Programmable logic controller Basic | Interview Question & answers

PLC interview , oral , viva examination questions For Mechanical and Electrical Engineering Students .

1.Define a programmable logic controller.

A Programmable Logic Controller (PLC) is a microprocessor based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and process

2. What are the main components parts of a PLC?

1. Central Processing Unit (CPU)
2.The input/output unit
3.The programming device
4. Memory Unit

3. What is the function of programming device?

The programming device is used to enter the required program using ladder logic into the memory of the processor. The sequence of operation and ultimate control of equipment or machinery is specified and determined by ladder programme.

4. List various types of PLC programming devices.

1. Use of hand held programmer
2.Terminal with video display unit
3. A personal computer with appropriate software

5. What is ALU? State is function.

The ALU is responsible for data manipulation and performs arithmetic and logical operations such as addition and subtraction. In addition, the ALU contains number of control inputs, which specify the data manipulation function to be performed. ALU is combinational logic circuit, whose output is an instantaneous function of its data and control inputs.

6. What are counters?

Counters allow a number of occurrences of input signals to count or record the number of times some event occurs. PLCs include some form of counting element and are set to some preset number value. When this value of input pulse has been received, it will operate its contact, the normally open contacts would be closed and a normally closed contact would be opened.

7. Write down various types of counters.

1. Down counters
2. Up-counters

8. List down PLC programming methods.

1. Structured text
2. Ladder diagrams
3. Function block diagram
4. Sequential function charts
5. Instruction list

9. List down the different types of Timers.

1. On delay timer
2. Off delay timer
3. Retentive timer
4. Pulse delay timer

10. What are the types of bus?

(a) Address bus
(b) Databus
(c) Control bus

11. What is ladder programming?

A ladder programming involves each program task being specified as though a Rung of ladder. Thus such a rung could specify that the state of switches A and B, the inputs, be examined and if A and B are both closed then a solenoid, the output is energized.

12. Write about the relay?

With the relay type, the signal from the PLC output is used to operate a relay and so is able to switch currents of the order of a few amperes in an external, circuit. The relay isolates the PLC from the external circuit and can be used for both D.C. and A.C. switching. Relays are, however, relatively slow to operate.

13. What is continuous updating?

This involves the CPU scanning the input channels as they occur in the program instructions. Each input point is examined individually and its effect on the program is determined. Each input is scanned with a 3 ms delay, before the program has the instruction for a logic operation to be executed and an output to occur. This process is called continuous updating.

14. Write about the mass input/output copying?

To allow a more rapid execution of a program, a specific area of RAM is used as buffer store between the control logic and the input/output unit. At the start of each program cycle the CPU scans all the inputs and copies their status into the input/output address in RAM. At the end of each program cycle all the outputs are transferred from RAM to the output channels. The outputs are latched so that they retain their status until the next updating.

15. Write about timer circuit.

A timer circuit is specified by stating the interval to be times and the conditions or events that are to start and/or stop the timer. They are commonly regarded as delays with coils which, when energized, result in the closing or opening of input contacts after some preset time.

16. What are the inputs of shift register?

There are three inputs of shift register. They are,
(a) One to load data into the first element of the register (OUT)
(b) One as the shift command (SFT)
(c) One for resetting (RST)

17. What is the criteria need for the selection of a PLC?

The criteria needed for the selection of a PLC are the following,
(a) Input/ Output capacity is required
(b) Types of inputs/outputs are required
(c) Size of memory required
(d) Speed and power is required for the CPU

18. What are the axes of a pick and place robot?

The robot has three axes about which motion can occur.
(a) Rotation in a clockwise or counter clockwise direction of the unit on its base.
(b) Arm extension or contraction and arm up or down
(c) Gripper can open or close.

19. Define Adaptive control.

The control system which can adapt changes and it can change its parameters depending on the situation. It is known as adaptive control system.

20. State the purpose of Jump controls

By causing Jump instruction, the processor scan time is reduced by Jumping over instruction not pertinent to the machine operation. It is achieved by missing intermediate program and can skip instructions when a production fault occurs.

21 How will you process the input and output of PLC?

From the input/output module, the information about in and out of PLC is obtained. The input module terminals receive signals from wires connected to input sensors and transducers. The output module terminals provide output voltages to energize actuators and indicating device.