Data Acquisition System MCQ Quiz in मल्याळम - Objective Question with Answer for Data Acquisition System - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Designing an efficient Programmable Logic Controller:

PLCs are classified into following the main categories based on the number of input and outputs they can handle:

Nano PLC: PLC that has less than 32 input/output (I/O) functions.

Micro PLC: PLC that has more than 32 Input/Output (I/O) points but not more than 128 input/output (I/O) functions.

Small PLC:

• PLC that has more than 128 Input and Output (I/O) points but not more than 256 I/O functions.
• It does not include any I/O enhancements fitted along with the basic system.
• Hence according to options, the maximum number of inputs and outputs that are there in a small PLC is 100.
   

Medium and Large PLC: PLC that controls a large number (>256) of discrete elements using very fast input/output (I/O) scan times. 

Concept:

PLC (Programmable logic controller):

It is a computer designed to be used in the industry.

It controls the different processes and is programmed according to the operational requirement of that process.

Block diagram of PLC:

Basic parts of the PLC:

• CPU (central processing unit)
• Programming device
• Input and output module

Central processing unit:

• CPU is the heart of the PLC system. It is a microprocessor-based control system.
• CPU reads input data from various sensing devices, executes the user program from memory, and sends appropriate output commands to control devices.
• A DC power source is required to produce the low-level voltage used by the processor and I/O modules.
• Subparts of CPU are:
  • Processor
  • Power supply
  • memory

Explanation:

• PLCs operate by continually scanning programs and repeat this process many times per second. When a PLC starts, it runs checks on the hardware and software for faults, also called a self-test. If there are no problems, then the PLC will start the scan cycle. The scan cycle consists of three steps: input scan, executing program(s), and output scan.
• Input Scan: A simple way of looking at this is the PLC takes a snapshot of the inputs and solves the logic. The PLC looks at each input card to determine if it is ON or OFF and saves this information in a data table for use in the next step. This makes the process faster and avoids cases where an input changes from the start to the end of the program.
• Execute Program (or Logic Execution): The PLC executes a program one instruction at a time using only the memory copy of the inputs the ladder logic program. For example, the program has the first input as ON. Since the PLC knows which inputs are ON/OFF from the previous step, it will be able to decide whether the first output should be turned ON.
• Output Scan (Write): When the ladder scan completes, the outputs are updated using the temporary values in memory. The PLC updates the status of the outputs based on which inputs were ON during the first step and the results of executing a program during the second step. The PLC now restarts the process by starting a self-check for faults.

Ladder Logic Program

• Ladder Diagram (LD) logic is a graphical programming language.
• Initially programmed with simple contacts that simulated the opening and closing of relays, but later it has been expanded to include such functions as counters, timers, shift registers, and math operations.
• A timer uses the frequency of the internal clock and generates delay.
• A counter uses an external signal to count pulses.
• The register is used to store data.
• Math operations include addition, subtraction, multiplying, and dividing to calculate the sum, difference, product, and quotient of the content of word registers.

Components of SCADA:

A basic SCADA system consists of the following components:

• Human Machine Interface
• Supervisory System
• Remote Terminal Units (RTU)
• Programmable Logic Controllers (PLCs)
• Communication Infrastructure
• SCADA Programming

Remote Terminal Unit (RTU):

It is a standalone data acquisition and control unit, generally microprocessor-based, which monitors and controls equipment at some remote location from the central station.

It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station.

Small-sized RTUs generally have less than 10 to 20 analog and digital signals.

Medium-sized RTUs have 100 digital and 30 to 40 analog inputs.

RTUs, having a capacity greater than this can be classified as large.

Block diagram of RTU:

Components of RTU:

• CPU and associated memory
• Analog output
• Digital input
• Communication interface
• Power Supply
• RTU rack
• Encloser

Ladder logic AND function:

• The AND function has multiple PLC inputs and has one resulting output.
• If we translate an AND function into a ladder diagram, we can express it symbolically in the form of two normally open (NO) contacts (PLC inputs A and B) and a relay coil (PLC output Y) in series.

 It is similar to logic expression

• If ‘A’ and ‘B’ then ‘Y’.

Here the ‘AND’ function examines if all the PLC inputs are TRUE, then the corresponding result is TRUE, however, if anyone of the PLC inputs is FALSE then the corresponding result is also FALSE.

PLC inputs A and B follows the binary concept and is part of the AND function there are four possible logic iterations.

The truth table of AND function is given below:

Note:

The number of logic iterations increases with an increase in the number of PLC inputs (2^{PLC inputs}). But it doesn’t matter too much with the AND function because the result can be TRUE only and only if all the PLC inputs are TRUE.

Ladder logic OR function:

If we translate on OR function into a ladder diagram we can express it symbolically in the form of two normally open (NO) contacts (PLC inputs A and B) and a relay coil (PLC output Y) in parallel.

The truth table of the OR function is given below:

Input of PLC:

• The two types of PLC inputs are commonly referred to as DI and AI (Digital and Analog).
• Analog Inputs include temperature sensors/transmitters, current sensors, voltage sensors, and others that can convert a physical quantity to an electrical signal.
• These electrical signals used for PLC input are typically 4-20 mA or 1-5 V DC.
• Digital Inputs include push buttons, limit switches, relay contacts, proximity switches, photo sensors (On/Off), pressure switches, and more. Digital inputs devices are available in both DC as well as AC and some are voltage-independent such as a switch contact.

PLC and SCADA:

• The primary difference between a PLC (Programmable Logic Controller) and SCADA (Supervisory Control and Data Acquisition) is that a PLC is hardware and SCADA is software.
• PLCs are designed to control complex industrial processes, such as controlling electrical machines.
• They are simple to program and fully scalable to an operation’s requirements.
• They’re also used to collect data from the systems they control.
• SCADA is a central system used to monitor and run the processes.
• SCADA is typically software installed on a computer, and one of its major functions is to act as an interface with industrial machines (or Human-Machine Interface, or HMI).
• SCADA systems are often used in conjunction with PLCs and other devices.
• Data from PLCs and Remote Terminal Units (RTUs) are relayed to the system, and commands are entered into the HMI to make adjustments to the processes they control.

The term PLC stands for the Programmable Logic Controller.

PLC is known as a ‘hard real-time system’.

The mass advantage of PLL over ‘hard-wire’ type relay control systems, is that PLC land themselves is functional changes very easily.

Mass components of PLC are:

• Rock or chassis
• Power supply
• Central Processing Unit (CPU)
• Communication modules
• Input and outputs

Modes of operation of PLC: There are two modes of operation:

• Program mode
• RUN mode

Program Mode: 

It is used to enter a new program, edit or update the existing program, upload files, and download files. Also, in this mode of operation, all outputs are de-energized.

Run Mode: 

It is used to execute the user program. Remote PLC programming or mode selection is dissolved when the keys are in the Run mode position. In this mode, the output is energized. It does not allow program entry editing in this mode of operation of PLC.