SensorsEvolution in Measurements

Sensors::

A sensor (also called detector) is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an (today mostly electronic) instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, most sensors are calibrated against known standards.

Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics.

A sensor is a device which receives and responds to a signal when touched. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer 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). Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats 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. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches.

Classification of measurement errors

Sensor Principles:

A good sensor obeys the following rules:

Is sensitive to the measured property only Is insensitive to any other property likely to be encountered in its application Does not influence the measured property

Ideal sensors are designed to be linear or linear to some simple mathematical function of the measurement, typically logarithmic. The output signal of such a sensor is linearly proportional to the value or simple function of 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 because the ratio is constant at all points of measurement.

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 will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning tunneling probe (a fine tip near a surface collects an electron tunneling current) can resolve atoms and molecules.

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 called a sensitivity error, but the sensor is still linear.

Since the range of the output signal is always limited, the output signal will 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 measured property.

If the output signal is not zero when the measured property is zero, the sensor has an offset or bias. This is defined as the output of the sensor at zero input.

If the sensitivity is not constant over the range of the sensor, this is called 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 of the full range.

If the deviation is caused by a rapid change of the measured property over time, there is a dynamic error. Often, this behavior is described with a bode plot showing sensitivity error and phase shift as function of the frequency of a periodic input signal.

If the output signal slowly changes independent of the measured property, this 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 direction,

but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.

If the sensor has a digital output, the output is essentially an approximation of the measured property. The approximation error is also called digitization error.

If the signal is monitored digitally, limitation of the sampling frequency also can cause a dynamic error, or if the variable or added noise changes periodically at a frequency near a multiple of the sampling rate may induce aliasing errors.

The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.

List of sensors::

1)Acoustic, sound, vibration:

Geophone Hydrophone Lace Sensor a guitar pickup Microphone Seismometer

2)Automotive, transportation:

Air-fuel ratio meter Blind spot monitor Crankshaft position sensor Curb feeler, used to warn driver of curbs Defect detector, used on railroads to detect axle and signal problems in passing

trains Engine coolant temperature sensor, or ECT sensor, used to measure the engine

temperature Hall effect sensor, used to time the speed of wheels and shafts MAP sensor, Manifold Absolute Pressure, used in regulating fuel metering. Mass flow sensor, or mass airflow (MAF) sensor, used to tell the ECU the mass

of air entering the engine Oxygen sensor, used to monitor the amount of oxygen in the exhaust Parking sensors, used to alert the driver of unseen obstacles during parking

manoeuvres Radar gun, used to detect the speed of other objects Speedometer, used measure the instantaneous speed of a land vehicle Speed sensor, used to detect the speed of an object Throttle position sensor, used to monitor the position of the throttle in an

internal combustion engine Tire-pressure monitoring sensor, used to monitor the air pressure inside the

tires Torque sensor, or torque transducer or torquemeter measures torque (twisting

force) on a rotating system. Transmission fluid temperature sensor, used to measure the temperature of the

transmission fluid Turbine speed sensor (TSS), or input speed sensor (ISS), used to measure the

rotational speed of the input shaft or torque converter Variable reluctance sensor, used to measure position and speed of moving

metal components Vehicle speed sensor (VSS), used to measure the speed of the vehicle Water sensor or water-in-fuel sensor, used to indicate the presence of water in

fuel

Wheel speed sensor, used for reading the speed of a vehicle's wheel rotation

3)Chemical

Breathalyzer Carbon dioxide sensor Carbon monoxide detector Catalytic bead sensor Chemical field-effect transistor Electrochemical gas sensor Electronic nose Electrolyte–insulator–semiconductor sensor Fluorescent chloride sensors Holographic sensor Hydrocarbon dew point analyzer Hydrogen sensor Hydrogen sulfide sensor Infrared point sensor Ion-selective electrode Nondispersive infrared sensor Microwave chemistry sensor Nitrogen oxide sensor Olfactometer Optode Oxygen sensor Pellistor pH glass electrode Potentiometric sensor Redox electrode Smoke detector Zinc oxide nanorod sensor

4)Electric current, electric potential, magnetic, radio:

Current sensor Electroscope Galvanometer Hall effect sensor Hall probe Magnetic anomaly detector Magnetometer MEMS magnetic field sensor Metal detector Planar Hall sensor Radio direction finder Voltage detector

5)Environment, weather, moisture, humidity:

Actinometer Bedwetting alarm Ceilometer Dew warning Electrochemical gas sensor Fish counter Frequency domain sensor Gas detector Hook gauge evaporimeter Humistor Hygrometer Leaf sensor Pyranometer Pyrgeometer Psychrometer Rain gauge Rain sensor Seismometers SNOTEL Snow gauge Soil moisture sensor Stream gauge Tide gauge

6)Flow, fluid velocity:

Air flow meter Anemometer Flow sensor Gas meter Mass flow sensor Water meter

7)Ionising radiation, subatomic particles:

Bubble chamber Cloud chamber Geiger counter Neutron detection Particle detector Scintillation counter Scintillator Wire chamber

8)Navigation instruments:

Air speed indicator Altimeter Attitude indicator Depth gauge Fluxgate compass Gyroscope Inertial navigation system Inertial reference unit Magnetic compass MHD sensor Ring laser gyroscope Turn coordinator Variometer Vibrating structure gyroscope Yaw rate sensor

9)Position, angle, displacement, distance, speed, acceleration:

Accelerometer Auxanometer Capacitive displacement sensor Capacitive sensing Free fall sensor Gravimeter Gyroscopic sensor Inclinometer Integrated circuit piezoelectric sensor Laser rangefinder Laser surface velocimeter LIDAR Linear encoder Linear variable differential transformer (LVDT) Liquid capacitive inclinometers Odometer Photoelectric sensor Piezoelectric accelerometer Position sensor Rate sensor Rotary encoder Rotary variable differential transformer Selsyn Sudden Motion Sensor Tilt sensor Tachometer

Ultrasonic thickness gauge Variable reluctance sensor Velocity receiver

10)Optical, light, imaging, photon:

Charge-coupled device Colorimeter Contact image sensor Electro-optical sensor Flame detector Infra-red sensor Kinetic inductance detector LED as light sensor Light-addressable potentiometric sensor Nichols radiometer Fiber optic sensors Optical position sensor Photodetector Photodiode Photomultiplier tubes Phototransistor Photoelectric sensor Photoionization detector Photomultiplier Photoresistor Photoswitch Phototube Scintillometer Shack-Hartmann Single-photon avalanche diode Superconducting nanowire single-photon detector Transition edge sensor Visible light photon counter Wavefront sensor

11)Pressure:

Barograph Barometer Boost gauge Bourdon gauge Hot filament ionization gauge Ionization gauge McLeod gauge Oscillating U-tube

Permanent Downhole Gauge Piezometer Pirani gauge Pressure sensor Pressure gauge Tactile sensor Time pressure gauge

12)Force, density, level:

Bhangmeter Hydrometer Force gauge Level sensor Load cell Magnetic level gauge Nuclear density gauge Piezoelectric sensor Strain gauge Torque sensor Viscometer

13)Thermal, heat, temperature:

Bolometer Bimetallic strip Calorimeter Exhaust gas temperature gauge Flame detection Gardon gauge Golay cell Heat flux sensor Infrared thermometer Microbolometer Microwave radiometer Net radiometer Quartz thermometer Resistance temperature detector Resistance thermometer Silicon bandgap temperature sensor Special sensor microwave/imager Temperature gauge Thermistor Thermocouple Thermometer

14)Proximity, presence:

Alarm sensor Doppler radar Motion detector Occupancy sensor Proximity sensor Passive infrared sensor Reed switch Stud finder Triangulation sensor Touch switch Wired glove

15)Sensor technology:

Active pixel sensor Back-illuminated sensor Biochip Biosensor Capacitance probe Catadioptric sensor Carbon paste electrode Digital sensors Displacement receiver Electromechanical film Electro-optical sensor Fabry–Pérot interferometer Fisheries acoustics Image sensor Image sensor format Inductive sensor Intelligent sensor Lab-on-a-chip Leaf sensor Machine vision Microelectromechanical systems Micro-sensor arrays Photoelasticity Quantum sensor RADAR

o Ground-penetrating radaro Synthetic aperture radar

Radar tracker Sensor array Sensor fusion

Sensor grid Sensor node Soft sensor SONAR Staring array Transducer Ultrasonic sensor Video sensor Visual sensor network Wheatstone bridge Wireless sensor network

Other sensors and sensor related properties and concepts

Actigraphy Analog image processing Atomic force microscopy Atomic Gravitational Wave Interferometric Sensor Attitude control (spacecraft), Horizon sensor, Earth sensor, Sun sensor Catadioptric sensor Chemoreceptor Compressive sensing Cryogenic particle detectors Dew warning Diffusion tensor imaging Digital holography Electronic tongue Fine Guidance Sensor Flat panel detector Functional magnetic resonance imaging Glass break detector Heartbeat sensor Hyperspectral sensors IRIS (Biosensor), Interferometric Reflectance Imaging Sensor Laser beam profiler Littoral Airborne Sensor/Hyperspectral LORROS Millimeter wave scanner Magnetic resonance imaging Moire deflectometry Molecular sensor Nanosensor Nano-tetherball Sensor Omnidirectional camera Optical coherence tomography Phase unwrapping techniques

Positron emission tomography Push broom scanner sensitive air-conductivity sensors Quantization (signal processing) Range imaging Scanning SQUID microscope Single-Photon Emission Computed Tomography (SPECT) Smartdust SQUID, Superconducting quantum interference device SSIES, Special Sensors-Ions, Electrons, and Scintillation thermal plasma

analysis package SSMIS, Special Sensor Microwave Imager / Sounder Structured-light 3D scanner Sun sensor, Attitude control (spacecraft) Superconducting nanowire single-photon detector Thin-film thickness monitor Time-of-flight camera TriDAR, Triangulation and LIDAR Automated Rendezvous and Docking Unattended Ground Sensors

Examples of each type of Sensors:

Acoustic, sound, vibration:

a)Hydrophone:

A hydrophone (Greek "hydro" = "water" and "phone" = "sound") is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials, or transducers can convert a sound signal into an electrical signal since sound is a pressure wave. Some transducers can also serve as a projector, but not all have this capability, and may be destroyed if used in such a manner.

A hydrophone can "listen" to sound in air, but will be less sensitive due to its design as having a good acoustic impedance match to water, which is a denser fluid than air. Likewise, a microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give similarly poor performance due to the similarly bad acoustic impedance match.

Automotive, transportation:

a) Air–fuel ratio meter

An air–fuel ratio meter monitors the air–fuel ratio of an internal combustion engine. Also called air–fuel ratio gauge, air–fuel meter, or air–fuel gauge. It reads the voltage output of an oxygen sensor, sometimes also called lambda sensor, whether it be from a narrow band or wide band oxygen sensor.

The original narrow-band oxygen sensors became factory installed standard in the late 1970s and early 80s. In recent years, a newer and much more accurate wide-band sensor, though more expensive, has become available.

Most stand-alone narrow-band meters have 10 LEDs and some have more. Also common, narrow band meters in round housings with the standard mounting 2 1/16" and 2 5/8" diameters, as other types of car 'gauges'. These usually have 10 or 20 LEDs. Analogue 'needle' style gauges are also available.

As stated above, there are wide-band meters that stand alone or are mounted in housings. Nearly all of these show the air–fuel ratio on a numeric display, since the wide-band sensors provide a much more accurate reading. And since they use more accurate electronics, these meters are more expensive.

b)Curb feelers

Curb feelers or curb finders are springs or wires installed on a vehicle which act as "whiskers" to warn drivers that they are too close to the curb or other obstruction.

The devices are fitted low on the body, close to the wheels. As the vehicle approaches the curb, the protruding 'feelers' act as whiskers and scrape against the curb, making a noise and alerting the driver in time to avoid damaging the wheels or hubcaps. The feelers are manufactured to be flexible and do not easily break.

Chemical:

Electrochemical gas sensor

The sensors contain two or three electrodes, occasionally four, in contact with an electrolyte. The gas diffuses into the sensor, through the back of the porous membrane to the working electrode where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. In addition to measuring, amplifying and performing other signal processing functions, the external circuit maintains the voltage across the sensor between the working and counter electrodes for a two electrode sensor or between the working and reference electrodes for a three electrode cell. At the counter electrode an equal and opposite reaction

occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.

Hydrocarbon dew point

The hydrocarbon dew point is the temperature (at a given pressure) at which the hydrocarbon components of any hydrocarbon-rich gas mixture, such as natural gas, will start to condense out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature at which such condensation takes place is called the cricondentherm.[1] The hydrocarbon dew point is a function of the gas composition as well as the pressure.

The hydrocarbon dew point is universally used in the natural gas industry as an important quality parameter, stipulated in contractual specifications and enforced throughout the natural gas supply chain, from producers through processing, transmission and distribution companies to final end users.

The hydrocarbon dew point of a gas is a different concept from the water dew point, the latter being the temperature (at a given pressure) at which water vapor present in a gas mixture will condense out of the gas.

In the United States, the hydrocarbon dew point of processed, pipelined natural gas is related to and characterized by the term GPM which is the gallons of liquifiable hydrocarbons contained in 1,000 cubic feet (28 m3) of natural gas at a stated temperature and pressure. When the liquifiable hydrocarbons are characterized as being hexane or higher molecular weight components, they are reported as GPM (C6+).[2][3]

However, the quality of raw produced natural gas is also often characterized by the term GPM meaning the gallons of liquifiable hydrocarbons contained in 1,000 cubic feet (28 m3) of the raw natural gas. In such cases, when the liquifiable hydrocarbons in the raw natural gas are characterized as being ethane or higher molecular weight components, they are reported as GPM (C2+). Similarly, when characterized as being propane or higher molecular weight components, they are reported as GPM (C3+).[4]

Care must be taken not to confuse the two different definitions of the term GPM.

Although GPM is an additional parameter of some value, most pipeline operators and others who process, transport, distribute or use natural gas are primarily interested in the actual HCDP, rather than GPM. Furthermore, GPM and HCDP are not interchangeable and one should be careful not to confuse what each one exactly means.

Electric current, electric potential, magnetic, radio

a)Magnetic anomaly detector

A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used by military forces to detect submarines (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field); the military MAD gear is a descendent of geomagnetic survey instruments used to search for minerals by the disturbance of the normal earth-field.

To reduce interference from electrical equipment or metal in the fuselage of the aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamic device. Even so, the submarine must be very near the aircraft's position and close to the sea surface for detection of the change or anomaly. The size of the submarine and its hull composition determine the detection range. MAD devices are usually mounted on aircraft.

There is some misunderstanding of the mechanism of detection of submarines in water using the MAD boom system. Magnetic moment displacement is ostensibly the main disturbance, yet submarines are detectable even when oriented parallel to the Earth's magnetic field, despite construction with non-ferromagnetic hulls. For example, the Soviet-Russian Alfa class submarine, whose hull is constructed out of titanium to give dramatic submerged performance and protection from detection by MAD sensors, is still detectable

This is due in part to the fact that even submarines with titanium hull will still have a substantial content of ferromagnetic materials as the nuclear reactor, steam turbines, auxiliary diesel engines and numerous other systems will be manufactured from steel and nickel alloys.

b)Radio direction finder

A radio direction finder (RDF) is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance from their destination (see Radio navigation). The distinct technology Range and Direction Finding was the abbreviation used to describe the predecessor to radar.

In use, the RDF operator would first tune the receiver to the correct frequency, then manually turn the loop, either listening or watching an S meter to determine the direction of the null (the direction at which a given signal is weakest) of a long wave (LW) or medium wave (AM) broadcast beacon or station (listening for the null is easier than listening for a peak signal, and normally produces a more accurate result). This null was symmetrical, and thus identified both the correct degree heading marked on the radio's compass rose as well as its 180-degree opposite. While this information provided a baseline from the station to the ship or aircraft, the navigator still needed to know beforehand if he was to the east or west of the station in order to avoid plotting a course 180-degrees in the wrong direction. By taking bearings to two or more broadcast stations and plotting the intersecting bearings, the navigator could locate the relative position of his ship or aircraft. Later, RDF sets were equipped with rotatable ferrite loopstick antennas, which made the sets more portable

and less bulky. Some were later partially automated by means of a motorized antenna (ADF). A key breakthrough was the introduction of a secondary vertical whip or 'sense' antenna that substantiated the correct bearing and allowed the navigator to avoid plotting a bearings 180 degrees opposite the actual heading. After World War II, there many small and large firms making direction finding equipment for mariners, including Apelco, Aqua Guide, Bendix, Gladding (and its marine division, Pearce-Simpson), Ray Jefferson, Raytheon, and Sperry. By the 1960s, many of these radios were actually made by Japanese electronics manufacturers, such as Panasonic, Fuji Onkyo, and Koden Electronics Co., Ltd. In aircraft equipment, Bendix and Sperry-Rand were two of the larger manufacturers of RDF radios and navigation instruments.

Environment, weather, moisture, humidity:

Humistor

A humidity sensor has a sensing portion which usually comprises a humidity-sensitive resistor composed of an organic polymer, such as a polyamide resin, polyvinyl chloride or polyethylene, or a metal oxide. A capacitive humidity sensor detects humidity based on a change of capacitance between two detection electrodes provided on a semiconductor substrate. The capacitance type humidity sensor detects humidity by measuring the change in the electrostatic capacity of an element corresponding to the ambient humidity. A resistive humidity sensor detects relative humidity by measuring the change in the resistance of an element corresponding to the ambient humidity. Most of the resistance type humidity sensors include an electrolytic, polymeric, or metallic oxide sensor element. An impedance humidity sensor changes its electrical impedance as the humidity of the surrounding environment changes, and the measured impedance is converted into humidity readings.

A humidity sensor measures the humidity level by measuring the change in the resistance of an element or the change in the electrostatic capacity of that element as it absorbs or releases moisture. Humidity sensors can be used not only to measure

the humidity in an atmosphere but also to automatically control humidifiers, dehumidifiers, and air conditioners for humidity adjustment.

Flow, fluid velocity

Mass flow sensor

A mass air flow sensor is used to find out the mass flowrate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each piston stroke.

Ionising radiation, subatomic particles:

Geiger counter

Geiger counter instruments consist of two main elements; the Geiger-Muller tube, and the processing and display electronics. The radiation sensing element is an

inert gas-filled Geiger-Muller tube (usually containing helium, neon or argon with halogens added) which briefly conducts electrical charge when a particle or photon of radiation makes the gas conductive by ionization. The tube has the property of being able to amplify each ionization event by means of the Townsend avalanche effect and produces an easily measured current pulse which is passed to the processing electronics.

Navigation instruments

a) Yaw-rate sensor::

A yaw-rate sensor is a gyroscopic device that measures a vehicle’s angular velocity around its vertical axis. . The angle between the vehicle's heading and vehicle actual movement direction is called slip angle, which is related to the yaw rate. The measurement of plane to the earth from the ground my home.

Yaw rate sensors are used in aircraft and in the electronic stability control systems of cars.

b)A ring laser gyroscope (RLG)::

A ring laser gyroscope (RLG) consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation. Interference between the counter-propagating

beams, observed externally, reflects shifts in that standing wave pattern, and thus rotation.

A certain rate of rotation induces a small difference between the time it takes light to traverse the ring in the two directions according to the Sagnac effect. This introduces a tiny separation between the frequencies of the counter-propagating beams, a motion of the standing wave pattern within the ring, and thus a beat pattern when those two beams are interfered outside the ring. Therefore the net shift of that interference pattern follows the rotation of the unit in the plane of the ring.

Examples of RLG applications:

Airbus A320[3] Agni III ASM-135 US Anti-satellite missile. Boeing 757-200. Boeing 777 MK39 Ship's Internal Navigation System used in NATO surface ships and

submarines

P3 Orion

Optical, light, imaging, photon:

Electro-optical sensors:

Electro-optical sensors are electronic detectors that convert light, or a change in light, into an electronic signal. They are used in many industrial and consumer applications, for example:

Lamps that turn on automatically in response to darkness Position sensors that activate when an object interrupts a light beam Flash detection, to synchronize one photographic flash to another

Light-addressable potentiometric sensor:

A light-addressable potentiometric sensor (LAPS) is a sensor that uses light (e.g. LEDs) to select what will be measured. Light can activate carriers in semiconductors and example is the pH-sensitive LAPS (range pH4 to pH10) that uses LEDs in combination with (semi-conducting) silicon and pH-sensitive Ta2O5 (SiO2; Si3N4) insulator. The LAPS has several advantages over other types of chemical sensors. The sensor surface is completely flat, no structures, wiring or passivation are required. At the same time, the "light-addressability" of the LAPS makes it possible to obtain a spatially resolved map of the distribution of the ion concentration in the specimen. The spatial resolution of the LAPS is an important factor and is determined by the beam size and the lateral diffusion of photocarries in the semiconductor substrate. By illuminating parts of the semiconductor surface, elctron-hole pairs are generated and a photocurrent flows. The LAPS is a semiconductor based chemical sensor with an electrolyte-insulator-semiconductor (EIS) structure. Under a fixed bias voltage, the AC (kHz range) photocurrent signal varies depending of the solution. A two-dimensional mapping of the surface from the LAPS is possible by using a scanning laser beam

.

Pressure:

a) Pressure sensor

A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed.

Pressure sensors are used for control and monitoring in thousands of everyday applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude. Pressure sensors can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators and piezometers, manometers, among other names.

b) Proximity

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.

A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target.

The maximum distance that this sensor can detect is defined "nominal range". Some sensors have adjustments of the nominal range or means to report a graduated detection distance.

Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object.

Proximity sensors are also used in machine vibration monitoring to measure the variation in distance between a shaft and its support bearing. This is common in large steam turbines, compressors, and motors that use sleeve-type bearings.

International Electrotechnical Commission (IEC) 60947-5-2 defines the technical details of proximity sensors.

A proximity sensor adjusted to a very short range is often used as a touch switch.

Applications

Parktronic, car bumpers that sense distance to nearby cars for parking Ground proximity warning system for aviation safety Vibration measurements of rotating shafts in machinery [1]

Top dead centre (TDC)/camshaft sensor in reciprocating engines. Sheet break sensing in paper machine. Anti-aircraft warfare Roller coasters Conveyor systems Touch screens on mobile devices that come in close proximity with the face

All these deviations can be classified as systematic errors or random errors. Systematic errors can sometimes be compensated for by means of some kind of calibration 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 sensor.