In the overall performance evaluation system of artillery, the twist rate of the barrel is a crucial indicator. Existing methods for measuring twist rate feature complexity and limited measurement accuracy. With the continuous development of optoelectronic technology, panoramic imaging devices with large field of view and high resolution have emerged to enable seamless 360° panoramic imaging in a single shot without blind spots, thus capturing comprehensive information of the measured object. However, research on measuring the twist rate of barrels based on panoramic imaging technology is limited, and existing methods mostly rely on conical refractive panoramic imaging systems. Due to the shape and material properties of the conical refractive mirror, such systems may introduce image distortion, affecting image quality and posing challenges to image stitching and unwrapping. Thus, we propose a panoramic imaging-based automatic tracking and measurement method for the twist rate of barrels using refractive panoramic imaging technology, which achieves twist rate calculation without the image unwrapping algorithms.

We put forward a panoramic imaging-based automatic tracking and measurement method for the twist rate of barrels. Refractive panoramic imaging technology is utilized to obtain a panoramic image inside the barrel. After obtaining the image, a detailed analysis of the characteristics of the wide-angle panoramic image is conducted, and the image is transformed from Cartesian coordinates to polar coordinates. In the polar coordinate system, the image enhancement mapping function for the panoramic ring-shaped bore image is derived to enhance the features of the twist rate. To achieve automatic tracking and measurement of the twist rate, we build an annular template matching model to track the edges of the pre-set positive and negative rifling twists along the direction of the barrel, thereby accomplishing coarse positioning of the twist rate. Next, an objective function is designed based on the contrast differences between the positive and negative rifling twists within a single cycle. Precise positioning of the twist rate is achieved by optimizing this objective function to acquire the twist angle. After obtaining the twist angle, the axial distance of the barrel is combined to calculate the twist rate of the barrel and complete the twist rate measurement.

To validate the effectiveness of the proposed measurement method, we generate simulation data to verify the accuracy of the rifling tracking and positioning algorithms for target rifling tracking. Additionally, practical twist rate measurement experiments are conducted with real test barrels. The simulation experiments demonstrate that, in most cases, the algorithm's positioning error is less than 3 arc minutes (3′) (Table 1 and Table 2). Even under strong noise interference [Fig. 8(a) at 13°], the maximum positioning error of the algorithm is only 4′12″. Before conducting the actual measurement experiments, the measurement system is calibrated, and the acquired panoramic images are enhanced by an enhancement algorithm based on the polar coordinate system. The practical experiments of twist rate measurement show that the measurement results tend to stabilize with the increasing axial distance of the barrel [Fig. 11(a)]. During the measurement, the maximum twist rate measurement error is less than 3 arc minutes (3′) [Fig. 11(b)]. The twist rate measurement precision is better than 1′ as indicated in Table 4, confirming the effectiveness and feasibility of the measurement method.

Given the significance of twist rate in the quality and performance evaluation of barrel manufacturing, a panoramic imaging-based automatic tracking and measurement method for the twist rate of the barrel is proposed. Refractive panoramic imaging technology is utilized to capture internal bore images within the barrel. By deriving an image enhancement mapping function in the polar coordinate system for the panoramic ring-shaped bore images, the features of the rifling twist are enhanced. The inherent characteristics of the rifling twist are analyzed, and a two-stage positioning algorithm is designed to address the challenge of accurately tracking the rotation angle of the rifling twist. Meanwhile, an annular template matching model is built to obtain the initial value of the rifling twist rate for coarse positioning. Based on the contrast relationship of the rifling twist, an objective function is formulated, and an optimization process is employed to achieve precise positioning of the rifling twist rate. The twist rate of the measured barrel is determined by combining the axial distance of the barrel. Experimental results demonstrate that the proposed method for measuring the barrel's twist rate achieves a measurement accuracy better than 1′, thus verifying the effectiveness and feasibility of the measurement method. The method exhibits vast application potential in military fields, such as artillery performance evaluation.