PLC : Introduction A programmable logic controller, which is the usually called a PLC or programmable controllers is solid- state, digital, industrial computer. Upon first glance, a programmable controller may seen to be no more than black box with wires bringing signals in and other wires sending signals out. It might also appear there some magic being done inside that somehow decides when field devices should be turned on. In actuality, there is no magic. PLC is a computer and someone had to tell it what to do. The PLC knows what to do through a program that was developed and then entered into its memory. The PLC is a computer, however without a set of instructions telling it what to do, it nothing more than a box full of electronic components. Without instructions, the black box that we call a PLC can do nothing. The user program is the list of instructions that tells the PLC what to do. 1.1 What is a PLC? The PLC can be classified as a solid state member of the computer family. A programmable controller is an industrial computer in which control devices such a limit switches, push buttons proximity or photoelectric sensors. Float switches or pressure switches to name a few, provide incoming control signals into the unit. An incoming control signals is called an input. Inputs interact with instructions specified in the under ladder program which tells the PLC how to react to the incoming signals. The user program also directs the PLC on how to control field device like motor starters, pilot lights, and solenoids. A signal going out of the PLC to control a field device is called an output. Figure 1 gives an overview of the interaction between the system inputs, PLC and its ladder program, and the pilot light output. A formal definition of a PLC comes from the National Electrical Manufacturers Association ( NMEA) : A programmable controller is a digitally operated electronic system designed for use in an industrial environment, which uses a programmable memory for the internal storage of use-orientation instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, though digital or analog inputs and outputs, various types of machines or processor.
1.2 Features The PLC is the hardened industrial computer. A PLC is made so that it can survive in the manufacturing environment. Notebook and desktop personal computers are not designed for continuous use in the manufacturing environment. When a computer needs to reside in the harsh manufacturing environment, an industrially hardened computer that is designed to withstand stress is the correct choice. Industrial computers are designed to withstand the dirt, shock, vibration, high temperatures and wash downs found in the factory environment by incorporating the following features:1. PLC is small, easy to install units. A PLC is easy to install because all inputs and outputs connections are connected to terminals strips in a central location. 2. Air filters on intake fans. 3. Fans with ball bearing. 4. National electrical manufacturers Association (NMEA) 12,4 and 4X enclosure rating. 5. Shock mounted hard drives. 6. 0 to 55 degree centigrade compared to 0 to 40 degree centigrade operating environment. 7. Industrial computers are modular, which results in faster and easier repair. 8. Industrial computers do not need to be placed in a fan- cooled or air- conditioned enclosure with a viewing window. 9. Easier troubleshooting.
10. Total Response time = Input response + Programme execution time + output response time. 1.3 History of PLC Early machines were controlled by mechanical means using cams gears, levers and other basic mechanical devices. As the complexity grew, so did the need for a more sophisticated control system. These elements were wired as required to provide the control logic necessary for the particular type of machine operation. Relay and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to provide for the new control scheme. This modification was difficult and time consuming to design and install and any small bug in the design could be a major problem to correct since it also required rewiring of the system. A new means to modify control circuitry was needed. The Development and testing ground for this new means was U.S auto Industry. The time period was the late 1960s and early 1970s and the result was the PLC. Automotive plants were confronted with a change of manufacturing techniques every time a model changed and, in some cases, for changes on the same model if improvements had to be made during the model year. The PLC provided an easy way to reprogram the wiring rather than actually rewiring the control system. The PLC that was developed during this time was not very easy to program. The language was cumbersome to write, requiring highly trained programmers. These early devices were merely relay replacements and could do very little else. Older PLCs were capable of only handling discrete inputs and outputs (i.e. ON-OFF type signals). While todays systems can accept and generate analog voltages and currents as well as wide range of voltage levels and pulsed signals. Unlike their personal computer cousin, they can typically withstand the vibration, shock, elevated temperature, and electrical noise to which manufacturing equipment is exposed. As more manufactures become involved in PLC production and developments, and PLC capabilities expand, the programming language is also expanding. Also, manufacturers tend to develop their own versions of ladder logic language. Comparison PLC 1. A plc is a type of MICROPROCESSOR 1. A microprocessor is the MICROCONTROLLLER 1. A microcontroller is a special
computer designed specifically for industrial applications. 1. All PLCs contain microprocessors.
Central Processing Unit (CPU) of a computer.
purpose computer system, usually programmed to perform a single task.
2. Not all microprocessors are used in PLCs.
2. Microcontrollers have microprocessors as a part of their system hardware.
2. A PLC can be stated as a 3. A microprocessor is only complete computer in itself with a microprocessor. one component of an electronic device and requires additional circuits, memories etc. before it can function.
3.A microcontroller is a small taskspecific computer. They contain microprocessors on-board to handle logic and instruction processing but they still must have all the essential elements of any computer system.
4. A PLC can be programmed or reprogrammed to control different types of devices using Ladder Logic et c. 6. Applications include automation and process based industries etc.
5. A general computer or microprocessor system can be tasked with a wide variety of jobs.
4. Usually a microcontroller is a programmed for a specific task and left alone to do it without further human input.
7. Applications include computers, telephone industry home appliances such as microwave.
5.Applications include monitoring water level in a tank etc.
Hardware Components: There are two fundamental uses of switches. First, switches are used for operator to send instructions to the control circuit. Second, switches may be installed on the moving parts of a machine to provide automatic feedback to the control system.Push Button: The most common switch is the push button.
It is also the one that needs the least description because it is widely used in an automotive.
The momentary push button switch is activated when the button is pressed and deactivated when the button is released. The deactivation is done using an internal spring. The maintained push buttons switch activates when press, but remains activated when it is released. To deactivate it, it must be pressed a second time. For this reason, this type of switch is sometimes called a push- push switch. The contacts on switches can be of two types: There are normally open (N/O) and normally closed (N/C). Whenever, a switch is in its deactivated position, the N/O contacts will be open ( non- conducting ) and the N/C contacts will be closed ( conducting ) shows the schematic symbols for (a) a normally open push buttons and (b) a normally closed ( pushButtons. There is no internal electrical connection between different contact pairs on the same switch. Most industrial switches can have extra contacts piggy backed on the switch, so as many contacts as needed of either type can be added by the designer.
Selector Switches: A selector switch is also known as a rotary switch. An automobile ignition
switch and an oscilloscopes vertical gain and horizontal time base switches are examples of selector switches. Selector switches use the same symbol as a momentary pushbutton, except a lever is added to the top of the actuator as shown. The switch is open when the selector is turned to the left and closed when turned to the right. The switch has two sets of contacts. The top contacts are closed when the switch selector is turned to the left position and open when the selector is turned to right. The bottom set of contacts works exactly opposite. There is no electrical connection between the top and bottom pairs of contacts. For the switch, the control panel would be labeled with the STOP position to the left and the RUN position to the right.Metal Detector Switch: It is another hardware component that works same as that of push-button
being sensitive to metal. It gets turned on when it gets into contact with metal and gets turned off when the metal contact is removed.
MANUFACTURE AND CLASSIFICATIONManufacturer PLC Memory I/D Count Communications options General Electric VersaMax Nano 2K 6 input 4 Output Siemens Simatic S7- 200, 4 K Program 2 K CPU 221 Mitsubishi FXO data 1.6K 6 input 4 Output Up to 16 Serial, Profibusinputs DP,CC-