Programmable Logic Controllers - Essay Example

An Overview of Programmable Logic Controllers An Overview of Programmable Logic Controllers A Programmable Logic Controllers (PLC) is a compact device used to monitor and control the functions of machines, assembly lines, and a myriad of other decision driven processes. The PLC usually contains a microprocessor to provide computing power which allows them to process information and provide a logical output. The PLC has the ability to be programmed for individualized tasks and is a versatile alternative to older hardwired relay arrangements. Its adaptability, size, and low cost have made the implementation of larger control systems possible. Packaging systems, pick and place robotics, warehouse control, and processing plants are a few of the applications of the PLC. Wherever there is an enterprise or industry, you will find a PLC. The PLC has seen dramatic evolution since the first PLC, the Modicon, was introduced by Bedford Associates in the late 1960s (Kuphaldt, 2003). Paralleling the evolution of larger data processing systems, Allen Bradley and General Electric introduced programmable units with terminal input in the 1970s (Morley, 2006). They are now available as stand alone units, flexible card driven modules, and rack mounted with standardized cards for large scale centralized control. The unitary style is a self enclosed, ruggedised unit that has all the circuitry necessary to operate independently. It is generally used near the machine it is controlling and is dedicated to a small number of tasks. Modular construction has the advantage of placing several control units into a single module which allows the units to share power supply and computing capabilities. Sharing support circuitry reduces the cost of the unit as well as allowing for expanded control capabilities. Rack mounted systems are used to provide centralized control of very large scale systems. With the circuitry on cards that are attached to a standardized bus, the rack mount system offers greater networking and communication possibilities. This allows PLCs to run lengthy programs and control complex automated systems. In its most basic configuration the PLC reads a set of inputs, processes the inputs through a program, and makes a decision as to what output switching is required. The inputs are configured to sense the presence of a voltage, often 12VDC, but may be 5VDC or 115VAC. These inputs come from switches or relays on machines that may indicate level, position, or status of the equipment. A PLC can be used to stop a conveyor belt when an assembly machine is turned off. Sensors are widely used in process control to detect parameters and adjust equipment accordingly. When interfaced through converters, pressure, level, flow, heat, PH, and light sensors are a few of the sources that can drive the PLCs input. High/low digital outputs from other machines can also be used as information to be input to a PLC. The communication between the PLC and the external switches and sensors is dependent on the needs and cost considerations of the application. Low speed applications with few devices may only require the use of standard twisted pair wiring. Higher speeds and greater distances require the use of improved communications efficiency as in the use of coaxial cable. For large scale systems where vast numbers of machines need to be accessed and distances are greater, fibre-optics offers a viable solution. The increased cost of fibre-optics in a complex system is offset by the ease of installation and low maintenance costs. Analogue to digital converters allows PLCs to control analogue processes as well as digital. A distributed intelligence network allows PLCs to be placed at remote locations, on individual machines, or networked together to create sophisticated sortation and handling systems. The ability to connect to computers and programming devices reduces programming time and cable costs, while recent protocol standards have simplified the design process (Dunn, 2005). The internal configuration of a PLC is similar to a computer, but uses different input and output devices. The PLC has voltage sensitive inputs that are optically isolated to electrically buffer the PLC from the input devices. This information is made available to the processing unit. The CPU has RAM to store the program and other information. It may contain a ROM area to store a program, diagnostics, and machine level code to initiate and control the PLC. As the PLC runs a program, it processes the input data and makes decisions on setting the status of the outputs. The PLC may have an arithmatic logic unit to do rudimentary calculations. Registers are used by the CPU as a temporary location to store data during a calculation or possibly used to maintain data during a power failure. Flags can be employed to signal an event or maintain a warning that an event has happened. Counters and shift registers can be utilized to enhance the programming capabilities of the PLC. As the program executes, the results are placed on the output switches, a series of switches that complete an external circuit. Properly interfaced these switches can be used to control relays, solenoids, motors, valves, or alarms. To control a simple level control in a basic configuration, the input would be connected to a level control sensor. As the level fell below the desired level, the PLC would sense the change of state at the level control input. The program would instruct the CPU to activate an output switch and complete the output circuit. This switch would activate a pump that would run until the tank reached the desired level, at which time the level sensor would change the input to the PLC. The program would re-evaluate the sensor, open the output, and shut off the pump. Due to the flexibility and programmability of the PLC, inputs can be read and processed as required by the individual application. The inputs may be scanned, one at a time, and the PLC may take action based on that individual input. Often times, a simple system will require no more than a logic operation to make a decision. More complex situations may require that all, or some, of the inputs be read, stored, and analysed by the program. As the program executes, the PLC has the capability to forward output information to the output switches as it is made available in a process called continuous updating (PLCs). It may be desirable to process all of the inputs simultaneously. The PLC can read all of the inputs and store them in memory. In a process called mass I/O, the CPU executes the entire program and places the output information in the output memory (PLCs). Output states are locked during the next cycle until the output is again updated. PLCs are the backbone of manufacturing and industry. Their rapid evolution has made large scale, complex systems possible. Their continued improvement has overcome many problems associated with their use. Program latency problems and phase errors can be overcome by considering the cycle time of the PLC and the change rate of the inputs (PLCs). Input change detection circuitry and interruption schemes can further enhance the performance by monitoring inputs during program execution (PLC, 1999). The CPU can then set a flag or branch to a given instruction when it senses a change in any input. These are the qualities that have led to the popularity of today’s PLC. Works Cited Dunn, D. J. (2005). Benefits of distributing intelligence. Retrieved June 24, 2006, from http://www.cimtalk.com/news/idc/idc107.html Kuphaldt, T. R. (2003). Programmable logic controllers Chapter 6. All About Electric Circuits. Retrieved June 24, 2006, from http://www.allaboutcircuits.com/vol_4/chpt_6/6.html Morley, R. (2006). PLC Timeline. PLC dev. Retrieved June 24, 2006, from http://www.plcdev.com/plc_timeline PLC has two interrupt inputs. (1999, September 6). Electronic Times, . Retrieved June 24, 2006, from http://www.findarticles.com/p/articles/mi_m0WVI/is_1999_Sept_6/ai_55714692 PLCs - Operation and programming. (n.d.). Bell College. Retrieved June 24, 2006, from http://floti.bell.ac.uk/logiccontrollers/plcs.htm Works Consulted Dunn, D. J. (2005). Benefits of distributing intelligence. Retrieved June 24, 2006, from http://www.cimtalk.com/news/idc/idc107.html Kuphaldt, T. R. (2003). Programmable logic controllers Chapter 6. All About Electric Circuits. Retrieved June 24, 2006, from http://www.allaboutcircuits.com/vol_4/chpt_6/6.html Morley, R. (2006). PLC Timeline. PLC dev. Retrieved June 24, 2006, from http://www.plcdev.com/plc_timeline PLC has two interrupt inputs. (1999, September 6). Electronic Times, . Retrieved June 24, 2006, from http://www.findarticles.com/p/articles/mi_m0WVI/is_1999_Sept_6/ai_55714692 PLCs - Operation and programming. (n.d.). Bell College. Retrieved June 24, 2006, from http://floti.bell.ac.uk/logiccontrollers/plcs.htm Programmable Logic Controllers Tutorial Outcome 1. (n.d.). Freestudy. Retrieved June 24, 2006, from http://www.freestudy.co.uk/plc/outcome1.pdf Read More