Programmable logic controllers are microcomputers developed to handle Boolean operations. A PLC produces on/off voltage outputs and can actuate such elements as electric motors, solenoids, fans, heaters etc. The basic operation of a PLC corresponds to a software based equivalent of a relay panel.
A PLC can also execute operations such as counting, delays and
timers. A single PLC replaces hundreds of relays. Many PLC these
days accept proportional signals and they can perform simple
arithmetic calculations and handle analog I/O and can replace PID
The Early Days Before the PLC
Before the days of the PLC the only way to control machinery was
through the use of relays. Relays work by utilizing a coil that, when
energized, creates a magnetic force to effectively pull a switch to
the ON or OFF position. When the relay is de-energized, the switch
releases and returns the device to its standard ON or OFF position.
So, for example, if I wanted to control whether a motor was ON or
OFF, I could attach a relay between the power source and the motor.
Then I can control whether the motor is getting power by either
energizing or de-energizing the relay. Without power, of course, the
motor would not run, thus I am controlling the motor. This type of relay
is known as a power relay. There could be several motors in one factory
that need to be controlled, so what do you do? You add lots of power
relays. So factories started to amass electrical cabinets full of power
relays. But wait, what switches the coil in the power relays ON and OFF
before the power relay turns the motor ON, and what if I want to control
that? What do you do? More relays. These relays are known as control
relays because they control the relays that control the switch that turns
the motor ON and OFF
Think about modern factories, and how many motors and ON/OFF power switches you would need to control just one machine. Then add on all the control relays you need and what you get is… Yes, machine control, but you also get a logistical nightmare. All these relays had to be hardwired in a very specific order for the machine to work properly, and heaven forbid if one relay would have an issue, the system as a whole would not work. Troubleshooting would take hours, and because coils would fail and contacts would wear out, there was need for lots of troubleshooting. These machines had to follow a strict maintenance schedule and they took up a lot of space. Then what if you wanted to change something? You would basically have to redo the entire system. It soon became clear that there were problems installing and maintaining these large relay control systems.
Let’s hear from a controls designer in the thick of things in the early ‘70s -
“Upon graduating from technical college in 1970, I began working as a controls designer, automating metal working machinery and equipment with industrial relays, pneumatic plunger timers, and electro-mechanical counters. Also included were fuses, control transformers, motor starters, overload relays, pushbuttons, selector switches, limit switches, rotary drum sequencers, pilot lights, solenoid valves, etc.
So what was the solution?
The popular forum PLCDEV.com outlines a list of requirements that GM engineers put out for a “standard machine controller.” It is this request that Dick Morley and his company, Bedford and Associates, were responding to when the first PLC was envisioned. Besides replacing the relay system, the requirements listed by GM for this controller included:
1.A solid-state system that was flexible like a computer but priced competitively with a like kind relay logic system.
2.Easily maintained and programmed in line with the already accepted relay ladder logic way of doing things.
3.It had to work in an industrial environment with all its dirt, moisture, electromagnetism and vibration.
4.It had to be modular in form to allow for easy exchange of components and expandability.
The programming look of the PLC required that it be easily understood and used by maintenance electricians and plant engineers. As relay-based control systems evolved and became more complicated, the use of physical component location wiring diagrams also evolved into the relay logic being shown in a ladder fashion. The control power hot wire would be the left rail, with the control power neutral as the right rail. The various relay contacts, pushbuttons, selector switches, limit switches, relay coils, motor starter coils, solenoid valves, etc., shown in their logical order would form the ladder’s rungs. It was requested that the PLC be programmed in this Ladder Logic fashion.
The modern PLC
The first PLCs had the ability to work with input and output signals, relay coil/contact internal logic, timers and counters. Timers and counters made use of word size internal registers, so it wasn’t too long before simple four-function math became available. The PLC continued to evolve with the addition of one-shots, analog input and output signals, enhanced timers and counters, floating point math, drum sequencers and mathematic functions. Having built-in PID (Proportional-Integral-Derivative) functionality was a huge advantage for PLCs being used in the process industry. Common sets of instructions evolved into fill-in-the-blank data boxes that have made programming more efficient. The ability to use meaningful Tag Names in place of non-descriptive labels has allowed the end user to more clearly define their application, and the ability to import/export the Tag Names to other devices eliminates errors that result when entering information into each device by hand.
PLC have these features
1. They are rugged and designed to withstand vibrations, temperature, humidity and noise.
2. The interfacing of the inputs and outputs are inside the controller.
3. They are easily programmed and have an easily understood programming language.
4. They are modular and they are event driven as compared to general purpose computers which are driven by stored information.
Two types of PLC are available in the market.
The small,compact, stand alone with limited functions and low cost.
The all powerful, networked multifunctional PLC.
Basic instructions and execution
The programming of a Plc mainly consists of mainly the definition of sequences. The input and output functions are already prepared. The instructions from a ladder diagram, a logical gate program or Boolean expression are translated to machine code. At execution, the program memory is run through in a cyclic manner infinitely. Every scan may take 15-30 ms and the scan cycle depends on the memory size and program complexity.
The response time of the PLC depends on the processing speed of thePLC. When the instructions and outputs are being executed, the computing system cannot read any new input signals. The ladder diagram can be considered as if every rung of the ladder were executed at the scan time. Thus it is not possible to visualize the ladder being executed sequentially on a row by row basis. The execution has to be very fast compared to the time scale of the process under control.
More at Sources