High-resolution position and displacement sensors

High-resolution position and displacement sensors are transducers dedicated to precision measurements of displacement, low-frequency vibration, positioning and deformation, with resolutions reaching the nanometer in the most advanced systems. The technologies used include capacitive, inductive (eddy current), magnetostrictive, interferometric and confocal optical sensors. They are used in laboratory metrology, in precision machinery, in semiconductors, optics, microscopy and materials R&D, wherever measuring sensitivity beyond the reach of standard industrial sensors is required.

Capacitive sensors

Capacitive sensors measure the change in capacitance between a sensor electrode and the conductive target. They offer nanometer resolution, bandwidth up to 100 kHz and a typical range of 50-500 μm. They are used for non-contact measurements of the highest precision on conductive surfaces, for example in high-precision tooling, semiconductors and metrological references. They require a clean environment, being sensitive to dust, humidity and contamination.

Eddy current sensors

Eddy current sensors generate a high-frequency magnetic field with a coil; the field induces eddy currents in the metal target, which oppose the original field and so change the impedance of the coil. Measuring that impedance gives the distance. Their advantages: they are insensitive to contamination (oil, water, dust) and work in extreme environments (high temperatures, vacuum), with a typical range of 0.4-80 mm and resolutions of 0.01-0.1 μm.

Other technologies

Magnetostrictive sensors exploit the Wiedemann effect on a ferromagnetic rod: a current pulse generates a mechanical wave which, intercepted by a movable magnet, produces an absolute position signal. Range extends to several meters, with repeatability in the μm. Interferometric sensors use optical interference for sub-nanometer measurements over limited travel. Chromatic confocal sensors exploit chromatic dispersion to measure the distance and thickness of transparent materials.

Applications

Typical applications include precision control on tooling and spindles, high-precision run-out measurements, monitoring of low-frequency vibration in machinery, dimensional measurements of micromechanical components, R&D in optics and photonics, nanotechnology processes, the calibration of other instruments, predictive maintenance of bearings and measurements on semiconductor components. Environmental requirements (controlled temperature, EMI shielding, low vibration) are often demanding.

Selection criteria

The choice of technology depends on the measuring range, on the resolution required, on the bandwidth needed, on the nature of the target (conductive, insulating, ferromagnetic, transparent), on the working environment (temperature, contamination, vacuum) and on the availability of a clean, parallel surface. For nanometrology applications, technical consultancy is generally recommended, to ensure that the sensor is correctly integrated and that the declared specifications are met across the whole measuring chain.

Integrating these sensors into high-speed digital measuring chains enables in-line metrology, dynamic dimensional control and advanced diagnostics, where the combination of high resolution and wide bandwidth makes it possible to detect transient phenomena normally invisible to conventional instruments.

The semiconductor, microelectronics and nanotechnology industries make extensive use of these sensors to qualify components, to control deposition processes and to verify dimensional tolerances in MEMS devices, where dimensional specifications reach levels that call for qualification instruments of the very highest performance.

Calibrating these sensors requires controlled environments (rooms with stabilized temperature, isolation from vibration, electromagnetic shielding) and the use of primary references such as metrology laser interferometers, certified according to ISO 17025 calibration procedures by laboratories accredited for high-precision dimensional metrology.