original purpose of PLC
replace electromagnetic relays with a solid state switching system that could be programmed
electrical relay
magnetic switch that uses electromagnetism to switch contacts
relays
- control relay
- motor starter
contactor
special type of relay designed to handle heavy power loads
motor starter
designed to provide power to motors, made up of a contactor with an overload relay
overload relays
- 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
manually operated switches
- controlled by hand
- examples: toggle switches, push buttons, knife switches, selector switches
dual in-line package switches
- small switch assemblies designed for mounting on printed circuit board modules
- use on/off settings to set parameters
mechanically operated switch
controlled automatically by factors such as pressure, position, or temperature
level switches
used to sense liquid levels in vessels and provide automatic control for motors
proximity switches
detect presence of object without physical contact
inductive proximity switch
used to detect both ferrous metals (iron) and nonferrous metals
capacitive proximity sensors
produce electrostatic field instead of electromagnetic and are actuated by both conductive/nonconductive materials
magnetic reed switch
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
photovoltaic cell
light produces voltage
photoconductive cell
light changes resistance
photoelectric sensor
- detect light beam and respond to change in received light intensity
- has receiver and transmitter
scan technique of photoelectric sensor
- 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
fiber optic sensors
use flexible cable containing tiny fibers that channel light from emitter to receiver, immune to electrical interference
bar code sensors
light source illuminates bar code symbol, bars absorb light and space reflects it, photodetector collects light in form of pattern, decoder converts data
ultrasonic sensor
sends high frequency sound waves toward target and measuring time it takes for pulses to bounce back, can detect solids/fluids
strain gauge
converts mechanical strain into an electric signal
strain gauge load cell
usually made with steel and sensitive strain gauges
most widely used temperature sensor
thermocouple
