Page 147 - Programmable Logic Controllers, Fifth Edition

CHAPTER 6 REVIEW QUESTIONS

1. Explain the basic operating principle of an electro- 15. Compare the operation of the reflective-type and
magnetic control relay. through-beam photoelectric sensors.
2. What is the operating difference between a nor- 16. Give an explanation of how a scanner and a decoder
mally open and a normally closed relay contact? act in conjunction with each other to read a bar code.
3. In what ways are control relay coils and contacts rated? 17. How does an ultrasonic sensor operate?
4. How do contactors differ from relays? 18. Explain the principle of operation of a strain gauge.
5. What is the main difference between a contactor 19. Explain the principle of operation of a thermocouple.
and a magnetic motor starter? 20. What is the most common approach taken with re-
6. a. Draw the schematic for an across-the-line AC gard to the measurement of fluid flow?
magnetic motor starter. 21. Explain how a tachometer is used to measure rota-
b. With reference to this schematic, explain the tional speed.
function of each of the following parts:
i. Main contact M 22. How does an optical encoder work?
ii. Control contact M 23. Draw an electrical symbol used to represent each of
iii. Starter coil M the following PLC control devices:
iv. OL relay coils a. Pilot light f. Heater
v. OL relay contact b. Relay g. Solenoid
7. The current requirement for the control circuit of a c. Motor starter coil h. Solenoid valve
magnetic starter is normally much smaller than that d. OL relay contact i. Motor
required by the power circuit. Why? e. Alarm j. Horn
8. Compare the method of operation of each of the 24. Explain the function of each of the following
following types of switches: actuators:
a. Manually operated switch a. Solenoid
b. Mechanically operated switch b. Solenoid valve
c. Proximity switch c. Stepper motor
9. What do the abbreviations NO and NC represent 25. Compare the operation of open-loop and closed-
when used to describe switch contacts? loop control.
10. Draw the electrical symbol used to represent each 26. What is a seal-in circuit?
of the following switches: 27. In what way is the construction and operation of an
a. NO pushbutton switch electromechanical latching relay different from a
b. NC pushbutton switch standard relay?
c. Break-make pushbutton switch 28. Give a short description of each of the following
d. Three-position selector switch control processes:
e. NO limit switch a. Sequential
f. NC temperature switch b. Combination
g. NO pressure switch c. Automatic
h. NC level switch 29. Compare the type of sensor signal obtained from a
i. NO proximity switch thermocouple with that from an RTD.
11. Outline the method used to actuate inductive and 30. Explain how a magnetic reed float switch works.
capacitive proximity sensors. 31. What is the function of an electrical interlocking
12. How are reed switch sensors actuated? circuit?
13. Compare the operation of a photovoltaic solar cell 32. What is the role of instrumentation in an industrial
with that of a photoconductive cell. process?
14. What are the two basic components of a photoelec- 33. You have been assigned the task of calibrating an
tric sensor? instrument. How would you proceed?

128 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

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