Modes of Operation

 Modes of Operation of a sensor are classified on the following basis:

  • Is the sensor Active or Passive?
  • Is its output signal Analog or Digital?
  • Is the sensor of Null type or of Deflection type?
  1. Active vs Passive

    Active Sensors are the sensors who do not require any external stimulus or supply for their operation.
    Just a change in the parameter being sensed is enough for them to give an output.
    e.g. Thermocouple for temperature measurement, Piezoelectric accelerometers,etc.

    Passive Sensors are the sensors which produce a signal on detecting change only when external stimulation is provided. If the supply is removed, their output signal stops.
    e.g. RTD for temperature measurement, Strain Guage, etc.
  2.  Analog vs Digital

    Analog sensor gives output in analog form, i.e its output is continuous in the given range; while Digital sensor gives digital output, i.e. Its output is in predefined steps in the given range. For example, if we need to measure some value in between 1.5 and 2.5, the Analog sensor will give the exact value, but the digital one will only show 2. The last digit in digital form will be rounded off to + 0.5, but the analog form will give the exact value it measures.We also need to consider the form of unit(A/D) to which the sensor is going to be interfaced. If it demands only digital type, we may need to use Analog to Digital Converters for Analog sensors, while Digital sensors could be interfaced directly. If it demands only analog type, then Digital to Analog Converters are used with digital types, while Analog ones are interfaced directly.Depending upon the type of output expected and the unit to which the sensor is going to be interfaced, we need to shortlist the type of sensors which are favorable for our process. A sensor giving binary output (0 or 1) is classified as a digital one.
  3. Null type or Deflection type

    In Deflection type, as the name suggests, deflection of the instrument provides a basis for determining the magnitude of quantity being measured. A mechanical deflection in proportion to the magnitude of the measured quantity is produced. A needle moves on a scale to give the respective reading.
    e.g. PMMC Ammeter, Voltmeter, etc.

    In Null type, a 0 or null indication leads to determination of the magnitude of the measured quantity. The transducer produces an opposing force to stay at null position, which is proportional to the magnitude of quantity being measured. A null detector (e.g. Current Galvanometer) is required to detect the null position. When the detector detects the null position of the indicator, the opposing force is equal to the applied change. The magnitude of opposing force is calibrated in terms of the quantity being measured.
    e.g. Potentiometer

    Please Note:
    i)   Accuracy of null type is more than that of deflection type.
    ii)  Sensitivity of null type is also higher than deflection type.
    iii) Respone of null type is slower than deflection type.
         Therefore, deflection type of instruments are more suited for dynamic measurements.