PLC FULL FORM (Programmable logic controller)

 

PLC FULL FORM (Programmable logic controller)

A Programmable Logic Controller, or PLC, is a ruggedized computer used for industrial automation. These controllers can automate a specific process, machine function, or even an entire production line. A PLC program is usually written on a computer and then is downloaded to the controller

Most PLC programming software offers programming in Ladder Logic, or “C”. Ladder Logic is the traditional programming language. It mimics circuit diagrams with “rungs” of logic read left to right. Each rung represents a specific action controlled by the PLC, starting with an input or series of inputs (contacts) that result in an output (coil). Because of its visual nature, Ladder Logic can be easier to implement than many other programming languages. “C” programming is a more recent innovation. Some PLC manufacturers supply control programming software.

In a traditional industrial control system, all control devices are wired directly to each other according to how the system is supposed to operate. In a PLC system, however, the PLC replaces the wiring between the devices.


Thus, instead of being wired directly to each other, all equipment is wired to the PLC. The use of a PLC to provide the wiring connections between system devices is called soft wiring. Then, the control program inside the PLC provides the “wiring” connection between the devices.

The control program is the computer program stored in the PLC’s memory that tells the PLC what’s supposed to be going on in the system.

PLC Advantages

• high reliability
• small space requirements
• computing capabilities
• reduced costs
• ability to withstand harsh environments
• expandability

PLC Basics

PLCs were invented by Dick Morley in 1964. Since then PLC has revolutionized the industrial and manufacturing sectors. There is a wide range of PLC functions like timing, counting, calculating, comparing, and processing various analog signals.

The main advantage of PLC over a “hard-wired” control system is that you can go back and change a PLC after you’ve programmed it, at little cost (just the cost of the programmer’s time). In a hard-wired control system, you’re essentially having to rip out wires and start from scratch (which is more expensive and takes longer). Let’s look at an example to better understand this advantage.

Imagine you have a light connected to a switch. In general, the light operates under two conditions – ON and OFF. Now you are given a task that when you turn ON the switch, the light should glow only after 30 seconds. With this hard-wired setup – we’re stuck. The only way to achieve this is to completely rewire our circuit to add a timing relay. That’s a lot of hassle for a minor change.

This is where a programmable logic controller comes into the picture, which doesn’t require any additional wiring and hardware to make sure of a change. Rather it requires a simple change in code, programming the PLC to only turn on the light 30 seconds after the switch is turned ON. So, by using a PLC, it is easy to incorporate multiple inputs and outputs.

INPUTS AND OUTPUTS OF PLC

This allowed the electrician to carry a simple and relatively inexpensive device called a “Wiggy” which was a solenoid voltmeter that was used by electricians to see if an input or output was on or off. Modern PLCs are adaptable devices. Stay tuned for the basics of PLC inputs and outputs explained.

In the early days, control systems were all 110VAC powered discrete inputs and outputs. Since every electrician had a “Wiggy” and 110VAC was pervasive in an industrial setting, every electrician and electrical supervisor wanted a control system to utilize AC circuits. Since ladder logic can work with analog values as well as on-off devices (called discrete inputs and discrete outputs), PLCs have evolved from their relay rack roots into very configurable systems. It had a solenoid that pulled against a spring equipped with a pointer to allow the electricians to visually see the magnitude of an AC voltage.

As we move toward the modern day, the shift has been to 24VDC since it does not present the same level of hazard to the electrician while still being a useful amount of current and voltage to power many devices.

INPUTS

Digital inputs are the most common types of inputs in PLC systems; due to the fact that PLCs are digital electronic devices themselves, thus, they’re able to easily process digital signals. A digital PLC input is basically a binary signal that is either ON or OFF, and which is applied to the PLC processor from a digital field input device. The concept of digital signals is based on the binary number system, which consists of only two possible digits, 1 or 0. Where 1 represents a HIGH state and 0 indicates a LOW state. However, in most PLCs, the HIGH signals are not represented by 1Volt but by 24Volts dc, because PLCs normally operate on 24V DC. In such a case, the PLC will indicate HIGH at its input side if the output voltage of the interfaced field input device is 24V DC.
Digital Input modules in PLC systems use the digital inputs to determine the status (i.e. ON or OFF) of specific field input devices. For example, if a certain PLC input module has an operating voltage of 24Volts DC and it receives a 0V DC input signal then it’ll identify the status of the connected field device as OFF. Whereas, a 24V DC input signal to that PLC input module will indicate an ON status of the field device.

OUTPUTS

While the much less costly PLC outputs are mere shadows of their former glory, the plethora of I/O available to a PLC more than make up for the inconvenience.  At this point, PLC control systems can deal with a comprehensive array of discrete I/O signals and a blinding array of complex sensors and actuators. 
Gone are the insane exposed circuit hydraulic amplifier cards that allowed the PLC to barely control actuator positions and in their place are completely solid state servo drives that can close the position loop in microseconds.  While PLCs used to interface with a variety of external single loop controllers that would control a single variable in a process like temperature or flow, now PLCs can control complex loops using standard PID statements from within the ladder logic. 
Furthermore, the control loops have been augmented with advanced mathematical functions that allow a control loop to operate without allowing the controlled device from creating any deviation from ideal.  Simply provide the type of input you want to control and the type of output you want to control it with and your closed loop control is nearly as good as running.