HIGH-SPEED CAMERAS: How to select a CCD camera for your applications

GREG FISK

Many industrial applications require the use of high-speed cameras. In manufacturing, such cameras are integral to parts handling, robotics, and surface-mount inspection. Test and measurement processes, such as nondestructive analysis and characterization of sprays and droplets, are common uses. The printing of reading materials, artwork, packaging materials, and fabrics is a major application for high-speed cameras, which can check the alignment of separate color plates in a high-speed, four-color printing press. And various other applications involve optical character recognition. The range of industrial uses for high-speed cameras is virtually unlimited.

High-speed imaging can be defined in different ways. From the users' perspective, a high-speed camera is one that allows large volumes of information to be processed in short periods of time (see photo). For such users, speed is usually measured in the number of parts per minute (ppm) scanned or the number of images that can be processed in that time.

For this reason, classification of high-speed cameras is generally broken down into two categories: applications below 1800 ppm (30 frames/s) and those above 1800 ppm. The selection breaks down into four basic types—area-array cameras, progressive-scan cameras, double-speed cameras, and line-scan cameras. Each type has properties that determine its suitability for a particular application.

Area-array cameras

All RS170-type cameras are considered area-array cameras. The RS170 standard refers to the timing specifications developed for black-and-white television systems. The standard specifies the horizontal and vertical scanning frequencies and a host of other parameters including tolerances allowed.

Area-array cameras are typically capable of capturing up to 1800 ppm. They capture images by fully charging the surface of the charge-coupled device (CCD) and then scanning the odd and even fields separately. Each video frame from an area-array camera is composed of the odd-numbered lines—1, 3, 5, and so on—interlaced with even-numbered lines. Generally, an interline-transfer-type CCD, a photosensitive array gate, is used for this purpose. Although they have limitations on their line-speed capabilities and vertical resolution, area-array cameras are popular for many applications because of their low cost and durability.

Progressive scanning

Progressive-scan cameras offer a higher level of performance than interlaced cameras by virtue of the way they capture images. Unlike conventional cameras, the CCDs in progressive-scan cameras read images from the top to the bottom—line 1, then line 2, then line 3, and so on. The entire image area is composed of only one field; there is no interlacing of odd and even fields. At higher speeds, interlaced scanning results in some blurring, because the object being imaged moves between the time the odd and the even fields are scanned. Scanning the lines sequentially eliminates such blurring. As applications approach the 1800-ppm benchmark, the inherent difficulty in matching the interlaced fields of RS170 cameras becomes more of a concern. Progressive-scan cameras, therefore, become a better option at higher scan rates.

In addition to higher scan rates, the scanning pattern of progressive-scan cameras makes them more compatible with computers, which do not use the interlaced pattern. Like area-array cameras, progressive-scan cameras can be applied in applications that call for up to 1800 ppm (30 frames/s).

Higher speeds

Double-speed cameras are progressive-scan cameras that scan at 60 instead of 30 frames/s. At that scan rate, they can process more parts per minute than their conventional counterparts. Double-speed progressive-scan cameras are well suited for applications that require up to 3600 ppm in partial-scan mode. One application where such cameras might be used is in scanning barcodes of parts being moved very quickly past the camera.

Line-scan cameras are for very fast applications and can scan single lines of information at a time. These cameras are expensive but well suited for applications such as continuous-web printing and paper manufacturing because they can selectively sample a small portion of the whole image area. Because these cameras scan progressively, they are also generally used in conjunction with computers to reconstruct and verify images.

For applications in which the parts move by the camera at a speed close to or higher than the 30-Hz rate at which the lines of the CCD are scanned, a 60-Hz camera is preferred. It can scan up to 3600 parts per minute in full-frame mode down to a single horizontal line scan. Such a camera emulates the function of a line-scan device, but is much less expensive.

Whichever camera is used, an external frame grabber and a triggering mechanism are needed to tell the camera when to capture an image. Our cameras, for example, are designed to accept an external asynchronous triggering signal set to a selectable shutter speed. When the reset is applied, a captured image (in the form of CCD current charges) is erased and the camera begins reading a new image from the top of the frame. The process repeats itself each time the trigger is activated. Captured images are continuously output from the camera to maximize the flow of data to the machine-vision processing software.

Some cameras use restart/reset triggering methods that signal the camera to shoot a single frame with each video reset signal received. Generally, these cameras offer four functions to select from: direct shutter control, accumulation of a single field, integration of two field images, and frame integration, in which the shutter is triggered every 30th of a second. Although these are effective ways to capture high-speed images, these types of restart/reset triggering inhibit the output of a continuous data stream from the camera and may increase processing time.

Future cameras

As camera technology evolves, several emerging trends will have a significant impact on their use. The transition to digital cameras will provide a new range of performance and feature parameters. For instance, digital cameras will allow users to selectively read data from more than one area of the CCD at a higher transfer rate than possible with conventional cameras. In addition, digital signals are more easily assimilated into computer network platforms.

A new high-speed imaging standard for transmission of data between camera and computer—called IEEE1394 and also known by its Apple Computer trademark, FireWire—is under development. This standard offers several benefits for high-speed applications. FireWire cameras will eventually eliminate the need for external frame grabbers and image-capture devices. We believe this will simplify the implementation of high-speed-camera technology while reducing costs.

The development of smart cameras is the ultimate goal of our camera-design team. Smart cameras will integrate and/or emulate the functions of specific hardware such as product sensors, digital interfaces, and strobe lights. Such smart cameras are being worked on widely.

In the meantime, a careful assessment of the requirements of a particular application will help determine what type of high-speed camera system best fits. Camera manufacturers also will be able to help users determine which models are appropriate.

Greg Fisk is machine-vision account manager at Panasonic Industrial/Medical Group, 1 Panasonic Way, Secaucus, NJ 07094; e-mail: [email protected].