PLC (Programmable Logic Controller) 

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.

Hence, 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 PLCis 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.

                                                            

                                                    Figure 1 Power supply and Rack

                                                                      Figure 2 Backplane 

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.

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 mode it accepts the downloaded logic from a PC.  The CPU is then placed in run modeso 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.




                                                   Figure 3 Components of a PLC

Comparison of PLC with other control devices

The main difference from other computers is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

PLCs were invented as replacements for automated systems that would use hundreds orressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remoteI/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a “generic” solution, and where the non-recurring engineering charges are spread over thousands of sales. 

For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.

A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.

Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.

PLCs may include logic for single-variable feedback analog control loop, a “proportional, integral, derivative” or “PID controller.” A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.