- Inductive Transducer Definition: An inductive transducer is defined as a device that measures variations in a quantity through changes in inductance.
- Operation Principles: The operation of inductive transducers involves three main principles: changes in self-inductance, mutual inductance, and the production of eddy currents.
- Measurement Calibration: The calibration of inductive transducers enables accurate measurement of quantities such as displacement.
- Applications: Inductive transducers are extensively used in proximity sensors for accurate position and motion detection.
- Practical Usage: These transducers are critical in industrial applications for metal detection, ensuring parts presence, and counting items.
Inductive transducers convert a physical quantity into a measurable change in coil impedance or magnetic coupling. Motion may change inductance, reluctance, coupling or eddy-current loss. An LVDT, for example, converts linear core displacement into the differential AC voltage of two secondary windings.
In an LVDT, AC excitation of the primary creates magnetic flux. A movable permeable core changes the mutual coupling from the primary to each secondary. At the centre null position, the series-opposed secondary voltages are nearly equal and cancel. Displacement increases one secondary voltage and decreases the other; output magnitude represents distance from null and phase represents direction. This illustrates the principle of inductive transducer based on mutual inductance.
The measurand must alter a defined magnetic or electrical parameter, and the readout circuit must excite, detect and calibrate that change. Three common operating principles are:
- Change of self inductance
- Change of mutual inductance
- Production of eddy current
Each principle uses a different sensor geometry and signal-conditioning method.
Change of Self Inductance of Inductive Transducer
For an ideal coil on a simple magnetic circuit, self-inductance is
Where,
N = number of turns.
R = reluctance of the magnetic circuit.
For a uniform ideal magnetic path, reluctance R is
Here μ is effective permeability. Real sensors also have air gaps, leakage flux, fringing, core loss and frequency-dependent effects.
Where,
G = A/l, the ideal geometric form factor.
A = magnetic cross-sectional area.
l = effective magnetic path length.
The self-inductance relation can change through:
- The winding turn count N, fixed during normal operation,
- Geometry G, including an air gap or movable core position,
- Effective permeability, changed by core insertion, material or magnetic conditions.
A variable-reluctance displacement sensor couples motion to geometry or permeability so that self inductance changes predictably. Calibration is valid only for the specified excitation, range, material, temperature and mechanical alignment.
Change of Mutual Inductance of Inductive Transducer
Mutual-inductance transducers use at least two coupled windings. For two coils, denote their self-inductances by L1 and L2.
Mutual inductance between the two coils is
Mutual inductance depends on both coil self-inductances and coupling coefficient K. Relative distance, orientation, air gap and core position can change K. A displacement sensor therefore fixes one magnetic structure and moves another part with the measured object. AC excitation produces an output whose magnitude and phase are demodulated and calibrated against position. The usable relation is linear only over the sensor’s specified range.
Production of Eddy Current of Inductive Transducer
An AC-driven coil placed near a conductive target induces circulating currents called eddy currents. Their magnetic field opposes the coil’s changing field and changes the coil impedance and oscillator loss. As the target approaches, coupling and eddy-current loss generally increase, changing oscillator amplitude or phase. The response also depends on target conductivity, magnetic permeability, thickness, shape, excitation frequency and temperature. This is the basis of inductive transducers for conductive targets. Signal conditioning converts the measured current or impedance change into distance. The inductance component may change, but resistive loss is also part of the response. Calibration must use the intended target material and geometry.
Real Life Application of Inductive Transducer
Inductive sensors are used for non-contact metal presence detection, analogue displacement measurement, machine position feedback and dynamic motion measurement. A switching inductive transducer can confirm that a metal part is present or count passing parts. An analogue sensor can estimate distance within a calibrated range. Target material, installation clearance, nearby metal and environmental limits affect accuracy.
