Prelocation image stitching method based on flexible and precise boresight adjustment using Risley prisms.

A prelocation image stitching method is proposed for the scan imaging system using Risley prisms. By confining the feature-based stitching procedure to several prelocated overlap areas in multiple subregion images, the proposed method reduces the computational complexity of feature extraction and image registration. Experiment results and analysis have validated the feasibility and robustness of the overlap prelocation algorithm, which can enhance the image stitching efficiency by 21.89% at least and by 39.38% at most. In addition, the composite image obtained from the prelocation stitching procedure can achieve a large field of view while maintaining high resolution.

Laser coarse-fine coupling tracking by cascaded rotation Risley-prism pairs.

Rotation Risley prisms are increasingly used for laser tracking due to high precision and good dynamic performance. In this paper, a novel laser coarse-fine coupling tracking method based on two pairs of rotation Risley prisms is proposed to perform the forward and inverse tracking function. The second pair of rotation Risley prisms with narrower wedge angle can achieve higher precision tracking with narrower field of view than the first one, enriching the coarse-fine coupling tracking trajectory patterns. Moreover, an inverse algorithm based on the two-step method and Newton's iterative method is applied to solve the inverse issue for laser coarse-fine coupling tracking. Two cases further demonstrate that the motion switching strategy executed with the inverse solutions can steer the beam to track the desired motion trajectory. An experiment validates the availability of the cascaded rotation Risley-prism pairs to perform the laser coarse-fine coupling tracking function.

Forward and inverse solutions for three-element Risley prism beam scanners.

Scan blind zone and control singularity are two adverse issues for the beam scanning performance in double-prism Risley systems. In this paper, a theoretical model which introduces a third prism is developed. The critical condition for a fully eliminated scan blind zone is determined through a geometric derivation, providing several useful formulae for three-Risley-prism system design. Moreover, inverse solutions for a three-prism system are established, based on the damped least-squares iterative refinement by a forward ray tracing method. It is shown that the efficiency of this iterative calculation of the inverse solutions can be greatly enhanced by a numerical differentiation method. In order to overcome the control singularity problem, the motion law of any one prism in a three-prism system needs to be conditioned, resulting in continuous and steady motion profiles for the other two prisms.

Performance characterization of scanning beam steered by tilting double prisms.

A pair of orthogonal tilting double prisms with a tracking precision better than submicroradian order exhibits a good application potential in laser tracking fields. In the paper, the beam scanning performance determined by both the structure parameters and the tilting motions of two prisms is overall investigated. The functional relation between the structure parameters and the exact beam scanning range is established, the capability of high-accuracy beam steering is validated together with the investigation of the scanning error sources and the nonlinear control laws, and the beam shape distortion degree under multi-parameter combinations is demonstrated. These studies can provide important references for the development of tilting double prisms.

Beam distortion of rotation double prisms with an arbitrary incident angle.

The distortion of beam shape in rotation Risley prisms is discussed in this paper. Using the ray-tracing method based on vector refraction theorem, a rigorous theoretical model of beam distortion with an arbitrary incident angle is established to explore the influencing factors. For a specified double-prism pair, the emergent beam is squeezed in one direction while stretched in the mutual perpendicular direction, the distortion of which is determined by the relative rotation angle. Moreover, the distortion of beam shape is greatly influenced by the wedge angles and the refractive indices of the prisms, as well as different double-prism configurations, while uncorrelated to the prism thickness and the distance between two prisms. This paper demonstrates the regular change of the beam shape with multiparameter variations in rotation double prisms, which can be applied to the design of rotation double-prism systems.

Investigation of beam steering performances in rotation Risley-prism scanner.

Rotation Risley-prism scanner appears to be the most promising solution to high-accuracy beam scanning and target tracking. In the paper, some important issues crucial to the function implementation are thoroughly investigated. First the forming law of scan blind zone relative to double-prism structural parameters is explored by a quantitative analysis method. Then the nonlinear relationship between the rotation speeds of double prisms and the change rate of beam deviation angle is presented, and the beam scan singularity is indicated as an essential factor that confines the beam scan region. Finally, the high-accuracy radial scan theory is verified to illustrate the important application owing to the high reduction ratio from the rotation angles of double prisms to the deviation angles of the emergent beam. The research not only reveals the inner mechanisms of the Risley-prism beam scanning in principle, but also provide a foundation for the nonlinear control of various beam scan modes.

Tilting double-prism scanner driven by cam-based mechanism.

A pair of orthogonal tilting prisms has been explored in our previous work to perform the orientation and position tracking function with tracking accuracy better than submicroradian order. Crucial to the function implementation, however, is the real-time nonlinear control of the tilting angles of double prisms for tracking a given target trajectory. In previous papers [Proc. SPIE5892, 1-5 (2005).PSISDG0277-786X; Appl. Opt.45, 8063 (2006).PSISDG0277-786X; Proc. SPIE6709, 41 (2007).PSISDG0277-786X; Appl. Opt.51, 356 (2011).10.1364/AO.51.000356APOPAI1559-128X; Appl. Opt.53, 3712 (2014).10.1364/AO.53.003712APOPAI1559-128X], a new driving method by a cam-based mechanism, which can transfer the control problem to the design of corresponding cam configuration, is investigated. The design process of a cam-based mechanism is explained from the mapping relation between the tilting angles of a prism and the configuration curve of a corresponding cam. Based on the designed cam-based mechanism, a tracking error less than 0.375% is depicted between the tracking trajectory and the original one. Moreover, the dynamic characteristic of the tracking mechanism is discussed in detail as well as the impacts of different tilting speeds on the tracking trajectory. The proposed tracking mechanism of a tilting double-prism scanner can create a new avenue for passively tracking a given target.

Inverse solutions for a Risley prism scanner with iterative refinement by a forward solution.

Risley prism scanners are increasingly used for laser beam steering due to their wide angular scanning range and high resolution. However, the inverse problem, which focuses on obtaining the required prisms' orientations for a given target position, has not been perfectly solved so far. The existing inverse solutions are not accurate or efficient enough for high-accuracy and real-time tracking. An iterative method that combines an approximate inverse solution with an iterative refinement by the forward solution is set forth in this paper. Two case studies indicate that the rotation motions of Risley prism pairs controlled by iterative solutions can slew the beam to create the desired tracking pattern quickly and accurately. Based on this method, a Risley prism scanner developed as a standard trajectory generator is implemented for the error measurement of a robotic manipulator in our experiments. The simulation and experimental results show that the inverse solution for one target point can be obtained within nine iterations for a prescribed tracking error threshold.