Tutorial: line scan cameras
Tutorial: line scan cameras
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教程：线扫描相机
DOI：
10.1108/eb007797
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发表时间：
1989
期刊：
影响因子：
--
通讯作者：
C. Loughlin
中科院分区：
文献类型：
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作者：
C. Loughlin
我们将首先看一下从连续行扫描生成二维图像所必需的程序。一旦获得了该图像，就可以应用与用于区域阵列图像处理和检查的相同或类似的技术，这些将在以后的文章中介绍。线扫描相机对它前面的场景进行重复扫描，这些扫描被发送到一个滚动的现场存储和计算机系统，该系统分析建立的二维场景。区域阵列的两个维度被线扫描相机的单维度和额外的时间维度所取代。如果产品以连续的速度移动过相机，那么使用线扫描相机比使用区域阵列有许多优点。首先也是最明显的是可以获得更高的图像分辨率。512 × 512的分辨率现在在区域阵列相机中普遍可用（图2），目前已知的最大的商业阵列是柯达的Megaplus相机，分辨率为1320 × 1035像素。相比之下，现在常用的线扫描相机多达2048个元素，4096个元素阵列随时可用。如果使用线扫描相机来监控连续移动的生产线，则捕获的图像基本上是生产线的宽度和无限长，而在区域阵列相机的情况下，获得的是生产线的直角快照，这些快照要么相互重叠，要么在连续帧之间缺少间隙（图3）。这可能会在产品计数等应用中造成混淆，因为零件可能会遗漏或计数两次。此外，线扫描阵列的曝光时间（即一次扫描和下一次扫描之间的时间）通常小于1毫秒，而区域阵列通常以16或20毫秒（60/50 Hz）的积分时间运行。长曝光时间导致图像模糊和尺寸测量不准确，以及使特征识别任务更加困难（图4）。线扫描相机在所有方面都优于区域阵列的印象是错误的，因为区域阵列相机非常适合许多应用，并且许多区域相机现在都具有非常高速的电子“快门”或比50/60 Hz视频标准更高的帧率，这有助于克服上面概述的一些困难。我们可以在以下基于实际应用程序的工作示例中演示二维图像捕获技术。线扫描相机具有2048个元素，每次扫描的最小周期时间为200微秒（5000次扫描/秒），用于监控一条生产线，该生产线运输的产品直径为200毫米，每5秒（200毫米/秒）移动1米。线扫描相机的2048个元素使我们能够在产品上实现大约0.1毫米的检查分辨率。如果我们还想以相同的分辨率检查运动方向，那么我们将需要在输送带移动的200毫米内采样2048次扫描。这需要大约500微秒的扫描时间，这在线扫描相机的规格范围内。如果已知产品以恒定且均匀的速度移动，则可以通过相机内的可编程时钟触发扫描（图5a）。然而，通常情况下，产品将被安装在传送带上，其速度将受到长期漂移和短期抖动的影响。在这些情况下，有必要有一个编码器安装在传送带系统的传动轴上，或以其他方式连接到传送带的运动，这样它发送一个线触发脉冲到相机每0.1毫米的实际传送带运动（图5b）。
We shall look first at the procedures necessary to generate a two-dimensional picture from successive line scans. Once this picture has been obtained, then the same, or similar techniques to those used for area array image processing and inspection can be applied, and these will be covered in a future article. The line scan camera takes repetitive scans of the scene in front of it and these are sent to a scrolling field store and computer system that analyses the two-dimensional scene that builds up. The two dimensions of the area array being replaced by the single dimension of the line scan camera and the extra dimension of time. If the product is moving past the camera at a continuous speed there are numerous advantages in using a line scan camera as opposed to an area array. The first and most obvious is the higher resolution of the image that can be obtained. Resolu tions of 512 by 512 are now generally available in area array cameras (Fig. 2), with the largest commercial array currently known being Kodak's Megaplus camera with a resolution of 1320 by 1035 pixels. By contrast line scan cameras with up to 2048 elements are now in common use and 4096 element arrays are readily available. If a line scan camera is used to monitor a continuously moving produc tion line, then the image that is caught is essentially the width of the production line and infinitely long, whereas in the case of an area array camera, rect angular snap shots of the production line are obtained, which will either overlap one another or have missing gaps between successive frames (Fig. 3). This can cause confusion in applica tions such as product counting when parts can be missed or counted twice. Further, the exposure time (i.e. the time between one scan and the next) for a line scan array is typically less than 1 ms, whereas area arrays generally operate at 16 or 20 ms (60/50 Hz) integration times. Long exposure times result in a blurring of the image and inaccuracies in dimensional measurements, as well as making the task of feature recognition more difficult (Fig. 4). It would be wrong to give the impression that line scan cameras are superior to area arrays in all respects as there are numerous applications for which area array cameras are ideally suited, and many area cameras are now available with very high speed elec tronic 'shutters' or higher frame rates than the 50/60 Hz video standards that help overcome some of the difficulties outlined above. We can illustrate the techniques of 2-D image capture in the following worked example that is based on a real application. A line scan camera with 2048 elements and a minimum cycle time of 200 microseconds per scan (5000 scans/s) is used to monitor a production line that transports products that are 200 mm across and moving at a speed of 1 m every 5 s (200 mm/s). The 2048 elements of the line scan camera enable us to achieve an inspec tion resolution across the product of approximately 0.1 mm. If we also want to inspect in the direction of movement at the same resolution, then we will need to sample 2048 scans within the 200 mm that the conveyor belt moves in the second. This requires a scan time of approximately 500 microseconds which is within the specification of the line scan camera. If it is known that the product is moving at a constant and even velocity, then it is sufficient for the scans to be triggered by a programmable clock within the camera (Fig. 5a). However, it is often the case that the product will be mounted on a conveyor belt the speed of which will be subject to both long term drifts and short term judders. In these cases it is necessary to have an encoder mounted onto the drive shaft of the conveyor system, or in some other way linked to the belt's movements, such that it sends a line trigger pulse to the camera for every 0.1 mm of actual conveyor belt movement (Fig. 5b).