Programmable Logic Controllers PDF

Programmable Logic Controllers Programmable logic controllers (Figure 1-1) are now the most widely used industrial process control technology. A programmable logic controller (PLC) is an industrial grade computer that is capable of being programmed to perform control functions. Benefits : (i) T...

Programmable Logic Controllers Programmable logic controllers (Figure 1-1) are now the most widely used industrial process control technology. A programmable logic controller (PLC) is an industrial grade computer that is capable of being programmed to perform control functions. Benefits : (i) The programmable controller has eliminated much of the hardwiring associated with conventional relay control circuits. (ii) fast response, (iii) easy programming and installation, (iv) high control speed, (v) network compatibility, (vi) troubleshooting and testing convenience, and (vii) high reliability The PLC is designed for multiple input and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs for the control and operation of manufacturing process equipment and machinery are typically stored in battery-backed or nonvolatile memory. A PLC is an example of a real-time system since the output of the system controlled by the PLC depends on the input conditions. PLCs and Conventional Relays: Programmable controllers offer several advantages over a conventional relay type of control. - Relays have to be hardwired to perform a specific function. When the system requirements change, the relay wiring has to be changed or modified. - In extreme cases, such as in the auto industry, complete control panels had to be replaced since it was not economically feasible to rewire the old panels with each model changeover. - The programmable controller has eliminated much of the hardwiring associated with conventional relay control circuits (Figure 1-2). It is small and inexpensive compared to equivalent relay-based process control systems. Modern control systems still include relays, but these are rarely used for logic. PLCs provide many other benefits including: Increased Reliability: Once a program has been written and tested, it can be easily downloaded to other PLCs. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error (Figure 1-3). The program takes the place of much of the external wiring that would normally be required for control of a process. Hardwiring, though still required to connect field devices, is less intensive. PLCs also offer the reliability associated with solid-state components. More Flexibility: It is easier to create and change a program in a PLC than to wire and rewire a circuit. With a PLC the relationships between the inputs and outputs are determined by the user program instead of the manner in which they are interconnected (Figure 1-4). Original equipment manufacturers can provide system updates by simply sending out a new program. End users can modify the program in the field, or if desired, security can be provided by hardware features such as key locks and by software passwords. Lower Cost: PLCs were originally designed to replace relay control logic, and the cost savings have been so significant that relay control is becoming obsolete except for power applications. Generally, if an application has more than about a half-dozen control relays, it will probably be less expensive to install a PLC. Communications Capability: A PLC can communicate with other controllers or computer equipment to perform such functions as supervisory control, data gathering, monitoring devices and process parameters, and download and upload of programs (Figure 1-5). Faster Response Time: PLCs are designed for high speed and real-time applications (Figure 1-6). The programmable controller operates in real time, which means that an event taking place in the field will result in the execution of an operation or output. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick-response capability.  Easier to Troubleshoot: PLCs have resident diagnostics and override functions that allow users to easily trace and correct software and hardware problems. To find and fix problems, users can display the control program on a monitor and watch it in real time as it executes (Figure 1-7)· Easier to Test Field Devices: A PLC control panel has the ability to check field devices at a common point. For example, a control system consisting of hundreds of input and output field devices may be contained within a very large manufacturing area. Thus, it would take a considerable amount of time to check each device at its location. By having each device wired back to a common point on a PLC module, each device could be checked for operation fairly quickly Parts of a PLC: A typical PLC can be divided into (i) central processing unit (CPU), (ii) input/output (I/O) section, (iii) power supply, and (iv) programming device as shown in Figure 1-8. The term architecture can refer to PLC hardware, to PLC software, or to a combination of both. An open architecture design allows the system to be connected easily to devices and programs made by other manufacturers. Open architectures use off-the shelf components that conform to approved standards. A system with a closed architecture is one whose design is proprietary, making it more difficult to connect to other systems. Most PLC systems are in fact proprietary, so one must be sure that any generic hardware or software you may use is compatible with their particular PLC. Also, although the principal concepts are the same in all methods of programming, there might be slight differences in addressing, memory allocation, retrieval, and data handling for different models. Consequently, PLC programs cannot be interchanged among different PLC manufacturers. There are two ways in which I/Os (Inputs/Outputs) are incorporated into the PLC: (i) fixed and (ii) modular. Fixed I/O (Figure 1-9) is typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together, and the I/O terminals will have a fixed number of connections built in for inputs and outputs. The main advantage of this type of packaging is lower cost. The number of available I/O points varies and usually can be expanded by buying additional units of fixed I/O. One disadvantage of fixed I/O is its lack of flexibility; they are limited in what you can get in the quantities and types dictated by the packaging. Also, for some models, if any part in the unit fails, the whole unit has to be replaced. Modular I/O (Figure 1-10) is divided by compartments into which separate modules can be plugged. This feature greatly increases the options and the unit’s flexibility. One can choose from the modules available from the manufacturer and mix them any way they desire. The basic modular controller consists of a rack, power supply, processor module (CPU), input/output (I/O modules), and an operator interface for programming and monitoring. The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack. The PLC processor is also connected to the backplane and can communicate with all the modules in the rack. (ii) The power supply supplies DC power to other modules that plug into the rack (Figure 1-11). For large PLC systems, this power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies. For some small micro PLC systems, the power supply may be used to power field devices. (iii) The processor (CPU) is the “brain” of the PLC. A typical processor (Figure 1-12) usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing user program instructions, numerical values, and I/O devices status. The CPU controls all PLC activity and is designed so that the user can enter the desired program in relay ladder logic. The PLC program is executed as part of a repetitive process referred to as a scan (Figure 1-13). A typical PLC scan starts with the CPU reading the status of inputs. Then, the application program is executed. Once the program execution is completed, the status of all outputs is updated. Next, the CPU performs internal diagnostic and communication tasks. This process is repeated continuously as long as the PLC is in the run mode. The I/O system forms the interface by which field devices are connected to the controller (Figure 1-14). The purpose of this interface is to condition the various signals received from or sent to external field devices. Input devices such as pushbuttons, limit switches, and sensors are hardwired to the input terminals. Output devices such as small motors, motor starters, solenoid valves, and indicator lights are hardwired to the output terminals. To electrically isolate the internal components from the input and output terminals, PLCs commonly employ an optical isolator, which uses light to couple the circuits together. The external devices are also referred to as “field” or “real-world” inputs and outputs. The terms field or real world are used to distinguish actual external devices that exist and must be physically wired from the internal user program that duplicates the function of relays, timers, and counters. A programming device is used to enter the desired program into the memory of the processor. The program can be entered using relay ladder logic, which is one of the most popular programming languages. Instead of words, ladder logic programming language uses graphic symbols that show their intended outcome. A program in ladder logic is similar to a schematic for a relay control circuit. It is a special language written to make it easy for people familiar with relay logic control to program the PLC. Hand-held programming devices are sometimes used to program small PLCs because they are inexpensive and easy to use. Once plugged into the PLC, they can be used to enter and monitor programs. Both compact hand-held units and laptop computers are frequently used on the factory floor for troubleshooting equipment, modifying programs, and transferring programs to multiple machines. A personal computer (PC) is the most commonly used programming device. Most brands of PLCs have software available so that a PC can be used as the programming device. This software allows users to create, edit, document, store, and troubleshoot ladder logic programs (Figure 1-15). The computer monitor is able to display more logic on the screen than can hand-held types, thus simplifying the interpretation of the program. The personal computer communicates with the PLC processor via a serial or parallel data communications link, or Ethernet. If the programming unit is not in use, it may be unplugged and removed. Removing the programming unit will not affect the operation of the user program. A program is a user-developed series of instructions that directs the PLC to execute actions. A programming language provides rules for combining the instructions so that they produce the desired actions. Relay ladder logic (RLL) is the standard programming language used with PLCs. Its origin is based on electromechanical relay control. The relay ladder logic program graphically represents rungs of contacts, coils, and special instruction blocks. RLL was originally designed for easy use and understanding for its users and has been modified to keep up with the increasing demands of industry’s control needs. Principles of Operation To get an idea of how a PLC operates, consider the simple process control problem illustrated in Figure 1-16. Here a mixer motor is to be used to automatically stir the liquid in a vat when the temperature and pressure reach preset values. In addition, direct manual operation of the motor is provided by means of a separate pushbutton station. The process is monitored with temperature and pressure sensor switches that close their respective contacts when conditions reach their preset values. This control problem can be solved using the relay method for motor control shown in the relay ladder diagram of Figure 1-17. The motor starter coil (M) is energized when both the pressure and temperature switches are closed or when the manual pushbutton is pressed.  Now let’s look at how a programmable logic controller might be used for this application. The same input field devices (pressure switch, temperature switch, and pushbutton) are used. These devices would be hardwired to an appropriate input module according to the manufacturer’s addressing location scheme. Typical wiring connections for a 120 VAC modular configured input module are shown in Figure 1-18. The same output field device (motor starter coil) would also be used. This device would be hardwired to an appropriate output module according to the manufacturer’s addressing location scheme. Typical wiring connections for a 120 VAC modular configured output module are shown in Figure 1-19. Next, the PLC ladder logic program would be constructed and entered into the memory of the CPU. A typical ladder logic program for this process is shown in Figure 1-20. The format used is similar to the layout of the hardwired relay ladder circuit. The individual symbols represent instructions, whereas the numbers represent the instruction location addresses. To program the controller, you enter these instructions one by one into the processor memory from the programming device. Each input and output device is given an address, which lets the PLC know where it is physically connected. Note that the I/O address format will differ, depending on the PLC model and manufacturer. Instructions are stored in the user program portion of the processor memory. During the program scan, the controller monitors the inputs, executes the control program, and changes the output accordingly. For the program to operate, the controller is placed in the RUN mode, or operating cycle. During the program scan, the controller monitors the inputs, executes the control program, and changes the output accordingly. Each symbol (looks like a normally open contact) is an instruction. The symbol is considered to represent a coil that, when energized, will energize the device that is wired to the respective output. In the ladder logic program of Figure 1-20, the coil O/1 is energized when contacts I/1 and I/2 are closed or when contact I/3 is closed. Either of these conditions provides a continuous logic path from left to right across the rung that includes the coil.  A programmable logic controller operates in real time in that an event taking place in the field will result in an operation or output taking place. The RUN operation for the process control scheme can be described by the following sequence of events: First, the pressure switch, temperature switch, and pushbutton inputs are examined and their status is recorded in the controller’s memory. A closed contact is recorded in memory as logic 1 and an open contact as logic 0. Next the ladder diagram is evaluated, with each internal contact given an OPEN or CLOSED status according to its recorded 1 or 0 state. When the states of the input contacts provide logic continuity from left to right across the rung, the output coil memory location is given a logic 1 value and the output module interface contacts will close. When there is no logic continuity of the program rung, the output coil memory location is set to logic 0 and the output module interface contacts will be open. The completion of one cycle of this sequence by the controller is called a scan. The scan time, the time required for one full cycle, provides a measure of the speed of response of the PLC. Generally, the output memory location is updated during the scan but the actual output is not updated until the end of the program scan during the I/O scan. Figure 1-21 shows the typical wiring required to implement the process control scheme using a fixed PLC controller. In this example, the Allen-Bradley Pico controller equipped with 8 inputs and 4 outputs is used to control and monitor the process. Installation can be summarized as follows: Fused power lines, of the specified voltage type and level, are connected to the controller’s L1 and L2 terminals. The pressure switch, temperature switch, and pushbutton field input devices are hardwired between L1 and controller input terminals I1, I2, and I3, respectively. The motor starter coil connects directly to L2 and in series with Q1 relay output contacts to L1. The ladder logic program is entered using the front keypad and LCD display. Pico programming software is also available that allows you to create as well as test your program using a personal computer.