ELECTRONIC SYSTEMS - polito.it...Electronic Systems - B1 23/04/2009 2008 DDC - 2006 Storey 1 23/04/2009 - 1 SisElnB1 - 2008 DDC ELECTRONIC SYSTEMS B – INFORMATION PROCESSING B.1

Electronic Systems - B1 23/04/2009

2008 DDC - 2006 Storey 1

23/04/2009 - 1 SisElnB1 - 2008 DDC

ELECTRONIC SYSTEMS

B – INFORMATION PROCESSINGB.1 – Systems, sensors, and actuators

» System block diagram» Analog and digital signals» Examples of sensors» Examples of actuators

Politecnico di Torino - ICT school

23/04/2009 - 2 SisElnB1 - 2008 DDC

Goup B - goals

• System block diagram – Sensors and actuators

– Analog and digital signals

• Examples of sensors• Examples of actuators

• Amplifiers taxonomy and parameters• Dual-port equivalent circuit• Time and frequency response

Storey, Electronics: A Systems Approach, 3rd Edition © Pearson Education Limited 200612.3

B1 - Sensors and actuators

� Introduction� Describing sensor performance� Sensors� Actuators� Laboratory measuring equipment.

Chapter 2


Introduction

� To be useful, systems interact with their environment using transducers: sensors and actuators.

� They convert one physical quantity into another.– mercury-in-glass thermometer

temperature � displacement of a column of mercury

– microphone sound � electrical signal

– Speakerelectric signal � sound

2.1

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Towards external world

• External world physical signals are in most cases analog

• Electronic sensors and actuators must handle analog signals:

ELECTRONICSYSTEM

INPUTSENSORS

OUTPUTACTUATORS

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Analog/Digital/Analog sequence

• The core of most electronic system is numeric• Signals must be translated from analog to digital, and

then back from digital to analog:– ANALOG/DIGITAL (A/D) conversion

– DIGITAL/ANALOG (D/A) conversion

ELECTRONIC SYSTEM

NUMERICSYSTEM

A/D D/A

ANALOG SIGNALS


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ELETTRONICSYSTEM

Analog �� Digital �� Analog

• Most electronic systems includes:– interfaces towards the analog external world (sensors)

– A/D conversion– Numeric signal handling

– D/A conversion – interfaces towards the analog external world (actuators)

ADC

Numericsystem DAC

Actuators:analog back-end

Sensors: analog front-end


Describing sensor performance

� Range– max and min values that can be measured.

� Resolution or discrimination– smallest discernible change in the measured value.

� Error– difference measured - actual values.

� Precision, accuracy, inaccuracy, uncertainty– measure of the maximum expected error.

2.2


� Precision– a measure of the lack of random errors

(scatter)

Sensor precision and accuracy

precision

accuracy


� Linearity– maximum deviation from a ‘straight-line’

response– expressed as a percentage of the full-scale value

� Sensitivity– a measure of the change produced at the output

for a given change in the quantity being measured

– Also called gain

Sensor parameters


Sensor types

� Physical property of a material that changes in response to some excitation are used in sensors.– resistive– inductive– capacitive– piezoelectric– photoresistive– elastic– thermal.

2.3


Temperature: Resistive thermometers

� typically use platinum wire (such a device is called a platinum resistance thermometers or PRT)

� linear but has poor sensitivity.

A typical PRT element A sheathed PRT

2.3.1


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� use materials with a high thermal coefficient of resistance

� sensitive but highly non-linear.

A typical disc thermistor A threaded thermistor

Temperature sensors: Thermistors


� a semiconductor device

� inexpensive, linear and easy to use

� limited temperature range(perhaps -50°C to 150°C) due to nature of semiconductor material.

pn-junction sensor

Temperature sensors: pn junctions


Light sensors: Photovoltaic

� light falling on a pn-junction can be used to generate electricity, as in a solar cell

� photodiodes are small devices used as sensors

� fast acting, but the voltage produced is not linearly related to light intensity.

A typical photodiode

2.3.2


� such devices do not produce electricity, but simply change their resistance.

� photodiodes (as described earlier) can be used in this way to produce linear devices.

� phototransistors act like photodiodes but with greater sensitivity.

� light-dependent resistors (LDRs) are slow, but respond like the human eye. A light-dependent resistor (LDR)

Light sensors: Photoconductive


Force sensors: Strain gauge

� stretching in one direction increases the resistance of the device, while stretching perpendicular to this has little effect

� can be bonded to a surface to measure strain� used within load cells and pressure sensors.

A strain gauge

Direction of sensitivity

2.3.3


Displacement sensors: Potentiometers

� resistive potentiometers are one of the most widely used forms of position sensor.

� can be angular or linear.

� consists of a length of resistive material with a sliding contact onto the resistive track.

� when used as a position transducer a potential is placed across the two end terminals, the voltage on the sliding contact is then proportional to its position.

� an inexpensive and easy to use sensor.

2.3.4


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Inductive proximity sensors

� coil inductance is greatly affected by the presence of ferromagnetic materials.

� here the proximity of a ferromagnetic plate is determined by measuring the inductance of a coil.

Displacement sensors: Inductive proximity


� simplest form of digital displacement sensor– many forms: lever or push-rod operated microsw, float

switches, pressure switches, etc.

A limit switch A float switch

Displacement sensors: Switches


� A pattern of light and dark strips printed on to a strip is detected by a sensor that moves along it.

– The pattern takes the form of a series of lines as shown below.

– It is arranged so that the combination is unique at each point.

– Sensor is an array of photodiodes.

Displacement: absolute position encoders


� uses a single line that alternates black/white– two slightly offset sensors produce outputs as shown below

– detects motion in either direction, pulses are counted to determine absolute position (which must be initially reset).

Displ. sensors: Incremental position encoder


� several methods use counting to determine position– two examples are given below.

Opto-switch sensorInductive sensor

Displacement sensors: pulse counters


Motion speed sensors

� Measure quantities such as velocity and acceleration.

– Can be obtained by differentiating displacement� Differentiation tends to amplify high-frequency noise.

– Some sensors give velocity directly� e.g. measuring frequency of pulses in the counting

techniques described earlier gives speed rather than position.

– Some sensors give acceleration directly� e.g. accelerometers usually measure the force on a

mass.

2.3.5


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Sound sensors: Microphones

� a number of microphone forms are available– e.g. carbon (resistive), capacitive, piezoelectric – moving-coil devices use a magnet and a coil

attached to a diaphragm.

2.3.6


Sensor interfacing: Resistive devices

� a potentiometer, with a fixed voltage across the outer terminals, voltage on the third related to position

� As device resistance changes, this change is converted into a voltage

� the output of this arrangement is not linearly related to the change in resistance– For small changes, can be

approximated with linear relation

2.3.7

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Interfacing: differential signals

• Signal from sensors connected by long wires

Differential signal

SignalSignal + noise

Differential signal

noise (same on both wires, removed by difference)

noise (cannot be removed from signal)

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Differential and common mode

GND

V1 V2

VD = (V1-V2)

VC = (V1+V2)/2

VD/2

VD/2

Noise affects the

common mode voltage

VC = (V1+V2)/2

Information is carried by differential signal

VD = (V1-V2)

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Differential amplifier

• A Differential amplifier removes common mode signals:

VO = AD VD

GND

V1 V2

VD = (V1-V2)

VC = (V1+V2)/2

VD/2

VD/2

V1

VO

V2

VD/2

VC

VD/2

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Example of differential signaling

• Strain gages are used in bridge configuration

• In steady state, the bridge is balanced: R1 = R2 = R3 = R4

– no differential signal Vd is developed– the common mode signal Vc is ignored

– A strain causes a change of R2 value, which in turn unbalances the bridge and generates a differential signal

Vu = AD VD

Vr

R1

R2

R3

R4

AD, AC

Vu


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� use a single resistor to produce a voltage output� all mechanical switches suffer from switch bounce� Electronics can remove bounces (part E)

Sensor interfacing: switches


Actuators

� An electrical or electronic system must be able to affect its external environment.

� This is done through one or more actuators.

� As with sensors, actuators are transducers, which convert one physical quantity into another.

� Here: actuators that receive electrical signals and from them vary some external physical quantity.

2.4


Heat actuators

� Most heat actuators are simple resistive heaters.

� For applications requiring a few watts ordinary

resistors of an appropriate power rating can be used.

� For higher power applications there are a range of heating cables and heating elements available.

2.4.1


Light actuators: lamps

� For general illumination: incandescent light bulbs or fluorescent lamps.

– power ratings range from a fraction of a watt

to perhaps hundreds of watts

– easy to use but relatively slow in operation

– unsuitable for signalling and communication

applications.

2.4.2


� produce light when electricity is passed through them.

� Can produce light of different colours.

� used individually or in multiple-segment devices such as the 7-segment display.

LED – seven-segment displays

Light actuators: Light-emitting diodes (LEDs)


� 2 sheets of polarised glass with a thin layer of liquid sandwiched between them.

� an electric field rotates the polarization of the liquid making it opaque.

� multi-element displays: e.g 7-segment displays

� matrix display to displayany character or image. A custom LCD display

Light actuators: Liquid crystal displays


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� used for long-distance communication

� Guiding removes the effects of ambient light– fibre-optic cables can be made of:

� optical polymer– inexpensive and robust– high attenuation, therefore short range (up to about 20 metres)

� glass– much lower attenuation, use up to hundreds of kilometres– more expensive than polymer fibres

– light source would often be a laser diode.

Light actuators: Fibre-optic communication


Force actuators: solenoids

� basically a coil and a ferromagnetic ‘slug’

� when energised the slug is attracted into the coil� force is proportional to current

� can produce force, displacement or motion

� linear or angular

Small linear solenoids

2.4.3


� moving-iron – effectively a rotary solenoid plus spring

– can measure DC or AC

� moving-coil– most common form

– deflection proportional to average value of current

– full scale deflectiontypically 50 µA – 1 mA

Moving-coil meters

Displacement and motion actuators: meters


� three broad classes– AC motors

� primarily used in high-power applications

– DC motors� used in precision position-control applications

– Stepper motors– a digital actuator used in position control applications.

Actuators: motors


Stepper motors

� a central rotor surrounded bya number of coils (or windings)

� opposite pairs of coils are energised in turn

� this ‘drags’ the rotor roundone ‘step’ at a time

� speed proportional to frequency

� typical motor might require 48-200 steps per revolution.


A typical stepper-motorStepper-motor current waveforms

Stepper motors driving


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Sound actuators

� Speakers– usually use a permanent magnet and a movable coil

connected to a diaphragm.– input signals produce current in the coil causing it to

move with respect to the magnet.

� Ultrasonic transducers– at high frequencies speakers are often replaced by

piezoelectric actuators– operate over a narrow frequency range.

2.4.4


Actuator interfacing

� Resistive devices– Interfacing involves controlling the power in the device.– In a resistive actuator, power is related to the voltage.– For high-power devices the problem is in delivering sufficient

power to drive the actuator (Group D).

� Switching regulation– High-power actuators are often controlled in an ON/OFF

manner.� This technique uses electrically operated switches

2.4.5


� Capacitive and inductive devices– Many actuators are capacitive or inductive

(such as motors and solenoids).

– These create particular problems –particularly when using switching techniques.

– We will return to look at these problems when we have considered capacitors and inductors in more detail.

Actuator interfacing - b

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Other functional units

• Input amplifiers � group B, C

• Analog/Digital converters � group F

• Digital processing devices � group E, F

• Digital/Analog converters � group F

• Output amplifiers � group C, D

• Power supply � group D

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Lesson B1: final test

• How do electronic systems interact with external world?

• Which is the general architecture of an electronic system?

• Define accuracy and precision of transducers

• Why do we need Analog/Digital and Digital/Analog converters?

• Describe some example of sensors with analog and digital output

• Describe some examples of actuators

Documents

ELECTRONIC SYSTEMS - polito.it...Electronic Systems - B1 23/04/2009 2008 DDC - 2006 Storey 1 23/04/2009 - 1 SisElnB1 - 2008 DDC ELECTRONIC SYSTEMS B – INFORMATION PROCESSING B.1