Transducer and Its Classification

In instrumentation transducers play a vital role. In this article the transducer and its classification is discussed.

  • Basically, transducer is defined as a device, which converts energy or information from one form to another. These are widely used in measurement work because not all quantities that need to be measured can be displayed as easily as others.
  • A better measurement of a quantity can usually be made if it may be converted to another form, which is more conveniently or accurately displayed.
  • For example, the common mercury thermometer converts variations in temperature into variations in the length of a column of mercury.
  • The variation in the length of the mercury column is rather simple to measure, the mercury thermometer becomes a convenient device for measuring temperature. On the other hand, the actual temperature variation is not as easy to display directly.
  • The transducer may be mechanical, electrical, magnetic, optical, chemical, acoustic, thermal nuclear, or a combination of any two or more of these.
  • Thus, the transducer is a device, which provides a usable output in response to specific input measured, which may be physical or mechanical quantity, property or condition.

The transducers are mainly classified into

  1. Mechanical transducers
  2. Electrical transducers

Mechanical Transducers

  • They are simple and rugged in construction, cheaper in cost, accurate and operate without external power supplies.
  • They are not advantageous for many of the modern scientific experiments and process control instrumentation owing to their poor frequency response, requirement of large forces to overcome mechanical friction, in compatibility when remote control or indication is required, and a lot of other limitations.
  • All these drawbacks have been overcome with the introduction of electrical transducers.

Electrical Transducers

  • A transducer is a device that converts one form of energy to other form.
  • An electrical transducer is a device which converts the measurand to a usable electrical signal.
  • In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current. The physical quantity may be mechanical, chemical, optical or thermal.
  • Basically, an electrical transducer is a sensing device by which a physical, mechanical or optical quantity to be measured is transformed directly, with a suitable mechanism, into an electrical signal (current, voltage or frequency).
  • The production of these signals is based upon electrical effects which may be resistive, inductive, capacitive etc. in nature.

The electrical transducers can be classified as

  1. Active and passive transducers
  2. Analog and digital transducers
  3. Primary and secondary transducer
  4. Transducers and inverse transducers
  5. On the basis of transduction principle used

Active Transducers and Passive Transducers

Based on the need of external power requirement the transducers are classified as active and passive transducers.

  • Active transducers do not need any external source of power for their operation. Therefore, they are also called as self-generating type transducers.
  • The active transducer are self-generating devices which operate under the energy conversion principle. As the output of active transducers, we get an equivalent electrical output signal
  • For example, temperature or strain to electric potential, without any external source of energy being used.
  • Passive Transducers need external source of power for their operation. So, they are not self-generating type transducers.
  • A DC power supply or an audio frequency generator is used as an external power source.
  • These transducers produce the output signal in the form of variation in resistance, capacitance, inductance or some other electrical parameter in response to the quantity to be measured.

Primary and Secondary Transducers

Transducers, on the basis of methods of applications, may be classified into primary and secondary transducers.

  • When the input signal is directly sensed by the transducer and physical phenomenon is converted into the electrical form directly then such a transducer is called the primary transducer.
  • For example, a thermistor used for the measurement of temperature fall in this category. The thermistor senses the temperature directly and causes the change in resistance with the change in temperature.
  • When the input signal is sensed first by some detector or sensor and then its output being of some form other than input signals is given as input to a transducer for conversion into electrical form, then such a transducer falls in the category of secondary transducers.
  • For example, in case of pressure measurement, bourdon tube is a primary sensor which converts pressure first into displacement, and then the displacement is converted into an output voltage by an LVDT. In this case LVDT is secondary transducer.

Analog and Digital Transducers

Transducers, on the basis of nature of output signal, may be classified into analog and digital transducers.

  • Analog transducer converts input signal into output signal, which is a continuous function of time such as thermistor, strain gauge, LVDT, thermo-couple etc.
  • Digital transducer converts input signal into the output signal of the form of pulse e.g. it gives discrete output.
  • These transducers are becoming more and more popular now-a-days because of advantages associated with digital measuring instruments and also due to the effect that digital signals can be transmitted over a long distance without causing much distortion due to amplitude variation and phase shift.
  • Sometimes an analog transducer combined with an ADC (analog-digital convertor) is called a digital

Transducers and Inverse Transducers

  • Transducer, as already defined, is a device that converts a non-electrical quantity into an electrical quantity.
  • Normally a transducer and associated circuit has a non-electrical input and an electrical output, for example a thermo-couple, photoconductive cell, pressure gauge, strain gauge etc.
  • An inverse transducer is a device that converts an electrical quantity into a non-electrical quantity. It is a precision actuator having an electrical input and a low-power non-electrical output.
  • For examples a piezoelectric crystal and transnational and angular moving-coil elements can be employed as inverse transducers. Many data-indicating and recording devices are basically inverse transducers.
  • An ammeter or voltmeter converts electric current into mechanical movement and the characteristics of such an instrument placed at the output of a measuring system are important.
  • A most useful application of inverse transducers is in feedback measuring systems.

On The Basis of Transduction Principle Used

On the basis of transduction principle used the transducer is further classified as

  1. Resistive transducer
  2. Inductive transducer
  3. Capacitive transducer

These transducers are explained in the next article.

Apart from this the transducers may be classified on the basis of

  • Applications
  • Method of energy conversion.
  • Nature of output signal

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