In this section, the system design and hardware selection criteria for the new SLS scanner are presented in Sec. 3.1. The selection criteria of the camera, lens, and projector are discussed in Sec. 3.2.
3.1 SLS System Design
This work uses the dual-camera form of SLS which has improved measuring accuracy compared with the single-camera form. The design of the system follows the conventional dual-camera system layout (Fig. 2(a)) which consists of two cameras, two lenses, and a projector. The spatial resolution requirement (5 µm) is used as the initial constraint for the hardware selection and it is determined by both the spatial resolutions of the cameras and the projector as follows,
where SR represents spatial resolution.
Camera spatial resolution. As Fig. 3(a) shows, a camera can capture images because its lens can pass the light reflected from the targeting object onto its internal image sensor. The image sensor consists of an array of small photosensors, each of which produces a pixel in the resulting image. The total number of the photosensors is termed pixel resolution, and the physical dimension of each photosensor is called pixel size. These are the two major specifications of an image sensor and they directly determine the sensor size as follows,
The camera spatial resolution is the physical distance between two adjacent pixels in the image. The smaller the distance is, the higher the camera spatial resolution will be. It is determined by both the internal image sensor and the camera lens as follows,
where SRcamera stands for the spatial resolution of a camera; FOVCamera is the field of view of the camera (the area camera can cover under the working distance); u is the working distance of the camera (the distance between lens and object) and f is the focal length of the lens (the distance between the lens and the sensor). The illustrations of all these terms are shown in Figure 3(a).
Based on Eqs. (1)-(4), it can be seen that the pixel size and pixel resolution are proportional to the camera spatial resolution and the focal length is inversely proportional to the camera spatial resolution. They also have the same type of influence on the camera FOV. The following basic methods can be used to improve the camera spatial resolution, as illustrated in Fig. 3(b). All of these methods are integrated into the proposed work and are presented in detail in Sec. 3.2.
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If the focal length of the camera lens is increased, then the FOV will be smaller, and consequently, the spatial resolution will be improved (see Figs. 3(b2)).
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If the sensor pixel size is reduced, then the FOV will be smaller, and consequently, the spatial resolution will be improved (see Fig. 3(b3)).
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If the sensor resolution is increased, then the cameral spatial resolution can be improved directly (see Fig. 3(b4)).
Projector spatial resolutions. The projector shares a similar principle with the camera in terms of spatial resolution. The two limiting features are the lens and microdisplay. Here the microdisplay is analogous to the sensor in the camera but it is used to project the image onto the object. In general, the resolution of a projector microdisplay (1280×720 pixels) is much lower than that of a camera sensor (3000×4000 pixels). Therefore, the projector is generally considered as the bottleneck for improving the SLS spatial resolution. However, this issue has been addressed by software and implementation techniques so that the resolution of the projector will not affect that of the SLS system. Specifically, two techniques from literature are adopted in our design to eliminate the impact of projector resolution to that of the 3D scanner, namely, the phase-shifting method and the defocusing technique [25, 26]. Instead of a single image projection, the phase-shifting method [25] projects multiple patterns (in this research, six patterns were used) with equally divided (2π ⁄ 6) phase shifts and using the combination of these six grayscale readings to distinguish adjacent points. Secondly, the defocusing technique [26] is utilized, which helps remove grayscale discontinuity (see the comparison between Figs. 4(a) and 4(b)). Thus, by adopting these two methods, the spatial resolution of our SLS is not affected by the projector but determined by the camera only, as long as the projector can focus on a similar FOV with the cameras. Correspondingly, Eq. (1) can be simplified as Eq. (5).
As exemplified in Fig. 1, in AM process monitoring, some small areas such as 15×15 mm2 should be covered by the FOV of the SLS. According to Eq. 3, the pixel resolution of the camera needs to be at least 3000×3000 to satisfy the SLS spatial resolution requirement (5 µm), which is the starting point of camera and lens selection.
3.2 SLS Components selection
A dual-camera SLS consists of two cameras, two lenses, and a project. The selection criteria for these components are discussed as follows.
Camera selection. A good SLS camera for use in metal AM in-situ monitoring should have a compact size, high frame rate, high pixel resolution, low noise, and small pixel size. The challenge here is to balance these requirements. Since the pixel resolution is set by the spatial resolution requirement as discussed in Sec. 3.1, so the selection begins with the sensor pixel size. A smaller pixel size will improve the spatial resolution given all other criteria are fixed. However, if the pixel size is too small, the noise level will be high. The sensor with a 3.45 µm pixel size was chosen for our system because it can capture melt pool details during the online monitoring without sacrificing imaging quality. To fit the cameras in the small FOV configuration required for metal AM, a machine vision camera (FLIR GS3-U3-123S6M-C) was selected due to its compact size and high pixel resolution (3000×4000) which can ensure a large coverage area without sacrificing the spatial resolution. Its frame rate is 30 Hz, which satisfies the scanning speed requirement mentioned in Sec. 1.1 (< 5 s for 45 images). The resulting FOV is 15×20 mm2 due to the aspect ratio of the sensor. This FOV is called “the desired FOV” in this paper and results in a camera (also SLS) spatial resolution of 5 µm.
Lens selection. To avoid the damage caused by the heat from the metal AM part surface, a relatively long working distance (u > 80 mm) needs to be maintained. According to Eqs. (3) and (4), given the pixel size (3.45 µm), pixel resolution (3000×4000), working distance (80 mm), and FOV (15×20 mm2), the focal length f needs to be at least 54 mm. Moreover, the lens should have an appropriate resolving power, which is the minimal distance between two lines or points that can be distinguished by the lens. It is determined by the optical polishing quality of the lens. In this work, the resolving powers of eight different lenses over 55 mm focal lengths are determined by using the 1951 USAF Resolving Power Test Target [27] as shown in Fig. 5(a). The conversion between the group & element number and resolving power can be acquired by using Table A.1 in the appendix. The lenses are all set to 65 mm focal lengths. Figures 5(b) and 5(c) show an example of the comparison between high and low resolving power lenses. A 55–75 mm zoom semi-telocentric lens is selected since it has the highest resolving power. It meets the Group 7 Element 2 standard and has a resolving power of 3.48 µm (Table A.1). This level of resolving power is very close to the determined sensor size of 3.45 µm. Therefore, it does not have a substantial influence on the spatial resolution of the whole system.
Projector selection. As for the projector, the one with the smallest micro-display (AAXA P2) was selected due to its compact size. The selected projector has a 1280×720 resolution and the lens was modified with an additional condenser lens to shift the projection area from 15×20 cm2 to 18×24 mm2, which is similar to the desired FOV. Even though the projector has a lower spatial resolution than the selected cameras, it will not affect the spatial resolution of the 3D scanner since the phase-shifting method and the defocusing technique were adopted in our work, which are discussed in Sec. 3.1.