Sensors

A device that detects the changes in electrical or physical or other quantities and thereby produces an output as an acknowledgement of change in the quantity is called as a Sensor. Generally, this sensor output will be in the form of electrical or optical signal.A sensor is a special kind of transducer which is used to generate an input signal to a measurement, instrumentation or control system. The signal produced by a sensor is an electrical analogy of a physical quantity like acceleration, temperature, pressure, distance, velocity, light level etc. A sensor is a device that detects/measures a physical quantity. The opposite device to the sensor is an actuators. The actuator converts an input signal into some kind of action, usually mechanical.

 

Types of Sensors

Active sensor:

  • An active sensor or self-generating sensor is one that can generate a signal without the need of any external power supply.
  • Examples of active sensors are Photovoltaic cells, thermocouples, piezoelectric device.

Passive Sensor:

  • A passive sensor is one which requires external power supply to generate the signal.
  • The typical example for this type of sensor is diaphragm used to convert the pressure or velocity oscillations of sound waves into movements of a solid sheet.

Resolution in sensors:
The resolution of a sensor measures its ability to detect a change in the sensed quantity. It is usually quoted in terms of the smallest change that can be detected.

Errors in sensors:
The sensing of any quantity is liable to error. The errors can be static or dynamic.

Static Error:

  • A static error is the type of error that is caused by reading problems like the parallax of a needle on a meter scale(which causes the apparent reading to vary according to the position of the observer’s eye). 
  • Interpolation error is another error of this type. It arises when a needle is positioned between two marks on a scale, and the used has to make a guess as to the amount signified by this position. When the scale is linear, the interpolation error is least.

Dynamic Error:

  • A typical error of this type is difference between the quantity as it really is and the amount that is measured, caused by the loading of the measuring instrument itself.
  • An example for this type of error is the false voltage reading measured across a high-resistance potential divider with a voltmeter whose input resistance is not high enough.
  • All forms of sensors are liable to dynamic errors if they are used for sensing only.
  • If they are used for measurement, both static and dynamic errors will occur.