I. Executive Summary
The purpose of this discussion is to define the difference and to understand the specific application of programmable logic controllers(PLC) vs. distributed control systems(DCS). For the manufacturers in the process industries, the procedure for selecting the best automation technology is challenging. It is imperative to understand the weaknesses and strengths of PLC vs. DCS due to advanced technologies and microprocessors presently available in the market. Finding the right automation technology is vital to minimize total cost investment.
II. Programmable Logic Controller (PLC)
A PLC is a standard unit with no dedicated application. The unit has to be connected to the various plant input and output devices. The controller then must be programmed with the tasks that the system must perform; this is achieved by a set of software instructions. The software contains sequences that are initiated by inputs from the plant, which then prompt the outputs to change the plant status. The programmable controller can consequently be applied to an extensive range of diverse applications. Once configured as part of the system, the controller will be able to handle tasks.
III. Features
The main difference from other computers is that PLCs are armored for severe conditions(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 or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the rewiring of hard-wired control panels when production models changed.
Many of the earliest PLCs expressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were 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 stackbased 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 remote I/O hardware, allow a generalpurpose desktop computer to overlap some PLCs in certain applications. Under the IEC 61131-3 standard, PLCs can be programmed using standard programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.
IV. PLC compared with other control systems
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 cost-effective due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the nonrecurring engineering charges are spread across thousands of places. 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 buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be exhorbitant. Very complex process control, such as that which is 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. Aircraft flight controls are one example. PLCs may include logic for singlevariable 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 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.
