Programmable Logic Controllers (PLCs)
Today’s modern era is an era of Automation. Automation is right from Mega Industries to Homes. Robots, Self-driving Cars , Home automation systems are everywhere. Lot of repetitive and complex tasks are done by Robots. In Industry due to Automation energy and raw materials are used efficiently, also the productivity and safety is has also increased. All of this has been possible because of PLC (Programmable Logic Controller) and SCADA(Supervisory Control And Data Acquisition).
In this Blog History, Major components, Scan cycle, some advantages , disadvantages , and basic instructions of PLC are covered.
The First programmable logic controller was specified by General Motors Crop Hydromantic division in 1968. The name for the first models was coined. The second models came in 1970 and the third model came in 1980, Standardization on protocol communication, 1990 Standardization on programming language.
The main Heart of any Automation system is PLC. PLC is an industrial computer that monitors inputs, makes decisions based on its program and controls outputs to automate a process or machine. A PLC has built-in operating system(OS). This OS is highly specialized to handle real-time incoming events. It is digital computer designed handle multiple input and output arrangements, immunity to electrical noise, extended temperature ranges and resists vibration and impact. PLC has input lines where sensors are connected to notify events(e.g. water level reached) and output lines to signal any reaction to the incoming events(e.g. open/close the valve). A Programmable memory is used by PLC to store the instructions and specific functions which include ON/OFF control, sequencing, timing counting, arithmetic and data handling.
Major Components of PLC:
Input modules accepts and converts signals from sensors are converted to Logic signal, e.g. switches, pushbuttons. As per the programming of the PLC, various types of inputs may be supplied to the PLC. The PLC input can also be split into analogue and digital classifications.
Output modules convert control instructions to a signal that can be used by actuators, e.g. lamps, LEDs, alarm. Outputs from the PLC at the receiving end are received in various ways. Depending on the input form, the output may be either analogue or digital.
The Central Processing Unit (CPU) is the brain of PLC and controls all the activities of entire PLC system. The Scan Cycle for PLC is performed by the CPU. It also consists an Arithmetic Logic Unit(ALU), Internal memory, Internal timers, counters and flags.
The Power supply provides the essential voltage required by the primary PLC components. Various types of power supplies are used for various types of PLCs. The different sources of power used in the PLC range from 24V DC to 120V AC.
The programming device or unit is a device which is used to program a PLC. Each PLC has its own programming software and code. The PLC is programmed with the help of programming unit by the programmer. The various logics for the PLC are created by the programmer and then applied to various units.
Scan time and Scan cycle of PLC:
Each PLC has a Scan time and Scan cycle. The scan cycle is the cycle that collects the inputs from the PLC, runs your PLC software, and then updates the outputs. This would take a certain amount of time, often measured in milliseconds or ms. The scan time of the PLC is called the amount of time it takes for the PLC to produce one scan cycle. Knowing about the PLC scan time and understanding how long the scan duration is, is important. Especially if you work with timers and delays for PLCs. The precision of the timers will also depend on the duration of the scan cycle. The following figure describes the steps in scan cycle of PLC.
While the PLC is running, the scanning process includes the four phases, which are repeated continuously as individual cycles of operation:
Input Scan: It is the first step. The states of inputs are scanned in this step.
Program Execution: In this step all the processes and program logic is executed.
Housekeeping: This step includes communications, Internal Diagnostics, etc.
Output Scan: This is the last step, in this step it is decided whether to energize/de-energize the outputs.
Programming in PLC:
A traditional programming language which mimics circuit diagrams is usually used by PLCs; this is called ladder logic. Programming languages such as Function Block Diagram (FBD), Instruction List (IL), Sequential Function Map (SFC), or Structured Text may be used by other PLCs (ST). The IEC 61131–3 standard describes these languages. PLCs are programmed using a PC or Laptop. RS Logix 500 is one of the IDE used for programming of PLC is shown below:
Math Instructions: The table below shows the Math instructions and their purpose in PLC.
Applications of PLC: PLC has endless applications in Automation. Some of the applications are listed below:
Robot manufacturing and control.
Process Automation plants and Boilers in Thermal power plants.
Automobile industry and Machine tools.
Glass and Paper industry.
Road Traffic signal, Conveyer belts and Automatic Doors
Automatic car wash and Self-driving Cars.
Advantages: PLCs have lot of advantages. PLCs offer high operation speed, high reliability and high accuracy.. PLCs provide flexibility in programming and reprogramming. PLCs are easier to install and troubleshoot. PLCs are extremely compact in comparison to hard-wired controller where electro-mechanical devices are used, hence they use less space. Average power consumption in PLCs is just 1/10th of power consumed by an equivalent relay based control, which is why PLCs are also energy saving. PLCs have remote control capability and can communicate with PCs and laptops. Complex systems can be easily handled with PLCs.
Disadvantages: Although PLCs have lot of applications and benefits, they do have some disadvantages. Initial cost of PLC is very high. Programming of PLCs is not an easy tasks, skilled technicians are required which can handle both the hardware and software of PLC. PLC derives are proprietary means that part or software of one manufacturer cannot be used in combination with parts of another manufacturer. Lot of work is required in connecting wires and error detection in PLC is a challenging task.
In the end it can be concluded that Programmable Logic Controllers have endless industrial and Daily Life applications. They have lot of advantages in the field of Automation and have really transformed the Era of Automation.
