A method will be described for using frame transfer (or in some cases full frame) CCD cameras to give improved images in applications where there is relative motion between the camera and object/target/web, etc. This method is similar to TDI (time delay and integration) mode imaging; however, it differs in several ways. It does not give a continuous image strip as does TDI, but instead gives sequences of conventional format images. On the other hand, the method to be described can be used with higher optical image velocities on the CCD since it is not limited by the pixel readout rate as is conventional TDI. Also, since imaging is completed while the object travels over a small portion of the field of view (FOV) rather than the full height of the FOV, the image quality with solid (3D) objects will be better than conventional TDI (assuming the same pixel array for both).
During the past year, several new computer camera methods (hardware and software) have been developed which have applications in machine vision. These are described below, along with some test results. The improvements are generally in the direction of higher speed and greater parallelism. A PCI interface card has been designed which is adaptable to multiple CCD types, both color and monochrome. A newly designed A/D converter allows for a choice of 8 or 10-bit conversion resolution and a choice of two different analog inputs. Thus, by using four of these converters feeding the 32-bit PCI data bus, up to 8 camera heads can be used with a single PCI card, and four camera heads can be operated in parallel. The card has been designed so that any of 8 different CCD types can be used with it (6 monochrome and 2 color CCDs) ranging in resolution from 192 by 165 pixels up to 1134 by 972 pixels. In the area of software, a method has been developed to better utilize the decision-making capability of the computer along with the sub-array scan capabilities of many CCDs. Specifically, it is shown below how to achieve a dual scan mode camera system wherein one scan mode is a low density, high speed scan of a complete image area, and a higher density sub-array scan is used in those areas where changes have been observed. The name given to this technique is adaptive sub-array scanning.
Cameras designed to work specifically with computers can have certain advantages in comparison to the use of cameras loosely defined as 'video' cameras. In recent years the camera type distinctions have become somewhat blurred, with a great presence of 'digital cameras' aimed more at the home markets. This latter category is not considered here. The term 'computer camera' herein is intended to mean one which has low level computer (and software) control of the CCD clocking. These can often be used to satisfy some of the more demanding machine vision tasks, and in some cases with a higher rate of measurements than video cameras. Several of these specific applications are described here, including some which use recently designed CCDs which offer good combinations of parameters such as noise, speed, and resolution. Among the considerations for the choice of camera type in any given application would be such effects as 'pixel jitter,' and 'anti-aliasing.' Some of these effects may only be relevant if there is a mismatch between the number of pixels per line in the camera CCD and the number of analog to digital (A/D) sampling points along a video scan line. For the computer camera case these numbers are guaranteed to match, which alleviates some measurement inaccuracies and leads to higher effective resolution.
Time Delay and Integration imaging offers a complete solution to the peripheral inspection/imaging of rotating cylindrical objects. Coupled with simple structured light schemes, the deformation or surface contour of the cylindrical object is highlighted and quantified. High speed TDI facilitate inspection of fast rotating objects, a feature preferred in industrial inspection systems. Experiments presented here are performed at rotation speeds of upto 2500 RPM. The experimental setup, influence of various system parameters are discussed in this paper. Examples using a food powder can as the object is provided.
Keywords: Machine vision, Digital imaging, Visual inspection, Moire methods
Machine vision systems routinely utilize structured light techniques for identifying the shapes of defects of the objects under inspection. The basic principle of the method is that any height difference from a reference plane causes a shift in the projection line of light either to left or right and up or down in the image plane of the recording camera. The height difference if due to a defect on an otherwise regular surface will result in a deformed light pattern corresponding to the dimensions of the defect. Moire patterns generated from this deformed light pattern can quantify the defect size, depth and shape. Existing machine vision systems use these techniques for the inspection of flat surfaces. Curved surface inspection although significant remains more or less unexplored. This paper presents the application of a TDI (Time Delay and Integration) camera for defect visualization on curved objects. The TDI operation and some applications of high speed TDI imaging will also be discussed.
A CCD computer-input camera is described which utilizes the virtual phase method for clocking. The camera was designed to acquire images by means of a PC, at low cost and with minimum parts count, and to allow for computer control of imaging parameters. Design components are listed, including ICs and image sensor, a low-resolution monochrome sensor, an 8-bit flash converter, and a programmed I/O. The software system is presented, including enhancements such as a method for avoiding streaking. Camera performance characteristics are listed. The camera resolution is shown to be 192 by 330 pixels with interlaced scanning. The camera permits electronic exposure control, and 8 bits per pixel gray scale. The camera is of interest in astronomical and other scientific applications, primarily due to its S/N and linearity.