as temperature and pressure), and the positions of complex positioning systems. PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC. PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed. Many of the earliest PLCs expressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control system. 1.1 PLC system

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Figure 1.1: Block diagram of PLC system. There are five basic components in a PLC system:

• The PLC processor or controller • I/O (Input /Output) modules • Chassis or backplane • Power supply • Programming software that runs in a PC • Network Interface

1.1.1. The PLC processor. It stores the control program and data in its memory. Reads the status of connected input devices. Executes the control program. Commands connected outputs to change state based on program execution For example: Turn a light on, start a fan, adjust a speed, or temperature and Comes in various physical forms. 1.1.2 I/O Modules. They Physically connect to field devices. Input modules convert electrical signals coming in from input field devices such as pushbuttons, to electrical signals that the PLC can understand. Output modules take information coming from the PLC and convert it to electrical signals the output field devices can understand, such as a motor starter, or a hydraulic solenoid valve. I/O comes in various forms. 1.1.3 Chassis/Backplane. All PLCs need some method of communicating between the controller, I/O and communications modules. Here are three ways used to accomplish this communications between the various components that make up the PLC system.

• Modules are installed in the same chassis as the PLC and communicate over the chassis backplane.

• Modules are designed to “plug” into each other. The interconnecting plugs form a backplane. There is no chassis.

• Modules are built into the PLC. The modules come together in one physical block. The backplane in this case is transparent to the user.

Fig 1.2: Backplane in a chassis based system 1.1.4. Power Supply. A power supply is needed to provide power to the PLC and any other modules. Power supplies come in various forms:

• Power supply modules that fit into one of the slots in a chassis.

• External power supplies that mount to the outside of a chassis.

• Stand alone power supplies that connect to the PLC or I/O through a power cable.

• Embedded power supplies that come as part of the PLC block.

1.1.5 Programming Software. Software that runs on a PC is required to configure and program PLCs. Different products may require different programming software. Software allows programs to be written in several different languages.

Fig 1.3: PLC program 1.1.6. Network Interface. Most PLCs have the ability to communicate with other devices. These devices include computers running programming

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software, or collecting data about the manufacturing process, a terminal that lets an operator enter commands into the PLC, or I/O that is located in a remote location from the PLC. The PLC will communicate to the other devices through a network interface.

Fig 1.4: Network connecting other devices 2. PLC Architecture

Fig 2.1: PLC architecture The PLC architecture is divided into three parts:

a. CPU : It is the brain of PLC system .It consists of the microcontroller, Memory IC and necessary circuit to store and retrieve information from the memory. The Job of CPU is to monitor status or state of input device, scan and solve the logic of a user program and control ON or OFF state of output device.

b. Memory: The type of RAM (Random Access Memory) normally used is CMOS (Complementary Metal Oxide Semiconductor) to store the program.

c. Input/output: Input is the one through which signal is send and result is observed at the Output.

PLC architecture design can be an open architecture design or a closed architecture design. An open architecture design allows the system to be connected easily to any device and also to programs developed by other manufacturers. A closed architecture which is also known as proprietary system is one whose design makes it more difficult to connect with other devices or programs developed by other manufacturers. When working on a proprietary PLC system, all hardware and software that is used should be compatible with the PLC. 2.1 Programming in PLCs Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lack of memory capacity.PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The very oldest PLCs used non-volatile magnetic core memory but now the program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Early PLCs were designed to replace relay logic systems. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams. The International standard IEC 61131-3 defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list) and SFC (Sequential function chart). These techniques emphasize logical organization of operations. While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible. 2.2. User Interface PLCs may need to interact with people for the purpose of configuration, alarm reporting or

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everyday control. A Human-Machine Interface (HMI) is employed for this purpose. HMI's are also referred to as MMI's (Man Machine Interface) and GUI (Graphical User Interface).A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser. 2.3. PLC Software The PLC software is manufacturer dependent and even when the manufacturer is the same, it may vary for the different models of the same brand. For, instance for a manufacturer like Allen Bradley the software may vary for its PICO Controller models and other models. For example, the software used for these controllers is PICO Soft whereas for its higher models it is RSLogix. Moreover, the HMI Interface may also vary for the different controllers. 2.4. Program Scan When a PLC executes a program, it must know in real time when external devices controlling a process are changing. During each operating cycle, the processor reads all the inputs, takes these values, and energizes or de-energizes the outputs according to the user program. This process is known as a program scan cycle. Figure 2.2 illustrates a single PLC operating cycle consisting of the input scan, program scan, output scan.

2.5. Ladder programs Ladder programs process inputs at the beginning of a scan and outputs at the end of a scan, as illustrated in Figure 2.3 . For each rung executed, the PLC processor will perform the following steps: The first step is to update the input image table by sensing the voltage of the input terminals. Based on the absence or presence of a voltage, a 0 or a 1 is stored into the memory bit location designated for a particular input terminal. The second step is to solve the ladder logic in order to determine logical continuity. The processor scans the ladder program and evaluates the logical

continuity of each rung by referring to the input image table to see if the input conditions are met. If the conditions controlling an output are met, the processor immediately writes a 1 in its memory location, indicating that the output will be turned ON; conversely, if the conditions are not met a 0 indicating that the device will be turned OFF is written into its memory location. And the final step of the scan process is to update the actual states of the output devices by transferring the output table results to the output module, thereby switching the connected output devices ON (1) or OFF (0).

Fig 2.3 Scan process applied to a multiple rung program.

Fig 2.4: Typical micro PLC.

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2.6. Latching Relays Electromagnetic latching relays are designed to hold the relay closed after power has been removed from the coil. Latching relays are used where it is necessary for contacts to stay open and/or closed even though the coil is energized only momentarily. 2.7 Ladder Logic Before a PLC can perform any control task, it must be programmed to do so. The most popular language used to program a PLC is ladder logic. It is a graphical, problem oriented programming language which replicates electronic switching blueprints or circuits. In a conveyor system, we have several “requirements” to accomplish; for example, timing and counting parts on the conveyor. Each of these requirements must be programmed into the PLC so that it knows how to respond to different events. The programmer develops the program, and connects their personal computer to the PLC through a network or cable and then downloads the program to the PLC. Ladder logic rungs are basically IF-THEN statements. Each individual rung is executed from the left to the right.

Fig 2.5: Example of Ladder Logic diagram

3. Block diagram of conveyor Controller

Fig 3.1: Bottle filling control process

The Figure 3.1 illustrates a bottle-filling motion control process. This application requires two axes of motion: the motor operating the bottle filler mechanism and the motor controlling the conveyor speed. The role of each control component can be summarized as follows: 3.1. Programmable Logic Controller • The controller stores and executes the user program that controls the process. • This program includes motion instructions that control axis movements. • When the controller encounters a motion instruction it calculates the motion commands for the axis. • A motion command represents the desired position, velocity, or torque of the servo motor at the particular time the calculations take place. 3.2. Motion Module The motion module receives motion commands from the controller and transforms them into a compatible form the servo drive can understand. In addition it updates the controller with motor and drive information used to monitor drive and motor performance. 3.3. Servo Drive The servo drive receives the signal provided by the motion module and translates this signal into motor drive commands. These commands can include motor position, velocity, and/or torque. The servo drive provides power to the servo motors in response to the motion commands. 3.4. Servo Motor • The servo motors represent the axis being controlled. • The servo motors receive electrical power from their servo drive which determines the motor shaft velocity and position. • The filler motor must accelerate the filler mechanism in the direction the bottles are moving, match their speed, and track the bottles. • After the bottles have been filled, the filler motor has to stop and reverse direction to return the filler mechanism to the starting position to begin the process again.

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4. Programming PLC for Conveyor

Fig 4.1: Example of conveyor

The simulation is done using omron which ensures that the program can be successfully implemented in PLC. Today delta PLC is available in market which helps in easier implementation of the above required purpose. It uses simple omron motion language for programming. A PLC program consists of rules that make logic relation between inputs and outputs of the controller. Basically it uses logic operands: AND, OR, negation. The structure of the rules is IF…THEN…ELSE. The PLC reads all field input devices via the input interfaces. Executes the user program stored in application memory, then, based on whatever control scheme has been programmed by the user. Turn the field output devices on or off, or perform whatever control is necessary for the process application. The PLC resolves the program rule by rule (sequential execution). The PLC operates in a synchronous way i.e. inputs does not change under a scan cycle. And the figure 4.1 shows that when a box is placed on the conveyor in front of Photoeye 1, Light 1, and Motor 1 will turn on, causing the box to move down the conveyor to the left. When the box passes in front of Photoeye 2, Motor 1 and Light 1 will turn off, stopping the conveyor.

Fig 4.2: Relay Ladder Logic Example

The figure 4.2 shows the program for the conveyor: The first line of code turns on the motor and the light when a box is detected by photoeye1. Likewise, the motor and light are turned off when photoeye2 detects the box in the second line of code.

Fig 4.3: Relay ladder logic

The third line begins a timer when the box passes by photoeye1, and if the box does not pass by photoeye2 in 30 seconds (the timer counts in milliseconds), the motor and light are shut off by line 4. This is the indication of a jam condition. 4.1. The control system A complete control system is made up of a combination of PLCs, networks, I/O, terminals and software. All the components work together to form a complete control system. The control system is the system that is responsible for the control of the process. This is the system that includes the PLC, all of the I/O and any Human Machine Interfaces (HMI).

Fig 4.4: The complete control system

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4.2. Data Acquisition System

Fig 4.5 : Data acquisition system The Data Acquisition system is generally responsible for collecting data about the control system, and storing it on master computers or servers, or displaying it on terminals. The data is often used later for reporting or charting purposes. They are made up of devices and networks which are responsible for acquiring data about the process but are not responsible for direct control of the process. The network used for data acquisition is often Ethernet. While data acquisition devices can exist directly on the control network, a gateway is often used to separate network traffic between the data acquisition system and the control system.

Fig 4.6: Omron PLC with modbus-RTU network

5. Advantages of PLC

• Cost effective for controlling complex system.

• Flexible and can be reapplied to control other system quickly and easily.

• Computational abilities allow more sophisticated control.

• Trouble shooting aids make programming easier and reduce downtime.

• Reliable components make these likely to operate for longer life.

6. PLC Applications

• Automotive Industry • Beverage Industry • Marine Industry • Packaging Industry • Intelligent Motor Control • Motor Control Applications

7. Conclusion PLCs are well-adapted to a range of automation tasks. These are typically useful in industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands of places. 8. References [1] Guo, L., Pecen, R., “Design Projects in a Programmable Logic Controller (PLC) Course in Electrical Engineering Technology”, ASEE Annual Conference & Exposition, 2008 [2] Johnson, C. D., Process Control Instrumentation Technology, Prentice Hall, 2006 [3] Petruzella, F. D., Programmable Logic Controllers, McGraw Hill, 2005 [4] Yang, G., Rasis, Y., “Teaching PLC in Automation – A Case Study”, ASEE Annual Conference & Exposition, 2003 [5] URL: http://read-out.net/signpost/oid.html; Education and Research in the area of Measurement, Control and Automation. [6] URL: http://www.plcs.net; Your Personal PLC Tutor Site - The BEST Place to Learn PLC Programming. [7] URL: http://www.me.ua.edu/ME360/plc; ME 360 - Programmable Logic Controller Module.

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Conveyor Control Using Programmable Logic Controller

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