The Programmable Logic Controller is a computer designed specifically for industrial applications that range from simple lighting projects to running entire manufacturing processes. This system can perform a wide variety of functions, providing a large number of analog and digital Input/ Output (I/O) interfaces; and various communication protocols. All of the PLC's components and functions revolve around the controller, which is programmed to perform a specific task.
Modern PLCs were introduced in the 1960s and was used in the automobile manufacturing industry. The present PLC has a lot of criteria to satisfy such as higher performance, smaller form factor, and greater functional flexibility. These functionalities along with its robust design make it the best choice for industrial control applications.
History and Development
As aforementioned, programmable logic controllers were originally invented for the U.S. automotive manufacturing industry to replace relay logic systems by General Motors (currently Schneider Electric). The automotive industry adopted PLCs initially for software revisions, which replaced rewiring of control panels that were used during the changing of production models.
Since then many companies have entered into the PLC manufacturing market like Siemens, ABB, Rockwell Automation (Allen-Bradley), Mitsubishi, GE, Omron, Delta, Bosch Rexroth, etc. The benefits and functions of the PLC are now found in industries beyond automotive manufacturing, including food processing, power generation, mining, etc.
The main hardware components of a Programmable Logic Controller system are Central Processing Unit (CPU), microprocessor, Input/ Output (I/O) ports and the power supply unit.
CPU- This is used to perform logical operations, arithmetic operations, computer interface and many other functions.
Microprocessor- It is divided into two areas namely RAM (Random Access Memory) and ROM (Read Only Memory). ROM stores the program and data permanently for the operating system. RAM stores the status information of input and output devices, and the values of timers, counters and other internal devices.
I/O ports- Input ports are used to interface and monitor field devices like sensors, switches, etc. while output ports are used to monitor and control devices like lamps, pumps, tanks, etc.
Power Supply Unit- Most PLCs work at 230VAC or 24VDC while some of them have an isolated power supply.
Features for a Promising Future
- Compact size, Cost Optimization Processing Time
With the recent trends in nanotechnology, smaller Programmable Logic Controllers have been introduced in the nano and micro classes offering features previously found only in larger of its kind. This has lead to abandonment of the larger PLCs, as even those in the nano class are capable of Ethernet communication, motion control, on-board PID with auto tune, remote connectivity and other features.
In response to market demands, many features and functions are now provided by lower end PLCs also. For example, we can expect that small PLCs will evolve to include many of the features of higher-level PLCs, and mid and high-end PLCs will offer a smaller, more compact and customized solutions to meet users' needs.
PLCs are also exploiting the sudden crash in memory chips cost and reduced size. This permits greatly increased local data storage, allowing the use of a PLC in many applications formerly requiring expensive data acquisition systems. It also opens the door to other features, such as the on-board storage of product information, which can expedite troubleshooting.
Today's PLCs are already benefiting from USB technology, permitting us to get online, program, and monitor the control system. This technology is continuing to evolve, and with the availability of micro and mini USB connectors, one can expect to see this communication option on more of the smaller PLCs.
Another example of a feature from the fast-moving consumer electronics world that's quickly penetrating the industrial controls field is non-volatile portable memory devices. These offer great benefit to the PLC customer by providing an enormous amount of additional memory in a small package. These options include USB devices to SD, mini SD, and micro SD cards, adding up 32 GB of additional memory to a PLC as needed by the end user, machine builder, or system integrator.
- Higher Integration
Programmable Logic Controller I/O ports can be increased adding certain additional modules by system extension through extension slots. Such a modified PLC that provides provision of extension modules is called a Programmable Automation Controller (PAC). Each module can contain both input and output ports. Extension modules can have inputs and outputs of a different nature from those on the PAC.
During this period of PAC versus PLC, we have seen a much faster advancement of both classes of the product. PACs have allowed users to increase the functionality of what is considered traditional industrial automation, encouraging suppliers to increase products to meet their demands.
These demands have challenged product designers to find new ways to support the available components and build them into a system that will be rugged and stand up to the harsh conditions of an industrial environment. The challenge continues with providing the connectivity, memory expansion, and processing power improvements required to handle ever more complicated applications, while maintaining or even lowering the cost of the end product.
- Programming Ease
An aspect of the Programmable Logic Controller (PLC) that reflects both the past and the future is programming language. The programming languages and classified as two graphical and two textual PLC programming language standards namely:
- Ladder logic (graphical)
- Function block diagram (graphical)
- Structured text (textual)
- Instruction list (textual)
The ladder diagram responded with advancements of its own and still continues to be the top choice for 96% of its users. There are applications where sequential function chart is better, particularly for process control. Structured text works well for data manipulation, and other IEC languages have their strong points. But ladder diagram forges on, and remains the leader by a wide margin in terms of PLC programming languages.
Regardless of the hardware involved, this language has gone a long way to making the PLC an industry standard, and this trend is expected to continue.