| Since a significant change involves an exchange of | | | | number. The gray code is a mutated pattern that |
| energy, sensors can be classified according to the | | | | ensures that only one bit of information changes with |
| type of energy transfer that they detect. Thermal | | | | each measured step, thus avoiding ambiguities. |
| temperature sensors: thermometers, thermocouples, | | | | |
| temperature sensitive resistors (thermistors and | | | | Initialized systems |
| resistance temperature detectors), bi-metal | | | | These require starting from a known distance and |
| thermometers and thermostatsheat sensors: | | | | accumulate incremental changes in measurements. |
| bolometer, calorimeter | | | | |
| | | | | Quadrature wheel- An disk-shaped optical mask is |
| Electromagnetic electrical resistance sensors: | | | | driven by a gear train. Two photocells detecting light |
| ohmmeter, multimeter | | | | passing through the mask can determine a partial |
| Electrical current sensors: galvanometer, ammeter | | | | revolution of the mask and the direction of that |
| Electrical voltage sensors: leaf electroscope, | | | | rotation.whisker sensor- A type of touch sensor and |
| voltmeter | | | | proximity sensor. |
| Electrical power sensors: watt-hour meters | | | | |
| Magnetism sensors: magnetic compass, fluxgate | | | | Classification of measurement errors |
| compass, magnetometer, Hall Effect device, | | | | A good sensor obeys the following rules: |
| Metal detectors | | | | the sensor should be sensitive to the measured |
| | | | | propertythe sensor should be insensitive to any other |
| Mechanical pressure sensors: altimeter, barometer, | | | | propertythe sensor should not influence the |
| barograph, pressure gauge, air speed indicator, rate | | | | measured property |
| of climb indicator, variometergas and liquid flow | | | | In the ideal situation, the output signal of a sensor is |
| sensors: flow sensor, anemometer, flow meter, gas | | | | exactly proportional to the value of the measured |
| meter, water meter, mass flow sensor mechanical | | | | property. The gain is then defined as the ratio |
| sensors: acceleration sensor, position sensor, selsyn, | | | | between output signal and measured property. For |
| switch, strain gauge | | | | example, if a sensor measures temperature and has |
| | | | | a voltage output, the gain is a constant with the unit |
| Chemical sensors detect the presence of specific | | | | [V/K]. |
| chemicals or classes of chemicals. Examples include | | | | |
| oxygen sensors, also known as lambda sensors, | | | | If the sensor is not ideal, several types of deviations |
| ion-selective electrodes, pH glass electrodes, and | | | | can be observed: |
| redox electrodes. | | | | |
| | | | | The gain may in practice differ from the value |
| Optical and radiation electromagnetic time-of-flight. | | | | specified. This is called a gain error. |
| Generate an electromagnetic impulse, broadcast it, | | | | Since the range of the output signal is always limited, |
| and then measure the time a reflected pulse takes | | | | the output signal will eventually clip when the |
| to return. Commonly known as - RADAR (Radio | | | | measured property exceeds the limits. The full scale |
| Detection And Ranging) are now accompanied by the | | | | range defines the outmost values of the measured |
| analogous LIDAR (Light Detection And Ranging. See | | | | property where the sensor errors are within the |
| following line), all being electromagnetic waves. | | | | specified range. |
| Acoustic sensors are a special case in that a pressure | | | | If the output signal is not zero when the measured |
| transducer is used to generate a compression wave | | | | property is zero, the sensor has an offset or bias. |
| in a fluid medium (air or water)light time-of-flight. | | | | This is defined as the output of the sensor at zero |
| Used in modern surveying equipment, a short pulse | | | | input. |
| of light is emitted and returned by a retro reflector. | | | | If the gain is not constant, this is called nonlinearity. |
| The return time of the pulse is proportional to the | | | | Usually this is defined by the amount the output |
| distance and is related to atmospheric density in a | | | | differs from ideal behavior over the full range of the |
| predictable way. | | | | sensor, often noted as a percentage of the full |
| | | | | range. |
| Ionizing radiation | | | | If the deviation is caused by a rapid change of the |
| | | | | measured property over time, there is a dynamic |
| Radiation sensors: Geiger counter, dosimeter, | | | | error. Often, this behavior is described with a bode |
| Scintillation counter, Neutron detection | | | | plot showing gain error and phase shift as function of |
| Subatomic particle sensors: Particle detector, | | | | the frequency of a periodic input signal. |
| scintillator, Wire chamber, cloud chamber, bubble | | | | If the output signal slowly changes independent of |
| chamber | | | | the measured property, this is defined as drift. |
| | | | | Long term drift usually indicates a slow degradation |
| Non-ionising radiation | | | | of sensor properties over a long period of time. Noise |
| light sensors, or photo detectors, including | | | | is a random deviation of the signal that varies in time. |
| semiconductor devices such as photocells, | | | | |
| photodiodes, phototransistors, CCDs, and Image | | | | Hysteresis is an error caused by the fact that the |
| sensors; vacuum tube devices like photo-electric | | | | sensor not instantly follows the change of the |
| tubes, photomultiplier tubes; and mechanical | | | | property being measured, and therefore involves the |
| instruments such as the Nichols radiometer. Infra-red | | | | history of the measured property. |
| sensor, especially used as occupancy sensor for | | | | If the sensor has a digital output, the signal is |
| lighting and environmental controls. | | | | discrete and is essentially an approximation of the |
| Proximity sensor- A type of distance sensor but less | | | | measured property. The approximation error is also |
| sophisticated. Only detects a specific proximity. May | | | | called digitization error. |
| be optical - combination of a photocell and LED or | | | | If the signal is monitored digitally, limitation of the |
| laser. Applications in cell phones, paper detector in | | | | sampling frequency also causes a dynamic error. |
| photocopiers, auto power standby/shutdown mode | | | | Sensor may to some extent be sensitive for other |
| in notebooks and other devices. May employ a | | | | properties than the property being measured. For |
| magnet and a Hall effect device.scanning laser- A | | | | example, most sensors are influenced by the |
| narrow beam of laser light is scanned over the scene | | | | temperature of their environment. |
| by a mirror. A photocell sensor located at an offset | | | | All these deviations can be classified as systematic |
| responds when the beam is reflected from an object | | | | errors or random errors. Systematic errors can |
| to the sensor, whence the distance is calculated by | | | | sometimes be compensated for by means of some |
| triangulation.focus. A large aperture lens may be | | | | kind of calibration strategy. Noise is a random error |
| focused by a servo system. The distance to an | | | | that can be reduced by signal processing, such as |
| in-focus scene element may be determined by the | | | | filtering, usually at the expense of the dynamic |
| lens setting.binocular. Two images gathered on a | | | | behaviour of the sensor. |
| known baseline are brought into coincidence by a | | | | |
| system of mirrors and prisms. The adjustment is | | | | |
| used to determine distance. Used in some cameras | | | | Resolution |
| (called range-finder cameras) and on a larger scale in | | | | |
| early battleship range-finderinterferometer. | | | | The resolution of a sensor is the smallest change it |
| Interference fringes between transmitted and | | | | can detect in the quantity that it is measuring. Often |
| reflected lightwaves produced by a coherent source | | | | in a digital display, the least significant digit will |
| such as a laser are counted and the distance is | | | | fluctuate, indicating that changes of that magnitude |
| calculated. Capable of extremely high precision. | | | | are only just resolved. The resolution is related to the |
| Scintillometers measure atmospheric optical | | | | precision with which the measurement is made. For |
| disturbances. | | | | example, a scanning probe (a fine tip near a surface |
| | | | | collects an electron tunneling current) can resolve |
| Acoustic sound sensors: microphones, hydrophones, | | | | atoms and molecules. |
| seismometers. | | | | |
| | | | | |
| Acoustic: uses ultrasound time-of-flight echo return. | | | | Biological |
| Used in mid 20th century polaroid cameras and | | | | |
| applied also to robotics. Even older systems like | | | | All living organisms contain biological sensors with |
| Fathometers (and fish finders) and other 'Tactical | | | | functions similar to those of the mechanical devices |
| Active' Sonar (Sound Navigation And Ranging) | | | | described. Most of these are specialized cells that are |
| systems in naval applications which mostly use audible | | | | sensitive to: |
| sound frequencies. | | | | |
| | | | | Light, motion, temperature, magnetic fields, gravity, |
| | | | | humidity, vibration, pressure, electrical fields, sound, |
| Other types motion sensors: radar gun, | | | | and other physical aspects of the external |
| speedometer, tachometer, odometer, occupancy | | | | environment; |
| sensor, turn coordinator | | | | Physical aspects of the internal environment, such as |
| Orientation sensors: gyroscope, artificial horizon, ring | | | | stretch, motion of the organism, and position of |
| laser gyroscopedistance sensor (non contacting) | | | | appendages (proprioception);an enormous array of |
| Several technologies can be applied to sense distance: | | | | environmental molecules, including toxins, nutrients, |
| magnetostriction | | | | and pheromones; |
| | | | | Many aspects of the internal metabolic milieu, such as |
| | | | | glucose level, oxygen level, or osmolality;an equally |
| | | | | varied range of internal signal molecules, such as |
| | | | | hormones, neurotransmitters, and cytokines;and even |
| Non Initialized systems | | | | the differences between proteins of the organism |
| | | | | itself and of the environment or alien creatures. |
| Gray code strip or wheel- a number of photo | | | | Artificial sensors that mimic biological sensors by using |
| detectors can sense a pattern, creating a binary | | | | a biological sensitive component, are called biosensors. |