A vacuum-compatible cylindrical inertial rotation sensor with picoradian sensitivity.

We describe an inertial rotation sensor with a 30-cm cylindrical proof-mass suspended from a pair of 14 µm thick BeCu flexures. The angle between the proof-mass and support structure is measured with a pair of homodyne interferometers, which achieve a noise level of ∼5prad/Hz. The sensor is entirely made of vacuum compatible materials, and the center of mass can be adjusted remotely.

An air suspension to demonstrate the properties of torsion balances with fibers of zero length.

We report on the design and characterization of an air-bearing suspension that has been constructed to highlight the properties of torsion balances with fibers of zero length. A float is levitated on this suspension, and its rotational and translational motion in the horizontal plane of the laboratory is controlled using magnetic actuators. We demonstrate the in situ electromagnetic tuning of the float's center-of-buoyancy to an accuracy of ±0.3 mm, which was limited by the noise in the air bearing. The rotational stiffness of the float, which is approximately zero by design, was also measured. We compare the observed behavior of the float with the predictions of a detailed model of the statics of the float-actuator system. Finally, we briefly discuss the application of these ideas and results to the construction of sensitive devices for the measurement of weak forces with short ranges.

A method for reducing the adverse effects of stray-capacitance on capacitive sensor circuits.

We examine the increase in voltage noise in capacitive sensor circuits due to the stray-capacitance introduced by connecting cables. We have measured and modelled the voltage noise of various standard circuits, and we compare their performance against a benchmark without stray-capacitance that is optimised to have a high signal-to-noise ratio for our application. We show that a factor limiting sensitivity is the so-called noise gain, which is not easily avoided. In our application, the capacitive sensor is located in a metallic vessel and is therefore shielded to some extent from ambient noise at radio frequencies. It is therefore possible to compromise the shielding of the coaxial connecting cable by effectively electrically floating it. With a cable stray-capacitance of 1.8 nF and at a modulation frequency of 100 kHz, our circuit has an output voltage noise a factor of 3 larger than the benchmark.