2009

Making the Move to Digital in Machine Vision

The situation has been changing over the last decade. Today’s digital cameras now offer improved interfaces to simplify system assembly. They also offer improved image sensors, capable of much higher speeds and resolutions than analog cameras. The digital nature of the sensors has also opened an opportunity for cameras to incorporate functions beyond image capture, increasing system design flexibility.

One of the first changes seen in digital cameras was the development of standard system interfaces. The proprietary digital interfaces limited developers to specific camera/system combinations. The rise of standard interfaces freed developers to mix and match components from different vendors as needed to meet their application requirements

altCameraLink was one of the first standard digital camera interfaces. Developed in 2000, CameraLink standardized
connector pinout and signal electrical characteristics for the interface cable. The cable was still bulky and expensive, however, comprising 26 strands that carry parallel digital bit and control signals. The cable was also still relatively short, with a 10-m length limit as compared to the 100-m length allowable under analog’s RS-170.

More recently, high-speed serial digital camera interfaces have arisen, including FireWire and Gigabit Ethernet (GigE). The move to a high-speed serial interface brought several advantages that addressed CameraLink’s limitations. One advantage was a reduction in cable complexity and cost. A 10-m CameraLink cable has a large, multi-pin connector and costs about $250. A GigE cable, on the other hand, is category 5 coax and costs around $15.

Of the two serial interfaces, GigE may be the most advantageous. The electronics industry’s extensive use of Ethernet ensures that expertise in and support for the GigE interface in machine vision systems is widely available, compared to the more limited availability of FireWire expertise and support. One indicator of the difference in support levels is that FireWire remains a specialty interface, while Ethernet is now a standard interface on almost all new PCs.

A second advantage of GigE over FireWire is the cable length supported. FireWire remains limited to a 10-m length, but GigE is virtually unlimited because it is the interface standard for networking. A camera with a GigE interface can be part of a machine vision system located on the far side of the world. The use of GigE also provides electrical isolation between camera and system and benefits from continuing innovation and technology developments that arise in the networking industry.

The development of standardized camera hardware interfaces has recently led to standardization in the software and control interface as well. Within the last three years, considerable progress has been made toward creating a common set of command options for digital cameras so that applications programs can become independent of the camera choice. Applications simply make standard calls to drivers that handle any data format or other hardware-specific differences.

Camera Capabilities Expand

In addition to improving system interfaces, modern digital cameras have expanded the capabilities of their image sensors. The best analog cameras today have a resolution limit of about 1 Mpixel with 30 to 60 frames per second (fps) image capture speed, for a data rate of about 40 MHz. Digital cameras, on the other hand, can easily achieve 100 to 200 fps with digitization speeds up to 160 MHz and resolutions that can go beyond 10 Mpixel.

Digital cameras also provide a much simpler and cheaper approach to color than analog cameras. In digital cameras, the three color signals (red-green-blue) are all automatically synchronized and use the same serial interface as monochrome cameras. Analog cameras, on the other hand, must provide three independent signals and synchronization of the digitization process requires careful handling in the frame grabber. A composite color video signal that needs only a single cable is possible, but at the cost of reduced resolution and color fidelity.

One of the latest innovations in digital cameras for machine vision is the availability of image preprocessing in the camera. A preprocessed video signal still has the data structure of an image, but has undergone some changes in the data content. The range of possibilities for the kind of changes a camera can introduce is wide open. For instance, a digital camera can readily put a time stamp on each image frame by selectively replacing data with white or black pixels to form numeric characters in the displayed image. Other possibilities include flipping the image vertically or horizontally, passing data through a threshold filter, or adjusting gain to increase contrasts. Many of these tasks are difficult or impossible to implement in an analog format.

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