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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.
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.
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.
Pushbuttons and Switches (Cont.) • Normally open (NO): • Always open—until they are forced to close.
Pushbuttons and Switches (Cont.) • Normally closed (NC): • Always closed—until they are forced to open.
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.
Pushbuttons • Momentary pushbuttons: • Also called spring-loaded pushbuttons. • Locked-position pushbuttons: • Also called latch pushbuttons.
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.
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.
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.
Switches (Cont.) • Pressure switches: • Used to detect low and high pressure in hydraulic and pneumatic systems.
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.
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.
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.
Switches (Cont.) • Foot switches: • Used as emergency or contact switches. • Placed in factory environments in which workers are using both hands.
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.
Proximity Switches • Proximity switches use: • Light-emitting diode (LED). • Phototransistor. (Siemens)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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. Relay
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.
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. Solid-State Relays (Siemens)
Solenoids • Control devices that use electro-magnetism to convert electrical energy into mechanical motion.
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.
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.
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.
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.
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.