Abstract
A method for automatic compensation of misalignment angles during matching the scale factors of two pairs of the accelerometers in developing the rotating accelerometer gravity gradient instrument (GGI) is proposed and demonstrated in this paper. The purpose of automatic scale factor matching of the four accelerometers in GGI is to suppress the common mode acceleration of the moving-based platforms. However, taking the full model equation of the accelerometer into consideration, the other two orthogonal axes which is the pendulous axis and the output axis, will also sense the common mode acceleration and reduce the suppression performance. The coefficients from the two axes to the output are δO and δP respectively, called the misalignment angles. The angle δO, coupling with the acceleration along the pendulous axis perpendicular to the rotational plane, will not be modulated by the rotation and gives little contribution to the scale factors matching. On the other hand, because of coupling with the acceleration along the centripetal direction in the rotating plane, the angle δP would produce a component with 90 degrees phase delay relative to the scale factor component. Hence, the δP component coincides exactly with the sensitive direction of the orthogonal accelerometers. To improve the common mode acceleration rejection, the misalignment angle δP is compensated by injecting a trimming current, which is proportional to the output of an orthogonal accelerometer, into the torque coil of the accelerometer during the scale factor matching. The experimental results show that the common linear acceleration suppression achieved three orders after the scale factors balance and five orders after the misalignment angles compensation, which is almost down to the noise level of the used accelerometers of 1~2 × 10−7 g/√Hz (1 g ≈ 9.8 m/s2).
Highlights
Over the past two decades, there has been a continuing and growing interest in the ability of moving-based gravimeters and gravity gradiometers, and this attention is focused on the remarkable achievements that has been made, and on the development of new methods and technologies in an attempt to find even better alternatives [1,2,3]
The output of the accelerometer a2 is multiplied by p and a4 is multiplied by q, and are injected into the torque coil of a2 for scale factor balance and misalignment angle compensation
The gravity gradient instrument (GGI) is mounted on a commercial rotary stage and the rotating frequency is set to 0.125 Hz
Summary
Over the past two decades, there has been a continuing and growing interest in the ability of moving-based gravimeters and gravity gradiometers, and this attention is focused on the remarkable achievements that has been made, and on the development of new methods and technologies in an attempt to find even better alternatives [1,2,3]. In our previous experiment of the scale factor matching [18], a small orthogonal signal at the spin frequency is found after the balance has been achieved. It will result in a residual sensitivity to the common linear acceleration. We report an alternative method to compensate the misalignment angles automatically during matching the scale factors of two pairs of the accelerometers in GGI. Experimental results show that the common linear level of the used accelerometers of 1~2 × 10 g/ Hz. scale factor matching and five orders after misalignment angle compensation, which almost reached the noise levelMisalignment of the used accelerometers of 1~2 × 10−7 g/√Hz
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