Optimization of an Active Leveling Scheme for a Short-Range Gravity Experiment

Abstract

At Cal Poly Humboldt, undergraduate researchers and faculty have constructed a torsion-pendulum experiment that seeks to measure gravitational interactions below test mass separations of 100 microns. The aim of this experiment is to look for deviations in the weak equivalence principle (WEP) and inverse-square law (ISL). The scale at which this experiment operates is within an untested range at the submillimeter scale. This apparatus’s torsion pendulum consists of equal masses with differing materials arranged as a composition dipole. The twist of this configuration is measured as an attractor mass oscillates in a parallel-plate arrangement nearby. The oscillation creates a time-dependent torque on the pendulum which can be studied for deviations in the WEP and ISL. At present, an active leveling scheme has been implemented to mediate the apparatus’s long-term tilt variations. This scheme has been optimized through the use of a power supply and proportional-integral-derivative (PID) loop that mitigates the variations in tilt by applying a voltage to a resistor attached to one of the apparatus legs. The applied voltage causes thermal expansion of the apparatus leg support structure, thus correcting and modulating the tilt of this experiment.