Types of sensors

Thermal sensors

• temperature sensors: thermometers, thermocouples, temperature sensitive resistors (thermistors and resistance temperature detectors), bi-metal thermometers and thermostats
• heat sensors: bolometer, calorimeter

Electromagnetic sensors

• electrical resistance sensors: ohmmeter, multimeter
• electrical current sensors: galvanometer, ammeter
• electrical voltage sensors: leaf electroscope, voltmeter
• electrical power sensors: watt-hour meters
• magnetism sensors: magnetic compass, fluxgate compass, magnetometer, Hall effect device,
• metal detectors

Mechanical sensors

• pressure sensors: altimeter, barometer, barograph, pressure gauge, air speed indicator, rate of climb indicator, variometer
• gas and liquid flow sensors: flow sensor, anemometer, flow meter, gas meter, water meter, mass flow sensor
• mechanical sensors: acceleration sensor, position sensor, selsyn, switch, strain gauge

Chemical sensors

Optical and radiation sensors

• electromagnetic time-of-flight. Generate an electromagnetic impulse, broadcast it, then measure the time a reflected pulse takes to return. Commonly known as - RADAR (Radio Detection And Ranging) are now accompanied by the analogous LIDAR (Light Detection And Ranging. See following line), all being electromagnetic waves. Acoustic sensors are a special case in that a pressure transducer is used to generate a compression wave in a fluid medium (air or water)
• light time-of-flight. Used in modern surveying equipment, a short pulse of light is emitted and returned by a retroreflector. The return time of the pulse is proportional to the distance and is related to atmospheric density in a predictable way.

Ionising radiation

• radiation sensors: Geiger counter, dosimeter, Scintillation_counter
• subatomic particle sensors: Particle detector, scintillometer, cloud chamber, bubble chamber

Non ionising radiation

• light sensors, or photodetectors, including semiconductor devices such as photocells, photodiodes, phototransistors, CCDs, and Image sensors; vacuum tube devices like photo-electric tubes, photomultiplier tubes; and mechanical instruments such as the Nichols radiometer.
• infra-red sensor, especially used as occupancy sensor for lighting and environmental controls.
• proximity sensor- A type of distance sensor but less sophisticated. Only detects a specific proximity. May be optical - combination of a photocell and LED or laser. Applications in cell phones, paper detector in photocopiers, auto power standby/shutdown mode in notebooks and other devices. May employ a magnet and a Hall effect device.
• scanning laser- A narrow beam of laser light is scanned over the scene by a mirror. A photocell sensor located at an offset responds when the beam is reflected from an object to the sensor, whence the distance is calculated by triangulation.
• focus. A large aperture lens may be focused by a servo system. The distance to an in-focus scene element may be determined by the lens setting.
• binocular. Two images gathered on a known baseline are brought into coincidence by a system of mirrors and prisms. The adjustment is used to determine distance. Used in some cameras (called range-finder cameras) and on a larger scale in early battleship range-finder
• interferometry. Interference fringes between transmitted and reflected lightwaves produced by a coherent source such as a laser are counted and the distance is calculated. Capable of extremely high precision.

Acoustic sensors

• sound sensors: microphones, hydrophones, seismometers.

Other types of sensor

• motion sensors: radar gun, speedometer, tachometer, odometer, occupancy sensor, turn coordinator
• orientation sensors: gyroscope, artificial horizon, ring laser gyroscope
• distance sensor (noncontacting) Several technologies can be applied to sense distance: magnetostriction

Non Initialized systems

• Gray code strip or wheel- a number of photodetectors can sense a pattern, creating a binary number. The gray code is a mutated pattern that ensures that only one bit of information changes with each measured step, thus avoiding ambiguities.

Initialized systems

• Quadrature wheel- An disk-shaped optical mask is driven by a gear train. Two photocells detecting light passing through the mask can determine a partial revolution of the mask and the direction of that rotation.
• whisker sensor- A type of touch sensor and proximity sensor.

Classification of measurement errors

1. the sensor should be sensitive to the measured property
2. the sensor should be insensitive to any other property
3. the sensor should not influence the measured property

• The gain may in practice differ from the value specified. This is called a gain error.
• Since the range of the output signal is always limited, the output signal will eventually clip when the measured property exceeds the limits. The full scale range defines the outmost values of the measured property where the sensor errors are within the specified range.
• 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 gain is not constant, this is called nonlinearity. Usually this is defined by the amount the output differs from ideal behaviour 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 behaviour is described with a bode plot showing gain 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.
• 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 the fact that the sensor not instantly follows the change of the property being measured, and therefore involves the history of the measured property.
• If the sensor has a digital output, the signal is discrete and 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 causes a dynamic error.
• The sensor may to some extent be sensitive for other properties than the property being measured. For example, most sensors are influenced by the temperature of their environment.

Sensor resolution

Biological sensors

• light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment;
• physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages (proprioception);
• an enormous array of environmental molecules, including toxins, nutrients, 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 the differences between proteins of the organism itself and of the environment or alien creatures.