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Chapter 4 Input/Output Devices and Motor Controls 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Pushbuttons and Switches (Cont.) • Normally open (NO): – Always open— until they are forced to close. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Pushbuttons and Switches (Cont.) • Normally closed (NC): – Always closed— until they are forced to open. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Pushbuttons and Switches (Cont.) © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Pushbuttons • Momentary pushbuttons: – Also called spring-loaded pushbuttons. • Locked-position pushbuttons: – Also called latch pushbuttons. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Switches • Manual 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Switches (Cont.) • Pressure switches: – Used to detect low and high pressure in hydraulic and pneumatic systems. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Switches (Cont.) • Foot switches: – Used as emergency or contact switches. – Placed in factory environments in which workers are using both hands. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Proximity Switches • Proximity switches use: – Light-emitting diode (LED). – Phototransistor. (Siemens) © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Contactors (Cont.) © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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) © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Solenoids • Control devices that use electromagnetism to convert electrical energy into mechanical motion. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. Motor Control Devices • Motor starter: – 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 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. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.