Here are the viva questions and answers for the PLC (Programmable Logic Controller)
and Conveyor Belt Control Experiment
Programmable Logic Controller (PLC) — Viva
Questions & Answers
1. What is a PLC?
A PLC (Programmable Logic Controller) is an industrial digital computer designed to
control machines and processes automatically.
It takes inputs from sensors, processes them according to a user-defined program, and
provides outputs to actuators such as motors, valves, or lights.
2. What are the advantages of using a PLC over a relay-based system?
PLC System
Relay-Based System
Compact, requires less space
Bulky wiring required
Easy to modify or reprogram
Hard to modify (requires rewiring)
Faster response time
Slower switching speed
High reliability and less maintenance More wear and tear
Easy troubleshooting using software Difficult fault tracing
In short: PLCs are more flexible, reliable, and efficient than traditional relay logic
systems.
3. What are the main components of a PLC?
1. CPU (Central Processing Unit):
Processes the logic program and controls all operations.
2. Memory:
Stores the program and data.
3. Input Module:
Receives signals from sensors, switches, or push buttons.
4. Output Module:
Sends control signals to actuators like motors, relays, or lamps.
5. Power Supply Unit:
Provides required DC power to the PLC.
6. Programming Device (HMI or PC):
Used to write and upload the control program.
4. What is a ladder logic diagram?
Ladder Logic Diagram (LLD) is the graphical programming language used in PLCs.
It resembles an electrical relay circuit diagram with:
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Vertical rails (power lines), and
Horizontal rungs (logic instructions).
Each rung represents a control operation using contacts, coils, timers, counters, etc.
5. What is the function of input and output modules?
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Input Module:
Converts physical input signals (like sensor states, switches) into digital signals the
PLC can process.
Output Module:
Converts PLC’s digital signals into physical actions (like turning ON motors, relays,
solenoids, etc.).
6. Explain the difference between “latching” and “unlatching.”
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Latching:
A latch keeps an output ON even after the input is released, until another condition
(like a reset) turns it OFF.
➤ Example: Start/Stop control of a motor — once started, it stays ON until a stop
button is pressed.
Unlatching:
An unlatch instruction is used to turn OFF a latched output when a specific
condition occurs.
➤ Example: Stop button signal or fault condition resets the motor.
7. What is a timer and a counter in PLC programming?
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Timer:
A timer delays an operation by a preset time.
o ON-delay timer (TON): Output turns ON after a delay.
o OFF-delay timer (TOFF): Output turns OFF after a delay.
Counter:
A counter counts the number of events or pulses.
o Up counter (CTU): Counts upward.
o Down counter (CTD): Counts downward.
8. What is the application of PLC in the conveyor belt experiment?
In the conveyor belt experiment, the PLC controls the movement and sequencing of the
conveyor system.
Typical functions include:
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Starting/stopping the motor automatically.
Detecting the presence of objects using sensors.
Counting or sorting items using counters.
Activating actuators at specific times or conditions.
Example:
If a sensor detects an object → PLC starts the conveyor → after a timer delay → the motor
stops or directs the object to another station.
Additional Conceptual Questions (for deeper viva
preparation)
9. What is the scan cycle in a PLC?
The scan cycle is the continuous process where the PLC:
1. Reads input data,
2. Executes the program logic,
3. Updates output states.
This cycle repeats several times per second (milliseconds).
10. What are the different types of PLC programming languages?
As per IEC 61131-3 standard:
1.
2.
3.
4.
5.
Ladder Diagram (LD)
Function Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL)
Sequential Function Chart (SFC)
11. What is the role of sensors in a PLC-controlled system?
Sensors act as input devices that detect physical parameters such as:
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Position (limit switches)
Light (photo sensors)
Temperature, pressure, etc.
They send signals to the PLC for processing.
12. What are actuators in a PLC system?
Actuators are output devices that perform mechanical actions based on PLC commands —
e.g., motors, solenoids, relays, valves.
13. What is the difference between analog and digital inputs in a PLC?
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Digital inputs: Have two states (ON/OFF) like switches or sensors.
Analog inputs: Have a continuous range of values (e.g., temperature, pressure
sensors giving 4–20 mA signals).
14. What are interlocks in PLC programming?
Interlocks are safety or logic conditions that prevent unsafe operations — for example, a
motor cannot start unless a safety door is closed.
15. What are the advantages of using timers and counters in PLC?
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Simplify process automation.
Eliminate the need for external timing relays.
Provide precise timing and counting control.
Allow easy modification via software.
16. What is the difference between PLC and microcontroller?
PLC
Used for industrial automation
Rugged and designed for harsh
environments
Microcontroller
Used for embedded control
Not suitable for industrial noise and
temperature
PLC
Programmed using ladder logic
Easy to reprogram and maintain
Microcontroller
Programmed using C, Assembly, etc.
Requires more technical programming
17. What safety measures should be taken while working with PLC systems?
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Ensure proper grounding and insulation.
Use correct input voltage levels.
Avoid touching terminals when powered.
Always simulate and test the program before applying it to machinery.
Would you like me to add a PLC summary sheet (with block diagram, ladder logic symbols,
timer/counter types, and typical conveyor program flow) for quick revision before viva?
Excellent — you’re preparing very systematically
Here are detailed Viva Questions and Answers for both Hydraulic System and Pneumatic
Trainer System — written in clear, exam-ready form
HYDRAULIC SYSTEM — Viva Questions &
Answers
1. What is a hydraulic system?
A hydraulic system is a power transmission system that uses pressurized fluid (usually oil)
to transmit power and perform mechanical work, such as lifting, pressing, or moving heavy
loads.
2. What is Pascal’s Law?
Pascal’s Law states that “Pressure applied at any point of a confined fluid is transmitted
equally and undiminished in all directions.”
Mathematically:
( P = \frac{F}{A} )
Where:
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( P ) = Pressure
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( F ) = Force
( A ) = Area
Application: Used in hydraulic presses, jacks, and brakes.
3. Name the main components of a hydraulic system.
1.
2.
3.
4.
5.
6.
7.
Reservoir (Tank): Stores the hydraulic fluid.
Pump: Converts mechanical energy into hydraulic energy (pressure).
Pressure Relief Valve: Maintains system pressure within safe limits.
Directional Control Valve (DCV): Controls fluid flow direction.
Actuator (Cylinder or Motor): Converts fluid energy back to mechanical work.
Pipes and Hoses: Carry the fluid between components.
Filter: Removes contaminants from the fluid.
4. What is the function of the relief valve?
The relief valve:
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Protects the hydraulic system from overpressure,
Opens automatically when pressure exceeds a set limit,
Allows excess fluid to return to the tank, ensuring safety.
5. What are the advantages of hydraulic actuation?
Advantages:
1.
2.
3.
4.
5.
High force-to-weight ratio — large forces with small components.
Smooth and precise control of speed and position.
Self-lubricating (due to oil).
Can transmit power over long distances.
Provides instant reverse motion with simple valve operation.
6. What type of fluid is used in hydraulic systems?
Usually, hydraulic oils are used, which are:
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Non-compressible,
Lubricating,
Rust and oxidation resistant,
Thermally stable.
🛢 Examples: Mineral-based oils, synthetic oils, or water-glycol fluids.
7. What is the function of an actuator in hydraulics?
The actuator converts hydraulic energy (pressure and flow) into mechanical motion:
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Hydraulic cylinder: Produces linear motion (push/pull).
Hydraulic motor: Produces rotary motion.
Additional Conceptual Viva Questions (Hydraulics):
8. What is hydraulic efficiency?
Ratio of useful power output to the total power input of the system.
9. What are some common hydraulic applications?
Hydraulic lifts, brakes, jacks, excavators, presses, and steering systems.
10. What is cavitation?
Formation and collapse of air bubbles in hydraulic fluid due to low pressure — causes
noise and damage to the pump.
PNEUMATIC TRAINER SYSTEM — Viva Questions
& Answers
1. What is a pneumatic system?
A pneumatic system uses compressed air as the working medium to transmit and control
power for performing mechanical work.
2. What is the function of a double-acting cylinder?
A double-acting cylinder performs work in both directions:
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One port extends the piston (forward stroke).
Another port retracts it (return stroke).
Used where both extension and retraction are power-driven, e.g., in robotic arms.
3. Why is air used as a working medium?
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Air is readily available, cheap, and clean.
It is non-flammable and safe for use.
It can be exhausted directly into the atmosphere.
Air is compressible, making systems lightweight and shock-absorbent.
4. What is the function of an FRL unit?
FRL = Filter, Regulator, Lubricator
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Filter: Removes dust and moisture from air.
Regulator: Maintains constant pressure in the system.
Lubricator: Adds fine oil mist to lubricate moving parts.
Ensures clean, dry, and lubricated air for smooth operation.
5. Difference between single-acting and double-acting cylinders:
Parameter
Single-Acting Cylinder
Double-Acting Cylinder
Working stroke Only one direction (return by spring) Both directions (by air pressure)
Ports
One air port
Two air ports
Power stroke One
Two
Force generated Less
Greater
Application
Clamping, lifting
Robotics, automation
6. What is the purpose of a 5/3 way DC valve?
A 5/3 Directional Control Valve:
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Has 5 ports and 3 positions.
Controls extension, retraction, and stop/hold positions of a double-acting cylinder.
Center position can be:
o Closed-center: Blocks all ports.
o Exhaust-center: Vents both sides.
o Pressure-center: Supplies pressure to both sides.
7. What are limit switches and proximity switches used for?
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Limit Switch:
A mechanical switch activated by the physical movement of a machine part.
➤ Used to detect end positions of cylinders or components.
Proximity Switch:
A non-contact sensor that detects nearby objects (metallic or non-metallic) using
magnetic, inductive, or capacitive fields.
➤ Used for object detection without physical contact.
Additional Conceptual Viva Questions (Pneumatics):
8. What is the main disadvantage of pneumatics?
Compressed air can compress easily → less precise control than hydraulics.
9. What is the working pressure range in pneumatic systems?
Usually 5–10 bar (0.5–1 MPa).
10. What are some applications of pneumatic systems?
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Packaging machinery
Pick-and-place robots
Air brakes
Dental tools
Automation in assembly lines
Comparison: Hydraulic vs Pneumatic Systems
Feature
Hydraulic System
Pneumatic System
Working medium Oil (liquid)
Compressed air
Pressure range High (up to 300 bar)
Low (5–10 bar)
Accuracy
High
Moderate
Cleanliness
Can leak oil
Clean and environment-friendly
Force output
Large
Limited
Speed
Slower
Faster
Typical use
Heavy-duty applications Light-duty, automation systems
Would you like me to give you a combined short-notes sheet (Hydraulic + Pneumatic
system summary with labeled diagrams and key formulas) for quick revision before your
viva?