A programmable logic controller is a special type of computing device that is used to control industrial devices and machines. It can be taken to be a small computer which has been designed to work in an industrial environment. They are integrated with special input and output interfaces which are used to undertake the required communication with the external environments. They are programmed with the use of a control programming language (Erickson, 1996).
There are different types of programmable logic controllers. Manufacturers produce PLC in five sizes. These include nano, micro, small, medium, and large. These are categorized according to the number of input/output points they have.
Nano programmable logic controller
Programmable logic controllers in this category have up to 16 input/output points. It has a full function like that of bigger programmable logic controllers. The measurements are 3.3” by 2.8” and the weight is 2.9 oz. It has an Ethernet port which supports MODBUS TCP/IP server and connections to client machines. Apart from this, it supports RS485 connection which is used to connect native host link commands. This can also be used to support MODBUS RTU and MODBUS ASCII. This makes this PLC communicate with other PLCs of different sizes. It also has the capability to communicate with PLCs which are on another network. The network should be connected to the network which they are operating. It is low in cost (approximately $119) and used extensively in small gadgets. It is used to control small gadgets. Most nano programmable logic controllers are used in controlling access controls for buildings (Erickson, 1996).
Micro-programmable logic controllers
This is a programmable logic controller which has more than 16I/O points and up to 64 I/O points. It has more power of computing when compared to the nano programmable logic controller. Some manufacturers offer web server modules which help in internet communications. They are used in sending messages to remote systems. With their capabilities, it is possible to control remote features and parameter. They allow control systems to be changed from remote locations. They are used to support discrete, analogue and simple motion applications. They are used in managing water systems and irrigation systems. It has processors which operational frequency of up to 400MHz. It also has a range of communication peripherals. It has standard peripherals like multimedia and Ethernet connections. They are suited for small and standalone machines and some distributed applications.
Small programmable logic controllers
These programmable logic controllers are compact and robust. They can be placed side by side with the device which is being controlled. They are used in replacement of relay logics which are hard-wired. This hard-wired logic is used to control individual items in plants a machinery. These are programmable logic controllers which have up to 960 I/O points in their operation. They support small footprints and systems which are used in small to mid-range applications. They are manufactured to be used in distributed environments. They have the capability to support many communication networks of communication. They have a conformal coating which allows the equipment to withstand being placed in harsh environments. One of the applications of small PLC is in checking the strokes per minute of oil pumps and other types of oil pumps (Bolton, 2009).
Medium programmable logic controllers
This is a PLC which has fiber optic capabilities and high speeds of storage. It also has one CF memory slot that is used for connection and has an Ethernet connector. It also has repeater capabilities which are wired to wire. It has 2M programming cable. They are able to support FINS protocol over the TCP/IP connection. This category of the PLC can be expanded using slots which are expandable. They also have modules that are plug-in and used for communication with external environments. It has the capability of communicating via wireless channels. It has an interface where it can communicate with GPRS/GSM modem. The customers can choose from the variety of I/O slots to find the needed count of I/O. It has the capability of having analogue measuring the right value. This is done using plug-in input modules. The PLC can time-stamp and log data that are used for measurements in its memory. This is commonly seen when the device is having communication breaks.
This is used when there is a need to control large systems and where there is a need to control a large number of inputs and output control points. This type of PLC is used when there is a need to control large machines and plants. Such plans and machines require continuous flow. They are also used as supervisory controllers where they monitor other smaller PLCs. The processor that is used is that of Modular card which has many function cards which are available. There are -new trends where there is a move towards the use of faster processors and multi-processors. This allows control of differing magnitude. With large PLC, performance of control for large systems is now optimized. The PLC is now capable of handling multiple programs which run in 100K instructions.
Bolton, W. (2009). Programmable logic controllers. Access Online via Elsevier.
Erickson, K. T. (1996). Programmable logic controllers. Potentials, IEEE, 15(1), 14-17.