sensor

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an 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, all sensors need to be calibrated against known standards.

Use

Sensors are used in everyday objects such as touch-sensitive elevator buttons 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'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. 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

A good sensor obeys the following rules:

• Is sensitive to the measured property
• Is insensitive to any other property
• Does not influence the measured property

Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value 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.

Transducer

A transducer is a device, electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic, that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer (for example: pressure sensors).
There are two kinds of transducers. A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical or digital signal), such as a tachometer. An actuator is used for the transformation of energy or in other words, actuator is the one which gets actuated or stands responsible for the output action, in that it converts electrical signal into generally nonelectrical energy. An example of an actuator is a loudspeaker which converts an electrical signal into a variable magnetic field and, subsequently, into acoustic waves. The third kind of transducer has both functions -- for example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves.

 Types of transducers

• Electromagnetic:
  • Antenna - converts electromagnetic waves into electric current and vice versa.
  • Cathode ray tube (CRT) - converts electrical signals into visual form
  • Fluorescent lamp, light bulb - converts electrical power into visible light
  • Magnetic cartridge - converts motion into electrical form
  • Photodetector or Photoresistor (LDR) - converts changes in light levels into resistance changes
  • Tape head - converts changing magnetic fields into electrical form
  • Hall effect sensor - converts a magnetic field level into electrical form only.

• Electrochemical:
  • pH probes
  • Electro-galvanic fuel cell
  • Hydrogen sensor
• Electromechanical (electromechanical output devices are generically called actuators):
  • Electroactive polymers
  • Galvanometer
  • MEMS
  • Rotary motor, linear motor
  • Vibration powered generator
  • Potentiometer when used for measuring position
  • Load cell converts force to mV/V electrical signal using strain gauge
  • Accelerometer
  • Strain gauge
  • String Potentiometer
  • Air flow sensor

• Electroacoustic:
  • Loudspeaker, earphone - converts electrical signals into sound (amplified signal → magnetic field → motion → air pressure)
  • Microphone - converts sound into an electrical signal (air pressure → motion of conductor/coil → magnetic field → signal)
  • Pick up (music technology) - converts motion of metal strings into an electrical signal (magnetism → electricity (signal))
  • Tactile transducer - converts solid-state vibrations into electrical signal (vibration → ? → signal)
  • Piezoelectric crystal - converts solid-state electrical moduluations into an electrical signal (vibration → ? → signal)
  • Geophone - convert a ground movement (displacement) into voltage - (vibrations → motion of conductor/coil → magnetic field → signal)
  • Gramophone pick-up - (air pressure → motion → magnetic field → signal)
  • Hydrophone - converts changes in water pressure into an electrical form
  • Sonar transponder (water pressure → motion of conductor/coil → magnetic field → signal)

• Photoelectric:
  • Laser diode, light-emitting diode - convert electrical power into forms of light
  • Photodiode, photoresistor, phototransistor, photomultiplier tube - converts changing light levels into electrical form

• Electrostatic:
  • Electrometer

• Thermoelectric:
  • RTD Resistance Temperature Detector
  • Thermocouple
  • Peltier cooler
  • Thermistor (includes PTC resistor and NTC resistor)

• Radioacoustic:
  • Geiger-Müller tube used for measuring radioactivity.
  • Receiver (radio)