digital sensors part-6
TRANSCRIPT
Transducer is a device which transforms energy from one type to another, even if both energy types are in the same domain.
Typical energy domains are mechanical, electrical, chemical, magnetic, optical and thermal.
Transducer can be further divided into Sensors, which monitors a system and Actuators, which impose an action on the system.
Sensors are devices which monitor a parameter of a system, hopefully without disturbing that parameter.
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Classification based on physical phenomena Mechanical: strain gage, displacement (LVDT), velocity (laser vibrometer),
accelerometer, tilt meter, viscometer, pressure, etc. Thermal: thermal couple Optical: camera, infrared sensor Others …
Classification based on measuring mechanism Resistance sensing, capacitance sensing, inductance sensing, piezoelectricity,
etc. Materials capable of converting of one form of energy to another are
at the heart of many sensors. Invention of new materials, e.g., “smart” materials, would permit the design of
new types of sensors.
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Physical phenomenon Sensor SystemSensor System
Measurement Output
Measurement output:• interaction between a sensor and the environment surrounding the sensor• compound response of multiple inputs
Measurement errors:• System errors: imperfect design of the measurement setup and the approximation, can be corrected by calibration• Random errors: variations due to uncontrolled variables. Can be reduced by averaging.
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Definition: a device for sensing a physical variable of a physical system or an environment
Classification of Sensors Mechanical quantities: displacement, Strain, rotation velocity, acceleration, pressure, force/torque, twisting, weight, flow Thermal quantities: temperature, heat. Electromagnetic/optical quantities: voltage, current, frequency phase; visual/images, light; magnetism. Chemical quantities: moisture, pH value
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Accuracy: error between the result of a measurement and the true value being measured.
Resolution: the smallest increment of measure that a device can make.
Sensitivity: the ratio between the change in the output signal to a small change in input physical signal. Slope of the input-output fit line.
Repeatability/Precision: the ability of the sensor to output the same value for the same input over a number of trialswww.techvilla.org.in
Dynamic Range: the ratio of maximum recordable input amplitude to minimum input amplitude, i.e. D.R. = 20 log (Max. Input Ampl./Min. Input Ampl.) dB
Linearity: the deviation of the output from a best-fit straight line for a given range of the sensor
Transfer Function (Frequency Response): The relationship between physical input signal and electrical output signal, which may constitute a complete description of the sensor characteristics.
Bandwidth: the frequency range between the lower and upper cutoff frequencies, within which the sensor transfer function is constant gain or linear.
Noise: random fluctuation in the value of input that causes random fluctuation in the output value
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Operating Principle: Embedded technologies that make sensors function, such as electro-optics, electromagnetic, piezoelectricity, active and passive ultraviolet.
Dimension of Variables: The number of dimensions of physical variables.
Size: The physical volume of sensors. Data Format: The measuring feature of data in time; continuous or discrete/analog or digital.
Intelligence: Capabilities of on-board data processing and decision-making.
Active versus Passive Sensors: Capability of generating vs. just receiving signals.
Physical Contact: The way sensors observe the disturbance in environment.
Environmental durability: will the sensor robust enough for its operation conditions
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THERE ARE SENSORS FOR MEASURING:LIGHTHEATSOUNDDISTANCETOUCH/PRESSUREDISPLACEMENT/ANGLELOCATION/HEADINGMOVEMENTACCELERATIONCHEMICALS/SCENTS
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For a sensor, we’re interested in the light power that falls on a unit area, and how well the sensor converts that into a signal.
A common unit is the lux which measures apparent brightness (power multiplied by the human eye’s sensitivity).
1 lux of yellow light is about 0.0015 W/m2. 1 lux of green light (50% eff.) is 0.0029 W/m2. Sunlight corresponds to about 50,000 lux Artificial light typically 500-1000 lux 11
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Simplest light sensor is an LDR (Light-Dependent Resistor).
Optical characteristics close to human eye. Can be used to feed an A/D directly without
amplification (one resistor in a voltage divider). Common material is CdS
(Cadmium Sulphide) Sensitivity: dark 1 MΩ,
10 lux 40 kΩ,1000 lux 400 Ω.
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Semiconductor light sensors include: photodiodes, phototransistors, photodarlingtons.
All of these have similar noise performance, but phototransistors and darlingtons have better sensitivity (more current for given light input).
Phototransistor:1 mA @ 1000 lux
Photodarlingtonsup to 100x this sensitivity. 13
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At the cutting edge of light sensor sensitivity are Avalanche photodiodes.
Large voltages applied to these diodes accelerate electrons to “collide” with the semiconductor lattice, creating more charges.
These devices have quantum efficienciesaround 90% and extremely low noise.
They are now made withlarge collection areas andknown as LAAPDs (Large-Area Avalanche Photo-Diode)
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Two solid-state camera types: CCD and CMOS. CCD is the more mature technology, and has the widest
performance range. 8 Mpixel size for cameras Low noise/ high efficiency for astronomy etc. Good sensitivity (low as 0.0003 lux, starlight)
CCDs require several chips,but are still cheap ($50 +)
Most CCDs work in near infraredand can be used for night visionif an IR light source is used. 15
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CMOS cameras are very compact and inexpensive, but haven’t matched CCDs in most performance dimensions.
Start from $20(!) Custom CMOS cameras
integrate image processingright on the camera.
Allow special functions likemotion detection, recognition.
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Many devices can measure temperature. Basic heat sensors are called “thermistors” (heat-sensitive resistors).
Available in a very wide range of resistances, with positive or negative resistance change/temp.
1-wire device family includes a thermometer.
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Heat can be “seen” at a distance. Recall temperature = heat/atom. At room temp each atom has average energy 6.3 x 10-21 J (lecture 10).
Some of this energy is emitted as photons. A photon of energy E and frequency f satisfies:
E = h f
where h is Planck’s constant = 6.63 x 10-34 J sec Thermal photons have frequency ~ 1013 Hz and
wavelength ~ 30 μm This is in the far infrared range. Sensors that respond to
those wavelengths can “see” warm objects without other illumination.
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Far infrared CCD cameras exist for 10 μm and above, but are much more sophisticated (and expensive) than near-infrared CCDs.
Generally many $1000s
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PIR (Pyroelectric InfraRed) sensors can detect IR heat radiation (7-20 μm typical).
They are simple, cheap and common. The basis of security system “motion detectors”.
Most PIR sensors contain two or four sensors with different viewing regions.
They detect a change in the difference between the signals and give a binary output. 20
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A few component PIR sensors are available that provide the PIR analog signals directly.
Eltec two-element sensor, shown with matching fresnel IR lens and mounting:
NAIS ultra-compact PIR sensor
Note: PIR sensors are slowwith time constants ~ 1 sec 21
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Microphone types: Dynamic (magnetic), high-quality, size, cost Piezoelectric, small, cheap, fair quality Condenser, good quality, cheap, small
Condenser mikes are the most common, and range from low-end to top-end in performance.
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Most condenser mikes include a built-in amplifier, and must be connected to a voltage supply through a resistor.
Almost any microphone will need further amplification before being fed to an A/D. Many audio preamp ICs can be used for this.
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Many kinds. At the low end, IR range sensors (Sharp sensor example).
An LED transmits (modulated) light, a phototransistor detects the strength of the modulated return signal. Good to a few ft.
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Sonar sensors. Polaroid sells several sonar modules that are very popular in mobile robot applications. Several pulses per second.
Can measure range up to 30’ or more.
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Phase delay light sensors. Light beam is modulated with radio frequency signal.
Phase shift of return beam gives distance. Can give very high accuracy (mm or better). Used in high-end laser systems ($100s-$1000s). Simple versions were available for ~ $100 several years
ago. Can be custom-built for this price.
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We have several overlay touch screens (< $100) for laptop screens.
Tactex makes high-performance touch surfaces: They respond to multiple
finger contacts, 8000samples/sec.
Intended for digital musicinput, and other expressiveinteractions.
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Piezoelectric film creates voltages in response to strain. It can be cut to custom shapes for special-purpose sensors.
Sensors include accelerometers, bend sensors, hydrophones,…
MSI (Measurement Specialists Inc.) sells a variety of piezo film products.
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A very simple way to measure displacement or angle is to use a potentiometer as a voltage divider with output to an A/D converter.
Precision potentiometers come in both linear and multi-rotation angular types.
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Encoders measure relative displacement. A pattern of light-dark bars is attached to the
moving element. Light sensors observe each region. The number of transitions encodes the movement
in either direction.
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AB
GPS provides location in LAT/LONG coords. Standard NAVSTAR systems good to ~ 5m. Survey grade GPS accurate to a few inches. Location calibration points may push consumer accuracy
toward the latter figure. Bluetooth GPS modules
now ~ $200. Cost increment for GPS
in CDMA cell phones ~ $531
Small magnetic compasses are available, such as the trekker ($65 kit):
Can be tricky to use magnetic compass data indoors, but we had good luck with it in non-metallic robots.
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Gyroscopes maintain direction information with fast response time.
Small gyros were developed for model helicopter use (~ $200). 270 Hz update.
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For motion tracking indoors, magnetic field systems are popular.
Ascension Technologies “Flock of Birds” systems are very popular.
Wired units are movedand all 6 degrees ofposition and rotationfreedom are tracked.
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Accelerometers are based either on MEMs or piezo-electric components.
Analog devices ADXL-series is a good example: ADXL202 2-axis 2 mg resolution, 60 Hz 6 kHz sensing range ~ $20 and dropping.
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