Programmable Logic Controller

Transducer

A transducer is any device that converts energy fromone form to another.

Input transducer (microphone) convertssound energy to electric energy

Output transducer (speaker) convertselectric energy to sound energy

Amplifier

Sensors

Sensors are input transducers used for detecting andoften measuring the magnitude of something. Theyconvert mechanical, magnetic, thermal, optical, andchemical variations into electric voltages and currents.

Photoelectricsensor

Sensors

Sensors provide the equivalent of eyes, ears, nose,and tongue to the microprocessor brain.

Microprocessor

Opticalsensor

Gassensor

Microphone

Probe

Proximity Sensor

Proximity sensors or switches detect the presence ofan object without making physical contact with it.

Proximity Sensor Applications

The object being detected is too small, lightweight, orsoft to operate a mechanical switch.

Rapid response and high switching rates are required.

An object has to be sensed through nonmetallic barrierssuch 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 typeBlock diagram

As the targetmoves into thesensing area, the sensorswitchesthe output ON

Capacitive Proximity Sensor

A capacitive proximity sensor can be actuated by bothconductive and nonconductive material such as wood,plastics, liquids, sugar flour and wheat.

Operation is similar to that of inductiveproximity sensor. Instead of a coil, theactive 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).

Common

NO

NC

The switch is actuated by a magnet.

Magnet

N S

Reed Switch Activation

Magnet

Reed switch

Proximity motion – movementof the switch or magnet willactivate 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 resistance5,000 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.

Object to besensed

Light detector

Light source

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.

Reflective Photoelectric Sensor

Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected.

Retro-reflective typeOperating 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 quicklythan manual methods, and arehighly accurate.

ScannerDecoder

PLC

DiverterThe 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 signalis electronically convertedto a 4 mA to 20 mA output,which supplies flow rate toexternal control devices.

Strain /Weight Sensors

A strain gauge transducer converts a mechanical strain into an electric signal.

ForceWire 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.

The load cell providessensor input to thecontroller, which displays the weight and controls the hopper chute.

Load cell

ControllerHopper

ChuteON/OFF

Control

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 pairof 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) temperaturesensor 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.

Flow Magnet

Turbine

Turbine Flow Meter

CoilThe turbine blades turn at a rate proportional to the fluid velocity and are magnetized to induce voltage pulses coil.

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.

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

LoadTachometers coupledto motors are commonly used in motor speed controlapplications to providea 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 SMagnetSensor

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 Solenoidvalve

Controlrelay

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.

AIR

Coil

Plunger

Solenoid Symbol The solenoid converts electric current into linear motion.

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

SOL A

Forward motion of piston

Directionalsolenoid valve

FWDCR

CR

SOL A

When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward

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.

ModuleStepper-motor translator

Stepmotor

Stepper motor control system

Communicates with the PLC and responds with pulse trains

Enables controlof the stepper motor The motor will move

one step for each pulsereceived