An optimized scheme can improve the navigation accuracy of RINS without changing the inertial devices. In the multi-position stop scheme, the IMU remains stationary for most of the time, which makes motor control easier. However, the installation errors and the scale factor errors of FOG can cause platform misalignment after a certain angle of rotation around the horizontal axis, resulting in a velocity error. Continuous rotation can suppress time-varying errors better, which is of particular importance for FOG, but it can also increase the sawtooth error of the navigation output, and the error in the direction of rotation cannot be offset. To integrate the advantages of both rotation schemes, we propose an improved rotational modulation scheme for tri-axis RINS. In this scheme, the inner gimbal rotates in a two-position and four-order manner, while the middle and outer gimbals rotate continuously in the order of forward-reverse-reverse-forward. Simulation and navigation test results demonstrate that this improved rotational modulation scheme can effectively improve navigation accuracy by 50% and 25% compared with continuous rotation around the azimuth axis and a 16-position scheme with the same inertial devices, which is of great importance for RINS with FOG.

Benefiting from the rotational modulation technique and the gimbal structure, rotational inertial navigation system (RINS) can improve navigation accuracy by about one order of magnitude compared with the strap-down inertial navigation system (SINS). By continuously rotating around a fixed axis, the constant and slow variability errors perpendicular to the direction of the rotation axis can be effectively restrained while the errors in the direction of the rotation axis cannot be suppressed. Therefore, a multi-axis rotation scheme needs to be designed. Meantime, the fiber optic gyro (FOG), as a very popular inertial device, is well suited for RINS but has a larger scale factor error and poorer stability than the laser gyro; thus, it is not suitable to flip the inertial measurement unit (IMU) around the horizontal axis too much in the rotation scheme to avoid the rapid accumulation of horizontal misalignment angle. Accordingly, we propose a multi-axis alternating continuous rotation scheme for tri-axis RINS with FOG. During the navigation process, the inner, middle and outer gimbals rotate forward and reverse in sequence. The IMU rotates continuously in the three-axis direction can effectively suppress the effects of constant and slow-varying errors and also avoid the large velocity errors oscillation caused by flipping the IMU. The simulations and navigation tests show that the multi-axis alternating continuous rotation scheme can achieve navigation accuracy comparable to the multi-position rotation scheme and reduce the oscillation of velocity errors, which has important research significance and engineering value for the research of RINS with FOG.

Rotating modulation technology is an effective method to improve the navigation accuracy of inertial navigation systems which modulate inertial sensor error by rotating inertial measurement unit (IMU). Benefiting from the rotational frame and high precision angle encoders, RINS could complete various rotating strategies relative to the carrier without a turntable. However, in dynamic environments, using traditional motor control methods could destroy the effect of rotation modulation easily and stimulate the errors associated with the fiber-optic gyroscopes (FOG) and accelerometers. To maintain the navigation accuracy, we propose a high-precision motor control method to track the wandering azimuth coordinate system determined by the initial heading for tri-axis RINS. The innovation of the control method is that the information of FOG and absolute grating is used simultaneously to calculate motor control instructions based on the orthogonal coordinate system defined by the IMU’s measuring axis direction, and the influence of non-orthogonal angles among different motor frames are considered. The correctness of the high-precision motor control method is demonstrated theoretically and experimentally. Experimental results show that this method can isolate three degrees of freedom angular motion and improve control accuracy effectively, which has important engineering value, especially for high dynamic and heavy overload applications.

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