Sensors and transducers

    The main function of sensor is to sense the environment around it, like temperature sensor senses the temperature around it. Most of the times some folks have misconception that sensors are those which converts one form of energy into another, where truth is that transducers are employed for the same. Sensor is the basic building block of transducer system. Again consider the temperature measurement system which displays current temperature. Here temperature sensors are used to sense the temperature and sensed temperature will be sent to further circuit which then converts the sensed quantity and convert it in suitable format to display the reading.

    Transducers have sensors alongwith supporting circuitry and the task of transducer is to convert one kind of energy into another form. Transducers are employed generally in automation systems.

    Choosing sensors and transducers is application dependant task and commonly depends on following point.


Operating Principle: The sensor/transducers are selected on the basis of operating principle it may be analogue, digital, active, passive, resistive, inductive, capacitive, optical etc.

Operating range: The range of sensor/transducer should be appropriate for measurement to get a good resolution.

Market availability: It should be available in market in case of replacement.

Accuracy: The accuracy of sensor/transducer should be as high as possible or as per the measurement.

Repeatability: The performance of sensor/transducer must be consistent throughout the time.

Less maintenance cost: it must be easy to repair/replace.

Range: The sensor/transducer can give good result within its specified range

Sensitivity: The sensor/transducer should be more sensitive to produce the output or sensitivity should be as per requirement.

Loading effect: The sensor/transducer’s input impedance should be high and output impedance should be low to avoid loading effect.

Errors: The error produced by the sensor/transducer should be low as possible.

Environmental compatibility: The sensor/transducer should maintain input and output characteristic for the selected environmental condition.

Speed of response: Must provide good speed of response.


Active transducers vs. passive transducers:

Active transducers: This type of transducers works on energy conversion principle and does not require external energy sources for their operation. Active transducers are mostly used surface roughness measurements. Some of examples of active transducers are thermocouple, piezoelectric transducers etc.

Passive transducers: This type of transducers works on energy controlling principle and does requires external energy sources for their operation. Passive transducers are mostly used in power measurement at high frequency applications. Some of examples of passive transducers are thermistors, strain gauge, RTD, inductive transducers, LVDT, LDR etc.


Analog transducers vs. digital transducers

Analog transducers: The output of analog transducer is analog in the nature and convert the input into analog output. Examples of analog transducers are strain gauge, potentiometer, LVDT, thermocouple, thermistor etc.

Digital transducers: The output of digital transducer is in the form of pulses and convert input into digital output. Example of digital transducers consists of rotary encoder.



I will like to conclude this post with one example of passive transducer i.e. strain gauge. (I wish to extend it but as per your demand. Please comment the transducer you want to learn about.)

Fig: Strain gauge using resistive bridge

    Figure shows strain gauge transducer in which divided bridge circuit is used to measure static or dynamic electrical resistance. To measure strain with a bonded resistance strain gauge, it is generally connected to an electric circuit which is capable to measuring the smallest changes in resistance due to strain. Strain gauge transducers use four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. The output voltage is shown in millivolts output per volt input. The Wheatstone circuit is also best candidate for temperature compensation. The number of active strain gauges that should be connected to the bridge depends on the application.