Basics of programmable logic controllers

Answer 1

PLCs were invented in the 60/70's for the automotive manufacturing industry. Since this time, they have developed into one of the most versatile tools used for industrial automation. A working knowledge of PLCs and other microprocessor based control systems are critical to technical personnel who are staying current with technology in industry. Programmable Logic Controllersor PLC are the hub of many manufacturing processes. These microprocessor based units are used in processes as simple as boxing machines or bagging equipment to controlling and tracking sophisticated manufacturing processes. They are in virtually all new manufacturing, processing and packing equipment in one form or another. Because of their popularity in industry, it becomes increasingly more important to learn skills related to these devices. Click on the buttons to learn more about industrial automation and this invaluable tool. The microprocessor or processor module is the brain of a PLC system. It consists of the microprocessor, memory

integrated circuits, and circuits necessary to store and retrieve information from memory. It also includes communications ports to other peripherals, other PLC's or programming terminals. Today's processors vary widely in their capabilities to control real world devices. Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more. One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process. The number of inputs and outputs PLCs can control are limited by the overall capacity of the PLC system hardware and memory capabilities. The job of the processor is to monitor status or state of input devices, scan and solve the logic of a user program, and control on or off state of output devices.

RAM or Random Access Memory is a volatile memory that would lose it's information if power were removed..

This is why some processor units incorporate a battery back up. The type of RAM normally used is CMOS or

Complementary Metal Oxide Semiconductor. CMOS RAM is used for storage of the user's program (ladder logic diagrams) and storage memory.

ROM or Read Only Memory is a non-volatile type of memory. This means you don't need an external power source to keep information. In this type of memory, information can be read, but not changed. For this reason the manufacture sometimes calls this firmware. It is placed there for the internal use and operation of processor units EEPROM or Electrically Erasable Programmable Read Only Memory is usually an add-on memory module that is used to back up the main program in CMOS RAM of the processor. In many cases, the processor can be programmed to load the EEPOM's program to RAM if RAM is lost or corrupted.

Input Module

There are many types of input modules to choose from. The type of input module used is dependent upon what real world input to the PLC is desired. Some examples of inputs are limit switches, electric eyes, and pushbuttons. DC inputs, such as thumbwheel switches, can be used to enter integer values to be manipulated by the PLC. DC input cards are used for this application. Since most industrial power systems are inherently noisy, electrical isolation is provided between the input and the processor. Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can cause severe problems in most solid state control systems. The component used most often to provide electrical isolation within I/O cards is called an optical isolator or optocoupler. The wiring of an input is not complex. The object is to get a voltage at a particular point on the card. Typically there are 8 to 32 input points on any one input module. Each point will be assigned a unique address by the processor. Analog input modules are special input cards that use analog to digital conversion (A to D) to sense variables such as temperature, speed, pressure, and position. The external device normally is connected to a controller (transducer) producing an electrical signal the analog input card can interpret. This signal is usually 4 to 20 Ma or a 0 to 10 volt signal.

Output Module

Output modules can be for used for ac or dc devices such as solenoids, relays, contractors, pilot lamps, and LED readouts. Output cards usually have from 6 to 32 output points on a single module. The output device within the card provides the

connection from the user power supply to the load. Usually silicon controlled rectifiers (SCR), triac, or dry contact relays are use for this purpose. Individual outputs are rated most often at 2 to 3 amperes. Output cards, like input cards have electrical

isolation between the load being connected and the PLC. Analog output cards are a special type of output modules that use digital to analog conversion (D to A). The analog output module can take a value stored in a 12 bit file and convert it to an analog signal. Normally this signal is 0 -10 volts dc or 4 to 20 Ma. This analog signal is often used in equipment such as motor operated valves and pneumatic position control devices PLC programs are made up of a combination of the above "gates" together with inputs, outputs, timers, counters, internal memory bits, analog inputs, analog outputs, mathematical calculations, comparators etc. We will now look closer at some of these and the operation of the PLC. INPUTS These are the physical connections from the real world to the PLC. They can be limit switches, push buttons, and sensors, anything that can "switch" a signal on or off. The voltage of these devices are usually, but not always, 24 Volt DC. Manufacturers make inputs that can accept a wide range of voltages both ac and dc. It should be remembered that an input will be ON, "status 1", when the voltage is present at the input connection and OFF, "status 0", when the voltage is no longer present at the input connection. OUTPUTS These are the connections from the PLC to the real world. They are used to switch solenoids, lamps, contactors etc on and off. Again they are usually 24 Volt DC, either relay or transistor, but can also be 115/220 Volt AC

INTERNAL MEMORY BITS (RELAYS)These so called relays only exist in the PLCs memory, they are used to remember certain "states" or functions that have been performed. They react exactly like ordinary relays and can be on, off or latching. Manufacturers of PLCs also pre-program certain internal relays with special functions such as, always on; always off; on whilst the PLC is powering up; on for the first scan of the program; on if there is an error. For details of other pre-programmed relays you will have to check the manual for the PLC you are going to use. THE PLC Now that we have a basic understanding of the workings of a PLC and some of the numbering systems used we can look at the operation of PLCs.

A PLC operates by continually scanning the program and acting upon the instructions, one at a time, to switch on or off the various outputs. In order to do this the PLC first "looks" at all the inputs and remembers their states ( i.e.. "1" or "0" / on or off) this information is then stored in memory. The PLC then scans the program instructions and decides if an output should be on or off BUT, and this is important to remember, the physical output is NOT turned on immediately. When the PLC has finished scanning the programmed instructions it will, finally, turn on the required outputs. This is called updating the process output image.

At this point the PLC then checks its own operating system and if everything is ok it then goes and checks the states of the inputs and starts all over again.

This is program scanning and the delay is called the program scan time.

From the above statement it should be seen that there is a delay from when an input is turned on; the program scanned and then a physical output being turned on or off. As the delay is only mS it is not usually a problem unless you are using the PLC in a high speed process. However it should be remembered that in a PLC with many thousand I/O an input could turn on and then off before the PLC has finished scanning the programmed instructions.

This could result in an output coming on for one or two scans when in fact it should be off. COMMUNICATIONS

There are several methods to communicate between a PLC and a programmer or even between two PLCs.

Communications between a PLC and a programmer (PC or Hand held) are provided by the makers and you only have to plug in a cable from your PC to the programming port on the PLC. This communication can be RS232; RS485 or TTY.

Communications between two PLCs can be carried out by dedicated links supplied/programmed by the makers (RS232 etc) or via outputs from one PLC to the inputs on another PLC.

This direct link method of communication can be as simple as, if an output on the first PLC is on then the corresponding input on the second PLC will be on and then this input is used within the program on the second PLC.

If a word of input/outputs (16 bits) are used then numerical data can be transferred from one PLC to the other (refer back to the section on numbering systems).

There are many other methods of communication between PLCs and also from PLC to PC. Please refer to the manuals supplied with the PLC that you are using for full details on communications

Answer 2

Programmable logic controllers (PLC) are microprocessors attached to sensors and actuators to control many parts of automated systems. They are programed to follow steps to complete its task.

Answer 3

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