Jawaharlal Nehru Technological University Hyderabad B.Tech IV INDUSTRIAL AUTOMATION Previous Question Paper (September - 2021 - R16)

A Programmable Logic Controller (PLC) is a specialized industrial computer designed to control manufacturing processes. Its architecture consists of several key modules that work together to execute control logic based on real-time inputs.

The primary components of the PLC architecture are:

• Central Processing Unit (CPU): The brain of the PLC. It executes the control program stored in memory, performs logical and arithmetic operations, and manages communication between other modules.
• Memory Unit: Stores the operating system, the user-written control program, and data tables (status of inputs/outputs, timers, and counters). It typically includes both ROM for system software and RAM/EEPROM for user programs.
• Input Module: Acts as an interface between field devices (sensors, switches, pushbuttons) and the CPU. It converts high-voltage or varying industrial signals into low-voltage logic levels compatible with the processor.
• Output Module: Acts as an interface between the CPU and field devices (actuators, motors, solenoids, relays). It converts the CPU's low-voltage control signals into the power levels required to drive industrial hardware.
• Power Supply Unit: Converts the incoming AC line voltage (e.g., 120V/230V AC) into the stable DC voltages (typically 5V, 12V, or 24V) required by the CPU, memory, and I/O modules.
• Programming Device: An external interface, such as a PC or handheld terminal, used to write, edit, and download the control program into the PLC memory.
• Communication Interface: Allows the PLC to exchange data with other PLCs, HMIs (Human-Machine Interfaces), or SCADA systems via industrial protocols like Modbus, Profibus, or Ethernet.

The operation follows a continuous cycle known as the scan cycle: reading the status of all inputs, executing the user program logic, updating the status of all outputs, and performing internal diagnostics.

Logic and sequence control are discrete control methods based on Boolean logic and state-based transitions, whereas analog control is continuous, relying on proportional, integral, and derivative (PID) actions to maintain variables at setpoints. In logic control, the system operates on binary states (ON/OFF), making it ideal for sequential operations like assembly lines, whereas analog control manages continuous variables such as temperature, pressure, or flow rate.

Comparison Table:

Merits and Demerits of Logic and Sequence Control:

Merits:

• High reliability and immunity to electrical noise due to discrete signal levels.
• Ease of programming and troubleshooting using ladder logic or state transition diagrams.
• Cost-effective for large-scale discrete manufacturing and batch processing.
• Flexibility to modify sequence logic without changing physical hardware wiring.

Demerits:

• Incapable of precise regulation of continuous variables without hybrid integration.
• Limited resolution; cannot handle fine-tuned adjustments between discrete states.
• Sequential logic can become difficult to manage if the state space grows excessively large.

Merits and Demerits of Analog Control:

Merits:

• Provides smooth, precise regulation of process variables.
• Essential for maintaining stability in dynamic systems through feedback loops.
• Allows for sophisticated control strategies like feed-forward and cascade control.

Demerits:

• Susceptible to signal drift and electromagnetic interference (EMI).
• Requires complex calibration and maintenance of sensors and actuators.
• Higher implementation cost due to the need for precision instrumentation and signal conditioning.