2. Input/Output Devices and Motor Controls4
Input/Output Devices and Motor Controls

3. Objectives Identify the electrical symbols for various switches.
Describe the operation of proximity, Hall effect, ultrasonic, and RADAR sensors.
Name the common indicators used in PLC systems.
Explain the purpose of using indicators in a PLC system.
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4. Objectives
Explain the difference between a power relay and a control relay.
Describe the operation of a relay.
Describe the operation of a solenoid.
Name the two major types of motor control devices.
Name two types of overload relays.
Explain the difference between temperature and thermal overload relays.
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5. Pushbuttons and Switches
Also called discrete input devices.
Independent devices that either allow or disallow electric current to flow through them.
Can be physically changed to the ON or OFF position.
Remains in that condition until its state is changed.
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6. Pushbuttons and Switches (Cont.)
Normally open (NO):
Always open—until they are forced to close.
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7. Pushbuttons and Switches (Cont.)
Normally closed (NC):
Always closed—until they are forced to open.
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8. Pushbuttons and Switches (Cont.)
Poles:
Shown in schematics as those contacts through which current enters the switch.
Connected to the movable contacts.
Number of places in which the switch opens or breaks the circuit.
Switches:
Single-break.
Double-break.
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9. Pushbuttons and Switches (Cont.)
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10. Pushbuttons Momentary pushbuttons: Locked-position pushbuttons:
Also called spring-loaded pushbuttons.
Locked-position pushbuttons:
Also called latch pushbuttons.
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11. Switches Manual switch: Automatic switch:
Requires an operator to change their state.
Automatic switch:
Controlled by a mechanical or electrical device.
Do not have to turn an automatic switch on or off manually.
Limit switches.
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12. Switches (Cont.) Switches are designed to work in harsh environments.
Industrial switches may be exposed to:
Extreme humidity.
Splashing liquid.
Gas or liquid immersion.
Dust.
Vibration.
High electrical noise.
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13. Switches (Cont.) Temperature switches:
Detect a specific preset temperature.
Normally open switch closes and normally closed temperature switch opens for temperatures above set point.
Use vapor pressure technology to sense changes in temperature.
Offer many features and modifications, allowing them to be used in a variety of applications.
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14. Switches (Cont.) Pressure switches:
Used to detect low and high pressure in hydraulic and pneumatic systems.
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15. Switches (Cont.) Liquid level switches:
Used to detect the liquid level in a chemical tank or water well.
Typically, placed at a specific height within a tank or well.
Normally open liquid level switch closes, normally closed liquid level switch opens above the set point.
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16. Switches (Cont.) Flow switches: Set to detect specific flow rates.
Normally open flow switch closes, normally closed flow switch opens above set point.
In pipes and heating, ventilation, and air conditioning (HVAC) air ducts.
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17. Switches (Cont.) Float switches:
Used to open and close contacts in response to changes in liquid level.
Used in water treatment plants, wastewater treatment plants, and storage tanks.
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18. Switches (Cont.) Foot switches: Used as emergency or contact switches.
Placed in factory environments in which workers are using both hands.
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19. Switches (Cont.) Limit switches: Physically touched by the part.
Open or close when objects physically hit their actuator.
Consists of an actuator that is mechanically linked to a set of contacts.
Used in a variety of applications for detecting the presence of parts.
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20. Proximity Switches Proximity switches use: Light-emitting diode (LED).
Phototransistor.
(Siemens)
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21. Proximity Switches (Cont.)
Light-emitting diode:
Transmitter that generates an infrared light beam.
Phototransistor:
Receiver that detects the presence of the light beam.
Switch triggered when the light beam is interrupted by the objects placed in the light beam’s path.
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22. Sensors Electronic sensors:
Used in PLC control systems to detect changes in the environment and industrial settings.
Can emulate the five human senses and convert the changes to electronic signals.
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23. Proximity Sensors Can detect the existence of an object.
Electronic signal used to detect an object so object does not have to be touched.
Can only have a state that is either true or false.
Can either be inductive-, capacitive-, or optical-type sensors.
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24. Inductive Sensors
Detects the presence of nearby metallic objects by measuring changes in magnetic fields.
Proved by Faraday in the 1800s.
Uses a coil to generate a magnetic field.
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25. Capacitive Sensors
Detects an object through the change in the sensor’s dielectric.
Value of a capacitor is directly proportional to the plate area and dielectric constant used between the plates.
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26. Capacitance Inversely proportional to the distance of the plates.
C = (8.85  10–12) A  k d
Area of the plates (A) and the distance (d) between them are fixed.
Dielectric constant (k):
Space around them varies as different materials are brought near the sensor.
Constant value that depends on the material is used to isolate the capacitor plates.
Oscillator is used to generate the electric field.
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27. Optical Sensors
Can use visible, ultraviolet, infrared, or laser lights.
Require both an emitter to generate the light beam and a detector.
Modern versions are very sophisticated and can even detect the type of object.
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28. Hall Effect Sensors
Semiconductor devices (transistors) that can be switched by magnetic fields.
Applications are similar to those of reed switches and relays.
Solid state, more rugged, and resists vibration.
Used in automated machines to complete initial calibration and detect end stops.
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29. Ultrasonic Sensors
Produces sound above the normal human hearing threshold of 16 kHz.
Uses this sound to detect the distance to an object.
Relatively accurate for short distances.
Economical.
Time required for sound to travel to the target and reflect back is proportional to the distance to the target.
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30. Electrostatic Ultrasonic Sensors
Use capacitive effects.
Short sound wave is transmitted to hit an object.
Longer ranges and wider bandwidths than piezoelectric ultrasonic sensors.
More sensitive to factors such as humidity.
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31. Piezoelectric Ultrasonic Sensors
Work by charge displacement strain on crystal lattices.
Rugged and inexpensive.
Effective for applications such as fluid levels in tanks and crude distance measurement.
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32. Piezoelectric Ultrasonic Sensors (Cont.)
When the reflected sound wave hits a crystal, the crystal produces a small AC voltage signal and the signal is then measured.
Frequency of the sound wave can be up to 1 MHz.
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33. Radio Detection and Ranging (RADAR) Sensors
Used to detect metallic objects.
Several cycles of high-frequency waves are transmitted into the environment and the reflection is picked up via a receiver.
Speed of traveling waves is too fast to use RADAR in factory for object detection.
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34. Indicators Pilot lights, ammeters, and voltmeters.
Help an operator to visually inspect the operation of an output device.
Different colored pilot lights show each stage or operating condition of an output device.
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35. Relay
Magnetized or demagnetized coil causes relay’s contacts to either close or open.
Electrically-operated control switches.
Classified according to use as power relay or control relay.
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36. Contactors Contactor:
Uses a relatively small amount of electrical power to control the switching of a large amount of power.
Used to control power in heavy power cables that are run to motors and other power devices.
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37. Contactors (Cont.)
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38. Solid-State Relays Similar to Hall effect switches
Uses transistors that can be switched on in the presence of a magnetic field.
Switching design uses no moving parts or contacts that can wear out.
Performs well in harsh environments.
(Siemens)
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39. Solenoids
Control devices that use electro-magnetism to convert electrical energy into mechanical motion.
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40. Solenoids (Cont.) Movement is used to:
Close a set of electrical contacts.
Cause the movement of a mechanical device.
Or do both at the same time.
Used to open and close a valve to control the flow of a liquid or gas.
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41. Motor Control Devices Motor starter: Motor drive:
Turns a motor on and off.
Incorporates a motor protective device to protect the motor from:
Low voltage.
High temperature.
Overload (high load current) conditions.
Motor drive:
Controls velocity, acceleration, and deceleration.
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42. Motor Control Devices (Cont.)
Overload relays:
Overload protective devices in the motor starter.
Placed in series between the contacts and the motor.
Used to detect excess motor current.
Contact opens when excess current is detected, breaking the motor starter coil circuit, and opening the motor starter contactors.
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43. Motor Control Devices (Cont.)
Temperature overload relay:
Also called a thermal overload relay.
Detects over current through the rise in excessive temperature generated by the line current.
Used in most motor control systems.
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44. Motor Control Devices (Cont.)
Magnetic overload relay:
Detects over current through the magnetization the line current generates.
Have faster response time than temperature overload relays.
More expensive.
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45. Glossary
Capacitive sensor: Sensor that detects an object through the change in the sensor’s dielectric.
Discrete input devices: Switches or pushbuttons that either block or allow the flow of electric current.
Electrostatic ultrasonic sensor: A sensor that uses capacitive effects where a short sound wave is transmitted to hit an object.
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46. Glossary
Float switch: Switch used to open and close contacts in response to changes in liquid level.
Flow switch: Switch that opens or closes for a preset flow rate of liquid or gas through a pipe.
Foot switch: Foot-operated switch placed in factory environments in which workers are using both hands.
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47. Glossary
Hall effect sensor: Semiconductor device (transistor) that can be switched by magnetic fields.
Inductive sensor: Sensor that detects the presence of nearby metallic objects by measuring changes in magnetic fields.
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48. Glossary
Light-emitting diode (LED): A diode device that emits light when forward biased. When the LED is forward biased (current flows through it), some of the electric energy is converted to light energy.
Limit switch: Switch that opens or closes when an object contacts the handle of the switch.
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49. Glossary
Liquid level switch: Switch that opens or closes when the liquid level in the reservoir tank reaches a preset level.
Locked-position pushbutton: Similar to a switch, it changes state permanently each time it is pressed. Also called a latch pushbutton.
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50. Glossary
Magnetic overload relay: Relay in which the overcurrent is detected using the magnetization the line current generates. Magnetic overload relays have faster response time, but are more expensive than temperature overload relays.
Momentary pushbutton: Pushbutton that changes status from open to closed or from closed to open. They remain at the changed positions as long as they are pressed. Also called a spring-loaded pushbutton.
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51. Glossary
Normally closed (NC): Normally closed switch or pushbutton. When opened, the switch or pushbutton will turn off.
Normally open (NO): Normally open switch or pushbutton. When closed, the switch or pushbutton will turn on.
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52. Glossary
Overload relay: Relay that is placed in series with power lines of motors to detect excess motor current.
Phototransistor: Device that allows current to flow from its collector to the emitter when a beam of light strikes its base.
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53. Glossary
Piezoelectric ultrasonic sensor: A sensor that works using charge displacement strain on crystal lattices where a reflected sound wave hits a crystal and the crystal produces a small AC voltage signal.
Pressure switch: Switch used to detect low and high pressure in hydraulic and pneumatic systems.
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54. Glossary
Proximity sensor: Sensor that can detect the existence of an object.
Proximity switch: Switch that consists of a light-emitting diode (LED) and a phototransistor. It opens or closes a circuit when the presence of an object is detected. The object breaks the path of the beam of light between the LED and the phototransistor.
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55. Glossary
Radio Detection and Ranging (RADAR) sensor: Sensor used to detect metallic objects where there are several cycles of high- frequency waves transmitted into the environment and the reflection is picked up via a receiver.
Relay: A device that uses a coil to generate a magnetic effect to close or open the relay’s contacts.
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56. Glossary
Solenoid: An electromechanical valve used to control liquid flow.
Temperature overload relay: Relay in which an overcurrent is detected through the excessive temperature rise generated by the line current. Also called a thermal overload relay.
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57. Glossary
Temperature switch: Switch that opens or closes for a preset temperature.
Ultrasonic sensor: Sensor that produces sound above the normal human hearing threshold of 16 kHz and uses this sound to detect the distance to an object.
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