Linear Variable Differential Transformer

Linear Variable Differential Transformer

LVDT is a passive inductive transducer and is commonly employed to measure force or weight, pressure and acceleration etc. Which depend on force in terms of the amount and the direction of displacement of an object.

LVDT

Construction

-It consist of one primary p and two secondary windings S1 and S2 which are placed on either side of the primary mounted on the same magnetic core. The magnetic core is free to move axially inside the coil assembly and the motion being measured is mechanically coupled to it.

-The two secondaries S1 and S2 have equal number of turns but are connected in series opposition so that e.m.f E1 and E2 induced in them are 180 degrees out of phase with each other and, hence cancel each other out.

-The primary is energised from a suitable A.C. source.

Lvdt

Working

-when the core is in the centre the induced voltage E1 and E2 are equal and opposite. Hence they cancel out and the output voltage Vo is zero.

-When the external applied force moves the core towards coil S2, E2 is increased but E1 is decreased in magnitude though they are still antiphase with each other. The net voltage available is E2-E1 and is in phase with E2.

Thus from above discussion, we find that the magnitude of Vo is a function of the distance moved by the core and its polarity or phase indicates as to in which direction it has moved. If core is attached to a moving object, the magnitude of Vo gives the position of that object.

Characteristics of a typical LVDT

Size of lvdt- Diameter-10mm and length-15mm.

Size of the core-Diameter 2.5mm and length 10mm. 

Primary winding resistance-120 ohm.

Primary winding inductance- 11.5 mH at null position.

Maximum input voltage sensitivity- 6 volt.

Linearity range- greater than 5% of full scale output voltage +-2.5mm.

Advantage

1. It gives a high output and therefore many a times there is no need for intermediate amplification devices.
2. The transducer possesses a high sensitivity as high as 40 V/mm.
3. It shows a low hysteresis and hence repeatability is excellent under all conditions.
4. Most of the LVDT consume a power of less than 1w.
5. Less friction and less noise due to absence of sliding contacts.
6. These transduces can usually tolerate a high degree of shock and vibration without any adverse effects.
7. It can operate over a temperature range from -265 degree celsius to 600 degree celsius.
8. It is available in radiation-resistance design for operation in nuclear reactors. 

Disadvantages

1. These transducer are sensitive to stray magnetic fields bu shielding is possible. This is done by providing magnetic shield with longitudinal slots.
2. Relatively large displacement are required for appreciable differential output.
3. The receiving instrument must be selected to operate on A.C. signals or demodulator network must be used if a D.C. output is required.
4. Several times, the transducer performance is affected by vibrations.
5. The dynamic response is limited mechanically by the mass of the core and electrically by the frequency of applied voltage. The frequency of the carrier should be ten times the highest frequency component to be measured.

Applications

• Measurement of material thickness in hot or slab steel mills.
• In accelerometer
• Jet engine controls in close proximity or exhaust.
• LVDT is not suited for fact dynamic measurements on account of mass of the core. 

Capacitive Transducer 

Operates on the principle of variation in capacitance produced by the physical quantity being measured.

capacitive transducer

Advantages

The major advantages of capacitive transducers are.

1. Require extremely small force for operation hence very useful for use in small system.
2. Extremely sensitive.
3. Require small power for operation.
4. High output impedance...therefore, loading effects are minimum.
5. Frequency response is good.
6. A resolution of the order 2.5 multiplying 10 to the power -3mm can be obtained.
7. Can be used for applications where stray magnetic fields render the inductive transducers.

Disadvantages- The principle disadvantages of capacitive transducers are.

1. The metallic parts must be insulated from each other. The frames must be earthed reduce the effects of stray capacitances.
2. They show non-linear behaviour several times on account of edge, huard rings must be used to eliminate this effect.
3. The cable connecting the transducer to the measuring points is also a source of error. The cable may be source of loading resulting in loss of sensitivity. Also loading makes the low frequency response poor.

Use-The capacitive transducers are used for the following purpose.

1. To measure both linear and angular displacement.
2. To measure force and pressure.
3. Used as a pressure transducers in all those cases where the dielectric constant of a medium changes with pressure.
4. To measure humidity in gases.
5. Used in conjunction with mechanically modifiers for measurement of volume,density,weight,liquid level etc.

Piezoelectric Transducers

Piezoelectric materials- A piezoelectric material is one in which an electric potential appears across certain surface of a crystal if the dimensions of the crystals are changed by the application of a mechanical force. This potential is produced by the displacement of external charges. The effect is reversible conversely, if a varying potential is applied to the properx axis of the crystal, it will change the dimension of crystal thereby deforming it.

piezoelectric

 This effect is known as piezoelectric effect. Elements exhibiting piezoelectric quantities are sometimes known as electro-resistive elements. Common piezoelectric materials are , ammonium dihydrogen phosphate, Rochelle salts, lithium sulphate, dipotassium tartrate, potassium dihydrogen phosphate, quartz and ceramics A and B. 

There are two main groups of piezoelectric crystal

1. Natural crystals- such as quartz and tourmaline.

2. Synthetic crystals- such as Rochelle salt, lithium sulphate, dipotassium tartrate etc.

Working

A crystal mode of operation of a piezoelectric devices employed for measuring varying force applied to simple plate is shown in figure above. The magnitude and polarity of the induced charge on the crystal surface is proportional to the magnitude and direction of the applied force. The charge at the electrode gives rise to voltage E. 

Advantages

1. High frequency response.
2. small size.
3. High output.
4. Rugged construction,
5. Negligible phase shift.

Disadvantages

1. Output affected by changes in temperature.
2. can not measure static conditions.

Applications

• Accelerometer.
• Pressure cells.
• Force cells.
• Ceranic microphones.
• photograph pick-up.
• Cartridges.
• Industrial cleansing apparatus.
• Under-water detection system.