Electrical Sensors - Basics, Definitions, Types

Introduction about Electrical Sensors:

Sensor - Definition:

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(usually electrical) into some kind of action, usually mechanical.

Transducer - Definition:

A transducer is a device that converts energy form one form into another form. The transducer whose output is electrical signal is known as electrical transducer.

Comparison of Sensor with Transducer:

  • The difference between sensors and transducers are often very slight.
  • A sensor performs a transducing action, and the transducer must sense some physical quantity. The difference lies in the efficiency of energy conversion.
  • The purpose of a sensor is to detect and measure the physical quantity and whether its efficiency is 5% or 0.1% is not important.
  • In contrast to this, a transducer is intended to convert energy and its efficiency is important ( though in some cases it may not be high). 
  • Plotting the output against the input is known as linearity response. The linearity response is important for a sensor.
  • But for transducers it is not important. 
  • By contrast, the efficiency of conversion is important for a transducer but not for a sensor.

Types of Sensors:

Sensors are classified as active sensor and passive sensor.

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.

Sensors can also be classified as Digital sensor and Analog sensor.

  • The output of a digital sensor is only two discrete states(either ON or OFF state). In other words either logic-1 or logic-0 etc.
  • The output of an analog sensor can be an infinite number of voltage or current levels. Thus it is said to be continuously variable.

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.

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