Displacement Measurement

What is LVDT and why is it used

An LVDT is an instrument that can be used to measure the linear displacement, LVDT stands for linear variable differential transformer basically it can be used to do the displacement measurement. This device is basically a transducer that can convert the linear motion into an electrical signal. An LVDT is an inductive transducer and it is very accurate too. It got its name because the output across the secondary of the transformer is differential. The operation of the LVDT is according to the principle of mutual induction. LVDT will create a linear electrical output which will be proportional to the position of the movable core.

What is linear displacement measurement

The movement that takes place in one direction along a single axis is called linear displacement. A linear displacement measuring device can determine the distance traveled by an object from a reference point, this measurement also shows the direction of motion.

What are the features of LVDT

  • No wear and friction because there is no contact between the parts
  • Damage resistant design
  • Robust to most of the environmental conditions
  • Null point repeatability is good
  • Absolute output
  • Limited frequency response

How LVDT’s are constructed

The major part of an LVDT is a transformer and a probe, this transformer has a primary winding and two secondary windings and this would be wounded to a cylinder that contains the core. The two secondary windings have the same number of turns and they are placed symmetrically to the primary winding. Ac source is given through primary windings. The windings are wound on a hollow cylindrical bobbin which is an insulating material and non-magnetic. The iron core will slide inside the hollow former and the core is made up of nickel-iron alloy.

How does an LVDT work? What is the LVDT principle

This device will produce an AC output voltage which is proportional to the relative displacement of the transformer and the armature. A voltage will be induced in the primary coil of the LVDT when an alternating current is applied to it and voltages will be induced in the secondary coil too. There is a magnetic flux path that connects the primary and secondary coils and it is achieved by the magnetic core inside the coil winding assembly. The two voltages that are created are of opposite polarity, the secondary coils are placed in a way that it will oppose the center. The output voltage will be zero because the voltages are equal and opposite in polarity. If the magnetic position is displaced from the null position then there will be an electromagnetic imbalance and because of this imbalance a differential AC output voltage is generated across the secondary coils and it would be linearly proportional to the direction and magnitude of the displacement.

How does a DC LVDT work

By using DC input we can operate an LVDT, mostly the operating principle is the same. The DC input is converted to AC input by conditioning hardware and this can be seen in the instrument case.

Why use an LVDT

  • LVDT will provide friction-free operation, the coil and core of an LVDT is not in contact
  • This device has infinite resolution, slight changes in core position can be determined by this
  • This device has less friction and because of this the mechanical life of this device is very high
  • Because of its construction material and features, the LVDT is not affected by most of the environmental conditions

What are the advantages of LVDT

  • Accurate measurement and infinite resolution
  • High output
  • High sensitivity
  • Very good linearity
  • Ruggedness
  • Friction
  • Low hysteresis
  • Power consumption is very low
  • Direct conversion to electrical signals
  • Simple, lightweight, and can be easily maintained

What are the disadvantages of LVDT

  • LVDT’s are affected by stray magnetic fields, so it would need an extra setup to prevent this
  • Temperature and vibrations would badly affect the LVDT
  • Requires large displacement for small output
  • The dynamic response is limited
  • LVDT drives are affected by amplitude variations

What are the applications of LVDT

  • Automation machinery
  • Civil engineering
  • Power generation
  • Metal stamping
  • Aeronautics
  • It can be used to do the measurement ranging from a fraction of millimeter to centimeter
  • This device acts as a secondary transducer so it can be used to measure the force, weight, and pressure.
  • Robotics
  • Computerized manufacturing
  • Die casting machinery

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