Chapter 6

Question 1

original purpose of PLC

Answer 1

replace electromagnetic relays with a solid state switching system that could be programmed

Question 2

electrical relay

Answer 2

magnetic switch that uses electromagnetism to switch contacts

Question 3

relays

Answer 3

• control relay

- motor starter

Question 4

contactor

Answer 4

special type of relay designed to handle heavy power loads

Question 5

motor starter

Answer 5

designed to provide power to motors, made up of a contactor with an overload relay

Question 6

overload relays

Answer 6

• designed to meet special protective needs of motor control circuit
• allow harmless temporary overloads that occur when a motor starts
• will trip/disconnect power if overload persists
• can be reset

Question 7

manually operated switches

Answer 7

• controlled by hand

- examples: toggle switches, push buttons, knife switches, selector switches

Question 8

dual in-line package switches

Answer 8

• small switch assemblies designed for mounting on printed circuit board modules
• use on/off settings to set parameters

Question 9

mechanically operated switch

Answer 9

controlled automatically by factors such as pressure, position, or temperature

Question 10

level switches

Answer 10

used to sense liquid levels in vessels and provide automatic control for motors

Question 11

proximity switches

Answer 11

detect presence of object without physical contact

Question 12

inductive proximity switch

Answer 12

used to detect both ferrous metals (iron) and nonferrous metals

Question 13

capacitive proximity sensors

Answer 13

produce electrostatic field instead of electromagnetic and are actuated by both conductive/nonconductive materials

Question 14

magnetic reed switch

Answer 14

when a magnetic field is created, the ends of the two reeds in a glass tube are attracted to one another and actuate the switch, they are unaffected by dust/humidity/fumes

Question 15

photovoltaic cell

Answer 15

light produces voltage

Question 16

photoconductive cell

Answer 16

light changes resistance

Question 17

photoelectric sensor

Answer 17

• detect light beam and respond to change in received light intensity
• has receiver and transmitter

Question 18

scan technique of photoelectric sensor

Answer 18

• through beam scan (direct): transmitter/receiver directly in line with one another
• retroreflective: transmitter mounted on top with receiver on bottom and reflective tape allows the beam to go from transmitter to receiver

Question 19

fiber optic sensors

Answer 19

use flexible cable containing tiny fibers that channel light from emitter to receiver, immune to electrical interference

Question 20

bar code sensors

Answer 20

light source illuminates bar code symbol, bars absorb light and space reflects it, photodetector collects light in form of pattern, decoder converts data

Question 21

ultrasonic sensor

Answer 21

sends high frequency sound waves toward target and measuring time it takes for pulses to bounce back, can detect solids/fluids

Question 22

strain gauge

Answer 22

converts mechanical strain into an electric signal

Question 23

strain gauge load cell

Answer 23

usually made with steel and sensitive strain gauges

Question 24

most widely used temperature sensor

Answer 24

thermocouple

Question 25

how do thermocouples work

Answer 25

when 2 dissimilar metals are joined, a predictable DC voltage will be generated that relates to the difference in temperature between the hot and cold junctions

Question 26

resistance temperature detectors

Answer 26

the hotter they become, the higher the value of the electrical resistance

Question 27

turbine type flowmeters

Answer 27

higher the flow rate means the faster the turbine will rotate

Question 28

tachometer

Answer 28

convenient for converting rotational speed into analog voltage signal

Question 29

encoder

Answer 29

convert linear or rotary motion into binary digital signal

Question 30

actuator

Answer 30

any device that converts electrical signal into mechanical movement

Question 31

stepper motors

Answer 31

rotates in discrete increments when electrical command pulses are applied

Question 32

open loop vs closed loop system

Answer 32

-open loop is control without feedback

Question 33

seal in (holding) circuits

Answer 33

method of maintaining current flow after momentary switch has been pressed and released

Question 34

latching relays (latching and unlatching)

Answer 34

- designed to hold relay closed after power has been removed from coil - both must have same address