Software powers Camera Link

JASON MULLINER is vision product manager, National Instruments, Austin, TX; www.ni.com.

Digital technology for acquiring images is pervasive, with applications from airport luggage scanners to postal-sorting machines. Imaging has been successfully applied to many challenging applications, and new imaging technologies are helping to solve new and complex problems. In that regard, the Camera Link digital imaging interface can help the machine-vision industry address current and future inspection requirements.

High-speed digital buses solve a need for high-speed and high-resolution image acquisition that isn't provided with the more established analog technology. Digital buses in conjunction with digital cameras provide large-scale flexibility because they are not bound by analog video specifications. Digital cameras provide higher resolution, greater pixel depth, and more variability in frame rates than do analog cameras. This increased availability of speed and spatial resolution makes it easier to incorporate vision systems into more demanding inspection applications.

The first digital cameras on the market were parallel format types that used a nonstandard LVDS or RS-422 electrical interface. However, along with the flexibility of parallel digital cameras comes complexity. A key challenge of parallel digital technology is connectivity. Typically, a custom cable had to be designed for every combination of camera and image-acquisition device because no standard digital imaging interface existed.

Camera Link

Now, the Camera Link standard defines the connectivity between a camera and an image-acquisition device, such as a frame grabber. Camera Link is based on National Semiconductor (Santa Clara, CA: www.national.com) Channel Link technology. This technology was originally developed for high-speed digital communication applications and was later used in digital RGB flat-panel displays. Pulnix America (Sunnyvale, CA; www.pulnix.com), Basler Vision Technologies (Exton, PA: www.baslerweb.com), and Dalsa (Waterloo, ON, Canada; www.dalsa.com) were some of the first camera companies to research Channel Link as a platform for machine-vision solutions.

In 2000, these camera companies, along with National Instruments and other frame-grabber companies, agreed upon the Camera Link specification. Since then, the Automated Imaging Association (Ann Arbor, MI; www.machinevisiononline.org) has endorsed this specification and, with help from member companies, is overseeing the development of the standard. A major benefit of the Camera Link specification is the definition of standard connectivity for both frame grabbers and cameras, thus greatly improving the users' interface experience. It also includes features that specifically address the needs of the machine-vision industry, such as provisions for triggering and precise real-time camera control.

Channel Link provided for the transmission of video data and video timing signals, such as frame- and line-synchronization signals. The Camera Link specification further defines the behavior of the timing and data signals to provide a consistent protocol. In addition to better defining the protocol for image-data transfer, six signals are included to specifically fit the needs of the vision industry. Four camera-control lines are also added to allow the frame grabber to control camera features, such as shutter speed and frame rate. The specification also provides for communications between the frame grabber and camera via six LVDS wire pairs based on the RS-232 protocol.

Camera Link features

Camera Link is versatile and can scale to higher bandwidths than those available today. Combining several Channel Link chips allows speeds up to 4.8 Gbits/s. Speed differences are indicated by different setup configurations such as Base, Medium, and Full. Image-acquisition devices are being designed to work with one or more of these configurations.

With one Channel Link receiver on-board, an image-acquisition device can see rates up to 1.6 Gbits/s. The Full configuration works with three Channel Link receivers and can deliver bandwidth up to 4.8 Gbits/s. Within each of these configurations, different tap arrangements can determine how the pixel data from the camera are arranged into an image. Currently, most cameras and image-acquisition devices support the Base configuration, while several devices also support the Medium configuration.

The challenge with high-speed image acquisition is that the PCI bus can rarely sustain data-transfer speeds greater than 100 Mbytes/s. This speed is hardly enough bandwidth for some Medium- and certainly all Full-configuration cameras. Currently, on-board buffers (with capacities of 8 to 128 Mbytes) store image data before transfer across the PCI bus. The design challenge is that continuous image acquisition is time-limited. New PC bus architecture using 64-bit data paths (compared to the current 32-bit paths) and faster bus speeds is on the horizon and will alleviate some of these speed concerns.

Software power

For PC-based systems, the selected software package must ensure that the Camera Link-based imaging system performs to specifications. A typical imaging software package must have the ability to acquire, process, and analyze an image and then make decisions, communicate with other measurement devices, and display results.

Acquiring an image relies on driver software that can communicate with a Camera Link-compatible device. This driver software is also the interface to the hardware for application software such as LabVIEW, Visual Basic, or C++. Common applications also require interfacing to other measurement devices (for example, making sound or vibration measurements in a cellular phone tester). Ease of use and flexibility also are important in evaluating Camera Link software.

Providing a configurable and interactive environment that can easily and rapidly change camera parameters is important. Setting up an imaging system requires connecting and configuring the camera and image-acquisition device. This process is greatly simplified by the use of interactive configuration software. Designers can select which cameras are connected to the devices and immediately acquire images from them while altering effects such as lighting and focus. Other changeable camera parameters include exposure control, triggering, and frame rate. After each device is configured, designers can access all devices directly in a LabVIEW, Visual Basic, C, or C++ program.

One of the caveats in working with a flexible standard such as Camera Link is that many different data configurations are possible. This situation requires that each camera must be assigned a unique interface file that dictates how information is being transmitted from the camera. The driver software then can correctly interpret the data as it streams across the Camera Link interface. These files are necessary because of the three Camera Link setup configurations (Base, Medium, and Full) and the myriad of tap configurations possible within each configuration. Most image-acquisition device vendors provide these interface files.

Camera Link also has a provision for serial communications between the image-acquisition device and the camera. Serial control enables designers to program change parameters such as exposure, trigger control, and a host of other functions. Typically, this is done over the several control lines that are built into the Camera Link specification.

Currently, there is a proposal before the Camera Link committee to provide a standard application programming interface (API) for serial control to both camera and image-acquisition device vendors. This API would ensure that, in the spirit of standardization, serial control would be identical for each camera regardless of vendor. These changes provide standard C and native Visual Basic support so that vendors can provide serial support with their products.

Camera Link applications

High speeds and easy connectivity are the noteworthy features of Camera Link. They translate into such benefits as flexibility, faster time to market, and low cost of ownership. Traditionally, large OEMs and machine builders have relied on analog cameras for their machines due to their easy connectivity and low prices. Previously, the expense of designing and manufacturing a custom cable for a digital camera introduced delays and unwanted costs. Many designers wanted to migrate to a digital format because of its inherent low signal-to-noise ratio and higher bandwidth. With the standard Camera Link cable, machine builders can easily integrate a digital solution into their machines and adopt the flexibility of achieving higher bandwidths in the future.

Whether it is microelectromechanical devices moving microscopic mirrors for guiding light through an optical switch or small organisms carrying much-needed vaccines, the need for inspection at the micron level has increased. The resolution requirements of tomorrow's applications will be served by the large bandwidth capabilities of Camera Link.