PROGRAMMABLE LOGIC CONTROLS (PLC)

Programmable logic controls (PLC) 

introduction to PLC         

The programmable logic controllers (PLCs) are microprocessor-based devices that come with a user programmable memory, which is used for storage of instructions in order to perform the functions of the relays, timers, counters and other components. It is specifically designed to replace the relays and other hard wired logic in machine operation processes in the industry.

PLCs are used in most factory processes such as motor controlling, pneumatic controlling, security and safety alarming systems and various other applications. They are most common in industrial automation due to great advantage that they processing.

PLC

PLC

Advantages of PLCs

v   Low power consumption and space requirements as PLCs are based on solid state electronics.

 vMaintenance free due to no moving parts.

v  Greater flexibility and reliability.

v  Low cost and enables expansion of basic system as requirement grow.

Can be programmed with out prior knowledge of any computer language

v v Flash ROM memory inside

v v Real time clock: With clock inside, Li battery power drop memory

v v Multi-COM ports can connect with inverters, instruments, printers etc

vv  Rich instructions, convenient to program

Statement Program

Statement program is the format which use “LD”, “AND”, “OUT” etc. These SFC instructions to input this format is the basic input form to compile the SFC program

Example :        Step                 Instruction       ID

0                      LD                   X000

1                      OR                   Y005

2                      ANI                  X002

3                     OUT                 Y005

Ladder Program

Use sequential control signal and soft unit’s ID to draw the sequential circuit’s graph on the screen, which is called ladder program. As this method uses trigger point’s symbols and coil symbols to denote the sequential control circuit, so it is easy to understand the program’s contents. At the same time it’s also available to monitor the PLC’s action via the status displayed in the circuit

Expansion’s constitution

Expansion

XC series PLC can be used independently or used along with the expansions. The following is the chart of a basic unit with seven expansions.

PLC Expansion

Constitution Rules

v  Digital Input/ Output quantity is Octal.

v  Analogue Input/ Output quantity is Decimal.

v  PLC main units can connect with 7 expansions and a BD module. The input/output type is not limited, both switch and analog quantity are available.

  

    how to PROGRAM A PLC

Ladder diagrams

        Ladder diagrams are simple graphical representation of similar electrical wiring diagrams. Normally open and closed contacts, relays, timers, counters are commonly used functions in ladder diagrams.  Some of the diagrams are as follows.

Ladder diagrams 

   Contacts

        Contacts are used for representing logic input elements such as switches, push buttons, sensors and limit switches etc. Normally open and closed contacts are available

   Out put relays

Out put relays are used for representing logic out puts. Logic out puts may be motor starters, solenoids, relays or lamps 

   Timers 

Timers are basically available as on-delay (TON) and off-delay (TOF) timers. TON counts time when the enabling input is on and when the timer value is equal to the preset value timer bit is on. TOF is used to delay turning an output off for a fixed period after input turns off.

   Counters

There also up and down counters are available in the PLC. Up counter is an incrementing counter instruction and conversely down counters are decrementing counters. Several parameters have to be set such as counter address, preset count value. 

    Input Devices for PLC

v  Selector switches

v  Push buttons

v  Limit switches

v  Circuit breakers

v  Level switches

v  Relay contacts

v  Motor starter contacts

v  Sensors (Metal detect, Proximity)

   Output Devices for PLC

v  Alarms

v  Control Relays

v  Solenoids

v  Lights

v  Horns

v  Valves

v  Motor Starters