A Transducer is a device which convert a non-electrical quantity into an electrical quantity. It comprises of a detecting / sensing element and a transduction element. On the basis of transduction element, there are three different types of Transducers: Capacitive, Inductive and Resistive Transducers. Apart from this, transducers are classified into following types:
- Primary and Secondary Transducer
- Passive and Active Transducer
- Analog and Digital Transducer
- Transducers and Inverse Transducer
This classification in shown below in pictorial form to have better grasp of the subject matter.
Explanation of Each Types of Transducers:
Classification based upon Principle of Transduction:
A transducer is classified as Resistive, Inductive or Capacitive depending upon how they convert the input quantity into resistance, inductance or capacitance respectively. They can be categorized as piezoelectric, thermoelectric, magnetorestrictive, electrokinetic and optical.
Primary and Secondary Transducers:
Primary Transducer is the detecting or sensing element which responds to the change in physical phenomena. Whereas the Secondary Transducer converts the output of primary transducer (output in the form of mechanical movement) into electrical output. Let us understand this type of transducer with an example.
Let us take an example for measurement of a compressive force with the help of load cell and strain gauge as shown in figure below.
Load cell is a short column or a strut with resistance wire strain gauge bonded to it. Here we are interested in measurement of applied force. When for is applied on the load cell, a strain is produced in it. The force is first detected by the column and is converted into the strain which is basically a mechanical displacement. The higher the applied force, the more will be strain generated in the column. This strain changes the resistance of strain gauge. Thus, we have output in the form of change in resistance which is an electrical quantity. Change in resistance of strain gauge is calibrated with the applied force and hence, output of strain gauge directly indicates the value of applied force.
The entire process of measurement of force with the help of load cell and strain gauge can be divided into two parts. First, the applied force generates a mechanical movement, called strain. Next, the strain gauge responds to the mechanical movement and process corresponding change in resistance value.
Thus, we see that the force is detected by the column in the first stage and hence it is called the Primary Transducer. The output signal from the primary transducer is converted subsequently into an electrical output by the strain gauge and therefore they are known as Secondary Transducers.
In most of the measurement systems, there is a combination of wherein a Mechanical device acts as primary transducer and the electrical device acts as secondary transducer with mechanical displacement serving as the intermediate signal.
Passive and Active Transducers:
A Transducer which requires external power supply to function is called passive transducer. This external power supply is called auxiliary power supply and required for the functioning of transduction element. This type of transducer is also called Externally Powered Transducer.
“POT” is an example of passive transducer. It is used to convert a linear displacement into electrical signal. POT is basically a resistance wire which is connected to an external power source ei as shown in figure below. The linear displacement is xi and the output voltage is eo.
Let the length of resistance wire be L and total resistance be Rt. When linear displacement is xi, the resistance of wire across the eo is (Rt/L)xi and hence, the output voltage eo will be given by voltage division rule as below.
eo = [ei(Rt/L)xi] / Rt
= ei(xi / L)
xi = (eo/ ei) L
From the above example, it is clear that the linear displacement is directly proportional to the output voltage for a given input voltage and resistance length. It can also be observed that a POT can not work if external power supply is not connected. This is the reason; it is a kind of passive transducer. Thus, we can say that, a passive transducer explicitly required external power supply otherwise it won’t work.
Active Transducers are those which does not require external power supply to work. This doesn’t mean that these types of transducers don’t require power at all. In fact, the power required for functioning is derived from the physical change itself. Example of active transducer is piezoelectric crystal. When external force is applied on a Piezoelectric crystal, a emf is developed across its face on which force is applied. This property is utilized to make an active transducer. This transducer is called Accelerometer used to measure the acceleration.
In an accelerometer, piezoelectric crystal is sandwiched in between the two metallic electrodes and the arrangement is fastened with the floor whose acceleration is to be measured. A fixed mass in placed on the top of the sandwich. Arrangement of accelerometer is shown in figure below.
When the floor travels with acceleration, a force proportional to the acceleration is exerted on the sandwich and hence a emf is generated across the electrodes. This emf is directly proportional to the acceleration and hence, indicates the acceleration of the moving body.
Here, it should be noted that no external source of power is required to work for an accelerometer. Hence, it is an Active Transducer.
Analog and Digital Transducers:
Analog Transducers are those whose output is continuous in time domain. This essentially means that the electrical output signal will be continuous function of time. Example of analog transducers are RTD, Thermocouple, LVDT, RVDT, thermistor etc. In RTD and Thermocouple, the output signal is in the form of voltage which is always available.
Transducers which convert the input quantity into an electrical output signal which is in the form of pulse is called Digital Transducers. Note that, the output is not continuous rather it is in the form of pulse which means that it is discrete. Example of digital transducer is a Shaft Encoder, Limit Switch, Digital Tachometer etc. Limit Switch is the simplest example of digital transducer.
Shaft Encoder is used to measure the angular position & velocity and its output is digital in nature. It is extensively used in the robotics, rotating machinery control like Crane.
Transducers and Inverse Transducers:
Devices which convert a non-electrical quantity into an electrical quantity is popularly known as Transducers.
Devices which convert an electrical quantity into non-electrical quantity is called Inverse Transducer. The name itself implies that the function of Inverse Transducer is inverse of the Transducer.
In this type of transducers, an electrical signal is intentionally converted into some sort of physical change. Example of Inverse transducers are piezoelectric crystal. When voltage across the surface of a piezoelectric crystal is applied, it changes its dimension. Thus, electrical quantity is changed into physical quantity. Another example is a coil carrying current and kept in magnetic field. Due to interaction of current of coil with the magnetic field, it starts to rotate or translate. Inverse Transducers are mainly used in control system to control various process parameters viz. pressure, temperature, displacement etc.