1. What is an LVDT?
An LVDT is an electromechanical sensor that converts an object's rectilinear motion into a corresponding electrical signal. It is mechanically connected to the object to be monitored.
It is the most widely used transducer that convert the linear motion into electrical signal(voltage). It is also known as linear variable displacement sensor.
2. Construction of LVDT
The transformer and LVDT have almost same construction. It consists of one primary winding and two secondary windings. The primary winding of the transformer lies in the middle of two secondary windings that are symmetrically placed around the primary and are both similarly wrapped.
Both secondary windings have an equal number of terms and are connected in series opposition, which means they are coiled in opposite directions but connected in series.
The coils are wound on a single-piece hollow form made of thermally stable glass reinforced polymer, sealed against moisture, covered in a magnetic shield with a high permeability, and then fixed inside a cylinder-shaped housing made of stainless steel.
Typically, LVDT sensor's stationary component is this coil arrangement and The moving part of the LVDT is a magnetic arm.
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| Typical LVDT Coil Construction |
2.1 Structure of an LVDT Coil
An LVDT coil assembly typically consists of three coils wound around a cylindrical, non-magnetic core:
- Primary Coil (P): This coil is positioned in the center and is powered by an AC source. It generates an alternating magnetic field.
- Secondary Coils (S1 and S2): Two identical secondary coils are symmetrically positioned on either side of the primary coil, each connected in a way that they generate voltages in opposite phases.
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| Source: electronics-tutorials.ws |
3. How does an LVDT work?
The LVDT operates according to Faraday's law of electromagnetic induction, which states that "the electrical power in the network induction circuit is proportional to the rate of change of magnetic flux in the circuit."
In operation, the LVDT’s primary winding is energized by an alternating current of appropriate amplitude and frequency, known as the primary excitation. Which produces the induced EMF on the secondary windings.
The LVDT’s electrical output signal is the differential AC voltage between two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.
3.1 Working Principle of LVDT Coils
The mutual inductance concept governs the LVDT's operation:
- AC Excitation: The primary coil receives an AC signal, creating a magnetic field.
- Induced Voltages in Secondaries: This magnetic field induces voltages in both secondary coils. The magnitude of the induced voltage in each secondary coil depends on the position of the magnetic core.
- Differential Output: When the core is precisely centered, the voltage in both secondary coils is equal and cancels out, resulting in zero output voltage. When the core moves to one side, the balance is disrupted. Therefore, the movement of the core and, consequently, the displacement of the object are represented by the differential voltage between the two secondary.
The relationship between the output voltage and the displacement of the core is highly linear within a specific range, making LVDTs ideal for precise and repeatable measurements.
4. Types of LVDT.
LVDTs, or linear variable differential transformers, come in a variety of forms according to their size, design, and function.
4.1 LVDT types based on armature
- Unguided armature
- Guided armature
- Spring Extended armature
4.2 LVDT types based on applications
- General purpose LVDT
- Precision LVDT
- Submersible LVDT
4.3 LVDT Types based on operation range
- Short storked LVDT
- Long storked LVDT
4.4 LVDT Types based on Excitation
- DC LVDT: This kind of LVDT uses direct current, or DC, as its input. These LVDT types are utilized in specific situations where it is more applicable to use a DC power supply.
- AC LVDT: It is the most popular and widely used kind of LVDT, it uses AC as its input and functions on the electromagnetic induction (EMI) principle. For measuring displacement in a variety of industrial applications, AC LVDTs are commonly employed.
- Miniature LVDT: As the name indicates, this kind of LVDT is tiny, with a short stroke length that although provides excellent precision. Subminiature LVDTs are ideal for applications with limited space because they are even smaller in size.
5. LVDT Applications.
An LVDT sensor is available in a variety of measurement ranges, may measure movements as little as a few millionths of an inch to as large as 20 inches.
These Sensors are also widely used for condition monitoring systems as a part of control and safety functions for applications in offshore platforms, desalination systems, mooring cables, seafloor wellheads, and oil and gas gathering systems.
Fuel Control: A LVDT helps to ensure that fuel is used as efficiently as possible according to fuel requirements. Later, it guarantees that the gasoline is delivered precisely when it is needed, giving you the best performance to fuel ratio.
Engine Bleed Air Systems: LVDT is used to assist in controlling the volume of air bleed into the bleed air systems of engines. Every gas turbine bleed valve must be facilitated with LVDT.
Thrust Vector Control: When it comes to thrust vector control, the LVDT effectively places the vector at the base of the missile or rocket. This allows you to vary the engine's thrust direction in order to control the rocket's angular velocity or the aircraft's altitude.
Valve Pitch: Actuators can be used to ensure that the valve actuator moved into the proper position and performed according to expectation.
Defense Satellites & Launch Systems: Defense satellites and launch systems are some of the most typical applications for LVDTs in space.
Valve position feedback: Linear displacement transducers give valve position data to help regulate the actuator as well as monitor and maintain the valve position.
6. Common issues with LVDTs
Linear Variable Differential Transformers (LVDTs) are highly reliable and accurate sensors, but they can face certain issues in specific applications.
Electromagnetic interference: One common problem is signal distortion caused by electromagnetic interference (EMI), especially in environments with high electrical noise.
Misalignment: improper alignment or mounting can lead to measurement errors, as LVDTs are sensitive to mechanical misalignment.
Ambient temperature: Temperature variations can also affect the core and coil materials, leading to drift in the output signal.
Wear & tear: Additionally, wear and tear in the core or supporting structures over time can degrade performance.
Electronic circuit: Faulty signal conditioning circuits or loose connections in wiring can introduce inaccuracies or even cause sensor failure.
Calibration: Regular calibration and maintenance, along with proper shielding and alignment, are essential to ensure optimal LVDT performance. Without proper calibration, it is not possible to get an accurate result.
All these are very common issues with LVDTs
7. How to Use an LVDT Sensor?
- Using an LVDT sensor effectively requires proper installation, calibration, and connection to a signal conditioning system.
- Begin by securely mounting the LVDT so that its core can move freely without friction or misalignment.
- Connect the primary coil to an appropriate AC excitation source, typically within the frequency and voltage range specified by the manufacturer.
- Ensure the secondary coils are connected to a signal conditioning unit that can demodulate the output signal and convert it into a readable DC voltage or digital format.
- Calibrate the sensor by setting reference positions to define the zero and full-scale points, ensuring accurate displacement measurements.
- Maintain consistent environmental conditions, or account for temperature variations if the application involves extreme conditions.
- Finally, regularly monitor the sensor for wear, misalignment, or electrical interference to ensure reliable and precise performance.
