Write short notes on the following: (a) Thermocouple, (b) Digital transducer, (c) D.C. Tachogenerator, (d) LVDT

(A) Thermocouple: 

The thermocouple effect is one of the most popularly used methods to measure moderately high temperature. When a pair of wires made up of different metals is joined together at one end, a temperature difference between the two ends of the wire produces a voltage between the two wires as illustrated in Figure below Seeback effect is one useful method for temperature measurement with Thermocouple. A current will circulate around a loop to make up of two dissimilar metals when the two junctions are at different temperatures as shown in Figure below.

When the circuit is opened, a voltage appears across the junction and this voltage is proportional to the observed see back current.There are four voltage sources and their sum is the observed Seeback voltage. Each junction is a voltage source, known as Peltier emf. Furthermore, each homogenous conductor has a self induced voltage on Thomson emf. The Thomson and Peltier emfs rise from the fact that, within conductors, the density of free charge carriers (electrons and holes) increases as well as temperature increases. If the temperature of one end of a conductor is raised above that of the other end, excess electrons from the hot end will diffuse to the cold end. This results in an induced voltage, the Thomson effect that makes the hot end positive with respect to the cold end.

Conductors which are made up of different materials which have different free-carriers densities even when at the same temperature. ure. When two dissimilar conductors are joined, electrons will diffuse across the junction from the conductor with higher electron density. When this happens the conductor losing electrons acquire a positive voltage with respect to the other conductor. This voltage is called the Peltier emf. A voltage is generated when the junction is heated and this phenomenon is known as the Seeback Effect. The Seeback voltage is linearly proportional for small changes in temperature. Various combinations of metals are used in Thermocouple's. The magnitude of this voltage also depends on the material which is used for the wires and the amount of temperature difference produced between the joined ends, and the other ends. The junction of the wires of the thermocouple is called the sensing ends. The junction of the wires of the thermocouple is called the sensing junction, and this junction is normally placed in or on the unit under test.

Since the critical factor is the temperature difference between the sensing junction and the other ends, the other ends are either kept at a constant reference temperature, or in the case of very low cost equipment at room temperature. In the latter case, the room temperature is monitored continuously and thermocouple output voltage readings are corrected for any changes in it.

Because the temperature at this end of the thermocouple wire is a reference temperature, this function is known as the reference, also called as the cold junction. A thermocouple therefore consists of a pair of dissimilar metal wires and these wires are joined together at one end (sensing or hot junction) and terminated at the other end (reference or cold junction), which is maintained at a known constant temperature (reference temperature). An emf is produced when a temperature difference exists between the sensing junction and the reference junction that finally causes current in the circuit.

Advantages of Thermocouple:

1. Thermocouples offer good reproducibility.

2. Speed of response is high compared the filled system thermometer. 3. Measurement accuracy is quite good.

4. Comparatively cheaper in cost.

5. Bridge circuits are not required for temperature measurement.

6. Calibration checks can be easily performed. 

7. It is rugged in construction.

8. It maintains a temperature range of-270°C to 2700°C.

9. Using extension leads and compensating cables, long distances transmission for temperature measurement can be made possible.

Disadvantages of Thermocouple:

1. To avoid stray electrical signal pickup, proper separation of extension leads from the thermocouple wire is required.

2. In many applications, the signals need to be ramified.

3. For accurate measurement, cold junction and other compensations are essential.

4. They exhibit non-linearity in the emf versus temperature characteristics.

5. Stray voltage pick-up is possible.

b) Digital transducer:

Digital transducers are those that convert the input into an electrical output which is in the form of "pulses."

As a binary number uses only two digits 0 and 1, it can be easily be represented by opaque and transparent areas on a glass scale Figure below or by non-conducting and conducting areas on a metal scale The complete binary number denoting position is obtained by "scanning" the pattern across the scale at a stationary index mark. The glass scales can be read by means of light sources, an optical system and the photocells. The metal scales are scanned by brushes making electrical contact with individual tracks The resolution depends upon the digits, comprising the binary number and is n/2 of full scale where n is the number of digits.

c) D.C. Tachogenerator:

 

Construction:

The construction of a D.C. tachometer is similar to a small D.C. generator. Its construction is classified as

i) stator part

ii) rotor or armature part

Stator part: As the name implies this part remains stationary. It comprises of several permanent magnet poles, which produce magnetic field.

Rotor or Armature part: A wound rotor is provided with a commutator and brushes. The output voltage is picked up from the brushes.

Rotor shaft is connected mechanically with the shaft of the rotating member whose speed is to be measured.

It generates electrical voltage whose

a) Magnitude is proportional to the magnitude of the angular velocity of the input shaft.

b) Polarity depends upon the direction of rotation of the input shaft as shown in Figure below.

Principle of operation: The operation of a DC tachometer is based on Faraday's law of

induction ie. when a conductor rotates in a magnetic field; it generates an electrical voltage across the conductor, which is proportional to the rotation of the conductor i.e. v 0 propto (d*theta)/(dt) where, v_{0} is the electrical voltage (volts) and (rad.) is the angular displacement of the conductor. So, when a conductor wound rotor, provided with a commutator and brushes, rotates in the magnetic flux due to permanent magnet poles, a D.C. voltage is produced.

In figure above (a), the terminal A is positive with respect to B when the armature rotates in clockwise direction (say). If the armature rotation is reversed then A becomes negatively polarized with respect to B (figure (b) above).