|Fig: LVDT cross-section (source: te.com )
Linear Variable Differential Transformer (LVDT) is an inductive transducer that we discussed in previous article. It is basically used to translate linear motion into electrical signals.
LVDT consists of one primary winding and two secondary windings. All these windings are wound on a cylindrical former. Secondary windings have equal number of turns and are placed either side of primary winding. The primary winding is connected to an AC supply.
There is a movable soft iron core inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. The whole assembly is having a stainless steel housing. The end lids provide electromagnetic and electrostatic shielding. The LVDT construction is shown in figure-1 as shown below.
Primary winding produces an ac magnetic field when it is excited with ac source. This ac magnetic field will induce ac voltage in the two secondary windings.
|Figure-1: LVDT Construction
Output voltage of secondary S1 is V1
Output voltage of secondary S2 is V2
The output of both the secondaries are connected in series opposition. So the differential LVDT output Vo is given by
Vo = V1 – V2
Case-1: When core is in normal position.
In this position the magnetic flux produced by primary winding linked with two secondaries coil equally (V1 – V2). Hence equal voltages are produced in both secondaries. Therefore, LVDT output is given by
Vo = V1 – V2
= V1 – V1
Case-2: When core moves towards right
In this case, more flux will links with secondary S2 and less with secondary S1. Hence voltage generated in secondary S2 is greater then S1 (V2 > V1). The magnitude of LVDT output voltage
Vo = V2 – V1
Magnitude is non-zero and is 180° out of phase with primary winding voltage.
Case-3: When core moves towards left
In this case, more flux will links with secondary S1 and less with secondary S2. Hence voltage generated in secondary S1 is greater then S2 (V1 > V2). The magnitude of LVDT output voltage
Vo = V1 – V2
Magnitude is non-zero and in phase with primary winding voltage.
|Figure-2: LVDT circuit diagram
The characteristics of LVDT is given in figure-3 below. It is a graph between LVDT AC output magnitude and core displacement.
|Figure-2: AC output of conventional LVDT versus core displacement
Practically, output voltage of LVDT is a linear function of core displacement up to a limited range of motion (say 5mm from null position). After this relationship becomes non-linear.
Available ranges: ±0.05 inches to ±25 inches
Sensitivity: measures displacements well below 0.001 inch
Temperature range: − 265℃ to + 600 ℃
Frequency range: 50 Hz to 20kHz
Applications of LVDT
LVDT used to measure force, strain, weight, tension, pressure, etc. These quantities are first converted into displacement by a primary transducer and LVDT is then used to give proportional output acting as a secondary transducer.
Advantages of LVDT
1. It has linearity up to 5mm and have high range for measurement of displacement.
2. Less friction and presence of electrical isolation.
3. Infinite resolution
4. High output- no need of amplification devices
5. High sensitivity
6. Ruggedness – can tolerate high degree of shocks
7. Low hysteresis – Repeatability is excellent
8. Low power consumption- most LVDTs consume less than 1W of power.
Disadvantages of LVDT
1. Temperature effects the operation of transducer.
2. Limited dynaic response
3. DC output needs demodulators
4. They are sensitive to stray magnetic fields.
Q. Why is the frequency of excitation of primary winding of an LVDT kept very high as compared to the frequency of the signal being detected?
The frequency of excitation of primary winding of an LVDT kept very high as compared to the frequency of the signal being detected to avoid low frequency noises.
Q. Where is LVDT used?