Basic of plc

Introduction to Programmable Controllers

It's always good to get an overview of where designs have been and were they are going.  To do this it's essential to get a bird's eye view of the concepts and processes that make the PLC so valuable in industrial control.  Pitting PLCs against other control types will also serve to show the pros and cons for different applications.

Definition of a PLC

What is a PLC? 

A Programmable Logic Controller, or PLC for short, is simply a special computer device used for industrial control systems. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. Where ever there is a need to control devices the PLC provides a flexible way to "softwire" the components together.

The basic units have a CPU (a computer processor) that is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control.  They are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures), compact and economical over traditional control systems.

A Simple Example 

Consider something as simple as a switch that turns on a light.  In this system with a flick of the switch the light would turn on or off.  Beyond that though there is no more control.  If your boss came along and said I want that light to turn on thirty seconds after the switch has been flipped, then you would need to buy a timer and do some rewiring.  So it is time, labor and money for any little change.

A PLC Saves the Day 

Now consider the same device with a PLC in the middle.  The switch is fed as an input into the PLC and the light is controlled by a PLC output.  Implementing a delay in this system is easy since all that needs to be changed is the program in the PLC to use a delay timer.

This is a rather simple example but in a larger system with many switchs and lights (and a host of other devices) all interacting with each other this kind of flexibility is not only nice but imperitive.  Hopefully a light bulb has now turned on over your head.

How PLCs Work

A programmable logic controller is a specialized computer used to control machines and processes.  It therefore shares common terms with typical PCs like central processing unit, memory, software and communications.  Unlike a personal computer though the PLC is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.

The components that make a PLC work can be divided into three core areas.

The power supply and rack

The central processing unit (CPU)

The input/output (I/O) section

PLCs come in many shapes and sizes.  They can be so small as to fit in your shirt pocket while more involved controls systems require large PLC racks.  Smaller PLCs (a.k.a. “bricks”) are typically designed with fixed I/O points.  For our consideration, we’ll look at the more modular rack based systems.  It’s called “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.

The Power Supply and Rack

So let’s start off by removing all our modules which leaves us with a naked PLC with only the power supply and the rack. 

The rack is the component that holds everything together.  Depending on the needs of the control system it can be ordered in different sizes to hold more modules.  Like a human spine the rack has a backplane at the rear which allows the cards to communicate with the CPU.  The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack.  The most popular power supplies work with 120 VAC or 24 VDC sources.

The CPU

The brain of the whole PLC is the CPU module.  This module typically lives in the slot beside the power supply.  Manufacturers offer different types of CPUs based on the complexity needed for the system.

The CPU consists of a microprocessor, memory chip and other integrated circuits to control logic, monitoring and communications.  The CPU has different operating modes.  In programming modeit accepts the downloaded logic from a PC.  The CPU is then placed in run mode so that it can execute the program and operate the process. 

Since a PLC is a dedicated controller it will only process this one program over and over again.  One cycle through the program is called a scan time and involves reading the inputs from the other modules, executing the logic based on these inputs and then updated the outputs accordingly.  The scan time happens very quickly (in the range of 1/1000th of a second).  The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.

I/O System

The I/O system provides the physical connection between the equipment and the PLC.  Opening the doors on an I/O card reveals a terminal strip where the devices connect. 

There are many different kinds of I/O cards which serve to condition the type of input or output so the CPU can use it for it’s logic.  It's simply a matter of determining what inputs and outputs are needed, filling the rack with the appropriate cards and then addressing them correctly in the CPUs program.

Inputs

Input devices can consist of digital or analog devices.  A digital input card handles discrete devices which give a signal that is either on or off such as a pushbutton, limit switch, sensors or selector switches.  An analog input card converts a voltage or current (e.g. a signal that can be anywhere from 0 to 20mA) into a digitally equivalent number that can be understood by the CPU.  Examples of analog devices are pressure transducers, flow meters and thermocouples for temperature readings

Outputs

Output devices can also consist of digital or analog types.  A digital output card either turns a device on or off such as lights, LEDs, small motors, and relays.  An analog output card will convert a digital number sent by the CPU to it’s real world voltage or current.  Typical outputs signals can range from 0-10 VDC or 4-20mA and are used to drive mass flow controllers, pressure regulators and position controls.

Programming a PLC

In these modern times a PC with specially dedicated software from the PLC manufacturer is used to program a PLC.  The most widely used form of programming is called ladder logic.  Ladder logic uses symbols, instead of words, to emulate the real world relay logic control, which is a relic from the PLC's history.  These symbols are interconnected by lines to indicate the flow of current through relay like contacts and coils.  Over the years the number of symbols has increased to provide a high level of functionality. 

The completed program looks like a ladder but in actuality it represents an electrical circuit.  The left and right rails indicate the positive and ground of a power supply.  The rungs represent the wiring between the different components which in the case of a PLC are all in the virtual world of the CPU.  So if you can understand how basic electrical circuits work then you can understand ladder logic.

In this simplest of examples a digital input (like a button connected to the first position on the card) when it is pressed turns on an output which energizes an indicator light.

The completed program is downloaded from the PC to the PLC using a special cable that’s connected to the front of the CPU.  The CPU is then put into run mode so that it can start scanning the logic and controlling the outputs.