CLASSIFICATION OF TRANSDUCERS

WHAT IS TRANSDUCER?

For most instrumentation systems, the input quantity will be a non-electrical quantity. The non-electrical quantity is converted into an electrical form for using electrical methods and techniques for measurement, manipulation and control. The device which when actuated, transforms energy from one form to another is called a transducer.

PARTS OF TRANSDUCER

The transducer consisting of two important and closely related parts

  • Sensing or Detection Element: A detector or a sensing element is that part of a transducer which responds to a physical phenomenon or a change in physical phenomenon. The response of sensing element must be closely related to physical phenomenon.
  • Transduction Element: A transduction element is one which transforms the output of a sensing element to an electrical output.

HOW ARE TRANSDUCERS CLASSIFIED?

Transducers are classified into 5 as,

1. On the basis of transduction form used

2. As primary and secondary transducers

3. As passive and active transducers

4. As analog and digital transducers

5. As transducers and inverse transducers

1. Classification based on the principle of transduction form used:

This classification is based on the principle of transduction as resistive, inductive, capacitive etc. depending upon their conversion into resistance, inductance or capacitance respectively. They can be classified as thermoelectric, piezoelectric, electrokinetic, optical and magnetorestrictive.

2. Classification as primary and secondary transducers

Primary transducer senses the input directly and physical phenomenon is converted into the electrical form directly. While in secondary transducer, initially input is being sensed by some detector or sensor and then its output in some form other than input signal is given to transducer to convert into electrical signal.Mechanical device acts as a primary detector transducer and the electrical device acts as the secondary transducer in most of the measurement systems with mechanical displacement serving as the intermediate signal.

As shown in the figure, the bourdon  tube which senses the pressure and converts it into displacement of its free end and hence acts as primary transducer.  The core of LVDT (linear variable differential transformer) is moved due to this displacement. Hence the LVDT produces output voltage proportional to that displacement of the free end of bourdon tube. Bourdon tube, which acts as primary transducer and LVDT acts as secondary transducer.

3. Classification as Active and Passive transducer

Active transducers are one which develop their output in the form of electrical voltage or current without any auxiliary source. They are also called self-generating transducers. Active transducers draw energy from the system under measurement and such transducers normally give very small output. Thermocouples used for the measurement of temperature, tacho-generators used for the measurement of angular velocity comes under this category.

In the example shown above, the crystal is sandwiched between 2 metallic electrodes, and the entire sandwich is fastened to a base which may be the floor of a rocket. On the top of the sandwich a fixed mass is placed which exerts a certain force due to acceleration on the crystal due to which a voltage is generated. The voltage output is proportional force and hence is proportional to acceleration. The property of piezo -electric crystal is that when a force is applied to them, they produce an output voltage.

Passive transducers are one which require external power source for energy conversion. In passive transducers electrical parameters like resistance, inductance, capacitance causes a change in voltage, current or frequency of external source. Resistive, inductive and capacitive transducers fall in this category.

This is an example for passive transducer. A linear potentiometer is a resistive transducer powered by a source voltage ei and is used for the measurement of linear displacement xi.

4. Analog and Digital transducer

Transducers, on the basis of nature of output can be classified as analog and digital.

  • Analog transducers: These type of transducers which convert input signal into output in the form of a continuous function of time such as thermistor, LVDT, thermocouple etc.
  • Digital transducers: These type of transducers which convert input signal into output in the form of pulses. The examples of digital transducers are linear displacement transducers using conducting and non-conducting contacts, opaque and translucent segments and shaft encoders. It can be easily represented by opaque and transparent areas on a glass scale or non-conducting and conducting areas as the binary system uses only two symbols 0 and 1.

5. Transducers and Inverse transducers

The basic requirement for control of physical quantities such as position, speed, temperature pressure and flow rate in an industrial plant is the ability to measure these quantities. The control action is only possible if the physical quantity can be measured. Transducers as early mentioned converts a non-eletrical quantity into electrical whereas inverse transducers converts an electrical quantity into some other forms.

An example is a current carrying coil moving in a magnetic field is an inverse transducer as the current carried by it is converted into a force which causes a rotational or translational displacement. A large number of data indicating and recording devices are inverse transducers. Such devices which include instruments like pen recorders, oscilloscopes that convert electrical into mechanical movement are placed at the output stage are called as output transducers. The below diagram shows a piezo -electric crystal acts as an inverse transducer. When a voltage is applied across its surfaces, a mechanical displacement is caused as it changes its dimensions.

WHAT ARE THE BASIC REQUIREMENTS OF A TRANSDUCER?

There are 8 basic requirements for the proper functioning of transducers;

  1. Ruggedness: It should be capable of withstanding over load and some safety should be provided for overload protection.
  2. Linearity: Its input-output characteristics should be linear and it should produce these characteristics in symmetrical way.
  3. Repeatability: It should reproduce same output signal when the same input signal is applied under fixed environmental conditions. For example pressure, temperature, humidity etc.
  4. Residual Deformation: There should be no deformation on removal of input signal after long period of application.
  5. No Hysteresis: It should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa.
  6. High Output Signal Quality: The quality of the output signal should be good. That is the ratio of signal to the noise is high and the amplitude of the output signal should be enough.
  7. High Reliability and Stability: It should give minimum error in measurement for temperature variations, vibrations and other various changes in surroundings.
  8. Good Dynamic Response: Its output should be faithful to input when taken as a function of time. The effect is analysed as the frequency response
News Reporter
Instrumentation & Control Engineer