Politecnico di Torino - ICT school Goup B - goals ELECTRONIC SYSTEMS B INFORMATION PROCESSING B.1 Systems, sensors, and actuators» System block diagram» Analog and digital signals» Examples of sensors» Examples of actuators 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 23/04/2009-1 SisElnB1-2008 DDC 23/04/2009-2 SisElnB1-2008 DDC B1 - Sensors and actuators Chapter 2 Introduction 2.1 Introduction Describing sensor performance Sensors Actuators Laboratory measuring equipment. 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 Speaker electric signal sound 12.3 12.4 Towards external world Analog/Digital/Analog sequence External world physical signals are in most cases analog Electronic sensors and actuators must handle analog signals: INPUT SENSORS ELECTRONIC SYSTEM OUTPUT ACTUATORS 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 A/D ELECTRONIC SYSTEM NUMERIC SYSTEM ANALOG SIGNALS D/A 23/04/2009-5 SisElnB1-2008 DDC 23/04/2009-6 SisElnB1-2008 DDC 2008 DDC - 2006 Storey 1
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) Sensors: analog front-end ADC Numeric system DAC ELETTRONIC SYSTEM Actuators: analog back-end 23/04/2009-7 SisElnB1-2008 DDC 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 12.8 Sensor precision and accuracy Precision a measure of the lack of random errors (scatter) precision accuracy 12.9 Sensor parameters 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 12.10 Sensor types 2.3 Temperature: Resistive thermometers 2.3.1 Physical property of a material that changes in response to some excitation are used in sensors. resistive inductive capacitive piezoelectric photoresistive elastic thermal. 12.11 typically use platinum wire (such a device is called a platinum resistance thermometers or PRT) linear but has poor sensitivity. A typical PRT element PRT A sheathed 12.12 2008 DDC - 2006 Storey 2
Temperature sensors: Thermistors Temperature sensors: pn junctions use materials with a high thermal coefficient of resistance sensitive but highly non-linear. A typical disc thermistor A threaded thermistor 12.13 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 12.14 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 Light sensors: Photoconductive 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) 12.15 12.16 Force sensors: Strain gauge 2.3.3 Displacement sensors: Potentiometers 2.3.4 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. Direction of sensitivity A strain gauge 12.17 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. 12.18 2008 DDC - 2006 Storey 3
Displacement sensors: Inductive proximity coil inductance is greatly affected by the presence of ferromagnetic materials. Displacement sensors: Switches simplest form of digital displacement sensor many forms: lever or push-rod operated microsw, float switches, pressure switches, etc. here the proximity of a ferromagnetic plate is determined by measuring the inductance of a coil. Inductive proximity sensors 12.19 A limit switch A float switch 12.20 Displacement: absolute position encoders 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. Displ. sensors: Incremental position encoder 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). 12.21 12.22 Displacement sensors: pulse counters Motion speed sensors 2.3.5 several methods use counting to determine position two examples are given below. Inductive sensor Opto-switch sensor 12.23 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. 12.24 2008 DDC - 2006 Storey 4
Sound sensors: Microphones 2.3.6 Sensor interfacing: Resistive devices 2.3.7 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. 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 12.25 12.26 Interfacing: differential signals Differential and common mode Signal from sensors connected by long wires Signal noise (cannot be removed from signal) Signal + noise V 1 V 2 V D = (V1-V2) V C = (V1+V2)/2 Information is carried by differential signal V D = (V1-V2) Differential signal noise (same on both wires, removed by difference) Differential signal GND Noise affects the common mode voltage V C = (V1+V2)/2 23/04/2009-27 SisElnB1-2008 DDC 23/04/2009-28 SisElnB1-2008 DDC Differential amplifier Example of differential signaling A Differential amplifier removes common mode signals: Strain gages are used in bridge configuration V O = A D V D V 1 In steady state, the bridge is balanced: R1 = R2 = R3 = R4 no differential signal Vd is developed V 1 V 2 GND V D = (V1-V2) V C = (V1+V2)/2 V C V 2 V O the common mode signal Vc is ignored Vr A strain causes a change of R2 value, which in turn R1 unbalances the bridge and generates a differential signal R2 R3 R4 A D, A C Vu Vu = A D V D 23/04/2009-29 SisElnB1-2008 DDC 23/04/2009-30 SisElnB1-2008 DDC 2008 DDC - 2006 Storey 5
Sensor interfacing: switches use a single resistor to produce a voltage output all mechanical switches suffer from switch bounce Electronics can remove bounces (part E) 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 12.31 12.32 Heat actuators 2.4.1 Light actuators: lamps 2.4.2 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. 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. 12.33 12.34 Light actuators: Light-emitting diodes (LEDs) produce light when electricity is passed through them. Can produce light of different colours. used individually or in multiplesegment devices such as the LED seven-segment displays 7-segment display. 12.35 Light actuators: Liquid crystal displays 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 display any character or image. A custom LCD display 12.36 2008 DDC - 2006 Storey 6
Light actuators: Fibre-optic communication Force actuators: solenoids 2.4.3 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. 12.37 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 12.38 Displacement and motion actuators: meters 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 deflection typically 50 µa 1 ma Moving-coil meters 12.39 Actuators: motors 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. 12.40 Stepper motors Stepper motors driving a central rotor surrounded by a number of coils (or windings) opposite pairs of coils are energised in turn this drags the rotor round one step at a time speed proportional to frequency typical motor might require 48-200 steps per revolution. 12.41 Stepper-motor current waveforms A typical stepper-motor 12.42 2008 DDC - 2006 Storey 7
Sound actuators 2.4.4 Actuator interfacing 2.4.5 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. 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 12.43 12.44 Actuator interfacing - b 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. Input amplifiers Analog/Digital converters Digital processing devices Digital/Analog converters Output amplifiers Power supply Other functional units group B, C group F group E, F group F group C, D group D 12.45 23/04/2009-46 SisElnB1-2008 DDC 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 23/04/2009-47 SisElnB1-2008 DDC 2008 DDC - 2006 Storey 8