Experimental characterisation of a combined LVDT position sensor and voice-coil actuator for gravitational wave detectors

TL;DR

This paper presents a detailed experimental and simulation-based characterization of a combined LVDT position sensor and voice-coil actuator, demonstrating high accuracy and linearity for seismic isolation in gravitational wave detectors.

Contribution

It introduces a validated measurement and modeling framework for a combined sensor-actuator device, enabling optimization and development of next-generation gravitational wave detector components.

Findings

LVDT response measured with 0.5% uncertainty

Force measurements matched simulations within 0.6%

High linearity over a +/- 5 mm range

Abstract

A detailed characterisation of a combined Linear Variable Differential Transformer (LVDT) position sensor and voice-coil (VC) actuator designed for seismic isolation systems in gravitational wave detectors is presented. A dedicated experimental setup and a FEMM-based finite-element simulation framework were developed to measure and model a representative ETpathfinder Type-A LVDT+VC assembly. The setup employs a precision translation stage and balance to quantify LVDT displacement response and VC force output under controlled conditions. We found good agreement between experiment and simulation: the measured LVDT response was determined with an uncertainty of 0.5% and differed by only 1.3% from the FEMM model prediction, demonstrating high linearity over a +/- 5 mm range. In addition, the VC force measurements agreed within the total uncertainty: the maximum normalised force was determined with a precision of 2.3% and matched the simulated value with only a 0.6% discrepancy. These results validate the combined sensor-actuator design and our measurement methodology. The demonstrated linear response and stable actuation confirm that this LVDT+VC device can be used for low-frequency suspension control. Our framework therefore provides a validated tool to optimise existing sensor and actuator designs and to study novel prototypes for next-generation gravitational wave detectors.