A superconducting six-axis accelerometer

A superconducting device that is capable of measuring acceleration in all six degrees of freedom with great sensitivity is discussed. The device makes use of the fact that a superconducting mass levitated over a set of superconducting coils by a persistent current behaves as if it were supported by spring with a very stable spring constant. The displacement of the levitated mass modulates the inductance of spiral coils in close proximity to it. By forming inductance bridges from sets of such coils and reading the bridge misbalance signal with a SQUID, it is possible to sense very small displacements and rotations of the proof mass. In the first full tests of the device, its scale factor for acceleration (angular acceleration) was found to be approximately 2×10⁵ Φ₀/g (10³Φ₀ /rad/s²) at the SQUID, in good agreement with values obtained from an analytical model. The inherent acceleration (angular acceleration) noise was found to be approximately 5×10^-10 g/√Hz (10^-7 rad/s²/√Hz) which corresponds to the expected input noise for the commercial RF SQUID used. A substantial augmentation of the scale factors is possible by increasing the area of the sensing coils and improving the resolution with which the proof mass can be positioned. Such improvements, along with the use of lower noise DC SQUIDs, should allow the inherent acceleration (angular acceleration) noise to be reduced to a value close to the Brownian noise limit, ~10^-13 g/√Hz (7×10 ^-11 rad/s²/√Hz)