Programmable Logic Controller Muhajir Ab. Rahim School of Mechatronic Engineering UniMAP

Relays

Electromagnetic Control Relays The PLC's original purpose was the replacement of electromagnetic relays with a solid-state switching system that could be programmed. The programmable controller was designed to replace physically small control relays that make logic decisions but are not designed to handle heavy current or high voltage. Electromagnetic relays, such as the lighting contactor shown, are still used as auxiliary devices to switch I/O field devices.

Electromagnetic Relay Operation An electromagnetic relay is a magnetic switch. It uses electromagnetism to switch contacts. A relay will usually have only one coil but may have any number of different contacts.

Electromagnetic Relay Operation With no current flow through the coil (coil de-energized), the armature is held away from the core by spring tension. When the coil is energized, the electromagnetic field moves the armature causing the contact points of the relay to open or close.

Input and Output Symbols Input or contact symbol Output or coil symbol

Relay Symbol Normally open Normally closed (NO) contact (NC) contact CR1-1 CR1-2 Normally open (NO) contact CR1 Coil Control relay Contacts are open when no current flows through the coil but close as soon as the coil is energized. Contacts are closed when no current flows through the coil but open as soon as the coil is energized.

Relay Circuit Operation S CR1 CR1-1 CR1-2 OFF R ON G With switch S open: coil CR1 is de-energized contacts CR1-1 are open light R is off contacts CR1-2 are closed light G is on

Relay Circuit Operation S CR1 CR1-1 CR1-2 OFF R ON G With switch S closed: coil CR1 is energized contacts CR1-1 are closed light R is on contacts CR1-2 are open light G is off

Magnetic Contactor A contactor is a special type of relay designed to handle heavy power loads that are beyond the capability of control relays. Contactors are designed to operate such loads as lights, heaters, transformers, capacitors, and electric motors for which overload protection is provided separately or not required.

Magnetic Contactor

PLC Used In Conjunction With A Contactor Pump Contactor power contacts L2 L1 High-current wiring Programmable controllers have I/O capable of operating the contactor but they do not have the capacity to operate heavy loads directly. Coil terminals Low-current wiring L2 L1 PLC output module

Magnetic Motor Starter A magnetic motor starter is a contactor with an overload relay attached physically and electrically. They are electromagnetically operated switches that provide a safe method for starting large motor loads. The overload relay will open the supply voltage to the starter if it detects an overload on a motor. Motor overload relay contacts are normally hardwired in series with the magnetic starter coil.

Magnetic Motor Starter Overload heaters are connected in series with the contactor. If the motor becomes overloaded they cause a mechanical latch to trip. Tripping this latch opens a set of contacts that are wired in series with the voltage supply and motor.

Across-The-Line AC Starter Operation The OL contact opens automatically when an overload condition is sensed, to de-energize the M coil and stop the motor. When the start button is pressed, coil M energizes to close all M contacts. The M contacts, in series with the motor, close to complete the high-current path to the motor. Control contact M also closes to seal-in the coil circuit when the START button is released. Low-current control circuit High-current power circuit

PLC Control Of A Large Motor Load When a PLC needs to control a large motor, it must work in conjunction with a starter. Motor starters are available in various standard National Electric Manufacturers (NEMA) sizes and ratings.

Manually Operated Switches Manually operated switches are controlled by hand. Pushbutton switches are the most common form of manual control found in industry. Normally Open (NO) pushbutton makes a circuit when it is pressed and returns to its open position when the button is released.

Manually Operated Switches Normally Closed (NC) pushbutton opens the circuit when it is pressed and returns to the closed position when the button is released. The abbreviations NO and NC represent the state of the switch when it is not actuated.

Manually Operated Switches The break-make pushbutton is used for interlocking controls. In this switch the top section is NC, while the bottom section is NO. When the button is pressed, the bottom contacts are closed as the top contacts open. L1 L2 R G ON OFF NOT PRESSED PRESSED

Selector Switch Selector switch positions are made by turning the operator knob – not pushing it. Contacts Position A B Off Hand Auto Selector switch positions may have two or more selector positions with either maintained contact position or spring return to give momentary contact operation. Hand X Off X Auto

Selector Switch Motor Reversing Selector switch used in conjunction with a reversing motor starter to select forward or reverse operation of the motor.

Dual In-Line Package (DIP) Switches ON OFF Are small switch assemblies designed for mounting on printed circuit board modules. Switch settings are seldom changed, and the changes occur mainly during installation or configuration of the system.

Mechanically Operated Switches A mechanically operated switch is controlled automatically by factors such as pressure, position, and temperature. The limit switch is a type of mechanically operated switch designed to operate only when a predetermined limit is reached, and is usually actuated by contact with an object such as a cam.

Limit Switch Operation Symbols NO Contact NC Contact Limit switches take the place of a human operator. They are often used in the control of machine processes to govern the starting, stopping, or reversal of motors.

Typical Limit Switch Circuit Stop Forward F R Reverse OLs Limit switches FWD Limit REV Limit Control circuit for starting and stopping a motor in forward and reverse with limit switches providing over travel protection.

Temperature Switch The temperature switch or thermostat is used to sense temperature changes and is actuated by some specific environmental temperature change. Responds to changes in temperature by opening or closing an electric circuit. Symbols NO Contact NC Contact

Temperature Switch Control of a Motor Closing ON Rising temperature Motor

Pressure Switch Pressure switches are used to control the pressure of liquids and gases and are activated when a specific pressure is reached. Opens or closes an electric circuit in response to a change in pressure. Switch Bellows Symbols NO Contact NC Contact

Starter Operated By A Pressure Switch Closing ON Rising pressure

Level Switch Level or float switches are used to sense the height of a liquid. Switch Opens or closes an electric circuit in response to a change in liquid level. Symbols NO Contact NC Contact Float Two-wire level switch control of starter.

1. The programmable controller was designed to replace relays designed to handle heavy current or high voltage. (True/False) 2. The NO (normally open) contacts of a relay are open when the relay coil is energized. (True/False) 3. A contactor is a special type of relay designed to switch heavy power loads. (True/False) 4. Motor overload relay contacts are normally hardwired in series with the magnetic starter coil. (True/False)

5. A _______________ pushbutton opens the circuit when it is pressed and returns to the closed position when the button is released. normally open normally closed break-make pushbutton make-break pushbutton 6. The abbreviations NO and NC represent the state of the switch when it is actuated. (True/False)

7. Which of the following would be classified as a manually operated switch? a. pressure switch c. level switch b. temperature switch d. none of these 8. A limit switch is usually actuated by: (a) hand (b) contact with an object (c) pressure (d) an electromagnet

9. The device shown is that of a: Float Switch Limit Switch Temperature Switch Pressure Switch

10. The symbol shown is that of a: NO float switch NC float switch NO temperature switch NC temperature switch

Transducer A transducer is any device that converts energy from one form to another. Input transducer (microphone) converts sound energy to electric energy Output transducer (speaker) converts electric energy to sound energy Amplifier

Sensors Sensors are input transducers used for detecting and often measuring the magnitude of something. They convert mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents. Photoelectric sensor

Sensors Sensors provide the equivalent of eyes, ears, nose, and tongue to the microprocessor brain. Microprocessor Optical sensor Gas sensor Microphone Probe

Proximity Sensor Proximity sensors or switches detect the presence of an object without making physical contact with it.

Proximity Sensor Applications The object being detected is too small, lightweight, or soft to operate a mechanical switch. Rapid response and high switching rates are required. An object has to be sensed through nonmetallic barriers such as glass, plastic, and paper cartons. Hostile environments conditions exist. Long life and reliable service are required. A fast electronic control system requires a bounce-free input signal.

Inductive Proximity Sensor Operation Barrel type Block diagram As the target moves into the sensing area, the sensor switches the output ON

Capacitive Proximity Sensor A capacitive proximity sensor can be actuated by both conductive and nonconductive material such as wood, plastics, liquids, sugar flour and wheat. Operation is similar to that of inductive proximity sensor. Instead of a coil, the active face of the sensor is formed by two metallic electrodes – rather like an "opened capacitor".

Magnetic Switch (Reed Switch) A magnetic switch (also called a reed switch) is composed of flat contact tabs that are hermetically sealed (air-tight). The switch is actuated by a magnet. Magnet N S NO NC Common

Reed Switch Activation Magnet Reed switch Proximity motion – movement of the switch or magnet will activate the switch Rotary motion – switch is actuated twice for every complete revolution Shielding – the shield short circuits the magnetic field; switch is activated by removal of the shield

Photovoltaic Or Solar Cell The photovoltaic cell, or solar cell, is a common light-sensor device that converts light energy directly into electric energy. Solar cell The solar cell converts light impulses directly into electrical charges which can easily be amplified to provide an input signal to a PLC.

Photoconductive Or Photoresistive Cell The photoconductive cell, or photoresistive cell, is is another popular type of light transducer. Light energy falling on this device will cause a change in the resistance of the cell. 20 Ohms Light resistance 5,000 Ohms Dark resistance Ohms

Photoelectric Sensor Operation Most industrial photoelectric sensors use a light-emitting diode (LED) for the light source and a phototransistor to sense the presence or absence of light. Light from the LED falls on the input of the phototransistor and the amount of conduction through the transistor changes. Analog outputs provide an output proportional to the quantity of light seen by the photodetector. Object to be sensed Light detector Light source

Reflective Photoelectric Sensor Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected. Retro-reflective type Operating range Reflector Operating range Diffused-reflective type Emitter/receiver Target

Through-Beam Type Photoelectric Sensor A through-beam photoelectric sensor is used to measure the change in light quantity caused by the target's crossing the optical axis. Operating range Target Emitter Receiver

Bar Code Systems Bar code systems can be used to enter data much more quickly than manual methods, and are highly accurate. Scanner Decoder PLC Diverter The decoder receives the signal from the scanner and converts these data into the character data representation of the symbol's code.

Ultrasonic Sensor An ultrasonic sensor operates by sending sound waves towards the target and measuring the time it takes for the pulses to bounce back. The returning echo signal is electronically converted to a 4 mA to 20 mA output, which supplies flow rate to external control devices.

Strain /Weight Sensors A strain gauge transducer converts a mechanical strain into an electric signal. The load cell provides sensor input to the controller, which displays the weight and controls the hopper chute. Load cell Controller Hopper Chute ON/OFF Control Force Wire type The force applied to the gauge causes the gauge to bend. This bending action also distorts the physical size of the gauge, which in turn changes its resistance.

Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The thermocouple consists of a pair of dissimilar conductors fused together at one end to form the "hot" or measuring junction, with the free ends available for connection to the "cold" reference junction. A temperature difference between the measuring and reference junction generates a small DC signal voltage. The resistance temperature detector (RTD) varies in resistance value with changes in temperature. RTD The thermistor varies in resistance value with changes in temperature The Integrated Circuit (IC) temperature sensor produces changes in voltage or current with changes in temperature.

Flow Measurement The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form. Electronic Magnetic Flow Meter Can be used with electrically conducting fluids and offers no restriction to flow. A coil in the unit sets up a magnetic field. If a conductive liquid flows through this magnetic field, a voltage is induced and sensed by two electrodes. Flow Magnet Turbine Turbine Flow Meter Coil The turbine blades turn at a rate proportional to the fluid velocity and are magnetized to induce voltage pulses coil.

Velocity/RPM Sensors A tachometer is a small permanent magnet DC generator which when rotated produces a voltage that is directly proportional to the speed at which it is driven. Controller Tach Motor M Load Tachometers coupled to motors are commonly used in motor speed control applications to provide a feedback voltage to the controller that is proportional to motor speed.

Velocity/RPM Sensors The rotating speed of a shaft is often measured using a magnetic (inductive) pickup sensor. 0 V Pickup coil Pole piece N S Magnet Sensor output A magnet is attached to the shaft. A small coil of wire held near the magnet receives a pulse each time the magnet passes. By measuring the frequency of the pulses, the shaft speed can be determined.

Output Control Devices A variety of output control devices can be operated by the controller output module to control traditional processes. These include: Pilot light Solenoid valve Control relay Alarm Heater Motor starter Small motor

Actuators An actuator is any device that converts an electrical signal into mechanical movement. The principle types of actuators are relays, solenoids, and motors. The solenoid converts electric current into linear motion. AIR Coil Plunger Solenoid Symbol

Solenoid Valve A solenoid valve is a combination of: a solenoid with its core or plunger a valve body containing an orifice in which a disc or plug is positioned to restrict or allow flow When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward SOL A Forward motion of piston Directional solenoid valve FWD CR

Stepper Motor A stepper motor converts electrical pulses applied to it into discrete rotor movements called steps. They are used to provide precise position control of movement. Module Stepper-motor translator Step motor Stepper motor control system Communicates with the PLC and responds with pulse trains Enables control of the stepper motor The motor will move one step for each pulse received

11. Sensors convert mechanical, magnetic, thermal, optical, and chemical variations into: electric voltages and currents. sound movement all of these 12. A proximity switch can be actuated without any physical contact. (True/False)

13. A thermocouple, when heated, increases its resistance value. (True/False) 14. The force applied to a strain wire gauge causes it to bend and change resistance. (True/False) 15. A capacitive proximity sensor may be actuated by nonconductive materials. (True/False)

16. When a magnetic pickup sensor is used to indicate the speed of a shaft, the speed is usually determined by measuring the: a. voltage output produced b. frequency of the pulses produced c. current output produced d. change in resistance that occurs

17. A through-beam photoelectric sensor is used to measure the change in ______ caused by the target crossing the optical axis. a. resistance c. current b. voltage d. light quantity

18. Which of the following devices would “not” be classified as an actuator? a. relay c. solenoid b. tachometer d. motor 19. Solenoid valves are available to control: a. oil flow c. water flow b. air flow d. all of these 20. A(n) ______ converts electrical pulses applied to it into discrete rotor movements. a. tachometer c. stepper motor b. solenoid d. electronic magnetic flowmeter