Hardware Components: Sensors, Actuators, Converters

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Hardware Components:Sensors, Actuators,Converters Sudeep PasrichaColorado State UniversityCS/ECE 561Fall 2012 Copyrighted Material adapted from Peter Marwedel, Rajesh Gupta, Frank Vahid and Tony Givargis1Simplified Block Diagramactuators21

Sensors and Actuators Sensors: Capture physical stimulus (e.g.,heat, light, sound, pressure,magnetism, or other mechanicalmotion) Typically generate a proportionalelectrical current May require analog interfacepressuremicspeaker Actuators Convert a command to a physicalstimulus (e.g., heat, light, sound,pressure, magnetism, or othermechanical motion) May require analog interfaceradarcameracompassaccelerometer3Sensors Processing of physical data starts with capturing datafrom sensors Sensors can be designed for virtually every physicalstimulus heat, light, sound, weight, velocity, acceleration, electricalcurrent, voltage, pressure, . Many physical effects used for constructing sensors. law of induction generation of voltages in an electric fieldlight-electric effects; magnetic effects; 42

Example: Acceleration Sensor MEMS deviceMicroelectromechanical systems( 1 to 100 µm) Small mass in center When accelerated: Mass displaced from centerResistance of wires connected tomass changeDetect change in resistance andmodel accelerationiPhones have MEMSaccelerometers5Example: Acceleration Sensor Alternative implementation: accelerometer measures distancebetween a plate fixed to the platformand one attached by a spring anddamper measurement is typically done bymeasuring capacitance Many uses! Navigation, orientation, drop detection,image stabilization, airbag systems Challenges Vibration Nonlinearities in the spring or damper Separating tilt from acceleration63

Measuring Changes in Orientation:Gyroscopes7Example: MEMS GyroscopeMEMS gyroscope - orientationVibrating-Wheel GyroscopeMEMS gyroscope forsatellite positioningDigital cameras using MEMSgyroscopes for image stabalizationWii's controller uses MEMS sensors, MEMSaccelerometers and MEMS gyroscope (MotionPlus)84

Inertial Navigation Systems Combinations of: GPS(for initialization and periodic correction). Three axis gyroscope measures orientation. Three axis accelerometer, double integrated forposition after correction for orientation. Typical drift for systems used in aircraft haveto be: 0.6nautical miles per hour tenths of a degree per hour Good enough? It depends on the application!9Example: Gait-Phase Detection sensorEmbedded in a Shoe Insole Measures the angular velocity of the foot Used to activate a functional electricalstimulator (FES) attached to the foot toactivate nerves Over 96% accuracy105

Example: Strain Gauges11Image SensorsMirror CoveringImage SensorMirror RaisedSensorExposed126

Example: Charge-coupled devices (CCD) Image is projected by lens on a capacitor array (photoactive) Each capacitor accumulates an electric charge proportional to the light intensity at that location Next, a control circuit causes each capacitor to ‘shift’ itscontents to its neighbor The last capacitor in the array dumps its charge into a chargeamplifier, which converts the charge into a voltage13How CCD Sensors Record Color Each CCD cell in the CCD arrayproduces a single valueindependent of color. To make color images, CCD cellsare organized in groups of fourcells (making one pixel) and a BayerFilter is placed on top of the groupto allow only red light to hit one ofthe four cells, blue light to hitanother and green light to hit theremaining two. The reasoning behind the two greencells is because the human eye ismore sensitive to green light and itis more convenient to use a 4 pixelfilter than a 3 pixel filter (harder toimplement) and can becompensated after a image capturewith something called whitebalance. Ex. A Bayer filter applied to theunderlying CCD pixel147

Example: CMOS Image Sensors Based on standard production process for CMOS chips,allows integration with other components Sensor array made of photo detector and amplifiertransistors at each pixel Many of the photons hitting the chip hit the transistorsinstead of the photodiode – lower light sensitivity15Source: B. Diericks: CMOS image sensor concepts.Photonics West 2000 Short course (Web)Comparison CCD/CMOS sensors168

Comparison CCD/CMOS sensors17Example: Biometric SensorsExample: Fingerprint sensor ( Siemens, VDE):Matrix of 256 x256 elem.Voltage distance.Resistance alsocomputed.No fooling byphotos and waxcopies!189

Example: Artificial eyes Dobelle Institute(www.dobelle.com)19Example: Artificial eyes (2) Retinal implants (BRI Project) array of electrodes to stimulate light sensing cells that do not workcamera on pair of eyeglasses captures image image processed by a microcontroller to producea simplified picture picture wirelessly beamed to the implant,which activates 15 electrodes inside the eye(to create a15-pixel image) Goal: 1000 pixel images implant also receives power wirelessly fromthe microcontroller electronics housed within a waterprooftitanium case similar to those used forheart pacemakers Boston Retinal Implant Project(http://www.bostonretinalimplant.org)2010

Example: Self Powering Sensors Nanoscale sensors piezoelectric zincoxide nanowires subtle movements canbend these wires soundwaves, wind, eventurbulence of blood flowover an implanted device Uses: Hearingaids, bonedensity loss monitors,“electric current fromclothes”21Other examples of sensors Heart monitoring sensors “Managing Care Through the Air”» IEEE Spectrum Dec 2004 Rain sensors for High-end autoswiper control Pressure sensors Touch pads/screens Proximity sensors Collision avoidance Vibrationsensors Smoke sensors Based on the diffraction of light wavesin the presence of smoke Thermal sensors (thermistors) SARS detection (“high fever”)2211

Design Issues with Sensors23Simplified Block Diagramactuators2412

Sensors and Actuators Sensors: Capture physical stimulus (e.g.,heat, light, sound, pressure,magnetism, or other mechanicalmotion) Typical generate a proportionalelectrical current May require analog interfacesolenoidspeaker Actuators Convert a command to a physicalstimulus (e.g., heat, light, sound,pressure, magnetism, or othermechanical motion) May require analog interfacelaser diode/transistordc motorLED display25Actuators Output physical stimulus varies in range andmodality Large (industrial) control actuators Pneumatic systems: physical motion Optical output IR, LEDs, displays, etc. Motor controllers DC, stepper, servo, Sound Loudspeakers, etc. Listgoes on .2613

Stepper Motor Controller Stepper motor: rotates fixed number of degreeswhen given a “step” signal Incontrast, DC motor simply rotates when power applied,and coasts to stop Rotation achieved by applying specific voltagesequence to coils Controllergreatly simplifies this Stepper motors used commonly in hard drives(traditionally) Headmotor (stepper), spindle motor (DC)27Stepper Motor ControllerSequence12345A B A’ B’ - -sbit SM A, SM B, SM AP, SM BP; // portsint curr pos; // tells us the current step positionvoid reset() { // must be called to synchronizecurr pos 0;for(int i 0; i 4; i ) {move one step(0);}}void move one step(int dir/*0 CW,1 CCW*/) {const int SM TBL[4][4] {1, 1, 0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 1 };curr pos (curr pos (dir 0 ? 1 : 3)) % 4;SM A SM TBL[curr pos][0];SM B SM TBL[curr pos][1];SM AP SM TBL[curr pos][2];SM BP SM TBL[curr pos][3];ms delay(50);}2814

Servo Motor Controller Actuator in a modern hard disk uses a device called a voice coil to move the head arms in and out over thesurface of the platters a closed-loop feedback system called a servo system to dynamicallyposition the heads directly over the data tracks works using electromagnetic attraction and repulsion How it works: Coil is wrapped around a metal protrusionon the end of the set of head arms This is mounted within an assemblycontaining a strong permanent magnet When current is fed to the coil, anelectromagnetic field is generated thatcauses the heads to move By controlling the current, the heads can betold to move in or out much more precisely than using a stepper motor29MEMS ActuatorsMicroelectromechanical systems ( 1 to 100 µm)Huge variety of actuators and output devices.Very small devices with low power consumptionE.g., applications: space telescopes and DLP projectors (moving micromirrors),miniaturized drug delivery inside human body (bio-MEMS), accelerometers,gyroscopesSilicon Gear and Chain –Chain Links 50 μm ApartSpider Mite Crawling onMicro-Mirror Device3015

Actuators: The Future? Cyborg Beetle giantflower beetle withimplanted processor ,microbattery, electrodes electrodes deliverelectrical jolts to its brainand wing muscles flight can be wirelesslycontrolled e.g.take off, turn, stopmidflight. Berkeley 9/?a f31Touch Bionics iLIMB ActuatorIt’s got an embedded computer, arechargeable battery, and five small dcmotors. It costs US 18 500. And it can dothings most other prosthetic hands justcan’t, like grabbing a paper cup withoutcrushing it, turning a key in a lock, andpressing buttons on a cellphone.The fingers of Touch Bionics’ iLIMB Handare controlled by the nerve impulses of theuser’s arm, and they operate independently,adapting to the shape of whatever they’regrasping. The hand can also do superhumantricks, like holding a very hot plate orgripping an object tirelessly for days. A skintone covering gives the bionic hand a lifelikelook, but some customers refersemitransparent models, to proudly flaunttheir robotic hands. “They like theTerminator look,” says Touch Bionics CEOStuart Mead. IEEE Spectrum, Oct. 2007.3216

Simplified Block Diagramactuators33Discretization of TimeSample and hold circuitSampling: how often the signal is converted?Quantization: how many bits used for samples?3417

Sampling Sampling: how often is the signal converted (sampled)to represent original signal? Twice as high as the highest frequency signal present in the input As much as 10 to 20 times for even better results Nyquist sampling theorem Typical Sampling Rates35Sampling Process – Time Domain Sampling multiplying with comb function3618

Sampling Process – Freq Domain37Aliasing Aliasing: erroneous signals, not present in analogdomain, but present in digital domain1.5Signal freq: 5.6 HzSampling freq: 9 Hz10.50-0.5-1-1.53819

Preventing Aliasing Sample at higher than necessary rate Use anti-aliasing filters Helpseliminate high frequency noise that can causealiasing of lower frequency input signal39Examples of Aliasing in computer graphicsOriginalSub-sampled, no filtering4020

Quantization Sample precision - the resolution of a sample value Quantization depends on the number of bits used tomeasure the height of the waveform. Audio formats are described by sample rate andquantization. Voice quality - 8 bit quantization, 8000 Hz mono(8 Kbytes/sec) 22kHz 8-bit mono (22kBytes/s) and stereo (44Kbytes/sec) CD quality - 16 bit quantization, 44100 Hz linear stereo (196 Kbytes/s) Quantization: how many bits used to represent a sample? Sufficient to provide required dynamic range 16-bit A/D 20 log10(216) 96 dB (human ear limit) Clipping: input signal beyond thedynamic .5V00111.0V00100.5V00010V4433analog output (V)Vmax 7.5Vanalog input (V)Analog-to-Digital Converter21timet10100t21000t3t40110Digital output010121t10100t2t31000 0110Digital inputt4time01010000proportionalityanalog to digitaldigital to analog4221

A/D Converters: Flash A/D ConverterDigital computers require digitalform of physical valuesA/D-conversion; many methodswith different speeds. Parallel comparisonwith reference voltage Speed: O(1) HW complexity: O(n) n # of distinguishedvoltage levelsEncodes input number ofmost significant ‘1’ as anunsigned number, e.g.“1111” - “100”,“0111” - “011”,“0011” - “010”,“0001” - “001”,“0000” - “000”(Priority encoder).43A/D Converters: Successive ApproximationKey idea: binary search:Set MSB '1'if too large: reset MSBSet MSB-1 '1'if too large: reset MSB-1 .Speed:O(log(n))Hardware complexity:O(1)with n # of distinguishedvoltage levels;slow, but high precision possible.4422

Successive ApproximationGiven an analog input signal whose voltage should range from0 to 15 volts, and an 8-bit digital encoding, calculate the correctencoding for 5 volts.½(Vmax Vmin) 7.5 voltsVmax 7.5 volts.00000000½(5.63 4.69) 5.16 voltsVmax 5.16 volts.01010000½(7.5 0) 3.75 voltsVmin 3.75 volts.01000000½(5.16 4.69) 4.93 voltsVmin 4.93 volts.01010100½(7.5 3.75) 5.63 voltsVmax 5.63 volts01000000½(5.16 4.93) 5.05 voltsVmax 5.05 volts.01010100½(5.63 3.75) 4.69 voltsVmin 4.69 volts.01010000½(5.05 4.93) 4.99 volts0101010145Quantization ards 0h(t)-w(t)4723

How Fast should ADC be?48Signal Processing Any interesting embedded system has toprocess some input signals and generatesome output signals Weuse the term signal in a general way Digital devices process signals in digitalform Auniformly sampled stream of data spread intime (e.g., audio) or space (e.g., image)4924

Signals and Systems Signal: set of information and data E.g., audio, video, radio, Continuous vs. discrete-timeAnalog and digital System: entity that transforms input signals tooutput signals Linear and non-linearConstant parameter and time-varying parameterCausal and non-causal Digital Signal Processing (DSP) Digitize signals and process them in the digital domain In software (e.g., DSPs, ASIPs, GPPs) or hardware (ASIC, FPGA)50Analog vs. Digital5125

General DSP ArchitectureEnvironmentEmbedded Systemf(t)SensorsA/Dfn Pu(t)ActuatorsMemoryD/A un52Signal ProcessingSampleHeight Digital signal S0, S1, S2 Sn-1samples What can we do with it? Transpose: e.g., Zi Si K Amplify: e.g., Zi Si Compose: e.g., Zi (S1i 1 K1) (S2i Filter: e.g, Zi (Si Si 1) / 2 Compress: e.g., using Huffman codes Archive, match against database, etc. 2 K2) Or, process after converting to frequency domain Spectralanalysis5326

Time Domain Any continuous periodictime varying signal canbe represented as thesum of cosine functionsof different amplitude andfrequency E.g.,input signalcaptured as the sum of 4cosine functions Capturedas aTrigonometric FourierSeries (continuous)54Time and Frequency Domain Signal can be represented infrequency domain Discrete values, representing amplitudeat each frequency for the periodic signal How about aperiodic signals? Linear function of infinite number ofsinusoidal functions 5527

(Continuous) Fourier Transform (FT) Represent periodic functions Input: h(t) continuous timedomain signal Output: H(f) continuousfrequency domain signal H ( f ) h(t )e j 2 ft dt H(f) is represented bycomplex number a b j Amplitude: (a2 b2) Phase-shift: arctan(b/a)h(t ) H ( f )e j 2 ft df e j cos( ) j sin( )56Discrete Fourier Transform (DFT) Input: ht discrete timedomain signal Output: Hf discretefrequency domain signal Hf is represented bycomplex number a b j Amplitude: (a2 b2) Phase-shift: arctan(b/a) DFT is computationallyexpensive 1 N 1 jH f ht eN t 0N 1ht H f ef 0j j2 tfN2 ftNDFTIDFTEuler’s Eq.e cos( ) j sin( )There is a fast softwareimplementation (FFT)5728

Discrete Cosine Transform A special version of the DFT Usedin JPEG and MPEG compression Why DCT and not DFT?58Simplified Block Diagramactuators5929

Digital-to-Analog (D/A) ConvertersVarious types, can be quite simple,e.g.:60Kirchhoff‘s junction ruleKirchhoff‘s Current Law, Kirchhoff‘s first rule Kirchhoff’s Current Law:At any point in an electricalcircuit, the sum of currentsflowing towards that point isequal to the sum of currentsflowing away from that point. Example:(Principle of conservation of electriccharge)i1 i2 i4 i3Formally, for any node in a circuit: ik ki1 i2-i3 i4 0 0Count current flowing away from node as negative.[Jewett andSerway, 2007].6130

Kirchhoff's loop ruleKirchhoff‘s Voltage Law, Kirchhoff's second ruleExample:The principle of conservation ofenergy implies that: The sum of the potentialdifferences (voltages) across allelements around any closedcircuit must be zero[Jewett and Serway, 2007].Formally, for any loop in a circuit: VkkV1-V2-V3 V4 0 0V3 R3 I3 if current counted inthe same direction as V3V3 -R3 I3 if current counted inthe opposite direction as V3Count voltages traversed against arrowdirection as negative62Operational Amplifiers (Op-Amps) Operational amplifiers (op-amps) are devices amplifying thevoltage difference between two input terminals by a largegain factor gSupply voltageVout (V - V-) g-op-amp V-VoutHigh impedance input terminals Currents into inputs 0V groundOp-amp in a separate package(TO-5) [wikipedia]For an ideal op-amp: g (In practice: g may be around 104.106)6331

Op-Amps with feedback In circuits, negative feedback is used to definethe actual gainIR1Due to the feedback tothe inverted input, R1reduces voltage V-.To which level?loop-RV1op-ampV- VoutgroundVout - g V- (op-amp feature)I R1 Vout-V- 0 (loop rule) I R1 - g V- -V- 0 (1 g) V- I R1 V I R11 gI R1 0g 1 gV ,ideal limV- is called virtual ground: the voltage is 0,but the terminal may not be connected to ground64Output voltage no. represented by xDue to Kirchhoff‘s laws:I x3 VrefR x2 Vref2 RDue to Kirchhoff‘s laws:Current into Op-Amp 0:I I'Hence:Finally:Vref4 R x0 Vref8 R3Vref xi 2i 3R i 0V R1 I ' 0 x1 V R1 I 0R 3R V Vref 1 xi 2i 3 Vref 1 nat ( x )R i 08 R6532

(Attempted) reconstruction of input signal** Assuming 0order hold66Limitations Actualfilters do not compute sinc( )In practice, filters are used as an approximation.Computing good filters is an art itself! Allsamples must be known to reconstruct e(t) or g(t). Waiting indefinitely before we can generate output!In practice, only a finite set of samples is available. Quantizationnoise cannot be removed.6833

Other examples of sensors Heart monitoring sensors "Managing Care Through the Air" » IEEE Spectrum Dec 2004 Rain sensors for wiper control High-end autos Pressure sensors Touch pads/screens Proximity sensors Collision avoidance Vibration sensors Smoke sensors Based on the diffraction of light waves

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