What Are Inductive Transducers?

Inductive transducers are sensors that convert a physical quantity, most commonly displacement, position, or proximity, into a measurable change in electrical inductance or mutual inductance. They operate through electromagnetic induction: a coil energized with alternating current produces a magnetic field, and any change in the geometry of that field, whether from a moving core, an approaching conductive surface, or a shifting coupling between coils, alters the coil's inductance in a predictable way. The resulting change in impedance or induced voltage provides an electrical signal that is proportional to the measured quantity. Inductive transducers draw on the same transformer and coil principles that underpin AC power engineering, applied here to precision sensing rather than energy transfer.

The family spans a wide range of operating principles and accuracy levels. Self-inductance transducers measure changes in a single coil's inductance; mutual inductance transducers measure changes in the coupling between two or more coils; and eddy-current transducers detect the effect of induced currents in a nearby conductive target on the driving coil. All three types share the advantage of non-contact operation, which means no wear, no lubrication requirements, and long service life in industrial environments.

Linear Variable Differential Transformers

The linear variable differential transformer, or LVDT, is the most widely used inductive transducer for continuous displacement measurement. It consists of one primary coil and two secondary coils wound symmetrically around a movable ferromagnetic core. When the primary is energized with AC, it couples energy to both secondaries; the differential output voltage, the difference between the two secondary voltages, is zero when the core sits at the electrical null point and varies linearly with core displacement in either direction. An MDPI review of inductive position and speed sensors documents that LVDTs can achieve repeatability of around 150 nm and linearity errors below 0.15% of full scale, making them standard references for calibration and machine tool feedback where reliable, high-resolution displacement data are needed. The absence of electronic components inside the sensing element allows LVDTs to operate across temperature extremes and survive mechanical overload.

Eddy-Current Sensors

Eddy-current sensors work by driving a coil at a frequency between a few kilohertz and several megahertz and measuring how the coil's impedance changes when a conductive target is brought within range. The alternating magnetic field induces circulating eddy currents on the target surface; those currents oppose the driving field and reduce the effective inductance and Q-factor of the coil by an amount that depends on target distance and material properties. The PMC review article on inductive position and speed sensors describes how eddy-current sensors achieve sub-micrometer resolution, with high-performance models reaching 10 nm resolution and 0.2% accuracy, and notes their particular suitability for measuring vibration and runout on rotating shafts in turbomachinery. Unlike LVDTs, eddy-current sensors require no mechanical coupling to the target and work through non-conductive barriers, which makes them useful for measuring inside sealed housings.

Inductive Proximity Sensors

Inductive proximity sensors are simplified inductive transducers that produce a binary output: the sensor detects the presence or absence of a metal object within a defined sensing distance, typically a few millimeters. A target approaching the face of the sensor disturbs the oscillating magnetic field generated by the internal coil, loading the oscillator and triggering a switch. The EDN overview of inductive proximity, displacement, and eddy-current sensors distinguishes proximity switches from continuous-output displacement transducers and explains how the same underlying eddy-current effect is scaled differently for detection and measurement tasks.

Applications

Inductive transducers have applications in a wide range of fields, including:

• Machine tool and CNC positioning feedback using LVDTs
• Turbine shaft vibration and rotor runout monitoring
• Automated assembly line part detection with proximity switches
• Aerospace landing gear and actuator displacement measurement
• Automotive engine and suspension component position sensing