Linear Variable Differential Transformer(LVDT)

Introduction:

It is a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. A LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±30 inches (±0.762 meter). Hence, a LVDT is a linear position sensor with a proportional analog output.

Construction:

LVDT is a transformer consisting of one primary winding P and two secondary winding S1 & S2 mounted on a cylindrical former. The two secondary winding have equal number of turns and placed identically on either side of the primary winding.

Construction of LVDT

A movable soft iron core is placed inside the former. Actually the movable core is made of nickel iron with hydrogen annealed. Hydrogen annealing is done to eliminate harmonics, residual voltage of core and thus provides high sensitivity. The movable core also is laminated in order to reduce eddy current loss. The assembly of laminated core is placed in a cylindrical steel housing and end lids are provided for electromagnetic and electrostatic shielding. The displacement to be measured is attached to this movable soft iron core.

Working of LVDT:

Since the primary winding of Linear Variable Differential Transformer (LVDT) is supplied with AC supply, it produces an alternating magnetic flux in the core which in turn link with the secondary winding S1 and S2 to produce emf due to transformer action.

Electrical Equivalent circuit of LVDT

Let us assume that the emf produced in secondary winding S1 is Es1 and that in S2 is Es2. The magnitude of Es1 and Es2 will depend upon the magnitude of rate of change of flux (dØ / dt) as per the Faraday’s Law. The lower the value of ‘dt’, the more will be the emf induced.
But lower value of ‘dt’ means that core is moving faster. Thus LVDT- Working Principle: we can say that the more is the movement of core, the greater will be the magnitude of emf induced in secondary windings.

To get a single output voltage from the Linear Variable Differential Transformer (LVDT), both the secondary winding are connected in series but in phase opposition. Due to this connection, the net output voltage E0 of the LVDT is given as below.
E0 = Es1 – Es2
Since the secondary windings of LVDT are identical and placed symmetrically on either side of core, therefore under normal position the flux linkage of both the secondary winding S1 & S2 will be same.
This means Es1 = Es2 and hence net output voltage E0 of LVDT = 0. This position of soft iron core is called NULL position. Thus NULL position of Linear Variable Differential Transformer is the normal position of movable core where the net output voltage is zero. Now, as the core can either be moved toward right or left to the null position. Let us now consider such movement of core under two cases.

secondary winding are connected in series but in phase opposition

Case-1: Core is moved left to the NULL position
When core of LVDT is moved to the left of the NULL position ‘O’ as shown in figure above, the flux linkage of secondary winding S1 will become more than that of winding S2. This means the emf induced in winding S1 will be more than S . Hence E > S2. Es1 Es2 and net output voltage,
E0 = (Es1 – Es2) = Positive.
This means that the output voltage E0 will be in phase with the primary voltage.

Case-2: Core is moved right to the NULL position
When the core of LVDT is moved toward right of NULL position the emf induced in secondary winding S2 will be more than that of S1. This means Es2 > Es1 and hence net output voltage,
E0 = (Es1 – Es2) = negative.
This means that the output voltage of LVDT will be in phase opposition (180 degree out of phase) with the primary voltage.

Applications of LVDT:

There are various applications of LVDT:

  1. LVDT is used in those applications where displacement ranging from fraction of a mm to few
    cm. As a primary transducer, it converts the mechanical displacement into electrical signal.
  2. The LVDT sensor works as the main transducer, and that changes dislocation to electrical signalstraight.
  3. This transducer can also work as a secondary transducer.
  4. LVDT is used to measure the weight, force and also pressure.
  5. Some of these transducers are used to calculate the pressure and load.
  6. LVDT’s are mostly used in industries as well as servomechanisms.
  7. Other applications like power turbines, hydraulics, automation, aircraft, and satellites.