Programmable logic controller (PLC)

Friday, 05 January 2007

A programmable logic controller (PLC) is an industrial computer used to control and automate complex systems. Programmable logic controllers are a relatively recent development in process control technology. It is designed for use in an industrial environment, which uses a programmable memory for the integral storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog inputs and outputs, various types of machines or processes. Programmable logic controllers are widely used in industry and process control. Programmable logic controllers are used in a wide spectrum of applications from factory automation to waste water treatment plant controls and from chemical process plant control to engine management systems. Each application has a unique program, written to the user's specifications, to control the sequence of events for that particular system. Programmable logic controllers are used throughout industry to control and monitor a wide range of machines and other movable components and systems. As a part of process control, a PLC is used to monitor input signals from a variety of input points (input sensors) which report events and conditions occurring in a controlled process. For example, a PLC can monitor such input conditions as motor speed, temperature, pressure, volumetric flow and the like. Programmable logic controllers are typically found in factory type settings. PLCs are used to control robots, assembly lines and various other applications that require a large amount of data monitoring and control. In addition to the programmable logic controllers itself, there are many different components that can be connected to the PLC using either an open protocol, such as, for example, RS-232, or a proprietary protocol. Both the PLC and it's associated peripherals are designed so that they can be easily integrated into an industrial control system and easily used in all their functions.

A typical programmable logic controller employs a backplane to serve as the communications bus for interconnecting the PLC processor with the array of individual input/output devices with which the processor interacts in terms of receiving input data for use in executing the control program and transmitting control data for use in controlling the targeted objects. A PLC includes a rack into which a plurality of input/output cards may be placed. A rack includes several slots into which these input/output cards are installed. Each input/output card has a plurality of I/O points. The I/O modules are typically pluggable into respective slots located on a backplane board in the PLC. An I/O bus couples the cards in the slots back to the processor of the programmable logic controller. The slots are coupled together by a main bus which couples any I/O modules plugged into the slots to a central processing unit (CPU). The CPU itself can be located on a card which is pluggable into a dedicated slot on the backplane of the PLC. The particular processor employed in a PLC together with the particular choice of input and output cards installed in the PLC rack are often referred to as the hardware configuration of the programmable logic controller. The hardware configuration also includes the particular addresses which the I/O cards employ. In typical PLC applications, a variety of input and output (I/O) signals, including analog signals, are processed using addressable I/O modules communicatively coupled to a processor. A typical PLC system includes at least one option module that performs input/output (I/O) functions. Each option module typically has a plurality of input/output points. The option modules are coupled through an interface bus, for example via a backplane, to a main controller having a microprocessor executing a user program. Option modules may also include a microprocessor and a memory containing separate user programs and data directed to a particular operation of the PLC system. During the execution of a stored control program, the PLC's read inputs from the controlled process and, per the logic of the control program, provide outputs to the controlled process. The outputs typically provide analog or binary voltages or "contacts" implemented by solid state switching devices. PLC's commonly utilize analog current regulator outputs for signaling and/or control purposes. In a typical PLC, an output stage of a linear circuit dissipates power thermally and regulates current by modulating a voltage drop caused by a loaded device in the output stage. Programmable logic controllers differ from conventional computers both in their reliability and flexibility. PLC's are normally constructed in modular fashion to allow them to be easily reconfigured to meet the demands of the particular process being controlled. The processor and I/O circuitry are normally constructed as separate modules that may be inserted in a chassis and connected together through a common backplane using permanent or releasable electrical connectors. To facilitate the necessary communication, the PLCs and related monitoring stations are connected by a computer network. Typically, a network is organized such that any computer may communicate with any other network computer. The communication protocol provides a mechanism by which messages can be decomposed and routed to a destination computer identified by some form of address.

Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the system in accordance with a stored control program. A control program is stored in a memory within the PLC to instruct the PLC what actions to take upon encountering particular input signals or conditions. In response to these input signals provided by input sensors, the PLC derives and generates output signals which are transmitted via PLC output points to various output devices, such as actuators and relays, to control the process. Programmable logic controllers can be programmed using different development platforms. One common development platform is called ladder logic. Ladder logic is a programming language based on executing commands on a line-by-line permission system. Each ladder logic program comprises one or more ladder logic statements. These ladder logic statements are often termed "rungs." Each ladder logic statement defines the relationship between an output variable and, in most cases, one or more input variables. The PLC responds to the input as directed by a ladder logic program which simulates the response of what used to be accomplished by older systems with a set of relays and other control devices. Ladder logic programs instruct the PLC how and when to react to the different signals it receives. As a PLC program executes, the processor checks to see if there is a permissive statement on the left side of a ladder rung. If that permissive statement is true or if there is no permissive statement, the processor performs the operation on the right side of the ladder rung. When one rung is executed, the processor moves to the next rung down of the ladder in a sequential fashion. This process is called a ladder scan. Input variables include variables corresponding to the inputs of the PLC, and output variables include variables corresponding to the signals at the output terminals of the PLC. PLCs operate by gathering information from various sensor inputs (analog and discrete) and processing the data using relay Ladder Logic. As sensor data is gathered and manipulated by the user program, the PLC sends appropriate output signals to control the operation of the equipment to which it is connected. The result is safer, more efficient operation of the monitored or controlled equipment. Ladder logic statements and programs are often expressed in terms of ladder logic graphs. The PLC is essentially a small computer specifically designed for a routine control system. After a user writes a program and stores it in a storage device, the CPU follows the control logic defined in the code to monitor and process the input signal from buttons, sensors, or threshold switches. After logic operations, output signals are sent to an external load, such as a relay, indicator, and motor. Sometimes, if necessary, the output signal can be fed back as the input signal to control other output devices.

Programmable logic controllers perform many of the control functions for assembly line machines, machine tools, and other types of industrial equipment. An industrial control system often includes a programmable logic controller for providing coordinated control of industrial control equipment, which includes various elements, that are often either sensors for providing inputs to the PLC or relays for receiving outputs from the PLC, each under the control of an element controller, and each connected to the PLC over a network via a network I/O device. The control program is stored in a memory and includes instructions which are read out in rapid sequence to monitor the condition of selected sensing devices on the controlled equipment. These instructions generally activate or deactivate selected operating devices on the controlled equipment as determined by the status of one or more sensors. The typical processor in a PLC rapidly executes programmable controller-type instructions which call for the manipulation of single-bit input data in the control of single-bit output data. Programmable logic controllers are finding increasing acceptance in a variety of environments ranging, for example, from process and apparatus control in a factory and recently to vending machines and the like. The modules can vary greatly in the types of signals they produce or receive. However, they generally fall into one of two categories, a digital I/O or an analog I/O. Digital ports in an I/O module are relatively easy and simple to isolate due to the nature of digital signals, i.e. a pulsed digital signal, an on/off switch and the like. Industrial control using a PLC requires what is termed rapid scanning, meaning the continuous, rapid execution by the PLC of three main steps executed repeatedly. Depending upon the manufacturer of the PLC, the exact scan sequence may be different. The PLC will determine if the states of the various peripherals, such as a remote I/O, have changed since the previous scan.

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