Programmable Logic Controller ( Plc )

Part A: Answer all question. 1. A PLC can effectively replace a number of components. Give examples and discuss some good and bad applications of PLCs. (2 marks) 2. Give an example of where a PLC could be used.

(2 marks)

3. Why would relays be used in place of PLCs?

(2 marks)

4. Give a concise description of a PLC.

(2 marks)

5. List the advantages of a PLC over relays.

(2 marks)

6. Explain the trade-offs between relays and PLCs for control applications.(2 marks) 7. Explain why ladder logic outputs are coils?

(2 marks)

8. In the figure below, will the power for the output on the first rung normally be on or off? Would the output on the second rung normally be on or off? (2 marks)

9. Explain why a stop button must be normally closed and a start button must be normally open. (2 marks) 10. Draw a simple hydraulic system that will advance and retract a cylinder using PLC outputs. Sketches should from the PLC output card to the hydraulic cylinder. (2 marks)

Part B: Answer 3 questions only. 1.

i- Is the ladder logic in the figure below for an AND or an OR gate?

ii-. Draw a ladder diagram that will cause output D to go true when switch A and switch B are closed or when switch C is closed. The devices are wired to the following locations. (10 marks) 2.

Draw a ladder diagram that will cause output D to be on when push button A is on, or either B or C are on. (10 marks)

3.

Consider a case where an input to a PLC is a normally closed stop button. The contact used in the ladder logic is normally open, as shown below. Why are they both not the same? (10 marks)

4.

Make a simple ladder logic program that will turn on the outputs with the binary patterns when the corresponding buttons are pushed. (10

marks)

5.

Convert the following Boolean equation to the simplest possible ladder logic. (10 marks)

6.

What is wrong with the following ladder logic? What will happen if it is used? (10 marks)

Part C: Answer 2 questions only. 1.

Write a ladder logic program to extend and retract a cylinder after a start button is pushed. There are limit switches at the ends of travel. If the cylinder is extending if more than 5 seconds the machine should shut down and turn on a fault light. If it is retracting for more than 3 seconds it should also shut down and turn on the fault light. It can be reset with a reset button. (25 marks)

2.

Convert the following flow chart to ladder logic.

3.

Design a garage door controller using a flowchart. The behavior of the garage door controller is as follows, (25 marks)

(25 marks)

- there is a single button in the garage, and a single button remote control. - when the button is pushed the door will move up or down. - if the button is pushed once while moving, the door will stop, a second push will start motion again in the opposite direction. - there are top/bottom limit switches to stop the motion of the door. - there is a light beam across the bottom of the door. If the beam is cut while the door is closing the door will stop and reverse. - there is a garage light that will be on for 5 minutes after the door opens or closes. 4.

A conveyor is run by switching on or off a motor. We are positioning parts on the conveyor with an optical detector. When the optical sensor goes on, we want to

wait 1.5 seconds, and then stop the conveyor. After a delay of 2 seconds the conveyor will start again. We need to use a start and stop button - a light should be on when the system is active. (25 marks)