Noise Analysis and In Situ Detection of the Electromagnetic Servo Actuator for a Nano-g Accelerometer With a Large Dynamic Range

Accelerometers are required to discern minute gravity-related variations when simultaneously measuring the large disturbing acceleration of the moving platforms in airborne or shipborne gravity gradient measurement. Closed-loop accelerometers with an electromagnetic servo actuator are commonly used to realize such a large dynamic range. However, the noise floor could deteriorate at high inputs and few studies have addressed this issue. This article investigates the noise origins of the electromagnetic actuator under large acceleration excitation using an in situ measurement scheme. The actuation unit is intentionally disconnected from the loop and an independent dc or ac current is fed into the integrated actuator coil to provide a drive force onto the proof mass (PM). The capacitive displacement detection unit independently operates to in situ monitor any induced positional variations of the PM. In order to identify potential actuation noise sources, actuator coils of two identical accelerometers are electrically connected in various configurations and stepwise tested. The common-mode disturbances such as electronic noise sources and ambient vibrations are canceled out by taking the displacement difference of the two PMs, whereas the intrinsic noises in each actuator would superimpose on each other. As a result, we implemented an accelerometer with a noise floor of sub ng/
$\sqrt {\mathrm{ Hz}}$
at any constant input within ±20 mg. On the other hand, our findings indicate that the noise floor deteriorates into a few ng/
$\sqrt {\mathrm{ Hz}}$
at a time-varying input and the deterioration results from the Barkhausen effect of the magnetic material in the actuator.