视频测量可实现高精度

由于光学器件中的像差可能被解释为零件中的测量误差，因此必须忠实地将图像忠实地呈现给相机。变焦镜头允许视频测量系统更改放大倍率，以最大限度地提高各种尺寸的特征的细节。必须校准放大率的每个变化，以实现测量精度 - 最好的系统会自动执行此操作。通过读取来自数字传感器（相机）中的每一个像素的信号电平来将图像转换为电信号。放大图像，直到感兴趣的特征覆盖许多像素，因此可以明确地确定各个细节。良好的视频测量系统使用子像素算法来扩展测量分辨率。在三维部分的情况下，有必要了解图像的图像的位置和整个系统的测量体积，需要在宏和微尺度上进行位置感测。单独采用Z运动，必须移动相机/光学组件，因此感兴趣的特征在于所使用的特定放大率的聚焦的光学深度（焦点通常随着放大率而降低）。整个相机/光学组件必须在移动时保持其临界对齐。在CNC系统中，Z-Motion是由电动线性载玻片，具有尺度以跟踪位置。 On a micro-scale, auto-focus takes over to maximize the sharpness of the image. System software notes the final position of the focused image relative to the original datum.To maximize image quality at the camera, proper illumination is important. Since measured features can be straight, curved, or be on the perimeter or the surface of the part, different methods of illumination are necessary. Back lighting (profile illumination) is best for through-holes and the perimeter of the part. Oblique angle top lighting is helpful for highlighting subtle surface features. For example, a ring-light made up of concentric rings of LEDs allows illumination at selectable angles of incidence as the different rings are illuminated. Segmenting the rings allows the light to be directional. This is especially helpful for features that lie in a particular orientation. It is rare for an entire part to fit within the optical field of view. This means the part must be moved under the lens until every feature to be measured is brought into the field. Since this is a measuring machine, any motion in the X-Y plane must be quantified. X-Y stage travels of video machines extend to as much as one meter or more. Parameters of importance include straightness of travel, stage speed, and positional resolution. Open loop motion uses counts of a known increment and assumes a position is reached based on the accumulation of a given number of counts. Closed loop motion adds a feedback device such as a linear scale to actually measure the stage position.Measurement accuracy depends on the structural integrity of the system. The three axes must be orthogonal (true 90∞ apart). This requires a precision design and exacting assembly. No part of the machine must move independently of any other or that offset will affect measurements.This requires a stable, damped mechanical structure, so video measuring systems are made of materials such as steel and granite for their structural stability.Video measurement consists of the choreographed motion of XYZ stages, magnification changes, setting illumination type, intensity, and angle, and the acquisition and processing of camera data, that ultimately lead to a digitized model which contains dimensional and angular relationships of the part under test. There is a lot going on, but user interaction with the machine need not be complex. Over the past 20 years, video measurement technology has advanced dramatically. Computers are faster. Cameras have higher resolution and better signal-to-noise ratios.Image processing software is better capable of ignoring background noise and artifacts. Stages are servo driven. Surface mounted components offer increased speed, lower noise and less energy consumption.Other sensor technologies are being added to video systems to expand their functionality. These multi-sensor measurement systems might use touch probes, allowing access to features that are beyond the range of the optics; laser sensors for auto-focus and surface scanning; micro-probes for miniature or fragile parts. Metrology software has evolved to accommodate multi-sensor measurement.This article was written by William Gilman, vice president, North American sales, Optical Gaging Products Inc., Rochester, NY.