Online time-space calibration method of hybrid wMPS-IMU system

Abstract

Limited by the principle of multi-angle intersection measurement, photoelectric scanning measurement systems such as workshop Measuring and Positioning System (wMPS) and indoor Global Positioning System (iGPS) have systematical positioning errors when measuring moving targets. Fusing target movement information can effectively compensate the positioning error introduced by the relative motion between the target and the transmitter. Inertial measurement systems are capable of high-frequency, all-weather and autonomous rotation and acceleration measurement. They have good measurement complementarity with photoelectric scanning measurement systems. Integrating movement information from inertial measurement unit (IMU) is an effective method to improve the dynamic measurement performance. Inertial information fusion needs two preconditions: the first one is clock synchronization and the second one is spatial alignment. In response to the two problems above, we research on wMPS and propose a wMPS-IMU integrated online calibration method. The time offset of the two systems is precisely calibrated by compensating the delay between the wMPS reference clock and the IMU clock. Due to the fact that wMPS receivers and the IMU are connected as a rigid body, the rotation R and the translation T between the wMPS and the IMU can be easily calibrated through a linear estimation method. Our novel method benefits to achieve real-time online temporal and spatial calibration without the help of any external measurement equipment and prior information and significantly improves the applicability of wMPS. Verification experiments were carried out. A three-axis turntable and a linear guide were utilized as the roundness and straightness references. The calibration experiment shows that the calibration accuracy of our method is at the same level as that achieved from the high accuracy coordinate measuring machine (CMM). The precision evaluation experiment shows that the hybrid wMPS-IMU system calibrated through the proposed method has slightly better dynamic performance than that calibrated on a CMM. This method is fully verified to be effective.