COMPLETE OVERVIEW OF PROGRAMMABLE LOGIC CONTROLLER PLC
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OVERVIEW OF PROGRAMMABLE LOGIC CONTROLLER PLC
A digital Electronic Device Uses a programmable memory to store instructions & to implement functions as
- In order to control machines & Process
- INPUT devices as switches & o/p devices as motors are connected to PLC.
- Controller monitors the inputs & o/ps according to the program stored in the PLC.
Uses Programmable Logic Controller
- Cost effective for controlling complex systems.
- Ladder logic programming can be applicable for other control system.
- Computational abilities allow more sophisticated control.
- Trouble shooting aids make programming easier and reduce downtime.
- Programming is easier and troubleshooting reduce downtime.
- Reliable components make these likely to operate for years before failure.
- Easy to modify a control system without rewiring the connections to input & o/p devices. Much faster than relay-operated systems
- Widely used for the implementation of logic control functions.
- Easy to use
- Reliability of PLC is greater
- Maintenance is easier
- PLCs take less floor space than do relay control panels.
- Can perform a greater variety of control functions than relay control
- Virtual real-time control becomes feasible
- Used for implementing automatic control of manufacturing systems
- Logical information is rapidly and repeatedly processed and immediately responded to with appropriate actions
- A series of logical decisions have to be made and a variety of actions taken on the basis of input that is obtained from appropriate sensors.
Features of PLC
- PLCs are designed to withstand with vibration, humidity, noise, temperature and any environmental changes.
- The main feature is interfacing for input and output is inside the controller.
- They are easily programmed.
Programmable Logic Control (PLC)
Definition – Dedicated computer for rapid processing of simple logic instructions in a defined time.
Purpose – Send and read signals that can be used to control and monitor devices.
Process – One of scanning all the devices (sensors, timers, etc.) in a cyclical time period.
PLC Control Approaches
Logic Control Method – This closed-loop method uses conditions and events to signal completion of a given step, and then triggers the execution of some other event. This is an asynchronous method of process control, because it does not always proceed in a constant time period.
Sequencing Method – This open-loop method uses timers to trigger the completion of one step and the beginning of the next. This is a synchronous control method.
Discrete Process Control
- Discrete process control systems deals with parameters and variables that change at discrete moments in time.
- Parameters and variables are also discrete in binary form.
- They can have either of two possible values, 1 or 0, values mean on/off, True/False, Object present or not present, high voltage value or low voltage value.
- The binary variables in DPC are associated with input signals to the controller & o/p signals from the controller.
- Input signals are generated by binary sensors, such as limits switches, Timer, Relay or photo-detector sensors that are interfaced to the process.
- O/p signals are generated by the controller to operate the process in response to the input signals & as a function of time.
- These o/p signals turn on or off switches, motors, valves, solenoid and other binary actuator related to the process.
- Discrete Process Control
Three conditions and Solution
- The first condition can be indicated by means of a simple limit switch that sense the presence of the part at the conveyor, stop & transmits an On signal to the robot controller.
- The 2nd condition can be indicated by the forge press, which sends an On signal after it has completed the previous cycle.
- The 3rd condition might be determined by the photo-detector located so as to sense the presence or absence of the part in the forging die.
Uses of Discrete Process Control
- Widely used in discrete manufacturing as well as process industries.
- In discrete manufacturing it is used to control the operation of:
- Automated Storage Systems
- Automated Transfer Lines
- Automated Assembly Systems
In process industries, discrete control is associated more with :
- Batch processing than with continuous processes
- Possible flow from one container to another during the cycle.
- Finally packaging
Logic Control Elements
AND, OR , & NOT GATE
A switch that works on principle of an electromagnet is called a “relay” and give on off contact.
For a process control, it is desired to have the process start (by turning on a motor) five seconds after a part touched a limit switch. When the second limit approached, the process is finished automatically. We can stop or shutdown the system anytime by pressing emergency switch.
Example of a relay in a simple control
A Simple Relay Controller
Counters and Timers
- Counters can be used in manufacturing to measure quantities such as production stock, inventory, and packaging
- Timers are used specifically to count clock pulses
- Timers and counters greatly expand the versatility of a PLC and allow the handling of some variable-type questions Virtually all PLCs on the market today include countering and timing capability
- Processor Microprocessor based, may allow arithmetic operations, logic operators, computer interface, local area network, functions, etc.
- Memory Measured in words.
EAPROM (Electronically Alterable Programmable Read Only Memory.
DC voltage I/P and O/P,
Low level analog I/P (INPUT),
High level analog (I/P)input and output(O/P),
Special purpose modules, e.g., high speed timers,
network communication interface
PLC SELECTION CRITERIA
- What I/O is required, i. e. the number of I/O, capability of expansion for future needs.
- What types of I/O are required, i. e. signal conditioning, on-board power supply for inputs, outputs, etc.
- What size of memory is required? This is linked to the number of I/O & a complexity of program used.
- What speed & power is required of CPU.
- Control actions are taken by making decisions depending on the values associated with various inputs or variables and the control logic in the program
- If a decision can be made by answering yes or no to a given question, it is referred to as a decision by attributes
- Is a part loaded in the machine?
- Is the tool path unobstructed?
- Is the AGV carrying a part?
- Decisions that cannot be made by answering yes or no are referred to as decisions by variables
- How long is the bar stock?
- What is the feed rate?
Ladder Logic Diagram
- Well established in industry in relation to the traditional electromechanical logic devices.
- Makes use of representations similar to electrical circuits in which a series connection represents
- a logical “and” and a parallel connection represents a logical “or”.
- Made up of inputs, outputs connected according to appropriate logic.
- Each rung in the ladder represents a set of logical relationships between the inputs that leads to a
- particular output.
- The output from one rung of the ladder could be used as an input in another rung of the same ladder.
- Except when special provisions are made, it is considered that all rungs in a given ladder logic diagram
- are executed simultaneously, so the order of the rungs on the ladder in general does not matter.
PLC WIRING DIAGRAM
Continuous and Discrete Control (comparison)
Ladder logic example
IF sensor 2 is not tripped THEN energize solenoid 1 (Ingr A)
Q. PROGRAMMING EXAMPLE
Light Switch Examples Test