ladder program

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INTRODUCTION TO PLC

Topics: Relays

Ladder Logic

PLC Programming

PLC Operation

Objectives: To be able to write simple ladder

logic programs

Understand the operation of a PLC


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Control engineering has evolved over time.

In the past, humans were the main methodfor controlling a system.

More recently electricity has been used for

control and early electrical control was basedon relays. These relays allow power to beswitched on and off without a mechanicalswitch. It is common to use relays to make

simple logical control decisions. The development of low cost computer has

brought the most recent revolution, theProgrammable Logic Controller (PLC). The

advent of the PLC began in the 1970s, and


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Scan Cycle of a PLC


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Major components of PLC

Power Supply - This can be built intothe PLC or be an external unit.Common voltage levels required by

the PLC (with and without the powersupply) are 24Vdc, 120Vac, 220Vac.

CPU (Central Processing Unit) - This is

a computer where ladder logic isstored and processed.

I/O (Input/Output) - A number of

input/output terminals must be


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Configurations:The configurationof the PLC refers to the packaging of thecomponents. Rack - A rack is often large (up to 18

by 30 by 10) and can hold multiplecards. When necessary, multiple

racks can be connected together.These tend to be the highest cost,but also the most flexible and easy to

maintain. Mini - These are smaller than full

sized PLC racks, but can have the

same IO capacity.


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O t t t t t ll PLC t


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Outputs to actuators allow a PLC tocause something to happen in aprocess. A short list of popular

actuators is given below: Solenoid Valves - logical outputsthat can switch a hydraulic orpneumatic flow.

Lights - logical outputs that canoften be powered directly from PLCoutput boards.

Motor Starters - motors often drawa large amount of current whenstarted, so they require motor

starters, which are basically large


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Outputs from PLCs are often relays,but they can also be solid stateelectronics such as transistors for DC

outputs or Triacs for AC outputs.Continuous outputs require specialoutput cards with digital to analog

converters.


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Some important terminology for relaysis given below

Contactor - Special relays for switching largecurrent loads.

Motor Starter - Basically a contactor in serieswith an overload relay to cut off when too much

current is drawn. Arc Suppression - when any relay is opened or

closed an arc will jump. This becomes a majorproblem with large relays. On relays switching AC

this problem can be overcome by opening therelay when the voltage goes to zero (while crossingbetween negative and positive). When switchingDC loads this problem can be minimized byblowing pressurized gas across during opening to

suppress the arc formation.


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Inputs come from sensors thattranslate physical phenomena intoelectrical signals.

Typical examples of sensors are listedbelow in relative order of popularity.

Proximity Switches - use inductance,capacitance or light to detect anobject logically.

Switches - mechanical mechanismswill open or close electrical contactsfor a logical signal.

Potentiometer - measures angularositions continuousl usin

ladder program

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