A Sensor Is a Transducer Whose Purpose Is to Sense (That Is, to Detect) Some Cha Racteristic of Its Environments
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A sensor is a transducer whose purpose is to sense (that is, to detect) some cha racteristic of its environments. It detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal; f or example, a thermo couple converts temperature to an output voltage. But a mer cury-in-glass thermometer is also a sensor; it converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated gl ass tube. Sensors are used in everyday objects such as touch-sensitive elevator buttons (t actile sensor) and lamps which dim or brighten by touching the base, besides inn umerable applications of which most people are never aware. With advances in mic ro machinery and easy-to-use micro controller platforms, the uses of sensors hav e expanded beyond the more traditional fields of temperature, pressure or flow m easurement,[1] for example into MARG sensors. Moreover, analog sensors such as p otentiometers and force-sensing resistors are still widely used. Applications in clude manufacturing and machinery, airplanes and aerospace, cars, medicine and r obotics. A sensor's sensitivity indicates how much the sensor's output changes when the i nput quantity being measured changes. For instance, if the mercury in a thermome ter moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C (it is basically the slope Dy/Dx assuming a linear characteristic). Some sensors can also have an impact on what they measure; for instance, a room temperature ther mometer inserted into a hot cup of liquid cools the liquid while the liquid heat s the thermometer. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages.[citation needed] Technological progress allows more and more sensor s to be manufactured on a microscopic scale as microsensors using MEMS technolog y. In most cases, a microsensor reaches a significantly higher speed and sensiti vity compared with macroscopic approaches.[citation needed] Contents 1 Classification of measurement errors 1.1 Sensor deviations 1.2 Resolution 2 Types 3 Sensors in nature 4 Chemical sensor 5 Biosensor 6 See also 7 References 8 Further reading 9 External links Classification of measurement errors An infrared sensor A good sensor obeys the following rules[citation needed]: Is sensitive to the measured property Is insensitive to any other property likely to be encountered in its applica tion Does not influence the measured property The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output , the sensitivity is a constant with the unit [V/K]; this sensor is linear becau se the ratio is constant at all points of measurement. For an analog sensor signal to be processed, or used in digital equipment, it ne eds to be converted to a digital signal, using an analog-to-digital converter. Sensor deviations If the sensor is not ideal, several types of deviations can be observed: The sensitivity may in practice differ from the value specified. This is cal led a sensitivity error. Since the range of the output signal is always limited, the output signal wi ll eventually reach a minimum or maximum when the measured property exceeds the limits. The full scale range defines the maximum and minimum values of the measu red property.[citation needed] If the output signal is not zero when the measured property is zero, the sen sor has an offset or bias. This is defined as the output of the sensor at zero i nput. If the sensitivity is not constant over the range of the sensor, this is cal led non linearity. Usually this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage o f the full range. If the deviation is caused by a rapid change of the measured property over t ime, there is a dynamic error. Often, this behavior is described with a bode plo t showing sensitivity error and phase shift as function of the frequency of a pe riodic input signal. If the output signal slowly changes independent of the measured property, th is is defined as drift (telecommunication). Long term drift usually indicates a slow degradation of sensor properties over a long period of time. Noise is a random deviation of the signal that varies in time. Hysteresis is an error caused by when the measured property reverses directi on, but there is some finite lag in time for the sensor to respond, creating a d ifferent offset error in one direction than in the other. If the sensor has a digital output, the output is essentially an approximati on of the measured property. The approximation error is also called digitization error. If the signal is monitored digitally, limitation of the sampling frequency a lso can cause a dynamic error, or if the variable or added noise changes periodi cally at a frequency near a multiple of the sampling rate may induce aliasing er rors. The sensor may to some extent be sensitive to properties other than the prop erty being measured. For example, most sensors are influenced by the temperature of their environment. All these deviations can be classified as systematic errors or random errors. Sy stematic errors can sometimes be compensated for by means of some kind of calibr ation strategy. Noise is a random error that can be reduced by signal processing , such as filtering, usually at the expense of the dynamic behavior of the senso r. Resolution The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit wi ll fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. F or example, a scanning tunneling probe (a fine tip near a surface collects an el ectron tunneling current) can resolve atoms and molecules. Types Main article: List of sensors Temperature Sensor Pressure sensor Ultrasonic sensor Humidity Sensor Gas Sensor PIR Motion Sensor The acceleration sensor Displacement sensor Holzer switch sensor Sensors in nature Further information: Sense All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that ar e sensitive to: Light, motion, temperature, magnetic fields, gravity, humidity, moisture, vi bration, pressure, electrical fields, sound, and other physical aspects of the e xternal environment Physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages (proprioception) Environmental molecules, including toxins, nutrients, and pheromones Estimation of biomolecules interaction and some kinetics parameters Internal metabolic indicators, such as glucose level, oxygen level, or osmol ality Internal signal molecules, such as hormones, neurotransmitters, and cytokine s Differences between proteins of the organism itself and of the environment o r alien creatures. Chemical sensor A chemical sensor is a self-contained analytical device that can provide informa tion about the chemical composition of its environment, that is, a liquid or a g as phase.[2] The information is provided in the form of a measurable physical si gnal that is correlated with the concentration of a certain chemical species (te rmed as analyte). Two main steps are involved in the functioning of a chemical s ensor, namely, recognition and transduction. In the recognition step, analyte mo lecules interact selectively with receptor molecules or sites included in the st ructure of the recognition element of the sensor. Consequently, a characteristic physical parameter varies and this variation is reported by means of an integra ted transducer that generates the output signal. A chemical sensor based on reco gnition material of biological nature is a biosensor. However, as synthetic biom imetic materials are going to substitute to some extent recognition biomaterials , a sharp distinction between a biosensor and a standard chemical sensor is supe rfluous. Typical biomimetic materials used in sensor development are molecularly imprinted polymers and aptamers. Biosensor Main article: biosensor In biomedicine and biotechnology, sensors which detect analytes thanks to a biol ogical component, such as cells, protein, nucleic acid or biomimetic polymers, a re called biosensors. Whereas a non-biological sensor, even organic (=carbon che mistry), for biological analytes is referred to as sensor or nanosensor (such a microcantilevers). This terminology applies for both in vitro and in vivo applic ations. The encapsulation of the biological component in biosensors, presents a slightly different problem that ordinary sensors; this can either be done by mea ns of a semipermeable barrier, such as a dialysis membrane or a hydrogel, or a 3 D polymer matrix, which either physically constrains the sensing macromolecule o r chemically constrains the macromolecule by bounding it to the scaffold..