THE UNIVERSITY OF TRINIDAD & TOBAGO

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1 THE UNIVERSITY OF TRINIDAD & TOBAGO FINAL EXAMINATIONS December 2012 MECHANICAL ENGINEERING DIPLOMA Course Code and Title: Date: MEMG 217D/PGCR 210D Programmable Controllers Time: 3 hours Read all instructions carefully before you begin this examination. Materials supplied: 1. Question Paper 2. Answer booklet Instructions to Candidates 1. You are required to follow the instruction in each Section. 2. The PLC programs for Section C are to be submitted on the diskettes provided or stored in the thaw space as per lecturer s directives. SFTI students will submit their programs on the program booklets. 3. You must return the question paper along with your answer booklet and other writing paper to the Invigilator at the end of the examination. 4. Answers to the multiple choice must be done on the answer sheet provided. (page 10) Key Examination Protocol 1) Students please note that academic dishonesty (or cheating) includes plagiarism, collusion, falsification, replication, taking unauthorized notes or devices into an exam, obtaining an unauthorized copy of the exam paper, communicating or trying to communicate with another student during the exam and being a party to impersonation in relation to an exam. 2) These and any other actions which compromise the integrity of the academic evaluation process will be fully investigated and dealt with in accordance with UTT s academic regulations. 3) Please be reminded that looking into another candidate s script is strictly prohibited. 4) Speaking without the Invigilator s permission is NOT allowed. The University of Trinidad & Tobago Page 1 of 10

2 Section B. You are required to answer all questions in this section 1) Outline the sequence of events involved in a single PLC program scan. (3 mks) 2) Answer the following questions: a. What does an EXAMINE IF CLOSED or EXMAINE IF OPEN instruction represent? b. What does an OUTPUT ENERGIZE instruction represent? c. The status bit of an EXAMINE IF CLOSED instruction is examined and found to be 0. What does this mean? d. The status bit of an EXAMINE IFOPEN instruction is examined and found to be 1. What does this mean? e. Under what condition would the status bit of an OUTPUT ENERGIZE instruction be 0? 3) Answer the following questions: (10 mks) a. How are the contacts and coil of a rung represented in a ladder logic program b. When is the ladder rung considered TRUE, or as having logic continuity? (4 mks) 4) Convert the decimal number 512 into the following i) Binary ii) Octal iii) Hexadecimal (3 mks) The University of Trinidad & Tobago Page 6 of 10

3 5) Write the ladder logic program, draw the logic gate circuit, and state the Boolean equation for the following two relay ladder diagrams. See Fig 3: PB1 PB2 H LS1 LS2 R LS1 LS3 CR1 (a) (b) Fig. 3 (10 mks) The University of Trinidad & Tobago Page 7 of 10

4 SECTION C This section consists of THREE questions You are required to attempt 2 questions out of the 3 questions. Each question is worth 25 marks. Question 1 Write a ladder logic program using PLC programming software to implement the activities as described in the following problems. You are required to download your program to the PLC and demonstrate its operation to your Instructor. Your program must be well documented and utilize symbolic addressing. Save your programs on the storage medium indicated and hand in to your Instructor at the end of the allotted time. A PLC is used to control the operations of an OPEN sign. The letters O, P, E, N come on with a 10s delay between each letter. When the entire word is illuminated it flashes OFF for 5s, then ON for 10s and OFF again. The sequence then restarts from O. Draw a timing diagram to illustrate the operation of the sign and then write the program to execute this operation. You are required to use only one timer for this application. The above sequence is to keep repeating until the PLC is placed in STOP mode. (25 marks) Question 2 Create, enter and test a program which uses a single start switch to control the operation of 4 lights using the following sequence of steps. 1 when the start switch is first pressed, the first light will turn on and remain on when the switch is released. 2 when the start switch is pressed for the second time, the first light will turn off and the second light will turn on and remain on when the switch is released. 3. when the start switch is pressed for the third time, the second light will turn off and the third light will turn on and remain on when the switch is released. 4. when the start switch is pressed for the 4th. time, the third light will turn off and the fourth light will turn on and remain on when the switch is released. 5. when the start switch is pressed for the fifth time the fourth light will turn off and all lights will be off. 6. The sequence now continues at Step #1 above. 25 mark The University of Trinidad & Tobago Page 8 of 10

5 Question 3 a) You are required to design test and implement a ladder logic programme that will control the operation of a car park. 5 marks i. The car park can hold 50 cars ii. Proximity sensors are imbedded at the input and output to the car park to sense incoming and leaving cars. iii. When the car park is full a light at the entrance must indicate so. b. i)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. ii)draw a ladder diagram that will cause output D to be on when switch A is on, and either B or C are on. iii) Design a ladder diagram that will behave as specified by the Boolean expression below, (do not simplify the expression). X = A B + (C. B) 10 marks c. Write a ladder logic program to do the following calculation.. If the result is greater than 16, then the output solenoid will be turned on. R=2A(C+D) Please move A=3, C=2 and D=4 into suitable memory locations using the MOV instruction 10 marks The University of Trinidad & Tobago Page 9 of 10