LVDT What it Is | How It works | Working | Applications

Linear Variable Differential Transformer (LVDT)  

It is also known as an inductive transformer, is defined as a process used for measuring displacement in instrumentation systems. The performance of sensory units drives the accuracy of the overall system.

The core phenomenon of LVDT is mutual induction generated between primary and secondary windings. The mutually coupled circuit concept derives the input and output characteristics of LVDT. The output responses captured across secondary windings will be in the form of voltage metrics and are measured using the net induced voltage across the secondary terminals.

Principle of Operation and Working

As the primary is connected to an AC source so alternating current and voltages are produced in the secondary of the LVDT. The output in secondary S₁ is e₁ and in the secondary S₂ is e₂. So the differential output is,

This equation explains the principle of Operation of LVDT.

Now three cases arise according to the locations of core which explains the working of LVDT are discussed below as,

• CASE I  When the core is at null position (for no displacement)
  When the core is at null position then the flux linking with both the secondary windings is equal so the induced emf is equal in both the windings. So for no displacement the value of output e_out is zero as e₁ and e₂ both are equal. So it shows that no displacement took place.
• CASE II When the core is moved to upward of null position (For displacement to the upward of reference point)
  In the this case the flux linking with secondary winding S₁ is more as compared to flux linking with S₂. Due to this e₁ will be more as that of e₂. Due to this output voltage e_out is positive.
• CASE III When the core is moved to downward of Null position (for displacement to the downward of the reference point). In this case magnitude of e₂ will be more as that of e₁. Due to this output e_out will be negative and shows the output to downward of the reference point.

Output Vs Core Displacement A linear curve shows that output voltage varies linearly with displacement of core.

Some important points about magnitude and sign of voltage induced in LVDT

• The amount of change in voltage either negative or positive is proportional to the amount of movement of core and indicates amount of linear motion.
• By noting the output voltage increasing or decreasing the direction of motion can be determined
• The output voltage of an LVDT is linear function of core displacement .

              LVDT Experiment and Calibrations

Assumptions and Specifications 

Several abbreviations are considered to define the working procedure of LVDT and they are as follows,

E_V=Voltage across the primary winding

E_V1=EMF induced across secondary winding S_1

E_V2=EMF induced across secondary winding S_2

E_o=Output EMF

Typical LVDT Characteristics

Case 1: Null Position

schematic-diagram-of-shaft-at-null-position

Fig. 2 illustrates the working procedure of LVDT at zero or null axial position. During this condition, the shaft is precisely placed at the midway of secondary windings S1and S2, which gives rise to equal flux generation and induced voltage across the secondary terminal, respectively. This position is also known as a null position.

The differentiation amongst output magnitude and output phase sequence with respect to input signal derives core movement and displacement.

The shaft placed at the null or neutral position signifies that the voltage induced across series-connected secondary windings are equal and inversely proportional to the net output voltage.

E_V1= E_V2 ———(1)

E_o=E_V1– E_V2=0 V———(2)

Case 2: Maximum Right Position

As shown in the following figure, when the shaft is moved towards the right side, more force is generated across S2, conversely minimum across S1.

schematic-diagram-of-shaft-at-extreme-right-position

Therefore, the induced Voltage E2 is significantly higher than E1. The equation for the resultant differential voltages are as follows,

E_o=E_V2-E_V1———(3)

Case 3: Maximum Left Position

The below figure depicts that the shaft is more inclined towards the left side, which in turn generates high flux across S1 and induced a voltage across E1 while decreasing E2. The equation for the same is,

E_o=E_V1-E_V2———(4)

schematic-diagram-of-shaft-at-extreme-left-position

The resultant output of LVDT can be measured in terms of voltage, current, or frequency. This circuit can also be designed using microcontroller enabled circuits such as Arduino, PIC microcontroller, and so on.

                 LVDT Graph and Measurements

The following figures showcase the graphical representation of LVDT shaft variations and their effect in terms of the magnitude of differential AC output from a null position and DC output from electronics.

graphical-representation-of-LVDT-shaft-variations-in-terms-of-differential-output-voltage

The maximum shaft displacement value from the core position is dependent on the amplitude of the primary excitation voltage and sensitivity factor. The shaft remains at the null location until a referenced primary excitation voltage is given to the primary winding of the coil. As shown in Fig, 6, the phase shift or DC output polarity defines the shaft position for the null point. It also represents the output linearity property of the LVDT module.

graphical-representation-of-LVDT-shaft-variations-versus-DC-output-from-electronics

LVDT Types

LVDT Sensors: It is categorized on the basis of output stage voltage parameters or a relative output current; evaluate the coil frequency as a function position or in frequency-based devices.

1). Captive Armatures: These procedures are generally used to measure long working displacement ranges. Captive alignment empowers users with low friction assemblies that avoid misalignment and ensure high reliability.

2). Unguided Armatures: Infinite resolution quality enabled unguided armature provides a no-wear design and facilitates design engineers with an unlimited resolution of measured data. This module is interfaced externally with the test specimen to be measured. It is flexible, and the user needs to guide the armature without any the interrelation between the edges.

3). Force Extended Armatures: In this mechanism, external support such as pneumatic force, spring mechanism, or electrical motors to dynamically propel armature to its utmost possible level. It is usually in slow-moving applications. This procedure eradicates the connection or interface between the test specimen and armature

Advantages

LVDT advantages are discussed below.

Nowadays, with an increased requirement of measurement units, LVDT is interfaced in the main circuit or used as an external source to measure the displacement of the object. The major advantages of leveraging LVDT circuit are as follows,

1). Smoother in operation, easy to measure and interface, and wide range displacement measurement with a range of 1.25mm to 250 mm.

2). The output value is highly sensitive and can be easily measured through the available voltage measurement devices. It reduces the requirement of the amplifier to filter or increase the output band range. The sensitivity range of the typical LVDT sensor is recorded at 40V/mm.

3). Minimal hysteresis loss that in-turn increases the reliability and offers excellent operating conditions.

4). Friction loss is approximately zero or considered as negligible due to the operation of the core is carried out inside the hollow former. This process yields actual output value with a high precision range.

5). It is capable of withstanding high wear and tear functionalities, especially during scenarios where the core is loaded with a spring or the system is under rugged operation.

6). LVDT is operated at a minimal power consumption of a range of 1w.

7). The output is obtained in terms of an electrical signal. Since most of the system input is dependent on the electrical signal, the output can be directly fed to other circuits, which reduces the requirement of other conversational elements.

8). The absence of friction enables faster dynamic response and high-core movement capability.

Disadvantages or Limitations

The limitations of LVDT are discussed below

1). Since LVDT works on the principle of the inductive transducer, a stray magnetic field is generated around the circuit. There is a requirement of an additional circuit to overcome the stray magnetic field.

2). Vibrations and temperature variations inside the electromagnetic device further inhibit the performance of the system.

Applications

The applications of LVDT include the following

1). LVDT sensors are majorly used in a myriad range of industries to measure the tension of spring, weight, displacement, and pressure, to name a few. The input factors achieved in the form of physical parameters are initially converted into displacement, followed by a corresponding electrical voltage signal.

2). It is deployed in industries to extract positive feedback from servomechanism.

3). It is used in machinery measurement tools, Aircraft industry, hydraulics, Satellite, and Industrial
Automation.

4). LVDT signal conditioner/conditioning is used to monitor and control the output waveform of the circuit.

5). The typical applications of LVDT sensors are as follows,

• Testing the strength of soil: The core of the material is engineered, softened, and manufactured using high permeability iron-nickel based alloy. Extension rod material is designed using nonmagnetic stainless steel. The voltage variation is observed during the external rod movement inside the material, which in turn generates an output pulse corresponding to displacement.
• Medical Field (Pill-making Machine): A hybrid operating mechanism of variable pitch secondary windings with a computer-controlled winding machine reduces the overall package length to stroke ratio, minimizes human error in measuring pill weight and thickness, and provides high accuracy in determining the eventual weight of medicinal powder.
• Automated Product Inspection Machine: Flat-panel displays eventually replaced the present PC and laptop monitors with high definition display systems. Small-package LVDT’s are utilized to undergo quality tests and a final check of flat-panel display dimensions.
• Aerospace: It is used to monitor flight controls, pilot control, and wheel steering mechanism.
• Robotic Cleaner: LVDT is used in leak detection systems for continuous monitoring of the fluid level. Especially systems that are submerged in nonconductive and noncorrosive fluids at ambient pressure conditions.
• Robotic Manipulator: It is used as a core part of joystick control based heavy equipment robotics.

With many such applications, Linear Variable Differential Transformer is driving the futuristic displacement and measurement units in numerous business spaces. The emerging-market sectors, such as power generation, water management, and structural safety, will be likely to implement LVDT to enhance the performance and operating principles of the overall system. Furthermore, disruption of power electronic modules enables easy calibration process in LVDT and boosts the production of distance measuring instruments such as magnetostrictive transducers.