Programmable Logic Controllers - Case Study Example

Programmable Logic Controllers (PLC) Programmable Logic Controllers (PLC) Styles of PLCs Unitary Modular Rack Mounting Unitary PLC A Unitary PLC is a complete PLC in a single case. All the inputs, outputs, main CPU are in a single package. Most of the Unitary PLCs have fixed outputs/inputs. Unitary PLCs are attached to the machine or a section of a machine having multiple logics. Mitsubishi LOGO, Siemens LOGO are unitary PLCs. Modular PLC A Modular PLC is a PLC that comes in modules for inputs and outputs. Basic structure of a modular PLC contains a main CPU and input and output modules. The main CPU contains a power supply and communication terminals that connects the modules to the main CPU and Connects the PLC to the other PLCs, computer or communication devices. Other Modules for different functions other than input/output can also be added to the PLC. The major advantage of a modular PLC is that modules can be added as per requirement of a particular machine. Modular PLCs are commonly arranged on a rack or railing that allows easy arrangement of modules. Siemens Step S& PLCs are modular PLCs. Systems can be expanded according to the need. Rack Mounting Rack Mounting PLCs are similar to the modular PLCs; however, rack mounting PLC is standardized as the particular cards that have a slot in a cabinet. Rack mounting PLCs can be expanded according to the requirement. Input voltages Typically, PLCs are designed to work with an external 24-volt power supply. However, the input voltages for operation and input relays may range from 10-30 volts DC for input relays (Commonly 12 volt and 24 volts) and 110 to 250 volts AC. Special types of PLCs may have different voltage range but mostly PLCs come in the above mentioned voltage ranges. Input Sensors Input sensors for digital (ON/OFF) operations are classified as: ON/OFF Selector Switches or Push buttons Limit Switches Reed Switches Level switches Proximity sensors Opto-Electric Sensors Relays Voltage Sensing Relays ON/OFF Selector Switches and Push Buttons Selector Switches and Push buttons open or close a circuit that power input relays or transistors of a PLC. The PLC is programmed to accepts the command and proceed with an operation as desired. Selector switches and push buttons are manually pressed by the operator to execute a PLC operation (Ab.rockwellautomation.com, 2015). Limit Switches Limit switches work on the similar principle as that of the selector switches and push buttons, but limit switches are pressed by the “machine (in a process)” or “arm of a machine”. Limit switches stops or starts a process. Like opening and closing of a machine door. A motor opens or closes the door until a limit. As the limit is approached, limit switches either open or close the circuit and send PLC command either to stop the motor or run the motor. Limit switched are either normally close (NC) or normally open (NO). NC terminals complete circuit when limit switch is in normal position and opens the circuit path when pressed. NO terminals complete circuit when switch is pressed (Bolton, 2006). Reed Switch Some sensors are fitted with the actuators to enable the total functionality of the actuator. Reed switches work like a normal switch when a magnet is placed near its vertical or horizontal axis. A non-magnetic cylinder holds the magnet. In the image below, a piston holds a magnet in an aluminium casing. While reed switched are just above the casing. Reed switched work as the RPM counter or may work in any other desired application. Level Switches Level Switches control the level of liquid in a tank and process regarding the level probes. Level switch is a set of probes that send a signal to on and off a relay. Probes are set at different desired levels. When the liquid touches a probe, it completes the circuit and switches on the relay that further process the operation (Bolton, 2006). Proximity Switches/Sensors Proximity switches/sensors are of two basic types: Proximity sensors have inbuilt transistors in them that allows and restricts the flow of current through collector and emitter terminals. A proximity sensor can be either PNP or NPN. A NPN type proximity sensor only works when positive volts are applied at gate terminal. However, PNP type proximity sensor works when negative volts are applied at gate terminal (Bolton, 2006). Inductive proximity sensors Capacitive proximity sensors Inductive proximity sensors Inductive proximity sensors work when a metal comes near its ferrite core or its head. It only sends signal when a metal comes near its head. It can be NPN or PNP. Capacitive Proximity Sensors Capacitive proximity sensors work when non-metallic materials i.e. plastic, rubber; etc comes near its head. The non-metallic materials function as dielectric. Capacity proximity sensors can either be NPN or PNP. Opto-Electric Sensors Opto Electric sensors or detective switches ON or OFF when a either a light beam hits an object or does not hit an object. In most of these types of sensors, a constant beam of infrared is maintained from an emitter that emits constant bean of infrared light. A receiver accepts the constant beam of light. If an object comes in between the transmitter and receiver, receiver switches either ON or OFF a relay or a transistor. Most of these types of sensors are attached to the conveyer belts to count the number of items leaving or entering the belt (Josifovska, 2004). Voltage Sensing Relays Voltage sensing relays switch ON or OFF when a certain voltage level is achieved. Voltage sensing relays come in different ranges according to the requirement. Voltage sensing relays are mostly used with analogue devices like motors. DC tachometer or a multi-turn variable POT can be connected to the motors to distinguish its desired speed. Output voltages Output switching voltages are limited to 10-30 volts DC for relay as well as transistor output terminals. While, relay output terminals can switch 110 to 250 volts AC. In order to pass more than rated voltages output terminals can be coupled with high voltage relays to deliver more than desired volts. Like in order to operate a motor with rated 380 volts and 9 amperes, a magnetic contactor is coupled with the output terminal of the PLC to execute the operation. In some cases, external transistors and thyristors are also coupled with the out put terminals for higher current surge (Josifovska, 2004). Output devices PLC switches ON and switch off the output devices. These devices may include: DC motors AC motors Linear Actuators Solenoid Valves Lights and Alarms Heating elements In most of the systems, PLCs are not directly connected with the motor switching. PLCs switch the variable frequency derives (AC motor drives) that process the command and run the motor according to the desired conditions. In case if there is no variable frequency drive (AC motor drive), a magnetic contactor is coupled with the output terminal of the PLC to run the motor. In the similar manner, DC drives are either switched or off to run the DC motors used for industrial process. Solenoid open and shut the valve. Solenoids are used to open valve for compressed air, steam and hydraulic fluids. Solenoids works either on normally open (NO) function or normally close (NC) function. When path is incomplete NO passes current while NC solenoids remain on until PLC output terminal completes the circuit. Opto-Isolators PLCs use the opto-isolators to isolate the input or output physical circuit and the PLCs’ internal circuitry. If any type of fault occurs at the input or output, the internal circuitry of the PLC remains unharmed. Opto-isolators use a light emitting diode (LED) and a phototransistor that closes the circuit path when the base terminal receives a signal from the LED. Opto-isolators are also known as opto couplers and these come in a single package (Ridley and Ridley, 2004). When the LED receives an input, it emits lights. When the light strikes the base terminal of the phototransistor, it allows the flow of current from collector to emitter terminals. In this way, both the circuits remain isolated from one another. On the other hand, the switching speed of an opto coupler is much higher as compared to that of relays (Ridley and Ridley, 2004). PLC is more like a computer. It has similar functional blocks as that of a computer. It has the following parts: CPU: central processing unit is the main processing IC of the PLC. It processes the information from inputs and develops required output according to the program and firmware. ALU: one part of the CPU is the arithmetic logical functions (ALU). It processes the information and develops logical arithmetic functions according to the installed firmware and produce output as per the logical operations. ROM: Read only Memory is the permanent storage of the program or the ladder program. When the PLC is switched ON, it reads the memory from the tem memory and executes the program according to the user-desired condition. RAM: Random Access Memory temporarily stores the data. Running programs always have a data location in the RAM. While uploading the programs, the programs have to be passing through the RAM and while retrieving the programs, the retrieved programs are accessed through the RAM. Registers: Registers hold data for the processor and make a sequence of the bits that are to be processed by the processor or processed by the processor. Busses: PLC CPU has two busses. Physical system bus connects and holds the inputs and outputs to the system input and output unit. However, data bus connects the RAM, ROM and CPU. Address bus gives address to the various inputs and outputs, RAM and ROM. The processor assigns address. Control bus is the sequential control of bits from input and output unit, RAM and ROM. Processor controls the control bus. Flag is a defined bit or a sequence of bits. It contains a binary value. It produces a differentiation between two different programs or bit sequences (Ridley and Ridley, 2004). PLC Communication Digital information from PLC to PLC to from PLC to computer or from computer to PLC is communicated with the use of: Twisted Pair Coaxial Cable Fibre Optics Wireless Networks Twisted Pair Communication Twisted Pair Communication is a simple type of communication. In this type of communication, two wires with opposite signals are twisted with each other. The twisting of the wires reduces the crosstalk. However, the interference increases if the distance of the wire is more than 100 metres. The major disadvantage of the twisted pair wire is that, the wire attains noise from a noise source. If one twisted pair has an attenuated signal, it disturbs the other twisted pair cables. The maximum data band from twisted pair is about 1000 MHz. Coaxial Cable Coaxial cable is not commonly used for PLC communication. It is most used fro the Cable TV communication due to its high transmission band, high impedance that does not allow signal distortion, low attenuation and distortion due to braided ground wire. The conductor for the communication is in the centre of the wire. Insulation protects the signal leakage to ground terminal. Coaxial cable is also used for computer communication. The major disadvantage of the Coaxial cable is the signal leakage, and higher radio frequency interference due to the other electromagnetic interfering devices nearby. Fibre optics Fibre optics is the most advanced mode of communication. In this communication system, a converter converts the electromagnetic signals into light waves. Fibre optics is single transparent threads that pass the light waves from one end of the wire to the other end of the wire. Fibre optic communication is fast, has low interference to electromagnetic radiations and high data rates. There is little loss in signal even at great distances. The major disadvantage is that the cable requires extreme care and may break at corners. Wireless communication is used in some types of the PLC for the communication between the PLCs and the variable frequency drives. “Lenze” frequency drives come with the optic fibre communication kit. Wireless networks Wireless networks are also used for the communication. Wireless LAN or Wi-Fi is the advanced technology that is capable of communication at high data rates. It has low disturbance but the communication is limited to small distances. On the other hand, microwave communication is highly precise, capable of working at high data rates and has low distortion. With increase in distance, the signal strength weakens. Convert the following: i. to binary 2 207 2 103 1 2 51 1 2 25 1 2 12 1 2 6 0 2 3 0 2 1 1 ii. into Hexdecimal 16 241 240 15 1 iii. Convert to haxa-decimal. iv. to decimal 11 X 160 + 1 X 161 + 12 X 162 + 8 X 163 11 +16 + 3072 + 32768 35867 A sequencing system is a system that allows the PLC to follow a pattern in a sequence. A sequencing systems is a ladder diagram or a sequence of operation that a PLC flows. The block diagram depicts the sequence between various inputs and outputs. The similar sequencing is in terms of ladder diagram below. “] [“symbol shows a normally open contact while “]/[“ symbol shows a normally closed contact, timers are denoted T (MITSUBISHI:PROGRAMMABLE CONTROLLERS: MELSEC-FX: FX-Series PLC Training Manual using GX-Developer, 2015). Other programmable devices Other programmable Logic Controller devices include microcontrollers. Microcontrollers can easily be programmed and programs can be executed on demand. Programs can be changed and modified. However, microcontrollers do not come in a package. A circuit board should be manufactured for the microcontroller. A microcontroller cannot be directly programmed. It should be out of the circuit board for programming or changing program. Bibliography Ab.rockwellautomation.com, (2015). Programmable Controllers. [online] Available at: http://ab.rockwellautomation.com/Programmable-Controllers [Accessed 18 Feb. 2015]. Bolton, W. (2006). Programmable logic controllers. Amsterdam: Elsevier/Newnes. Josifovska, S. (2004). What next for the PLC?. Manufacturing Engineer, 83(4), pp.10-11. MITSUBISHI:PROGRAMMABLE CONTROLLERS: MELSEC-FX: FX-Series PLC Training Manual using GX-Developer. (2015). 1st ed. [ebook] Mitsubishi Electric. Available at: http://midas.herts.ac.uk/helpsheets/fx_training.pdf [Accessed 18 Feb. 2015]. Ridley, J. and Ridley, J. (2004). Mitsubishi FX programmable logic controllers. Oxford: Newnes. Read More