Sensing and Sensors: Overview and Fundamental Concepts I

Transcription

1 Sensing and Sensors: Overview and Fundamental Concepts I MediaRobotics Lab, January 2010 Sensor: a device the receives and responds to a stimulus; a device that detects a changing condition (change in presence of absence of something) Stimulus: something external that elicits activity - anything that may have an impact on a system; an input to the system.. So, what is NOT a sensor? References: Fraden: Handbook of Modern Sensors Press et alt: Numerical Recipes The Art of Scientific Computing Nilson: Electric Circuits Dunne: Hertzian Tales

2 Sensing natural, human, synthetic Natural systems Sensing mostly reflexive (as far as we humans know) Human beings Sensing intuitive, reflexive and intimately linked to cognition and every aspect related to cognition (philosophy, culture) Synthetic systems Inspired by natural systems, modeled by laws of physics, implemented in mechanical and electronic media, commonly known as sensors and transducers

3 In the 'natural' world, sensing means developing a highly specialized apparatus to perceive certain signals. The more refined the apparatus becomes, the better use it can be put to. Sonar, for example, measures acoustic location in air or water. Echolocation is the term used to describe sonar in mammals such as whales and bats. In echolocation (aka biosonar), ranging is done by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. Echolocating animals make use of the fact that they have two ears positioned slightly apart. The echoes returning to the two ears arrive at different times and at different loudness levels, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive direction and distance. With echolocation the animal can see not only where it's going but can also discern size and texture of objects in its path even when other sensors (such as vision) utterly fail (on a dark night). Most insectivorous bats call with dominant frequencies between 20 khz and 60 khz. Lower frequencies are avoided because echoes from insect-sized targets are weak when the wavelength is longer than the insect wing length. Bat echolocat ion calls: adapt at ion and conver gent evolut ion Gar et h Jones* and Mar c W. Holder ied, Proc. R. S oc. B (2007) 274,

4 SONAR (SOund NAvigation and Ranging) sonar is a technique that uses sound propagation under water to navigate, communicate or to detect other vessels. There are two kinds of sonar active and passive. Active sonar Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections (echo) of the pulse. To measure the distance to an object, the time from transmission of a pulse to reception is measured and converted into a range by knowing the speed of sound. To measure the bearing, several hydrophones are used, and the set measures the relative arrival time to each. Passive sonar Passive sonar listens without transmitting. It is often employed in military settings, although it is also used in science applications, e.g. detecting fish for presence/absence studies in various aquatic environments. Active an passive sonar techniques are widely used in naval warfare, including submarine, anti-submarine devices, mines and torpedoes.

5 Non-military applications of sonar include robot navigation and orientation for visually impaired people. An example of an experimental navigation design is the Electronic Travel Aid by Maroof, Choudhury, Aguerrevere and Barreto (2004). Pockt-PC based Electronic Travel Aid helps a blind individual navigate through indoor environments. The system detects surrounding obstacles using ultrasonic range sensors and the travel direction using an electronic compass. The acquired information is processed by a Pocket-PC to generate a virtual acoustic environment where nearby obstacles are recognizable to the user. This virtual environment is played back through stereo headphones, so that the user can perceive surrounding obstacles and the direction of the Earth's magnetic North using spatialized 3D sounds.

6 As mentioned previously, sensors are usually highly specialized and tuned to a particular task (hearing, seeing, feeling in animals, for example). Building (or evolving) a sensor entails taking a slice through reality (and the many ways it can be perceived) and choosing certain things, events, energies to focus on in lieu of others. There are often many ways to sense a given physical signal. Ideally one chooses the most reliable method with the highest sensitivity and resolution (more on this below). There are often more than one way to deal with the output of a sensor. The choices are mostly guided by the particular application in which the sensor is used. Sonar sensors for robot navigation are usually not used for electronic music design (but can be). Because of this, sensor design is both a technical domain as well as a cultural domain. Show me your sensors and I will tell you who you are.

7 Haptic interfaces and Devices Sensor Review Volume 24 Number pp

8 A sensor is a device that receives a stimulus and responds with an electrical signal A transducer is a converter from one type of energy into another one. Loudspeaker: electric signal -> variable magnetic field -> acoustic wave / sound Direct sensor: converts a stimulus into an electrical signal by using a physical effect (photo effect, for example) Complex sensor: needs in addition one or more transducers before the sensor can generate an electrical signal.

9 Sensors do not 'function' alone; they are always part of a larger system -

10 ... these other systems include: other sensors signal conditioners signal processors memory devices data recorders actuators... but also: material properties engineering constraints monetary constraints political issues... fear beliefs (prejudices)

11 Anthony Dunne Hertzian Tales, 2004

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14 Classifying sensors passive vs active passive sensors needs no additional power, generates an electric signal directly in response to an external stimulus >> examples: photodiode, piezoelectric sensor active sensors require external power for their operation (an excitation signal). This signal, in turn, is modified by the sensor to produce an output. So their own properties change in response to the external input (they are parametric). >> examples: thermistor (temperature sensitive resistor) contact vs non-contact contact sensors sensors that detect change through direct physical contact with a target object >>examples: limit switches, safety switches non-contact sensors sensors that create an energy field or beam and react to disturbances in that field; no physical contact is required >>examples: ultrasonic sensors

15 Basic concepts Sensitivity the minimum input (of a physical parameter) that will create a detectable output Range the minimum and maximum values of a given parameter the sensor can measure. Precision the ability of the sensor to reproduce the same results in repeated tests of identical conditions Accuracy the maximum difference between the actual value and the value indicated by the sensor Resolution the smallest detectable incremental change of input that can be detected in the output signal Offset the output existing in the absence of input Hysteresis the effect of direction of the input on the output Response Time Time required for a sensor to change from a previous state to a new state (within a tolerance band)

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17 Six basic units of the international measurement system, the meter (unit of length), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity) are defined in terms of properties of nature and can be measured at any suitably equipped laboratory. >> The seventh unit, the kilogram (mass), cannot...

18 - creation of the decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the metre and the kilogram, on 22 June 1799, in the Archives de la République in Paris -concept: mass of a cubic decimeter of water at maximum density. But: 1 decimeter of water at its max density = % of the mass of the provisional kilogram... -new approach: the former royal jeweler, Marc Etienne Janetti, is asked to create the kilogram reference -> platinum cylinder 39.4mm x 39.7mm high -an improved version, made of platinum-iridium alloy, closely match the original, is today the International Prototype Kilogram... -but: the prototype's mass is changing (50 parts per billion per 100 years) -> make an electronic kilogram! (not successful todate..) R. Steiner, E. R. Williams, D. B. Newell and R. Liu. Towards an electronic kilogram: an improved measurement of the Planck constant and electron mass. Metrologia, vol. 42, pages (2005)

19 Help save the kilogram - two story high apparatus - designed to redefine mass in terms of fundamental physics and quantum standards -measures the force required to balance 1-kilogram artifact against the pull of Earth's gravity, as well as two electrical values. These measurements are used to determine the relationship between mechanical and electrical power, which can be combined with several equations to define the kilogram in terms of basic properties of nature...

20 Up until the late 1800's, the wave picture of light was the prevalent theory as it could explain most of the experiments done on light. An exception was associated with blackbody radiation, which is the characteristic radiation that a body emits when heated. I theory: I ~ 1/lambda^4 experiment lambda In 1900 Planck devised a theory of blackbody radiation which gave good agreement for all wavelengths. In this theory the molecules of a body cannot have arbitrary energies but instead are quantized - the energies can only have discrete values. Plank's postualte states that the magnitude of these energies is given by the relationship: E = h f* f (hf : Planks constant, f: frequency of radiation)

21 frequency = de / h h: Planck s constant m2 kg / s

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23 Human Physiology in Space" by Barbara F. Abuja and Ronald J. White, 1994

24 Camera: Image formation (lens), image capture (CCD) CCD: charge coupled device Reprinted from the January 2001 issue of PHOTONICS SPECTRA