What is Sensor?
A sensor is type of transducer that converts the measurand (a quantity or a parameter) into signal carrying information.
"Something which can sense and response accordingly is called as Sensor". Human is a typical example of Sensor.
A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy to another for various purposes including measurement or information transfer. In a broader sense "a transducer is sometimes defined as any device that converts a signal from one form to another".
Types of Sensors
Since a significant small change involves an exchange of energy, sensors can be classified according to the type of energy transfer that they detect.
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
- 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 proportion Sensors: oxygen sensors, ion-selective electrodes, pH glass electrodes, redox electrodes, and carbon monoxide detectors.
Optical radiation Sensors
- Light time-of-flight. Used in modern surveying equipment, a short pulse of light is emitted and returned by a retro reflector. The return time of the pulse is proportional to the distance and is related to atmospheric density in a predictable way - see LIDAR.
- Light Sensors, or Photo Detectors, 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-finders
- Interferometry. Interference fringes between transmitted and reflected light waves produced by a coherent source such as a laser are counted and the distance is calculated. Capable of extremely high precision.
- Scintillometers measure atmospheric optical disturbances.
- Fiber optic Sensors.
- Short path optical interception - detection device consists of a light-emitting diode illuminating a phototransistor, with the end position of a mechanical device detected by a moving flag intercepting the optical path, useful for determining an initial position for mechanisms driven by stepper motors.
Ionizing radiation Sensors
- Radiation Sensors: Geiger counter, dosimeter, Scintillation counter, Neutron detection
- Subatomic particle Sensors: Particle detector, scintillator, Wire chamber, cloud chamber, bubble chamber.
- Acoustic: uses ultrasound time-of-flight echo return. Used in mid 20th century Polaroid cameras and applied also to robotics. Even older systems like Fathometers (and fish finders) and other 'Tactical Active' Sonar (Sound Navigation and Ranging) systems in naval applications which mostly use audible sound frequencies.
- Sound Sensors: microphones, hydrophones, seismometers.
All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:
- 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.
- Artificial sensors that mimic biological sensors by using a biological sensitive component are called biosensors.
- Motion Sensors: radar gun, speedometer, tachometer, odometer, occupancy sensor, turn coordinator
- Orientation Sensors: gyroscope, artificial horizon, ring laser gyroscope
- Distance Sensor (non contacting) Several technologies can be applied to sense distance: magnetostriction
Non Initialized Systems
Gray code strip or wheel- a number of photo detectors 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.
These require starting from a known distance and accumulate incremental changes in measurements.
- Quadrature wheel- A 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
A good sensor obeys the following rules:
- the sensor should be sensitive to the measured property
- the sensor should be insensitive to any other property
- the sensor should not influence the measured property
It is often ideal that the output signal of a sensor is proportional to the value of the measured property. The gain 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 gain is a constant with the unit [V/K]. If the sensor is not ideal, several types of deviations can be observed:
- 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 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 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.
- 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.
Resolution of Sensors
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 probe (a fine tip near a surface collects an electron tunneling current) can resolve atoms and molecules.
Sensors are used in everyday life. Applications include automobiles, machines, aerospace, medicine, industry and robotics. In general, there is a large variety of sensors. Sensors have uncertainty that needs to be effectively handled, such as with hysteresis. Sensors values can be used to learn to classify the percepts.
Technological progress allows more and more sensors to be manufactured on the microscopic scale as micro sensors using MEMS technology. In most cases a micro sensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches.
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Rajesh Kumar Gupta on 2009-03-07 03:05:52 wrote,
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