A laser scanning projector can deflect a laser beam quickly and accurately, and the path of the laser spot can be shaped into a pattern, thereby facilitating processing and assembly. To ensure the precise calibration of the projection system, several cooperative targets should be scanned to solve the coordinate transformation equations between the world and projector frames. Typically, two calibration methods are employed: One involves manipulation of the laser to scan the cooperative target area, identify points on the edge of the target, and determine the center of the circle through least-squares fitting. This approach requires numerous scanning points, leading to an ineffective calibration process. The alternative approach utilizes binocular cameras for simultaneous multipoint positioning, which results in a reduced calibration time. However, identifying cooperative targets requires significant arithmetic resources. In addition, limited by the performance of the camera, this method is restricted to the calibration distance, and its ability to adapt to ambient light is poor. The correlated double-sampling (CDS) technique can be adopted to enhance the anti-interference ability of the system. Furthermore, a discontinuous scanning method based on the bisection principle is proposed. This technique can precisely identify the boundaries of cooperative targets as well as considerably reduce the number of scanning points, thereby guaranteeing the precision of cooperative target localization.
CDS was investigated to realize band-pass filtering and enhance the adaptability of the detection system to ambient light. Subsequently, an integral sampling circuit was designed to reduce the effects of high-frequency noise. The silicon photodiode operates in a zero-bias state and can produce an output signal proportional to the incident light intensity. According to the abovementioned features, a change in the signal light can be detected under different lighting conditions. TINA-TI was used for circuit simulation (Fig.3), and a circuit prototype (Fig.5) was constructed to verify the performance of the detection module. The control program is written to a 32 bit microcontroller to realize integrated functions, such as the output of control signals, signal acquisition, and information transmission. The designed printed circuit board was placed in the light-exit window of a laser scanning projector (Fig. 9). When the scanning point is within the high-reflection area of the target, the CDS module can stably detect the reflected signal. According to this feature, a scanning method based on the bisection principle was proposed (Fig.13). This can improve the positioning speed and accuracy of the scanning projection systems for target detection. The theoretical error of this scanning method was analyzed, and a comparison experiment between the grid-scanning method and the proposed method was conducted. The grid-scanning method can be used to obtain detailed point-cloud data for cooperative targets. The Canny operator and triangulation algorithm were implemented in MATLAB to extract the edges of the targets. These measurement results were adopted as benchmarks, and the same cooperative targets were measured using the proposed method under the same conditions. Finally, the number of scanned points and positioning deviations of the two methods were compared.
The sampling circuit designed for this study is capable of withstanding the power ripple influence of 60 mVp-p (Fig.4). Furthermore, although the signal-to-interference ratio (RSI) of -29.5 dB was calculated (Tables 1,2), the CDS detection module can stably output the signal. In this study, the scanning positioning error resulting from the perspective projection relation was determined as less than 1/10 of the galvanometer resolution. This suggests that the theoretical accuracy of laser scanning positioning is sufficiently high. Compared with the 10000 scanning points of the grid scanning method (Fig.18), the number of scanning points in the proposed approach is decreased by 97.4%. Moreover, the positioning deviation of the cooperative target is less than 0.06 mm (Tables 5,6). The new scanning method minimizes the irrelevant scanning regions and eliminates the image calculation process, thereby reducing the need for computational resources.
In this study, a novel method for detecting cooperative targets using CDS is proposed, which can achieve reliable detection even in the presence of ambient light interference (RSI=-29.5 dB). In addition, a discontinuous scanning method based on the bisection principle is introduced and verified. The results show that with the reduction of 97.4% in the number of scanning points, the deviation of the proposed method is better than 0.06 mm and simplifies the arithmetic process. Applying this technique to the developed laser scan projection system can improve the anti-interference performance during calibration. In addition, the proposed method can reduce time consumption and ensure position accuracy.
